Crevice Corrosion of Alloy 625 in Chlorinated ASTM Artificial Ocean Water^⋆

Factors controlling the initiation and propagation of crevice corrosion on alloy 625 (UNS N06625) in ocean water of ambient temperature were explored within the contexts of the Oldfield-Sutton model for critical crevice corrosion solution (CCS) development and the ohmic criterion for crevice corrosion initiation. Data supported an earlier claim that a critical potential drop must be exceeded to initiate crevice corrosion. Steady-state crevice corrosion propagation was found to be under ohmic control. Chlorine decreased the time required for initiation and possibly raised propagation rates in ASTM artificial ocean water. Addition of molybdate (MoO₄^2–) to ocean water (simulating the dissolution of alloyed molybdenum [Mo]) delayed initiation and reduced propagation rates temporarily, but not sufficiently to explain fully the inhibiting effects of alloyed Mo. Beneficial effects on crevice corrosion susceptibility of alloyed Mo and equivalent chromium (Creq) concentration were examined critically in the context of the two models by comparing alloys 625, C-276 (UNS N10276), and G-3 (UNS N06985) in several simulated CCS. Alloyed Mo lowered passive current densities, decreased the tendency for the primary passive potential to increase with increasing acidity, and lowered the anodic dissolution rate in the active polarization region. This implied that Mo alloying restricted initiation to crevices with large depth-to-gap aspect ratios and lowered propagation rates for crevices that initiated.