Determining the onset of crevice corrosion of iron in a sodium-acetate acetic-acid buffer solution

摘要

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 buffer solution. This material-environment combination shows an active-passive transition in the polarisation behaviour around -150 mV_(SCE) and, hence, is susceptible to crevice corrosion. Indeed, a potential drop down the crevice might bring part of the crevice in the active region, resulting in crevice corrosion. The potential drop down an iron crevice in a sodium-acetate acetic-acid buffer solution has been computed and successfully compared to experimental measurements. Mathematically, the potential drop down the crevice is a Poisson-type field problem with non-linear boundary conditions and it has been described in a one-dimensional finite difference framework. The subsequent critical depth calculations, determining the onset of crevice corrosion, were solely based on the geometry of the crevice, the conductivity and the polarisation behaviour of iron in a sodium-acetate acetic-acid buffer solution. In order to achieve a Weibull transition function has been used to describe the active-to-passive transition in the polarisation behaviour. Due to the strongly non-linear and inherently non-monotonic character of the boundary conditions (the active-passive transition in the polarisation behaviour) the resulting equations had to be solved using a homotopy method. Based on the numerical computations, two new parameters, important to crevice corrosion, have been identified. The first one, the electrochemical diameter D_(EC), allows one to compare different crevice geometries. The second one, the characteristic dimension CD (=L~(2)/D_(EC)), allows one to identify the occurrence of crevice corrosion in the given system. Indeed, a critical value of the characteristic dimension, CD_c, determines the onset of crevice corrosion. It is believed that the concept of such a critical characteristic dimension is not only relevant to this particular system, but also to crevice corrosion of other material-environment combinations showing an active-passive transition.