The mechanisms underlying the effect of tempering time on the pitting behavior of high-nitrogen martensitic stainless steel

High-nitrogen martensitic stainless steel (HNMSS) is widely used in chemical, marine, and medical applications due to its excellent mechanical properties and corrosion resistance. However, pitting corrosion remains a major issue, particularly in chloride-containing environments. The tempering process after quenching affects both the mechanical properties and microstructure of HNMSS, but its influence on pitting behavior is not well understood. This study investigates the effect of tempering duration on the pitting corrosion of HNMSS in 3.5 wt% NaCl solution. The HNMSS was solution-treated at 1030 degrees C for 30 min, followed by oil quenching and cryogenic treatment, and then tempered at 500 degrees C for 1-100 h. Microstructure characterization was performed using XRD, SEM, and TEM. Potentiodynamic polarization and EIS were used to assess pitting behavior, while in-situ electrochemical techniques and SEM were employed to observe corrosion morphology. The results showed that after solution treatment, HNMSS contained significant undissolved primary Cr2N, with no chromium carbides. Secondary Cr2N appeared only after 100 h of tempering. Pitting sensitivity increased with longer tempering times, with the pitting potential decreasing from 0.348 V (cryogenic treatment) to 0.124 V after 2 h of tempering. Beyond 5 h, pitting occurred without anodic polarization, resulting in loss of passivation. This behavior is related to the decrease in chromium content in the matrix surrounding the primary Cr2N, which drops to 7.47 at% after 100 h of tempering. The primary Cr2N had a more significant impact on pitting sensitivity than the secondary Cr2N.