Mossbauer spectroscopy study on the corrosion resistance of plasma nitrided ASTM F138 stainless steel in chloride solution

Plasma nitriding of ASTM F138 stainless steel samples has been carried out using dc glow discharge under 80% H-2-20% N-2 gas mixture, at 673 K, and 2, 4, and 7 h time intervals, in order to investigate the influence of treatment time on the microstructure and the corrosion resistance properties The samples were characterized by scanning electron microscopy, glancing angle X-ray diffraction and conversion electron Mossbauer spectroscopy, besides electrochemical tests in NaCl aerated solution. A modified layer of about 6 mu m was observed for all the nitrided samples, independent of nitriding time. The X-ray diffraction analysis shows broad gamma(N) phase peaks, signifying a great degree of nitrogen supersaturation. Besides gamma(N), the Mossbauer spectroscopy results indicated the occurrence of gamma' and E phases, as well as some other less important phases Corrosion measurements demonstrate that the plasma nitriding time affects the corrosion resistance and the best performance is reached at 4 h treatment It seems that the epsilon/gamma' fraction ratio plays an important role on the resistance corrosion Additionally, the Mossbauer spectroscopy was decisive in this study, since it was able to identify and quantify the iron phases that influence the corrosion resistance of plasma nitrided ASTM F138 samples (c) 2010 Elsevier Inc All rights reserved