Effect of Tempering Treatments on Microstructure and Intergranular Corrosion of 13 wt% Cr Martensitic Stainless Steel

Susceptibility to intergranular corrosion (IGC) of 13 wt% Cr martensitic stainless steel, with different tempering treatments was studied by potentiodynamic and potentiostatic polarizations and a modified double loop electrochemical potentiokinetic reactivation (DL-EPR) tests in sulfuric acid solution. Further, the electrochemical results are correlated with immersion test corrosion rates. Microstructural studies showed formation of nano-sized and sub-micrometer sized chromium-rich carbides at lath interfaces, and also increases in size and volume fraction of carbides with tempering temperature. Hardness decreased with tempering temperature resulting from increase in carbide volume fraction. Tempering at 550°C for 2.5 h resulted in a higher corrosion rate and DL-EPR value, and the preferential attack along the martensite lath interfaces and potentials causing preferential attack were established. Immersion, polarization, and DL-EPR tests confirmed desensitization above tempering temperature of 550°C. The corrosion rates and DL-EPR values corresponding to IGC and uniform corrosion have been established.