Quantifying Alloy 625 Crevice Corrosion using an Image Differencing Technique: Part II. A Diffusive Transport Model of Crevice Cation Concentration using Surface Current Density

Abstract

In this paper we present a diffusive transport model of alloy 625 crevice corrosion for simulated ocean water. The model uses the empirical electrochemical kinetics (local surface current density) and empirical damage profile to determine the source of cations from surface oxidation reactions and then calculates the total cation concentration in the crevice solution as a function of time. To gain insight into the solution speciation at the calculated cation concentrations, a thermodynamic software package was used. In those calculations, it was assumed that the anodic dissolution products of the individual alloy components form their corresponding metal salts in the crevice solution. The calculations, based on activity, predicted that precipitation occurred in the same time frame as the experimentally observed increase in active area, decrease in surface current density and transition to diffusion control. The salts that precipitated out of the model solution were NaCl, at 5.3 m, and NiCl2·6H2O, at 5.7 m. It is shown that the presence of the Cr3+, Fe2+, Mo3+ and Nb5+ in the model crevice solution increase the cation and chloride activities resulting in precipitation at the same Ksp values as in the pure salts, albeit at lower Na+ or Ni2+ concentrations. Implications of the speciation results as they relate to actual crevices are discussed.