The relationship between pit chemistry and pit geometry near the critical pitting temperature

In pitting corrosion, the pit stability product is the product of anodic current density and depth, ia, which for an open hemispherical cavity would be proportional to the product of the cation diffusivity and some interfacial concentration of the dissolving cations, DC. Many authors believe that pit growth is diffusion-controlled, i. e., C = C-s (solubility of the metal salt). Temporarily, prior to salt film precipitation, supersaturation is possible up to some maximum value: C = C-ss. Estimates of ia for real pits can be used to analyze their degree of occlusion, that is, by how much the effective value of pit radius (a(eff) = alpha nFDC(s)/i, where alpha is a geometrical factor close to 0.5) differs from the geometrical pit radius calculated using Faraday's law (a). Small pits are found to be highly occluded, i. e., a(eff) >> a. For metastable pits formed in a high-alloy 904L stainless steel, the value of ia increases linearly with pit depth, and for a temperature 7 degrees below the critical pitting temperature (CPT), approaches DCs at the largest observed pit depth of ca. 10 mu m, but very large metastable pits formed closer to the CPT have ia values that exceed even DCss. We show that this is consistent with the time-dependent development of a dish-shaped pit geometry, and that by suitable correction the stability products become consistent with diffusion-controlled pit growth. (c) 2006 The Electrochemical Society.