Chemistry of the Corrosion of Metals in Presence of Molten Vanadium Pentoxide

CHEMISTRY OF THE CORROSION OF METALS IN PRESENCE OF MOLTEN VANADIUM PENTOXIDE

THESIS Submitted for the Degree of

DOCTOR OF PHILOSOPHY

THE UNIVERSITY OF LONDON

KAILATHUVALAPPIL INNIRI VASU

February, 1964 Department of Metallurgy, Royal School of Mines, Imperial College, London, S.W.7. The author is grateful to Dr. D. A. Pantony, who supervised this research project, for his constant encouragement, numerous suggestions and stimulating discussions; to his colleagues and the members of the teaching and technical staff of the Department of Metallurgy, Imperial College, for their helpful co—operation; and to the British Petroleum Company Limited for financial assistance. CONTENTS

ABSTRACT

NOMENCLATURE vi I. INTRODUCTION

A. HISTORY 1

B. NATURE OF CORROSION 2 C. ACTIVATION - AND DIFFUSION -

CONTROLLED PROCESSES 4

D. RELEVANT PREVIOUS INVESTIGATIONS 6

E. APPROACH TO THE PROBLEM 9 (a) PHYSICAL-CHEMICAL STUDIES ON VANADIC MELTS (1)Dissociation Equilibrium 10 (2)Kinetics of the Oxidation of Vanadium Dioxide 11 (3)Cryoscopy 11 (4)Conductivity of Vanadic Melts 11 (5)Viscosity and Density 12 (b) CORROSION OF METALS IN THE PRESENCE OF MOLTEN VANADIUM PENTOXIDE (1)General Nature 12 (2)In Relation to Gas-Turbine Corrosion 13 (3)A Model for Study 14 (3a) The Corrosion Layer 19 (4)Metal Surface and Corrosion 23 (5)Prevention of Corrosion 23 II. EXPERIMENTAL A. MATERIALS (a)Vanadium Pentoxide 25 (b)Metals 25 (c)Metal Oxides 27 (d)Oxygen, Nitrogen, and Oxygen- Nitrogen Mixtures 28 B. SPECIMENS FOR CORROSION STUDIES 30 C. EXPERIMENTAL PROCEDURE (a)Container for Molten Vanadium Pentoxide 31 (b)The Thermobalance Assembly 32 (c)Dissociation Equilibrium in Molten Vanadium Pentoxide 34 (d)Velocity of Oxidation of Vanadium Dioxide 36 (e)Cryoscopic Work 38 (f)Viscosity and Density 40 (g)Conductance Measurements 42 (h) General Corrosion Studies 47 (i)Effect of Nitrogen on the Vanadic Corrosion of Metals 50 (j)Convection Effect on the Corrosion of Metals 51 (k)Diffusional Corrosion Studies 51 (1.) Action of Molten Vanadium Pentoxide on Metals in the Absence of Oxygen 53 (m)Compatibility of Sodium Sulphate (Chloride) - Vanadium Pentoxide System 54 (n)Electrochemical Work 55 (o)Electron Probe X-ray Microanalysis 56 III. STUDIES ON MOLTEN VANADIUM PENTOXIDE - RESULTS AND DISCUSSION A. DISSOCIATION EQUILIBRIUM 1. RESULTS (a)From Weight loss under Different Oxygen Pressures 58 (b)Weight-Loss on Mass-Flow Thermobal- ance 64 (c)Chemical Analysis of the Melt 64 (d)Effect of Sodium Oxide on the Dissociation Equilibrium 65 2. DISCUSSION 66 B. KINETICS OF THE OXIDATION OF VANADIUM DIOXIDE 1. MATHEMATICAL EXPRESSION 70 2. RESULTS 73 3. DISCUSSION 75 C. CRYOSCOPY IN MOLTEN VANADIUM PENTOXIDE 1, MATHEMATICAL RELATIONSHIP 82 2. RESULTS 82 3. DISCUSSION 84 D. CONDUCTANCE OF VANADIC MELTS 1. RESULTS (a)Pure Vanadium Pentoxide under Oxygen 86 (b)Effect of Oxygen Pressure on the Conductance of Molten Vanadium Pentoxide 89 (c)Effect of Sodium Oxide on the Conductance of Vanadium Pentoxide 91 (d)Effect of Cobaltous Oxide on the Conductance of Vandium Pentoxide 92 (e)Effect of Nickel Oxide on the Conductance of Vanadium Pentoxide 94 (f)Effect of Ferric Oxide on the Conductance of Vanadium Pentoxide 95 (g)Effect of Tungstic Oxide on the Conductance of Vanadium Pentoxide 96

2. DISCUSSION 96 E. VISCOSITY AND DENSITY OF MOLTEN VANADIUM PENTOXIDE 1. RESULTS 108 2. DISCUSSION 109

IV. RESULTS ON VANADIC CORROSION OF PURE METALS A. GENERAL EXPRESSIONS 114 B. RESULTS ON THE CORROSION OF COBALT 1. General Nature of the Corrosion 117 2. The Product of Corrosion 120 3. Kinetics of the Corrosion 121 C. RESULTS ON THE CORROSION OF IRON 1. General Nature of the Corrosion 124 2. Kinetics of the Corrosion 128 D. RESULTS ON THE CORROSION OF TITANIUM 1. General Nature of the Corrosion 130 2. Kinetics of the Corrosion 130 E. RESULTS ON THE CORROSION OF TUNGSTEN 1. General Nature of the Corrosion 133 2. Kinetics of the Corrosion 133 F. RESULTS ON THE CORROSION OF MOLYBDENUM 1. General Nature of the Corrosion l35 2. The Product of Oxidation 136 3. Kinetics of the Corrosion 137 G. RESULTS ON THE CORROSION OF VANADIUM 1. General Nature of the Corrosion 139 2. The Product of Oxidation 140 3. Kinetics of the Corrosion 141 H. RESULTS ON THE CORROSION OF NICKEL 1. General Nature of the Corrosion 143 2. Kinetics of the Corrosion 144 I. RESULTS ON THE CORROSION OF CHROMIUM 145 J. ACTION OF MOLTEN VANADIUM PENTOXIDE ON METALS IN THE ABSENCE OF OXYGEN 146 K. DIFFUSIONAL CORROSION STUDIES 146 L. EFFECT OF SALTS AND METAL OXIDES ON THE CORROSIVE PROPERTIES OF VANADIUM PENTOXIDE 1. SODIUM SALT-VANADIUM PENTOXIDE MIXTURES (i)Sodium sulphate-Vanadium Pentoxide System 150 (ii) Sodium Chloride-Vanadium Pentoxide System 153 2. Metal Oxide-Vanadium Pentoxide Mixtures • 156 M. ELECTRO CHEMISTRY OF VANADIC CORROSION 157 N. SHAPE OF THE CORRODED SAMPLES l59 O. EXAMINATION OF THE METAL-SLAG INTERFACE 160 P. EFFECT OF GRAIN-SIZE ON CORROSION 162

V. DISCUSSION ON VANADIC CORROSION OF METALS A. INTRODUCTORY 165 B. PROPERTIES OF THE OXIDATION PRODUCTS 166 C. NATURE OF THE CORROSION PROGRESS 168 D. GRAIN-SIZE AND CORROSION 169 E. STORY OF VANADIC CORROSION 170 F. FACTS ABOUT DIFFUSIVITY 172 G. ENERGETICS OF CORROSION 175 H. POSSIBLE MECHANISMS OF DIFFUSION 187 I. EFFECT OF OXYGEN PRESSURE ON VANADIC CORROSION 188 J. PROTECTIVE MEASURES A(7-AINST VANADIC CORROSION 193

VI. SUMMARY AND CONCLUSION 197 BIBLIOGRAPHY 209 INDEX TO TABLES 219 INDEX TO FIGURES 222 ABSTRACT ABSTRACT

-This thesis embodies work carried out on the corrosion of selected transition metals in presence of molten vanadium pentoxide. In every case there is an initial fast corrosion step, and the major part of the work is concerned with the kinetics of this process• The mechanism of the corrosion has been investigated by the study of the transport properties of the melt and of the characteristics of the metal-slag interface.

Molten vanadium pentoxide dissociates to the dioxide, and the equilibrium composition of the melt is governed by the temperature and the oxygen pressure• This dissociation equilibrium has been examined gravimetrically and its existence confirmed by cryoscopic work on molten vanadium pentoxide. The heat of dissociation is comparable with the energy of activation for the vanadic corrosion of some of the metals•

At constant oxygen pressure, the oxidation of vanadium dioxide dissolved in the molten pentexide is found to be kinetically of the first order with respect to the dioxide concentration. The energy of activation is very close to that for the vanadic corrosion of vanadium metal which indicates that both might involve the same mechanism. The exponent of oxygen pressure in its relation to the velocity constant has a value close to that expected from considerations of the semiconductance of vPmadium pentoxide.

The conductivity of molten vanadium pentoxide and its variation with oxygen pressure is interpreted in terms of Wagner's model for an n-type semiconductor. The explanation for the variation of conductance after the addition of metal oxides is based on an interstitial occupation of the added cations and a consequent increase in the dissociation of vanadium pentoxide.

By the application of the Fia"NernPt concept of diffusion-controlled corrosion processes, an equation has been derived for the linear corrosion of some of the metals in the presence of molten vanadium pentoxide. Under stipulated conditions, this a measure of the diffusion coefficient which compares well with those calculated from the measured viscosity as well as from the concentration gradient obtained by the electron probe microanalysis of the corroded metal-slag system. Nickel is found- to obey a' logarithmic rate low, and chromium is the least corroded.

Electron probe X-ray microanalysis of the metal-slag interface for chromium and nickel revealed the existence a a. coherent corrosion layer; this is a- protective barrier for these metals, and is absent for other metals. Intergranular attack is generally absent for the vanadic corrosion of metals.

The variation of the corrosion rates with temperature follows an Arrhenius relationship. The energies of activation and the reaction constants for corrosion under comparable conditions vary from metal to metal. These results have been interpreted in terms of two rate-controlling sequential mass= transfer processes, viz. an inward diffusion of oxygen and an outward diffusion of the corrosion products, and a rational analysis of the different activation energies and reaction constants has been carried out. In the presence of excess of vanadium pentoxide and with the easily corroded metals the second step loses~ its significance, but it is of prime importance for the resistant metals and when a deficiency of vanadium pentoxide allows development of low diffusivity barriers- at the metal-slag interface. The diffusivity of this layer is governed by the nature of the corrosion iv. product.

An inverse variation of corrosion rate with the depth of melt vindicates the concept of a diffusion- controlled process: and although there is evidence that, at low oxygen pressures, transport of elemental oxygen predominates, at high oxygen pressures the transport is by an exchange between vanadium entities. Abrupt changes of corrosion rates are observed when the oxygen partial pressure is less than about 0.01, and this has been attributed to a change in the diffusion mechanism. At all oxygen pressures, however, th the corrosion rate varies with the n root of oxygen pressure, and the significance of n depends on the semiconductance of vanadium pentoxide and the dissolved corrosion products.

Vandium pentoxide causes decomposition of sodium sulphate and chloride at about 500 °C. The residue absorbs oxygen on fusion and evolves it on solilification, and its corrosive activity has been correlated with this oxygen absorption capacity. The-effect of the addition of other oxides on the corrosive property of vanadium pontoxide - invariably it is- increased - has been found to be due to a change in the transport properties of the melt. v.

Analysis of the results on the corrosion of metals and on the physical-chemical properties of molten vanadium pentoxide lead: to the conclusion that the' rate-controlling mechanism of vanadic corrosion involves- an inward diffusion of oxygen (or other active species) and an outward diffusion of the corrosion products: the intrisic properties of the metal itself are of little importance. vi. NOMENCLATURE

Symbn16 Chemical Reaction

T-- Chemical Equilibrium Equal to Equivalent to Vacant (ions in the lattice)

V02.5 or V205 Vanadium Pentoxide V02 Vanadium dioxide V204 Vanadium tetroxide p02 Oxygen Partial Pressure or oxygen potential

Expressions Vanadic: (i) containing vanadium pentoxide or vanadates, e.g. vanadic melt or slag (ii) caused by such melts, e.g. vanadic corrosion Nascent, Labile: distinct from and more reactive than allied entities, e. g. labile oxygen in an oxide.

Volume Ratio: molar volume of the oxide, MOx atomic volume of equivalent amount of metal CHAPTER I

INTRODUCTION 1.

INTRODUCTION

A. HISTORY The presence of vanadium and sodium compounds in the heavy fractions of fuel oils has been recognised for at least fifteen years as a major 1 - 4 cause of severe corrosion of gas-turbine blades. Vanadium pentoxide and sodium vanadates have been isolated from the solidified oil-ash residues and the's', mixed in the molten state, have been found 5, 6, to possess "catastrophic" corrosive properties. The nature of corrosion caused by sulT,hate-choride slags has been stated to be different from that caused by vanadium pentoxide in a reasonably pure 7. form or diluted with low-melting sodium salts. Oxides of lead, bismuth and molybdenum have also been found to possess corrosive properties similar 8 to those of vanadium pentoxide.

Much of the published work is concerned with qualitative investigation or empirical corrosion studies on commercial alloys. Little thought seems to have been given to the mechanism of corrosion, and work on pure metals is uncommon. However, there is general agreement on: 2.

(a) the unusual property of vanadium pentoxide in causing "accelerated" corrosion, (b) the necessity of the presence of a molten phase for accelerated corrosion, and (c) the inefficiency of the normal methods for preventing this type of corrosion. It is therefore thought that a fundamental investigation on: (a) the physical-chemical properties of molten vanadium pentoxide, and (b) the corrosion of single metals in its presence, might be more informative and might indicate the type of alloys which can withstand the corrosion or point to a method of preventing the corrosion. The results and the conclusions to be drawn from these systematic experiments can still be regarded as representative of the likely mechanism of this type of corrosion in practical conditions.

B•. NATURE OF CORROSION When a metal is exposed to oxygen continued reaction is possible by a counter-current diffusion of metal or oxygen atoms (or ions and electrons) through the grains, along grain boundaries or surfaces or pores of an intermediate oxidation layer at the metal-gas interface. The characteristics of this oxidation layer determine the mechanism of oxidation: in particular, the thicker the scale, the longer is the diffusion path. Catastrophic oxidation occurs only when there is a molten phase of the 1, 9 - 12 vanadium slag and under these conditions it is difficult to assume the continuous growth of an oxide layer. The oxide scale is unlikely to form or remain because it might (a) react with the melt to form a solution or low-melting "vanadates" (if these exist in the melt), or (b) be carried away from the metal by convection currents set by thermal or density effects, or by diffusion currents due to a concentration gradient.

In the first instance, thermodynamics of the metal-vanadium pentoxide system govern the oxidation of a metal; in other words, the feasibility of formation of metal oxides (or the corresponding vanadates) in molten vanadium pentoxide is a question of the stability of these oxides under these environments. However, the rate' at which this occurs is mainly a kinetic problem concerned with the mobility of the melt or the ions therein 40 and by other associated transport phenomena in the melt. It will be shown that, thermodynamically, all the metals studied should form oxides, and that the fluxing action of molten vanadium pentoxide is the cause of the rapid corrosion of metals which oxidise extremely slowly in the presence of gaseous oxygen alone.

C. ACTIVATION - AND DIFFUSION - CONTROLLED PROCESSES When a fluid (gas, liquid or both) acts upon the surface of a solid (as in the case of the corrosion of metals in presence of molten vanadium pentoxide), the rate of any reaction occurring is influenced by: (a) the chemical nature of the reactants, (b) the physical nature of the solid =face and the impurties therein, (c) the activation energy of reactions taking place between solid and fluid phases, and (d) the rate of transfer of fluid phase reactant to the surface of the solid. In turn, the transfer of the fluid phase reactant to the surface of the solid is governed by: (i) films of inert fluid, (ii) layers of solid reaction products, and (iii) ffiffusion coefficient of the reacting fluid component through the inert fluid component. It is recognised that certain of these processes or conditions can be rate-controlling. If, for example., the activation energy barrier of the chemical reaction is high, the rate of transfer of reactant to the solid surface has little bearing on the over-all reaction rate. Such reations are "activation-controlled". However, many reations between solid and fluid have such low activation energies that the controlling process in the: over-all reaction is transfer of reactant to the solid- surface. These reactions are "diffusion-controlled". If the product of the reaction between the liquid and solid dissolves in the liquid, this might influence the diffusion process at an early stage of the reaction, and i, almost certain to have a marked effect as its concentration increases. Generally, systems under diffusienal control: (i) have low activation energy for the surface reaction, follow the equation for a first-order process, for diffusion is a first-order process, and (iii) have reaction velocities independent of the nature of the solid, since reaction rate is a function of the diffusion coefficient and the- thickness of layers impeding diffusion. 6.

13' 'Diffusion can be treated as a rate-process , and hence the reaction rate for such processes should obey an Arrhenius typo of relationship. However, its interpretation may rest on the temperature-dependence of the "diffusion layer" thickness or of the diffusion coefficient in any part of the system.

D. RELEVANT PREVIOUS INVESTIGATIONS Rapid or catastrophic corrosion is encountered characteristically when: (a) metals are alloyed with vanadium, 'molybdenum, 8, 9, 14, 15 lead, bismuth etc. (internal contamination) or (b) the surface of the alloys are in contact with a'low.:rnelting oxide (external (..onAlmina, • 1, 9, 11, 16-28' tion) , and (c) a molten or sintered phase of the corrosive 9 contaminant is present . Moreover, the corrosion rate is not reduced by the presence of a deliberately laid-down protective oxide scale but is linear (or even accelerating) with time. At least two attempts have been made to explain the. 2 accelerated corrosion. One school of thought holds' that certain oxides dissociate to form a sub-oxide and "nascent" oxygen; the latter is exceptionally reactive and hence cause rapid corrosion. The second school of 1, 10-12, 21 thought holds that a molten oxide contaminant at the metal-oxide interface destroys any corrosion- resistant layer and leads to napiA corrosion; the molten phase acts primarily as an oxygen carrier and a destroyer of protective films.

The first step in a laboratory investigation is an attempt to reproduce the effect obsOved in practical gas-turbines. One simple test that has been used consists merely of placing a small heap of the vanadic ash on an alloy specimen heated in air. A more elaborate test can be conducted in an experimental rig consisting of a combustion chamber. Between these extremes, a variety of methods has been used to evaluate the behaviour of metals or to investigate other factors related to the problem: 1, 9 (a) crucible tests , with the specimens wholly or partially immersed in the vanadic melts, and (b) immersion tests, with specimens alternately dipped in and withdrawn from vanadic melts, or coated with a thin slurry representative of the vanadic ash, in order to siffUlate continual ash- 23, 27, 29-33. deposition in gas-turbines 9 Schlapfer, et. al. measured the loss in weight of the specimen after using a standardised electrochemical descaling technique. Measurement of the reduction in 34 thickness of the specimen is also possible. An oxygen- 8.

17. consumption method has also been employed . These methods are satisfactory for estimating corrosion of the. surface oxidation type, and are not generally suitable in those cases where intergranular penetration of the metal has occurred. For this reason some workers prefer to use the ultimate tensile stress of a specimen as a measure of the mechanical deterioration due to 35, 36 corrosion The most satisfactory method of determining rates of oxidation is continuous weighing 37 during oxidation. Burns has carried out such determinations and has detected an inflection in the corrosion progress versus time plot before the onset of the linear curve.

So far as the attack by irmrlium ashes is concerned, the base metals have been placed in the following order of increasing resistance: iron, cobalt and nickel; and there. is a long list of proprietary alloys. The protective properties of clir,odum as an alloying agent against high—temperature scaling are said to be considerably diminished in the presence of vanadic deposits.

Numerous attempts have been made towards the prevention of this type of accelerated oxidation of alloys in the presence of molten vanadium pentoxide: (a) additives, organic or inorganic, to the oil or to the combustion chamber, (b) ceramic or metal coatings on the alloy surfaces, and (c) chromising, aluminising and siliconising. So far, none of these has been proved to be commercially successful.

E. APPROACH TO THE PROBLEM The process of oxidative corrosion of metals in presence of molten vanadium pentoxide can be pictured as follows:

------> Diffusion of Diffusion of active slpecies active agent through almost through V205 pure V205 :corrosion product 10

Diffusion of Diffusion of products of products of corrosion away corrosion through through almost V205 +- corrosion pure V205 products GAS MELT "DIFFUSION LAYER" METAL 10.

Preliminary investigation on the corrosion of metals in presence of molten vanadium pentoxide has shown that the physico-chemical properties of the metal-slag interface (including the diffusion layer) influence the corrosion of nickel and chromium, and the physical properties of the melt control the corrosion of iron, cobalt, tungsten, molybdenum, titanium and vanadium. Hence two distinct but convergent lines of research have been adopted: (a) a general investigation on the physical and physico-chemical properties of vanadic melts, and (b) corrosion of pure metals in presence of molten vanadium pentoxide.

PHYSICAL-CHEMICAL STUDIES ON VANADIC MELTS Diocociation Equilibrium Vanadium pentoxide dissociates slightly in the molten state, and this defective nature results in deviations from exact stoichiometry. The changes in chemical potentiql across the homogeneous regions of the nonstoichiometric phase are likely to constitute the driving force of the oxidation reactions, and therefore the thermodynamics of the thermal dissociation: VO VO + r1 0 2.5 2 2

should be known to a much greater extent and accuracy than are available to date. Most of the earlier investigators have usually considered vanadium pentoxide as stoichiometric. 11.

(2) Kinetics of the Oxidation of Vanadium Dioxide The velocity of the reaction

V02 1 .,..(2) 7 02 V02.5 and its variation with temperature and oxygen potential in the environment are likely to throw light on the mechanism of the vanadic corrosion of metals, since the interaction between the metal and vanadium pentoxide is likely to reduce the pentoxide and itr replenishment is controlled by the kinetics of the reaction (2). (3) CrTescoLy A study of the state of aggregation of vanadium dioxide and pentoxide as investigated by a cryoscopic method throws light on the existence of the entities of the lower oxide in molten vanadium pentoxide. (4) Conductivit,, of Vanadic Melts Most of the metal oxides are generally semiconductors and exhibit electronic as well as ionic conductivity. The semiconductivity of some of these 38-40' oxides persists even in the molten state This T)roperty is to be expected in a marked form for vanadium nentoxide with its augmented nonstoichiemetry in the molten state. Since ionic and electronic diffusion is involved in many corrosion -„rocee,ses, the type of lattice defects in the structures, which dominate the diffusion mechanism, must be known. These can be deduced from the electrical properties of molten vanadium pentoxidb. '50 40 pptirs. 1.1„...... Imm._ill 1.I.1 30 .1...... caplemmin . .. 20 PILLIII rpp lA Ji GAS 9,41/00111"t gplil A .4111 4 I RESISTANCE AMIE IPA • -MELT

NERNST • • • LAYER 11 (RESISTANCE ra ) poyl Ali METAL COBALT PELLE S WITH 0.2g V 0 IN 207. CXYG .2 5 FIG.Z. MODEL FOR CORROSION (IA WITH 0.659 V205 ). STUDIES. I lA - 700°, 2 -750° 3-850°,4-900°,5-950!

., . .L 2 3 L 5 6 7 R 9 1 CORROSION TIME (hours) FIG.1. GENERAL NATURE OF VANADIC CORROSION OF COBALT. 12.

The absolute value of th,: conductivity reveals little, and additional information on the changes of .onductivity with temperature, environment (oxygen pressure) and changes of composition with added oxides has been collect in the present work. (5) ViJcosity and Density Any study of transport phenomena in the molten state is incomplete without a knowledge of the viscosity and density of the melt and their variation with temperature. (b) CORROSION OF METALS IN PRESENCE OF MOLTEN VANADIUM PENTOXIDE (1) General Nature In the early stages of the corrosion of metals in presence of a small amount of vanadium pentoxide, a remarkably rapid attack occurs (Fig. 1). This is due to the highly oxidising properties of vanadium pentoxide and the ease of transport of oxygen through the melt. Moreover, molten vanadium pentoxide acts both as a corrosive agent as well as a solvent for the corrosion products. In most cases the corrosion progress is linear with respect to time. The initial rapid oxidation slows down after a while and changes to one which cannot be explained by any of the normal rate equations. The corrosion progress is asymptotic towards a constant value in this case and is possibly due to the 13. consumption of the active molten phase and the consequent scale-thickening, total solidification or marked reduction in diffusivity. Experibents on rates of corrosion under these conditions should give an indication of the rate-controlling process and of the character and build-up of any protective barrier formed at the metal surface. From the experiments carried out it appears that the layer is highly characteristic of the corroding metal and the rates are in agreement with the general findings in published data on the vanadic corrosion of various alloys.

(2) L_. Relatim to Gas-Turbine Corr-sion In practice, however, where a continuous deposition of vanadium-bearing ashes is likely to occur as in the gas-turbine operations, only the initial stages of the oxidation are important, and much attention has been paid to this part of the corrosion process. In fact, investigations in the initial stages of oxidation will really reveal the basic mechanism which is likely to be merely modified by the corrosion products in the later singes. The equilibrium properties of the scales or slags, particularly with respect to temperature, are, governed by the phadt-aa.gram of the respective metal 28 oxide / vanadium pentoxide system . However, due to the slow dissolution of the corrosion products (metal oxides) homogeneous composition is not necessarily 14. attained, and graded high concentrations of metal oxide are likely to be f,.und near the metal surface. The end- product of any attack by vanadium pentoxide would therefore be solid solutions if it were not for the restraining effect of scale thickness on the oxygen- and metal- ion diffusion process as is certainly the case with nickei and chromium. The compounds present in the slag are, however, unlikely to affect the nature of the reactions at the slag-metal interface, since it seems certain that a metal vanadate will pass on its vanadium component to the oxide to form a solid solution as fresh oxide forms. However, the transport properties of the slags and consequently the rates of the reactipns at the slag-metal interface will depend on the composition of the slag or diffusion layer.

(3) f:-or Stu The system adopted for the corrosion studies is a flat pellet of a metal immersed in and known to be at the base of molten vanadium pentoxide contained in a crucible, and an atmosphere of variable but known oxygen pressure is maintained at the surface of the melt. The model adopted is represented in Fig. 2. The whole is weighed, and the increase in weight is a measure of the oxidative corrosion. The hydrodynamic diffusion layer at the. gas-melt interface is neglected since it has been shown to have no bearing on vanadic corrosion of metals. 15.

If Fick's first law is written for linear diffusion to a plane, the flux, which is a measure of the rate of dw reaction, Lt,77 1in the present system, is proportional to the extent of the area (A) and to the concentration gradient (R) of the reaction species measured in a direction normal to the plane: dw (3) - = A D aX .... where D is the diffusion coefficient. For the velocity of diffusion-controlled reactions between solids and 41, 42 fluids, Nernst expressed Equation (3) as (See Fig. 2): - FdwIE = A D (Co Cx) (4) rI where r1 is the constant diffusiunal resistance which is itsolf a function of the thickness, x . Since reaci4g species disappear at the surface, Cm * 0, and Equation (4) reduces to: dw C A D o • (5) 1 When a solid reaction product or a product of very high viscosity is being formed and accumulated on the surface (as in the case of the corrosion of nickel and chromium), or when there is a Nernst's "diffusion layer" separating the melt and metal, this can represent an additional resistance (r2) to the diffusim of the active fluid component to the solid surface. Equation (5), with the assumption that D associated with rIis virtually identical with D of r2, is then r_idifiod to: dw Co - 71T =AD *SOO (6) ri r2 16.

If there is a continuous supDly of the reactant, maintaining a constant concentrati n, and if the solid reaction product goes into solution, A and D are virtually constant in the initial stages of corrosion, and the. diffusional resistance, (r1 + r2), of Equation (6) is rate-controlling. This can then be most conveniently written to express the disawearance of the metal where w is the weight of the metal at time t, and integration with the condition that w = wo when t = o, gives: w -w= ADC o o__ t r1 r2 Thus, a plot of loss in weigh of the metal (or corres- pondingv.quantities such as the increase in weight of the whole system) against t should be a straight line, with ADCo the slope equal to the group r1 r2 which can be called the velocity constant of the corrosion process under the stipulated conditions.

In all these cases the diffusion coefficient has been assumed to be constant and the same for all the different diffusion paths. A constant concentration gradient and a linear distribution of concentration exist only if the diffusion coefficient can be treated as 43 constant . A steady state with these imposed conditions can hardly be realized in the corrosion of metals in presence of molten vanadium pentoxide. In Ch. V, evidence will be given which shows that the diffusion 17. coefficient of the metal ions does indeed vary from a high value in the slag to a low value at the slag-to-metal layer (the "diffusion layer"). However, under special conditions of oxygen diffusion when large amounts of vanadium pentoxide are present, the validity of Equation (7) has been demonstrated.

There are two special conditions arising from Equation (7), depending on the quantity and the depth of the melt of resistance r1•; the resistance of the Nernst's diffusion layer, rn, is a constant characteristic of the c metal in question. In either case, the velocity of corrosion obeys a linear rate law except in the case of nickel and chromium.

Conlition (i) For small depths of the melt, r1 is small compared with r and the corrosion rate constant becomes ADC' 2, r2 which is highly characteristic of the metal in study.

Condition (ill In the presence of a large excess of melt and when the depth of melt (x) is largo and has a qiffusional resistance rl (which is proportional to x) considerably greater than r2, Equation (7) can be written as: A D C w OW (8) o r1 o and the corr-,sion rate becomes independent of the metal in study (excluding nickel and chromium which form solid products on the surface and hence r2 is not negligible). 18.

- w) represents weight change in unit time for If (wo constant area and concentration (Co = 1 when p02 = 1, and oxygen diffusion is rate-controlling), then: A w a D r1 D a x Hence, if D is constant, the weight change per unit time, i.e., the corrosion rate is inversely proportional to the 4epth of melt and is independent of the metal in study. This is particularly the case when diffusion of oxygen or vanadium (v) or possibly vanadium (iv) is rate- controlling. On the other hand, if the diffusion of the metal ions is rate-controlling (electronic diffusion being rapid) the total diffusional resistance will be a characteristic of the metal as well. This means that the inverse proportionality to thickness will still hold, but the rate should also be dependent on the nature of 'the corroding metal. Exact obedience to the inverse proportional relationship also indicates that no rate- controlling process occurs at the surface of the metal.

Con:lition ( i1 a) Incidentally, an interesting outcome of Equation (9),

under the assumption that r1is equal to x, is: A w.x .... (10) In any case, A w.x is a measure of the diffusion coefficient, and a method is thus available for the study of this quantity.

19.

(3 a). The Corrosion Layer (i) In the derivations made from the model in Fig. 2, a uniform concentratirn gradient of the diffusing s-ecies from Co to Cx has been assumed, and if oxygen is the diffusing species, Cx = 0 at the metal surface. The diffusion coefficient has also been assumed as remaining substantially constant. However, if there is a "corrosion layer" of thickness m separating the melt from the metal (identical with or similar to the Nernst's diffusion layer represented by the resistance r2 of Equation (6) ), two separate layers must be considered in the extreme case. These are: (i) the melt column of depth (x - m) with the concentration difference of . (Co - C1) and with a diffusion coefficient of, say, Do, and (ii) the corrosion layer of thickness a where the concentration difference is (C1 - C2), probably with a different diffusion coefficient, say, Dm (Fig. 2). According to Fick's first law in its simllest sense, the flux for any diffusion species is given by: = - D dc .... (11) (x,t) lx The flux will be equal to the corrosion rate, - v (per unit area and unit time) under steady state conditions. Hence, integration of Equation (11) for the two cases gives: v (x m) = D (C - C ) for case (i) o 1 (12) 20. and v . m-Dm (01-C2) for case (ii) .... (13) The value of m will be highly characteristic of the metal. For nickel and chromium, m is the predominant diffusion barrier, whereas for other mota1, m is negligible in presence of an excess of vanadium pentoxide, and Equation (12) is applicable. Under the conditions where C1 ct! Cx = 0, and (x m) x, this becomes: v .x=DC0 • • .• (14) This reduces to Equation (10) when Co = 1 at D02 = 1; under these conditions both methods of treatment give the same final result. However, the treatment with the simple model adopted in the previous section gives a meaning to the velocity constant (Equation (7) ) for linear corrosion which is the case with most metals where the corrosion layer does not produce a significant second diffusion barrier when excess vanadium pentoxide is present. Clear distinction can also be made between the depth of melt and the diffusional resistance. According to the present treatment the corrosion rate should be linear with Co or the corresponding p02 when the depth is kept constant because Do can be assumed to be constant for minor changes of p02. Under these conditions, the single-layer treatment shows that the C velocity constant should vary with -21 (Equation (8) ) and this takes into account the change in the

21.

diffusional resistance with the oxygen pressure; this is as it should be since the diffusional resistance of any semiconducting oxide melt will vary with the L,xygen pressure.

(ii) Exact dependence on any of the equations developed above cannot, however, be expected because the simplifications involved in the deductions are not strictly applicable to the normal conditions of vanadic corrosion of metals. In fact in many cases, the rate of corrosion is likely to be dependent on a combination cf the two processes represented by Equations (12) and (13) and this is demonstrated in all but extremes of conditions. Moreover, it will be demonstrated that diffusion processes in vanadium pentoxide are remarkably sensitive to small additions of impurities. Under steady state conditions, substitution of the value of v.m from Equation (13) in (14) gives, after simplification and on the legitimate assumption that 02 = 0, the relation: v.x = DoC0 + Cl (Dm - Do) .... (15A)

Dmel + Do (Co - C1) (15B) Equation (15A) reduces to Equation (14) only when Moreover, since Dm Do since C1 is not equal to zero. Do is relatively large, C1 must be very nearly equal to Co, so that Co - 0 and Equation (15B) shows that the corrosion rate is governed by the diffusivity in the corrosion layer (Dm), i.e., 22.

vox -DC1 ctv.m (according to Equation (13)

when C2 - 0) • • • • (16) The growth of the corrosion layer can be assumed to be linear with time, i.e., m - kit (where k1 is a constant) (17) When the velocity (v) is expressed as 7dw. in terms of experimental change of weight with time, Equation (16) can be written as: dw dt x kit = DmCi .... (18) so that w D 1 t dt f dw km C f t (183) we 1 o or A w = k to t .... (19) D where k ----m1 and is a5s.umed to remain constant. Thus 1 in those cases where the corrosion layer is rate- controlling, a logarithmic rate law is to be expected. This has been demonstrated in the case of the vanadic corrosion of nickel. This treatment is a gross over- simplification of the situation since it can be argued that Do also varies with time and that m will not necessarily be directly proportional to time so that Ci is not likely to remain constant. Nevertheless, for short periods of time the approximations are likely to be reasonably valid and the deduction of a logarithmic rate law is justified. 23.

(4) li,otal Surface and Corrosion If diffusion is not the rate-controlling step, it may be that the nature of the surface of the metal, particularly with respect to grain-size, may have a bearing on the corrosion rate.

(5) Stu'.ius on Prevention of Corrosion (i) In the absence of an alloy with the necessary resistance to corrosion, the usual way of the prevention of corrosion of gas-turbine parts in practice is by fuel additions which will change the corrosive properties of the vanadic slags. The value of an additive is explored simly by mixing it with vanadium pentoxide and comparing the corrosive attack by the combination with that by pure vanadium pentoxide. In this connection, additions of nickel compounds to the feed oil have been more effective than others: this partial success has indirectly received attention in the present work.

(ii) An electrochemical method of the prevention of corrosion is possible if the attack by molten vanadium pentoxide can be regarded as electrochemical in nature. In effect, the chemical reaction is made the slow process by applying a potential (i.e. energy difference) in opposition to the chemical reaction. CHAPTER II

EXPERIMENTAL EXPEFIii

A. :=ERI2ILS

(a) V.-.nadium Pent)xide "High Purity" vanadium pentoxide (99.99% pure) was used for most of the work, except for the study of . the effect of addition of nickel oxide, tungstic oxide, iron oxide and sodium oxide on the conductance of vanadium pentoxide: for this, normal grade pentoxide was used. The main impurities in the high purity vanadium pentoxide (Johnson, Matthey & Co., Ltd., Lond)n) are silicon and magnesium.

(b) "Spectrographically Standardized" iron, cobalt, nickel, molybdenum, titanium and tungsten rods were used. The limits of impurities, as furnished by the suppliers (Johnson, Matthey & Co., Ltd., London) are given in Table I. Single crystals of chrotlum and 99.5% vanadium rods were supplied by L. Leight & Co., Ltd., England. For certain electrochemical work, Remko iron rods (0.03% C, 0.01% Si, 0.12% Mn, 0.005% P and 0.01% S) were. used. (Ernst B. Westman, Ltd., London). Molybdenum specimens of different grain-size were made from samples supplied by Murex Co., Ltd., England (0.008% Fe, 0.002% (0), 0.004% C, 0.0001% H, 0.001% N). Single crystals were made by "electron beam floating zone melting" and polycrystals by different thermal treatments; 26.

TABLE I. IMPURITIES IN THE SPECTROGRAPHICALLY STANDARDIZED METALS

,etal Irmuritiosl. D.D.. 1 "..; Co Ni Cr Mo M.r.-. Cv,. Ag Na Ca Mg AlI. Si Others 10 .. 5 3 41 e, .. 2 .. .. 7 0n.03%; (0) 0.01%; N2 0.01 c); Co 2 ...... 1 1 41 2 .. 8 ta'SI ,I Ni 6 .. ''' ild, .l 41 . . 1 1 7 Mo 60 .. 20 10 - 1 <1 ...... 1 .. 20 it :Ti 70 .. .. 10 .. 30 10 2 .. .. 41 30 10 20 It W30 .. 40 4...... 41 .. it 27. acknowledgements are being made to Messrs. Johnson and Guir. fJr their help in this connection. Single, bi-, and tri-crystals of iron and cobalt wore obtained from Metals Reserrch kta.

(c) Metal OxiThs (1) Ferric Oxi,M, Fo203 A.R. Ferric ammonium sulphate was heated to 600°C, and the yellow product with the partially formed ferric oxide was 7)owdered and heated 44' to 1000°C in air in a silica crucible . When powdered, it is brown-black in colour. (2) Nicol Oxxi10 Ni0. Nickel oxide (green) was prepared by heating A.R. Nickel nitrate in a platinum crucible at 1000°C. (3) Cobalt Oxide.t. Co0 Cobaltous oxide wasepared 415'. by the method described by Pantony and Siddii . "Spec-pure" cobalt sponge was dissolved in nitric acid, evaporated to dryness at 150°C and dried at 300°C for 8 hours to give black cobaltic oxide, Co203. Ignition of cobaltic oxide in a platinum crucible at 800°C in air for 12 hours gave the dark blue cobalto-cobaltic oxide, Co304. Decomposition of this at 950)C in oxygen-free nitrogen gave the light brown cobaltous oxide, CoO.

(4) Sodium Oxide, Na20. Addition of sodium oxide to vanadium pentoxide was effected by introducing sodium

28. metavanacrte, NaVO3, to the melt. In some cases, sodium oxide was formed in situ by the decomposition of sodium sulphate in molten vanadium 7entoxide.

(5) Tungstic Oxide, W03 and Niobium Pentoxide, Nb205. "High purity" oxides (Johnson, Matthoy & Co., Ltd., London) were used. (b) Aluminium. Chromium other Oxiles. These were of reagent grade. (d) Oxyonl. Nitroi,on and O,en-Nitro7on Mixtures. Pure oxygen, and "oxygen-free" nitrogen were used for most of the studies. Equilibrium studies were conducted with "extra-pure" oxygen. All gases were dried with silica gel and magnesium perchlorate before use.

Since the partial pressure of oxygen was very critical for many of the equilibrium studies, the oxygen content of the stock nitrogen was analysed by measuring 46, 47. the e.m.f. of the cell gasi Pt solid electrolyte - NiO C (lime-stabilized zirconia aargon) at 698° and 800°C. The apparatus used had bedirl.5eb up by 47 Steele . An atmosphere of purified argon was kept over the Ni - Ni0 pellet and the dried gas to be analysed was passed over the platinum electrode. The e.m.f. of the cell was measured by moans of a Vibron Electrometer. Knowing the oxygen potential of the Ni - Ni0 system 29.

48 (2EF = AGn - 113, 300 + 41.5 T) , the partial '2 pressure of oxygen in the gas was calculated. Typical results are given in Table II.

TABLE II. ANALYSIS OF STOCK NITROGEN FOR ITS OXYGEN CONTENT

TQM-) Pot. of cell Pot. of Pot. of Pt/gas p02 (UK) (v) Ni - Ni0 system (v) System (v) 971 0.541 0.7910 - 0.2500 6.46 x 10-67 -6 1003 0.518 0.7766 - 0.2586 6.34 x 10 -6 1073 0.466 0.7404 - 0.2744 6.97 x 10

For nitrogen-oxygen mixtures of low oxygen potentials, allowance was made for this oxygen-content of the nitrogen used.•

As an inert gas for dilution of oxygen, carbon dioxide was not selected because of the complications due to the

CO2-CO equilibrium at high temperatures; carbon dioxide also slightly interferes with the corrosion of metals. Nitrogen was therefore chosen. Since the molecular weights of nitrogen and oxygen are close, segregation of the components resulting from thermal diffusion was assumed to be negligible and, therefore, the equilibrium oxygen pressure over the melt was taken to be that of the flowing gas-mixture itself. PRESSURE GUAGE

A11CONTROL VALVE

. 1C ONTROL VALVE

SILICA GEL

NITROGEN PRESSURE CYLINDER TUBING GAS CYLINDER r{TO MANOMETER

_DRY ICE AND ALCOHOL 11 TO OXYGEN - DEWAR FLASK

I TO PUMP

FIG. 3. GAS MIXING APPARATUS . 30.

Mixtures with martial pressure of oxygen greater than about 0.05 atm were made by randomly mixing the dried gases by passing through a bed of glass-beads and then long lengths of tubing after individual monitoring in simple capillary flow-meters with di-butylphthalate as the manometric and blow-out liquid. A special technique was adopted to prepare mixtures of low oxygen potentials: oxygen, dried over silica gel kept in a cooling mixture of dry ice and alcohol was passed into an evacuated cylinder kept in a thermostat, and its exact pressure was read in an ordinary mercury manometer; nitrogen, dried similarly was then led in and the total pressure read in a Budenberg Gauge, which had been standardized against the vapour pressure of liquid 49 carbon dioxide at known temperatures (-6 to +-25°C). This gas-mixing apparatus is shown in Fig. 3. The parts ABC wore made of pressure copper tubing and part D of glass.

B. SPECIMENS FOR CORROSION STUD 4,S Discs of the metals, about 0.15cm thick and polished on both sides, were prepared by standard metallurgical techniques. One side of the representative samplec was etched for microscopic examination. This procedure gave a set of similar metal discs for study. The particulars of the different metal specimens are 31.

given in Table III. All specimens were degreased with chloroform before use. The grain-size given in Table III was obtained by comparing the etched sample 50' at 100X magnification with standard ASTM charts .

TABLF III. PARTICULARS OF THE POLYCRYSTALLINE METAL SPECIMENS

Metal Etchant ASTM Grain Size Diameter Thickness No. diam. (mm) (mm) (mm) Co HC1-HNO -HOAc 5 0.065 5.0 -2.- 0.05 1.5 ± 0.05 3 Fe Alcoholic FeC1 6 0.045 5.0 ± 0.05 1.5 ± 0.05 3 Ni Fe01 -HC1-HOEt 1 0.25 5.0 ± 0.05 1.5 ±- 0.05 3 Mo K3Fe(CN)6-NaOH 7 0.03 5.0 ± 0.05 1.5 ± 0.05 W K3Fe(CN)6-NaOH 7 0.03 4.0 f 0.05 1.5 ± 0.05 V HC1- FaC1 0.03 5.1 ± 0.05 1.5 ±- 0.05 3 7 HF-HNO -glycerol 0.03 3.0 ± 0.05 1.5 t0.05 Ti 3 7 Cr - Single Cut into ---- Crystal cubes

C. EXPERIMEN= PROCEDURE (a) Container f[Jr l'hlton Vanadium PontDxide The instability of molten vanadium pentoxide in inert atmospheres and its oxidizing capacity preclude the use of those container materials such as tungsten, molybdenum and graphite (or vitreous carbon) which are otherwise suitable for high temperature work. Most of the oxide ceramics such as alumina, zirconia and thoria are unsuitable because of the fluxing action of the Thermo — balance

Water-cooled plate

Thermocouple B.10.

Platinum 7. "------:;- wire

B.40. FIG.5. CRUCIBLE FOR CORROSION WORK.

B.10. GAS.

V 0tg

FIG.4. SILICA SHEATH FOR THERMOBALANCE. 32. melt. Silica is appreciably attacked and glass has low softening points. The only material showing reasonable resistance to these conditions is platinum. Even this was found to be attacked, though extremely slowly, by the malt; moreover, it generally showed signs of deterioration after prolonged use. Iridium has the disadvantage of being converted to the dioxide in an oxygen-containing atmosphere: a thick but not wholly cohesive or adhesive scale was formed. Analysis of the melt which had been kept in contact with iridium for 24 hours at 80000 in air showed the solubility of the iridium oxide in molten vanadium pentoxide to be negligible. This does not preclude the chemical and physical contamination of the melt by prolonged contact with the iridium oxide. Hence, for all studies except on corrosion of metals, suitably designed platinum crucibles were used. For electrochemical reasons, silica crucibles- were selected for the corrosion studies.

(b) The Thormobalance Assemblz The commercial form of thermobalance (Stanton Si recording balance, Stanton Instrument Co., Ltd., London) was adapted to the present studios; the main feature of alterations was that the crucible was suspended from the rear pan of the balance and the furnace assembly was underneath. With the silica sheath (Fig. 4), any atmosphere could be maintained in the reaction system. Y).

In order to avoid stray convection heating of the balance the suspension wire passed through a small hole in a baffle consisting of a water-cooled plate much 51 larger in area than the balance

The flow rate of gas mixtures was normally kept 100m1 per minute; this was chosen as an optimum to meet the.• conflicting requirements of the desirability of a. large flow-rate to avoid the inward diffusion of air from the outside and of a small flow-rate to eliminate, any buoyancy effect on the crucible which might upset smooth weight recordings. Since the gas passed slowly (12cm / minute) through the annular space of the sil- sheath, it was preheated to the same furnace temperaLL. by the time it reached the crucible. Accumulation of static electricity on the silica crucibles used in corrosion work upset the smooth weight-recording; laic:J was effectively eliminated by a simple expedient of winding some platinum wire round the crucible and. attaching it to the suspension wire (Fig. 5).

Except for studies on the cunvection effect caused by thermal gradients, the position of the crucible was- always selected to be in the hot zone, about Gcm long, of the furnace. Though set in a position, it wouJ .d be gently moving up and down over about 2 - 3 jom jia the hot zone of the furnace when the balance changed weight E.ncl 34. arrested / released automatically. In other words the crucible was not remaining stationary at a particular position in the furnace. This is an inherent weakness of the thermobalance, but since the furnace had an excellent hot zone of constant temperature (I- 100) of about 6 cm long, the small movement of the crucible would not appreciably affect the result.

For the equilibrium: 1 V 02.57---%. V 02 + 7 02 1 0*.0 (1)

K1 * (V 021 K 1'02 CV 024 The amount of the dioxide was determined from the change in weight due to the loss of oxygen from the system at a known -partial nressure of oxygen using the stoichiometry of Equati)n. (1). Weighed quantities of vanadium pentadde, were kept in a platinum crucible at fixed temperatures- inside the silica sheath (Fig. 4) underneath the thermobalance. Mixtures of nitrogen and oxygen of.o known oxygen pressure (0.00,058 to 1 atm) were passed over the molten sample of vanadium pentoxide until equilibrium was =hed as indicated by a constancy of weight at the temperature.

(ii) By Vacuum Thormobalance In using the vacuum balance, -ure oxygen was maintaj.ned at different low pressures and the equilibrium loss in weight of the melt at the particular oxygen 35. pressure was read to give the necessary data for calculation. Acknowledgement is made to Stanton Instruments Ltd., London who provided facilities for using the prototype of their Mass-flow Thermobalance.

(iii) pi Chemical final is For the chemical analysis of the molt after keeping it in platinum crucible in a horizontal furnace until equilibrium was reached under a stream of a gas-mixture of known oxygen pressure, it was chilled by drawing it out rapidly to the cooled end of the reaction tube. The chilled sample was then dissolved in 2N potassium hydroxide under an inert atmosphere of nitrogen. The dark residue, usually left undissolved, was analysed qualitatively to see whether it was pure vanadium oxide or was contaminated with platinum or its oxide. For quantitative estimation of vanadium (OF), vanadium (IV) and possibly vanadium (III) in this vanadium oxide, it was dissolved in warm dilute sulphuric acid and titratcd potentiometrically against standard potassium permanganate again under nitrogen. The amount of vanadium (IV) in the chilled melt was determined by the above method, and the total vanadium by a procedure involving the addition of a known excess of ferrous sulphate, complexing with hydrofluoric acid and titration with standard potassium dichromate using diphenylamine sulphonate as indicator: 36. ferrous sulphate reduces vanadium (V) to vanadium (IV) (E° = 1.15v) and potassium dichromate (E° = 1.30v for VI III Cr to Cr ) reacts only with the unused excess of fellous sulphate and not with vanadium (IV); however, potassium VII II permanganate (E° =1.52 for Mn to Mn ) oxidizes vanadium (IV) quantitatively, though slowly.

(d) Volocity of Oxidation of Vanadium Dioxide The reaction: 1 V 0 +- 0 2 4 2 V 02.5 .... (2) with the vanadium dioxide dissolved in molten vanadium pentoxide was followed by measuring gravimetrically the rate of oxygen absorption on the recording thermobalance at constant selected temperatures and oxygen pressures. Vanadium pentoxide was initially taken in a platinum crucible suspended in the silica sheath (Fig. 4) within the furnace. For uniformity of conditions, the amount of vanadium pentoxide taken was always the same, viz.,

3.6g (0.037 moles of V02.5). From the dimensions of the crucible used, the approximate surface area exposed at the reaction phase was about 2.0 ± 0.1 sq.cm; the' melt having a depth of 1.2 +`0.2 cm. Nitrogen was initially passed over the vanadium pentoxide at about 900°C to convert it partially to the dioxide which remains dissolved in the pentoxide. The amount of the dioxide produced was kept small lest it should separate 3r/ • as a solid: it never exceeded 0.00625 moles (corresponding to 50 mg. oxygen loss from the molten vanadium pentoxide), i.e., the mole-fraction of the dioxide in the melt was 0.158. Reacti,n at any suitable temperature between 700° and 950°C was started by changing the flow of nitrogen to one of nitrogen and oxygen of suitable composition. In all cases, the flow-rate was kept constant at 100 ml per minute except wh= the effect of the rate of flow on the velocity of the reaction was studied in detail. For different initial concentrations of vanadium dioxide, the half-life period at constant oxygen flow was the same indicating that the reaction is of the first order with respect to vanadium dioxide: the concentrations could therefore be expressed in terms of mg. of oxygen absorbed in the process. At higher temperatures and lower pressures, the difference between the initial weight and the constant final weight could not be taken as equivalent to the total concentration of vanadium dioxide since the final weight corresponds only to the equilibrium composition of the melt (predicted by Equation (1) ) at the selected temperature and oxygen pressure. Hence in every case, the melt was cooled to 680°C in pure oxygen until constant weight was attained. The product was taken to be almost exactly 100% vanadium pentoxide (V02.5), 38 . and the difference between its weight and the initial weight before the oxidation at the reaction temperature gave the total amount of the vanadium dioxide, say 'a' (mg.02). If x(mg.02) was the increase in weight after time t, the concentration at that instant was given by (a - x).

The reaction progress as given by the plot of log (a x) against time (for this first order reaction) was sometimes sigmoid in nature even though the main middle portion was linear. The shape may be explained as follows: in the initial stages, the transfer of the nitrogen flow to one of the nitrogen-oxygen, mixture is likely to introduce some inhomogeneity in the gas- stream, and at the final stages, the rate of the reverse reaction becomes significant enough to compete with the forward reaction.

(e) C32-33CO'AC Work The depression of freezing point of pure vanadium pentoxide produced by the presence of its lower oxides formed at different oxygen pressures was studied by the simple cooling curve method with a Pt/Pt-Rh thermocouple dipping in the melt which was being continuously stirred during the course of the experiment. In spite of continuous stirring and slow cooling, supercooling was appreciable, sometimes amounting to tens of degrees, and FIG. G. APPARATUS FOR CRYOSCOPIC WORK. 3,1. a suitable correction for it was difficult because of the sudden rise followed by an immediate and equally sudden fall in the temperature. An ingenious method of seeding a slightly supercooled melt (2° to 3°C) partially overcame the errors due to supercooling. The apparatus is shown in Fig. 6. Vanadium pentoxide, about 25g, was contained in a platinum crucible (2) resting on a pyrophyllite mould (3) fixed to a pedestal of nickel rod (4). Two silica tubes fixed to a pyrophyllite cover (5) of the reaction tube (6) directed the thermocouple (1, 1) into the melt. In the centre of the cover was another 1 cm diameter silica tube with the arrangement (7) for dropping crystals into the melt. A few crystals of vanadium pentoxide were kept in the limbs (8, 8), and by rotating them crystals were allowed into the melt. The furnace temperature was controlled by a variable transformer; the output voltage was adjusted to give a temperature 2° to 3°C below the melting point of pure vanadium pentoxide; the V02 V02.5equilibrium was. maintained at this supercooled condition of the melt by passing a necessary gas-mixture for a long time. (8 hrs normally). When equilibrium could be assumed to have been attained, a few crystals were dropped in by turning one of the limbs (8). In most cases, several attempts had to be made before solidification was found to occur. In some cases, solidification Water-cooled plate Tr- Platinum lead to crucible

Silica tube

Crucible Bob Sliding furnace

Thermocouple

FIG.7. VISCOSITY - DENSITY APPARATUS. did not occur if the temperature was not several degrees below the solidification temperature. However, these results were more reproducible than those with the stirring arrangement. Potential of the Pt/Pt-Rh thermocouple was- measured by a vernier potentiometer (Type 3126B, H. Tinsley & Co., Ltd., London).

(f) Viscosityjmd Density Attempts to use an Ostwald-type capillary 52 viscometer were unsuccessful because the molten vanadium pentoxide significantly attacked the silica from which the apparatus was made. A MacKenzie-type 53 of apparatus was therefore set up which, in principle, enabled simultaneous measurement of viscosity, density and conductivity of molten vanadium pentDxide to be made (Fig. 7). A gold-platinum bob was suspended from one arm of a balance -,)laced on a water-cooled plate over a vertical furnace. The melt was contained in a platinum crucible, 5.2 cm diam. and 6.5 cm high, resting on the bottom of a silica tube clamped vertically in the furnace. Density was measured by the Archimedean principle. The corrections in density results due to the effect of surface tension on the suspension wire were eliminated by the use of two bobs of different dimensions but similar wires. Upward or downward 41. motion of the bob in the melt was produced by changing the weights placed on the other ;an of the balance. An automatic weight-loading mechanism assisted attainment of a greater accuracy. For a given bob and crucible, the rate of movement of the bob through a certain fixed distance in the melt is directly dependent on the viscosity. The slope of the load-velocity plots, which are linear, is a measure of viscosity. The viscosity of vanadic melts in the present study IAT - obtained from 54 calibrations against standard glycerine solutions at 20°, 25° and 30°C. This is not wholly satisfactory, but no other standards in the temperature and viscosity range are quoted in the literature. The method was 55 actually developed for highly viscous glasses and borates (ranging from 5 to 10,000 poise). Owing to the very low viscosity of vanadium pentoxide (ranging from 0.06 to 0.13 poise) the measurements and calculations were more involved in the present work: thus, the plot of logarithm of viscosity against the logarithm of the slope of load-velocity curve was not linear for the standard glycerine s;lutifns of this viscosity range, unlike the linear plots obtained for the highly viscous 53. borates . It seems that the method is somewhat unsatisfactory for this low viscosity range and comparisons with other methods may be open to criticism. However, for internal comparisons such as those required 6A

O O

11 15 5

8 O

15 -2;7 ;112

FIG.Z.APPARATUS FOR CONDUCTANCE MEASUREMENT. 44%. in activation energy calculation, the method is satisfactory.

(g) Criductance Measurements Attempts to measure the conductance of molten vanadium pentoxide using the platinum container as one electrode and an immersed gold-platinum bob as the other electrode during the viscosity-density study (Sect. (f) ) were unsuccessful due to the inconsistnet and unfavourable cell constants. With platinum as the material for the 56 container and electrodes, the method of Bockris et al was adopted for accurate measurements. The apparatus assembly is shown in Fig. 8. The main change was that the electrodes were fixed in position so as to maintain a highly reproducible cell-constant, and the position of the crucible could be altered by an attached rack- and-pinion arrangement coupled with a reduction gear.

The Kanthal wire was wound on a 95 cm 1.D. alumina 57 tube (5) in such a way as to give a long hot zone. , and surrounded by a hard asbestos box (42 x 42 x 56 cm) packed with alumina.. The inner "reaction" tube of impermeable alumina (4) just fitted into a specially grooved pyrophyllite cover (6) resting on a Syndanyo. platform (7) fixed to the furnace at the bottom; the reaction tube was covered with an earthed nickel foil (8) which annuled the stray electric fields and helped to maintain a uniform temperature. An electronic controller (Type MK4, Kelvin & Hughes, Ltd., London) coupled with a variable transformer was used to maintain constant (±4 2°C) temperature.

The platinum wire electrodes, 2 mm diam. (1, 1), were spot-welded to platinum lead-wires, 0.6 mm diam. (2, 2). The electrodes were firmly held in a thick (2 mm bore) silica tube attached to a thick silica capillary holding the lead-wire (3, 3). The silica tubes with the electrodes were fixed to the pyrophyllite cover (6A) of the reaction tube by means of screw-clips and araldite cement. The inter-electrode distance was- about 2 cm. The cover also contained another silica tube (9) through which the necessary gas-mixture could be passed.

The melt, about 250 g of vanadium pentoxide, with or without added metal oxides, was contained in a platinum crucible, 6 cm diam. x 6 cm high, so that the electrodes would be at least 2 cm away from the sides of the crucible. The crucible was kept in a silica mould at the top of a silica tube pedestal (11) resting on the brass top of the rack-and-pinion arrangement (12). The rack-and-pinion arrangement coupled with a 100 to 1 reduction gear (13) proved to be an excellent device for upward or downward movement and for keeping the. crucible permanently in any desired position. The vernier (14) attached to the base could read to 0.01 cm and the success of the device was reflected in the highly reproducible conductance results obtained. The temperature of the melt was measured by a Pt/Pt-Rh thermocouple (15) just below the _platinum crucible.

The cell constants were determined with 0.1 M potassium chloride solution having a specific conductance -1 -1 of 0.012856 Jhm cm at 25°C, and were checked with 58. molten potassium chlorile . Occasional checks were done on the cell constant which changed by a maximum of 0.5% over the course of all the experiments. To overcome polarization, the electrodes were platinized when the conductance of aqueous potassium chloride was measured. For molten salts, platinizing was unnecessary. The electrodes were tapered to a point at the tip so that the amount of melt adhering to the ti-) when the crucible was lowered was a minimum, and there was little difficulty or error in reproducing exactly the depth of immersion of the electrodes in the melt. The crucible was raised slowly and the pint where the electrodes touched the surface of the melt was determined by noting the sharp change in resistance between the electrodes.

In high temperature conductance work, a correction 45. has to be made for the resistance of the electrode assembly. For this, the electrodes were shorted and their, resistance (r) was measured at different temperatures. The resistance of this shorted system was deducted from the. apparent resistance (R) of the melt to give the actual resistance (R - r) due to the melt alone.

The resistances were measured by means of a.. Capacitance-Resistance Bridge, Tripe No.1862, Cinema- Television, Ltd., London. In every case, the capacitance. was adjusted to zero so that the Parker effect, if any, 59' did not introduce appreciable error in the values

The- resistances were normally measured for depths of 0.5,- 1.0, 1.5 and 2.0 cm, and the average value was- taken as- the correct conductance at the selected. 60 temperature. The standard- deviation in adjusting the depth and taking the readings was only ± 0.014- ohm, and this could introduce a maximum error of only 0.6 per cent in the Conductance.

Electrolytic polarization generally decreases with increasing temperature and is expected to be negligible at the high temperatures used. This was tested in two ways: the cmductance was measured (i) for various depths of immersion (up to 35 cm) of 46.. the electrodes, and (ii) for various frequencies between 3000 and 14,000 c/s. The conductance varied linearly with the depth (and hence the area) of immersion of the electrodes, thus indicating the absence of polarization as well as the absence of any interaction between the electrodes and the crucible. In support of this, change of frequency affected the conductivity to a negligible degree..

The main source of error was the creep of the melt up the electrodes which changed the effective cell constant giving s7)urious and sporadic divergence in the conductance of some melts. These anomalies were. observed and were neglected.

Table IV gives the values of the cell constants and the values of the specific conductance of molten vanadium pentoxide at 809°0 in pure oxygen, 4 .

TABLE IV CELL CONSTANTS OF THE CONDUCTANCE ELECTRODES

-. . epth of Immersion Cell Constant Sp. Conductance of -1 -1 (cm) V205 (ohm cm ) at 809°C

0.5 1.2081 0.1874 1.0 0.7183 0.1868 1.5 0.5123 0.1863 2.0 074002 0.1866 2.5 0.3279 0.1860 3.0 0.2797 0.1866

Mean Sp. Conductance 0.1866 ± 0.0004

(h) C3noral Corrosion Studios Vanadic attack is an oxidative type of corrosion, and the rate of attack was followed by noting the increase in weight with time, as was recorded on the thermobalance. The increase in weight was independently found by weighing the metal pellets to be equivalent to the decrease in weight of the metal due to the oxidation; discrepancies were negligible in most cases and where divergence occurred, results were studied separately.

The specimen was kept in the centre of the cylindrical 48.

silica crucible (Fj4-4. 5), 1 cm diam. x 1.5 cm high, which was suspended inside the silica sheath arrangement (Fig. 4) of the thermobalance. The specimen was gnerally attacked from all sides, and the change in area due to the continuous oxidation was taken into consideration in all calculations. It was, 'however, generally observed that the top portion was corroded more and the specimen became conical in the final stages. There was no increase in weight in extremely low partial pressures of oxygen; the reaction of the metal with molten vanadium pontoxide under those conditions has been studied separately and found to be finite but extremely slow. Hence, for all experiments, the temperature of the specimen in presence of vanadium pontoxide was raised in a current of nitrogen, and at the required constant temperature nitrogen was replaced by a gas—mixture of known oxygen pressure. Slight decomposition of vanadium pentoxide is inevitable in the short period when the melt is in an atmosphere of nitrogen; but this was reduced to the minimum by a judicial adjustment of the heating time in all experiments. Almost immediately after the introduction of the oxygen mixture, the melt acquires its equilibrium composition and a uniform corrosion rate was observed. Accelerated oxidation occurs only above the melting point of vanadium pentroxide. 49.

Most of the experiments were carried out in silica crucibles. Unfortunately, silica was slightly attacked by molten vanadium pentoxide so that most of the present corrosion work may be regarded as that on the corrosion of metals under a very dilute solution of silica in vanadium pentoxide. Generally for a'. two—hour experiment the amount of silica dissolved in 1 g. of vanadium pentoxide was: 20 10 mg at 900°C, 15 ±- 5 mg at 800°C and 10 ± 5 mg at 700°C. In order to discover the electrochemical influence of one metal (platinum) on the corrosion of the other, a few experiments have been performed with platinum crucibles under almost similar conditions.

The depth of molten vanadium pentoxide was calculated from the dimensions of the crucible, the geometry of the sTecimen and the density of the melt. The depth of the melt was normally 0.3 to 0.4 cm with 0.8 to 1.0 g. of vanadium pentoxide. But the oxides formed and the silica dissolved during the course of corrosion are likely to change the depth in proportion. These effects were at a minimum when the initial weight changes were considered in subsequent calculations. In spite of all the precautions, an error cf 0.01 to 0.03 cm is to be exl-ected in the depths reported. 50.

To find out the change in weight of the metal, the slag was removed by boiling in 4 N sodium hydroxide solution for a long time; this loosened the slag, though it did not completely dissolve, and a light rubbing with a soft material helped to remove the slag completely.

The effect of additions on the corrosion of iron was studied by mixing them thoroughly with vanadium pentoxide and determining the corrosion of iron in their presence.

(i) Effoct of Nitro,:qen on the Vanadic Corrosion of Metals Nitride formation is not an infrequent observation with the metals particularly when they are in direct contact with nitrogen at temperabures. However, in the present study the metal specimen was under a considerable depth of the melt. The unlikely diffusion of nitrogen through the oxide melt, the rather poor solubility of nitrogen in these metals as compared with the considerable oxygen-absorption during their oxidation and the definite decrease of the corrosion rate with decrease of the oxygen partial pressure (i.e_. increase of nitrogen partial pressure) in the gas mixture - all these leaa to the conclusion that nitrogen was an inactive gas in the study. This was further confirmed by the absence of any change in corrosion rate for some of the metals studied in argon-oxygen mixtures

-- --_-- - -- 847 °C (847 - t )°C __--____ Melt —>-_-__ = (uniform) -_-= ______.-z Metal —>i. • 847 °C. , 847 °C. 847°C. A DIFFUSION ONLY. INCREASED DIFFUSION DECREASED DIFFUSION

DECELERATED CONVECTION ACCELERATED CONVECTION.

FIG.9A.CONVECTION EFFECT ON THE CORROSION OF VANADIUMS%.1.2riTErS)

2.5

Top of the melt at lower temperature. (C).

Top of the melt at higher temperature. (B).

(A).

15 +16 +8 0 -8 -16 -24 TEMPERATURE DIFFERENCE OF TOP AND BOTTOM OF MELT. Fla9BCONVECTION EFFECT ON THE CORROSION OF VANADIUM UNDER MOLTEN VANADIUM PENTOXIDE IN OXYGEN. DEPTH OF MELT = 2.2cm. METAL AT THE BOTTOM, VANADIUM. 51.

instead of nitrogen-oxygen mixtures (Chapter IV).

Convocti,n Effect on the Corr:si)n of Metals Thermal gradients in the column of melt might produce convection currents and augment the corrosion rates of metals under oxygen. Diffusion processes will also be altered, and the combined effect was studied under the following conditions (Fig. 9A): a column of 2.2 cm of molten vanadium pentoxide in a narrow crucible with vanadium specimen at the bottom; the metal at the bottom was always at 847°C and the top of the melt at lower (-t) or higher (+t) temperature by a proper selection of the position of the crucible in a calibrated furnace.

The values are represented in Fig. 9B. It is evident that convection currents have a profound effect on the corrosion rates when the thermal gradients are beyond 4 or 5 degrees, which, however, never occurred in the normal work on corrosion. Moreover, convection could be predominant only when the melt columns are very long, and for small depths of melt, it is almost absent or negligible in analogy with heat transfer 61. processes .

(k) Diffusi,nal Corrosion Studios The method used for the general corrosion studies 52.

was adapted to study the oxygen diffusion through molten vanadium pentoxide. In order to minimize c nvecti n currents, the cross-socti)n of the melt column was made as small as practicable. A long, narrow tubular crucible of silica, 0.5 cm diam. and 2 to 4 cm high, with a suitable metal specimen at the bottom was filled with vanadium ;.entoxide- and heated to the desired temperature in a current of nitrogen. The metal reacted but slowly with vanadium pentoxide al.onp; even s), this was reduced to tho- i7limum in all cases. Nitrogen was suddenly replace.I. by oxygen, and it diffused through the melt. Since the column was it might be taken that the oxygen reaching the bottom was completely consumed by the metal (i.e. C[17 0). The increase in weight was thus, in effect, the amount of oxygen diffusing through the melt and reaching the bottom. Since there was sufficient vanadium pentoxide to dissolve the metal oxide efficiently, there was no complication arising out of barrier oxide formation, particularly if the initial weight changes were. taken. The depth of molt was obtained from density and quantity of vanadium pentoxide, the dimonsims of the crucibles being known, or from actual measurement of the mark on the silica crucible left by the attack of molten vanadium pent oxide. 53.

(1) Acti n ,f Molten Vanadium Pentoxide on Metals in t'2.e Lbsence of Ox en To study the action of molten vanadium pentoxide on metals in the absence of oxygen, onygen must be removed from the environment. Evacuation or a continuous flow of an inert gas such as nitrogen is an easy answer; unfortunately molten vanadium pentoxide decomposed under these conditions. Hence, the only possibility was to remove the oxygen by means of nitrogen in an enclosed system containing the metal and vanadium pentoxide at a sufficiently low temperature for no decomposition to occur, and then heat the system to the required temperature with the enclosed nitrogen therein. Depending on the volume of the system and the extent of decomposition, the ambient atmosphere would be kept to a low equilibrium oxygen potential. In the present studies, the system had a capacity of about one litre, and the enclosed nitrogen, ultimately containing the Equilibrium pressure of oxygen, was continuously circulated over the metal under molten 62 vanadium pentoxide by means of a simple pumping device .

The metal specimen and vanadium pentoxide in a silica crucible was kept in the centre of a horizontal furnace with a suitable magnetic device to pull out the specimen for chilling. Air was displaced by 54. purified nitrogen, and when the temperature was around 650°C and the system was completely free of air, the enclosed nitrogen was allowed to circulate and the temperature was raised to the required value. After a specified period, the crucible was drawn out, and the slag removed by boiling in 4 N s ,dium hydroxide solution* The loss in weight of the metal due to the attack by molten vanadium pentoxide under these conditions was obtained by weighing the corroded sample.

(m) Com:Datibility of Sodium Sulj2hate (Chloride) - Vanadium Pont oxide System Mixtures of known composition were made from dried- and finely powdered samples of vanadium pentexi'Te and either sodium sulphate or chloride. Known quantities were taken in a platinum crucible and their decomposition studied on the thermobalance first in a flow of nitrogen and the behaviour of the residue on co)ling or heating in • a current of oxygen was examined later. The amount of the vanadium dioxide (or the tetroxide, V204) was determined from the amount of oxygen liberated on cooling the molten residue or reabsorbed on heating the solid residue in oxygen, In some cases, it was checked by a chemical analysis (Sect. C (iii) ). In the case of sodium chloride - vanadium pentxide system, the possibility of attack of the platinum by the liberated VARIABLE RESISTANCE.

COVER TO FIT REACTION TUBE SAND HOLD ELECTRODES.

FIG.10.APPARATUS FOR ELECTROCHEMICAL WORK. 55.

chlorine was a deterrent for a complete study; part of the decompositions were therefore done in silica crucibles in a muffle furnace. In order to avoid decomposition of molten vanadium pentoxide in inert atmospheres, oxygen was used for temperatures above about 700°C. However, decomposition was found to be faster in a stream of nitrogen than of oxygen; probably this has some bearing on the mechanism of decontosition.

(n) Electrochemical Work Three iron rods, 0.65 cm diam. and about 25 cm long, were placed equidistantly (2 cm) in a large excess of vanadium pentoxide (50 g.) in a cylindrical silica vessel (4 cm diam. and 10 cm tall) (Fig. 10) supported on an insulating alumina pedestal within a vertical tube furnace. The area of the electrodes exposed to the melt was about 20 sq. cm. One of the rods was made the anode, a second the cathode, and a third as a neutral reference under identical conditions. The experiments were carried out in air. Current was taken from storage batteries and was adjusted by a variable resistance in series. After the experiment, the electrodes were drawn out, dipped in strong caustic soda solution, and finally boiled in it to remove the slag completely before measuring the changes in weight.

To find out the decomposition potentials of 56. molten vanadium pentoxide, the iron rod electrodes were replaced by two platinum s7)iral electrodes and the potential between them was measured by means of a valve. polentiometer.

(1) El ctren Probe X - ray Microanalysis Metal specimens, partially corroded in presence of small amounts of molten vanadium pentoxide under selected conditions, were rapidly chilled and cut longitudinally into halves so that the metal-slag interface could be exposed for examination. The samples had to be suitably mounted, polished and aluminium-coated (to make the slag-surface conducting) before the investigation. A Cambridge Microscan X-ray Analyser (Mark 2) was employed.

Irradiation of a selected part of the metal-slag interface with a fine-focus beam of electrons and spectro- metric analysis of the resulting emission of the x-rays excited in the sample and characteristic of its constituent elements enabled both a qualitative and quantitative analysis of the sample to be made. Incorporation of deflection coils in the electron-optical system of the instrument enabled the electron-probe to scan the surface of the sample and show the distribution of selected elements over the area scanned. For quantitative analysis the x-ray spectrometer was set 57• to the Bragg angles of the selected elements while a scaler included in the display electronics plotted peaks corresnding to the intensities of the characteristic lines emitted by the elements; these peaks were then compared with similar peaks produced from pure samples. CHAPTER III

STUDIES ON MOLTEN VANADIUM PENTOXIDE RESULTS AND DISCUSSION 58.

STUDIES ON MOLTEN VANADIUM PENTOXIDE RESULTS AND DISCUSSION

A. DISSOCIATION EQUILIBRIUM 1. RESULTS (a) From Wei:r ht-Loss under Different Olsy',en Pressures 63, T4 Literature data indicate that at temperatures above its melting point, vanadium pentoxide has an appreciable vapour pressure, particularly above 900°C. 65 However, close inspection shows that this pressure might equally well be ascribed to oxygen resulting from the dissociation equilibrium: 2V205(l) ;=:= 2V2O4 (solution) -H-02(g) .... (20) This reaction is displaced to the right at high temperatUres, particularly when the transpiration 63, 64 method is adopted to determine the vapour pressure. In practice, heating in a stream of reasonably pure nitrogen is sufficient to convert vanadium pentoxide to the tetroxide (dioxide) in a reasonably short length of time. Even in pure oxygen, molten vanadium pentoxide loses s.me weight fairly rapidly until it attains a constant value. This loss in weight increases as the oxygen pressure is reduced or when the temperature is increased, and is highly reproducible and reversible. Thus in all cases of a definite temperature and oxygen pressure, there is a definite an: constant loss for 59. unit mass, rather than a continuous loss with time (except at temperatures beyond 90000 where evaporation iscgpreciable). These observations lead to the conclusion that the melt is an equilibrium mixture of vanadium tetroxide (dioxide) and pentoxide, the composition of which is governed by the partial pressure of oxygen at a particular temperature. That this picture is correct can be seen by applying the phase rule to the equilibrium. There are two components, vanadium pentoxide and oxygen, and two phases, a gas and a homogeneous mixture of vanadium tetroxide in the pentoxide - a uniform solution. Hence f = c - p + 2 = 2 - 2 2 = 2 (21) The system is defined, therefore, when two of the three variables (oxygen pressure, temperature and composition) are specified. This means that at a definite temperature and oxygen pressure, the melt will have a definite comppsition. In this treatment, the condition that a single molten phase of uniform composition exists must be maintained throughout; if another phase, as when a solid separates from the melt, is present, the equilibrium will be distwrbed and the application of the phase rule niedified.

60.

For a complete study of this equilibrium in molten vanadium pentoxide, the nature of the species present in the melt and the manner of expressing their activities are to be ascertained first. Temkin's 66 suggestion of the use of mule-fractions for activities has been found to beewaerally valid in the molten 67. state . Temkin's model of the fused salt mixture represents a higher degree of disorder than the ordinary solid solution; from a statistical point of view, the particular species concerned are supposed to be randomly distributed in the mixture regardless of their electrical charge.

Assuming that the decrease in weight on heating vanadium pentoxide is equal to the loss of oxygen from the melt until equilibrium is attained at a particular temperature and oxygen pressure, the general expression for the decomposition can be written:

V o 2.5— V 0 (2.5 - n) 41- 02. (22) for which n/2. K = p02 . xVO n) SOSO (23) x VO 2.5 or

log K = log p02 +-log xV0 - log x. VO (2',5 - n) 2.5 .... (24) where x- representsmole fraction. If wt g or 61.

w 1 moles of the nentoxide has a loss of 90.95 (V02.5) w2 g. or 2 moles of417ygen, one mole of oxygen will be . w w evolved from 1 - 2 moles of V02.5. This quantity, -775 37 say't" moles, will be equal to the sum of the moles of

VO2.5 and VO(2.5 n) in the final state of the equilibrium according to the stoichiometry of Equation (22). One mole of oxygen liberated will also correspond to 2moles of VO and hence the mole- n (2.5 - n)' fraction of the latter is given by: - n) = 2/a V 0(2.5

= 2/an in the equilibrium mixture . is: and the mole-fraction of V02.5 • 5 ._ 2 an - 2 VO2. an an Therefore, Equation (24) can be written as: log K = 7n log. p02 +-log-7-2 - log f an - 2\ an or I • log (an - 2) = 41 log 102 + log 2 - log K .... (25) "a" and p02 are known in the exi.:eriment, and hence a plot of log (an - 2) against log 1:02 will give a straight line for the proper value of n (say, 0.5, 1.0, 1.5 etc) and the slope of the line should be 5; if the selected value of n is correct, parallel straight lines should be obtained for data at different temperatures. The value of n evaluated by this method (Fig. 11) gives: 0.50 at 700°, 0.49 at 750°, 0454 at 800°, 0.52 at 850° FIG.12. REACTION ISOBAR OF THE FIG.11. EVALUATION OF THE n :

VO2r; V02+402 EQUIL f BRIUMM. VO VO 401 . 2.5 (2.5--q 2 ( LOG K r 7') EQUILIBRIUM .

—1.6 —PLOT FOR n • 0.5

-~ •5

—2.0

—2.2

—2.4

—26

—2.8 1.6 —LOG p 02

30 8.0 8 4 88 92 96 10 10.4 VT) X 104

62.

and 0.52 at 900°C, and since this is quite close to 0.5, the equilibrium is to be represented as: V 02.5 .-=!t, V 02 +- 71 02 (1)

and 1 K = (1 024 . x / x V 2 2.5 1 5+ .... (26) - (p027 . xV 4+) / xV Lest solid vanadium dioxide (V02) be separated from the melt, the partial pressure 'f oxygen was never reduced to such a level that vanadium dioxide in amounts exceeding its solubility limits in molten vanadium pentoxide was produced. The tem:: rature variation of solubility N1 (in mole-fractions) of pure vanadium dioxide in an ideal solution (whatever the solvent) is 68 given by : 1 1 AHf - T) In Ni 69.... (27) R where AHf is the heat of fusion (13,600 cal) , and Td is the melting -;oint (1545°C) of vanadium dioxide. The values of the maximum solubility (in mole fractions) of vanadium di.oxide in molten vanadium pentoxide calculated on the above basis are: 0.032 at 677°, 0.038 at 700°, 0.097 at 850° and 0.16 at 950°C.

The results from the equilibrium studies by this dravimetric method are presented in Table V and Fig. 12. 63.

TABLE V. EQUILIBRIUM CONSTANT OF THE REACTION V02.5-v==±V02 +- 72 02 (g) IN THE MOLTEN STATE.: Platinum Crucible with about 3.5 g. of vanadium pentoxide; oxygen-nitrogen mixtures; rate of flow ca.100 ml/minute.

1 Equilibrium Constant (K in att7) x 103 p02 694° 760° 800° 863° 900° 956°C

1.0 1.08 2.49 3.99 8.01 12.1 16.8 ..2 1.22 2.55 4.21 8.11 12.3 1777 87048 1.07 3.97 11.8 15.8 - - 6.044 - 2.71 - 8.20 - 18.8 70185 - 2.81 8.25 - 18,4 ' - * 18.1 •,0146 1.24 4.22 13.4 6.00373 2.62 8.32 - 17.5 -. 1.00358 1.19 4.38 12.5 - - - •,00210 2.68 8.30 19.8 6.0007 1.23 4.54 12.2 - - 1.00062 - 2.96 - 8.39 - -

lean K 1.19±0.07 2.69±0.17 422t0.13 8.23±0.14 12.40.3 17.8±1.1

Note: The standard deviations were estimated from "Range Tables" * after rejection of wild results 64.

(b) Wei-_;ht-Loss Mass-Fl ;w Thermobalance In the studies with the nitrogen-oxygen mixtures on the Thermobalance (Section (a) above), the effect of the nitrogen on the melt is not known. Hence a few experiments were performed on a Vacuum Thermobalance with -,ure oxygen ke7,t at low pressures. The results are presented in Table VI. The loss in weight is again taken to c)rres::)nd to the amount of vanadium dioxide in the melt. Very low oxygen pressures could not be used because the loss due to evaporation under reduced pressure is appreciable. Even though the melt is not under isobaric conditions, the activity of the species is taken to be equal to their mole fraction, as in the previous calculations.

(c) Chemical Anal2is of the melt As a check on the validity of the assumption that vanadium is present only in the tetravalent and pentavalent states in the melt, a chemical analysis of the chilled sample of the melt under equilibrium was carried out. On dissolving the sample in 2N potassium hydroxide under an inert atmosphere of nitrogen, a dark residue was invariably left undissolved, and by microanalysis, it was found to contain only vanadium. When this residue was titrated potentiometrically against potassium permanganate, only one peak was 65. obtained in the lotential change indicating that there 70 was only one valency change , calculated to be vanadium (IV) .R.-=-A vanadium (V). The values of the equilibrium constants obtained from the analysis for vanadium (IV) and vanadium (V) are included in Table VI.

TABLE VI. EQUILIBRIUM CONSTANT OF: V02.5 i= V02 4 02. (By weight-loss in Mass-Flow Thermobalance and by Chemical Analysis).

1 Equilibrium Constant (K in atm4) Temp-- (CC) Thermobalance Mass Flow Balance Chemical Analysis

750 0.00232 0.0043 0.0016 850 0.00708 0.0093 070047 950 0.01820 0.0249 0.0180

The values are the average of several analysis: insufficient data make estimates of errors for columns 3 and 4 (Table VI) impossible, but the random and- constant errors of the last two methods are expected to be much greater than those of the thermobalance.

(d) Effcct Jf S:lium Oxide ,n the Dissociation Equilibrium The variation of the equilibrium constant with the added sodium oxide is given in Table VII. 66.

TABLE VII. DISSOCIATION EQUILIBRIUM OF VANADIUM PENTOXIDE IN PRESENCE OF SODIUM OXIDE

1 4 7 Heat of Mole % Equilibrium Constant (atm ) x 10' dissociation Na20 (kcal) 694° 760° 800° 863° 900° 956°C

0 1.19 2.69 4.22 8.23 12.4 17.8 25.6 0.064 1.59 3.23 5.76 9.72 15.4 20.7 23.4 0.78 2.08 3.77 6.85 11.16 16.1 22.6 21.3 1.42 2.49 4.50 7.19 11.78 16.8 23.7 20.4

The Table also contains the values of the heat of dissociation calculated from a plot of logarithm of the equilibrium constant versus reciprocal temperature. In order to avoid a large variation in activity coeffici7 ents and an undue excess of the complex sodium vanadates, the amount of sodium oxide added is kept very low. There was a tendency for the equilibrium constant to increase with decreasing oxygen pressure and this tendency is greater here than in the case of pure vanadium pentoxide (Table V).

. DISOU3SIO1\ Table VI shows that the equilibrium constants computed by the different methods are all of the same order of magnitude. The inefficiency of chilling, 67. particularly when the reverse reaction has a high velocity, is the main source of error in the chemical analysis method: for this reason, the equilibrium constants are smaller than expected. Excessive loss due to evaporation under reduced pressure, the uncertainty of the activities under these conditions and the unreliability of the balance performance lead to some divergence in the results from the Vacuum Thermobalance: the values are therefore expected to be high and this is found in practice. The results on the Thermobalance with the oxygen-nitrogen mixtures are therefore the more reliable for further thermodynamic studies on the system, and the vacuum balance and analytical results serve as confirmation of the satisfactory nature of the system used with the thermobalance.

The plot of log K against reciprocal temperature (Fig. 12) isa straight line between 694° and 956°C showing that the heat of reaction doer not apreciably change with temperature and the equilibrium constant obeysvan't Hoff's reaction isobar. Tho heat of reaction calculated from the slope is 25.6 ± 0.5 kcal/g.mole V02.5. For the reaction: V 02.5(1) = V 02 (s) +- 24: 02 (g) .... (28) 69 the heat of reaction calculated from reported values of the heat contents, latent heats of various transitions 68. and the specific heats of V02, V02.5 (V205) :nd 02, comes to 7.8 ±- 9.0 kcal. at 863°C. The summation of error in this calculation is done according to the 60 method described by Pantony : the error is so large that the value of the heat of reaction calculated by this method is quite unreliable. However, the difference between the -resent result and this calculated value is 17.8 ±'9.0 kcal. This re:resents- the partial molar heat of solution of vanadium dioxide in molten vanadium 2entoxide. This large value shows the strong interaction between vanadium di)xile and vanadium pentoxide in the molten state, aifact which can be expected from the existence of so many vanadyl 71 vanadates in the solid state. The activity coefficient of vanadium dioxide in the melt can be calculated from the relation: 611 (s 7g cs ) AGIcs = RT (29) which is true when the entropy does not change appreciably on solution. With the above data for the partial molar heat of solution of vanadium dioxide in vanadium pentoxide: log = 17800 ± 9800 4.576 x 1136 or 2667 ± 4 at 863°C Such a high value for X shows that there is appreciable entropy change on the dissolution of vanadium dioxide in molten vanadium pentoxide. The solution is 69. therefore not 'In ideal one, which accounts for the trend of the equilibrium constants (Table V) towards higher values with decreasing oxygen pressure, i.e., increasing vanadium dioxide: the solution becomes more non—ideal as the dioxide c-incentration increases. However, the reasonable constancy of the values of the dissociation constants at a given temperature but over a range of com)ositions and gas pressures indicates that the activity coefficients of vanadium dioxide and vanadium pentoxide remain approximately equal over the. range of conditions investigated.

Addition of sodium oxide enhances the dissociation of vanadium pentoxide giving larger values for the dissociation constant (Table VII). The heat of dissociation is also found to decrease for increasing amounts of sodium oxide in the melt. The effect is likely to be found with other metal oxide additions. If this dissociation equilibrium is assumed to play a part in the transport of xyr_en through the melt, an enhanced corrosion of metals can be expected in the presence of molten vanadium pentoxide when sodium oxide, and therefore other oxides, are added to it. 70.

B. :INETICS OF THE OXIDATION OF VANADIUM DIOXIDE.

1. MATHEMATICAL EXPRESSION The rate of the reaction: 1 n (2) V 02 + 7 u2 V 02.5 (dissolved) (g) (molten) in the most general sense is given by: m n - dc.7 = k C . 1>02 (30) where c is the concentration of the vanadium dioxide in the melt, p02 is the oxygen potential, and m and n are the exponents of these cJncentratins in the rate equation. The treatment of this heterogeneous reaction is considerably simplified if it is approached with the following two conditirns:

(a) C_nditi.n 1 If the reaction occurs in a constant flow of pure oxygen, an ideal condition is there where the concentration of one of the reactants is kept constant (similar to Ostwald's "is)lation method" of determining the order of a reaction), and theanetics of the reaction is then governed by the change in concentration of vanadium dioxide alone. That is, dc m .... (31) - cat k1c where m is the order of the reaction with respect to the concentration of vanadium dioxide. The half-life 72 period, tj, of a reati)n of this type is given by : 71.

log t1 (1 - m) log a ± log 2m-1 - 1 .... (32) 7 1777-17-1 - where 'a' is the initial concentration of the reactant. It can be easily seen fr,L this relatimship that, for a reaction of the first order (m = 1), the half-life is independent of the initial cAacentrati- n and the velocity constant is given by:

k1 In 2 / t1 7 0.693 / t1 .... (33) 7 The values of the half-life -periods for the present reaction at 940°C in pure oxygen are presented in Table VIII. It is quite evident that the reaction is

TABLE VIII. HALF-LIFE PERIODS OF THE REACTION:

V02 420 2 -4 V02.5 UNDER PURE OXYGEN AT 940°C

Initial Conc. :f V0 Half-Life Period Mean Velocity' 2 Constant, k1 (= mg.02) (minutes) (sec-1)

45*7 14.82 35.4 15.62 25.4 15.42 0.00075 23.7 15.69 22.9 15.58 16.1 15.23 ___-- 72. kinetically of the first order with res)ect to vanadium dioxide. For this pseudo-uninolecular reaction the rate is given by: dc _ . 1c .... (34) where c is the concentration at the instant t. This, on integration, gives:

In c k1 c constant (35) A plot of log c against t should give a straight line with a slope equal to - k1/2.303.

(b) C_,ILlition 2 The rate of reaction will certainly depend on the oxyten ,t. ntial also. The general expression can then be written as: -.7lc = k c p02n .... (36) For a constant exygen pressure, not necessarily equal to unity:. - 77dc (k p02n) c

k2 c (37) Integration gives a relation similar to (35), and the value of k2 can be determined for different, but constant, oxygen pressures. The values of k2 thus obnined is actually equal to (k p02n), where k is the specific . rate constant characteristic of the process. Hence,

k2 = k p02n or log k2 = log k n log p02 (38) rtei.i3.KINECTICS OF THE REACTION,(ARRHENIUS PLOTS ). VOA — NO 2 4 2 2.5 • 4.3

4.1

3.9

3.7

0 0 11 3.5

3.3

3.1 8.2 6.6 90 94 98 10.2 10.6 l/T-) x 104 73.

A plot of log k2 against log p02 should be linear with a slope of n, and when p02 = 1, k = k2 = kl

2. REGUITS Table IX and Fig. 13 contain the effect of temperature on the kinetics of the reaction. Included- in the Table are other thermodynamic quantities of the reaction

1 TABLR IX. KINETICS OF THE REACTION: VO2 -4 02 VO2.5 UNDER PURE OXYGEN at 1 atm.

_____ i Temperature Velocity Velocity 2.303 flog AH As Constant Constant RT -1 kh ( ) (soc-1) (sec ) (Integration) (From half- kbT life)

• 1•.••• _ . I 700 0.0001228 0.0001283 -41.9531 3.7689 -75.89 739 0.0001816 0.0001916 -'513.3 3.6235 -71-3 800 0.0002880 0.0002850 -Y1.0 3.4175 -11.a 855 0.0004283 0.0004183 -3'3'7 3.2509 - 701 880 0.0004816 0.000459 -38'4, 3.1804 -70:g 900 0.0005433 070005250 40.6544 3.1262 -74.58 940 0.0007142 0.00075 ....3g.3 3.0231 -10.13 1...... _

The effect of oxygen pressure on the rate of the reaction at 700°, 800° and 900°C is presented in Table X an.1 rig. 14 FIG.K.EFFECT OF OXYGEN PRESSURE ON THE RATE OF THE REACTION., 2 V0,2102 0 5 •

1.2 1.6 2.0 24 2.8 3.0 -LOG p02 74.

TABLE X. EFFECT OF OXYGEN PRESSURE ON THE KINETICS OF 1 THE REACTION V02 4-1- .4 02 V02.5

700° 800° 900° , -:1zity -I" Velocity Velocity Constant Constant Constant -1 -1 1)02 (rlin-1) (min ) D02 (min ) -11- 13°2 Int.„;- From Integ- Half- Integ- Half- ration half- ration life ration life Mc: t' •life Method Method Method Method

1 '6,00737 040077 1 Q0173 Q0170 1 Q0326 0.032 Q545 000635 00070 050 0.0154 00152 050 0.0313 0.030 0209 Q00056 Q0069 0209 00147 Q0146 0.121 00273 Q026 0.09.0-3 0.00322 ao060 0.109 0.0138 1.0134 Q061 00241 0024 00648 000589 ap059 0.0609 0.0127 00125 0.0226 Q0154 0015 002 95 000558 00058 00295 001150 Q0112 00141 Q00978 0.10097 Q0226 0.00553 00057 00158 000851 000823 000778 000522 Q0051 Q0113 000397 00040 00129 000674 00069 000331 Q00251 Q0020 Q00811 0.00316 0.0033 000798 000547 Q0058 Q00136 000123 00012 0.00015 000237 00025 000616 000374 00038 000375:000164 00018 75.

3• There are several factors to be considered in this process, particual;rly the semi-conducting property of the melt and the equilibrium between the pentavalent and tetravalent vanadium in the melt. The reaction can be assumed to take -)lace at the gas-melt intertkce, and the reaction progress as determined by the weight- gain is not linear, but rather exponential with time. There is, therefore, at least one "slow" step in the process, either chemically-controlled or diffusion- controlled in nature. A few experiments with different flow-rates of oxygen (10, 50, 1039 200, 300 ml per minute) showed that the reaction rate is essentially independent of the flow-rate and hence the diffusion through the hydrodynamic layer at the gas-molt interface has no bearing on the reaction rate. The other rate-controlling processes are (a) the dissolution of oxygen and its chemical interaction with the vanadium dioxide, and (b) the diffusion of oxygen and/or vanadium (V) ions towards the bottom of the melt, or the diffusion of vanadium (IV) ions to the surface of the melt. The former is mainly a chemical process and will be rate-controlling under high oxygen potentials when the diffusion of ion and electron vacancies will be very fast because of the large difference in oxygen potentials at the bottom 76. and surface of the melt. This does not preclude the diffusional contribution under these conditions and this effect is clearly understood from the variation of the reaction rate with oxygen pressure. Under very low oxygen potentials, diffusion alone is likely to be, the slow-step and will be rate-controlling. The distinction between diffusional- and chemical-controlled processes will be mainly in the energy of activation: the latter is likely to have, a.considerably larger value than the former. For a chemical-controlled process, the reaction should also obey any of the kinetic equations for a first, second or higher order reactions with respect to the vanadium dioxide concentration.

The data presented in Tables VIII, IX and X show that the reaction is pseudo-unimolecular with respect to vanadium dioxide. From the transition state theory of rate processes, the energy or heat (AH) and entropy 73 (AS) of activation is given by : k = kbT e - AH/RT AS/R • e WO (39) rh The transmission coefficient is assumed to be unity which is justifiable because the reaction rates are reasonably high. The equation becomes: kh - AH AS 2.303 log (39A) kbT RT F 77. where k is the velocity constant per second, k, is the Boltzmann constant (1.38 x 10-16 erg. deg.-1), °IT is the Planck's constant (6.625 x 10-27 erg. sec.), Lis the temperature (°K) and R is the gas constant (1.988,cl. deg.-1 mole-1). A plot of log k against reciprocal temperature should give a straight line with a slope equal to - AH/2.303R, where AH is usually called' the energy of activation for the reaction. Fig. 13 shows that this is valid for the reaction in pure oxygen and AH is calculated to be 16.8 +` 0.5 kcal for the range, 700° to 940°C. The entropy of the reaction is approximately - 75 E.U. (Table IX). Since AG = AH - T.AS, the value of AG at 855°C is 99.4 kcal. The activated state in a unimolecular reaction is expected to resemble. the initial state, and hence the entropy of activation should be small. However, the present reaction occurs with an appreciable decrease of entropy and this could only mean that the activated state involves the formation of a chemical bond, and gives support to the view that at reasonably high oxygen pressures, the rate-controlling stop is essentially a chemical one.

Fig. 14 shows that the variation of the rate of the reaction with oxygen pressure at constant temperature obeys a two-term expression corresponding to the distinct break in the plots:

78.

n n k Bip,2 ,2p02 m ( 40) One term is applicable for the high and the other for the low oxygen pressure regions, probably due to different mechanisms in these regions. The change- over from one mechanism to the other occurs at different oxygen pressures for different temperatures; this indicates that the defect structure of vanadium pentoxide and its dissociation equilibrium are responsible for the two mechanisms.

Table XI gives the values of the exponents of the oxygen pressure and other details of the process. The exact values of the exponents are

TABLE XI EFFECT OF OXYGEN PRESSURE ON THE RATE OF OXIDATION OF VANADIUM DIOXIDE DISSOLVED IN MOLTEN VANADIUM PENTOXIDE (Ref. Fig. 14)

'sm . c noxe yngteonf pprOe3sl f ors aVat igaonfsip00 Exponentoxyge no fp prtsfsourr 0 ( C)) high t i "o n low region

700 0.083 0.0158 0.83 800 0.098 0.0240 0.91 900 0.102 0.0436 0.91 79. determined by the method of least squares60, and the values agree well with those obtained from graphs.

Reference to Figs. 13 and 14 gives the information about the energies of activation for the two processes. Down to an oxygen pressure of about 0.05, the heat of the reaction (the energy of activation) remains practically constant: for p02 = 1, AH = 16.8; for p02 = 0.1, AFR, = 16.3 and for p02 = 0.05, AH = 16.3 kcal. However, for lower pressures, log k vs I plot is not T linear and the heat of reaction seems to decrease with increase of temperature: for p02 = 0.01, AH between 700° and 8000C is approximately 9.g kcal, and between 800° and 900°C it is 244 kcal. This gives further support to the view that at the larger oxygen pressures, the rate-controlling step is chemical whereas at lower pressures diffusional processes with very low energy of activation dominate the rate-controlling step. It must be emphasized that diffusional processes of some type must be operating at all oxygen pressures, and this is evident from the effect of oxygen pressure on the reaction rates.

The question of the effect of oxygen pressure on the rate of reaction as measured by the amount of oxygen uptake is mainly concerned with the semiconductivity of

80. the melt. The oxygen absorption is controlled by the diffusion of vanadium (iv) ions (or vanadium (v) vacancies), eletron holes and anion vacancies towards the surface of the melt. Quantitatively, one oxygen molecule would react with four V5-1- vacancies (m V5+) and four electron holes (®) and would fill in two of the anion holes O -)

02 a: Lk 0.1/5+ ;F 2 D 0= (41) and therefore from mass action considerations applied to vacancies and holes, 1 2 p02 = const. [4:z V51 4 x. [el 4 x [00 (42)

By considering the relation between the concentration of the electron and ion vacancies in the absence of any other disorder equilibria ( [4:1 V5-j = fel = 1/2 [t:301) , it becomes evident that:

C®1 = const. p021/10 = const. p020'1 (43) Since the rate of the oxygen uptake is proportional to the concentration of eletron holes, it should be proportional to the 10th root of the oxygen pressure. Considering the simplifications and assumptions involved, this is certainly very close to the experimental values of the exponents (Table XI). The reaction can thus be written as: 16800 k = A.p021/10 e - RT (44)

81.

At very low oxygen pressures, the diffusional processes aro the only slow and rate-determing step, but because of the complications involved it is difficult to give any concrete physical meaning to the exponents. Two possible mechanisms are there: (a) the dissolved oxygen and hence the activated state for diffusion might be in the form of atoms (Sievert's law):

1/2 02 0 so that 10] = const. p021/2 (45) (b) The anion vacancies alone might determine the rate of diffusion of oxygen: 1/2 02 a 0= so that Lti 01= const. p021/2 (46) In either case the corrosion rate should be proportional to the square root of the oxygen pressure. However, if the activated state for diffusion is the molecule or two atoms or two anion vacancies associated together, the corrosion rate should vary linearly with the oxygen pressure. The experimental values of the exponents (Table XI) at low oxygen pressures are close to unity so that moecular or associated-atom or vacancy diffusion is the predominant factor.

82.

C. CRYOSCOPY IN MOLTEN VANADIUM PENTOXIDE 1. MATHEMATICAL RELATIONSHIP According to the dissociation equilibrium in molten vanadium pentoxide: K = p021/4. xv02 (26)

2.5 so that K. Y: . •--• • • • (47) X VO2 . 5 VO2 p021/4

The depression in freezing point for this amount (molefraction)' of vanadium dioxide is given by: 2 AT RTo .xV0 (48) AHf 2 2 K.x ' = RTo . V024 (49) ITf p021/4 so that AT.p021/4 = constant (50) which is valid if x . is approximately equal to VO2.5 unity. This in fact is a close approximation since K has a low value of 8.9 x 10-4 at the melting point

(To) of vanadium pentoxide.

2. RESULTS The data obtained for a range of oxygen pressures are presented in Table XII. The melting point in pure oxygen (677.96°C) is assumed:to be that of pure vanadium 83.

TABLE XII. DEPRESSION IN FREEZING POINT OF VANADIUM PENTOXIDE UNDER DIFFERENT OXYGEN PRESSURES.

K x 104! A T A T (Experimental) AT. p0 2 4,1 p.02 8yEga) (Theoretical) Stirring Seeding technique technique:

1.0 8.90 0.010 - , - 0.25 8.90 0.290 0.45 0.33 .0.233 0,188 8.90 0.311 0.51 0.45 0.296 0.179 8.88 1 0.315 0.69 0.51 0.331 0.169 8786 i 0.316 1704 0.76 0.487 0.0895 8.81 0.371 1.46 - 0.798. 07.0601 8.61 0.399 1.95 - 0.955 0.0295 8.51 0.478 2.80 - 1.161

Nolte: The first four values in the last column are obtained from the A T values of the 'Seeding Technique'. 84. pentoxide; this is not quite correec, but for internal comparisons this does not introduce a significant error. Assuming the heat of fusion (AHf- 7800 cal. for V02.5)69 and the value of the dissociation constant (K) obtained by extrapola- tion to the melting point, the amount of vanadium dioxide present in the melt at different oxygen pressures and the expected depressions have been calculated in each case. These are also included in the Table. All the values have been corrected for supercooling even when the seeding technique was used to avoid excessive supercooling. At the end of the experiment the amount of platinum dissolved in the melt was found to be 4 mg. which might produce an error of only 0.001°C in the depression of freezing point.

3. DISCUSSION In spite of the difficulties- in (a) avoiding excessive supercooling, (b) maintaining the equilibrium conditions at the melting point, and (c) measuring such low temperature changes, the values of AT.p021/4 are fairly constant at moderately high oxygen pressures. The melting point of vanadium pentoxide depends not only on its purity but also on the oxygen pressure which determines the amount of the lower oxide in the melt. Further credence to the assumptions is obtained from 85. the value of the heat of fusion of vanadium pentoxide calculated from the above data applied to Equation (49): T = 951, K = 8.9 x 10-4, AT.p021/4 — 0.2333, and is assumed to be unity; AHf is calculated to be xvo2 .5 6900 cal. for V02.5 or 13.8 kcal. for V205. The reported value69 is 15.6 kcal., based on calorimetric experiments.

If the experimentation can be assumed to be satisfactory the results confirm the Temkin model of the melt, at least at low concentrations of the dioxide. At higher concentrations divergence from constancy of 1/4 the product AT.p02 could be due to difficulties associated with supercooling - better technique gave a better series of values - and a theoretical interpretation is difficult. An experimental value higher than the expected AT cannot be explained by, say, polymerization of vanadium dioxide, and only an increased number of species (e.g. resulting from ionization of vanadium pentoxide) could account for the results. Increased ionic character of the melt in the presence of added oxides such as those of sodium, cobalt, nickel and iron is discussed later in connection with the conductance experiments: vanadium dioxide could behave 1/4 i similarly. The apparent rapid rise in AT.p02 n the neighbourhood of 0.05 for the oxygen partial pressure

86.

is probably fortuitous, and a logarithmic plot of AT vs p02 is a definite curve.

As a tentative conclusion from this work the freezing point depression of vanadium pentoxide depends approximately on the inverse fourth root of the oxygen pressure at low concentration of dioxide; and at higher concentrations it is suggested that high concentrations of ions (e.g. 02- V5+ and V0 ) result and these lead 3 to an abnormally high depression of freezing point.

M. CONDUCTANCE OF VANADIC MELTS 1. RESULTS (a) Pure Vanadium Pentoxide Under Oxygen The specific conductance, the apparent activation energy and the logarithm of the pre-exponential reaction constant for the conductance are given in Table XIII. The activation energy (E) and the reaction constant (A) are calculated on the assumption that the specific conductance (X) obeys an equation of the Arrhenius type74:

X = A exp(-E/RT) (51) An equation of this type is applicable to an electrolyte where the same species conduct the current through the entire temperature range and following the same mechanism. For reasons stated below, this is unlikely in the case of molten vanadium pentoxide. However, the

87.

TABLE XIII. SPECIFIC CONDUCTANCE OF MOLTEN VANADIUM PENTOXIDE UNDER OXYGEN. ______,.. Temp Cc) Sp. Conductance Activation! log A, Energy (ohm-1 cm-1) (kcal)

ii•••••••••••• 692 0.0658 710 070767 17.2 2.70 739 0.0963 759 0.1175 20.2 3.3 784 0.1492 809 0.1860 23.3 3.97 828 0.2274

847 0.2929 24.5 4.24 855 0.2992 889 0.3909

909 0.4973 25.9 4.47 (909) (0.4976)* OW. 942 - 0.6674 (40,75,76) Reported Values (40) 700 ca. 0.2 (from graph) MO. 900 ca. 0.6 ( (75) 690 0.0234 29.4 899 0,1590 964 0,2888 (76) 1000 1.180 12.3 ( Sp. conductance of the melt after keeping for 25 hours at this temperature showing the negligible effect of the dissolved platinum) F/GZSPECIFIC CONDUCTIVITY OF VANADIUM PENTOXIDE IN OXYGEN

0_0 PRESENT WORK , L1 A REPORTED VALUES .

o--o VALUES FOR 1.24 molo% V02 VALUES FOR 1.6 7 mole % VO .A--L 2

0.9

r< 0.7

C.)

0.5

0•3

0.1 0.2 6.4 6.6 6.6 90 92 .9.4 .9.6 9.4 10.0 102 104 ( VT ) X lo4 88. values of E and A have been calculated for purposes of comparison. Expressing Equation (51) in the modified form: X = A *exp (-E /2kT) ...7 (52) where k is the Boltzmann constant (1.38 x10-16erg.deg-1), the activation energy can be calculated in ergs. The values obtained are used to estimate the number of current carriers, n (cm-3). In the molten state, where intrinsic conduction predominates, the following 40 formula is employed for the calculation:

(2n m kT)/ 2 n = 2 . exp. (-E*/2kT) • • • • (53) h3 where h is the Planck constant (6.625 x 10-27erg.soc) and m is the effective mass of the electrons which can be assumed equal to its rest mass (9.108 x 10-28g). The result at 80VC shows that the activation energy is 3.23 x 10-12 ergs and the number of current carriers is 3.43 x 1015 cm-3.

The temperature variation of the specific conductance of molten vanadium pontoxide is represented 75' in Fig. 15, together with a few reported values . In view of: (i) the purity of the vanadium pentoxide used in the present work, (ii) the non-contamination by silica or , alumina

89.

or zirconia which was almost certain in the 75,76' previous works , and (iii) the use of an oxygen atmosphere, the values of the present work are cetainly the more reliable. The specific conductance of the melt does not change much even after keeping the melt for days together at high temperatures. This indicates that the platinum container does not contaminate the melt.

(b) Effect of Oxygen Pressure on the Conductance of Moltun Vanadium Pentoxide The variation of specific conductance of molten vanadium pentoxide with oxygen partial pressure at 855°C is shown in Table XIV and Fig.16. The specific conductance of the : lt,on rejuvenation with pure

TABLE XIV. VARIATION OF CONDUCTANCE OF MOLTEN VANADIUM PENTOXIDE WITH OXYGEN PRESSURE AT 855°C. -- i . • . 1 Oxygen i 1.0 05013 02592 0/012 00436 Q0264 00159 0011 Pressure H— , -----,------• E Sp.2.1Conc.21. 0,2592 03548 01-0_03 0.523E00.6313 07652 L0462 L3007 Kohm cm (0.2998) 1

oxygen after the rest of the experiments extending over a period of two weeks is seen to be the same as that of the FIG.1SVARIATION OF CONDUCTANCE FIG.17. EFFECT OF SODIUM OXIDE OF MOLTEN VANADIUM PENTOXIDE ON THE CONDUCTANCE OF WITH OXYGEN POTENTIAL AT 855°C . MOLTEN VANADIUM PENTOXIDE' 0-0 SR CONDUCTANCE . APPARENT DENSITY 0-0 UNDER OXYGEN. 045

0.10

-0.1

-0.2

-0.3

-0.4

-0.5

4 8 /2 16 18 SODIUM OXIDE CONC.Imcie per cent ) —0.6 0-300 —1.6 0 LOG p02 90.

starting material (marked with an asterisk in Table XIV). This shows the reproducibility of the results in conductance measured in a platinum container.

It is evident from Fig.l6 that the specific conductance of molten vanadium pentoxide at constant tmperature obeys a two-term expression corresponding

-t.) the distinct break in the plot:

X1 nOk 2 + X 2ip0 2 - Above an oxygen pressure of 0.033, the exponent has a value of -0.24 or close to -0.25, and below 0.033 (3.3% oxygen), the exponent is -0.58.

During the conductance study, the position of the electrodes and the crucible was not disturbed, but the depth of the total melt under each oxygen pressure was found to be different. Assuming that this is due to the change in density, tentative v,aluos could be suggested for the density of various vanadium pentoxide- vanadium dioxide melts, the compositions of which--. are calculated from known equilibrium data (Section (A) ). The values are collected in Table XV and plotted in Fig. 16 along with the specific conductivity; there is a distinct break in the curve below an oxygen partial Pressure of 0.05 which indicates a structural change in the melt below this value. 91.

TABLE XV. APPARENT DENSITY OF VANADIUM PENTOXIDE - VANADIUM DIOXIDE MELTS AT 855°C.

Amount of V02.5 = 214 g.

PC2 Amount of V02 Depth Volume Density (mole 56) (cm) '(from geometry (g.cm-3) of crucible) • 1.0 0.0735 3.76 106.3 2.01 0.2592 0.1028 3.75 106.0 2.02 0.1012 0.1297 3.73 105.4 2.03 0.0264 0.1806 3.68 104.0 2.06 0.0110 0.2238 3.60 101.7 2.10

(c) Effort Sodium Oxide on the Conductance of Molten Tlnadium Pentoxide Varying amounts of sodium oxide in the form of pure dry sodium metavanadate added to vanadium pentoxide affect its conductivity considerably. The data are presented in Table XVI and Fig. 17. The data collected are for normal grade vanadium pentoxide. The density of the sodium oxide-vanadium pentoxide melt did not appear to be significantly different from that of pure vanadium pentoxide. The melt, however, evolved copious quantities of oxygen on solidification, and the whole melt sometimes frothed uncontrollably on 92.

TABLE XVI. EFFECT OF SODIUM OXIDE ON THE CONDUCTANCE OF MOLTEN VANADIUM, PENTOXIDE (NORMAL GRADE) UNDER OXYGEN.

SD. Conductance (ohm-1 cm-1) Activation Mole % o i o Energy log 72:A 750° 7750 800 !825` C Na 20 (kcal ) .; , 0 1 0.122 0.143i0.177 0.225 0.292 23.8 4.20 0.52 0.178 0.220 0.278 0.363 0.478 18.8 3.36 1.53 0.226 0.259 0.320 0.418 0.571 17.7 3.00 3.44 0.290 0.347 0.428 0.532;0.669 17.2 3.21 5.20 0.363 0.423 0.507 07618 07770 14.7 2.77 8.36 0.463 0.526 0.603 0.7020.815 11,9 2.27 11.09 0.551 0.618 0.698 0.806 ; 0.933 11.9 2.32 13.49 0.570 0.647 0.71 1 0.862 1.000 11.7 2.31 16.58 0.729 0.815 0.926 1.06311.230 10.8 2.21 .._ solidification. The activation energies are calculated from the temperature variation of conductance with the aid of Equation (51).

(d) Effoct of Gobaltous Oxide on the Conductance of Meltn Vanadium Pentoxide. By successive additions of cobaltous oxide to the high purity vanadium pentoxide, the composition has been changed from 1.18 mole per cent to 39.3 mole per 1-0

0-6

875 °C. 0.7

775 °C.

E 03-

02— /

A .1 10 15 20 25 30 35 40 COBALT OXIDE CONCENTRATION ( Mole °/0). Flag. SPECIFIC CONDUCTANCE OF COBALT OXIDE -VANADIUM PENTOXIDE MELTS AT 775°C AND 875°C UNDER OXYGEN. 930

TABLE XVII. EFFECT OF COBALTOUS OXIDE ON THE CONDUCTANCE OF MOLTEN VANADIUM PENTOXIDE (PURE) UNDER OXYGEN

Sp. Conductance (ohm-1 cm-1) Apparent Mole % 775°C 800°C 825°C1 850°C 875°C Activation log A Co0 i Energ 1...._ (kcal 0 0.137 0.172 0.220 0.285 0.363 23.3 3.97 1.18 0.205 0.260 0.3331 0.421 0.528 22.6 4.03 2.34 0.303 0.389 0.495 1 0.624 0.805 22.4 4.17 . I , 3.47 0.245 0.307 0.382, 0.465 0.564 19.9 3.55 4.58 0.257 0.313 0.389 0.482 0.590 20.8 3.73 5.66 0.283 0.350 0.430 0.529 0.640 2071 3.64 6.71 0.384 0.481 0.602, 0.753 0.937 20.4 3.84 10,71 0,335 0,410 0,4931 0,585 0,700 17,7 3,23 14.38 0.364 0.420 0.493; 0.585 0.690 16.4 2.96 17.75 0.400 0.525 0.662; 0.785 0.910 17.6 3.32 23.77 0.377 0.502 07540 0.628 0.720 15.5 2.83 31.31 0.375 0.442 0.520 0.612 0.717 14.2 2.54 39.31 0.355 0.415 0.485 0.565 0.660 14.7 2.63 ------

cent cobalt oxide, and the conductance was measured at a series of temperatures so that the apparent activation energy in the range 850 ±1 50°C could be calculated in each case and compared with that of the pure vanadium pentoxide. The values are recorded in Table XVII and 09 900°C.

0 050 °C.

0.5

c.) 0.3

Q U)

8 10 12 NICKEL OXIDE CONC. (Dale %). FIG.1.S. SPECIFIC CONDUCTANCE OF NICKEL OXIDE — VANADIUM PENTOXIDE MELTS UNDER OXYGEN.

U x_0.5 875°C. c.);• Z O U 750 °C. 850°C. -LI.- 0.3 IZ Lv cf)Q 775°C. 0.1 14: 0 4 6

FERRIC OXIDE CONCENTRATION. tr) (Mole 0.1 %). 0 2 4 6 TUNGSTIC OXIDE CONCENTRATION. F.G.20.EFFECT OF FERRIC (Mel e %J. OXIDE ON THE CONDUCTANCE FIG.2.-SPECIFIC CONDUCTANCE OF MOLTEN VANADIUM OF TUNGSTIC OXIDE—VANADIUM PENTOXIDE. PENTOXIDE MELTS UNDER OXYGEN. 94.

Fig. 18. The apparent activation energies are calculated from the slopes of the plots of log X against reciprocal temperature on the assumption that they obey the Equation (51).

During the course of the experiments it was found that the volume of the melt did not change on the addition of cobaltous oxide; the cobalt oxide- vanadium pentoxide melt is thus denser than pure vanadium pentoxide.

(e) Effect of Nickel Oxide on the Conductance of clten Vanadium Pontoxide The data are presented in Table XVIII and Fig. 19. Because the nickel oxide-vanadium pentoxide melts have increased melting points on the progressive addition of nickel oxide, values at lower temperatures could not be obtained. This effect is indicated in the Table. The volume of the melt and the solidified slag is considerably reduced on the addition of nickel oxide; the nickel-oxide-vanadium pentoxide slags possess a density larger than that of pure vanadium pentoxide in the solid and molten states.

95. TABLE XVIII. EFFECT OF NICKEL OXIDE ON THE CONDUCTANCE OF MOLTEN VANADIUM PENTOXIDE (NORMAL GRADE) UNDER OXYGEN.

-1 -1 S. Conductance (ohm cm ) ! Activation Mole % 7 r, 0 -1 Energy log A NiO r7")r 7° C 180o°a 825°C1850°c875- 900 C (kcal) ommr4.....m.m. i i 0 0.177 041225 0.292 f;00380 i°6495 0.625 23.8 4.20

0.71 0,1929.240 0.31010.40010.527 0.685 22.9 4.05 1.41 0.20219.255 0.325 !0.410 9.520 0.645 22.9 4.07 2.78 0.240 0.291 0.355 10.438 .54010.655 21.9 3.93 . , . 4 i 4 ; 4 i 4 3.15 5.20 0.260 0.320 0.395i10.490 9.56510.650 17.8 1 7.92 0.268 0.350 0.4451 0.5659.695 0.815 22.9 4.20 , . , 10.28 Solid up to 0.495 0.555 10.67510.825 33.8 ,6:32 780°0 12.53 Solid up oi0 555 b.660 10.830 30.2 5.55 i e I 825 C i i 1 ----:—...... ----- 444=1.1104MMOMMIN1414Mm.111MIMIMION.10,

(f) Effect of Ferric Oxide on the Conductance of DiLlt,J11 Vanadium Pentoxide Owing to danger of attack of the platinum crucible by high concentrations of ferric oxide in the melt, variation in the composition was restricted to less than 6 mole per cent of ferric oxide. The data are presented in Table XIX and Fig. 20. The ferric oxide-vanadium pentoxide slag seems to be less dense than pure vanadium pentoxide. 96.

T;2LE XIX. EFFECT OF FERRIC OXIDE ON THE CONDUCTANCE.. OF MOLTEN VANADIUM PENTOXIDE (NORMAL GRADE) UNDER OXYGEN. 1

Sp. Conductance (ohm-1 cm-1 ) 1,Activation Mole % Energy , log A Fe 03 750°7-775° 800° 82507-8500C (kcal) 2 1 ' 1 i I 0 0.143'0.177 0.225 0.292 0.380 23.8 4.20 , ! . , 0.71 0.1851065.2 :03.362 0.482 0.615 27.0 5.14 ' 1.41 0.2150.250 '0.295 0.352 0.428 13.7 2.32

2.09 0.22010.265 "0.3251 0.39510.476 17.9 3.15 . , t , , , . ! 3.44 0.28210.312 0.360 0.44810.560 20:8 3.79 1 6.00 0.308 10.369 10.464 0.65210.660 1 23.. 5 4.45

(g) Effect of Tungstic Oxide on Conductance The results are tabulated below (Table XX) and presented in Fig. 21. The molten slag on cooling expands a little: probably the solid has a lower density than the liquid since there is no other sign of gas evolution as in the case of sodium oxide-vanadium pentoxide melts.

2. Many solid oxides are semiconductors. So too is vanadium pentoxide. In the solid state, vanadium pentoxide has in fact been found to be an intrinsic semiconductor with an activation energy of approximately 77' 0.4 eV or 9i2 kcal per mole . This semiconducting property may be expected to remain in a slightly 97.

TABTJE XX. EFFECT OF TUNGSTIC OXIDE ON THE CONDUCTANCE. OF MOLTEN VANADIUM PENTOXIDE (NORMAL GRADE) UNDER OXYGEN. - Sn. Conductance (ohm-1 cm-1) Activation Mole % Energy log A 875 W03 775°C 800°C 825°C 850°C °C (kcal) , 4 O 0.177 0.225 0.292 0.380 0.495 23.8 4.20 , 0.71 0:167 0.220 0.285 0.365 0.465 22.9 4.02 1.40 0.177 0.230 0.300 0.390 0.495 24.9 4.44 2:77 0.258 0.328 0.420 0.535 0.708 23.8 4.36 4.74 0.205 0.310 0.425 0.557 0.745 18.3 3.34 6.65 0.213 0.264 0.338 0.430 0.545 24.7 4.44

modified form even after fusion, since the appearance of conductance electrons depends mainly on short range order in the arrangement of atoms. This principle has 78' been shown to hold for several oxides . This is further confirmed by the absence of an abrupt change of 40 conductance on fusion and, in the present work, by the effect of oxygen potential and metallic oxides on the electrical conductance of molten vanadium pentoxide. Vanadium pentoxide could be assumed to possess n-type semiconduction since a stoichiometric excess of metal favours the formation of donor levels in the oxide. 98.

The activation energy of conductance is not constant but increases with increase of temperature (Fig.15). This is probably due to a larger degree of disseciatipn of vanadium 7entoxide at higher temperatures causing an increase in the width of the forbidden gap. Diffusion of electrons might be partly balanced by a flow of holes. In the case of intrinsic conduction the number of free electrons (n) is normally equal to the number of holes (p) in the valence band; but the high temperatures and the consequent greater nonstoichiometry of the melt might bring about a predominance of the diffusion of electrons over the diffusion of holes. Moreover, it may well be that the short-range "jumps" at the low temperatures change to longer-range jumps at the high temperatures.

The anion lattice of vanadium pentoxide contains several vacant sites (holes), and electrical neutrality is established by the formation of cations of lower valency, namely vanadium (IV) ions. The number of lower valency ions is identical with the number of 'electron defects', and the number of electron defects determine the electron conductivity. It is useful to think of these ions of lower valency as "mobile valency states". The movement of the electrons takes place by exchange between the pentavalent and tetravalent vanadium ions. 99.

From the electronegativity values it can be seen 79' that vanadium pentoxide is about 42 percent ionic Molecular melts usually have boiling points close to 80 their melting points (for example, mercuric chloride and molybdic oxide); due to its d*composition, the exact value of the boiling point of vanadium pentoxide has not been determined. It is, however, not close to its melting point, and hence vanadium pentoxide is not a. pure molecular melt; it is partially ionic. In ionic melts one species not infrequently transports nearly all the current (ti = 1); in pure semiconductors, the transport number with respect to the electrons (te) is almost equal to unity, and the transport numbers of cations and anions (t* t_) is approximately zero. In many instances the unipolar conduction changes to dipolar conduction upon increase of temperature, when the thermal motion brings about sufficient disorder of 81' the whole lattice . Hence, we can conclude that the increase of (i) nonstoichiometry and (ii) the ionic nature causes a change in the conducting species and a consequent change in the mechanism. This, to a certain extent, explains the deviation from the simple Arrhenius relation for conduct4Lce ai given by Equation(51) (Fig.15). The effect due to the nonstoichiometry alone on conductance can be studied by measuring at different temperatures the conductance of vanadium pentoxide 100.

containing the same amount of the dioxide controlled by a judicial adjustment of the oxygen pressure (Section A, this Chapter). Table XXI shows the remarkable results obtained, also plotted in Fig. 15.

TABLE XXI. CONDUCTANCE OF VANADIC MELTS OF THE SAME COMPOSITION AT DIFFERENT TEMPERATURES.

No. Temp (°C) 942 909 855 750

1 102 for 1.24 mole % V02 ,1 0,12 0,001 0.479 Sp. Conductance (ohm-lcm-1) - 0.4973 0.4955

2 p02 for 1.67 mole % V02 ,1 0.302 0,0363 Sp. Conductance (ohm-lcm-1) 0.6674 0.664 0.661 -

It is evident that the conductivity of melts of the same composition does not change appreciably with temperature, thus showing that the main contribution for conductance comes from the nonstoichiometry of the melt and not from any ionic migration. This conclusion is not antagonistic towards the views expressed on the. results from cryoscopic work because the conductance is not highly sensitim with respect to small changes in ionic character whereas the depression of freezing point is extremely sensitive to such changes. 101.

The specific conductance and hence the concentration of electrons in the conduction band increase with decrease of oxygen pressure. The results discussed above and the increased nonstoichiometry at low oxygen pressures show clearly that the concentration of electrons in the conduction band is associated with an excess of metal. Variation of specific conductance with oxygen potential (Table XIV and Fig. 16) follows 82, 83' the Wagner explanation of semiconductors The qualitative explanation for this rests in the decrease of oxygen pressure creating vacant vanadium (V) lattice sites (which incidentally are converted to vanadium (IV) ions), thus promoting the migration of vanadium (V) and electrons, forming new defects and, consequently, resulting in an increase of conductance. Decrease of oxygen pressure also creates vacant anion sites, thus facilitating the diffusion of the conducting ions. Quantitatively, one oxygen molecule would effect the migration of four vanadium (V) ions and four electrons with the formation of four vacant V5+1- sites( ciV5+) and four electrons (e); one oxygen molecule would also fill in two anion holes ( a 02-)R 5-14- 0 +- 4 =V5+- 1+4 * 2 002- 4 V 2 02. - 2 ' .... (55) Since, however, the existence of cation vacancies and anion holes could not normally be expected to contribute significantly to conductance in the molten state of a 102. semiconductor, particularly at moderately high oxygen potentials, the actual equilibrium will be: 0 4+ 46) 4 4 V5* *2 02-- 4 n V5+- 20027 2 .... (59 The right hand side of this expression is a Constant, and from mass action considerations:

, . p02 x (6)1 4 constant i.e., 1 L.1 a 1 (57) :202

Electrical conductivity is proportional to the concentration of electron vacancies, [0] , and hence should be inversely proportional to the fourth root of oxygen pressure if ion vacancies are absent or ineffective in conductance. The experimental value of the exponent is - 0.24 and so is very close to the expected - 0.25, thereby vindicating the view that ion vacancies are absent or absttin from conductance in molten vanadium pentoxide at moderately high oxygen pressures.

At very low oxygen potentials, the nonstoichiometry is so predominant that the anion vacancies do play a role in conductance. In fact, the contribution from ionic migration would be the predominant factor in conduction. Two anion holes are filled in by one oxygen molecule: 103.

2- 2- 02 +- 2 o 0 2 0 .... (58) or 1

tp0 = const. p02 (59)

The ionic contributions to conductance should thus be inversely proportional to the square root of the oxygen pressure. Below an oxygen pressure of 0.033, the experimental value of the exponent (Fig. 16, Table XIV) is found to be - 0.58 or close - 0.5, showing a remarkable similarity between the theoretical and the experimental values. The greater divergence in the expected and found exponents can be ascribed to contributions from other disorder equilibric.•

Small additions of metal oxides have a profound effect on the conductance of molten vanadium pentoxide (Tables XVI to XX; Figs. 17 to 22). The explanation for this behaviour lies in the influence of these impurities on (i) the defect structure, and (ii) the increased ionic conductance, the latter arising from the acidic nature of madium pentoxide which results in the formation of vanadate ions plus cations on the addition of the metal oxides. Oxides of sodium, iron, cobalt and nickel increases the conductance, indicating that these act as electron donors for minor concentrations, and highly conducting ions when present in large quantities. The excess of metal in molten 104.

vanadium pentoxide is probably due to vacancies in the oxygen lattice. When these are filled on the addition of metal oxides and the cations occupy interstitial positions, the equilibrium between V5* and V is disturbed in favour of formation of more of V44'. ions; this increases the concentration of electrons in the conduction band. Formation of the various vanadyl vanadates and several spinel could be= 71, 84, 85' cited as evidence for this proposition • Experiments on the dissociation equilibrium of vanadium pentoxide in presence of sodium oxide hava given conclusive vindication of this; the degree of dissociation increases with increase of added sodium oxide, and the heat of dissociation is considerably reduced (Table VII, Section A of this Chapter).

In addition to the increased electronic contribution, formation of ions is an added factor to the increased conductance. When a large excess of a basic metal oxide is present, the vanadium pentoxide melt is no —longer a semiconductor but only an electrolyte, and the conductance will depend on: (i) the degree of ionization and dissociation of the electrolyte, and (ii) the mobility of its component ions. These, to a certain extent, will be governed by the

0-9 825°C. CNa

0.7 Fe 775 °C. t.Co

FeA Co

XV/.

__I 1...... _ I 0.10 I 2 4 6 2 4 6 8 10 MOLE %METAL OXIDE. MOLE % METAL OXIDE. FIG .2.3. COMPARATIVE CONDUCTANCES OF METAL OXIDE -VANADIUM FENTOXIDE SYSTEMS. 105. nature of the metal ion and by the position of the melt in the corresponding temperature-composition diagram. In the present study, the influence of the basic metal oxides in increasing the conductance of molten vanadium pentoxide is in the order: sodium ?- iron > cobalt > nickel (Fig. 22). And it is a. reasonable conclusion that these metal oxides will increase the corrosive property of vanadium pentoxide in the same order.

In the case of cobalt oxide-vanadium pentoxide system (Fig. 18), the initial sharp increase (AB) in conductance is due to the increase in the electron conductance on the basis of the above reasoning. But with 2.5 mole per cent of cobalt oxide, the structural deformities reduce the electron conconcen- tration and the conductance falls to C. (Similar effects are observed with nickel oxides (Fig. 19) and iron oxide (Fig. 20) at higher temperatures). The points D and F in Fig. 18 indicate the existence of highly mobile species since there are no reasons to believe that they are experimental errors: creep of melt up the electrode is not likely to produce so great an 87- error. According to Burdese , the eutectic in the system is at 25 mole per cent cobalt oxide, and after this composition, the conductance is found to decrease 106. with increased cobalt oxide (Fig. 18).

For nickel oxide-vanadium pentoxide system, a similar behaviour would have been observed if it were not for the smaller degree of ionization of the dissolved nickel oxide and the higher fusion points of the melt. If, on the other hand, nickel oxide is assumed to form a substitutional solid solution, the nickel dissolved in the melt might be expected to be an acceptor of electrons according to the reaction: 2+-

V4* 11- Ni0 -H- 00 + e Ni (v4+ ) . VQ2 r- (electron) or O ,00 (60) -4.1- V V - +HINiO e. 0 - Ni2*, VO2 2' e 2 (V".44) (electron Dole) O 000 (60A) Thus substitution of nickel ion on vanadium (IV) sites- in the melt tends to increase the electron hole concentration in the valence band. Since there is the following relation between holes and electrons:

1 = const. O040 (61) an increase in will result in a decrease of ce3 and consequently a decrease in electronic conductance. Increase in ionic conductance might partly compensate for the decrease in the electronic conductance. The same might be expected to be the result of additions of other metal oxides. However, it seems that sodium, iron, cobalt and nickel are likely to take up interstitial positions because of their basic nature. The difference 107. in their behaviour is then to be attributed to the differences in (i) their ionic nature, (ii) degree of ionization and (iii) the product of interaction with 88 vanadium pentoxide. According to the phase disgram the eutectic occurs at about 20 mole per cent of nickel oxide; however, in the present work it is found that the conductance does not change much for compositions of 8 to 13 mole per cent nickel oxide (Fig. 19). The melting points of some of the melts are seen to be 88 higher than those reported in the literature (Table XVIII)

Tungstic oxide is similar to vanadium pentoxide in its acidic character. The formation of a vanadate or similar ions is therefore quite unlikely and ionic contribution to conductance is negligible. With smaller amounts of tungstic oxide, the anion vacancies are filled in and tungsten occupies interstitial positions. The dissociation equilibrium of vanadium pentoxide is shifted in favour of the vanadium (V) ions, unlike the previous cases; this decreases the concentration of electrons in the conduction band. The conductance is thus decreased. For higher amounts of tungstic oxide, other disorder equilibria enter the scene, some concerned with the tungstic oxide itself, but on the whole the conductance does not seem to change appreciably with composition. 108.

It may be concluded that vanadium pentoxide is a transition semiconductor implying that addition of both higher and lower valency ions increases the electrical 89 conductance and it is therefore extremely unlikely that any additive would reduce the semiconductivity and hence the corrosive properties of molten vanadium pentoxide unless a secondary effect such as an increase of melting point or decrease of diffusivity become operative, say with high concentrations of the additive.

E. VISCOSITY AND DENSITY OF MOLTEN VANADIUM PENTOXIDE 1. =SULTS The mean viscosity and density values of molten vanadium pentoxide in an atmosphere of oxygen are given in Table XXII. A few values of viscosity with 15 weight per cent of alumina in the melt are also inrauded in the Table. TABLE XXII. VISCOSITY AND DENSITY OF VANADIUM MELTS.

Temp Pure Vanadium Pentoxide Viscosity with 15% (°C) Density Viscosity alumina (F;cm-3) (centipoise)

701 2,27 13,1 14,2 720 2,22 11,3 13,1 762 2,15 10,0 ft, 790 2,12 8,2 •• 820 2,09 7,0 •• 865 1,97 6,1 up, 950 1.90 •. •• ti

740 780 820 860 900 TEMPE RAT URE FIG.a•. EFFECT OF TEv;PE.RATUR7 ON THE DENSITY OF MOLTEN VANA,DIUM PENTOXIDE.

9.1 95 9.9 10 3 x 10 4 EFFECT OF TEMPERATURE ON THE VISCOSITY OF MOLTEN VANADIUM PENTOXIDE. 109.

2. DICCU3SION In the temperature range of 750° to 850° the 76 reported viscosity values vary from 30 to 20 centipoises for molten vanadium pentoxide. The discret» pancy might be due to the slight decomposition of the pentoxide in air and the possibility of dissolved 90 alumina from which other workers' apparatus was made as well as for reasons discussed in Chapter II, Section C (f); the effect of alumina is evident from 5 the few values given in Table XXII. Reports of decreased corrosive activity of vanadium pentoxide containing added alumina find a possible explanation in the increased viscosity of these melts, for an increased viscosity is parallelled by diminished transport properties.

The density values of molten vanadium pentoxide 76 are very close to those of Zyasev and Esin ; the rather low values at the high temperatures may be attributed to the condensation of vanadium pentoxide on the suspension wire which is usually observed in all cases in the present work. The non-linear variation of density with temperature is shown in Fig. 23.

Viscous flow may be treated as a reaction rate 91" process , and the viscosity '7, is represented by: 110.

,7 .. 1p 4 AH' exp (- ) exp (RT) .... (62) where N is the Avogadro number (6.025 x 1023 mole-1), h is the Planck's constant (6.625 x 10-27 erg.sec) and V is the molar volume. This reduces to: - AS A H 2.303 log -= R. RT #4,•. (63) The energy of activation (AH) for viscous flow is obtained from a plot of log'? against reciprocal temperature which should be linear according to the above equation. In the present work, the plot (Fig. 24) is linear between 700° and 850°C, and the activation energy for viscous flow is found to be 10,100 cal, as against 13,000 cal. at 1000°C reported 76' by Zyasev and Esin . If V, the molar volume of the flowing entity is known, substitution in Equation (63) will give the value of AS, and the free energy of activation (AGvis) will be AH - TAS. From the empirical relationship: AE__.p 2.45 AGvis 91- which is obeyed by almost all liquids the heat of vapourization of molten vanadium pentoxide can be calculated. The calculated values are given in Table XXIII assuming that V205 is the flow unit and evaporates as such from the melt. Viscous flow takes place when a molecule finas itself next to a hole in the structure of the liquid. This hole need not be TABLE XXIII. THERMODYNAMICS OF VISCOUS FLOW.

Energy of Entropy of Free Heat of Vapourization Activation Activation Energy of _ AE _(.oal) Temp Activation Calculated Reported (°C) AH (cal) AS (E.U) AG (cal)

720 10,100 -5.2 15,300 37,500 23,400 820 10,100 -5.2 15,800 38,700 23,400

as large as a molecule, for if there is sufficient room fora group of atoms to revolve together about the point midway between them, such a motion can produce flow. We can, in such cases, assume units of flow smaller than 72,051 say V025, with a molar volume of 40.95 at 920°C. The values of ASvis,AGvis and AEyap calculated on this basis are - 3.89 E.U., 13,900 cals. and 34,000 cals. respectively. In view of the relationship between entropy and probability, it is apparent that the negative value of entropy in all the cases im-olies a small probability of the formation of the activated state. The high value of heat of vapourization calculated from the viscosity indicates that the vapour pressure is actually small, and certainly smaller than those repirted. This is 63, 64 understandable because the authors in their transpiration methocl for the determination of vapour pressure failed to take into account the oxygen loss 112. due to the dissociation equilibrium existing in molten vanadium pentoxide.

The mobility, , acquired by a molecule or ion moving through a medium is independent of the nature of the force, whether it is an electrical force, as, in conductance measurements, or an osmotic force, as in diffusion. There are several reasons to believe that vanadium pentoxide is mainly a semiconductor in the molten state and the molecules are present as such in the melt. It is therefore of doubtful validity to calculate the self-diffusion coefficient of the conducting ions with the data on conductance and the use of the Nernst-Einstein relationship.

A more reasonable approach is by means of the 92- Stokes-Einstein relatinship: according to this , the diffusion coefficient D for large spherical molecules of molecular weight M and density d moving in a medium of viscosity "? at temperature T is given by: 1

RT (4TuNd)7. .... (65) 6%7(3M)3 where R is the gas constant (8.315 x 107 erg deg-1) and N is the Avogadro number (6.025 x 1023). With a value of 0.1126 fOr the viscosity of molten vanadium pentoxide at 720°C, the self-diffusion coefficient is 113. calcul:Ltea to be 2.0 x 10-6 cm2 sec-1; the value at 850°C is 4.0. x 10-6 cm2 sec-1. Those values are rational and it may be assumed that i'ns and molecules dissolved at low concentrations in the melt possess similar (say, 10-5 to 10-7) aiffusi- n coefficients. CH2,PTER IV

RESULTS ON VANADIC CORROSION OF PURE METALS 114.

RESULTS ON V=.DIC CORROSION OF PURE MET S

C=3RLL EXPRESSIONS There are two distinct classes of metals with regard to their behaviour towards vanadic corrosion. Motals like vanadium, tungsten and molybdenum form oxides which offer no a-)nreciable resistance to corrosion under molten vanadium pentoxide, whereas metals like iron, cobalt, nickel and chromium possess oxides or form complex vanaf'_ates which influence the corrosion rate, the extent 'Le-pending the conditions. Cobalt is a median example of the second class, and Fig. 1 represents its behaviour in Dresence of a small amount of vanadium pentoxide. The use of a fairly large excess of vanadium 'entexide eliminates the decelerated process of oxidation, as it does in the cases of iron, titanium, and molybdenum, tungsten and vanadium. Under these conditions, these metals Mow a linear oxidation rate: Ow = kt c .... (66) where Aw is the weight increase par sq.cm. at time t, k is the velocity constant, and c is another constant. In most cases c- is found to be zero-, Equation (66) then reduces to Equation (7) deduced in Chapter 1. Nickel is found to obey a simple logarithmic law:

Aw = k log at 0004 (67) similar to Equation (19), Ch. 1.

Chromium is the most resistant of all metals studied, the corrosion rate of which is so low that it could not be studied by the present method. In all cases, the velocity constant is found to obey the Arrhenius equation: I I k = A exp(TTE) .... (68) The expression "activation energy" is retained for AE, which in effect is the difference in heat contents between the activated and inert species taking part in the process. The pre-exponential reaction constant, A, has the dimensions of k and includes the entropy term, the oxygen pressure term, the diffusion coefficient and diffusional resistances (see Equation (7) as well as geometric factors so that it becomes impossible to analyse it for the entropy contribution alone. However, an attempt has been made to isolate all the individual contributions.

Complete corrosion of the tablets of metals by pure molten vanadium nentoxide showed a gradual but persistent divergence from a simple rate law. antrally, the complete sequence of the catastrophic corrosion is not explicitly expressed by linear, parabolic or logarithmic time functions. However, 116. the initial stage of the corrosion adhere; closely to a linear rate law (except for chromium and possibly fcr nickel also), and obeys with exactitude the Arrhenius relaticnshiD for its temperature variation. Apparently, therefore, a single, fundamental corrosion mechanism over the temperature range is induced by the vanadium pentoxie, and the divergence is due to secondary effects caused by the products of the oxidation, and this has been explained fully in Ch. V.

The results on the vanadic corrosion of metals and allied investigations are given in the following pages. Cobalt is a typical element and it is treated first. Iron is close to cobalt in its behaviour towards vanadic corrosion. Then comes titanium, tungsten, molybdenum and vanadium where the effect due to the corrosion or diffusion barrier is decreasing until it vanishes in vanadium. All these metals obey a linear rate law in presence of moderate excess of molten vanadium pentoxide, while nickel behaves differently and hence is treated separately. Corrosion of metals in presence of a large excess of vanadium pentoxide is dealt with separately in view of its significance in explaining the diffusional processes in vanadic corrosion. Then follows the effect of additives on vanadic corrosion and other methods of (1) 700 °C, 0.56g V2 05,,a02 = 0.2. (2) 650 °C, 0.63g V905, p02= 0.2. (3) .900 QC, 0.708 '605, p02 = 0.2. 2A (2) with 0.9.9g V205. 28 (2) at p0.2 = 0.05. 2C (2) at p02 = 0.01.

6. r.

.**

40 60 120 730 200 240 200

200 400 600 000 1000 1200 CORPOSION T114.E (minute:J. FIG.25. GENERAL NATURI-7 OF THE CORROS!ON OF COBALT. 117.

prevention of corrosion. The chapter ends in the experiments exploring the mechanism of corrosion, particularly the examination of the metal-slag interface and the effect of grain-size on corrosion.

B. RESULTS ON THE CORROSION OF COBALT

1. General Nature of the Corrosion (a) Corr-si,n in nresence of Small Am,unts of Vanadium Pent oxide The general nature of the corrosion of cobalt in presence of small amounts of vanadium pontoxide f= a 40-hour experiment is shown in Fig. 1, and the effect of the amount of vanadium pentoxide is further amplified in Fig. 25. There is an initial fast reaction which slows down unless the temperature is raised or further addition of vanadium 'entoxide is made. Increased depth of the molt however decreases the corrosion rate. 93 There are reports that corrosion of alloys is proportional to the rate of deposition up to 0.1 mg per cm2 of vanadium pentpxile in a gas-turbine but that linear law does not hold good for higher deposition rates: the excess ash in the dep_sit appears to give 19, 94 protection. An attempt has been made to explain this on the basis of an increased diffusional resistance with excess of vanadium pentoxide. This aspect is 118. being extended in the present study: with small amounts of vanadium pentoxide, continued oxidation is governed by ,liffusion through the corrosion products whereas with large excess of vanadium 1-)entoxide, it is governed by the diffusion through and therefore, inter alia, the thickness of the melt.

There is a definite relation between the amount of vanadium pentoxide and the extent of the initial rapid attack (measured in terms of cobalteus oxide) at a particular temperature; this is evident from Table XXIV. The end of the rapid reaction seems to correspond to the formation of a solid product since the results are comparable with the melting point—composition diagram 87 of the cobalt oxide—vanadium pentoxide system .

(b) CerrosiDn in Presence of Moderate Excess of Vanadium Pent:)xide In the presence of a moderate excess of vanadium pentoxide, corrpsiJn proceeds unimpeded to the very last (Curves 2 & 2A, Fig. 25). The corresim progress obeys a linear rate law, and most of the work is therefore concentrated on this aspect of corrosion. However, this linear oxidation process changes to a remarkably accelerated stage in course of time. Similar instances of accelerated linear corrosion have been reported earlierl' 7 The changes in the 119.

TABLE: MXIV. RELATION BETWEEN THE AMOUNT OF VANADIUM PENTOXIDE AND THE EXTENT OF THE RAPID CORROSION CF COBALT.

Amount of Increase in Moles V20 Mean mole Teuz, ,_ 2 V20 !weight due to ------.4 per cent of co) ' oxidation of Co Moles Co0 Cobalt oxide (C) i (g)

700 0,2 0,1434 010028 4,53 " 0,2 0,1945 0,0040 4,28 18 " 1,0 0,3197 i 0,0062 4,53 " 0.2 0.6731 j 0.0134 4.42 750 0,2 0,2044 0,0086 2,09 " i0„2 0,7006 ' 0,0244 2,52 31 " 1.0 0.3197 0.0127 2.21 800 0,2 0,3197 0,0162 1,73 35 " 1.0 0.1130 0.0057 1.74 850 0,2 0,1748 010101 1,52 42 " 0.2 0.2958 0.0101 1.52 900 0,2 0,1890 0,0157 1,06 48 " 1.0 0.3197 0.0270 1.04 940 0,2 0,1815 010178 0.89 " 0,2 0,1945 i 0,0200 0,485 53 " 1.o 0.3197 i 0.0322 0.87 8.4 8-8 9.6 10.0 10.4

FIG.2G.EFFECT OF TEMPERATURE ON THE CORROSION OF COBALT (1)IN OXYGEN WITH 0.4cm DEPTH OF VANADIUM FENTOXIDE. (2) WITH 0.3cm DEPTH OF VANADIUM PENTOXIDE. p00 = 0.20. 120. transport properties of the melt on the introduction of cobalt oxide (the corrosion product) and, possibly, the convep.tion currents set by the heat evolved in the linear oxidation could be responsible for this accelerated stage. It has been suggested that the rates of the reactions are so rapid that the liberated heat of the reaction raises the temperature of the metal and melt 95' and results in a further acceleration of oxidation This has been experimentally found to be true in the case of the corrosion of iron and cobalt: temperature increases by 4 to 6°C at this stage of the corrosion carried out at about 900°C.

2. The Product of Corrosion -Cobaltous Oxide is the product of corrosion as is evident from Table XXV. When the depth of vanadium

TABLE. XXV. RELATION BETWEEN WEIGHT INCREASE OF THE CORROSION-SYSTEM AND WEIGHT-LOSS OF COBALT.

Depth of Time of Metal Increase. Observed Tem p02 V205 •V 205 Corrosion loss for Co.0 increase ormation (g) (cm) (mins) (g) (PC) (E) (g)

900 1 0.88 0.4 90 0.2254 0.0612 0.0607 850 •2 0.90 0.4 120 0:1387 0:0377 0.0351 940 1 1.08 1.44 150 0.1098 0.0298 0.0140 940 1 1.05 1.52 120 0.0810 0.0220 0.0090 57

I I 4 7 I I i 0 0.4 0.8 1.2 1-6' 2-0 24 - LOG p02. FIG.27. EFFECT OF OXYGEN PRESSURE ON THE CORROSION OF CO2ALT gi 3.50 °C WITH 0.4cm DEPTH OF VANADIUivi PENTOXIDE. 121. pentoxide is very large, there is some discrepancy between the weight-loss of the metal and the tial weight- gain on the thormobalance. In order to explain the difference, it ,an be assumed that the ro7ction of cobalt with vanadium 7,,entoxide is going on all the time, but the replenishment of oxygen is inadequate because of the large diffusion path (probably a combination of more than one diffusion layer). However, for moderate depths of melt, as in the normal kinetic studies, the increase in weight of the corrosion system corresponds to the formation of cobaltous oxide. The method adopted for the corrosion studies is therefore sound as far as the kinetics of the vanadic corrosion of cobalt is concerned, and this will be further illustrated for other metals.

3. :inotics of the Corrosion The effect of temperature, depth of molten vanadium pentoxide and oxygen pressure on the linear oxidation of cobalt is given in Table XXVI and Figs. 26 and 27. The oxygen pressures used were not too low to have any significant effect on the corrosion rates due to decomposition of vanadium pentoxide. This condition is preserved in the case of other metals as well.

122,

TABLE XXVI. EFFECT OF TEMPERATURE, DEPTH OF MELT AND' OXYGEN PRESSURE ON THE VANADIC CORROSION OF COBALT

Depth Vel.icity Constant Act- Appa- 1 hir . of . . iva- rent, '''' V205.. (g.cm -2.sec -1) x 106 tion Act- '(A.x) o Ene- ion 0.88. P02 melt, ---o'co847°696 745 795 897° 940 C rgy Cons- p02 x (kcal) Cant (cm) A 0.2. •- 0.5 13.30 6 , I • 6 0.2 0,3 23.40 3.80 7.41 1259 3981 58.88 26.5 3.465 1189 1.0 0,4 12.00 2.58 4.83 950 28.33 37.08 27.0 2.875 1.150 0.40 " 11.00 0.24 " 10.67 0,095 U 10.00 0.048 " 8.67 0.0295 II 7,20 0,01 " 4.17 0.0109 .' 3.34

0.0073, • " 2.75 0.2 0.3 23.22' 4.4)) , , .1.0 1 0.4 12.1 l(PIatinum crlcible) 123.

(a) A=lication of the Arrhenius Equation The enertjy of activation, calculated from the Arrhenius plots (Fig.26), is found to be 26.5 kcal for p02 = 0.2 with 0.3 cm depth of melt (vanadium pentoxide and 27.0 kcal for p02 = 1.0 and depth = 0.4 cm; the corresponding reaction constants are 3.465 and 2.874 respectively.

(b) Effect of Oxygen Pressure The variation of the velocity constant with oxygen pressure at 847°C obeys a two-power relation similar to Equation (40);

• • • • ( 40) k = Bi p02 n ,-B 2 p02 m the exponent for the high oxygen pressure region is 0.083 and that for the low oxygen pressure region is 0,71

(c) Effect of depth of melt The velocity constant is found to be inversely proportional to the depth of melt as envisaged in Equation (10), Ch. I for a diffusion-controlled process For 0.3 and 0.5 cm depths of melt, the velocity constant at 847°C in 20% oxygen are 23.4 x 10-6 and 13.3 x 10-6 respectively, the products of the depth and velocity -6 constant being 7.02 x 10-6 and 6.65 x 10 .

Corrected Representation of Velocity Constant Taking into account the inverse proportionality (1) ,D0= 1, DEPTH OF V2 05 =0.2cm.

230—

. 240— c\7_

1:1) E 200— , cc) LI I

0 150 -

0 (f)

cc 120— o L)

80—

. - 0 40 80 120 160 200 240 280 CORROSION TIME (minutes).

FIG. 2.3. GENERAL Ni-UURE OF THE COI;),ROS:ON c IRON gi" E50 °C IN THIL-: PRESENCE OF: VAN'ADIUM PENTOXIDIE. 124.

with depth. (x) and relationship with the oxygen pressure, the velocity constant can be written as:

• • I k Ak x 1x p02n x exp.(-AE/RT) .... (69) instead of the simple Arrhenius relationship represented by Equation (68). Ak might be regarded as a representative of the entropy of the reaction. Table XXV contains the values of A and Ak and shows the closeness of the values of Ak as against those of A in the simple Arrhenius expression.

C. RESULTS ON THE CORROSION OF :BON 1. General Nature of the Corrosion (a) The c-,.^.er 1 nature of the vanadic corrosion of iron is shown in Fig. 28, and is similar to the behaviour of cobalt but for the enhancement of the accelerated stage of the corrosion progress even at low oxygen pressures.

(b) Corrosi.n in presence of Small Amounts of Vanadium Pentoxide In presence of insufficient amounts of vanadium pentoxide, the reaction rate decreases markedly after reaching a definite stage. The relative amounts of vanadium pentoxide and ferric oxide at the slowing- down stage at different temperatures are given in Table XXVII. It is evident that the slowing-down is 125.

TABLE XXVII. RELATION BETWEEN THE AMOUNT OF VANADIUM PENTOXIDE AND THE EXTENT OF THE RAPID CORROSION OF IRON.

Temp p02 Amount of V205 in weight Moles V202_ for rapid cr= Moles Fe 0 (°0) (g) (g) 2 3 700 1.0 0.8896 0.0088 26.7 (mean) 750 1.0 0.8968 0.0158 15.0 (mean) 800 1.0 0.8979 0.0277 9.5 (mean) 850 1.0 0.2061 0.0130 4.2 850, 1.0 0.5478 0.0250 5.8 850 1.0 0.8927 0.0470 5.0 850 0.206 0.8961 0.0486 4.9 850 0.064 0.8970 0.0488 4.8 900 1.0 0.8951 0.0649 3.6 (mean) 940 1.0 0.9001 0.0742 3.2 (mean) somewhat dependent on the relative amounts of the two oxides. However, the data do not correspond with the 87 - temperature-composition diagram , probably because all the corroded iron is not present in the ferric form alone: this is illustrated below.

(c ) _u_io Product of Corrosion of Iron Iron can exist in several oxidation states - ferrous, ferric and a mixed-valency state. However, 126. the ferrous oxide is oxidized by vanadium pentoxide to ferric oxide. Even in aqueous solutions, ferrous salts are oxidized by vanadium pentoxide or the corresponding salts, and an attempt at the chemical analysis of the chilled slag for ferrous oxide was unsuccessful, but it did prove the existence of tetravalent vanadium in the melt.

Table XXVIII shows that ferrous oxide is the initial product of the vanadic corrosion and is being converted to ferric state in due course, except when the oxygen partial pressure is lOss than 0.01. However, 9, 17 there are contradictory reports that x-ray examination of the scale formed on steels oxidized in the presence of vanadium pentoxide showed that it consists almost exclusively of ferric oxide, particularly the inner layers.

C:rrosion in presence of Moderate Excess of Vanadium Pentcxide The corrosion progress is linear with respect to time, accelerates in due course and slows down finally. The accelerated stage and the final slowing-down ore due to secondary effects and hak) been explained in Sect. B, on the 'Corrosion of Cobalt'. Most of the kinetic studies are therefore carried out on the initial linear step. 127.

TABTR XXVIII. CORRELATION BETWEEN WEIGHT-LOSS OF METAL AND WEIGHT-GAIN FQR DIFFERENT PERIODS OF CORROSION OF IRON. 850°C; 1.0 g.. V205 Underlined values indicate the. closeness with the observed . p02 ..--- 1 except for those indicated. I Increase in weight (g) Corrosion time Loss ofiWeigth-gain ex7Dected for the Metal observed formation of i (min) (g) (g) FeO Fe304 Fe203 limmiumn• 10 0.0233 0.0052 0.0062 0.0089- 0.0100 20 0.0350 0.0091 070100 0.0134 0.0150 40 0.0540 0.0155 070122 0.0206 0.0232 60 0.0732 0.0240 0.0210 0.0278 0.0315 80 0.0989 0.0318 0.0233 0.0378 0.0425 100 0.1210 0.0464 0:0347 0.0462 0.0520 125 0.1280 0.0498 0:0367 0:0489 0.0550 150 0.1311 : 0.0529 I070376 0.0501 0.0563 175 0.1357 0.0571 0.0389 0.0518 0.0583 300 0.1386 ! 0.0600 0.0397 0.0530 0.0596 600 0.1633 0.0741 I0.0468 0.0624 0.0702 350 0.1552 0.0662 0.0445 0.0593 0.0667 (p02=0.063) 150 0.0482 0.0147 .0:0138 0.0184 0.0207 !(1-)02=0.-0105) 150 0.0373 0.0100 .0:0102 0.0143 0.0160 p0 =0.0061) 2 240 0.0300 1 0.0120 ;0.0085 0.0114 0.0128 (Na20-48 mole% V205 )) -4.4 5.0

(1) p02:.. 1. (2) p02 0-0066,9. -4.6 5.6

5.4 O

0 O

I 5-2

1 I I ; r -6'0 J' 6.2 6-6 9.0 9.4 9.6 102 _._1 x 104. T FIE) .2(:;. 17.17FECT aF TEMPEt"Y-TUP.E: 4. 0.4 0.6 1.2 1.6 20 CORVZOS1ON QF 1FZON \,71T1-1 - LOG p02. DEPTH OF VANADILliVi FENT0.1.)1.7.. EFFECTCdr o::vorEi\si PREssuri,-;E WITH 0.4cFn DETEPTP. 0: Vi-%NADIUM AND Al Of3.0 °C. 128.

2. Kinetics of the Corrosion The effect of temperature, depth of melt and oxygen pressure on the corrosion of iron isl given in Table XXIX, and Figs. 29 and 30.

(a) Application of the Arrhenius Equation For the vanadic corrosion of iron in pure oxygen the Arrhenius plot gives an activation energy of 21.2 kcal and an apparent reaction constant of 0.1735. However, the zinetics of the vanadic corrosion at a. low oxygen pressure is governed by a low activation energy. (7.4 kcal) and apparent reaction constant of 7.1 x 10-5

(b) Effect of Oxygen Pressure For the two-power expression (Equation (400) for- the-variation of corrosion rate with oxygen pressure (Fig. 30), the exponents are 0.11 and 0.91 respectively for the high and low oxygen pressure regions.

(c) Effect of Depth of Melt An inverse relationship is found for the depth of melt and the velocity constant. The products of -2 -1 the velocity constant (g.cm sec ) and the depth -6 -6 -6 (cm) are 5.0 x 10 1 4.9 x 10 , 5.0 x 10 and 4,8 x10-6 respectively for depths of 0.2, 0.4, 0.6 and 0.8 cm. This is as it should be if corrosion is

129.

TABU, XXIX. EFFECT OF TEMPERATURE, DEPTH OF MELT AND. OXYGEN PRESSURE ON THE CORROSION OF IRON

7-- Depth1 Velocity Constant Act-- Appa- pf • 1 -2 -1) x 106 iva- rent V 0 (gm.cm sec tion Act-. 2 5 p02 -7 - - Ene- ion smelt o' ol o Cosn x 347°1696 745 °.1795 1897 !940 C acca tant (cm) A 1 1.0 0.2 125.0 1 , I , 1.0 0.4 11273312.96 4.6 18.0 1.6.3124.0 21.2 0.1735 ao694 -51 0.008691 0.4 2.5111.48 1.86 2.191 2.351 3.23 7.4 7.1xioi 1 i 1.0 0.6 1 8.30! 1 11.0 0.8 1 6.001 1 0.30 0.4 i10.6T 0.206 10.33; 0.100 9.83' 0.0635 8.50 0.(10 6.60 1 0.0140 3792 0.0105 3!00! 0.008691 2.511 HO.0061 1.83 , 1.0 0.4 112.4 (Pt crucible) 10.3 0.4 110.,7

-4-8

160 cc"

(1) 0.2g. V205, -50 (2) 0-9g. V2 05.( 0-4cm DEPTH OF MELT). tn E 120 U) ct -5.2 0 0,• 80 0

0 U) -5.4 0

O 40

- 5.6

0 o 40 80 120 160 200 CORROSION TIME (minutes). -5.6 FIG.31. VANADIC CORROSION OF TITANIUM 8.0 8.4 8.8 92 9.6 10.0 AT 850°C IN OXYGEN. 7 x 104 FIG.32-EFFECT OF TEMPERATURE ON THE VANADIC CORROSION OF TITANIUM WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE IN OXYGEN. 130. diffusion-controlled and the diffusional resistanca is proportion to the depth of the melt (Equation (10), Ch. I).

(d) Corrected Reaction Constant Since the velocity constant can be represented by Equation (60), the reaction constants could be corrected for the effect of depth of melt and the variation with oxygen pressure. The corrected values are 0.0694 for the high oxygen pressure region and 2.67 x 10-6 for the kinetics at low oxygen pressures.

IT. RESULTS ON THE CORROSION OF TITANIUM

1. General Nature of the Corrosion Even in the presence of only small amounts of vanadium pentoxide, titanium has a very fast (linear) corrosion rate which becomes increasingly fast after some time (Fig. 31). It is evident that there is- no corrosion barrier on the surf-Ice of the metal, as in the case of cobalt and iron. Decrease of the weight of metal always corresponds exactly to the formation of titanium dioxide.

2. Kinetics of Corrosion Table XXX and Figs. 32 and 33 represent the I I I 1 I

56 - -

5.5 - -

54 - -

o .4 5.3 1

5:2

5.1

5.0 0 0.4 0.6 1.2 1.6 2.0 2-4 - LOG p02

FIG.33. EFFECT OF OXYGEN PRESSURE ON THE CORROSION WITH 0.4 cm DEPTH OF VANADIUM PENTOXIDE—iTITANIUM . 131. variation of the corrosion rate with temperature, depth of melt and oxygen pressure. (i) Application of the Arrhenius Equation The activation energy calculated from the Arrhenius plot (Fig. 32) is found to be 24.0 kcal with an apparent reaction constant of 0.3574; the true reaction constant (Ak) corrected for the effect of the depth of melt and oxygen pressure (Equation (69)) is 0.143.

(ii) Effect of Oxygen Pressure For the variation of the corrosion rates with oxygen pressure, the two-power expression (Equation (40)) has the exponent of 0,09 for the high and 0.59 for the low oxygen pressure regions (Fig. 33).

(iii) Effect of the_auth of Melt Values of the velocity constants with different depths of melt show no inverse relationship even though there is a general decrease of the corrosion rate with increasing depths (Table XXX). The specimens used in this study were very small and considerable error is involved in the computation of depths of melt; titanium is a "light" metal and could not be expected to remain at the bottom of the crucible when heavy corrosion products are formed. 132.

TABTRXXX. EFFECT OF TEMPERATURE AND OXYGEN PRESSURE ON THE VANADIC CORROSION OF TITANIUM

Depth Velocity Constant Act- Act- A = of -2 -1 6 iva- i ion lc V 0 (g.cm sec ) x 10 lion :Cons-- (A.x) 2 5' Ene- tant P02 melt, ---- rgy 77777d x 863° 713° 761°; 812° 912°1954°C (kcal); A 2 (cm)

m•••••••••••••

1 0.2 12.17' 0.4 8!59 1.67 2.881 5.33 14.33;17.33 24.00.3574 0.143. 1 0.6 7.50

0.486 0.4 8.00, 0.0945 6.83 0.040 6.33 0.024 5.19 0.0184 4.33 0.00955 " 2.98 1 1.0 0.4 11.671 (Pt crucible) 5.9 ON trt E 160 2 5.7

55 0 k 80 0 0

0 40 80 120 160 20 0 CORROSION TIME (minutes). 4-9 VANADIC CORROSION OF TUNGSTEN AT 850 °C IN OXYGEN. (1) 0-2g. V205. 47 (2) 0-88g V205 (0.4cm DEPTH). 8.0 64 81 9.2 .9.6 10.0 10.2 4 Tx 10 FIG.35. EFFECT OF TEMPERATURE ON THE CORROSION OF TUNGSTEN IN OXYGEN WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE. 133.

These facts partly explain the divergence observed.

E. RESULTS ON THE CORROSION OF TUNGSTEN 1. General Nature of the Corrosion The corrosion product of tungsten is its trioxide in almost all cases as is indicated by the loss in weight of the metal in comparison with Gains in weight in presence of molten vanadium pentoxide. Tungstic oxide possesses properties similar to vanadium pentoxide and hence does not hinder the accelerated corrosion of tungsten even with only small amounts of vanadium pentoxide (Fig. 34). In fact, vanadic corrosion acquires acceleration when more and more of tungstic oxide is produced.

2. Kinetics of the Corrosion The effect of temperature, depth of melt, and oxygen pressure on the vanadic corrosion of tungsten is represented in Table XXXI and Figs 35 and 36.

A,T)lication of the Arrhenius Equation From the Arrhenius plot (Fig. 35), the. activation energy is found to be 28.2 kcal. The apparent reaction constant is 1.99, and when corrected for the effect of the depth of melt and oxygen pressure is found to be 0.796. 5.6

5.3

5.1 0 0.8 12 1.6 20 - LOG p02• FIG. 36. EFFECT OF OXYGEN PRESSURE ON THE CORROSION OF TUNGSTEN AT 850°C WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE.

134.

TABLE XXXI. EFFECT OF TEMPERATURE, DEPTH OF MELT AND OXYGEN PRESSURE ON THE VANADIC CORROSION OF TUNGSTEN

1 Depth Velocity Constant Act- Act- A . of -1 6 iva- ion k melt (.cm-2.sec ) x 10 tion Cons-j (A.x) P02 x Ene tint' (cm) 863° 712° 760° 812° 916° 956°C rgy A P020."

1 0.4 7.59 1.25 2.0 3.89 12.1 17.83 23.2 1.99 0.796 0.295 rr 6.92 0.095 6.20 0.052 6:03 0.01845 4.33 0.0138 3.35 0.01 r. 2.57 1.0 8.42 (Pt crucible) 0.295 0.4 6.891 (A-02) 1 46 (I) 0.2g V 2 05. (2) 0.57g V2 05, (1) 1- 269 V 2 05,

I u in* —5.0

ac — 52 -160 O ON

U) —5.4 ct 120 a. 5.6 82 8.6 9.0 9.4 9.8 10.2 -; 80 x104 0 FIG.38.EFFECT OF TEMPERATURE ON THE CORROSION OF MOLYBDENUM WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE.

0 40 80 120 160 200 CORROSION TIME (minutes).

FIG.37. VANADIC CORROSION OF MOLYBDENUM AT 850 °C IN 2O% OXYGEN. 135.

(ii) Effect of Oxygen Pressure The variation of the corrosion rate with oxygen pressure obeys a two-power oxpression similar to Equation (40). The index of the power of oxygen pressure is 0.077 for the high and 0.83 for the low oxygen pressure regions (Fig. 36).

(iii) Effect of Depth of Melt An inverse proportionality between the depth of melt and the velocity constant is observed.

F. RESULTS ON THE CORROSION OF MOLYBDEITUM 1. General Nature of Corrosion (i) Similar to tungsten and titanium and unlike iron and cobaltv molybdenum is linearly oxidized in presence of even very small amounts of molten vanadium pentoxide (Fig. 37). The product of oxidation, molybdic oxide, does not therefore form a corrosion barrier, probably due to its low melting point and properties similar to those of vanadium pentoxide. Molybdenum itself is oxidized quite appreciably in the absence of vanadium pentoxide at the temperatures of the present experiments.

(ii) Molybdic oxide volatilizes and condenses on the cooler parts of the silica sheath and this is 136. particularly observed in long-term experiments. This vapourization is not appreciable in the initial stgaes of oxidation because the molybdic oxide is in very dilute solution in the vanadium pentoxide. Therefore no correction need be made for this effect in the calculation of the velocity constant using initial rates.

(iii) The initial rates are normally linear with time but changes to an accelerated process after some time because of the alterations in the transport properties of the melt.

2. The Product of Oxidation The oxidation product of the vanadic corrosion of molybdenum is molybdic oxide, and there is parity between the loss in weight of the metal and the gain in weight on corrosion (T'lble XXXII). Deviations are found only for longer duration of corrosion and very low oxygen =assures. The loss in weight due to volatilization of the molybdic oxide and decomposition of vanadium pentoxide would partly explain the disparity; under these conditions the observed increase in weight is seen to be less than that expected for the loss in weight of the metal. 60

56 Ac 0 0 ...1 1 54 .=,

•

5? =II

50 1 I 0 64 0.8 12 1.6 2.0 2.4 — LOG p02 FIG.39. EFFECT OF OXYGEN PRESSURE ON THE CORROSION OF MOLYBDENUM WITH 0.4 cm DEPTH OF VANADIUM PENTOXIDE. 1137.

TABLE XXXII. CORRELATION BETWEEN WEIGHT LOSS OF METAL AND WEIGHT GAIN ON VANADIC CORROSION OF MOLYBDENUM

Depth Period of Metal Increase for Obsaved of Mo03 Temp p02 V205 Melt Corrosion Loss (oc) formation incrctis (g) (cm) (min) (g) (g) (g)

700 1.0 0.90 0.4 160 0.0250 0.0125 0.0104 750 1.0 0.90 0.4 140 0.0330 0.0165 0.0138 850 1.0 0.90 0.4 160 0.0490 0.0245 0.0280 850 1.0 0.85 1.1 280 0.0202 0.0101 0.0070 850 1.0 1.47 2.2 450 0.0310 0.0155 0.0040 , 850 0.0295 0.93 0.4 150 0.0520 0.0260 0.0230 850 0.0108 0.91 0.4 140 0.0340 0.0170 0.0133 850 0.0036 0.90 0.4 150 0705201 0.0260 0.0070 940 1.0 0.90 0.4 160 0.0490 0.0245 0.0280

3. Kinotics of the Corrosion Table XXXIII and Figs. 38 and 39 present the data on the effect of temperature, depth of melt and oxygen pressure on the corrosion of molybdenum.

(i) An7licability of the Arrhenius Equation The activation energy does not change appreciably with the oxygen pressure provided it is not too low: the values obtained from the Arrhenius plots are 18.3 138.

TABTR, XXXIII. EFFECT OF TEMPERATURE, DEPTH OF MELT AND OXYGEN PRESSURE ON THE VANADIC CORROSION OF MOLYBDENUM

11••••IM•IMMYSImm• •••mo...1.111.1.0001••••••1••••••••••YMI Depth( Velocity Constant Act- Act A of -1 6 iva- io lc= melt (g.cm-2.sec ) x 10 tion Con (A.x) P02 0 Ene- tan (cm) 844° 694° 749° 797° 8940 932 C rgy (kcal) A

0.20 074 8.0 2.67 4:25 5:75 12.33 16.5 18.4 c4.7,62 au_ 1.00 0.4 9.33 2.69 4.33 6.17 13.34 17.78 18.3 0.0319 ,9045 0.20 0.27 15.0 0.20 0.6 5.25 0.248 0.4 8.1 0.102 7:41 0.031 5.25 0.01459 3.95 0.0108 3.0

0.0100 H 3.0

0.0073 It 2.5 0.003627 1.25 0.a 0.4 10.2. (Pt crucible 0.2 0.41 8.2 I I (A-02) mixttre ' 139. and 18.4 kcal. respectively in p02 = 1 and p02 = 0.2; the corresponding apparent reaction constants are 0.0319 and 0.0362 respectively. The reaction constants corrected for the effect of oxygen pressure. and depth of melt are also given in Table XXXIII.

(ii) Effect of Oxyp;en Pressure The reaction rates obey a two-power expression (Fig. 39; Equation (40) ), and the exponents of the; oxygen pressure are 0.1 for the high and 0.77 for the low oxygen pressure regions.

(iii)Effect of Depth of Molt When the molybdenum specimen is fully covered with molten vanadium pentoxide, the rate constant is inversely proportional to the depth of the melt. With depths of 0.2, 0.4, and 0.6 cm, the products of the depths (cm) and the velocity constants (g.cm-2 sec-1) are 3.0 x 10-61 3.2 x 10-6 and 3.15 x 10-6 respectively (Table XXXIII).

G. RESULTS ON THE CORROSION OF VANADIUM 1. Goril Nature of the Corrosion The oxidati n of vanadium in the presence of molten vanadium pentoxide gives a product of the same composition as the melt. There is thus an ideal case of an absence of a corrosion barrier together with -445 (1) 0.4cm DEPTH OF MELT, p02= 1. (2) 08cm u , p02= 1.

- 5-0 (3) 0.4 cm p02= 0-00869.

-52

o •4

- 5-8

20 40 60 80 100 8.0 8.4 8.8 92 9.6 10-0 1 4 CORROSION TIME (minutes). -- x 10 . T FIG.40. EFFECT OF TEMPERATURE ON THE F1G.41. ARRHENIUS PLOT FOR THE CORROSION CORROSION OF VANADIUM IN OXYGEN OF VANADIUM IN THE PRESENCE OF WITH 0.4cm DEPTH OF VANADIUM MOLTEN VANADIUM PENTOXIDE. PENTOXIDE. 140.

absence of a change in the transport properties of the melt. Always a smooth; linear corrosion rate is observed (Fig. 40).

2. Thu ,r ,-act of Oxidation The increase in weight observed during corrosion compares well with the loss in weight of the metal if vanadium pentoxide is assumed to be the product of oxidation. The necessary data are presented in Table XXXIV.

TABLR XXXIV. CORRELATION BETWEEN WEIGHT-LOSS OF METAL AND WEIGHT-GAIN ON CORROSION OF VANADIUM

Depth Period of Metal Increase for °item-void of V205 Temp p02 V205 Melt Corrosion Loss ( q) formation illml ° (g) k (cm) (min) (g) (0) (g)

850 0.50 0.86 0.4 180 0.0444 0.0343 0.0340 850 0.207 0.86 0.4 240 0:0433 0.0340 0.0338 850 0.0609 0.36a 0.4 140 0.0273 0.0214 0.0210 850 0.0216 0.86 0.4 150 0.0244 0.0191 0.0167 850 0.0105 0786 0.4 150 070200 0.0157 0.0133 350 0f00606 0736 0.4 120 0.0143 0.0112 0.0077 850 1.0 1.36 2.2 60 0.0051 0.0040 0.0038 O

04 0.8 1.2 1.6 2.0 24 - LOG p02 FIG.42. EFFECT OF OXYGEN PRESSURE ON THE CORROSION OF VANADIUM AT 850 °C WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE. 141.

3. Kinutics of the Corrosion The effect of temperature, depth- of melt and oxygen pressure on the corrosion of vanadium is shown in Table XXXV, and Figs. 41 and 42.

TABLE XXXV. EFFECT OF TEMPERATURE, DEPTH OF MELT AND OXYGEN PRESSURE ON THE CORROSION OF VANADIUM

Depth Velocity Constant Act- Act- ' of - (F.;.cm-2.sec-1) x 106 iva- ion (A , p02 melt, tion Cons 1.-A) x I Ene- tan' 0.12 (cm) 033° 712° 760° 812° 916°1956°C ru le (kcal) A

41C9 0,4 2,4611,38 1,62 2,04 2,95 3,31 8,9 01 000 • 7-- 1.0 0.4 7417 2.58 3.75 5.33 10.3 13.4 16.5 0.0114 000456 , , , ' (7,01 0 0,8 3,91 1.27 1.95 2.70 5.431 7.4 16.5 0.00573 453 .0 0,2 12,33 _ 1 1.0 0,6 5,17 /5 0.4 6,33 0,297 " 6,25 0,207 " 5,60 p,104 " 5,56 b,o6o9 ” 4,97 o,o3o3 n 4,55 0,0216 " 4,00 0,0140 ft' 3,33 0,0105 " 2,30 Q.00605 " 1.33 1,0 0.4 3.33 (Pt crucible) 1 142.

(1) :,lication of the Arrhenius Equation The activiation energy calculated for the kinetics of corrosion in pure oxygen with 0.4 and 0.3 cm depth of molten vanadium pentoxide give the same value of 16.5 kcal. However, for the corrosion at the low oxygen partial pressure of 0.00369, the activation energy is considerably smaller than that for corrosion at the 'Lich oxygen pressure region. This shows that the mechanism of the transport of oxygen is different in these two cases.

(ii) Effect of Oxygen Pressure The vaeiation of the velocity constants with oxygen pressure (Fig.41) can be represented by a two- power relation (Equation (40) ), and the exponents of oxygen pressure are 0.12 and 0.71 respectively at the high and low oxygen pressure regions.

(iii) Effect of the Depth of Melt An inverse relationship exists between the depth of melt and the velocity constant; the products of depth and velocity constants are: 2.57 x 10-6 for 0.2 cm 1 -6 2.37 x 10 for 0.4 cm, 3.09 x 10-6 for 0.6 cm, and 3.13 x 10-6 for 0.3 cm (Table XXXV) depth of melt.

(iv) Corrected Reaction Constants (Ak) Assuming that the velocity constant is governed (1) 0.4cm DEPTH OF MELT, 699T. (2) a , 844 'C. (3) a ,932T. (4) 0.6cm DEPTH OF MELT, 844 'C. (5) 0.8 cm DEPTH OF MELT, 844 T.

N20 0 • --- -. •, U) • ..-- .....

2 6 0 ce 5 ct cj0 4

04 0.2 03 0.4 0.5 1 2 3 4 5 6 7 8 9 10 CORROSION TIME (hours). FIG.43.VANADIC CORROSION OF NICKEL IN 20% OXYGEN. 143. by Equation (69) where the effect of the depth of melt and oxygen pressure on the velocity has been accounted for, the reactin constants have been corrected and given in Table XXXV. The corrected reaction constants are remarkably similar while the apparent reaction constants differ considerably.

H. RESULTS ON THE CORROSION OF NICKEL 1. Genoral Nature of the Corrosion Nickel differs from other metals in that the initial fast oxidation process slows clown immediately even when a fairly large excess of vanadium pentoxide is present. Thermal shock, either a sudden decrease or increase, brings about an increase in the corrosion rate, and it can be assumed that an adherent oxidation layer controls the corrosion progress. A log-log plot of the corrosion progress against time (Fig.43) is not linear, but the initial stages of oxidation obeys the logarithmic rate law (Equation (67) ). The value of 'a! in the equation, L1w = k log at is not equal to unity. From Fig. 431 it seems that another mechanism operates immediately after the initial fast corrosion, and the slope of the log-log plot gives values of 0.1 to 0.4, always considerably less than 0.5 expected for a process controlled by oxygen diffusion through a soli.' layer. Probably several processes operate in 2.7

. 2•3• 4c to 0 1' 1.9

1.5 8? 8.6 9.0 .9.4 9.8 10? 2 104 T x FIG.44.EFFECT OF TEMPERATURE ON THE CORROSION OF NICKEL WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE IN 20% OXYGEN.

2•8

I 2.0

1.6 I I I I I 0 0.4 0.8 1.2 16 2.0 2•4 — LOG p02. FIG.45. EFFECT OF OXYGEN PRESSURE ON THE CORROSION OF NICKEL AT 850 °C. WITH 0.4cm DEPTH OF VANADIUM PENTOXIDE. 144.

this region. The accuracy of the present experimental method is not satisfactory enough to make any positive conclusion; the same is true with the extremely low corrosion rate of chromium.

2. Kintics of the Corrosion The effect of temperature, depth of melt and oxygen pressure on the corrosion rate is shown in Table XXXVI and Figs 44 and 45.

TABLE XXXVI. EFFECT OF TEMPERATURE, DEPTH OF MELT AND: OXYGEN PRESSURE ON THE CORROSION OF NICKEL

Depth Velocity Constant Act- Act- Mean of - iva- ion p02 molt, (g.cm-2) x 103 tion Cons- Value x Ene- tant of (cm) `2,44° 699° 749° 797° 094° 932°C rgy Iran, (kcal) . , , , 0,2 0,4 10,9 1.62 3.21 5.16 16.3 25.7 27.9 2971 ••595 1,0 0,4 14,13 0,2 0,6 6,2 0,2 0,8 3,35 0,055 0.4 8,03 0,019 " 6,92 0,010 " 4,77 0,00708 " 3,16 0.00501 " 1.74

(i) Jf the Arrhenius Equation The activation energy is found to be 27.9 kcal. The reaction constant in this case has dimensions 145.

different from those for other metals which corrode linearly with time and therefore is not comparable with them.

(ii) Effect of Oxygen Pressure The exponents of the two-power relation (Equation (40) ) for the variation of the corrosion rate with oxygen pressure (Fig. 45) are 0.15 and 1.66 for the high and low oxygen pressure regions respectively.

(iii) Effect of the Depth of Melt Increase of the depth of melt decreases the corrosion rate (Table XXXVI). As the corrosion proceeds with time, the rate becomes almost constant and similar for the different depths (Fig. 43). The rate is, therefore, mainly a chtlpacteristic of the corrosion layer and not of the bulk melt.

I. RESULTS 0N THE CORROSION OF CHROMIUM The behaviour of chromium is similar to nickel, but the corrosion rates are too low to be studied by the present method. The savoles tested were single crystals of chromiun, and probably this has some bearing on this reduced corrosion rates; but from experiments on single crystals of other metals, this seems unlikely. It has been reported that chromium-nickel steels, with other alloying elements, appear to have the best 146.

9' resistance to vanadic corrosion-. However, contra- 28 dictory views- have also been expressed .

J. ACTION OF MOLTEN VANADIUM PENTOXIDE ON METALS IN THE ABSENCE OF OXYGEN On the basis of thermodynamics, molten vanadium pentoxide is expected to have some effect on the metals however low the oxygen partial pressure. This aspect will be discussed later, but, for the present, the results from the investigation on the extent of the reaction are discussed. Circulation of a fixed quantity (in practice, about one litre) of nitrogen over the metal in contact with molten vanadium pentoxide obviates the continuous decomposition of the melt and kc;eps the system under a low equilibrium oxygen pressure. Under these conditions, the general trend of the attack on the metals for an 8 and 24 hour experiment at 8500C is shown in Table XXXVII. Chromium is the only metal which is not appreciably affected by the melt. These results are in contradiction to some statements which 5, 7, 9, 18' have appeared in the literature

K. DIFFUSICNAL CORROSION STUDIES The inverse proportionality between the velocity constants and the depth of melt during the vanadic corrosion of metals has been exemplified by the data on iron, cobalt, molybdenum, tungsten and vanadium. Tho I47.

TABLE XXXVII. ATTACK OF METALS BY MOLTEN VANADIUM PENTOXIDE AT 35000.

Initial Initial VMetal2 5 dissolved Metal Metal Weight Surface i in 3 hours dissolve.1 Area 1 r in 24 hour- (g) (sq.cm) (8) (g) (8)

Fe 0.23 0.63 j 0.81 0.0216 0.0403 Co 0.25 0.62 0.38 0.0202 0.0396 Ni 0.25 0.61 0.87 0.0142 0.0295 Mo 0.29 0.64 0.37 0.0168 0.0656 V 0.17 0.63 0.77 j 070071 0.0185 Ti 0.041 0:23 F 0.75 i 0.0070 0.0280 W 07 33 0744 0.80 0.0178 0.0432 Cr 0.073 0.48 i 0.67 Practically unaffected i

conditions in these experiments are not conducive to undisturbed diffusional processes represented by Equations (10) and (14); the change in area of the specimen during corrosion and the change in height and transport properties of the melt brought about by the corrosion nroducts affect the rates. In the experiments designed to study diffusion through the melt alone, the narrow cross-section of the crucible (Section Ch.II) with the specimen just fitting the bottom minimizesconvection currents, and for long columns of melt, the oxygen reaching the specimen -1.9

-1.7

-1.5 0

-13

8.6 9.0 9.4 9.8 10•? 1 x 104 T FIG.46.PERMEABILITY OF OXYGEN THROUGH MOLTEN VANADIUM PENTOXIDE. x. = DEPTH OF VANADIUM PENTOXIDE COLUMN (c m ). Aw g OXYGEN UPTAKE BY METAL (mg). 143. is consumed immediately. The increase in weight per unit time is then the amount of oxygen diffusing through the column and reaching the bottom. Since there is sufficient vanadium pentoxide to dissolve the metal oxides efficiently, there is no complication arising out of the oxide formation, particularly if the initial weight-changes are taken.

Representative data are presented in Table XXXVIII. The column depths for each experiment are given approximately; exact individual depths have been used in the calculations. The mean value is calculated from several experimental values while the Table contains only representative ones. The underlined values in the Table deviate considerably from the mean and are not used in the evaluation of the mean. Deviations are mainly observed for iron and cobalt at higher temperatures and smaller depths of melt. Convection currents caused by the heat of the reaction and a change in the diffusing species, diffusion mechanism or transport properties of the melt could be the main cause of deviation.

According to Equation (9), Chapter I, the product of the depth and the weight increase per unit time is a measure of the diffusion coefficient. An Arrhenius relation is applicable only below 900°C (Fig. 46). The activation energy of diffusion between 700' and 900°C 149.

TABLE XXXVIII.' DIFFUSION, STUDIES ON MOLTEN VANADIUM PENTOXIDE. (Depth in cms; Weight increase in mg. per hour)

..pproximat Product of depth (cm) and Depth of weight-increase (mg) per hour Metal ,nelt ----7--t------(cm)i 694" 745° 795° 347° 397° 940°C

1,0 0,0212 0,0247 0,0281 0,0320 0,0380 0,069.- V 1,6 0,0153 0,0213 0,0247 0,0296 0,0378 0,058" 2.2- 0.0151 0.0188 0.0249 0.0293 0.0347' 0.0624 , 1 , . 1.0 0.0192 10.02111 0.0269 0.0570 0._0640 0.0912 1 , 1 Mo 1.6 0.0220 10.0176 0.0293 0.031110.0381 0.057* , , I ' 2.2 0.0178 10.0183 0.0222 0.0304 0.0364 0.0630 i ---t--, , . 1,0 0,0192 10,0290 0,0384 0,0555 0,0767: 0 091 1,6 0,0139 1010180 0,0303 0,0347 0,0530 0,0720 2.a 0.0202 10.0222 0.0260 0.0336 0.0456 0.0616 .- ' , , . 1t0 0,0200 10,0200 0,0378 0,0567 0,1180 0,1684 I Co 1,6 0,0135 10,0216 0,0259 0,0570 0,0576 0i2760 2.2 0.0130 10.0155 0.0250 0.0555 0.0555 0.0750 1 , --- ' . , Mean-Product 0.0177'10.0203 0.0267 0.0351 0.0494 0.0601 1

is calculated to be 14.7 ± 2.0 k.cals, and this tends to decrease with increase of temperature. 30

0 0 10 20 30 40 50 Na 0 CONCENTRATION (Mole %) 2 FIG.41. OXYGEN ABSORPTION CAPACITY AND CORROSION PROPERTY OF SODIUM OXIDE—VANADIUM PENTOXIDE MIXTURES. (CORROSION OF IRON IN OXYGEN AT 850 °C WITH 0.4cm DEPTH OF MELT.) 150.

4. EFFECT OF SALTS AND METAL OXIDES ON THE CORROSIVE PROPERTIES OF VANADIUM PENTOXIDE 1. Sodium Salt - Vanadium Pentoxide Mixtures (i) 3,dium Sul-Dhate - Vanadium Pentoxide System Foster et al have studied the decompostion of sodium sulphate in molten vanadium pentoxide and constructed the compatibility diagram for the 96, 97. Na20 - SO3 - V205 system Their finaings are further amplified in the present work, and tha corrosion rates of iron in various mixtures are determined. The results are presented in Table XXXIX and Fig. 47. The main observations of importance are:

(a)all mixtures decompose at about 5000C by sintering and solid-state reactions and sulphur trioxide is produced;

(b)the eutectic sintering point depends on the. mole ratio of vanadium pentoxide to sodium sulphate;

(c)decomposition is very fast in molten vanadium pentoxide;

(d)decomposition in the solid state produce a certain amount of "bronze" containing vanadium tetroxide as found by chemical analysis and by the uptake of oxygen in the molten state if the- decomposition occurs in oxygen;

151.

TABLEXXXIX. INTERACTION BETWEEN SODIUM SULPHATE AND VANADIUM PENTOXIDE. i Velocity' 1 Wt. Wt. Wt. Mole% V204: Constant, pecrease, for Mole% Wt. of I for ! decrease decrease V 0 Na20: coreosio 'decompo- on 4 V 0 of iron No2SO4 Mixture sition iobserved cooling solid 2 5 by the to re$idue Na20 (g. cm (g) (g) j (g) (g) sec_1 ) x 106 1 . 0 - 1 - i - - 12.0 , 1 . . - 1.26 3.6014 '0.200 1 0.0202 0.0032 0.91 L/1.25/97 15.8 1 a , 1.28 ' 0.0076 0.0015 1.26 1/1.21/77 16.2 , 1.3454 0.0076 , 3.241.5904:0.02280 0.0234 0:0 043 3.05 1/1:0 6/31 21.2 6.65 1.7253 0.0513 1 0.0521 0.0097 6.30 1/1.05/14 27.0 11.28 1.7000 0.0865 0.0391 0.0172 11.22 1/1/7 1 29.5 14.45 1.6052 10.1054 1 0.1190 0.0168 11.52 1/1:,25/64 28.1 24.89 1.4133 0.1637 1 0.1650 0.0063 4.79 1/52/14.7 20.0 a . 28.19 1.8923 0.2502 i 0.2537 0.0084 4.73 1/595/14.2 14.5 1 a , 32.19 1.1476 0.1750 , 0.1778 0.0038 3.50 1/92/18.4 11.2 , 1 a , , 4 • 36.13 1.5730 0.2717 0.2854 0.0050 3.33 1/10.948.2 9.5 40.44 1.4677 ,0.2867 I 0.2916 0.0026 1.84 1/22/31.4 6.0 52.15 1.5182 10.3933 0.3425 0.0003 0.2 1459/236 2.2 1 1 152.

(e)decomposition of sodium sulphate is complete if it is 50 mole per cent or less in the mixture.;

(f)free sodium sulphate can exist in the melt only- for mixtures of more than 50 mole pa-cent of the salt; (g)the sodium-oxide-vanadium pentoxide melt (residue after the decomposition) solidified with evolution of oxygen, and the solid reabsorbs the same amount of oxygen on fusion and the cycle can be repeated;

(h)chemical analysis of the solidified mixture shows that it contains vanadium tetroxide exactly equivalent to the oxygen evolved;

(i)the corrosive activity of the melt towards iron could be correlated with its oxygen absorption capacity; and

(j)a reduced corrosion rate is observed with mixtures containing more than 30 mole per cent of sodium oxide.

These findings agree reasonably well With those. 2,16-19,2,16-19,24,71,96-100' reported piecemeal in the literature • 34 The observation that aluminium bronze specimens partially immersed in a 90/10 vanadium pentoxide/sodium sulphate mixture at 5009-0 were attacked severOy above 153. the level of the powder mixture finds a convincing explanation in terms of the sulphur trioxide evolved. Any tendency of the evolved sulphur trioxide to break down to sulphur dioxide and oxygen, acomang to equilibrium requirements, would make additional oxygen available for oxidation at the metal surfaces; this oxygen is claimed to be present in a highly reactive form.

(ii) Sodium Chloride - Vanadium Pentoxide System Like sodium sulphate, sodium chloride is also decomposed by vanadium pentoxide at about 500°C, and the decomposition rate becomes faster when vanadium pentoxide is molten or when the decomposition is carried out in a stream of nitrogen instead of oxygen. The chlorine evolved attacked the platinum parts of the experimental arrangement and hence a complete study was not carried out on the thermobalance. Part of the work on the decompositions were therefore carried out in a muffle furnace. The results obtained arc presented in Table XL. The residue obtained by decomposition of sodium chloride-vanadium pentoxide mixtures has properties similar to those from sodium sulphate-vanadium pentoxide mixtures. 154.

TABLE XL. INTERACTION BETWEEN SODIUM CHLORIDE ANTI VANADIUM PENTOXIDE.

IWt of Wt of Mole %, Wt I Wt -1 Method I decrease i decrease Remarks- V2C5 Na2C12 Na2C12 for observed chlori ne

(g) (g) 1((g) (g) , a , Thermo- 2.2113 0.0270 1.86 0.0164 0.0170 Decomposed' in balance N2 below 700°C " 11.0570 0.0285 4.03 0.0173 0.0179 U "' 0.1818 0.0189 14.3 0.0115 0.0115 11. n 0.1818 0.0369 25.01 0.0230 j 0.0202 Ts' "' 0.0909 0.0369 40.02 0.0230 0.0187 II rti n' 0.0455 0.0369 57.16 0.0230 0.0190 . , . . Muffle 1 6753 0.0865 7.4 0.0525 0.0597 Decomposed in , . , . , • , air at 900°C " 1.9131 0.1540 11.15 0.0934 0.1048 . , a 11.5060 0.2031 17.36 0.1232 0.1147 " a "' 3.0577 0.4972 20.17 0.3016 j 0.2781 " .1.5278 0.3911 28.51 0.2373 0.1940 " 1.5192 0.5398 35.59 0.3275 0.2635 11.' 0.6422 1.5722 79.2 0.9538 0.3873

The decomposition is a complex process since sodium chloride, unlike sodium sulphate, is a non- oxygenated compound. It is evident from the Table that all mixtures with 14.3 mole per cent or less of sodium chloride (Na2C12) decompose completely with' 155.

loss of chlorine. The 14.3 mole per cent composition corresponds to the reaction:

NEL-C12 6V205 -4 Na20 . V204. 5V205 Clz • • • C70) ' If this reaction is carried out at about 550°C in nitrogen, the product is a stable solid which under- goes no reaction with oxygen; however, when the temperature is raised to melt the solid, oxygen is absorbed corresponding to the change of Na20.V204.5V205 to Na20.6V205. Mixtures of sodium chloride and vanadium pentoxide with more than 14.3 mole per cent

of sodium chloride (Na2C12) are only incompletely decomposed. Several vanadyl-vanadates have been 71 isolated including Na20.V204.V2-05, the formation of which may be expected from the reaction

Na2C12 2V205 Na20 • V204. V205 + C12, ..•• (71) And between the extremes represented by reactions (70) and (71), there might be many other complex- cases.

The structure of solid sodium oxide-vanadium 86 oxide bronzes is reported to contain about 0.4 per cent of the oxygen sites vacant in the lattice. The order-disorder transformation on fusion makes possible the absorption of oxygen to fill in the vacant sites. This oxygen is highly labile and unlike the oxygen anions in pure molten vanadium pentoxide. Consequently the oxygen transport 60 60

0 '/.

2.5%

4.8% 17.47... 9.4% 60%

0 40 80 120 0 40 80 120 CORROSION TIME (minutes). CORROSION TIME(minutes). (a) COBALT OXIDE. (b) NICKEL OXIDE.

60 60

. 40 40 - .?,- -6): e E LU U) 14.1 •:/ .cU) LU Lu 1: 20 cr 20 - 12.7% - L.) (..3 Z Z

14 k i 0 40 80 120 0 40 80 120 CORROSION TIME (minutes). CORROSION TIME (minutes). ( C ) CHROMIUM OXIDE. (d) NIOBIUM PENTOXIDE. FIG.4g. EFFECT OF METAL OXIDE ADDITIONS ON THE VANADIC CORROSION OF IRON. (850°C IN OXYGEN WITH 0.4cm DEPTH OF MELT. INITIAL SURFACE AREA OF SPECIMEN = 0.60sq•cm.) 156. properties of the melt and its corrosive nature are profoundly increased (Fig. 47). The decrease of corrosion rate in mixtures with a large excess of sodium oxide or salt is probably due to a combination of the dillition effect and the reduced activity and 17, 10 fluidity due to increased melting points.

2. r':etal Oxide - Vanadium Pentoxide Mixtures In imitation of the use of additives to prevent or partially reduce the residual fuel oil ash corrosion, the results on the corrosion rates of iron in presence of molten vanadium pentoxide with added cobalt oxide, nickel oxide, chromium oxide and niobium pentoxide are presented in Fig. 48 (a, b, c & d). It is difficult to express the corrosion progress by means of a single or combined rate law and thence discover the characteristic rate constant which could be compared with others. Hence comparison of the effect of additives on the vanadic corrosion of iron is made by plotting the weight increase against time for similar specimens under similar conditions, particularly with regard to the initial depth of melt.

In general, small additions enhance the corrosion rate. This is because vanadium pentoxide is a transition semiconductor and its conductance is 157. normally increased by the addition of metal ions of lower or higher valency; the effect of ions of different valencies are in the order: mono >di >tri (Fig.22). The reduction in the corrosion rates for larger quantities of the added metal oxide is to be attributed to the dilution and the attendent reduction in activity of the vanadium pentoxide; the increased melting points of the slags also play a part in affecting its corrosive property.

EIECT=CHEMISTRY OF VANADIC CORROSION 102 Ilschner-Gensch has observed an enhanced corrosion of nickel immersed in a molten borate when it is touched by a platinum wire. However, the data on the vanadic corrosion of metals in a platinum crucible under conditions comparable to those in silica crucibles (Tables XXVI, XXIX - XXXI, XXXIII and XXXV) show that the corrosion rate is only very slightly increased by contact with the noble metal. In spite of the difficulty of simulating the conditions with the two types of crucibles (one conical and the other cylindrical), the values show a trend towards an enhancement but this may well be due to dissolved silica reducing the corrosion rate.

The extent of corrosion of an iron rod anode dipping in molten vanadium pentoxide is higher than 158. that of a cathode or an uncharged reference rod under similar environments. The relative extents of corrosion of an anode, a cathode and a neutral rod under a variety of conditions are presented in Table XLI.

TABLE XLI. VANADIC CORROSION OF IRON IN AIR WITH AND WITHOUT CURRENT PASSING. (Area exposed = 6 sq.cm; inter-electrode distance=1.5cm)

Temp Current Passing Time Wt loss due to Corrosion (°o) (A) (hours) Neutral Anode Cathode

750 0.5 8 2.51 3.12 4 1.98 750 1.0 8 2.60 3.06 1.96 750 2.0 7 2.45 3.26 1.90 800 2.0 12- 3.84 5.12 2.71 850 4.0 5 4.47 6.13 3.24 850 1.0 6 4.58 6.02 3.44

The results are not highly promising as regards the cathodic protection, but they do indicate the electrochemically decelerated corrosion of the cathode. The problem of vanadia corrosion is mainly due to the oxidising and semiconducting capacity of molten vanadium pentoxide. This is further illustrated by the only partially successful attempts to find out the decomposition potential of molten vanadium Anode Cathode A B

flizle-

C D Chemically Controlled Diffusion Controlled

FIG. 49. SHAPE OF CORRODED SAMPLES 159. pentoxide. The only break, albeit ill-defined, in the current-voltage curve is at 60mV, which might therefore be assumed as the decomposition potential of the melt. Being a semiconductor, there is appreciable flow of current through the melt even at as low a potential as 5 mV. Beyond 60 mV, the current- voltage curve is linear up to 2.4 V when a current of 4 amperes is passing through. All these, in a way, prove that there is a greater element of electronic than of ionic conduction in molten vanadium pentoxide. It is due to this fact that the cathodic protection of metals from vanadic corrosion is inefficient.

N. SHAPE OF THE CORRODED SAMPLES There is a rounding-off at the sharp edges of the tablet specimens, especially at the upper perimeter during corrosion (Fig. 49A). A rod of iron dipped in molten vanadium pentoxide (Section M, above) corrodes to the maximum at the melt-air interface, the corrosion decreasing apparently exponentially with the depth (Fig. 49B). An attempt to correlate the change in diameter of the rod with the depth of immersion was not successful. Probably secondary effects such as the accumulation of denser corrosion products at the lower parts of the rods invalidate a mathematical relation. These observations have however bben 1, 29' reported earlier with different explanations • V, 43 °A _ ..• , ..,.. a # ..,..-..--qr-ts,i,,7-' a) Slag 4-, -1,/-1: 1 .t.:'.. a a.) r IS‘ (1) Ti, 5°A . k-,44:•., jib,: 4 ,s4

A. X- Ray picture for V. B. X-Ray picture for Ti. C. Scanning for V and Ti.

FIG.50ELECTRON PROBE MICROANALYSIS OF A TITANIUM -SLAG SYSTEM. (Ti + 0.4g.V205;4hrs at 800°C in 02).

ELECTRON PROBE MICROANALYSIS.

Fe,100 Mo,100°A

Slag Metal V,40% Metal Slag Fe, 25% Mo;10% V,0°/0

A. X-Ray picture for V. B. Scanning for Fe. Scanning for Mo. and V.

FIG.52IRON-SLAG. FIG.5,4- MOLYBDENUM -SLAG. (Fe + 0.5g.V205;4hrs at 850°C In 02 ) (Mo + 0,5gV205,4hrs at 850°C in 02). 160.

Corrosion above the immersion level has been attributed to the attack by the vapour; but this seems to be due to the creep of the melt rather than due to its vapour. Thus if all the facts are taken into consideration, these observations illustrate qualitatively the presence of a diffusional step in the vanadic corrosion of metals. If it were a simple chemical dissolution of the metal in molten vanadium pentoxide, only a uniform decrease in the size of the specimen can be expected (illustrated in Fig. 49 C & D) during the course of corrosion. The activity of oxygen (or of corresponding reactive species) at the bottom of the crucible will be less than that at the top if it were a diffusion-controlled process; this greater corrosion and the rounding-off at the top is therefore to be expected.

O. EXAMINATION OF THE METAL-SLAG INTERFACE Figs. 50 - 56 represent the results on the examination of the metal-melt interface by means of electron probe X-ray micro-analyser. The main observations are:

(a) There is a very sharp interface between the metal and slag and no evidence of grain-boundary penetration is observed (Figs. 50 & 52).

A. FROM METAL TO SLAG. B. DIFFUSION LAYER. 1000

60 60 ""

, o Ti 0 200 400 600 2 4 6 8 10 DISTANCE FROM METAL TO SLAG(p). DISTANCE FROM METAL TO SLAG(p). FIG.51.X-RAY SCANNING FOR VANADIUM AND TITANIUM IN TITANIUM-SLAG INTERFACE. (Ti CORRODED FOR 4HOURS IN OXYGEN AT 800 °C WITH 0.49. V205). 100 100

80 MIW 80

60 —1 Q Lai X ilt 40

20 20

100 200 300 400 100 200 300 400 DISTANCE FROM METAL TO SLAG (N). DISTANCE FROM METAL TO SLAG( p). FIG.53.X- RAY SCANNING FOR VANADIUM F1G.55.X-RAY SCANNING FOR CHROMIUM AND IRON AT IRON-SLAG AND VANADIUM AT CHROMIUM-SLAG INTERFACE. INTERFACE. (Fe CORRODED FOR 3 HOURS AT (Cr CORRODED FOR 8 HOURS AT 800 °C IN 02 WITH 0- 6g.V205). 850 °C IN 02 WITH 1 g. OF V200. 161.

(b) For titanium, the corrosion layer, if at all present, is not more than 5 micron thick.

(c) Definite segregation of titanium occurs occasionally in the slag because lumps of titanium concentrations are observed in the slag; this gains support from a report of the negligible solubility of titanium dioxide in 103' vanadium pentoxide

(d) The diffusion barrier is more developed in thickness and homogeneity in the iron-slag system.

(e) The diffusion layer is not thick in molybdenum- slag system, and the concentration profiles show non-uniformity, probably due to blow-holes in the slag, perhaps those arising from condensation of high local pressures of molybdenum oxide vapour.

(f) Nickel and chromium show a corrosion layer of approximately 50 micron thick separating the metal and the slag (Fig. 55 & 56).

The corrosion layer in the case of nickel and chromium contains a high proportion of the metal and can be assumed from compatibility data to be a layer which is solid or, at least, semi-solid over a high 100 200 300 400 DISTANCE FROM METAL TO SLAG (p). FIG.56.X- RAY SCANNING FOR NICKEL AND VANADIUM IN NICKEL- SLAG INTERFACE. (Ni CORRODED AT 850°C IN OXYGEN FOR 4 HOURS WITH 162. proportion of its extent and which could inhibit continued accelerated corrosion of these metals. This layer is so characteristic that it would be a significant factor in controlling the corrosion by allowing low rates of diffusion through itself or its vanadation products: in the case of the easily corroding metals there is no similar hindering surface layer and corrosion is mainly controlled by diffusion through the melt. However, the presence of a compara- tively large proportion of vanadium in the corrosion layers of nickel and chromium is likely to distort the structure so that its ionic conductivity is increased and its protection against corrosion is thereby decreased in comparison with the protection afforded in the absence of vanadium.

P. ETrECT OF GRAIN-SIZE ON CORROSION Instances of integranular corrosion and consequent fracture are rare in many of the case-histories of oil-ash corrosion: in most cases, only a gerierdl " 1,8,34,104,105. surface attack and slag-thickening occurs A conclusive evidence for the distinction between the general and intergranular corrosion could be obtained by a comparison of the corrosion rates of metals of varying grain-sizes but under identical conditions (Table XLII). 163.

TABLE XLII. EFFECT OF GRAIN SIZE OAT THE CORROSION OF METALS AT 850°C WITH 0,4 CM DEPTH OF MELT

Metal Grain Diameter p02 Velocity Constant (mm) (g.cm-2 sec-1)

Molybdenum Single Crystal 0.2 10-6 , 7.9 x u 0.2222 0.2 8.0 x- 10-6 u 0.1066 u 8.0 x 10-6 u 0.0633 u 8.0 x 10-6 u 0.0417 u 8.01 x 10-6 u 0.03 11 8..3 x 10-6

\l'ickel 0.25 1.0 1.4 x 10- II 0,03 1,0 1,41 x 10-4 -4 11 0.022 1.0 1.41 x 10

Cobalt 0.065 1.0 1 12.0 x 10-6 I u 0.030 1.0 12.0 x 10-6 , 1 , Iron Single Crystal 1.0 12.26 x 10-6 1r. Bicrystal 1.0 12.3 x 10-6 , 11: Tricrystal 1.0 12.3 x 10-6 u 0.045 1.0 12.3 x 10-6 164.

It is evident that the corrosion rates under comparable conditions are not appreciably altered by the extremes of grain-sizes, and are, incidentally, extremely reproducible. Probably the corrosion rates are so rapid as to offset the effect of grain-size on corrosion under these conditions. In other words, the rate of advance of the bulk oxide is faster than the rate of intergranular attack especially where local rupture of the oxide barrier has occurred. CHAPTER V

DISCUSSION ON VANADIC CORROSION OF METALS 165. DISCUSSION ON VANADIC CORROSION OF METALS

A. INTRODUCTORY A proper understanding of the mechanism of the vanadic corrosion of metals is most essential for an effective prevention of the attack. Any attempt at explaining the mechanism must aim at: (1) an interpretation of the physical and chemical properties of the melt and their changes resulting from the introduction of of the metal oxides during the course of the corrosion, (2) a study of the metal-melt interface and its immediate neighbourhood in order to understand the local accumulation of the corrosion products, for, in final analysis, it is this that is most likely to govern the corrosion rate, and (3) an analysis of the results of the kinetic studies on the vanadic corrosion of metals, particularly with regard to (i) the significance of the reaction constants and the activation energies and (ii) the effect of oxygen pressure and depth of melt. 166.

B. PROPERTIES OF THE OXIDATION PRODUCTS Results on the reaction between metals and molten vanadium pentoxide in the absence of oxygen (Table XXXVII) indicate that all metals (except probably chromium) are attacked by the melt to a greater or less extent depending on the nature of the metal. In an oxidizing atmosphere, the reduction product, namely vanadium dioxide, is subsequently converted to the pentoxide at a fairly rapid rate (Sect. B, Ch. III), and the attack continues unless the oxidation product of the metal does not dissolve in the melt at least as rapidly as it is formed and can thus form a coherent corrosion layer through which the metal ions cannot diffuse out nor can vanadium pentoxide penetrate. An examination of thevolume ratios" ofthe metal oxides (Table XLIII) reveals that they are exceptionally high; there is always the possibility of porosity due to the development of stresses which lead to buckling and subsequent cracking. This will also aid the dissolution of the oxide in the melt by causing greater surface areas of oxide to be exposed to the fluxing agent. It is therefore inadequate to compare the vanadic corrosion rates of these metals with the volume ratios or heats of decomposition of their oxides since they interact with vanadium pentoxide 167.

TABLE XLIII VOLUME RATIO AND HEAT OF DISSOCIATION OF THE METAL OXIDES IN RELATION TO VANADIC CORROSION RATES

Activation Velocity Energy (kcal) Constant -2 Oxidn. Metal Rate (E--- em Pri- Vana- . -1.-1) nary Vol- dic in sec Oxidn ume AH dec. Corr- pure 02 Law (850°C; 0 • Pro- Ratio (kcal) ;sion 0.4ann duct -6 Fe lin= 12.3x10 FeO 1.7' 126.5 21.2 22.4

Fe203 2.14 110.2 6 Co " 12.0x10 Co0 1.86 114.2 t 27.0 20.0 -6 Ti " 8.6 x10 TiO2 1.73 176.2 24.0 24.3 77.6 1 18.3 - Mo " 9.3 x10-6 Mo03 3.3

W " 7.6 x10-6 W03 3.35 134.1 ! 28.2 44.0 -6 IT " 7.2 x10 V205 3.19 60.0 16.5 - Ni logari- 1.4 x190 4 Ni0 1.65 115.0 i 27.9 34.7 thnic (g.on- ) 168. to give an entirely different product. In fact, accumulation of the metal oxide is observed only in the case of nickel and chromium (Sect. 0, Ch. IV).

C. NATURE OF THE CORROSION PROGRESS With moderate excess of molten vanadium pentoxide, corrosion progress is generally linear with respect to time except for (a) a very fast uptake of oxygen in the very first stage (2 to 3 minutes) of changing the inert to an oxidising atmosphere, (b) an accelerated stage following the main linear process, and (c) the final subsidence of the accelerated stage: these are to be attributed to the inherent properties of the melt:

(a) The initial rapid uptake of oxygen is probably due to the oxidation of vanadium dioxide which has already been formed either by the decomposition of molten vanadium pentoxide in the inert atmosphere or by the interaction of the metal and melt; there is only negligible difference in behaviour for specimens of different grain-sizes and therefore this initial fast uptake of oxygen does not seem to be due to any extra-energy associated with the surface grains of the metal.

(b) The enhanced conductance of molten 169. vanadium pentoxide with dissolved metal oxides (Tables XVI - XIX, Ch. III) and the possible self- acceleration due to local accumulation of the liberated heat explain the accelerated stage of the vanadic corrosion of iron and cobalt, and possibly of other metals as well. Attention is drawn, in passing, to the parallelism between protective action 106 and poor conductivity of oxide skins. Also, for solid vanadium pentoxide - potassium sulphate mixtures the rate of exchange with molecular oxygen is greater for those with higher electrical conductivity107: an increased conductance, therefore, means an enhanced "lability" of oxygen and a consequent increase in its corrosive property.

(c) When a large excess of the metal oxide is present, the conductance of molten vanadium pentoxide does not change appreciably; this is why the accelerated stage of corrosion of iron, cobalt and nickel disappears with time and the rate falls to a new approximately linear relationship.

D. .:_GRAIN-SIZE AND CORROSION In the presence of molten vanadium pentoxide (not contaminated with sodium sulphate) only a general surface attack and slag-thickening have been 17 0 . observed.1, 8, 34, 104, 105 Presence of a sharp boundary between the metal and slag, and absence of any vanadium in the grain-boundaries at the metal surface, as revealed by the electron probe microanalysis of the 'metal-slag interface (Section0, Ch. IV), prove that only general attack occurs in the vanadic corrosion of pure metals. Moreover, the corrosion rates under comparable conditions are not appreciably altered by the extremes of grain sizes (Table XLII, Ch. IV). This is a conclusive evidence that only a general attack occurs in the vanadic corrosion of metals.

E. STORY OF VANADIC CORROSION Since the nature of the surface of the metal, particularly with respect to grain-size, does not have appreciable influence on the corrosion rate, it is apparent that other chemical or diffusion process is rate-controlling. The actual picture of the mechanism of vanadic corrosion, ignoring the gas/liquid surface diffusion, involves (Sect. E., Ch. I):

(a) diffusion of oxygen (or other active species) in through the bulk of pure melt, (b) diffusion of the active species in through a "diffusion layer" at the surface of the metal, 171.

(c) interaction of the active species with the metal, (d) diffusion of the corrosion product out through the "diffusion layer", and (e) diffusion of the corrosion product out through the bulk of the almost pure melt. Of these, one or several might be operating in the corrosion process. Provided the diffusion coefficients are the same (or, at least their activation energies are nearly the same) in the bulk as well as in the diffusion layer, processes (a) and (b) could be combined into a single step for the diffusion of oxygen or other active species, and processes (d) and (e) could be combined into a single step for the diffusion of corrosion products.

The chemical reaction::

Metal + V02.5---4 Metal Oxide + V02 .... (72)

is thermodynamically possible in all cases as is evident from a perusal of the free energy of the reactions (Table XLIV)‘-q ' 108

Metal + 02 Metal Oxide .... (73) and

VO (1) ---4 V02 :/-; 02 2.5 .... (74) 1 71 .

The chemical reactiont

Metal + V02.5---4 Metal Oxide + V02 .... (72)

is thermodynamically possible in all cases as is evident from a perusal of the free energy of the reactions (Table XLIV)69' 108:

Metal + 02 —*Metal Oxide .... (73) and V02.5 (1) ---4 V02 + 4 02 .... (74) 172.

TABLE XLIV CHANGES IN FREE ENERGY FOR THE OXIDATION OF METALS &IIEDUCTION OF VANADIUM PENTOXIDE

Reaction AG1125(kcal)

V02.5(1) --4V02 + *02 + 9.8 Co + -102 — 36.9 Fe + 202 — 45.3 Ni + 1.02 --Ni0 — 42.0 Mo + 1202-9 Mo03 — 94.2 W + 1202 --4W03 — 133.5 Ti + 202 -->TiO — 98.4

All these metals possess oxides of high free energy of formation, and hence the chemical reaction is not likely to be rate-controlling; it certainly is not in the case of oxidation of metals in pure gaseous oxygen. This aspect will be amplified later. F. FACTS ABOUT DIFFUSIVITY (a) From the viscosity of molten vanadium pentoxide, application of Stokes-Einstein relationship gives 4 x 10-6cm-2sec-1 for the self-diffusion coefficient at 850°C; similar values can be allotted 173. to the diffusivity of ions and molecules of the same dimensions. (b) For a purely diffusion-controlled process, the velocity of corrosion should be independent of the nature of the surface, and under ideal conditions the rates should be independent of the nature of the metal and be inversely proportional to the depth of the melt (Equation (10), Ch. I). Table XLIII shows that the corrosion rates do not vary much from metal to metal; -6 at 85000 they vary only between 7.2 x 10 for vanadium and 12.3 x 10-6 for iron. And the inverse proportionality between the depth of melt and the corrosion rate has been amply proved by the data in Tables XXVI, XXIX - XXXI, XXXIII and XXXV, Ch. IV. According to Equation (10), the product of the velocity constant and the depth of melt is a measure of diffusivity under ideal -6 conditions. For vanadium this is 3 x 10 and for iron 5 x 10-6; these are of the same order as the self-diffusion coefficient in molten vanadium pentoxide.

(c) The diffusivity has been calculated from the steady-state flux (i.e. the corrosion rate under the stipulated conditions) and the concentration gradient read from the electron probe X-ray profiles (Figs. 51, 53 and 56) with the aid of Fick's first law (Equation 174.

(11), Ch. I). The values are collected in Table XLV. Two facts emerge from the Table:

TABLE XLV. DIFFUSIVITY OF METAL IONS IN DIFFERENT LAYERS DURING VANADIC CORROSION

2 -1 Steady State Flux Diffusivity (cm sec ) Metal -2 (g(M).cm .sec-1) Bulk of Melt Corrosion Diffusio Layer Layer

Ti 2.5 x 10-5 1 x 10-7 - 2 x 10-9 Fe 4 x 10-5 5 x 10-6 - 7 x 10-7 Ni 9 x 10-7 4 x 10-7 3 x 10-9 3 x 10-9

(i) the diffusivity in the bulk of the melt is of the same order of magnitude as those above, and

(ii) the diffusivity in the "diffusion layer" is considerably smaller than that in the melt, and hence this is likely to influence the factors governing the corrosion of metals.

Scarcity of data for a variety of conditions and for other metals is a deterrent to make a categorical conclusion at this stage. However, this situation bears a striking analogy to that arising in anodic 109 polishing of metals. Pantony has shown that the diffusion coefficient of any ion in the layer adjacent 175.

to the anode undergoing polishing must be 100 - 1000 times smaller than that in the bulk of the solution. Anodic polishing is controlled by the diffusion processes in this layer, and as a result the dissolution of the anode is not intergranular. In anodic polishing the diffusion layer is quite thick and the lack of polishing in the vanadic corrosion of metals may be connected with the thinness and the ease of local rupture of the associated diffusion layer.

G. Energetics of Vanadic Corrosion 1. A better insight into the mechanism of vanadic corrosion can be gained from a comparison of thecner:y of activation for the various processes, for it is this that determines the driving force of the procecc. In general, energetic considerations will play a dominant role in determining the most effective mechanism, since the probability of the occurrence of any particular mechanism is large if the corresponding energy of activation is small. The necessary data are collected in Table XLVI.

2. In view of the high reproducibility of the results, a rational estimate of the errors in the various values of activation energies, calculated on 45" the basis of the method adopted by Pantony , will be

176.

TABLE XLVI. COMPARISON OF VARIOUS RATE PROCESSES (700° - 950°C)

1 . 1 1 !Depth Activation Roaction I Exponent of n of Energy Constant I NTgenPressurai Process t 1:)k-i 2- ;J Melt Air i High 1(cm) (kcal) (corrected Pros-1 Pros-L-(371 sure i sure , 1102'..5;.‘VO2 + .302 - - 25.6 - - , . 1 - V0 + 10, --,V2O 5 1.0 - 16.8 (0.754) 04083;10483; 2 4 2. 04098;10491; . b.102;10.91 II 0,1. - 16,3 - - I - 11 0,05 - , 16.3 , - - - II 0,01 - 3.4 to 90 --i - , ii Conductance 1,0 - 17.2t925.9 - b.24 10.59 Viscous flow 1,0 - 10,1 - - - Diffusional 1.0 - 14.7 - - - Corrosion . , Corrosion of Co 1,0 0,4 27.0 1.150 D.083 10.71 it 0,2 0,3 26.5 1.189 ,_ ,1i _ Corrosion of Fe 1.0 0,4 21.2 0.0694 0.11 0.91 II Q09869 0,4 7.4 267x10-6 - ,- Corrosion of Ti 1,0 0,4 24.0 0.143 p.091 :0.59 Corrosion of W i 1,0 0,4 28.2 Q.796 b,o77 0.83 Corrosion of Mo!l 1,0 0,4 18.3 0.01276 0.1 0.77 II 1 0,2 0,4 18.4 0.01701 I ,- .- Corrosion of VI 1,0 0,4 16,5 0.00456 b.12 10.71 ti. A.0 0,8 16.5 Q.00458 - i - Tr. i Q09869 0,4 8,9 4.81x1Cr6 ,- f ,- Corrosion of Nil 0.2 0.4 27.9 (2971) p.15 1.67 i 177.

between ± 0.5 and ± 1,0 kcal for corrosion studies and may rise to ± 2.0 kcal for diffusion, conductance and viscosity studies: most of those errors arise from the use of logarithms in the calculations. Even by taking the maximum errors, the values (i) are so divergent, (ii) vary so much from metal to metal, and (iii) are not so low, that it would be an obvious temptation to consider vanadic corrosion as a chemically controlled process.

3. However, in view of the facts that:

(a)these metals undergo oxidation with a considerable decrease in free energy which imply the thermodynamic ease of the reaction (Table XLIV, Section E);

(b)the energy of activation for vanadic corrosion is very close to that for corrosion in goseous. oxygen which has been well-established to be diffusion-controlled (Table XLIII, Section B);

(c)the energy of activation for vanadic corrosion is considerably reduced at very low oxygen pressures (Table XLVI);

(d)the reaction constants are so discordant (Table XLVI) that the entropies of reaction, 178.

which should have approximately the same value for all corresponding reactions, vary far too much to be attributed to the same type of reaction; in addition, the reaction constants become extremely low at very low oxygen pressures; and (e)metals of different valency states and chemical behaviour obey the same rate law for oxidation, one can certainly dispense with the view that vanadic corrosion is a chemically controlled process.

4. On the other hand, the following observations lead to the conclusion that vanadic corrosion is definitely a diffusion-controlled process:

(a) Different metals obey the same rate law deduced for a diffusion-controlled process (Ch. I). (b)Corrosion rates do not vary very much from metal to metal (Table XLIII).

(c)Corrosion is independent of the nature of the surface, particularly with respect to grain size (Section D, this Chapter).

(d)An inverse proportionality is observed between the depth of melt and the corrosion 179.

rates (Ch. IV).

(e)The qualitative illustrations of the shape of

corroded samples -re in conformity with a diffusion-controlled process (Sect. N, Ch. IV & Fig. 49).

(f)Diffusivity calculated from different methods comes to be of the same order of magnitude (Sect. F, this Chapter).

(g)The reaction constants and energies of activation for corrosion at very low oxygen pressures are small (Table XLVI) 110 And, there are reports where the activation energies for self-diffusion have been found to be as high as those observed for vanadic corrosion, so that the conclusion that vanadic corrosion is a diffusion- controlled process is a sound and satisfactory one. Nevertheless, the divergence in the values of the reaction constants and activation energies is so great that it is necessary to assume the existence of different diffusion processes in different systems. As discussed in Section E, the progress of corrosion is governed by two diffusion processes:

(a) the diffusion of oxygen (or other active species) inward towards the metal, and 180.

(b) the diffusion of the corrosion products away from the metal.

A detailed examination of this fact follows now.

5. In the case of the vanadic corrosion of vanadium metal, the species diffusing away from the surface of the metal can only be vanadium (V) or vanadium (IV) ions. These are already the constituents of the melt and therefore do not in effect introduce any additional contributions to the corrosion mechanism which can hence be assumed to be determined only by the inward diffusion of oxygen (or a corresponding reactive species). This view is further supported by the close similarity between the values of the energy- ofactivation for the corrosion of vanadium, the oxidation of vanadium dioxide and the diffusion processes in corrosion with much vanadium pentoxide (16.5 ± 0.5, 16.8 4-0.5 and 14.7 ± 2.0 kcal respectively), all being determined by the amount of oxygen uptake. In passing, it may be observed that the effect of vanadium dioxide near the metal surface is common to all processes.

6. However, for the vanadic corrosion of metals other than vanadium, the inward diffusion of oxygen through the bulk of the melt and the diffusion layer is followed by a sequential outward diffusion of the 181. corrosion products, the metal ions. In effect, two diffusion processes are operating sequentially, but in opposite directions, and therefore one process has a negative flux with respect to the positive flux of the other: (dcN = 0 at the interface .... (75) 'D1 ` dx 11 D2 (g.i)2 so that do D1 (4i)l - D2 (1)24 11000 (76) 7. Each of these processes has its own characteristic rate constants, say kl and ka respectively. The experimental value of the rate constant (k) can be equated to these two separate processes by means of reciprocal additions in analogy. 111 to mass-transfer processes

.... (77) which becomes:

1 .... (78) A -E 1/ e RT A2.eE2/RT

Here Al' E1 and A2, E2 represent the respective reaction constants and energies of activation. Equation (78) can be written as: A .e El/RT E2/RT 1 1 — 2*e .... (79) AlA2.e -tE14-E2)/RT so that: 182.

El+E2 El/RT E2/RT) in k = In A1 A 2 --±--- RT - in (A16e A2..e- (80) This equation can be analysed for:

(a)the reaction constants, and

(b)the activation energies.

(a) Reaction Constants According to Equation (80), a plot of log k against reciprocal temperature gives an intercept of Al log r--;=T- when T1 = 0. This should be equal to the 1 '2 logarithm of the reaction constants (Ak) given in Table XLVI, i.e., log Ak log Al .A2 Al +- A2 Or

Ak Al A2 44100 (81) A1 +•- A2 For the vanadic corrosion of the different metals, there is a common process of the diffusion of oxygen (or a corresponding reactive species) towards the metal; this is characterised by a reaction constant, Al and an energy of activation, E1. The sequential process of the outward diffusion of the product of oxidation, namely the metal ions, has A2 and E2 as the reaction constant and theenergy of activation respectively. For the corrosion of vanadium in the presence of 183.

molten vanadium pentoxide, the product of oxidation has the same composition and its outward diffusion is therefore evidently absent cr has no effect on the corrosion mechanism or is common to all such processes.

Hence the value of Ak for the vanadic corrosion of vanadium metal could be taken to be equal to Al, which is the reaction constant characteristic of the inward diffusion of oxygen: this is further assumed to be common to the vanadic corrosion of all metals.

Hence, an evaluation of A2 (the reaction constant characteristic of the outward diffusion of the product of oxidation) could be carried out with the aid of Equation (81) from the known experimental values of

Ak (Table XLVI). The calculated values of these reaction constants are collected in Table XLVII.

TABLE XLVII. REACTION CONSTANTS CHARACTERISTIC OF THE OUTWARD DIFFUSION OF THE PRODUCTS OF OXIDATION DURING VANADIC CORROSION OF METALS

Reaction Reaction Constant for Reaction Constant for Metal Inward Diffusion of .Outw6rd Diffusion of Constant Oxygen Oxidation Products (Ak) (Al) ) (A2)

V 0.00457 0.00457 7 Co 1.150 n - 0.00458 Fe 0.0694 n - 0.00488 Ti 0,143 It - 0.00471 W 0.796 n - 0.00459 Ile 0.01276 n - 0.0071 184.

The negative sign of the reaction constants is as expected for a process with a negative flux with respect to the positive flux for the forward diffusion of oxygen (see para 6, Equation (76), above). This confirms that vanadic corrosion is a two-stage diffusion controlled process, namely, an inward diffusion of oxygen (or other corresponding reactive species) towards the surface of the metal and an outward diffusion of the corrosion products away from the surface. For either process, the entropy should be small and should not differ much from each other since. there are no appreciable divergencies of changes in disorder. The values of the individual reaction constants calculated by the method discussed above are in remarkable conformity with this view (Table XLVII). The highest and only abnormal value of the reaction constant is for molybdenum. But this is to be expected because the oxidation product, namely, molybdic oxide, has an appreciable vapour pressure in the temperature range studied, and, inter 'alia, increases the effective diffusion owing to vapour formation.

According to Equation (8), Ch. I, the velocity constant is equal to A D Co For unit area, unit rl concentration (Co.= 1 at p02. = 1) and unit diffusional resistance ( =unit depth of the melt), this gives the 185.

value of D. The variation of k with Temperature therefore expresses the variation in D only. 13, 112 ryring has derived the following expression for the diffusion rate in solids with defect-structure:

2 kbT AS/RT - AE/RT 1 • D = p•di . .... (82) where d1 = the interatomic distance, kb and h are Boltzmann's and Planck's constants, and the factor p takes into account the type and contribution of the defect structure. The pre-exponential reaction

constant term Al or A2 thus entails an entropy of diffusion, AS. However, in view of the difficulties in assuming any value for p and di for molten vanadium pentoxide or for the corrosion products, an analysis of the two reaction constants (Table XLVII) for the entropy of diffusion is impossible at this juncture.

(b) Energy of Activation Temperature variation of the rate constant is given by the differentiation, by the "chain-rule" method, of Equation (80) with respect to reciprocal temperature: d_....,___- ln .k -E / -E d (...)T 1 RT RT . A1 A2 83) which, on rearranging and collecting into one term, becomes: 186.

(2 E1. .11 A-2. e (El-E2)/RT d la k (84-) d (T) A ( 1-E2)/RT 1 e

(i) When El2, e (El - E2/RT = 1, and Equation (84) reduces to: E - E d ln k -7-1 or --N-2 (85) d (T)

(ii) When E1 >E29 and E2 is small, Equation (84) gives"

a ln k -El =.... (86)

(iii)When E2>E1 and 1 small, Equation (84) can be written as:

d In__T_ k - E2 d (T) **so (87)

Equation (84) is of such a form that even when E2 is only sMightly smaller than E1 (or E1 slightly smaller than E2), Equation (86) or (87) still holds.

Thus it becomes evident that when two processes occur sequentially, the energy of activation calculated from the plot of the logarithm of rate constant against reciprocal temperature corresponds either to the process having the higher energy of activation or to either of the processes when they have equal activation energy. Hence the measured value is of one process only, and that is of diffusion in the presence of 187. corrosion products in the diffusion (or corrosion) layer since all measured values are greater than that of corrosion of vanadium metal. It should also be clear that it is unnecessary, indeed it is misleading, to invoke chemical reaction as one of the consecutive processes contributing to values of Al or A2. The change of thickness of the diffusion layer with temperature is also unimportant, at least for the easily corroded metals.

H. Possible Mechanisms of Diffusion Exchange of oxygen between neighbouring vanadium dioxide and vanadium pentoxide groups, possibly accom— panied by the rotation of=these, is a possible diffusion mechanism. The exchange of oxygen atoms between the holes in the vanadate lattice can also be postulated. 79 However, the ionic radii of 0=- = 1.40A, of V5* .59A. and of V4* = 0.62A. and the molecular radii (assuming that they are spherical) of V02 and V205 in the molten state are 2.46. and 3.201 respectively. Hence, the small vanadium ions embedded in the oxygen anion lattice are likely to diffuse easily in the melt through the vacant space present. In addition, it is possible to assume the migration of vanadium pentoxide molecules. Vanadium pentoxide is a polar molecule as a result of 12 asymmetry , and as such has strong tendency to be 188. adsorbed on transition metal surfaces.. Such adsorption and consequent interaction would tend to form a gradient in chemical potential which cause the migration of vanadium pentoxide molecules to the interface; low viscosity and volatility of this oxide give the necessary mobility.

I. Effect of Oxygen Pressure on Vanadic Corrosion (1). The semiconductivity and the ease of electron transport in molten vanadium pentoxide is another important aspect of vanadic corrosion. The variation of corrosion rate with oxygen pressure can in all cases be represented by a two-power expression similar to Equation (40):

n n m k= Bi . pw2 -1T ,2 p02 , feet (40) where B1 Ale -El/RT and B 2 = A2e -E2/RT; these represent two separate mechanisms operating individually at two oxygen pressure regions. Quantitative treatment of this expression is difficult. Substitution for Bl and B2, n and m with experimental data shows that there is a delicate balance between the two mechanisms, as is shown below for the vanadic corrosion of iron (Table XXIX) at 850°C with 0.4 cm depth of melt: . , _5 . -5 A1 =0.1735x0.4=0.07 A2 7..x10 x0.4=2.84 x 10 n = 0.1 m = 0.9 El4 /1-9.5 ' -5 -EAT? -3.3 . -2 El =21,200 ; e =e =7.5x10 E2 =7400 ; e '"=e =3.69x10 189.

Substitution in Equation (401: k = 0.07x1x7.5x10-5 -1-2.84x10-5x1x3.69x10-2 at P02=1 = 5.25 x 10-6 + 1.05 x 10-6 k = 5.25x10-6x(0.01)c4 + 1.05x10-6x(0.01)°*9 at DO =0• 01 - 2 = 3.3 x 10-6 +-2.10 x 10-8

(b) Reciprocal Addition:

(i) = 1 4, 2 7 5.2 x 10-6 1.05 x 10-; at p0

(ii) = 1 1 = 0.01 3.3 x 10- 2.10 x 10-8 at p Clearly, slight errors in the values of the coefficients and exponents can have a marked effect on the values of the individual terms in the expression (40); so much so these could decide which of the terms is rate- controlling and which exponent of oxygen pressure is applicable at a given oxygen pressure. At present, it might be more realistic to separate the terms and state that at high oxygen pressures, k = Bl pO2n, and at low oxygen pressures, k = B2.p02m The effect of the corrosion products in the melt also causes failure of a rigorous treatment.

(2) For the high oxygen pressure region, the value of the index of power varies between 0.08 and 0.15 and for the low pressure region it lies between 190.

0.59 to 1.67 for the several metals. A rigid treatment of the significance of the individual values is impossible because of the alterations in the properties of the melt brought in by the corrosion products. However, all of them stem from the semiconductivity of molten vanadium pentoxide. Corrosion of metals is controlled by the diffusion of vanadium (V) ions, and oxygen anions towards the metal or by the diffusion of vanadium (IV) ions and anion holes away towards the surface of the melt. Decrease of oxygen pressure will decrease the vanadium (V) ions and oxygen anions at the surface and will thus decrease their flux towards the metal. Consequently, the rate of corrosion will decrease with decrease of oxygen pressure. Quantitatively, one oxygen molecule would react with four vanadium (V) and four electron vacancies and would fill two of the anion holes; hence:

02 .1; 4 In V5+' A, 4 + 2s 0 0= 4000 (88) and therefore, on mass action considerations: 4 2 p02 const. {0 V51 [cd 4 x rici (89) and considering the relation between the concentrations of electron and ion: vacancies, [ aV and in the absence of any other disorder equilibria, it becomes evident that: 191.

, 1 EA] const. p02,10 i.e. const. pOz.0 .1 .... (90) The corrosion rate, proportional to the concentration of electron vacancies, should thus vary with the 10th root of the oxygen pressure. Depending on the structure of the metal oxide-vanadium pentoxide system, contributions from the ion vacancies may be clbsent or present. In spite of the existence of several other disorder equilibria in the metal oxide-vanadium pentoxide systems, the variation of the experimental value of the exponent, viz. from 0.08 to 0.15, as against the expected value of 0.1, is not too significant to repudiate the validity of the assumptions. The variation is merely a question of degree in that 2- the proportion of vanadium (V) or 0 holes (or electron vacancies) changes with the addition of extraneous cations from the corrosion products. This effect has already been demonstrated in the conductivity work.

(3) However, these assumptions are valid only if the melt possesses the structure of the solid vanadium pentoxide, even though of a short-range order. This will be the case when the oxygen pressure is not too low. At very low oxygen pressures, the melt will contain a largo excess of vanadium dioxide and the 192. anion vacancies will determine the rate of diffusion of oxygen through the melt; one oxygen molecule is equivalent to two anion vacancies: 1 7 02 = DO= .... (91) so that 1

ra const. p027 .... (92) It can also be that at low oxygen pressures, the solubility of oxygen and hence its activity is governed by Sievert's law: n 7 '2 0 (93) so that 1 7 f..01 = const. p02 0000 (94) Consequently, when the dissolved gas and hence the activated state for diffusion is in the form of atoms (Sievert's law) or when the anion vacancies determine the rate of diffusion, the corrosion rate should be proportional to the square root of the oxygen pressure. However, the activated state for diffusion can as well be the molecule or two atoms associated together, and in this case the corrosion rate should vary directly with the oxygen pressure. Lack of obedience to Sievert's law, the complex nature of molten vanadic slags, and the experimental difficulties at these low oxygen pressures make a full assessment of the results impossible. However, the values of the exponents 193.

(Table XLVI) indicate that a combination of the two process (of atom and molecular diffusion) operates to varying degrees in the system.. In this connection, it is highly significant to observe that iron and vanadium have almost similar values for both the activation energy and the reaction constant for the corrosion at low oxygen pressures (Table XLVI); this indicates that a single fundamental mechanism operates under these conditions.

J. Protctive Measures Against Vanadic Corrosion (1) One of the most attractive ways to protect a metal from oxidation in normal circumstances is by making use of its stable oxides as barriers. As is well-known, ouch oxidation is mainly a diffusion process- continued oxidation proceeds by transport of ions and electrons through the oxide layer. Failure of these to leave the metal and enter the oxide lattice, either interstitially or via vacant lattice positions, leads to formation of a protective film. Protection by oxide films is usually specific for one atmosphere or one temperature. Thermodynamically, the alloy underneath such an impervious skin certainly has the tendency to oxidize, but the kinetics of oxidation are considerably affected by the obstruction to diffusion. The interfacial energy of two adjacent 194. grains of the crystalline phase of the oxide skin is usually so great that the presence at the grain boundaries of molten vanadium oentoxide which easily adapts itself to the local atomic structure leads to a reduction of the total interfacial energy. In simple- chemical terms, the interaction of these oxides with molten vanadium pentoxide reduces the chances of protection considerably unless (i) a completely new kind of high-melting oxide or slag can be formed on the surface, such as nickel vanadate, or (ii) the oxidising capacity and the transport properties of the bulk of melt are reduced considerably by effective additions.

(2) Normally, minor additions of certain metal oxides to the melt (Fig. 48) enhances its corrosive property. This is because vanadium pentoxide is a transition semiconductor and addition of ions of lower or higher valency increases its conductance, i.e. diffusivity. The reduction in the corrosion of iron by the addition of nickel oxide to the melt in moderately large quantities could be due to the. change in the transport properties of the melt and also to interaction between the oxide and iron. However, no layer of nickel is found on an iron specimen corroded in a vanadium pentoxide—nickel oxide melt. It seems 195. that a nickel oxide (or vanadate) barrier is not formed at the metal surface under these conditions and the effect of nickel oxide can only be ascribed to its raising the melting point of the melt and reducing its bulk diffusivity.

(3) Cathodic protection cannot be highly effective again because 3f the semiconducting property of the vanadium pentoxide. However, the improvement observed in this respect (Table XLI, Ch.IV) could be incorporated with other methods to give an effective overall protection.

Now that the results of this broad-based work have put the nature, the causes and the mechanism of vanadic corrosion on a sounder theoretical footing and have offered an explanation for failures to prevent the corrosion, it is possible to suggest that any further attempts towards the prevention of vanadic corrosion should be directed along the following avenues:

(a) a choice of chromium-nickel alloys with some selected metals which will be resistant to this type of corrosive environment owing to the resistance offerred by the resulting diffusion or corrosion layer, and not to the resistant properties of the alloy itself; 196.

(b)impoverishment of the oxidising capacity of vanadium pentoxide by reducing it to lower oxides;

(c)conversion of vanadium pentoxide to stable and high-melting compounds so that the bulk- diffusivity is reduced; and

(d)reduction in the solubility of oxygen and the oxygen-transport properties of the melt by suitable additions to fill in the lattice vacancies. 197.

SUMMARY AND CONCLUSION

1. The nature and the extent of the corrosion of metals in the presence of molten vanadium pentoxide have been studied in relation to (a) the temperature, (b) the amount and depth of the melt, and (c) the composition of the ambient atmosphere. It is found that the physico-chemical properties of the metal-melt interface influence the general corrosion rate in presence of small amounts of vanadium pentoxide, particularly in the case of nickel and chromium. On the other hand, the physical properties of the melt, when present in moderately large quantities, control the corrosion of cobalt, iron, titanium, tungsten, molybdenum and vanadium. For these metals in the presence of sttfficent excess of molten vanadium pentoxide, the corrosion process has been found to be entirely diffusion-controlled, and hence much attention has been paid to the study of the physical and chemical characteristics of the melt, particularly those with a bearing on its transport properties and its interaction with other oxides.

2. All mixtures of sodium sulphate and sodium chloride with vanadium pentoxide decompose at as low a temperature as 5000 C and evolve sulphur trioxide and

198.

chlorine respectively. These gases are themselves powerful corrosive agents at such low temperatures whereas vanadic corrosion occurs significantly only when there is a molten phase (678°C with pure vanadium pentoxide). It is also observed that molten sodium oxide-vanadium pentoxide (up to 25 mole per cent sodium oxide) mixtures have stronger corrosive activity than pure vanadium pentoxide. These melts evolve oxygen suddenly on solidification and reabsorb it rapidly on melting; their corrosive activity was correlated vAthis oxygen absorption capacity, and therefore it is to be assumed that part of the oxygen content in vanadic melts is highly labile and reactive.

3. The concentration of labile oxygen in vanadic melts is partly governed by the equilibrium:

VO 2.5T -- VO2 + 4 02 (95) This has been studied to a large extent by the loss in weight when the melt is in equilibrium with a gas- mixture of definite oxygen partial pressure. The heat of the reaction (25.6 kcal) has been found comparable to the activation energies (16.5 - 28.2 kcal) of the vanadic corrosion of some of the metals.

4. The freezing point of vanadium pentoxide is found to depend on the ambient oxygen pressure and the 199. consequent amount of vanadium dioxide in the melt. Discrepancies from an expected relationship have been qualitatively correlated with increased ionic character of the melt following introduction of more vanadium dioxide.

5. Since ionic and electronic diffusion is involved in many of the corrosion processes, the type of lattice defects in the structure of molten vanadium pentoxide has been deduced from its electrical conductivity and its variation with temperature, oxygen pressure and added metal oxides. The activation energy of conductance is compared to those of other rate processes. Increased nonstoichiometry with decrease of oxygen pressure results in an increase in the conductance. There is a distinct break in the plot of conductance against oxygen partial pressure showing that different mechanisms operate in the two regions. The exponent of oxygen pressure for the high pressure region is — 0.24, and that for the low pressure region is — 0.58. The significance of these values has been explained on the basis of Wagner's theory of semiconductors. Increase of conductance on the addition of oxides of sodium, cobalt, nickel and iron finds an interpretation in 200. terms of the interstitial occupation of the added cations and the shift of the equilibrium represented by Equation (95) towards more of nonstoichiometry; there is an increase of ionic conductance as well.

6. The oxidation of vanadium dioxide at constant oxygen pressure,

V02 + 0 V0 (96) 4 ' 2 2.5 is found to be kinetically of the first order with respect to the concentration of the dioxide in the melt. Interaction between a metal and vanadium pentoxide is likely to reduce the latter and its replenishment is controlled by the kinetics of the reaction (96). The energy of activation for this reaction is in fact very close to those for the vanadic corrosion of molybdenum and notably, vanadium. The variation of the velocity constant with the oxygen pressure can be represented by a two-power expression:

k = B1 p02 n +pB2 02 m (97)

One term operates at the high oxygen pressure region and the other at low pressures. The values of the exponents of oxygen presence (n = 0.083 to 0.102; m = 0.83 to 0.91) have bearing on the semiconductance of vanadium pentoxide. 201.

7. Molten vanadium pentoxide has the relatively low viscosities of 13 to 6 centipoises between 700° and 900°C. The energy of activation for viscous flow is 10.1 kcal. Because of such low viscosities, molten vanadium pentoxide can easily penetrate through porous metal oxide layers and cause catastrophic corrosion at the metal surface. The self-diffusion coefficient -6 (4 x 10 cm2sec-1 ) calculated from the viscosity is of the same order as those obtained from velocity constants of vanadic corrosion under stipulated conditions.

8. (i) By the application of the Fick-Nernst treatment of diffusion-controlled processes adapted to a model selected for the present study, an equation has been derived for the corrosion of the metals in terms of the change in weight:

Aw = A Do(C. Cx) t (98) r1 + r2 where A is the area exposed, D is the diffusion coefficient (assumed in this equation to be the same for the different reacting species and for the different layers), Co and Cx are the concentrations of the reacting species at the surfaces of the melt and metal respectively, r1 and r2 are the diffusional resistances of melt column and the diffusion layer 202. respectively and Aw is the change in weight in an interval t. If r2, the resistance of the diffusion layer is negligible in comparison with r1 (which is proportional to the depth, x, of the melt) and if all the active species disappear at the surface of the metal (i.e. 0x = 0), then: Aw = A D000 t (99) x

For constant oxygen pressure (Co = const.), this reduces to a linear rate law:

Aw = kt (100) where k is the velocity constant.

(ii) If, on the other hand, the diffusion coefficient of the active species through the diffusion layer (or an additional corrosion layer) is different from that for the diffusion through the melt, and assuming it to remain constant over the diffusion layer, it is shown that:

v.x = DmC1 + Do(Co — Ci) (101) where v = velocity of corrosion, C1 = concentration of the active species at the surface of the diffusion layer and Dm is the diffusivity in this layer.

When C1 ce Co , the corrosion rate is found to be 203. controlled by the diffusivity (Dm) through the corrosion layer (diffusion layer), and assuming a linear growth of the corrosion layer with time, it has been possible to deduce a logarithmic rate low for such a corrosion process.

(iii) The simple linear rate law has been found to be applicable for the vanadic corrosion of cobalt, iron, titanium, molybdenum, tungsten and vanadium in presence of a moderate excess of molten vanadium pentoxide. It is thus clear that one of the main rate-controlling steps is the rate at which oxygen diffuses through the molten slag to the metal surface.

(iv) For nickel (and chromium) the resistance of the corrosion layer (i) is comparable with the resistance of the melt (r1) and one diffusional process is superimposed on another which in practice, results in a logarithmic rate law for nickel for the initial oxidation process.

9. The velocity constant is found to obey an Arrhenius relation. As is to be expected from Equation (99), the velocity constant is found to be inversely proportional to the depth of melt for most 204. metal corrosions. However, the variation of the velocity constant with oxygen pressure which is a measure of the driving force (Co ) is conditioned by the semiconductance of vanadium pentoxide. The variation is represented by a two—power expression similar to Equation (97). The index of power of the oxygen pressure varies slightly from metal to metal; but its significance has been explained in terms of the semiconductance of vanadium pentoxide. Combining all the observations, the velocity constant of the vanadic corrosion of metals can be written as:

k = Ak. 1 . p02n A E) (102) x AT where x is the depth of melt and Ak contains terms for the diffusion coefficient and the entropy function.

10. Comparison of the activation energies of the various rate processes shows that there is a single mechanism underlying all the corrosion processes (probably the inward diffusion of oxygen) and this is being conditioned by another mechanism characteristic of the particular metal, probably the outward diffusion of the corrosion products. In analogy to mass transfer processes, the rate constants of these two sequential processes has been combined by the method of reciprocal addition. It became obvious that the 205. energy of activation corresponds to the process having the higher value or either of the processes when their activation energies are equal. On the other hand, the reaction constants characteristic of the two processes have been separated from each other; and as is to be expected, since they are based on similar diffusion processes, found to be approximately equal within each group.

11. An electron probe X-ray microanalysis of the metal-melt interface revealed the existence of a coherent corrosion layer separating the melt from metals like nickel and chromium; this contains a high proportion of the respective metal and is a protective barrier against accelerated attack. With iron, titanium and molybdenum, the corrosion product is distributed in the melt except for an extremely thin diffusion layer, and no protective barrier is observed. However, the diffusion coefficient of the metal ions calculated from the steady-state flux and the concentration gradient is considerably smaller in the diffusion layer than in the bulk. The resistance of the diffusion layer is therefore not insignificant. This supports the postulation of two sequential processes controlling the mechanism of corrosion. 206.

12. No evidence of intergranular attack is obtained from the electron probe X-ray microanalysis of the metal-melt interface. Further work on the corrosion of iron, cobalt, molybdenum and nickel of varying grain sizes has shown that the nature of the surface of the metal, particularly with regard to grain size, has no appreciable bearing on the corrosion rate. This effect is explained in terms of the diffusivity relationships in the system.

13. Going further into detail of the mechanism of diffusion it is suggested that the semiconductivity associated with other transport properties of the melt plays a major role in its corrosive properties. This is evident from the closeness of the values of the exponent in the relationship of the diverse rate constants with the oxygen pressure. In every case, the rates are influenced by the added oxides, the ionic contribution to the conductance and the dissociation equilibrium of vanadium pentoxide.

14. Attempts have been made to reduce the corrosive activity of vanadium pentoxide by changing the transport properties of the molten phase with added metal oxides. Smaller additions always enhances the transport properties, but larger 207.

additions, particularly nickel oxide, do reduce the corrosion rate of iron. Dilution of the melt, reduction in its corrosive activity, interaction of the metal oxide with iron and the alteration in the melting point and transport properties of the melt - all these, to a greater or less extent, are operative in reducing the corrosion rate. Cathodic protection, though ineffective by itself because of the semiconductivity and the unusual oxidising and fluxing properties of the zielt, could be incorporated to other methods for a successful prevention of the corrosion.

15. In summary, therefore, vanadic corrosion of pure metals, and presumably alloys, has been demonstrated to be diffusion-controlled irrespective of the corroded metal. Surface, grain-size and other intrinsic properties of the metal are irrelevant to the mechanism. The ease of diffusion is related to the non-stoichiometry of vanadium pentoxide, and because it has n-type transition seniconductance the diffusivity of the melt cannot be reduced unless its melting point is sharply raised by the corrosion products or additives. Metals which show some resistance to the attack act by forming an adherent 208. and coherent layer of very low diffusivity: it appears that this barrier cannot be generated by external means. 209. BIBLIOGRAPHY

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38. Kubaschewski, 0., & Hopkins, B. E., "Oxidation of Meta14sand Alloys", Sec. Ed., London: Butterworths, 1962, p. 15 39. Tomlinson, J. W., "The Physical Chemistry of Melts" London: Institution of Mining and Metallurgy, 1953, 22 - 34 40. Manakov, A. I., Esin, O. A., & Lepinskikh, B. M., Russian J. Phys. Chem., 1962, 36, 1481 - 85; see also: Russian J. Inorg. Chem., 1962, 7, 1149 - 1152 41. Nernst, W., Z. Phys. Chem., 1904, 47, 52 42. Gardner, G. S., Corrosion, 1963, 19, 81t - 90t

43. Jost, W., "Diffusion in Solids, Liquids and Gases", New York: Academic Press Inc., 1952, p8 44. Mellor, J. W., "A Comprehensive Treatise on Inorganic and Theoretical Chemistry", London: Longmans, Green & Co., 1934, Vol. XIII, p. 774 45. Pantony, D. A., & Siddiqi, A., Talanta, 1962, 9, 811 - 821 46. Weissbart, J., & Ruka, R., Rev. Sci. Instr., 1961, 32, 593 47. Steele, B. C. H. (Imperial College, London), private communication 48. Markin, T. L., & Bones, R. J., A.E.R.E. (Harwell) R.4042, 1962, part 1, 7 213.

49. Hodgman, C. D., Ed., "Handbook of Physics and Chemistry", 32nd Ed., Ohio: Chemical Rubber Publishing Co., 1950, p. 1933 50. "Metals Handbook", 1948 Ed., American Society for Metals, 401 - 404

51. Blair, A. J., Pantony, D. A., & Minkoff, G. J., J. Inorg. Nuclear Chem., 1958, 5, 316 - 331 52. Bloom, H., Harrap, B. S., & Heymann, E., Proc. Royal Soc. (London) 1948, A194, 237 53. MacKenzie, J. D., Rev. Sci. Instr., 1956, 27, 297; see also: Trans. Faraday Soc., 1956, 52, 1564 - 1568 54. Ref. 49, p. 1841 55. Rait, J. R., Trans. Brit. Ceram. Soc., 1940, 40, 157 - 204 56. Bockris, J. O'm., Kitchener, J. A., Ignatowicz, S., & Tomlinson, J. W., Disc. Faraday Soc., 1948, No. 4, 265 - 281

57. Drillant, J., & Mahissye, J. T., Research & Development, 1963, No. 18, 26 - 27 58. Yaffe, I. & Van Artsdalen, E. R., J. Phys. Chem., 1956, 60, 1125 - 1131 59. Glasstone, S., "An Introduction to Electrochemistry", First Ed., third printing, New York: Van Nostrand Co., Inc., 1947, p. 137 214.

60. Pantony, D. A., "A Chemist's Introduction to Statistics, Theory of Error and Design of Experiment", Lecture Series, 1961, No. 2, London: The Royal Institute of Chemistry. 61. McAdams, W, H., "Heat Transmission", London: McGraw-Hill Book Co., p. 181 62. Alcock, C. B., & Richardson, F. D., Acta Metallur- gica, 1958, 6, 385 - 395; see also: "Physico- chemical Measurements at High Temperatures", Bockris et. al. Ed., London: Butterworths Sci. Publn., 1959, p. 135 63. Polyakov, A. Yu., Zhur..Fizt, Khim., 1946, 20, 1021 64. Mathews, D., Ph.D. Thesis, May 1956, University of Birmingham 65. Milan, E. F., J. Phys. Chem., 1929, 22, 498 66. Temkin, Acta Phys. Chico. URSS., 1945, 25, 411; See also, Ref. 79, p. 281 67. Flood, H., Forland, T., & Grjothein, "The Physical Chemistry of Melts", London: Inst. Mining & Metallurgy, 1953, p. 46 68. Moore, W. J., "Physical Chemistry", 3rd Ed., London: Long/Dans, Green & Co. Ltd., 1959, p. 129 69. Kubaschewski, 0., & Evans, E. Ll., "Metallurgical Thermochemistry", 3rdEd. London: Pergamon Press, 1958 215.

70. Ref. 44, Vol. IX, Ch. LIV, p. 740 71. Canner!, G., Gazetta Chim. Italiana, 1928, 58, 6-25 72. Ref. 68, p. 543 73. Ref. 13, p. 295; se also: Maron, S. H., & Prutton, C. F., "Principles of Physical Chemistry" New York: The Macmillan Company, 1958, p. 629 - 630 74. Bockris, J. 01 m., Ed., "Modern Aspects of Electro- chemistry", No. 2, London: Butterworths Sci. Publn., 1959, p. 168 75. Van Arkel, A. E., Flood, E. A., & Bright, N. F. H., Canad. J. Chem., 1953, 31, 1009 - 1019 76. Zyazev, V. L., & Esin, 0. A., Zhur. Neorg, Khim., 1958, 3, 1381 - 1385; see also: ibid, 1958, 3, 2143 - 2149 77. King, B. W., & Suber, L. L., J. Amer. Ceram. Soc., 1955, 38, 306 - 311 -78. Baynton, P. L., Rawson, H., & Stanworth, J. E., J. Electrochem. Soc. 1957, 104, 237 - 240 79. Darken, L. S., & Gurry, R. W., "Physical Chemistry of Metals", New York: McGraw-Hill Book Co., 1953, p. 84 80. Delimarskii, K. IU., & Markov, B. F., "Electro- chemistry of Fused Salts", Washington: The Sigma Press, Publishers, Ch. 1. 216.

81. Kortum, G., & Bockris, J. O'm., "Textbook of Electrochemistry", Vol. I, London: Elsevier Publishing Company, 1951, p. 40 82. Wagner, C., "Atom Movements", Ohio: American Society for Metals, 1951, 153 - 173

83. Swann, R. A., "Thermodynamics of Solids", New York (London): John Wiley & Sons, Inc., 1962, Ch. 15. 84. Volzhenskii, D. S., Pashkovskii, M. V., & Svekolkina, L. G., Zhur. Neorg. Khim., 1963, 8.4., 255 - 257; Chem. Abs., 1963, 58, 9931 85. Rogers, D. B., Arnott, R. Wood, A., & Goodenough, J. B., J. Phys. Chem. Solids, 1963, 347 - 360 86. Ozerov, R. P., (& Kildisheva, E. V.), Russian J. Inorg. Chem., 1959, 4, 476; Zhur. Neorg. Khim., 1959, 4, 1047 - 54 87. Cirilli, V., Burdese, A., & Brisi, C., Atti della Accademia delle Scienze di Torino, 1960 - 61, 95, 1 - 32

88. Brisi, C., Annali di Chimica, 1957, 47, 806 - 816 89. Preece, A., & Lucas, G., University of Durham, Dept. of Metallurgy, Report, Feb. 1954 90. Burdese, A., Annali di Chimica, 1957, 47, 797 - 805 91. Ref. 13, Ch. 9 217.

92. Kolthoff, I. M., & Lingane, J. J., "Polangraphy", Vol. I, 2nd ha--. New York: Interscience Publishers, Inc., 1955, P. 56 93. McFarlane, J. J., & Whitcher, F. S. E., N.G.T.E. Report No. R. 193, July 1956 94. McFarlane, J. J., & Stephenson, N., Discussion of Paper by Frederick & Eden, Corrosion, 1955, 11, 30t (46); see also: Ref. 18 95. Hafer, A. A,, Trans. Amer. Soc. Mech. Eng ., 1953, 75, 127 - 136 96. Foster, W. R., Leipold, M. H., & Shevlin, T. S., Corrosion, 1956, 12, 539t - 548t 97. Wickert, K., Brennstoff - Warme - Kraft, 1959, 11, 266 - 79; see also: Erdol & Kohle, 1960, 13, 658 - 664 98. Tseft, A. L., & Salibaev, T. 0., J. Applied Chem. USSR, 1950, 23, 1113 - 1116; Chem. Abs. 1951, 46, 7859 99. Johnson, L. B., Ind. Eng. Chem., 1960, 52, No. 8, 55A - 56A 100. Phillips, N. D., & Wagoner, C. L., Corrosion, 1961, 17, 102 - 106 101. Konopicky, K., Brennstoff Chimie, 1955, 36, Na 9/10, 151 102. Ilschner- Gensch, G., J. Electrochem. Soc., 1958, 105, 635 - 638 218.

103. Belyaev, I. N., & Nesterova, A. K., Zhur. Obshchei Khim., 1952, 22, 396 - 403; Chem. Abs., 1952, 46, 8497 104. Foley, R. T., Corrosion, 1958, 14, No. 8, 369t 372t 105. Logan, H. L., Corrosion, 1959, 15, No. 8, 443t - 446t; see also: ibid., 1961, 17, 185t - 187t 106. Vervey, E. J. W., Haayman, P. W., & Romeyn, F. C., Philips Techn. Review, 1947/1948, 9, 239 - 248

107. Kasatkina, L. A., Boresko, G. K., & Sokolov, P. N., Russian J. Phys. Chem., 1960, 34, 168; Zhur. Fiz. Khim., 1960, 34, 360 - 366 108. Rostoker, W., "The Metallurgy of Vanadium", New York: John Wiley & Sons, 1958, p. 7 - 10 109. Pantony, D. A. (Imperial College, London), private communication 110. Shewmon, P. G., "Diffusion in Solids", New York: McGraw-Hill Book Co., 1963, p. 66 111. Richardson, F. D., Iron and Coal, 1961, 1105 - 1116 112. Ref. ,38, p. 36

- 219. INDEX TO TABLES

No. Content Pa e I. Impurities in Spectrographically Standardized Metals 26 II. Analysis of Nitrogen for its Oxygen Content 29 III. Particulars of the Metal Specimens 31 IV. Cell Constants of the Conductance Electrodes 47 V. Equilibrium Constant of the Reaction VO2.5 V02 + 4 2(g) in the Molten State - Oxygen-loss Method 63

VI. Equilibrium Constant of: V02. 1-2 + -140 2 (By Mass Flow Thermobalance and Chemical Analysis) 65 VII. Dissociation Equilibrium of Vanadium Pentoxide in presence of Sodium Oxide 66 VIII Half-life Period for the Oxidation of Vanadium Dioxide under Pure Oxygen 71 IX. Kinetics of the Oxidation of Vanadium Dioxide under Oxygen 73 X. Effect of Oxygen Pressure on the Oxidation of Vanadium Dioxide at 700°, 800°, and 90000 74 XI. Effect of Oxygen on the Oxidation of Vanadium Dioxide (Fig. 14) 78 XII. Depression in Freezing Point of Vanadium Pentoxide at Different Oxygen Pressures 83 XIII. Specific Conductance of Molten Vanadium Pentoxide under Pure Oxygen 87 XIV. Variation of Specific Conductance of Molten Vanadium Pentoxide with Oxygen Potentials at 855°C 89 XV. Apparent Density of Vanadium Pentoxide - Vanadium Dioxide Melts at 855°C 91 XVI. Effect of Sodium Oxide on the Conductance of Molten Vanadium Pentoxide 92 XVII..Effect of C Oxide on the Conductance ,of Molten Vanadium Pentoxide 93 XVIII.Effect of Nickel Oxide on Conductance 95 220

XIX. Effect of Ferric Oxide on the Conductance of Molten Vanadium Pentoxide 96 , XX. Effect of Tungstic Oxide on the Conductance of Molten Vanadium Pentoxide 97 XXI. Conductance of Vanadic Melts of the Same Composition at Different Temperatures 100

XXII. Viscosity and Density of Vanadic Melts 108

XXIII. Thermodynamics of Viscous Flow 111 XXIV. Relation between the Amount of Vanadium Pentoxide and the Extent of the Initial Rapid Corrosion of Cobalt 119 XXV. Relation between Weight-increase ob3erved and Weight-loss of Cobalt 120 XXVI. Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadic Corrosion of Cobalt 122 XXVII. Relation between the Amount of Vanadium Pentoxide and the Extent of the Initial Rapid Corrosion of Iron 125 XXVIII.Correlation b3tween Weight-loss of Metal and Weight-gain for Different Periods of Corrosion of Iron 127 XXIX. Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadic Corrosion of Iron 129 XXX. Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadic Corrosion of Titanium 132 XXXI. Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadium Corrosion of Tungsten 134 XXXII. Correlation between Weight-loss of Metal and Weight-gain on the Vanadic Corrosion of Molybdenum 137 XXXIII.Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadic Corrosion of Molybdenum 138 XXXIV. Correlation between Weight-loss of Metal and Weight-gain on the Vanadic Corrosion of Vanadium 140 XXXV.L . Effect of Temperature, Depth of Melt and Oxygen Pressure on the Vanadic Corrosion ofVtmalium 141. XXXV1. Variables on Vanadic Corrosion of Nickel 144 221.

XXXVII. Attack of Metals by Molten Vanadium Pentoxide 147 XXXVIII. Oxygen-Diffusion Studies on Molten Vanadium Pentoxide 149 XXXIX. Interaction between Sodium Sulphate and Vanadium Pentoxide 151 XL. Interaction between Sodium Chloride and Vanadium Pentoxide 154 XLI. Vanadic Corrosion of Iron in Air with and without Current Passing 158 XLII. Effect of Grain-size on the Vanadic Corrosion of Metals 163 XLIII. Volume Ratio and Heat of Dissociation of the Metal Oxides in Re;ation to Vanadic Corrosion 167 XLIV. Changes in Free Energy for the Oxidation of Metals and Reduction of Vanadium Pentoxide 172 XLV. Diffusivity of Metal Ions in Different Layers 174 XLVI. Comparison of Various Rate Processes 176 XLVII. Reaction Constants Characteristic of the Outward Diffusion of the Oxidation Products during Vanadic Corrosion of Metals 183 222.

INDEX TO FIGURES

No. Particulars Facing Page 1. General Nature of Vanadic Corrosion 12 2. Model for Corrosion Studies 12 3. Gas-mixing Apparatus 30 4. Silica Sheath for Thermobalance 32 5. Crucible Arrangement for Corrosion Studies 32 6. Apparatus for Cryoscopic Work 39 7. Apparatus for Studies on Viscosity and Density 40 8, Apparatus for Conductivity Measurements 42 9. Convection Effect on Corrosion: Systems and Results 51 10. Apparatus for Electrochemical Work 55 11. Evaluation of n of: VO2.5(2.5 - n) + 7n 0 2 62 12. Reaction Isobar of: VO T--=0 + 02 62 2 * 2 13. Kinetics of Oxidation of Vanadium Dioxide: Arrhenius plot 73 14. Effect of Oxygen pressure on Oxidation of Vanadium Dioxide 74 15. Effect of Temperature on Conductance of Molten Vanadium Pentoxide 88. 16. Variation of Conductance with oxygen potential 90 17. Conductance of Sodium Oxide - Vanadium Pentoxide Melts 90 18. Conductance of Cobalt Oxide Vanadium Pentoxide Melts 93 19. Conductance of Nickel Oxide Vanadium Pentoxide Melts 94 20. Conductance of Ferric Oxide - Vanadium Pentoxide Melts 94 21. Conductance of Tungstic Oxide - Vanadium Pentoxide Melts 94 223.

22. Comparative Conductances 105 23. Density of Molten Vanadium Pentoxide 109 24. Viscosity of Molten Vanadium Pentoxide 109 25. General Nature of Vanadic Corrosion of Cobalt 117 26. Kinetics of Vanadic Corrosion of Cobalt 120 27. Effect of Oxygen Pressure of Vanadic Corrosion of Cobalt 121 28. General Nature of Vanadic Corrosion of Iron. 124 29. Kinetics of Vanadic Corrosion of Iron 128 30. Effect of Oxygen Pressure on vanadic Corrosion of Iron 128 31. General Nature of Vanadic Corrosion of Titanium 130 32. Kinetics of Vanadic Corrosion of Titanium 130 33. Effect of Oxygen Pressure on Vanadic Corrosion of Titanium 131 34. General Nature of Vanadic Corrosion of Tungsten 133 35. Kinetics of Vanadic Corrosion of Tungsten 133 36. Effect of Oxygen Pressure on Vanadic Corrosion of Tungsten 134 37: General Nature of Vanadic Corrosion of Mo. 135 38. Molybdenum: Kinetics of Corrosion 135 39. Oxygen Pressure .on Corrosion of Molybdenum 137 40. General Nature of Corrosion of Vanadium 140 41. Kinetics of Corrosion of Vanadium 140 42. Effect of Oxygen Pressure on Corrosion of Vanadium 141 43. General Nature of Vanadic Corrosion of Nickel 143 44. Kinetics of Vanadic Corrosion of Nickel 144 45. Effect of Oxygen Pressure on Vanadic Corrosion of Nickel A.44 46. Diffusion of Oxygen through _Molten Vanadium pentoxide 148 47. Oxygen-Absorption Capacity and Corrosion Property of Sodium Oxide- Vanadium Pentoxide Mixtures 150 224.

48. Effect of Metal Oxides on Vanadic Corrosion of Iron 156

49. Shape of Corroded Samples 159. 50. Electron Probe Microanalysis of Titanium- Slag System 160 51. X-ray Scanning of Titanium-Slag System 161 52. Electron Probe Microanalysis of Iron-Slag System 160

53. X-ray Scanning of Iron-Slag System 161 54. Electron Probe Microanalysis of Molybdenum- 160 Slag System

55. X-ray Scanning of Chromium-Slag System 161

56. X-ray Scanning of Nickel-Slag System 162