Critical Potential for Growth of Localized Corrosion of Stainless Steel in Chloride Media

A series of investigations were carried out to determine the critical potentials for the growth of localized corrosion on Type 316 and Type 316L stainless steel in chloride media using artificial specimens having the macroscopic local anode separated from the cathode. The critical potential value for the pitting in 0.5N NaCl solution at 70 C (158 F) was shown to be approximately −0.25 V to a saturated calomel electrode and that for the crevice corrosion was about −0.36 V SCE. The latter was nearly equal to the critical potential for the crack propagation of stress corrosion cracking (SCC) in boiling 35% MgCl₂ solution. These results indicate that the critical potential can vary, depending on the degree of occlusion on the anode. The critical potential seems to be determined mainly by the hydrogen ion concentrations inside the occluded anodes. The critical potential for pit growth was not affected by the environmental factors as the pitting potential.