Experimental Procedures and Sensitization Diagnostics of AISI304 Steel by Double Loop Electrochemical Potentiodynamic Reactivation Method

Abstract

This paper is dedicated to the establishment of experimental conditions for practical use of Double Loop Electrochemical Potentiodynamic Reactivation method (DL-EPR) to characterize the degree of sensitization in AISI 304 austenitic stainless steel based in the standard ISO 12732. The steel undergoes heat treatments to solubilize the commercial material and later controlling its sensitization degree. Microstructure of the samples with quantification of the sensitization was obtained according to ASTM E1245. The standards ASTM A262 and ASTM G48 were used, respectively, to rank the sensitization and the susceptibility to pitting corrosion of the different samples. Solutions of sulfuric acid 0.5mol.dm−3 with three contents of KSCN (0.001, 0.01, 0.05mol.dm−3) and three different potential scan rates (0.56, 1.67 and 4.2mV.s−1) were used for DL-EPR measurements. Results illustrate the high dependence of Ir/Ia on KSCN concentration and scan rate, even respecting the standard limits. Therefore, table presented by ISO 12732 addressing Ir/Ia to sensitization degree can be misleading as it is not linked with specific experimental conditions. On the other hand, DL-EPR can be direct related to sensitization when reference materials are used. For that, heat treated steel must be used to obtain DL-EPR curves considering electrolyte composition and potential scan rate. These results are the base to establish a reference curve of Ir/Ia versus the sensitization degree. This curve can be used later to detect sensitization of the same kind of steel in-service with Ir/Ia values measured in-situ. The diagnosis of susceptibility to localized corrosion obtained by various standard procedures is critically evaluated.