Calculating chemical concentrations during the initiation of crevice corrosion

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