Prediction of Critical Crevice Potentials for Ni-Cr-Mo Alloys in Simulated Crevice Solutions as a Function of Molybdenum Content

Critical potentials for crevice corrosion initiation were predicted for Ni-22Cr-XMo alloys (X = 0, 3, 6, 9, 13%), Alloy 625 (UNS N06625), and Alloy 22 (UNS N06022) based on electrochemical oxidation behavior in various artificial crevice solutions using an adaptation of a previously developed approach. Simulated occluded site crevice solutions, high in chloride and H+, were simulated using both hydrochloric acid (HCl) alone and HCl with addition of a high concentration of chloride to replicate the low pH and high-chloride content of stabilized crevice solutions in this alloy class. The chloride content and pH were controlled independently to simulate critical solutions using lithium chloride (LiCl) and HCl solutions. Critical potentials were derived and compared to experimental values from the literature. The effects of alloyed Mo on crevice stability in this group of alloys are discussed, especially its significant impact when present in quantities larger than 6%. Distinct differences were observed between Alloy 625 plate and wire suggesting a strong role of minor alloying elements.