Hot corrosion of materials: Fundamental studies

Abstract

Hot corrosion is the accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit. Because of its high thermodynamic stability in the mutual presence of sodium and sulfur impurities in an oxidizing gas, Na2SO4 is often found to be the dominant salt in the deposit. The corrosive oxyanion-fused salts are usually ionically conducting electrolytes that exhibit an acid/base chemistry, so that hot corrosion must occur by an electrochemical mechanism that may involve fluxing of the protective oxides. With the aid of high-temperature reference electrodes to quantify an acid/base scale, the solubilities for various metal oxides in fused Na2SO4 have been measured, and these show remarkable agreement with the theoretical expectations from the thermodynamic phase stability diagrams for the relevant Na-Metal-S-O systems. The solubilities of several oxides infused Na2SO4-NaVO3 salt solutions have also been measured and modeled. Such information is important both in evaluating the corrosion resistance of materials and in interpreting any oxide fluxing/reprecipitation mechanisms. Various electrochemical measurements have identified the S2O7 2− anion (dissolved SO3) as the oxidant that is reduced in the hot corrosion process. Electrochemical polarization studies have elucidated the corrosion reactions and clarified the corrosion kinetics of alloys. Mechanistic models for Type I and Type II hot corrosion are discussed briefly.