Effects of Cl−Concentration and Temperature on Pitting of AISI 304 Stainless Steel

Abstract

Measurements of the pit nucleation potential and the induction time for pit initiation under open circuit conditions were made for AISI 304 solution-annealed stainless steel (SS) specimens upon exposure to aerated solutions with different Cl⁻ concentrations at temperatures ranging from 20 to 200 C. For comparison, impedance measurements were made in aerated NaCl solutions at various temperatures for pitted and unpitted specimens. Linear relationships between the pit nucleation and log chloride ion concentration, and between log induction time and log chloride ion concentration were obtained at all temperatures tested; however, the slopes of those two relationships were less at 150 and 200 C than in the lower temperature range of 20 to 80 C. At all the Cl⁻ concentrations and temperatures studied, the electrochemically measured pit nucleation potential was a linear function of log induction time. The impedance measurements showed that the charge transfer resistance decreased with increasing temperature. Pitting resulted in decreased R_t, values. Results of this study emphasize the essential role of temperature-affected variations of the protective properties of oxide films on the steel surface in contact with air-saturated, Cl⁻-containing solutions. Impedance measurements indicate that films formed in the presence of different Cl⁻ concentrations exhibit different properties; the higher the Cl⁻ concentration, the lower the charge transfer resistance, hence the less protective the oxide film.