Depassivation pH of Austenitic Stainless Steels in NaCl Aqueous Solution

Abstract

The combination of the potentiometric pH measurement and potentiostatic polarization was carried out for the type-312L and 316L austenitic stainless steels during decreasing pH by dropping hydrochloric acid in sodium chloride solution. The acidification changed the surface condition of the austenitic stainless steels. The significant acidity of the solution induced a rapid increase in the anodic current passing through the steel at a critical pH, pHdepass. The value of pHdepass was increased by elevating the solution temperature or concentrating the chloride ions in solution. In the potential - pHdepass diagram, the surface condition could be classified into four domains; immunity, active, passive and transpassive/pitting states. The boundary between the active and passive states was strongly dependent on solution pH but independent of potential, indicating that the depassivation of the steel surface proceeded with a chemical reaction participating protons rather than the electrochemical one.