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Home , Hydroxide, Sodium hydroxide

U#GLASSI FIE0

Contract No. W-7405ang-26

METALLURGY DIVIS1QN

CORROSION OF MATERIALS IN FUSED

G. P. Smith

OAK RIDGE NATIONAL tADORATORY Operated by UNION CARBIDE NUCLEAR COMPANY A Division of Union Carbide and Carbon Cwporation Post Office Box ? Oak Ridge, Tennsceo

3 4456 0350099 5

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OR NL-2048 Metallurgy and Ceramics

1/47 E RNAL DlSTRl BUT10 N

1. C. E. Center 46. 0. W. Cardwell 2. Biology Library 47. W. D. Manly 3. Health Physics Library 48. E. M. King 4. Metallurgy Library 49. A. J. Miller 5-6. Central Research Library 50. D. D. Cowen 7. Reactor Experimental 51. P. M. Reyling Engineering Library 52. G. C. Williams 8-20. Laboratory Records Department 53. R. A. Charpie 21. Laboratory Records, ORNL R.C. 54. M. L. Picklesimer 22. A. M. Weinberg 55. G. E. Boyd 23. L. 8. Emlet (K-25) 56. J. E. Cunningham 24. J. P. Murray (Y-12) 57. H. L. Yakel 25. J. A. Swartout 58. G. M. Adonison 59. 26. E. H. Taylor M. E, Steidlitz 27. E. D. Shipley 60. C. R. Boston 28. F. C. VonderLage 61. J. J. McBride 29. W. C. Jordan 62. G. F. Petersen 30. 6. R. Keini 63. J. V. Cathcart 31. J. H. Frye, Jr, 64. W. H. Bridges 32. R. 5. Livingston 65. E. E. Hoffman 33. R. K. Dickison 66. W. H. Cook 34. S. C. Lind 67. W. R. Grimes 35. F. L. Culler 68. F. Kertesz 36. A. H. Snell 69. F. A. Knox 37. A. Wollaender 70. F. F. Blankenship 38. M. h. Kelley 71-90. G. P. Smith .39. K. Z. Morgan 91. N. J. Grant (consultant) 40. J. A. Lane 92. E. Creutz (consultant) 41. T. A. Lincoln 93. T. S. Shevlin (consultant) 42. A. S. Householder 94. E. E. Stansbury (consultant) 43. C. S. Harrill 95. ORNL - Y-12 Technical Library, 44. C. E. Winters Document Reference Section 45. D. S. Billington

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96. R. F. Bacher, California Institute of Technology 97. Division of Research and Development, AEC, OR0 98. R. F. Kruk, University of Arkansas, Fayetteville, Arkansas 99. Douglas Hill, Duke University, Durham, North Carolina 100. L. D. Dyer, University of Virginia, Charlottesville 101. R. A. Lad, National Advisory Committee for Aeronautics, Cleveland 102. L. F. Epstein, Knolls Atomic Power Laboratory 103. E. G. Brush, Knolls Atomic Power Laboratory

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184. D. D. Williams, Naval Research Laboratory 105. R. R. Miller, Naval Research Laborotory 106. E. M. Simons, Battelle Memorial Institute 107. 11. A. Pray, Battelle Memorial Institute 108. P. D. Miller, Battelle Memorial Institute 109. M. D. Banus, Metal , Inc. 110-428. Given distribution as shown in TID-4500 under Metallurgy and Ceramics category

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iv U SI UNCLASSIFIED CONTENTS ABSTRACT ...... 1

1. INTRODUCTION ...... 1

2. ATOMIC NATURE OF FUSED ...... 2

3, OF CERAMICS ...... 2

Solubility Relations in Fused ...... 2 - Donor-Acceptor Reactions ...... 3 Ceramics with Saturated Cations and Saturated Anions ...... 3 Ceramics with Acceptor Cations and Saturated Anions ...... 4 Ceramics with Saturated Cations and Acceptor Anions ...... 6 Oxidation-Reduction Reactions ...... 7 Corrosion of Other Kinds of Ceramics ...... 7 Secondary Corrosion Phenomena ...... 7 Summary ...... 8

4. CORROSION OF METALS ...... 8 Corrosion of Metals by Oxidation ...... 8 Corrosion by Hydroxyl ...... 8 Corrosion by Metal Ions ...... 11 Corrosion by Ox idiz ing Solutes ...... 11 ...... Corrosion of Alloys ...... 11

5. MASS TRANSFER ...... 16 1\1 icke I ...... 17 Other Elemental Metals ...... M Alloys ...... 20 Bimetal I ic Effects ...... 20 Mechanism of Mass Transfer ...... 21 Differential (Mechanism I) ...... 21 Oxidation-Reduction Processes (Mechanisms Il-VI) ...... 21 Local Cell Action (Mechanism Il)...... 22 Reduction of Hydroxyl lons(Mechanism Ill) ...... 22

Reduction of Ions (Mechanism IV) ...... 23 Reduction of Solutes (Mechanism V) ...... 23 (Mechanism VI) ...... 23 Summary of Mechanisms ...... 24 Summary ...... 25

REFERENCES ...... 27

UNClASSlFI ED V

CQRRBSION OF MATERIALS IN FUSE5 HYDROXIDES G. P. Smith

ABSTRACT

Some of the fused alkali-metal hydroxides are of potential interest in reactor technology both as coolants and as moderators. The property which most discourages the use of these substances is their corrosiveness. The corrosion of ceramics and ceramic-related substances by fused hydroxides occurs both by and by . A few ceramics react with hydroxides by oridation- reduction reactions, but most have been found to be attacked by oxide-ion donor-acceptor re- actions. oxide was the most corrosion-resistant of the ceramics which have been tested, although several other ceramics are sufficiently resistant to be useful. The corrosion of metals and alloys by fused hydroxides takes place primarily by oxidation of the metal, accompanied by reduction of hydroxyl ions to form and oxide ions. Three side reactions are known, but two of them are not important in most corrosion tests. The corrosion of alloys involves either the farmatian of subsurface voids within the alloy or the formation of complex, two-phase corrosion products at the surface of the alloy. Nickel is the most corrosion- resistant metal which has been studied in fused . Mass transfer, induced by differentials, has been found io be the primary factor limiting the use of metals which, under suitable conditions, do not corrode seriously.

1. INTRODUCTION measured. Steidlitz and Smith4 have shown that thi 5 is relatively small. For example, Some of the fused alkali-metal hydroxides are of potential interest in reactor technology as moder- at 750°C the partial pressure of in equi- librium with fused sodium hydroxide is the order ators, moderator-coolants, and, possibly, moderator- of fuel vehicles.’ The heat-transfer properties of of magnitude of 0.1 mim Hg. The radiotion stability of fused hydroxides has these substances are sufficiently good for them to have been recommended as industrial heat- been reported by HochanadelS with regard to transfer media? For use as a high-temperature electron bombardment and by Keiiholtz et d.6 with moderator without a cooling function, sodium hy- regard to neutron bombardment. The radiation levels reported were very substantial, and no droxide possesses the virtues of being cheap and available as contrasted with its competitors, either evidence was found for radiation damage. liquid or solid. Furthermore, an essential sim- The property of fused alkali-metal hydroxides plicity in reactor design is achieved by combining which most discourages their application at high in one material the two functions of slowing IS their corrosiveness. It is the neutrons and ~ooling.~There are only a few purpose of this discussion to review the current substances which will do both at high tempera- scientific status of studies of corrosion by fused tures. hydroxides. Of the high-temperature moderator-coolants, the During the past five years, knowledge of the fused ailkali-metal hydroxides are among the most corrosive properties of fused hydroxides at high stable with respect to both thermal dissociation temperatures has increased con sidera bly . Never- and radiation damage. It has been known for theless, this is a new field of research and one some time that the alkali metal hydroxides should in which there has been found a remarkable variety be stable toward thermal dissociation. However, of corrosion phenomena, so that the areas of not until recently have techniques been developed ignorance are more impressive than the areas whereby this dissociation could be quantitatively of knowledge. For this reason, the writer has

1 chosen to place emphasis on some of the problems in the past, together with a proper regnrd for the which are most in need of solution. This review reactivity of many corrosion products in contact will, accordingly, be concerned with the kinds of with moisture and , is essential for corrosion phenomena which hove been observed further progress. rather than with the compilation of engineering The cortosion of ceramics and of ceramic-related reference data. Moreover, with a view toward substances by fused hydroxides has been observed stimulating further research, the writer has taken to take place by solution and by chemical reaction. the liberty of presenting a number of quite specu- A few of the chemical reactions which have been lative ideas on the origins of some of the corrosion observed were found to he oxidation-reduction re- phenomeno. Only in this way can many important actions, These will be described briefly near the possible accomplishments of further research he end of this section. The majority of the known indicated. reactions of ceramics and ceramic-related sub- In his report, the term "fused hydroxides" is stances with fused hydroxides have been found taken to mean the fused alkali-metal hydroxides; to be oxide-ion donor-ncceptor reactions. This the exception is lithiuin hydroxide, which, like kind of reaction in fused hydroxides may be treated the fused hydroxides of the alkaline-earth metals, in a quantitative although formal way by appli- is less stable with regard to thermal dissociation cation of - analog theory. Such a treat- than the other alkali metal hydroxides. Most of ment is beyond the scope ob this report. However, the available corrosion data are for fused sodium some of the concepts derived from the theory of oxide-ion donor-acceptor reactions are very useful hydroxide. Information on corrosion in fused po- tassium hydroxide indicates that this substance in describing the reactions between ceramics and behaves qualitatively like sodium hydroxide. There hydroxides and wi II be considered. are very little data available on the hydroxides Solubility Relations in Fused Hydroxides, - A of rubidium and cesium. ceramic material which is chemically inert toward fused hydroxides might be liinited in usefulness 2. A'FOMlC NATURE OF FUSED SODIUM because of appreciable solubility in these media. HYDROXIDE There have besn very few quantitative meusure- ments of solubility relations in fused hydroxides, Studies of e[ectrical conductivity7 and of the and none of these measurements have been made freezing-point depression by solutes* for substances which are of importance as ccrarnics, indicate that fused sodium hydroxide is a typical However, in a discussion of the corrosion of fused electrolyte which obeys, at least approxi- ceramics, such solubility relations must inevitably mutely, the model of ideal behavior proposed for be mentioned if only in a qualitative way. such substances by Ternkin.' Thus, sodium hy- There is a fundamental difference between solu- droxide consists of sodium ions and hydroxyl ions bility relations in nonelectrolytes such as water helid together by coulombic attraction such that and in fused such as hydroxides, no cation may be considered bound to a particular of a simple ionic substance in water may anion, although, on the average, eoch cation has be described in terms of a binary system, while only anion neighbors and vice versa. When an such solutions in fused hydroxides are reciprocal ionic sodium compound is dissolved in fused systems which require the specification of sodium hydroxide, the cations the solute become of four components provided that the solute does not indistinguishable from those of the solvent. The have an ion in common with the solvent. Under hydroxyl anion is not a simple particle of unit special conditions reciprocal salt systems may negative charge but has a dipole structure and, be treated as quasi-binasy systems, but it is under suitable conditions, a polypole structure." unwise to assume such behavior in the absence of confirmatory experimental evidence. CERAMICS 3. CDRRO510M OF Metathesis reactions in aqueous media are, of Studies which have been made of the behavior course, the manifestation of reciprocal salt solu- of ceramic materials in fused hydroxides ure bility relations. The corresponding lype of re- limited in scope both as regards the variety of action in a fused hydroxide solution is complicated substances tested and the information obtained on by the fact that six composition variables would any one substance. The use of petrography and in general be needed to describe the possible x-ray diffraction, which have been largely ignored solid phases which could precipitate from solution.

2 Oxide-Ion Donor-Acceptor Reactions. - Many may not form. It means that an chemical substances have a significant affinity for such as dissolves as sodium ions oxide ions. Two well-known examples are carbon and chloride ions rather than as sodium chloride dioxide and water, which react with oxide ions to . This concept of ionization 8s the form, respectively, carbonate ions and hydroxyl foundation of most of the modern chemistry of fused ions, Such substances are referrid to as “oxide- electrolytes, On the basis of this ccncept it IS ion acceptors.’’ Once an acceptor has reacted postulated that, when an ionizable solute is dis- with an oxide ion, it becomes a potential oxide-ion solved to form a dilute solution in a fused donor and will give up its oxide ion to a stronger hydroxide med iurn, separate ox i d e-ion off in it ies acceptor. Thus, the hydroxyl ion is an oxide-ion may be ascribed to the cations and to the anions donor which is conlugate to, or derived from, the of the solute. Although there are definite limi- oxide-ion acceptor, water. Carbon dioxide is a tations to the application of this postulate, it is stronger acceptor than water and hence will react very useful for two reasons. First, it provides a with hydroxyl ions to form carbonate ions and satisfactory qualitative description of the known water. Such a reaction may be viewed as a compe- reactions between solutes and hydroxyl ions in tition for oxide ions between the two acceptors, fused hydroxides. Second, to the extent that this carbon dioxide and water: postulate is true, it allows inferences to be made about the reactivity of an untested ionic compound (3.1) CO, i-20H- = CO,-- + H,O composed of cations C+ and anions A-, provided that the behavior of Cc and A- IS known separately acceptor I + donor II = donor I + acceptor II from the reactions of compounds which have as The reaction goes in that direction which produces cations only C+ and other cornpounds which have the weakest donor-acceptor pair. as anions only A-. Many substances wi II behave toward hydroxyl Considerations of the origin of the oxide-ion ions like carbon dioxide in Eq. 3.1; that is, they affinity of cations, such as has been presented will capture oxide ions and liberate, water. This by Dietzel’’ and by Flood and F6rland,l2 indicate is not necessarily because the acceptor in question that cations of very low ionic potential should is intrinsically stronger than water. An acceptor have low oxide-ion affinities. The cations to be which is stronger than water must successfully found in this category are the alkali-metal and compete with water for oxide ions when water and alkaline-earth cations. These ions have ac- the acceptor in question are at the same concen- cordingly been found to show only a weak-to- tration. Frequently reactions like Eq. 3.1 take negligible tendency to react with hydroxyl ions in place because water is easily volatilized from fused hydroxide solution. fused hydroxides at high temperatures, and conse- Likewise, certain kinds of anions have such quently its is very low compared small oxide-ion affinities that they have been with the concentration of the competing oxide-ion found to show only a weak-to-negligible tendency acceptor. However, the water concentration in a to capture oxide ions from hydroxyl ions in fused fused hydroxide may be maintained constant by hydroxide solution. The anions in this category fixing the partial pressure of water over the melt. include the oxide ion, the halide ions, and some Under this condition a comparison may be made oxysalt anions such as carbonate and sulfate. of the relative abilities of two oxide-ion acceptors Obviously, ortho-oxysalt anions belong in this to capture oxide ions from hydroxyl ions in the category inasmuch as they are all saturated with presence of the same fixed concentration of water. oxide ions. Ions which show little or no tendency Thus it is meaningful to speak of the relative to accept oxide ions from hydroxyl ions in fused oxide-ion acceptor strengths of two substances in hydroxides will be referred to, for convenience, f u sed hy drox ide s 01 ution s. as “saturated” ions. As was pointed out in the preceding section, Ceramics with Saturated Cations and Saturated when an ionic substance is dissolved in a fused Anions. - Ionic compounds with saturated cations hydroxide, dissociution, in the sense of separation and anions have thus far all proved to be too of ions, occurs, and the cations and anions of the soluble for use as solid components in fused dissolved substance behave as chemically separate hydroxide media. The alkali metas oxides13 and entities. This does not mean that complex ions halide~~v’~have been found to be quite soluble.

3 Barium ~hloride’~showed significant solubility stability of magnesiuni oxide in fused sodium in sodium hydroxide at 350°C. A sample of hydroxide is not surprising, since magriesiutn is oxide fired at 1900°C to give an apparent not known to occur in an oxysalt anion.” porosity of 3% was reported” to be severely Magnesium oxide is not only chemically stable attucked by fused sodium hydroxide at 538”C, in fused sodium hydroxide but is also quite in- although other tests13 indictlte that its solubility soluble. In corrosion tests, Steidlitz and Smith4 is sinall at 350°C. found that single-crystal specimens of magnesium CeiGmiCS with Acceptor Cations and Saturated oxide in a large excess of sodium Anions, - A substantial proportion of $he ceramics hydroxide at 800°C. decreased in thickness by less and ceramic-related substances which huve been than 0.001 in. in 117 hr. tested in fused hydroxides is composed of cations Water in fused sodium hydroxide has been found with appreciable oxide-ion acceptor strengths and to attack magnesium oxide. Boston’* reported that anions which are saturated. Cations which accept anhydrous sodium hydroxide has no visible effect oxide ions fiom hydroxyl ions should do so by the on cleavage planes and polished surfaces of mag- formation of either or oxysalt anions, nesium oxide crystuls up to 700°C but that the -Ihe formation of an oxide by the reaction of an presence of water vapor over the melt caused rapid acceptor cation with a fused hydroxide has been etching of the crystal surfaces. This effect of observed in studies13 of the reactions of mag- water is entirely consistent with the oxide-ion nesium chloride and nickel chloride with fused donor -acceptor concept. sodium hydroxide. ’These compounds reacted very The equilibrium between magnesium oxide and rapidly at 400°C to form insoluble magnesium oxide water in the presence of sodium hydroxide may be and insoluble nickel oxide. The reaction of mag- expressed as nesium chloride is given by the equation (3.3) MgQ(s) + HpO(rneIt) (3.2) MgC12(solid) + 20ki-(melt)

= MgO(solid) -t 2CI-(melt) -+ t-l,O(gas) The activity of solid magnesium oxide, MgO(s), is Considerable data are available on the reaction a constctnt. Furthermore, the hydroxyl ions, of oxides with hydroxyl ions. Three oxides, those Ol.i-(melt), QFZ present in great excess and should of magnesium, , and , have been tested have approximately constant activity. Conse- up to substantial temperatures without giving quently, the application the action evidence of reaction. Four oxides, those of of low of i-nass to shows that the activity of magnesium ceriutti(lV), nickel, , and aluminum, have Eq. 3.3 ions in the melt should he proportional to the been shown under some conditions to have a sig- activity of water in the melt; that is, an increase n ificant res istence toward reaction with hydroxyl in the concentration of water in the melt should ions, although they are all known to be capable of reacting completely. Two oxides, those of increase the apparent solubility of magnesium (V) and titanium, have been found to react oxide. relatively rapidly at lower temperatures. Further The practical application of magnesium oxide as details on the above oxide-hydroxide reactions will a ceramic material for service in fused anhydrous be given below, hydroxides presents difficulties. Most sintered The action of fused sodium hydroxide on mag- compacts of pure magnesium oxide are sufficiently nesium oxide was strrdicd by D’hs and L6ffIerl6 porous to absorb appreciable quantities of sodium at temperatures up to 800”C, but they were unable hydroxide. Large magnesium oxide single crystals to detect any water evolution. Steidlitz and can be machined into fairly complicated shapes Smi?hf4 whcr made riiass spectrometric determi- when a special need for o particularly corrosion- nations af the gases evolved on heating sodium resistant past justifies the expense, At the Oak hydroxide to 800°C in a vessel cut from a mag- Ridge National Laboratory, reaction vessels have nesium oxide single crystal, found water vapor, been machined frorrt massive muynesiiim oxide but its presence was accounted for quantitatively crystals, and in every instance such vessels have in terms of the theriiia! dissociation of hydroxyl proved to be very satisfactory containers far fused ions in the presence of sodium ions. The chemical sodium hydroxide.

4 Qualitatively, zinc oxide behaves much like by exposure to fused sodium hydroxide for 25 hr magnesium oxide in the presence of fused sodium at 538°C. Stabilized zirconium oxide differs from hydroxide. D’Ans and LoCfler16 were unable to the pure substance in having n different crystal find evidence of a reaction between zinc oxide structure at the temperatures of interest here and and anhydrous sodium hydroxide at 600°C. How- in having a few weight per cent of calcium or ever, oxysalt anions of zinc are known to exist,17 magnesium oxide in solid solution. However, it and it is possible that Q reaction occurs at temper- is difficult 1.0 see how these differences could atures above 600°C. D’Ans and Loffler report significantly alter the thermochemistry of the zir- that water greatly increased the apparent solubility conium oxide-sodium hydroxide reraction, although of zinc oxide in sodium hydroxide. This behavior they may have an effect em the rate of reaction. may be analogous to that of magnesium oxide, or Aluminum oxide was found hy D’Ans and Loffler’6 it may result from the formation of complex zinc to react with sodium hydroxide to give the oxysalt anions involving water.’’ In a corrosion test” Na,AI,O,. Craighead, Smith, Phillips, and Jaffee” compressed and sintered zinc oxide was found to exposed a single crystal of aluminum oxide to have lost 8% in weight at 538°C. sodium hydroxide at 538°C and reported that the Chemical studies16 on thorium oxide indicate crystal decreased in weight by 0.40% after 24 hr. that it does not react with fused sodium hydroxide Simons, Stang, and Logedrost22 used hot-pressed up to 1000°C. However, no corrosion or solubility A120, as a pump bearing submerged in fused data are available. sodium hydroxide at 538°C. After 8 hr of pump As pointed out above, (lV) oxide, nickel operation they were unable to detect any signs of oxide, zirconium oxide, and aluminum oxide are bearing wear or other damage. Tests have also all known to react completely with sodium hy- been rep~rted’~for hot-pressed and for sintered droxide under suitable conditions, However, they aluminurn oxide. Weight changes which were all have been reported to show considerable re- observed were less than those for single-crystal sistance toward reaction under other conditions. material under the same conditions. This sur- D’Ans and LofflerI6 found that cerium(lV) oxide prising result may have been caused by absorption reacted very slowly with an excess of sodium of hydroxide into pores of the hot-pressed sample, hydroxide, the interaction beginning first between which compensated for a weight loss. 950 and 1OOO”C, but, surprisingly enough, an The relatively good corrosion resistance of excess of cerium(lV) oxide reacted more easily aluminum oxide found in these corrosion tests is at 900’C to give Na2Ce03 and water. confirmed by day-to-day experience at ORNL. Nickel oxide was found13 to be unreactive in Vessels made of pure, binder-free, recrystallized fused sodium hydroxide at 40O”C, at least for short aluminum oxide without porosity have been found periods of time. Williams and MillerZo did not find to be excellent containers for fused sodium hy- any reaction even at 8OOOC for a period of 2 hr. droxlde under a wide variety of service condrtions However, Mathews, Nauman, and hh2’ found that where a small contarnination could be at 800°C nickel oxide reacted rapidly with sodium tolerated. It should be remembered, however, that hydroxide to give water and Na2Ni02 according to most commercial alumina and some so-called the equation ($ corundum ware” have a silica or binder which is readily attacked by fused hydroxides. (3.4) NiO(s) + 2NaOH(I) Niobium(V) oxide has been found to react very --1 Na,NiO,(s) c H20(g) readily with fused sodium hydroxide. Spitsyn and Lapit~kii~~heated niobium(\/) oxide with fused The difference between the results of these two sodium hydroxide for I-hr periods at temperatures groups of experimenters is not understood, of 350 to 650°C and found the reaction Zirconium oxide was found by D’Ans and Loffler’ to react with excess sodium hydroxide to form (3.5) Nb20, + 1ONaOH = 2Na5Nb0, + 5ti20 Na2ZrO3* However, corrosion tests on stabilized zirconium oxide have shown this substance to be reacts readily with fused sodium resistant to attack. Craighead, Smith, Phillips, hydroxide at 800°C. D’Ans and Laffler16 studied and Jal‘fee15 found that stabilized zirconium oxide, this reaction and concluded that when an excess of fired at 1700°C in either air or to give an hydroxide was present the reaction was not quanti- apparent of less than 0.5%, was unaffected tative but that an equilibrium existed, probably

5 between sodium orthotitanute and a metatitanate. like niobiutn(V) oxide, reacts with sodium hy- Such an equilibrium might be a5 follows: droxide to give sodium orthoniobate(Y). This reaction is as follows: (3.6) Na2Ti,0, i 2NoOt-4 = 2Na,TiB, + H,O (3.7) NaNbO, + 4NoOt-l -- Na,NbO, t 21-1,O In corrosion tests, l5 titanium dioxide ceramics sintered at 760°C were found to be very severely Some information is available on two nonmetallic attacked by sodium hydroxide at 539°C. systems of sodium oxysalts. It has beet7 known Inasmuch as appears to be for a long time that dioxide, boron oxide, stable in fused sodium hydroxide, it might be the with small ‘2 values, the of supposed that the aluminate anion is of itself inert. boron(ll1) with sinall Z values, and the It has been shown’5 that ceramics with compu- based on these substances nearly all react readily sitions in the neighborhood of MgAI,O, are quite with fused hydroxides to form water and the more corrosion-resistant in fused sodium hydroxide. alkaline silicates and borates. This reactivity hus This result is not surprising, because sodium been utilized for many years in the solubilization aluminate has a low solubility and the magnesium of minerals by a process known as alkali fusion. ion wowld tend to react to give magnesium oxide, On the other hand, very little information is which is very corrosion-resistant. available on the action of fused hydrsxides on the more alkaline silicates and borates, many of Ceramics with Saturated Cations and Acceptor Anions, - The saturated cations, as painted out which should be relatively unreactive toward fused above, are those of the alkali and alkaline-earth hydroxides provided that they contain unreactive cations. metals. Only o few tests have been made on compounds with cations which are saturated and From a knowledge of the end products of alkali anions which will accept oxide ions from hydroxyl fusions it would be possible to estimate the ions, and they have been limited to the sodium reactivity of the silicates and borates with high oxysalts with high oxide-ion affiniiies. Obviously, C values by application of the X postulate. Un- such substances shou Id not be corrosion-resistant. fortunately, there are very few data of this kind. However, an examination of their behavior will Usually the products of alkali fusions are not serve to illustrate the kinds of reactions which examined until after they have been hydrolyzed. anions undergo. The results of two experiments are available, The chemical composition of a sodium oxysalt however. First, Morey and MerwinZ4 mention, can always be expressed in terms of the nuniber without giving experi mental conditions, th ut sod i um of equivalent weights of per equivalent pyrobasate is stable in fused sodium hydroxide. weight of the other oxide of which the oxysalt is Second, D’Ans and Loffler’6 found thut silicon sto ich iometr icaI ly composed. For sonveni ence, dioxide reacts with an excess of sodium hydroxide this quantity will be designated as X. to yield an equilibrium between the pyrosilicate In general, the greater the value an oxysalt, X of and the orhosi licate as follows: the less will be the oxide-ion affinity of its anion as compared with other tlnions composed of the (3.8) Na,Si,O, + 2NaOH = 2Na4§i0, + H,O same elements. Therefore, if an oxide is found From these data it is concluded that borates with to T~QC?with fused sodium hydroxide to yield at C less than 2 and silicates with C iess thun 1.5 equilibrium an oxysalt with a value of C equal to will react in B similar fashion. This rather terse u and if there are other oxysults of the oxide in conclusion con be interpreted in terms of the question with values of C, less than 0, it may be structural chemistry of the borates and the sili- presumed that the other oxysalts will also react cates, with fused sodium hydroxide. This well-known postulate, herein called the ‘‘2postulate,” under- in simple borates and silicates ore lies all phasediagram work in whish an oxide-ion of two kinds: those which are bound to two boron donor such as is used as a or silicon atoms and form so-called “oxygen SOVAC~of oxide ions. bridges” and those which are bound to only one In agreement with this postulate Spitsyn and boron or silicon and carry o negative charge. LopitskiiZ3 showed that sodium mstaniobate(V), ’Thus, the pyroborate ion has one oxygen bridge

6 and may be structurally represented as As shown by LeBlanc and Weyir25 silicon reacts violently when displacing hydrogen from fused hydroxide at 400'C. The over-all re- action was not determined, but an obvious gue55, on the basis of the information in the preceding section, is that the orthosilicoie is formed ac- When an oxide ion reacts with a or sili- cording to the equation cate, an oxygen bridge is broken and is replaced (3.9) Si t 4NaOH = Na,SiB, 3 by two nonbridge oxygen atoms. Thus, the pyro- 2H 2 borate ion shown above can react with one oxide Carbon in the form of massive graphite shows ion to form two orthoborate ions each with the very little tendency to react at temperatures up to fo Ilow irig s tructirre : 500°C but does absorb some hydroxide because -0 of its porosity. However, it has been demonstrated \/*- that oxidation of graphite does occur, although not I rapidly, at temperatures as low as 500°C. Ketchen 0- and Overholser26 made a careful study of the The C. postulate implies that the greater the number action of a large excess of sodium hydroxide on of oxygen bridges attached to a given boron or graphite (possible oxidunts other than the hy- silicon atom, the greater the oxide-ion affinLty of droxide were excluded) and found that, above that atom. Since simple borates and silicates with 500"C, graphite is oxidized to form sodium car- 2 greater than 2 and 1.5, respectively, react with bonate. At 815"C, graphite has been shown to sodium hydroxide, it is concluded that for these react quite rapidIy.l5 compounds a boron or silicon atom with more than D'Ans and LofflerI6 report that under an atmos- one oxygen bridge has sufficient oxide-ion affinity phere of , (ll1) oxide reacts with fused sodium hydroxide to give mostly sodium ferrate(l1 I), to accept an oxide ion from a hydroxyl ion. There are other alternatives to the structural with a small amount of an oxysalt in a higher valence state, which they suggest may have been interpretation of and borate reactions in fused hydroxides. For example, equilibrium 3.8 sodium ferrate(V1). is thermodynamically consistent with the as- Corrosion of Other Kinds of Ceramics. - The of sumption that, rather than the oxygen bridge in action sodium hydroxide on a number of other the melt being a physical entity, the pyrosilicate kinds of ceramics such as carbides has been ions (known to exist in solids) dissociate into studied15 in corrosion tests. However, no experi- orthosilicate and SiQS2- ions. Another alternative mental information was obtained which would is that there is an equilibrium in the melt between i dent ify the c hem ica I react ions. Si,O,6- on the one hand and SiOA4- and Si0,2- Secondary Corrosion Phenomena. - The three on the other without either one or the other pre- causes of the deteriorution of ceramic bodies, dominating. The present technique of studying which will be considered secondary in the sense fused electrolytes does not make it possible to that they do not reflect the intrinsic chemical determine which reuction takes place. reactivity of the primary ceramic constituents, are Oxidation-Reduction Reactions. - Only a few spalling, binder attack, and pore penetration. examples have been found of oxidation-reduction It is well known that many ceramics spall when reactions of ceramics and ceramic-related sub- subjected to severe thermal shock. Occasional iy, stances in fused anhydrous hydroxides. Further- however, spalling which resulted from the im- more, all oxides and oxysalts which contain metals mersion of a cold specimen into a hot liquid has been confused with corrosion. capable of existing in more than one valence state in fused hydroxides should undergo oxidation or Many ceramic bodies contain binders which have reduction reactions. There are very few studies different chemical properties from those of the of oxidation-reduction reactions of oxides or oxy- principal constituent. Some of the more common in fused hydroxides. In the former category binders are among the most reactive materials in the behavior of silicon and carbon, two elements fused hydroxides, so that attack on the binder which might be considered ceramics, has been ntegration of the ceramic body. An studied. example of this is the attack of fused hydroxides

7 on magnesia and alumina. The pure oxides MgO by small amounts of water. Zinc oxide is not SO and AI,O, are resistant to corrosion by fused corrosion-resistant as magnesium oxide. No cor- hydroxides. Hove~er,these substances are fre- rosion data are available on thorium oxide. quently fabricated witti the oid of a high-silica Oxides of cariuni(lV), nickel, zirconium, ~nd binder, which is strongly attacked, aluminum have been shown under some conditions Fused hydroxides show a pronounced copillary to have n significant resistance toward reaction activity, and they very readily penetrate sniull with hydroxyl ions, ~lthoughthey are all known to pores and cracks in cesarnic moterials, There is be capable of reacting by an oxide-ion donor- no evidence that such pore penetration will of acceptor mechanism. Under many conditions alu- itself cause disintegration of the ceramic. How- iiiinuiii oxide is sufficiently inert to serve CIS CI ever, ceramic specimens are frequently woahed useful container material for fused hydroxides. with woter aftor test to remove adhering hydroxide Oxides of niobium(\/) and titanium react fairly and are exposed to air during examination. Under readily with fused sodium hydroxide by an oxide- these conditions a hydroxide-impr23nated specimen ion donor-acceptor mechanism at relatively low will disintegrate, since tho hydroxide rapidly forms teinpsratiires. or hydrated carbonates with appreciable There have been very few studies of compounds volume expansion. Such behnvior has been uh- contaiiling reactive anions but unreactive cations. served foi POFOUS specimei?~~~ade of pura mag- In tSe few examples known, the reactions all nesium oxide and pure aluminuin oxide, proceeded by oxide-ion donor-acceptor mechanisms. The pronounced capillary activity of fused hy- Indications are that the oxide-ion affinity of simple droxides lends to other difficulties in corrosion borates in fused hydroxides is not satisfied until tests. Fused hydroxides wet a very wide variety the pyroborate anion is formed. Simple silicates of substances, both metallic and nonmetallic, and react by a mechanism like that of the borates, but show a strong tendency to creep, This property the end product seems to be an equilibrium between has not been observed to bc serious near the the pyrosilicate and orthosilicate anions. of the hydroxide, but at higher The corrosion resistance of a fabricated ceramic temperatures it presents significant experimental body is dependent on the intrinsic solubility and d iff icuI t ie s. chemical stability of the primary constituents and Summsry. - Studies which have been made of the is also determined by porosity and the resistance behavior of ceramic materials in fused hydfoxides to attack of any binder material it may contain. are limited in scope both os regards the vnrie:y of 4. CORROSlBM OF METALS substances tested and the informution obtained on any one substance. The corrosion of ceramics and Studies have been made of the action of fused ceramic-related materials hiss been found to occur hydroxides on ot least 31 elemental metals and both by solution and by chemical reaction. A few 65 alloy compositions. The results of these substances, among them graphite and silicon, were studies show a wide variety of corrosion phe- found to undergo oxidation-rediict ion reaction s, but nomena. In the following discussion, only the for the majority of compounds the attack proceeded more common types of corrosion will be treated, by an oxide-ion donor-acceptor reaction. These types will be illustrated by the better known Compounds of the alkali and alkaline-earth examples. metals such cis oxides, halides, and saturated Corrosion of Metals by Oxidation. - The oxi- oxysolts hove not been found to react with fused dation of metal atoms to fori11 ions is a basic step sodium hydroxide but have usually shown ap- in the corrosion of metals by fused hydroxides. preciable solubility. Compounds of other metals Oxidation niny be caused by the action of hydroxyl with the above anions were found to react by an ions, alkali metal ions, or foreign substances oxide-ion acceptor-donor mechanism to form on dissolved in the hydroxide, to oxide of the metal or form an oxysalt, Corrosion by Hydroxyl Ions. - Oxidation by Oxides of magnesium, zinc, and thorium have hydroxyl ions is the most comiiion form of corrosion been tested at substantial temperatures wihout of metals in fused hydroxides. In such rewtions giving evidence of chemical reaction. Magnesium the hydroxyl ions are reduced to hydrogen and oxide is very corrosion-resistant in anhydrous oxide ions, while the metal is oxidized to form sodium hydroxide but is susceptible to mi!d attack metal ions. These metal ions inay dissolve as

8 such or may act as oxide-ion acceptors and form sodium hydroxide under suitable conditions at an oxide or an oxysalt. Oxide-ion acceptor re- 500°C. This group includes aluminum, bismuth, actions were discussed in the preceding section chromium, cobalt, coppern , indium, iron, , on ceramics. nickel, , platinum, silver, and zirconium.

For a metal M of valence ~t the basic step in The second group is, of course, of more interest. corrosion by hydroxyl ions may be represented as Some of the riietaIs appear to be corrosion-resistant 1, because of protective film formation, while others (4.1) M + vOH- = Mu+ t- ~0”‘ + - H, 3 appear to be corrosion-resistant under suitable conditions because the thermodynamic driving force It has been demonstrated that this reaction occurs is small. Aluminum will serve as an example of for a wide variety of metals from the most reactive a metal which seems to be protected by film to the most inert. formation, and nickel is an example of a metal In a i’ew instances this hydroxyl ion reaction is which can be maintained almost in thermodynamic accompanied by a side reaction which produces a equilibrium with the melt under isothermal con- of the metal. If the hydride is volatile, it ditions. Film formation will be considered first. may escape; thus arsine and stibine are evolved from the reactaons of arsenic and antimony, re- There is no thermodynamic barrier to the reaction of spectively, with fused sodium hydroxide.” If the aluminum with sodium hydroxide no matter hydride is not volatile, it may react with the whether the reaction product is aluminum oxide hydroxide as do the alkali metal hydrides. This or sodium aluminate or whether the product occurs side reuction does not seem to be important in the as a solid phase or is dissolved in the melt. Free- energy changes which accompany the possible corrosion of most metals. Water is farmed in a second side reaction which is sometimes observed. reactions between aluminum and sodium hydroxide The possible processes involved here will be have been estimated and were shown to be quite discussed later, negative. The hydrogen pressure required to reverse these reactions was likewise shown to be Attempts to account for the relative corrosion resistance of different metals toward fused hy- much greater than any hydrogen pressures thus far droxides have had only limited success. There encountered in corrosion tests. Revertheless, at is some correlation between the standard free Iower temperatures the reaction between a I urn i num energies of formation of the oxides and corrosion and fused hydroxides proceeds slowly. Cra ighead, resistance, as has been pointed out by Brosunas.28 Smith, and -toffeeZ9 tested 2s and high-purity The metals whose oxides are most stable are aluminum in fused sodium hydroxide for 24 hr at frequently the least corrosion-res istant and vice 538’63. The specimen of 25 aluminum lost 0.337 versa. Thus calcium with a very stable oxide mg/cm2 in weight and pitted slightly, while the reacts vigorously with fused hydroxides, while high-purity aluminum lost 1.68 mg/cm2 but showed silver with a relatively unstable oxide is fairly no other signs of corrosion on metallographic corrosion-resistant. This correlation, however, is examination. The origin of the corrosion re- sistance aluminum is not known, but in view i10 more than a rough, general guide. For example, of nickel is more corrosion-resistant to fused hy- of the corrosion resistance of aluminum oxide (see droxides than gold is, although gold oxide is much Sec. 3, “Corrosion of Ceramics”) it seems probable less stable than nickel oxide. that the reaction of hydroxyl ions with aluminum For present purposes, elemental metals will be produces at first a thin, protective film of oxide, classified in two groups according to corrosion which then very slowly reacts to form the slightly resistance toward fused sodium hydroxide. The 5 ol ubl e a lu mi note. first group consists of those metals which seem Nickel represents a metal which can be held, to be strongly and irrepressibly attacked by hy- even at high tempera tures, a I mo st at thermody no mi c droxyl ions at 500°C under all conditions which equilibrium with the melt. The corrosion rate of have been studied. This group includes alkaline- nickel at high temperatures has been found to be earth metals, antimony, arsenic, , cerium, dependent on the partial pressure of hydrogen over niobium, magnesium, manganese, molybdenum, tan- the melt. Under 1 atm of hydrogen, nickel shows talum, titanium, tungsten, and vanadium. The only negligible signs of corrosion in fused sodium second group consists of those metals which show hydroxide for 100 hr up to 815OC, provided that a measure of corrosion resistance toward fused thermal gradients are absent, as shown by Smith,

9 Steidlitz, and Hoffman.” However, if the hydrogen Miller, and Hani~un,~~namely, the reduction of atmosphere is not maintained, the corrosion rate oxide ions by hydrogen when the latter is present of nickel in fused sodium hydroxide may become in sufficient concentration. Such a reduction of very rapid at high temperatures. Williams and oxide ions would have to be accompanied by c1 AAiller2’ and Petersen and Smith13 hove studied reduction of positive ions such as nickel ions this reaction undw such conditions that the hy- either in NiQ or in No,NiO,. drogen which formed was removed by high-speed There are several kinds of evidence which vacuum pumping. It was found that below 408°C support the plausibility of this second postulate. the reaction is exceedingly slow but that above It is known from the work of Pray and Miller3, 800°C it is exceedingly rapid, The only gaseaus that the ;odium nickelate(ll) which is formed from product found was hydrogen. The metal formed a the hydroxide-nickel reaction can be reduced to sodium nickelate(l1) of uncertoin composition but metallic nickel by a partial pressure of hydrogen WQZ probably Na,NiQ,. This reaction might be of 0.1 atm at 950°C. In reactions between nickel reprc sen ted by and sodium hydroxide, particulate nickel is fre- (4.2) Ni(s) -L 2NaOH(I) = Na,NiO,(s) + H,(g) quently found in the hydroxide phase. Mathews, Nauman, and Kruh” found particulate nickel in Williams and Miller” suggested thut the product the experiments from which they deduced Eqs. 4.3 of the above reaction might be either Q compound and 4,4. Williams and Miller,’ reported that in or a mixture of sodium oxide and nickel oxide. their experiments there was a definite correlation Petersen and Smith13 isolated single crystals of between the occurrence of particulate nickel in the reaction product and were able to obtoin the the melt and the occurrence of water in the gas unit cell dimensioiis by x-ray methods. The results phase and that neither particulate nickel nor water left no doubt that the reaction product which they occurred if the hydrogen evolved was removed obtained was a sodium nickelate(l1) compound. rapidly enough for its partial pressure over the In other studies at 950°C the hydrqen which melt to remoin low, formed was not rapidly evacuated from the system Data an the action of hydroxyl ions on the metals but was removed slowly so that a significant bismuth, chromium, cobalt, , gold, indium, partial pressure existed over the melt. Under these iron, Isad, palladium, platinum, ond silver are very Conditions, Peoples, Miller, and Wannan” and frugmentary, and no attempt at a review will be Mathews, Naurnan, and Kruh” report that con- made here except to point out two facts which are siderable water vapor wos evolved. Mothews et a!. important in evol uoti ng corrosion mechanisms found that water and hydrogen were evolved ap- geiPerils ly. proximately in an equal molar ratio and suggested First, Lad33 has shown that, when chromium that the corrosion process consisted in two steps: reacts wich fused sodium hydroxide, it is oxidized the formation of nickel oxide with the evolution of first to a lower valence state and is followed by hydrogen, a very slow oxidation to a higher valence state. (4.3) Ni + 2NaOH = Ni8 + Na,O + H, Similar data are not available for other metals which have more than one in fused follo?aed by hydroxides, but the work of Lad suggests that the (4,4) NiO + 2NaOt-l = Na,NiO, + H,O reaction kinetics of such metals in fused hy- droxides may be quite complicated. The latter reaction was discussed in Sec. 3. The Second, although the reaction between nickel and over-all reaction would he sodium hydroxide con be inhibited by hydrogen, (4.5) Ni + 4NaQH Williams and Miller” hove shown that hydrogen has no inhibiting effect on the reaction of iron with 7 Nrr,NiO, + Na,O i H,O + H2 sodium hydroxide. Furthermore, they were unable Peoples, Miller, and Hc~nnan~~did not check the to find any metallic iron resulting from attempts hydrogen-wafer correlation, but they, a1 so, sug- at hydrogen reduction of iron corrosion products gested equations similar to those above as a in the presence of fused sodium hydroxide. This source of water. result is important in a consideration of the An odditiano! source of water was also postu- mechanisms of moss transfer in fused hydroxides lnted by both Williams and h4illcrZ0 and by Peoples, and will be discussed in Sec. 5.

10 Corrosion by Alkali loris. - The ability The action of weak reducing agents such as of metals such as iron tu reduce alkali metal ions nickel ori alkali metal ions in fused hydroxides in fused alkali-metal hydroxides ~QSbean known would be brought to equiiibriirrn by a very small for more than a century and WBS at one time us& concentration sf the alkali metal. Hence, the of in the laboratory preparation of the alkali metals. generation appreciable quantities of alkoli metal LeBlainc and Wey!25 reported that chromium, mo- is only possible under conditions in which the lybdenum, tungsten, magnescum, and sodaurn were alkali metal can escape from the reacting mixture. found to displace potassium metal, as well as Gold forms an alloy with sodium, so that in the hydrogen, from fused . Viilard34 sodiuni hydroxide-goid reaction 5onie sodium may found that fused sodium hydroxide was reduced to be removed from the field of reaction by alloy grve sodium or by magnesium, formation, as was pointed out by Wrlliams and chromium, tungsten, iron, cobult, nickel, and Gsrso- Miiier,20 In corrosion tests at temperatures above manganese. Brasunos28 found that zirconium 800°C small amounts af sodium metal have same- times been found cooler parfs of the apparatus reduced sodium hydroxide to give sodrbsrn metal OR and hydrogen. above the liquid. Presumably, this metal had Willrams and Miller2' reported that nickel acts distiiled out of the n~el~.However, under the on fused sodium hydroxide liii a two-step process conditions of ITIQS~corrosion tests on relatively by whnch hydroxyl ions are seduced and, offer they c orros I on-res ista nt meta 1s in fused hydrox ides are consumed, sodium ions are reduced. This below 800°C, appreciable quantities of alkali metal sequence of events corresponds to the reaction ore never found. Therefore, from the stondpoint of in which sodium ncckelate(ll) is Farmed and then the thermodynamics of corrosion, ~ver-allreactions the sodium ions are reduced in the nickelate, in which alkali metals are produced would seem From the stondpoint ot over-all reactions it might to be of little in:portance, Nevertheless, such be concluded that (1) mild reducing agents such reactions may be quite significant in the kinetics as nicked act first on hydroxy! ions and when they of corrosion. are reduced to a low concentration, sodium ions Corrosion by Oxidizing Solutes. - Dissolved are attacked; (2) very strong reducing agents like oxygen and the peroxide ion are known to Le zirconturn act on both kinds of ions at the some powerful oxidizing agents 10 fused hydroxides. By time. However, the experimental findings are also using such solutions, Dyer, Borie, and Smith35 consistent with the postulate that hydroxyl and were able to cause very rapid corrosion of nickel sodium ions are simultaneously .educed by all with the production of oxysalts in a valence state metals, Sodium-ion reduction may be represented greater than 2. as Water at sutficrent seems to act as rg weak oxidizing agent. Peoples, Miller, and (4.6) Hannan3 reported that at 950°C small additiaiis The sodium thus generated con react with hydroxyl of water to the blanketing atmosphere over fused ions as follows: sodium hydroxide had Iittle effect on the corrosion rate of nickel but that substantial amounts of water made hecorrosion much more severe. If reaction 4.7 proceeds as rapidly as sodium is Corrosion of Alloys. - Although nickel has a generaied, the net result is as though only hydroxyl superior corrosion resistance in fused hydroxides ions hod been reduced. As the concentrotion of at high temperatures, it has a low mechanical hydroxyl ions decreases, reaction 4,7 will proceed strength. Gnsequenfiy, studies have been made more slowly, until finally sodium is generated of the effect of alloying constituents on corrosion faster than it is ccnsumed. In the case of the very behavior with the ultimate aim of obtaining a mo- active metals such as zirconium, the rate ~f gener- teriai which is both corrosion-resistant and use- ation oC sodium may be much more rapid than the fully strong. rate of reaction 4.7 even for a high concentration There is no evidence as yet that the corrosion of of hydroxyl ions. Petersen and Smith studied alloys by twsed hydroxides ~IWQ~V~Sany kind of the reduction of sodium hydroxide by sodium orid cheniicoi reaction which is different from that sn- found that it was not by any means so rapid as countered with elemental metals. However, an reduction by such active metais as calcium. examination of microstructures shows two corrosion

11 phenomena distinctive of alloys: (1) the formation metal but also dew-lopsd subsurface voids to a of pores or voids beneath the sutfuce of the cor- depdr sf 3 to 5 x lom3in. roded alloy and (2) the fonnation of complex, two- Subsurface void formation is not an uncommon phase, corrosion products at the surface and along phenoaenon in high-temperature corrosion. Manly the grain boundaries of the corroded alloy. Ex- and Grant36 proposed o rncchnnism for this form of amples of each of these forms of corrosion will be corrosion based on :he well-established theory of cited below from the work of Smith, Steidlitz, and void formation during interdiffusion of metals. Ho Bfinan. Bra~unas~~hos discussed this theory and has The formation of pores or subsurfacc voids was presrriicd some pwtirlent experimental data. Quali- found to be characteristic of the attack of fused tatively, the theos- can be applied to hydroxide sodium hydroxide on highpurity nickel-iron alloys. corrosion of nickel-iron ullsys as follows. During Nickel exposed to fused sodium hydroxide at 815'C the early stages of corrosion, the iron atoms at the for 100 hr in an evucuerfed capsule was found to he surface of the metal specimen are oxidized much slightly etched, with an average loss in thickness more rapidly than the nickel atoms. As a result, of 4.5 x in. as computed from weight-change the surface becomes depleted in iron. This deple- data. Iron under the same conditions :vas found tion establishes a concentration gradient in the to lase about 100 tirntes as much in thickness. A proper direction to ccmse a diffusion of iron from high-purify nickel-iron alloy containing 2Q wt % tho interior of the specimen to the surface, where iron not only suffered cnttuck by removal of surface reaction continues. The diffusion of iron is pre-

mplex Grrosian P~eductLayer Fsrmed an ~~~~~~~ of Carpenter Compensator 30 Alloy (M~misra!~~~~~~i~~~~ 30% Mi, 78% Fe) by Action of Fused NaBH for 100 br at 815"C, 250X. (From the studies of Smith, Steidlitr, and Hoffman, ORNL)

12 sumed to oeciir by a vacanbioitice-site mechisin a specimen of Carpenter Comaansator 30 cs~loy so that there is a flux of vacancies diffusing from (nominal composition 30% i, 70% Fe) exposed to the surface into the interior of the specimen. Since fused sodium hydroxide for 100 l-2 at 815OC. The the crgpst~l lattice cannot maintain more than a corrosion paoduct layer consisted of three zones: certain concentration of vacant lattice sites, the (1) an irregular, gray layer of nonmetallic rnotertoi excess vacancies, which occumuiate from inward in contact with tile fused hydroxide; (2) beneath diffusion, “precipitateo9 as voids. These voids this layer, a zane consisting of a mixture of rne- continue to grow as long as the inward flux is tallic and nonmetallic phases; (3) lastt a zone of m ai n ta ined. intergranular penetration. The +carmation af such Subsurface void formation hos been found not complex iaye~sof corrosion products was charec- only in high-purity nickel-iron alloys but also teristic of the corrosion of u number OF a!ioys, in- in high-purity nickel-molybdenum and nickel-iron- c lud i n g the coinmi erc i a/ ni c keI- I am-c hrom i u m a I I oy s. moiybdenum alloys with a high nickel content. Very few alloys show all three zones seen in Pure molybdenum under he same test conditions Fig. 1. Usually the zone OB massive nonmetallic was severely attacked. material is absent, and, under suitable conditions, As an example of the second kind of corrosion one of the other twa zones may be absent. phenomenon characteristic of the corrosion of Figure 2 shows the corrosion product formed on alloys by fused hydroxides, Fig. 1 shows in cross Timken 35-15 allay (nominal cornposition 35% Ni, section a corrosion product layer which formed on 15% Cr, 50% Fe) after exposure to fused sodium

Fig. 2. Corrosion Product Layer Formed on Timken 3515 Alloy (Nominal Composition 35% Hi, 15% Cr, 50% Fe) by the Action of fused NoOH for 100 hr at 815O6 IOOX. (From the studies of Smith, Steidlitz, and Hoffman, ORNL)

13

______._...... ~ ...... - hydroxide for la0 hr nt 815OC. Here, only the zone fncoi;e! showed a slight amount of corrosion after consisting of a metnIRic and nonmetallic phose is 100 hr at 600T. From $00 to 8OOOC the corrosion to be seen, with a rudimentary grain boundary at- increased very rapidly. At all temperatutes cor- tack appeaiing at the corrosiorl product-base alloy rosion began as gwin boundary attack, on example in te sface. of which is shown in Fig. 3. When these grain Of the various rnctalr; which forin complex cor- boundary regions were examined at a high mognifi- rosion-product layers in fused hydroxides, only cation, it wns found hat a corrasion product had lnconei has been extensively studied. Resuits formed in these regions and that this corrosion with this metal illustrate the structural coinplexity product consisted of Q metallic matrix within which of the layers. Most of the tests have heen con- were acicular particles of a nonmetallic phose. ducted for various times up to 100 hr, at various Ah: grain boundary altack had advanced a few temperatures up to 8QO°C, and under blanketing thousandths of an inch into the alloy, a massive, atmospheres sf purified helium and of purified hy= two-phase, corrosion product layer had begun to drogen at 1 ah. The results obtained for tests fonn on the suiface and to thicken with Pime. Fig- under helium were quite different from those ob- ure 4 shows the microstructure of this fvw-phase tained for tests under hydrogen with regard to bo& layer at a high magnification. corrosion rate and microstructure of the corrosion The corrosion rate of lnconel at a given tempera- product. ture was significantly greater under a blanketing Under a blanketing atmosphere of hydrogen, atmosphere of helium than under hydrogen. A mas-

&? **

14 Sl omdon lrncsne! by Exposure to Fused H Under Q Blanketing Atmosphere af Helium for Pwiods 04 up $0 100 hr at Temperatures above 650°C. The cu plate was applied after test to protect the edge of the specimen during metallographic polishing. 1000X. (From the studies of Smith, Stridlitm, and Hoffman, ORNL)

The chemistry of the reaction between nickel and 5. MASS TRANSFER sodium hydroxide has been extensively investi- When liquids are circulated through metal plumb- gated but is still not well understood. For all ing systems at high temperatures, it is often found metals other than nickel, details on the chemistry that the hot parts of the plumbing are corroded and of corrosion reactions in fused hydroxides are very that the metal thus removed is deposited in the fragmentary. cool parts of the system. This corrosion-related The corrosion of allays by fused sodium hydrox- phenomenon, usually called “mass transfer,” is a ide does not seem to involve any kind of reaction particularly serious problem with fused hydroxides. which is different from the reactions encountered Mass transfer has been observed in sodium hydrox- with elemental metals. However, he inicrostwc- ide for nickel, iron, copper, silver, gold, and a tures of corroded alloys show the distiiictive cor- nuinbar of alloys. Moreover, nickel has been ob- rosion phenomena of the formation of subsurface served to mass-transfer in all the fused alkali- voids and the formation of complex, tws-phase COP metal hydroxides and, it might he mentioned, in rosion products at the surface of he alloy and nil the fused alkaline-eo& hydroxide^.^^ Mass along grain boundaries near the surface. transfer is the primary factor limiting the use of

16 metals which, under suitable conditions, do not corrode seriously. The mechanisms of mass transfer in fused hy- droxides are not known. However, several pos- sibilities exist, and they will be discussed at the end of this section. First, however, some of the empirical facts of mass transfer will be presented. Nickel. - Some of the characteristics of mass transfer of nickel in fused hydroxides are illus- trated with the results of thermal-convection loop test^.^^^^^ Figure 6 is a photograph of a nickel thermal-convection loop which was cut open to show the appearance of the inside wall surfaces aher test. During test the loop was filled with fused sodium hydroxide and placed in on upright position. One side of the loop was heated and the other cooled to maintain the temperature distribu- tion shown in Fig. 6. The highest temperature measured was 825"C, near the bottom of the hot leg (left side in Fig. 6), and the lowest tempera- ture measured was 535OC, near the bottom of the cold leg (right side in Fig. 6). This temperature distribution caused a flow of fused hydroxide around the loop because of thermal convection. The numbers in Fig. 6 from 5 to 45, at intervals of 5, identify positions around the loop. Before the loop was exposed to the hydroxide, the inside walls were somewhat rough and had a dull appearance. After test, as seen in Fig. 6, the hot leg from position 25 to position 45 became polished, whereas the cold leg from position 5 to position 20 became encrusted with a deposit of dendritic nickel crystals. Grain boundary grooving occurred in the polished zone, but the groove angle was wide and there was no serious grain boundary attack. The polishing observed in the hot leg is Fig. 6. Nickel Thermal-Convection Loap Cut characteristic of the corrosion step of mass trans- Open to Show Appearance of hide Wail Surfaces fer at high tempemtures in a number of different After Test. The numbers 5 through 45 identify kinds systems, including some containing liquid of positions around the loop. (From the studies of metals. Smith, Cathcart, and Bridges, ORNL) Figure 7 shows a deposit of dendritic nickel crystals from a cold-leg section of a loop with the sotidified hydroxide intact. In the initial stages of very long dendrites begun to appear, Because Qf deposition it was found that the crystals grew as a the dendritic brm of the thicker deposits, 0 rela- dense deposit, forming an almost continuous plate tively small amount of nickel was found to have a over the cold-leg surface. The crystal grains in large effect in restricting flow through a pipe. this plate frequently continued the orientation of When the flow of liquid was sufficiently rapid, the grains in the base metal, which necessitated dendrites became dislodged and they collected in the LJS~of special techniques for metallographic irregular mosses which formed plugs that effec- deternilnations of the place at which the base metal tively stopped fluid flow altogether. Figure 8 ended and he deposit began. When this dense shows a section of pipe from Q Barge loop which plate became several thousandths of an inch thick, contained several,such plugs, and Fig. 9 shows a UNCLASSIFIED Y -5543

2 3

Fig. 7. Section of Tubing Removed from the Cool Parts of a Nickel Th~~rna~~~~nv~~~~~nLoop and Cut Open After Test with the Frozen NaOH Intoet. Dendritic nickel crystals may be seen attached to the inside WQIISof the tube. (From the studies of Smith, Cathcart, and Bridges, ORNL)

Fig. $. Section of Nickel Pipe Cut from the Cool Portions of Q ~~~~ma~-~o~~~c~io~Loop llrregular masses of nickel dendrites which stopped the Flow of fwsed NaOH in the pipe may be seen. (From the studies of G. M. Adomson, ORNL)

18 tem, while Smith, Steidlitz, and Hoffman micro- scopically measured the thickness of deposits formed in the coldest parts of the system. Never- theless, confirmatory resuits were obtained when the two studies overlapped. Nickel, because of its excellent corrosion re- s istance, has received more attention than other metals in mass-transfer research. Studies have been made of the effect of several variables an the rote of mass transfer of nickel in fused sodium hydroxide. These variables were temperature level of the system, temperature differential, time of op- eration, composition ~f the atmosphere over the melt, and composition of the melt itself. Lad and Simon4' and Smith, Steidlitz, and Hoff- man3' found that the rate of mass transfer accel- erated with linearly increasing temperature level, Smith, Steidlitz, and Hoffman found that mass transfer of nickel in fused sodium hydroxide under helium could be detected by their techniques after 100 hr at a maximum system temperature of 6OO0c and a temperature differential of lOoOC but not at Fig. 9. Plug Formed of Nickel Dendrites Taken a maximum system temperature of 550°C, other from the Section of Nickel Pipe Shown in Fig. 8. conditions being constant. The lowest maximum The plug was about 1 in. across. (From the studies system temperature for which Lad and Simon re- ported data was 538*C, at which temperature under of 6. M. Adamson, ORNL) helium with a temperature differential of 46OC they found a very small amount of mass transfer after single plug removed from the loop. about 100 hr. Because the corrosion accompanying mass trans- The studies by Lad and Simon showed that, fer tended to occur uniformly, while deposition under helium, mass transfer of nickel was de- produced long dendrites, the constriction of flow in pendent upon the time of operation and the tempera- the cool parts of the system usually reached seri- ture differential. They found that the rate of trans- ous proportions lony before the corrosion in the fer decreased slightly over the first 50 hr and hot parts of the system became troublesome. thereafter remained constant. The initial tronsient Nearly all studies of mass transfer in fused hy- in the rate agreed with the change of nickel con- droxides have been carried out in thermal-convec- centration in the melt; both rates built up to a tion systems, although same forced-circulation steady-state value after 100 hr. The rote of trans- work has been rep~rted.~' Several different de- fer increased rapidly with temperature differential s igns of thermal-convection apparatus and several and then became independent of the differential at different methods of measuring mass transfer have higher values. Lad and Simon suggested that at been used. The only extensive studies in which higher temperature differentials the rate of flow numerical values of the amount of mass transfer in their apparatus was too great to permit complete were obtained were the work of Lad and Simon4' saturation of the fluid with nickel compounds. on the mass transfer of nickel in sadium hydroxide Because of the ability of hydrogen to suppress and the work of Smith, Steidlitz, and Hoffman3' on corrosion it might be expected that this element several metals and alloys in sodium hydroxide. would be effective in inhibiting mass transfer. These two groups used not only different designs This inhibiting effect was demonstroted by Smith, of therma I-c onvecti an apparatus but a Is o d ifferen t Cathcart, and Bridges39 in 195'3, but only recently mans of measuring mass transfer. Lad and Simon ha5 it been subjected to extensive quantitative measured mass transfer hy determining the weight test. Lad and Simon found that at 81YC the loss of a specimen in the hottest part of the sys- inhibiting action of hydrogen on the mass transfer

19 of nickel was considercblr. Smith, SteidBitz, and fused sodium hydroxide under hydrogen, but at Hoffman confirmed this result for temperatures elevotcd temperatures coirosion is a serious prob- bebw 81Tc and found that the meximuin system lem with these metals whether ~QSStransfer occur5 temperature at which no mass transfer was rileas- or tiof. Monel also showed3' mass transfer under urable WQS about 75°C: above that found under both hydrogen and helium. helium in comparable tests. In addifinn, Lad and The most extensive studies of muss transfer of Sininn found that 3% water vapor in the blanketing an alloy have Ocen those made on lncoiae1.30 It atmosphere is beneficial with and without hydrogen was found that Incone! showed corrosion in fused at 81S°C, sodium hydroxide at roughly the same rete whether Lad and Simon4' studied the effect an mass mass transfer took plmx or not and that mass transfer at 815°C: of additions to the melt of t~pto trimsfcur took place at roughly the SDM~rate as for 5 wt 76 of 33 substances. Many of the iiiaterials pure nickel under the same conditions. Mass- which they added re5ct with sodium hydroxide at traiisfer deposits formed in lnconel systems con- this temperature; sorne are soluble, while co me taining fused sodium hydroxide under an atmosphere are insoluble and relatively inert. SodicJiri car- of helium viere virtually devoid of chromium. Hy- bonate, a common contaminant in commercial drogen suppressed IIIQSS transfer of lnconel with sodium hydroxide, was found to have no effect for about the some effectiveness os it did for pure additions up to 1 wt %. Above this concentration, nickel. sodium carbonate WQS found to accelerate CRUSS Binaetallic Effects, '- When two different solid transfer, Sodium chloride and sodium orthophos- metals ore immersed in a liquid - for example, a phate, both soluble, had no effect. Sodium and liquid metal - under isothermal conditions at high hydrides had vary detrimental effects, Lad tampcratures, it is frequently observed hat one of and Simon suggested that- these compounds react to the solid metals contnrninates the other metal or form sodium oxide whish is "very corrosive." that the metals contaminate each other. This tiowever, tests made by adding sodium oxide, as process of metal tionsport without a temperature such, caused only about half os much mass transfer differential is usually referred to os "isothermaI as wa5 criiised by the addition of omounts of hy- mass transfer.'^^' In the case of liquid-nietaI dride which would give roiighiy the same oxide systems, the driving force for isothermal mass concentration after reaction. transfer seems to be the tendency of the two solid Other Elemental MetoBr. - Data on elemental metals to form alloys. metals other than nickel are meager. The moss A number of experiments have been conducted in transfer of iron in fused sodium hydroxide has been which two different metallic elements were im- found to be mie severe than that of nickel under mersed in the same hydroxide melt under pre- compatable conditions. For example, Smith, Stcid- sumably isothemal conditions. LeEblanc and litz, and I-~offrnar~~'found that under hydrogeri at ~ergmann~~performed i7 series of experiments in 5m"C the nass transfer of iron was worse than which different metals were immersed in di~sed that of nickel at 800°C, other conditions being sodium hydroxide contained in u gold crucible ot constant. However, iron was not observed to form 70oOC under nitrogen. When copper was immersed the long dendrites $hot were found for nickel. in the malt, they found that the gold crucible be- Rather, ?he crystals in the iron deposits remained came alloyed with copper; when silver was im- soniswhat equiaxed in habit. mersed, the silver specimen became alloyed with Copper in sodium hydroxide under hydrogen was gold; but when nickel was immersed, nothing was found3' to mass-transfer appreciably in 100 hr at observed to happen. ErasiJnas28 heated sodium 600°C- with a temperature differential of 100°C. hydroxide i:i an evacuated nickel capsule together I hus, copper moss-transfers mere rendily than with a copper specimen at 815OC. He reported nickel under these conditions but not so severely that nickel plated out an the copper. as iron. It would Sa tempting to conclude that the in- The mass transfer of silver and of gold has not stances of metal transport cited above arc onolo- been quantitatively studied, but it has been ob- gous to isothermal mass transfer in liquid metal served repeatedly. systcmrj if it were not for some studies by WiIIiaoriis Mhp. - Hastellay B and the stainless steels and MiIIer.2Q They immersed strips of nickel in have been observed3' to undergo inass transfer in fused sodium hydroxide contained in a gold vessel

20 under a hydrogen atmosphere and found that the la is postulated here, however, that metals have nickel became gold-plated. However, they gave a negligible solubility as metal atoms in fused convincing evidence that this plating occurred only hydroxides. This pos+uiate is dlificuIt to iustjfy during cooling. This observation shows that the by Bosrnel argument, Nevertheless, the differences transfer of gold to nickel wh~chwas observed was which exist in the internal prsgsures and cn the not an example of isothermal inass transfer, nl- binding forces for metals and fused hydroxides though Williams and Miller’s techniques wou strongly suggest that the solubility of the metal have detected small amaunts of alloying which as otorris in the hsed hydroxide will be exceed- might have taken place in addition to the much ingly small. grosser plating effect, There are no direct experimentill studies of the Craighead, Smith, arid JaFfee2’ report the trans- solr~bility of any metals in Fused hydroxides. fer of nickel from a container vessel onto D number There is indirect evidence’ 3fiTB that sodium metal of metals immersed in sodium hydroxide at 677 and has u siynrflcmt solubility in fused sodium hy- 8 15OC. However, they point out that thertnad gradi- droxide and that It dissolves before reacting. This ents, which were known to exist, could have ex- phenomenon appears to &e analogous to the solu- plained their results, tion of a metal in its fused hai~de.~~This process, however# not simply case the solution of Mechanism of Mass Transfer. - The mechanisin IS a of ineta! atoms but dependent some interaction of mass transfer of metals in fused hydraxides IS IS on of not known nor do the proper kinds of data exist between tile valence electrons the metal atoms which are necessary to distinguisia amang several and the ions of ttie same metal in the melt. The dissolving of sodium in a sodium halide and possible mechanisms. The purpose of the faCajlGw- ing discussion isto point out the various processes possibly in sodium hydroxide can be schematically by which a chemically reactive fused electrolyte represented by the equation such as sodium hydroxide might transport metals under IIie influence of a temperature differential. These mechanisms are all possible in the sense where the electrons O(meh) may be thought of as II that they do not violate known principles. How- excess” or electrons which are more ever, they are all quite speculative. or less associated with the cations of the melt. In It is assumed that the mode of metal transport some instances he dissolution of a mt?tQl in its involves either solution of the container material fused halide seems to involve the formation of a as metal atoms or its solution os metal ions plus diaiomic cation; for example, Cd2’+ 1s formed electrons. If the solute consists of metal atoms, when cadmium is dissolved in cadmium chloride. mass transfer could be considered in tenns of diC It is possible, in principle, for any metal to dis- ferential solubility. If the solute consists of rnetuI solve to some extent in a fused electrolyte as metal ions plus electrons, several possibilities exist, ions plus excess electrons which may Le con- depending on he mechanism of transport or the sidered as more or less associated with the cations electrons. These various possibilities will be of the electrolyte. This process will be discussed discussed in the following sections. further in the section “Mechanisn, lV.*’ Differentia1 Solubility (Mechanism - In a Oxidation-Reductian Processes (Mechanisms 11- superficial way the mass transfer of metals in VI). - The mechanisms considered most probable for mass transfer in fused hydroxides all involve fused hydroxides appears to obey CI simple solu- bility-temperature relation. Mass transfer in liquid- oxidation-reduction processes. The two basic steps metal systems usually follows such a relation, and for all such processes may be schematically repre- v it can also, in principle, applied to fused hy- sented for a metal M of valence by the following be At hot droxide systems. Every metal must have a finite equations, the metal-melt interfaces there occurs the oxidation solubility in a fused hydroxide, just as every metal must have a finite at ai/ tempera- tures, although this solubility may be so small as (5.3) to have only a statistical meaning. Symbolrcally this process may be represented as where 0 indicates an electron and where MV(melt) represents a metal in whatever form in which it may (5.1) M(s) = Mfmelt) he stable in the melt, that is, us an uncornplexed

21 cation or as a suitable oxysalt anion. At the cool and that, at least in the case of. nickel, Mv(mtslt) metal-melt interfaces there occurs the reduction ions may be reduced to the inetal again by hydra- gen. It would not be surprising, therefore, to find cool (5.4) Mv(melt) + 1.4 > M(s) that hydrogen rnolasii les serve to transport elec- trons, the hydcogon acting together with an oxide which is the reverse of Eq. 5.3. For all kinds of ion as aa electron donor conjugate to a hydroxyl oxidation-rediiction mechanisms the b+\”(melt) ions ion. ’ This 1nezhai7ism was proposed independently will be trarisportcd by diffusion through boundary by Skinner4’ in 1951 and by Williams and Miller in layers at the interfaces and hy fluid flow within 1952. Since then it has received wide acceptance the bulk of the melt, HOW~VW,the electroils may as the only mechanism for the mass transfer of> move by two quite different routes. They may be nickel. conducted through the metallic parts of the system, There is no doubt that Eq. 5.5 represents o likely or {hey iimy be transported through he malt by mechanism of mass transfa: yndes many conditions. some chemical species which is reduced at the hot However, the evidence in favor of this mechanism metal surfaces and oxidized ot the cool surfaces. is ambiguous, and mass transfer has been observed The first method of electron transport represents to occur mder conditions where the operation of local cell action. The second method inight he this rnechilnisrri seems doubtful. ‘lhe evidence most effected by any of a number of chemical spccies. frequently cited in its support is thc effectiveness It is convenient to classify these species into four of a blanketing atmosphere of hydrogen in suppress- groups according to the oxidized form of the elec- ing mass transfer. However, regardless of the tron carrier: hydroxyl ions, sodium ions, foreign nature of the electran carriers, hydrogen should, to solutes, and a higher -valence state of the metal a greater or lesser extent, suppress all oxidation undergoing transport. I hese five modes of elect:on processes in fused hydroxides and thereby depress transport may be considered as representing five the concentration of nickel ions moving through the additional mas s-ti-ail sfcr mechan isms. f Iu id, Furthermore, Smith, Cathcart, and Bridges39 L~calCell Action (Mt.rhrrniam !a). - If the elec- have conducted several kinds of experiments in tions we transpo;ted through the metallic parts of which nickel preferentially deposited on a surface the system, Eq. 5.3 represents the anodic dissolu- at which the hydroxide was saturated with otinos- tion of MI and Eq, .jid the cathodic deposition of M. pheric oxygen. In these experiments mass transfer The driving force for this process con be thought cannot he accoi.inted for hy Eq. 5.5. Accsrding to of as (s ‘IIieriiioCIectric potential. Such potentids this equation, hydrogen must be present at the have been measured in fused electrolyte^.^^ In interfsce of deposition at o conce~trationgreater ~rd~for such a process to tuke place at all, thce thai the miiiiniurn necessary to reduce Mv(meIt) transported ions M”(melt) must be already present. ions. In the experiments of Smith, Catlricart, and However, any oxidizing substunce, including the Bridges, this condition could not have heen ful- hydroxide itself, could serve this purpose. filled. In other words, if mass transfer occurred Inasmuch as Pray and Millers2 have induced only according to Eq. 5.5, the absence of hydrogen nickel to deposit preferentially on an /?Plundtim at the hot surfaces would accelerate the solution ring immersed iri a sodium hydroxide melt, local step, but an absence of hydrogen at the cool sur- cell action canriot be the exclusive mode of mass faces-.- would likewise prevcnf the deposition step. transfer. I-iowever, there is no reason why local 1 he quantitative specification of the hydrogen cell action might not be the primary niechanism mnricsntrations or, via Henry’s law, of the hydrogen under sei ected corid it ions, pressures necessary to cause Iliass transfer under Reduction of Hydroxyl Dons (Mechanism 111). - given circumstances is dependent, among other In Sec. “Corrosion of it was pointed 4, Metadls,” things, on the temperatures and nickel concentro- out that the most common over-all corrosion reac- tions at both hot and cold interfaces, Data are not tion wos the reduction of hydroxyl ions iepreserited yet available with which to make such specifica- by Eq. 5.5 tions, but outstanding progress is being wade toward this end for the case of nickel by Kertesz, M(s) vOH-(rnc.lt) = M”(msIt) (5.5) + Knox, and Iron ha5 been shown30 to undergo mass transfer in SO~~~JIIIhydroxide much more rapidly than nickel did under the same conditions. Idowever, WilB~rms kinetics of Merrranism IF/. and Miller2’ hove pointed out that the reaction ho- ~~~~~%~~~ OF sosLDte5 { echanism V). - In some tween iron and sodium hydroxide is nor rsvsnsed or instances dr ssaB~.ied substances wlaich are more even inhibited by hydrogen at pressures up to 1 atm. eosiiy reduead than fhe fused hydroxide may serve It is, therefore, doubtful that hydtogen is ail rm- as electron acceptors in the mass-trcansfer inecho- portant electron carrier in this qnstancc. nism, Such a substance migi18 he the ions of Reduction cf Alkali Meta another mefa!, A, with two avai labie valence states IV). - It does not seem likely that alkali metal y and y - 1. Then a reaction such as ions would be effective agents rn niilss tmnsier rs.8) kliis) -+ z~~(rna~t)= ~~jmeil*\l .+ vAY-’(melt) under most conditions, because hey are thermo- dynamically inferior oxidizing agents compared wouId he *o occur. Displacement of this with oiher species such as hydroxyl ions. expecjsd possible equilibrium could cause tsncister. This mech- On the other hand, alkali metab ions arid alkali mass anesm nai accr3unt for the wass transfer ob- metal atoms could, at least in psincilpis, act as WIBI served in mast of the systems investigated, since conjugate electron acceptor-donor pairs for electron suitably high cancentrotions af anions A7 do not transpart in fused hydroxides. Therefore, +his usually exist. However, such ions present, possible mechanism will drscussed sf. are be briefly. it IS possible that niass tronsfer would be acceler- Every metal must be capable of effectang the crted, reduction of a fincte number of sodiutii ~n fused ians One source of ions ~iA is from impuri- sodium hydroxide. Hence, it is possible for moss possible ties in the original sample of nie~al undergoing transfer to occur by the following process: mass transfer, ~itliamsond ~siier~’have shown thut iron is selectively leached from the surfaces (5.6) M(s) 4 vNa’(melt) -=_ MY(rneIt) 4- vNa(melt) af samples of coinmercial nickel which originally where Na(meIt) is to be ccnsidered os sodium ions corituined 8,89 0.41 X iron as an impurity. plus excess electrons, as discussed tinder Meclia- i 5 pix0parti onati on (Mechran i om V nism I. However, sodium dissolved in fused sodium M exists in more than one valence state, it is pos- hydroxide will reacf with hydroxyl ions: s ibie far disproportionation to CalI5e mass transfer. For example, with M having the valence states Y

The reverse of this reaction is, also knuwngQ7SQ %-orthis reactson to OCCLIT, ions of M musf be pres- that Eq. 5.7 may be taken as representing ern ent in the malt, CIS was true for the local cell equilibrium. (For reasons of siniplicity, the inter- merlaamni.sm. Any of the oxidation mechanisms just mediote equilibrium involving hydride ions is descriLwd could generate them, The possibility omitted.) Therefore, if local thermodynamic equi fi- that (djsproporticsnation might serve os a mechanism brium exists in the melt, both Eqs. 5.6 and 5.7 Ser mass transier in fused hydroxides was first must be simultaneously satisfied, and the over-all pointed out by Grimes and Will.4B equilibrium (the summation of Eqs, 5,6 and 5.7) It was noted above that liydrogeii is an unlikely becomes identical with that represended by Eg. 5.5. electsan carrier in the mass transfee. of iran, How- Therefore, if local thermodynurnic equilibrium ever, a disproportionation mechanism might be exists, Mechanism IV is the same as Mechanism very eS6ective irz this case. Little is known about Ill; that is, the same end 9ssult is achieved if it is the valence states of iron which are stable. in fused the assumed that either the sodium ions or the hydroxyl hydruxldssp but indications are that more than ions are the electron acceptors. On the other hand, one valence stuje occurs. Dahs ond Laffler16 the reaction between sodium metal and sodium hy- report that, under an atmosphere of nitrogen, re.ac?s droxide has been found to be surprisingly slow,d8 iron(ill) oxide with sodium hydrmjde to give and it is possible that Nu(rnelt) is not iri local rnosfly sadiisrn fe~~~~~~~i~)~with CI small amount of thermodynamic equilibrium according to Eq. 5-7- axysalt its II higher valence state, which they sug- Under this latter circumstance, the kinetics of gest may be sodium ferrate(V1). Thus it is pos- Mechanism IV could be quite different from the s &le that n ~~~~r~~~~t~o~~t~~nprocess takes place

23 involving iron(ll1) and iron(VJ). Under reducing mechanism. Observations of mass transfer of conditions, such as under a blanketing atmosphere nickel in sodium hydroxide saturated with atmos- of hydrogen, iron(l1) and iron(ll1) might be the pheric oxygen were cited in Sec. 5, “Mass Trans- active valence states. fer - Nickel.” From what little is known,35 it ap- Nickel has been to occur in both the pears that the presence of nickel(ll1) ions in fused valence states II and Ill in fused sodium hydroxide hydroxides is favored by oxidizing conditions, and in the presence of oxidizing agents. It has also in the absence of these conditions its concentration been found35 that under suitable conditions, the is quite low. ratio of nickel(l1) to nickel(ll1) in the fused hy- Summary of Mechanisms. - Six mechanisms were droxide changed with thermal cycling in the way discussed as possible modes of metal transport in required by a mechanism based on Eq, 5.9. The fused hydraxide medica. These mechanisms ore absence of hydrogen at the cool interfaces does not summarized, and some of their interrelations shown interfere with mass transfer by this mechanism. by the following outline. The symbolic processes Consequently, it is possible that mass transfer of and reactions presented in this summary are to be nickel in a hydroxide saturated with atmospheric considered as taking place from left to right at the oxygen may take place by a disproportionation hot mefaI-melt interfaces. The interpretation of the symbols WQS discussed in the preceding section.

A. Transport as metal atoms, that is, differential solubility - Mechanism I

M(a) M(rnelt) (See Eq. 5.1) 6. Transport as metal atoms plus electrons M(s) eMv(me!t) i- v6 (See Eqs. 5.3, 5.4)

1. Electrons transported by metallic conduction, that is, local cell action - Mechanism II dhot metal) --+ cool metal)

2. Electrons transported by the reduced form of some species in the melt a. Reduced form of hydroxyl ions - Mechanism Ill v I,@ + dH-(malt) &

Over-all reaction v M(s) + bQti-(melt) Mv(melt) + bQ?--(melt) t - H2(melt) (See Eq. 2 5.5)

b. Reduced form of sodium ions .- Mechunism IV + vNa+(rnel+)e vNa(melt)

Over-al I reaction

M(s) + vNa*(melt) eMv(melt) i vNa(rnelt) (See Eq. 5.6)

C. Reduced form of a solute Ay(me1t) - Mechanism V 18+ vAY(rnelt) ,-‘vAY*’(melt)

Over-all reaction

M(s) t vAY(rnelt),Irfr Mv(melt) + vAY-’(mel+) (See Eq. 5.8) d. Disproportionation - Mechanism VI v8 + vMv+’(melt) vMv(melt)

Over-all reaction

M(s) t vMv+’(melt) e(1) t 1) Mv(melt) (See Eq. 5.9)

24 Mechanism I, transport of metnl ntoms as SIJC~, in some way srach as his, it wolild be effective in was discounted inasmuch as the solubility of suppressing inass transfer by almost any mecha- metals as atoms in fused hydroxides wouId be ex- niS4ll. pected to be very small. Summary. - Mass transfer has been observe; in The remaining mechanisms, SI through VI, are all fersed sodium hydroxide for i>scka%,ii-011, copses, oxidation-reduction processes, No si~iyleone of sil~e~.~and a number a6 alloys. kd~reover, nickel these mechanisms wi II account for all observations has keen observed to mass-transfer in all the fused of metal transport in fused hydroxide media. It alkali-metal and alkaiine-earth hydroxides. In $he seems that 110 one process is always the exclusive ma j ority of investigation5 under*oken, mass transfer mode of transport but rather that different mecb was and~rced by ca temperature differential in the nisms predorninete under different c~nditions. Much s yslem. further research will be needed before specific The sate of the mass transfer which takes place modes of metal transport can be proposed as the under the nnniiuerice of a terilwrature differential probable mechanisms of mass transfer under stipu- was found $0 be affected by the fallowing variables: lated conditions. nature of the metal undergoing transfer, composition Two substances, hydrogen and water, have been of the melt, composition of the atmosphere over the noted to suppress mass transfer of nickel in fused melt, temperature level of the system, temperature sodium hydroxide. This information cannot be diffeiential wittian the system, and geometry of the cited in support of any particular one of the oxida- system. Undoubtedly, the rate ot fluid flaw is a tion-reduction mass-transfer mechanisms. Hydrogen very important factor, Gut existing data are in- should suppress the step all oxidation- corrosion for adequate for evaluation oi this variable. reduction me han i ms s s . Under a particular set of comparable conditions, The role of water in mass transfer is unknown. copt-~~was found to mass-transfer faster ntian It is possible that water functions to suppress the i-iicLel, and iron to ninss-transfer faster than copoer. oxide ion concentration in the melt and thereby ith alloy systerris, moss transfer and corrosion causes the corrosion steps to produce a film of were found to occur simultarieously. relatively insoluble nickel oxide. The action of mechanisms of mass transfer in fused hy- water might be expressed by an equilibrium such us ?he droxides are not known. Some 0f the possible NaNiO,(meIt) H28(melt) modes of mass transfer were examined, and five (5.10) t i: NiO(s) mechanisms were proposed as being possible. It -i 2NaOH(tnelt) was suggested that no one process is always the This film could function as D diffusion barrier ko exclusive mode of transport but that different slow down the corrosion step. if water were to act mech~lnismspredominate under different conditions.

25

REFERENCES

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31. R. S. Peoples, P. D. Miller, and H. D. Hannari, Kcactir,tls o/ Nickel in Moltrm Sodium ffy- dmxide, BMI-1041 (Sept. 1955). 32. M. A. Pray and P. Miller, private communication, Battelle Memoria! Institute (8952). 33. R. A. Lad, privafe communication, Lewis Flight Propulsion Laboratory (1955). 34. P, ViRlard, Cnmpt. rend. 193, 681 (19311, 35. L. D. Dyer, B. 5. Boric, and 6. P. Smith, J. Am. C:he,z. YOC. 76, 1499 (1954); and unpvb- I ished research, Oak Ridye National Laln~ratory(19531, 36. W. D. Manly and N. J. Grant, private communication, Oak Ridge National Laboratory (1951). 37. A. de$. Brasunas, Metal Progr. 82(6),88 (1952). 38. G. M. Adamson, private cominunication, Oak Ridge National Laboratoty (‘1951). 39. G. P. Smith, J. V, Catticart and W, H. Bridges, unpublished research, Oak Ridge National Laboratory (1951). 40. H. A. Lad and 5. L. Simon, Corrosion IO# 435 (1954). 41. W. D. Manly, “Fundamentals of Liquid MetaA CorrosionI” Corrosioa (in press). 42. M. LeBlanc and L. Bergmaim, Brr, deut, rbcm. Ges, 42, 4725 (1909). 43. M, A. Brediy, J. W. Johnson, and W. T. Smith, /. Am. Chem. Fvc. 77, 307 (11955);K. Griotheim, F, Gronvold, and J. Krogh-Nioe, j. Am. Ghcm. SOC. 77, 5824 (1955); E. Heymann, R. J. L. Martin, and M. F. K. Mulcaily, 1. Phy~.Chcm. 47, 473 (1943); 48, 159 (1944). 44. H. Reinhold, Z. nnorg, ullgenz. Chem. 171, 181 (1928). 45. E. N. Skinner, private communication, International Nickel Co. (1951)- 46. F. Kertesz, F. A. Knox, and W, H. Grimes, private communication, Oak Ridge National Laboratory (1955). 47. H. c\I. Gilbert, U. S, Patent 2,377,876, June 1945. 48. N. 0. Scott, U. S. Patent 2,202,270, May 1940. 49, W. R. Grimes and D. Hill, private communication, Oak Ridge National Laboratory (1951).

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