Microstructural evolution and response to double-loop reactivation testing of heat-treated PH 13-8 Mo martensitic stainless steel

Compared to the austenitic stainless steels, relatively few studies have been reported of the intergranular corrosion suceptibility of martensitic stainless steels, particularly those containing < 0.1% carbon. For two such alloys, Custom 450 and PH 13-8 Mo, preferential pitting along a narrow band in the heat-affected zone was ascribed to Cr-depletion of the matrix as a result of general carbide precipitation. However, analytical verification of Cr-depletion surrounding these sub-microscopic carbides was not reported. The authors have used the double-loop electrochemical potentiokinetic reactivation (DL-EPR) test to investigate the intergranular and interlath corrosion susceptibility of PH 13-8 Mo as a function of heat-treatment. The DL-EPR procedure was developed by Akashi et al and characterized by Majidi and Streicher for use on austenitic stainless steels. They found this test to be a sensitive method of quantifying mild degrees of sensitization with good reproducibility. Degree of sensitization is measured by the ratio of the peak current on a reverse (reactivation) scan to that on the forward anodic scan. Austenitic stainless steels free of sensitization exhibit a ratio of < 0.001, while a ratio of > 0.05 corresponds to a ditch structure in ASTM A 262-A (oxalic acid)