The kinetics of Na2SO4-induced corrosion were measured by accelerated oxidation tests on Co-30Cr and Ni-30Cr as a function of temperature from 600~ "C, SO3 in the environment and deposit composition. The alloys were rapidly attacked at temperatures between 650 ~ and 750~C when a liquid sulfate phase was obtained from an initially pure solid Na2SO4 deposit. The rapid rate of attack resulted ~rom sulfation of the transient surface nickel or cobalt oxides and the dissolution of these transition metal sulfates into Na2SO4 to yield a liquid phase. This retarded the formation of a protective Cr203 scale. The exposure conditions under which liquids could form from Na2SO4-CoSO4 mixtures were calculated from thermodynamic considerations.Many fossil-fueled devices for energy conversion are susceptible to rapid degradation of high temperature alloy components when subjected to molten salt deposits. Because of the nearly universal presence of sulfur and alkali metals in fossil fuels, the corrosive deposits usually contain alkali sulfates (Na2SO4, K2SO4). The mechanism of Na2SO4-induced corrosion at high temperatures, where Na2SO4 (mp _--884~ is liquid, has been extensively studied. This form of attack, commonly known as hot corrosion, results in typical sulfidation with an aluminum and/or chromium depletion zone in the alloy.Recently, a different, but related, form of hot corrosion has been identified in marine propulsion gas turbines under service conditions where the surface temperatures of the first stage blades and vanes were in the range of about 650~176(1). The attack on Co-base coatings resulted in an unusual morphology: pitting, little or no depletion of A1 or Cr in the alloy, and corrosion products which contained water soluble Co and Ni. Burner rig tests and other studies have been able to reproduce these characteristic features of intermediate temperature attack only when Na2SO4containing deposits are used in conjunction with sufficient SO3 in an oxidizing environment (2, 3).Only a few investigators have studied the mechanism of alkali sulfate induced corrosion at intermediate temperatures (600~176 (4-6). Umland and Voigt (4) investigated the formation of soluble Ni, Co, and chromate ions in crucible-type corrosion tests. Several nickel-, cobalt-, and iron-base alloys were exposed to melts containing Na2SO4, K2SO4, or mixtures thereof in air or in air containing SQ over the temperature range 650~176They found a maximum in corrosion rate at 750~ which coincided with the highest recovery of Ni and Co sulfates from the melt. This study was subsequently extended by Balajka and Danek (5), who observed enhanced corrosion of Ni in alkali sulfate melts containing CoSO4 at intermediate temperatures. They interpreted their results in terms of Umland and Voigt's suggestion that cobalt * Electrochemical Society Active Member.

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The solubility of NiO in mixtures of Li2CO3-K2CO3 and Na2CO~-K2CO3 was measured as a function of the calculated basicity of the solvent mixture and as a function of the temperature. Acid and basic dissolution were found in all mixtures of the Na2CO3-K2CO3 system and in the K2CO3-rich mixtures of the Li2CO3-K2CO3 system. The slopes of the acid solubility lines were markedly different from those in the single salts. The solubilities in mixtures were not quantitatively related in a simple manner to the solubilities in the pure salts of which the mixtures were composed. However, useful qualitative trends may be discerned.Materials degradation owing to corrosion is an important issue for virtually all energy-producing systems, including the advanced molten carbonate fuel cells now under development. Current collectors and other metallic components of the fuel cell may be rapidly degraded at high temperatures if molten salt contacts the surface. This form of attack, known as hot corrosion, is thought to be due to "fluxing," wherein the normally protective oxide scale on an alloy is destroyed by a dissolution reprecipitation process. To help identify hot corrosion mechanisms and to evaluate fluxing quantitatively, data on the chemistry of oxide dissolution are needed.Although the solubility of transition metal oxides in pure molten carbonates has been previously reported (1), in practical terms the solubilities in mixtures are often more important. Baumgartner (2), Kaun (3), and Doyon et al. (4) have reported solubilities of I~iO in mixtures of K2CO3 and Li2CO3. The purpose of the present paper is to investigate the effects of basicity, composition, and temperature on the solubility of NiO in binary mixtures of Na2CO3-K2CO3 and Li2CO~-K2CO3. Theoretical BackgroundWhen NiO dissolves in a single molten alkali metal carbonate the solubility is a function of the basicity of the solvent. This basicity is determined experimentally by the cation of the melt and the pCO2 in the gas phase above the melt (1). In a mixture of two or more salts the basicity of the melt is function of these same variables but since two or more cations are present the basicity is also a function of the mole fractions of the components. For example, in a binary mixture of Na2CO3 and K2CO~ the basicity will be a function of the pCO2 above the melt and of the mole fractions of Na2CO3 and K2CO3. If the pCO2 is varied for a mixture of fixed composition the basicity is directly proportional to log (pCO2), just as for the single salts. However, when the composition of the solvent is varied at a fixed pCO2, the basicity is a nonlinear function of composition, expressed as activities of the components. This relationship was explored in a Previous paper (5) and is summarized in Fig. 1 and 2 for mixtures of Li2CO3-K2CO3 and Na2CO3-K2CO3. The M20 described in these figures is the neutral counterpart of a common oxide ion activity calculated from the free energies of dissociation of both salts at the indicated pCO3, and the activity of each component in the mixtu...

The solubility of NiO in molten Li2CO3, Na2CO3, K2COa, and Rb2CO3 was measured at 910~ as a function of the basicity or -log a(O 2-) of the salt. The basicity was controlled by varying the pCO2 in the gas phase above the melt. Acidic and basic dissolution were observed in all salts except Li2CO3, which underwent only basic dissolution. The slopes of the solubility lines in both acid and basic regime were related to the dissolution products where possible. The dependence of NiO solubility on the activity of 02 in the system was determined for both regimes of all the melts. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 132.174.254.159 Downloaded on 2015-05-27 to IP Vol. 135,No. 7