Electrochemical Behavior and Passive Property of 13Cr Martensitic Stainless Steel in Nitric Acid Solution

Intergranular corrosion behavior of a 13Cr martensitic stainless steel (MSS) in tempered condition was studied. Tempering was done in the temperature range of 300-700 degrees C after austenitization, and microstructure was established using scanning and transmission electron microscopy. Electrochemical characterizations to establish localized corrosion susceptibility were performed in 5% HNO3 solution. The MSS tempered at 550 degrees C showed sensitization and significant localized attack during tests in HNO3. Characterization of surface film on this condition revealed an oxide with a higher defect density (through electrochemical impedance spectroscopy-Mott-Schottky analysis) and Fe content (x-ray photoelectron spectroscopy) as compared to that for the austenitized condition. The inability of sensitized MSS tempered at 550 degrees C to form a protective surface film was attributed to the formation of considerable nano-sized Cr-rich carbide precipitates associated with narrow Cr depletion zones. On the other hand, the MSS tempered at 300 and 700 degrees C did not show any localized attack in HNO3 and this was attributed to lack of sensitization at 300 degrees C and de-sensitization at 700 degrees C.