Abstract
Factors controlling the initiation and propagation of crevice corrosion on alloy 625 (UNS N06625) in ambient temperature seawater have been explored within the context of the Oldfield-Sutton Model for critical crevice corrosion solution (CCS) development and the Ohmic Criterion for crevice corrosion initiation. Environmental and alloy compositional factors influencing both the initiation and propagation stages of crevice corrosion are experimentally examined within the context of these two theories. It has been shown that a critical ohmic potential drop must be exceeded to initiate crevice corrosion and this claim is supported here. Moreover, once initiation occurs crevice corrosion propagation is found to be under ohmic, possibly IR control. Concerning the effects of solution alterations, chlorination decreases the time required for initiation and possibly raises propagation rates. Addition of molybdate to seawater (simulating the dissolution of alloyed Mo) is beneficial in both delaying initiation and temporarily reducing propagation rates but does not fully explain the inhibiting effects of alloyed Mo. The beneficial effects of alloyed Mo and, as an alternative, equivalent Cr concentration were critically examined in the context of the two models for crevice corrosion in several simulated CCS by comparing alloys C-276, 625, and G-3. Alloyed Mo lowers the passive current density, decreases the tendency for the primary passive potential to increase with increasing acidity, and lowers the anodic dissolution rate in the active polarization region. This implies that Mo alloying both restricts initiation to crevices with large depth to gap aspect ratios and lowers propagation rates for those crevices that initiate.
