Species-Dependent Crevice Corrosion Modeling of Ni-625

A coordinated experimental and mathematical modeling effort to develop a three-stage model for determining the spatial and temporal potential, current, ionic species, and damage profiles for nickel 625 crevice corrosion applications in seawater solutions is presented. Stage one is oxygen depletion inside the crevice, stage two is the development of a critical crevice solution (induction stage), and stage three is long-term aggressive dissolution that is consistent with a nearly well-mixed, IR controlled crevice system. In stage one, deoxygenation allows separation of the anodic and cathodic sites. In stage two, local initiation occurs at the crevice tip that diffuses toward the crevice mouth. Minimal dissolution occurs in stage two and complete initiation occurs when the total anodic current is such that the critical IR drop is observed. In stage three, the crevice is saturated at or near the critical crevice solution and metal salt precipitates allow stable crevice corrosion propagation. The key inputs to the three-stage model are concentration-dependent polarization curves, and hydrolysis and hydroxide precipitation reaction data. Model calculations are compared with experimental damage histories and total current measurements. Analytical solutions and 2D modeling efforts are also used to validate the proposed 1D approach for simplified systems leading up to the complex Ni-625 system. All results agree under a thin-film approximation. Further, it is shown that with appropriate experimental input data, and knowledge from solution of the species-dependent system, a damage evolution well-mixed model provides comparable results.