Effect of plasma nitriding parameters on corrosion performance of 17-4 PH stainless steel

This study investigates the effect of plasma nitriding parameters on corrosion susceptibility of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. In this regard, 17-4 PH stainless steel was plasma nitrided at 400 degrees C for 5 and 10 h, 450 degrees C for 5 h and 500 degrees C for 5 h. Cross-sectional images after nitriding process showed that a uniform nitrided layer has been formed on steel substrate. Depending on the temperature and time of the nitriding process, different phases were formed in the nitrided layer. This affected general corrosion and pitting corrosion performance of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. While precipitation of chromium nitrides for nitrided specimens at 450 degrees C and higher increased the susceptibility to pitting and general corrosion, formation of expanded martensite ( EM) in nitriding at 400 degrees C improved the pitting corrosion resistance of 17-4 PH stainless steel. This is believed to be due to the release of nitrogen atoms from EM phase to form ammonium ions and increase the pH of the solution, supressing pit growth.