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

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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 by 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 intersti- tial 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.
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