Pit Growth in Pitting Corrosion of Austenitic Stainless Steels

Growth of a single pit developed on austenitic SUS 27 (type 304) and SUS 28 (type 304 L) stainless steels has been studied potentiostatically and microscopically in 1 N-H2SO4 containing 0.5 N-NaCl at 25°C. The single pit was developed in the specimen surface of 0.95 cm2 by using, for example, a SUS 27 specimen pretreated in 1 N-H2SO4 at 1200 mV (SCE) and treated for repassivating film-breakthrough sites at 250 mV in the solution containing chloride.The critical potential for pitting is 400 mV for SUS 27 and 250 mV for SUS 28, respectively. This is the boundary potential between the repassivation and pit-growth regions.A distribution hystogram of diameters of film-breakthrough sites occurring in the repassivation region is given. A peak lies at 30∼40 μ.Concernning pit growth, the potential dependence of growth rate was determined. The current density per mouth area of the single pit is 0.3 A/cm2 for both steels at the critical potential. The pit grown near the critical potential has a thick lid (Deckel) over its mouth.The critical potential for pitting seems to be a minimum potential which allows the flow of current enough to concentrate chloride in the pit to a critical value. The lid would be convenient to maintain the high chloride concentration.