Crevice Corrosion in High‐Temperature Aqueous Systems Potential/pH Measurements in Alloy 600 Crevices at 288°C

In situ potential/ p H measurements on Alloy 600 crevices have shown that p H decreased in both sulfate‐ and chloride‐containing crevices during exposure to aerated water at 288°C as extrapolations of available thermodynamic data predicted. A local redox corrosion process then consumed the acid and the systems attained neutral p H within a period of several days. This unexpected behavior required that Ni(II) and Fe(II,III) corrosion products be present as positively charged species or as complexes with Cl − , , or anions and not hydrolyze to any significant extent. Subsequent p H measurements indicated that iron corrosion products were primarily responsible for preventing hydrolysis. The p H increases occurred principally in a potential/ p H region where water is thermodynamically stable, but previous crevice experiments had suggested that acid loss by diffusion and reduction of dissolved oxygen were not important factors. Since the equilibrium hydrogen fugacity was extremely low, it is possible that the metal acted as a sink for nascent hydrogen, but the nature of the p H‐increasing local redox reaction and the form of the corrosion products at 288°C remain unclear.