Effect of ionic interactions on the initiation of crevice corrosion in passive metals

In the crevice corrosion process, oxygen reduction occurs faster than diffusion into an occluded crevice, causing deoxygenation of the crevice solution. Once oxygen is depleted in the crevice, oxygen reduction can only occur on the metal surface outside of the crevice. The cations. produced by metal dissolution are hydrolyzed, which causes the pH of the crevice solution to drop. This increases the rate of metal dissolution, forming an autocatalytic coupling that causes concentrated electrolyte solutions to form in the crevice. The focus of this paper is the prediction of the effect of nonideal solution behavior on crevice corrosion using the ionic interaction model of Pitzer coupled with an electrolyte mass-transport model. This mathematical model was used to simulate the type 304 stainless steel crevice corrosion experiment of Alavi and Collis. The results are in excellent agreement with experimental observations. The model was then applied to simulate the crevice corrosion initiation period of a titanium crevice. Comparison of the predictions to those generated via an ideal solution crevice corrosion model indicates that interionic forces draw chloride ions into the crevice and hinder the transport of hydrogen ions out of the crevice. (c) 2005 The Electrochemical Society. All rights reserved.