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, i a , 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 i a 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 = α n F D C s ∕ i , where α 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 i a increases linearly with pit depth, and for a temperature 7 ° below the critical pitting temperature (CPT), approaches D C s at the largest observed pit depth of ca. 10 μ m , but very large metastable pits formed closer to the CPT have i a values that exceed even D C ss . 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.