Electrochemical study of nickel-based alloys in high temperature water chemistry

The electrochemical behavior of nickel-based alloys was investigated using a potentiodynamic technique in sulfate and/or chloride environments at 316 degrees C. It is found that in the co-existence of 8000 ppm sulfate anions the pitting resistance of Alloy 690 to chloride attack at pH 4 was up to 8000 ppm. Whereas, a concentration of 4000 ppm chloride anions is enough to generate pit on 8% Cr alloy, which is designed to simulate the grain boundary composition in sensitized condition, and on Alloy 600. The polarization curves of nickel-based alloys in alkaline electrolytes (pH(25) = 14) were similar to those in neutral environments (pH(25) = 6.5). Nevertheless, the first passive current density at pH 14 was one order of magnitude higher than that in neutral solutions, suggesting the preferential dissolution of Cr. With knowing the heat capacity, the entropy, and the chemical potential of interest species, the redox equilibrium lines for Cr, Ni, and Fe were derived at 316 degrees C.