Chemistry and electrochemistry of hot corrosion of metals

Materials Science and Engineering, 87 (1987) 319-327

319

Chemistry and Electrochemistry of Hot Corrosion of Metals* ROBERT A. RAPP Department of Metallurgical Engineering, The Ohio State University, Columbus, OH 43210 (U.S.A.) (Received May 27, 1986)

ABSTRACT

gives

H o t corrosion beneath fused salt films may involve the fluxing o f the protective oxides as either acidic or basic solutes. High temperature Pourbaix-type diagrams can be used to interpret the basicity-dependent solubilities. The measured solubilities for NiO, Cos04, Al2Os, iron oxides, Cr20 s and Si02 in fused Na2S04 at 1200 K show remarkable agreement with the expected behavior. The presence o f strong acids and their salts, e.g. NaV03, in Na2S04 greatly increases the acid solubilities o f oxides and stabilizes (buffers) a more basic solution, Corrosive oxyanion fused salts are usually ionically conducting electrolytes so that the corrosion attack must exhibit an electrochemical mechanism. Various transient electrochemical measurements have identified $2072- (dissolved SOs) as the oxidizing agent which is reduced in hot corrosion attack. The chemical changes occurring at a polarized platinum electrode in Na2S04 are measured and interpreted.

log aN~o + log Pso3 -

1. INTRODUCTION Metals and alloys may experience accelerated oxidation when their surfaces are coated by a thin film of fused salt in an oxidizing gas. This mode of attack is called h o t corrosion, and the most usual or d o m i n a n t salt involved is Na2SO 4 because of its high t h e r m o d y n a m i c stability. The equilibrium constant at 1200 K for the dissociation reaction Na2SO 4 = N a 2 0 + SOs(g )

on

(1)

*Paper presented at the International Symposium High Temperature Corrosion, Universite de P r o -

vence, Marseille 13331, France, July 7-11, 1986. 0025-5416[87/$3.50

AG1 °

2.303RT = -- 16.7

(2)

This extreme stability of Na2SO4, with a K1 value nearly three orders of magnitude smaller than t h a t for water at 298 K, implies that chemical reactions involving the species Na20 or SO s could readily and importantly shift the chemistry of a pure Na2SO4 melt. Oxyanion fused salts exhibit an acid-base chemistry and, according to a Lewis-type description, the basicity of pure fused Na2SO4 can be defined as -- log as~o, or the acidity as the quantity + log Pso3. The use of these parameters avoids the ambiguity involved when the activities of ionic species are used to define basicity or acidity. Since the early studies by DeCrescente and Bornstein [1], Bornstein and DeCrescente [2, 3] and Goebel and coworkers [4, 5], most researchers would admit that the fluxing (dissolution) of one or more oxides comprising the protective scale is important, or at least involved, in the damage to the scale which results in accelerated kinetics. For a given deposit and a certain set of environmental conditions, h o t corrosion attack is then minimized by the selection of an alloy or coating which is able to form a protective oxide despite the presence of the salt film. Implicitly then, the selection process should include a match of the solubilities of the oxides involved in protective scales to the acid-base conditions imposed by the fused salt solvent in the given environment. Thermodynamic phase stability for an oxide in an M-Na-S-O system (M = metal) can be described by high temperature Pourbaix-type diagrams, and these can be used to interpret the basicity-dependent solubilities © Elsevier Sequoia/Printed in The Netherlands

320

of oxides. In this paper the detailed measurements recently published by Zhang [6] and Zhang and Rapp [7] of the solubilities of CruOa and iron oxides and by Shi and Rapp [8] of the SiO2 solubility in pure NazSO4 at 1200 K will be discussed. Similarly, new measurements and interpretation for the solubilities of CeO2, HfO2 and Y2Os in fused Na2SO,-30mol.%NaVO s offer insight into vanadate hot corrosion, and more generally into the acidic fluxing mechanism [9]. Corrosive fused salt deposits are usually ionicaUy conducting electrolytes; so the corrosion attack must exhibit an electrochemical mechanism, i.e. a spatial separation of the partial anodic and cathodic reactions. In this conference, Rahmel [10] has discussed the electrochemical theory and experimental testing of hot corrosion reactions. Various transient electrochemical measurements have identified $2072- (dissolved SO8) as the oxidizing agent which is reduced in hot corrosion attack. In this paper, electrochemical polarization studies of metals in deep salt melts are discussed and compared with the local chemical changes observed for the polarization of a non-reactive platinum electrode. For more detailed discussion of hot corrosion mechanisms, several review articles are recommended. [11-14].

2. STABILITYDIAGRAMS, OXIDE SOLUBILITIES AND SALT CHEMISTRY Prior to a discussion of the solutes of oxides in fused Na2SO4, examination should initially be made of the phase stability in the Na-S-O solvent system, including identification of the regimes of dominance for the minority solutes, as is shown in Fig. 1. As for the Pourbaix diagrams of E vs. pH for aqueous systems, the coordinates in Fig. 1 correspond to oxidizing potential and melt acidity. The auxiliary ordinate scales on the right-hand side provide values for the voltages of cells consisting of a working electrode in fused Na2SO4 and either a reference ZrO2 electrode (horizontal isopotentials) or a reference Na ÷ ion conducting electrode (sloping scale) [15]. The indication of predominance regimes for ionic species in Na2SO4 provides guidelines for the interpretation of possible chemical and electrochemical reactions in fused Na2SOa.

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(900 °c). The reaction of non~ominant species would always be expected to form the dominant ionic solute. This obvious rule emphasizes the interest and need to identify the coordinates in Fig. 1 for any fused Na2SO4 studies. The problem is complicated by the thin film geometry of hot corrosion which might support significant gradients in oxidizing potential and basicity. For several years, closed solid state electrodes have been used to measure the oxygen activity or sodium activity in fused melts of Na2SO4 at equilibrium. As explained later, recent work by Park and coworkers [16, 17] has clarified the interpretation of the cell voltages for melts containing gradients. At 1173 K the voltage between an inert working electrode (denoted by WE in parentheses after the parameters aNa~Oand Po~) in Na2SO4 and a sodium probe consisting of a silver wire immersed in a 0.9Na2SO4-0.1Ag2SO4 solution contained in a closed Na ÷ conducting tube (muUite, ~-Al2Os or Vycor) is given by RT [. aN~o(WE_).-I EI = 1.427 - - - - In (3) 2F [-(Po~(WE))1/2J Sloping isopotential lines (constant as~) are shown in Fig. 1. Similarly, at 1173 K the equilibrium voltage between a working electrode in fused Na2SO4 and a porous platinum electrode contacting air within a closed-end stabilized ZrO2 electrode is given by R T . [{Po~ (air)) 1/2aNa~o(WE ) ] En = m - - 2F L{Po~(WE)) 1/2aN~o(ZrO2-Na2S04)~ (4)

321 For an equilibrium melt lacking any basicity gradient, the two terms involving aNa~O cancel so that the ZrO2 electrode measures only Po, (WE), corresponding to the auxiliary ordinate scale of Fig. 1. However, if gradients exist in the melt, then the entire eqn. (4) must be used. As will be explained later, this fact permits the measurement of the shift in basicity at a porous working electrode (or corroding metal) attached to the ZrO2 tube. In any case, by subtracting Eli from El, by measuring the voltage between the internal lead wires of the two reference electrodes, the resulting voltage E m indicates the local basicity at the ZrO2-Na2SO4interface:

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Thus, combination of a pair of reference electrodes provides a measurement of the basicity of the melt, but only at the ZrO2-Na2SO 4 interface. For equilibrium melts, this limitation presents no problem. In earlier studies, these electrochemical probes have been used for the determination of the basicity dependences of the solubilities of the oxides NiO and CoaO4 [ 1 8 , 1 9 ] , Y2Oa [20] and AI20 s [21] in fused Na2SO 4 at 1200 K and an 02 pressure of 1 atm. The basicity dependences of the solubilities for the individual acidic and basic solutes of these oxides have been used for correlation with theoretical expectations to identify the nominal ionic solute species. The measurement and interpretation of the solubility of Cr203 in fused Na2SO 4 offered a special challenge because of the possibility of forming four different solute species, Cr2(SO4)3, CrS, Na2CrO 4 and NaCrO2, depending on basicity and Po:, as indicated in the stability diagram in Fig. 2. As is conventional, the broken lines in the field of Cr20 s stability indicate the activities for the various solutes. Zhang [6] measured the solubility of Cr20 s at 1200 K as a function of basicity for two different oxygen activities (1.01 X 105 and 3.19 X 10 -9 Pa). Figure 3 demonstrates the precision of those measurements for solute concentrations generally in the range from 100 ppm to 1 mol.%. The simple dissolution equations written in Fig. 3 can be compared with the experimental slopes to

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demonstrate both the ideal behavior of these dilute solutions (constant activity coefficients) and the exceptional utility of the electrochemical probes for basicity measurements. Indeed, the ratios of the known solute activities (Fig. 2) to the measured equilibrium

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solute concentrations provided values for the activity coefficients o f the solutes. Figure 4 is a plot of measured and calculated (extrapolated) values for Cr203 solubility in Na2SO4 throughout its range of existence in Fig. 2. Such a solubility map should prove to be important in interpreting any hot corrosion fluxing mechanisms, especially a mechanism involving gradients in the fused salt film [22]. As seen in Fig. 5, the solubility behavior of the iron oxides in fused Na2SO4 should be complicated both by the existence of four solutes and by two oxide phases (Fe203 and FesO4). However, atomic absorption measurements of equilibrium solute concentrations in combination with basicity measurements by two reference electrodes again provided extremely well-behaved solubility values which are compiled in Fig. 6 for the entire range of F e 2 0 3 and Fe304 stabilities [7]. Figure 7 divides the field of oxide stability into regimes of dominance for the iron oxide solutes. Once more, Figs. 6 and 7 should aid in the interpretation of hot corrosion mechanisms. By similar methods (but with greater analytical problems), the solubility of SiO2 in acidic Na2SO4 melts was also determined [8].

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323

ponding Na-Si-S-O stability diagram. The low SiO~ solubility and its independence of melt basicity (no solubility gradients in thin fused Na2SO 4 films) are consistent with the known excellent resistance of SiO 2 to acidic fluxing, Figure 8 presents a compilation of the solubilities in 02 at 1 atm of the oxides of greatest interest to the hot corrosion of engineering alloys by fused Na2SO 4. The difference of six orders of magnitude in basicity between the minima for the most basic Co304 to the acidic SIO2, Cr2Oa and A120 s

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is striking and is consistent with important known generalizations about alloys and coatings in hot corrosion. Cobalt alloys are more subject to acidic fluxing than are nickel-based alloys. In particular, Cr20 a is resistant to acidic fluxing because the minimum in its solubility curve corresponds approximately to the acidity of gas turbine environments. Figure 4 suggests that the solutes of Cr203 may be able to support a buffering action and to resist large shifts in the basicity of the fused salt film by changing the ratios of concentrations for the several Cr20a solutes. (Chromate fuel additives are effective in combating hot corrosion.) Because the solubility of Cr203 is higher as Na2Cr04 (external interface) than as NaCrO2 (internal interface), dissolution and reprecipitation of Cr20 a in the salt film cannot occur. Finally, although not evident from oxide solubility values, Cr20 a protective scales are preferable t o A12Oa, probably because of their faster growth rate which permits protection of the alloys at a steady state or more rapid recovery of protection after a scale failure. Recent measurements [ 9 ] of the solubilities at 1200 K of Ce02, HfO2 and Y2Oa in fused Na2SOa-30mol.%NaVOa and, for comparison, of CeO2 in pure Na2S04 have provided insight into both the corrosive effect of vanadate melts and the mechanism for acid fluxing [5]. Without presenting the original data, Fig. 9 is a plot comparing the solubilities of CeO2 in pure Na2SO4 and in Na2SO4-30mol.%NaVO 3 at 900 °C and Po~ = 1.01 X 105 Pa. In each

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14

324 case the slopes of the solubility curves for basic fluxing correspond to the dissolution reaction CeO2 + Na~O = Na2CeOs (6) and the magnitudes do not differ importantly, However, in pure Na2SO4 the acidic dissolution of CeO2 follows the reaction CeO 2 -{- 2Na2SO 4 = Ce(S04) 2 + 2 N a 2 0 (7) while in the sulfate-metavanadate melt the acidic dissolution reaction is 3CEO2 + 4NaVO3 = Ces(VO4)4 + 2 N a 2 0

(8)

whereby the magnitude of the acidic solubility is increased greatly over a wide regime of melt basicities. At the minimum solubility for CeO2 in pure Na2SO4, the solubility is increased by three orders of magnitude. The increase does not result because Ce 4÷ cations complex importantly with vanadate anions, but rather because the metavanadate anions strongly complex oxide ions to form orthovanadate anions. For this reason, the general effect of NaVOs on oxide solubility should be similar for all oxides. From Fig. 9, any h o t corrosion reaction which involves acidic oxide fluxing would be greatly assisted for nominally acidic melts by the presence of vanadate anions or, by inference, o f the salts of other strong acids such as molybdate, tungstate etc. This increase in solubility is probably not the only important difference introduced b y the strong acids, but it has not been demonstrated previously, Somewhat less obvious than the increased acidic solubilities of oxides is the implication of the measurements on the accepted mechanism for acidic fluxing in hot corrosion. Goebel e t al. [5] proposed that the introduction of a strong acid such as V205, MoO3 or WO3 into fused Na2SO 4 would serve to complex oxide ions and thereby to lead to a more acid solution in which the oxide solubilities would be higher. Implicitly the oxide solubility plot was considered to be fixed with respect to basicity and, as the acidity increased, the acidic solubility would increase. In fact, while strong acids do complex oxide anions, the resulting solutions are more basic than those for pure Na2 SO4 because the salts of strong acids are stable in more basic solutions than are weaker acids, (On the oxidation-reduction scale the equiva-

lent generalization is that the more stable oxides are stable in the most reduced environments.) The presence of V205 in Na2804 will generate equilibrium activity ratios of V205 to NaVO3 or NaVO3 to NasVO4 etc. which generally buffer more basic solutions than Na2SOa equilibrated with SO3. For example, at 1200 K a melt buffered with equal activities of NaVO s and NasVO 4 assumes a calculated value for log aNa20 equal to --8.57. Equal activities of V205 and NaVO s provide log aNa~O ------ 13.9. However, a melt of pure Na2SO 4 equilibrated with SO3 at I atm stabilizes a much more acid salt with log aN~O = --16.7. A revised acidic fluxing mechanism would state that the introduction of strongly acidic oxides into Na2SO4 (or other oxyanion salts) leads to the formation of anion complexes which increase the acidic solubilities of oxides while increasing the melt basicity.

3. ELECTROCHEMICAL STUDIES Because :pure or moderately d o p e d fused Na2SOa is an ionic conductor, electrochemical measurements in the laboratory can clarify certain aspects of the h o t corrosion mechanism. Earlier reviews should be referred to [14, 15, 23]. The determination of limiting cathodic currents on platinum beneath thin fused Na2SO 4 films in various O2-SO2-SO 3 gas mixtures proved that dissolved SO3 (as $2072ions) serves as the dominant oxidant in acidic environments and that the dissolution, transport and reduction of 02 are relatively small [24]. A.c. impedance studies [23], as well as cyclic voltammetry and chronopotentiometry measurements, are consistent with the following sequence of steps for the reduction reaction: SO 3 Jr 8042-= 82072- (chemical equilibrium) (9) 82072--~ - e - ~ SO42- Jr- 8 0 3 - (reduction) (10) followed b y the gain of a second electron given by

SO8- + e- ~ SO2 + 0 2 -

(reduction)

(11)

The release of SO2 gas during hot corrosion has been quantitatively measured [25]. Numata e t al. [26] have shown that water vapor can also serve as the oxidant.

325 An interesting question concerning electrochemical experimentation in fused Na2SO4 arises on consideration of the alternative use of either of the specific ion electrodes (oxygen or sodium probes) as the reference electrode in a three~lectrode arrangement, From Fig. 1, the isopotentials for the oxygen electrode are horizontal, while those for the sodium electrode are sloping. Therefore, it might be questioned whether polarization studies of a working electrode, e.g. platinum, in Na2SO 4 would give comparable results for the two reference electrodes, and how the results are interrelated. Figure 10 shows polarization curves for a platinum working electrode in fused Na2SO 4 for an equilibrated SO3-10%SO2-O 2 gas at 900 °C. Obviously the two polarization plots are virtually identical except for a displacement in potential corresponding to a difference in standard electrode potential. As explained in detail by Park e t al. [16], in fact, the ZrO 2 electrode can also he considered as a sodium activity probe in Na2SO 4 of given basicity, and the difference between the two plots in Fig. 10 depends in a predictable manner on the melt basicity. Thus, either specific ion electrode can be used as the reference for a threeelectrode cell, and the interpretation is thus far unambiguous, However, to interpret electrochemical polarization experiments, understanding of exactly what chemical conditions, e.g. in terms of the coordinates of Fig. 1, are created

at the working electrode during anodic and cathodic polarization is necessary. For aqueous solutions, where shifts in pH resulting from electrode reactions are generally small because o f the higher dissociation product (10-14), polarization generally results in a local change in oxidizing potential with little pH change except perhaps in pits and crevices. The same result cannot be expected for the more stable, and therefore more "sensitive", fused Na2SO 4. Park and coworkers [16, 17] used auxiliary sodium and oxygen electrodes to follow the local chemical potentials of O2 and Na20 during the polarization of a porous platinum electrode sintered onto the ZrO2 probe. The resulting Fig. 11 is a plot of the local chemical activities on platinum in pure Na2SO 4 during polarization. The initial stage of cathodic polarization (reduction reaction) generated a reduction in local acidity as $2072- was reduced and depleted. Then the oxidizing potential dropped sharply as the limited dissolved O2 was reduced. Finally, at a low potential where the sulfate ion should be reduced, the melt increased in basicity as expected. In accord with the earlier discussion of the ionic dominance fields in Fig. 1, changes in behavior occurred as the local chemistry approached or crossed boundary lines. Anodic polarization of platinum initially increased the melt acidity relative to the open-circuit condition, but high anodic potentials attacked the platinum electrode and its adherence to the Zr02.

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326

The change in local chemistry on polarizing a nominally inert electrode shows that polarization causes large chemical shifts in both oxidizing potential and basicity. For reactive electrodes such as nickel-base alloys, even larger changes must be expected and, if the volume of salt is limited to a thin film, then t h e uncertainty in interpreting the local chemistry at an electrode polarized in pure Na2SO4 is enormous. However, a salt buffered b y salts of strong acids, e.g. NaVOs-NaaVO4, should follow a vertical polarization trace on Fig. 1. Shores [27] and Rahmel and coworkers [28-30] have reported useful correlations of h o t corrosion performance to polarization studies in deep Na2SO4 melts for engineering alloys. However, it m u s t b e a d m i t t e d t h a t the importance of the method is empirical. The interpretation [28] t h a t t h e m e l t maintains an equilibrium with oxygen activity of the gas phase (O2 is only slightly soluble) and that polarization only shifts the local melt basicity cannot be correct.

4. CONCLUDING REMARKS

In this paper, I have reported on measurements of oxide solubility in Na2SO 4 and on

electrochemical experimentation without much direct discussion of h o t corrosion kinetics, morphologies or mechanisms. However the results may serve to interpret hot corrosion mechanisms and to guide meaning-

ful future experimentation. In particular, the solubility studies seem to offer a needed quantification of the acid-base concept and

the stabilities of oxides in oxyanion salts, Obviously, the methods and interpretations should be applicable to other oxyanions salt systems, e.g. carbonates, nitrates and hydroxides.

ACKNOWLEDGMENTS

The experimental results and discussions w i t h W. C. F a n g , C. O . P a r k a n d Y, S. Z h a n g are acknowledged, This work was sponsored by the National

Science Foundation, Metallurgy Program of the Division of Material Research, under Grants 75-17204 and 791190. This paper

was written while the author spent a sabbatical leave at the Universit~ de Paris-Sud, Orsay, France.

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