A Study on Pitting Corrosion of Sensitized 316 Stainless Steel in Aqueous 0.01 M NaCI Solution Using the Abrading Electrode Technique and Ac-Impedance Spectroscopy

摘要

Pitting corrosion of sensitized 316 stainless steel has been investigated as a function of the degree of sensitization in aqueous 0.01 M NaCl solution at room temperature. The squared rod specimens of 316 stainless steel were thermally annealed at 700 °C for various durations (0 h : non-sensitized specimen A; 8 h : moderately sensitized specimen B; 96 h : severely sensitized specimen C) and the degree of sensitization was assessed by using electrochemical potentiokinetic reactivation (EPR) test technique. The pitting corrosion resistance of the three kinds of specimens was evaluated by the potentiodynamic anodic polarization method, abrading electrode technique and ac-impedance spectroscopy. The measured potentiostatic decay current transient obtained just after interrupting the abrading action showed that the repassivation rate of the oxide film on the fresh bare surface of the specimen decreased in the order of specimens A, B and V C in the early stage of the film formation. From the results of ac-impedance spectroscopy, the charge transfer resistance R_(ct) and film capacitance C_f of specimens B and C were evaluated and found to be lower and higher, respectively, than those of specimen A. Based upon the combined results of the abrading experiment and ac-impedance measurement, it is indicated that the instantaneous formation of the passivating oxide film is more retarded by the impoverished chromium (Cr) concentration over the wider Cr-depleted zone adjacent to the grain boundaries in sequence of specimens A, B and C. This means that the thin oxide film formed on the sensitized specimens B and C is less protective than that formed on the non-sensitized specimen A.