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Abstract

A critical transition potential is shown to be associated with the abrupt increase in the number of metastable pits and the onset of spatial clustering of pits amongst nearest neighbor defects in both commercial and model stainless steels. This critical transiton potential is correlated with the parameter, Epit, commonly observed in near-neutral NaCl solutions when stainless steels form pits. The explosive growth of pit sites, the onset of clustering of pits, and Epit, all depend on the diffusion length associated with aggressive corrosion products formed at pits and thus on the strirring rate. Epit is observed to increase significantly and pit sites remain random to high potentials in both experiments and modeling when the diffusion length is decreased below the nearest neighbor distance (NND) associated with surface defects such as non-metallic inclusions that form a random array of susceptible sites across the otherwise passive surface. This phenomenon is confirmed by experiments and modeling using model stainless steel alloys with decreased inclusions densities and, thus, increased defect NNDs. If the NND is greater than the lateral diffusion length, then, Epit will increase, clustering of pit sites will decrease and explosive growth in the number of pits will be suppressed at a given applied point.