Effects of environmental factors on key kinetic parameters relevant to pitting corrosion

Experiments using stainless steel artificial pit (lead-in-pencil) electrodes in ferric chloride and lithium chloride solutions were performed in order to determine the effects of key environmental factors such as chloride concentration and pH of the bulk solution on the central parameters utilized to characterize the pitting phenomenon—the repassivation potentialErpand the pit stability product under a salt film (i · x)saltfilm. For all the stainless steel alloys studied, a relative independence of theErpto the pit depth was observed once sufficient anodic charge had been passed. The pit stability product under a salt film was seen to be largely insensitive to the pH of the bulk solution.Erp, on the other hand, was fairly independent of bulk pH only at the lower chloride concentrations of both lithium chloride and ferric chloride solutions. The two parameters were affected differently by variation in the chloride concentration of the bulk solutions. Increasing the chloride concentration resulted in a decrease in the value of (i · x)saltfilmfor all alloys in both solutions. In ferric chloride, the value ofErpincreased with increasing chloride concentration for Custom 465 and the austenitic steels, whereas it decreased across the same range for 17–4 pH. These trends were explained qualitatively using solution conductivity and alloying composition arguments. Finally, the results obtained from this study allowed for a rationalization of the phenomenology, enabling a method of measurement of the diffusion coefficient and the concentration at saturation of the “stainless steel cation” within the pit, both of which agreed well with values obtained from the existing literature.