The effect of tempering temperature on the microstructure and electrochemical properties of a 13Â wt.% Cr-type martensitic stainless steel

The effect of tempering temperature on the microstructure and electrochemical behavior of 13 wt.% Cr-type martensitic stainless steel (MMS) in 3.5 wt.% NaCl aqueous solution has been studied. Based on XRD, SEM and TEM-EDS analysis, it has been found that the precipitates in the steels that were tempered at 300 °C, 500 °C and 650 °C, are nano-sized ϵ-M3C carbides, nano-sized Cr-rich M23C6 carbides and micron or submicron-sized Cr-rich M23C6 carbides, respectively. Potentiodynamic polarization curves and electrochemical impedance spectroscopy studies indicate that the pitting potential of the as-quenched steel is higher than those of the tempered steels, and decrease with the increase in tempering temperature. Especially when tempered at 500 °C, the corrosion resistance of the steel decreases abruptly. It shows that the precipitation of the massive-amount of nano-sized Cr-rich M23C6 carbides would provide lots of interface of carbides/matrix as the pitting initation and then hinder the formation of protective passive film on the steel surface due to the short interspace between the tempered carbides.