Investigation of the Intergranular Corrosion Behavior of UNS S42000 Martensitic Stainless Steels Using Potentiodynamic Electrochemical Impedance Spectroscopy

In this research, UNS S42000 martensitic stainless steel (SS) samples were subjected to tempering heat treatment at constant temperature of 550 oC for different time duration, ranging between 0.5 and 24 h, so as to assess the intergranular corrosion (IGC) behavior of the alloy. The microstructural characteristics and electrochemical properties of the alloys were evaluated using scanning electron microscopy (SEM) and anodic polarization method, respectively. The localized attack of the samples was investigated by potentiodynamic electrochemical impedance spectroscopy (PD-EIS) in the transpassive potential region. The results indicated that tempering up to 2 h would be followed by severe intergranular attack at martensite lath interfaces; however, the effect of tempering on corrosion performance of the alloys would be suppressed at prolonged tempering due to desensitization. Moreover, the PD-EIS results revealed that the charge transfer resistance (Rct) values of specimens at 1.10 V can be employed as a primary criterion in order to assess the localized attack of the martensitic SS. In this regard, the lower the Rct value at this DC bias potential, the lower the corrosion resistance to the IGC. These results were in good agreement with the SEM morphologies from the surfaces of corroded samples, indicating uniform corrosion, severe attack and desensitization for samples experiencing no-tempering, short tempering time and prolonged tempering, respectively.