Determining the critical crevice depth for iron in a dodium acetate-acetic acid buffer solution

Alloys that depend on oxide films or passive layers for corrosion resistance are susceptible to crevice corrosion. This is for instance the case for iron in a sodium acetate-acetic acid (NaAc-HAc) buffer solution. This material-environment combination shows an active-passive transition in the polarization behavior around -150 mV (SCE) and, hence, is susceptible to crevice corrosion. Indeed, a potential drop into the crevice might bring part of the crevice into the active region, resulting in crevice corrosion. The potential drop into a crevice in a Fe/NaAc-HAc buffer system has been computed and compared to the experimentally measured profile. Mathematically, the potential drop into the crevice is a Poisson-type field problem with nonlinear boundary conditions and it has been described in a one-dimensional finite difference framework. The subsequent critical depth calculations, determining the onset of crevice corrosions, were solely based on the geometry of the crevice, the conductivity and the polarization behavior of iron in a NaAc-HAc buffer solution. In order to achieve this, a Weibull transition function has been used to describe the active-to-passive transition in the polarization behavior. Due to the strongly nonlinear and inherently non-monotonic character of the boundary conditions (the active-passive transition in the polarization behavior), the resulting equations are solved using a generally applicable homotopy method.