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Borucka, This Journal, 124, 972 (1971). 32. S. H. Lu and J. R. Selman, ibid., 137, 1125 (1990). 33. R. S. Nicholson and I. Shain, Anal. Chem., 36, 706 (1964). 34. T. Berzins and P. Delahay, J. Am. Chem. Soc., 75, 555 (1953). Electrochemical Impedance Studies of Hot Corrosion of Preoxidized Ni by a Thin-Fused Na2S04 Film Yiing Mei Wu*" and Robert A. Rapp** Department of Materials Science and Engineering, The Ohio State University, Columbus, Ohio 43210 ABSTRACT A thin-film electrochemical arrangement has been developed to investigate the hot corrosion of preoxidized Ni by a fused Na2SO4 film at 1200 K in a catalyzed 0.1% SO2-O2 gas mixture. A Pt counterelectrode enables polarization measure- ments using the three-electrode configuration as well as open-circuit potentiometry. To extract mechanistic information without disturbing the system, the electrochemical impedance technique was used. Results of three distinct modes of hot corrosion (passive, pseudo-passive, and active) resulting from different preoxidation conditions are discussed. The degradation of metals or alloys at elevated tempera- tures by a thin-fused salt film in the presence of an oxidiz- ing gas is called hot corrosion. Several authors have pro- posed mechanisms for hot corrosion (1-7). The most widely discussed is the oxide fluxing model (4-11), by which oxide scales which are normally protective in gaseous oxidation are dissolved or penetrated by the molten salt. As dis- cussed by Rapp (8, 9, 11), the solubility of a given oxide and the type of dissolution (acidic or basic) depend strongly on the basicity of the molten salt solvent. In the basic dissolution regime, where an oxide forms an anionic solute by complexing with oxide ions, its solubility is higher when the salt is more basic. The opposite is true in the acidic dissolution regime where the product solutes are cations and oxygen anions. When the dissolution of the scale is combined with a negative gradient for the oxide solubility in the salt film, caused by gradients in basicity or oxygen activity, the dissolved species migrate away from the dissolution site and reprecipitate within the salt film (7). Thus, a porous and nonprotective oxide is precipitated, and the compact adherent barrier oxide scale is destroyed. At this stage, direct contact between the metal and the salt film may produce sulfides and oxides, at least for attack by fused alkali sulfates. Formation of a sulfide locally de- pletes the sulfate melt of sulfur and thereby increases the melt basicity, perhaps significantly. The electrochemical aspects of the metal-salt attack in- volved in hot corrosion can be divided into two half-cell re- actions. The cathodic reduction reaction should generally be expected to create a condition of locally high basicity, * Electrochemical Society Student Member. ** Electrochemical Society Active Member. i Present address: Mobil Research and Development Corpora- tion, Paulsboro. New Jersey 08066-0486. with oxide ions as reaction products. If the salt film con- tains only a low concentration of transition metal ions, then the reduction of the oxidant species dissolved in the salt film must occur at the oxide/salt interface, where elec- trons are supplied directly from the metal oxidation reac- tion by inward hole migration through the oxide film. In turn, the arrival of the oxidant to the oxide/salt interface requires its dissolution and diffusion through the fused salt film. Since oxide dissolution and precipitation are also involved in the reaction sequence, the mechanism of hot corrosion, which includes diffusion, chemical and electro- chemical reactions, is quite complicated. Electrochemical methods such as scanning polarization, cyclic voltammetry, and chronopotentiometry have been employed in hot corrosion studies. Electrochemical im- pedance spectroscopy (EIS) is a technique which has proved effective in investigating reaction mechanisms and kinetics for other electrochemical phenomena. But suc- cessful application of the method requires suitable models for fitting the impedance data. Only a limited number of impedance investigations of hot corrosion, mostly prelimi- nary work, have been reported. Farrell et al. (12) have em- ployed electrochemical noise and impedance techniques to monitor the corrosion behavior of Nimonic 75 in bulk Na2SO4 and in Na2SO4/1% NaC1 at 900 and 750~ By com- paring the impedance at a fixed low frequency (50 mHz), which was deliberately defined as the estimated polariza~ tion impedance Zp, the authors concluded that the corro- sion rate was higher at 900~ and with the addition of so- dium chloride. The shape of the impedance diagrams also indicated a diffusion-controlled reaction, which results be- cause of the separation of the specimen from the gaseous environment by a deep melt. Gao et al. (13) also used im- pedance and electrochemical noise techniques to monitor ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 173.59.47.29 Downloaded on 2019-04-05 to IP