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

Abstract

Plasma nitriding of ASTM F138 stainless steel samples has been carried out using dc glow discharge under 80% H2–20% N2 gas mixture, at 673 K, and 2, 4, and 7h time intervals, in order to investigate the influence of treatment timeon themicrostructure andthecorrosion resistance properties. The samples were characterized by scanning electron microscopy, glancing angle X-ray diffraction and conversion electron Mössbauer spectroscopy, besides electrochemical tests in NaCl aerated solution. A modified layer of about 6 μm was observed for all the nitrided samples, independent of nitriding time. The X-ray diffraction analysis shows broad γN phase peaks, signifying a great degree of nitrogen supersaturation. Besides γN, the Mössbauer spectroscopy results indicated the occurrence of γ′ and ε 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 4h treatment. It seems that the ε/γ′ fraction ratio plays an important role on the resistance corrosion. Additionally, the Mössbauer 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.