Electrochemical Interaction of Lead with Alloy 600 and Alloy 690 in High-Temperature Water

Anodic polarization behaviors of Alloy 600 (UNS N06600) and Alloy 690 (UNS N06690) have been studied as a function of lead content in a solution of pH 4 and pH 10 at 90°C. As the amount of lead in the solution increased, and critical current densities and passive current densities of Alloy 600 and Alloy 690 increased, the breakdown potential of the alloys decreased. The high critical current density in the high-lead solution was thought to come from the combination of an enhanced dissolution of constituents on the surface of the alloys by the lead and an anodic dissolution of metallic lead deposited on the surface of the specimens. The morphology of lead precipitated on the specimen after the anodic scan changed with the pH of the solution; small, irregular particles were precipitated on the surface of the specimen in the pH 4 solutions while high-density, regular-sized particles formed on the surface in the pH 10 solutions. Lead was observed to enhance chromium depletion from the outer surfaces of Alloy 600 and Alloy 690 and to increase the ratio of O^2–/OH^– in the surface film formed in the high-lead solution. The stress corrosion cracking (SCC) resistance of Alloy 600 and Alloy 690 may have decreased because of the poor quality of the passive film formed and the enhanced oxygen evolution in the solution containing lead.