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 degrees C, 500 degrees C and 650 degrees C, are nano-sized epsilon-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 degrees 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. (C) 2015 Elsevier Ltd. All rights reserved.