Calculating chemical concentrations during the initiation of crevice corrosion

Numerical simulations of solution chemistry and electrochemical potential v ariations within crevice corrosion sites on stainless steel have been carri ed our using a method recently developed by Friedly and Rubin, Potential di fferences in the crevice solution have been calculated via the Nernst-Planc k equation rather than by making the usual assumption of electroneutrality at any point, although this assumption is proven to be justified for rime s cales of > 10(-13) s. Results of these simulations have been compared with the experimental data of Alavi and Cottis and the modelling results reporte d by Sharland. It is shown that typical passive current densities for stain less steels in near-neutral solutions are too low to induce local chemistry , or potential, changes capable of causing depassivation in the crevice. Ho wever, once active corrosion has been initiated, physically reasonable anod ic current densities can be used to match modal simulations with experiment . The precipitation of Cr(OH)(3) is the critical reaction responsible for r educing pH within the crevice. and consideration of the effect of potential on anodic current density allows the model to reproduce a minimum in the p H profile along the crevice, as found experimentally. (C) 2000 Elsevier Sci ence Ltd, All rights reserved.