The integrity of Inconel alloys in high temperature water chemistry

The electrochemical behaviours of nickel-based alloys were investigated by potentiodynamic technique in sulphate and/or chloride environments at 316oC. It was found that the pitting resistance of Inconel-690 to chloride attack at pH 4 was up to 8000 ppm, whereas, 8% Cr alloy and Inconel-600 would suffer from pitting corrosion when the chloride concentration was ≥ 4000 ppm. The polarization curves of nickel-based materials in alkaline electrolytes (pH = 14) were similar to that in neutral environments (pH = 6). Nevertheless, the first passive current densities at pH 14 was one order of magnitude higher than in neutral solutions. With knowing the heat capacity, the entropy, and the chemical potential of interest species, the redox equilibrium equations for Cr, Ni, and Fe were derived to correlate the measured passivation potentials at 316oC. (author)