Implications of internal cathodic reactions for crevice attack of stainless steels in chloride environments

Abstract

Modelling of the crevice chemistry in stainless steels in chloride environments at ambient temperatures has been conducted in order to evaluate the impact of internal cathodic reactions on the potential and pH within the crevice. Consideration of the internal cathodic reactions is of particular importance in predicting the behaviour of crevices formed between dissimilar metals. The model is based on mass conservation of species and incorporates a range of chemical and electrochemical reaction processes. Hydrogen ion reduction within the crevice has a marked effect on the time evolution of the pH in the crevice and consequently on predictions of the time to passivity breakdown based on the critical pH concept. The impact on the steady state pH depends on the corrosion potential and on crevice dimensions. In the latter context, decreasing the gap or increasing the length does not invariably create more severe conditions for passivity breakdown. There is an optimum magnitude of the dimensions for inducing crevice attack. Coupling the stainless steel to a more noble material within the crevice, when the main cathodic reaction is reduction of hydrogen ions, may be more likely to retard crevice attack by virtue of the higher crevice pH, provided that effective coupling of the materials in the external environment is constrained. A specific bulk conductivity exists below which the critical crevice chemistry does not develop, but this is associated with potential drop in the bulk solution rather than in the crevice.