Improvement of corrosion resistance of nitrided low alloy steel by plasma post-oxidation

Post-oxidizing treatments can be performed to improve the corrosion resistance of nitrided steel samples. In this paper, plasma nitriding treatments were performed at 540 degrees C for 4 h using ammonia as the working gas, and plasma post-oxidizing treatments were carried out at temperatures ranging from 350 degrees C to 500 degrees C for 2 h in oxygen gas. The treated samples were characterized by using optical microscopy, SEM, XRD, and electrochemical polarization. The X-ray analysis revealed the formation of iron-nitride phases of epsilon-Fe(2-3)N and gamma'-Fe(4)N during plasma nitriding and iron oxide phases of hematite (Fe(2)O(3)) and magnetite (Fe(3)O(4)) through the post-oxidizing treatment. In particular, it was found that the very thin magnetite layer 0.8-1.5 mu m in thickness on top of the compound layer was obtained by plasma post-oxidized at 400 degrees C and 450 degrees C. It was also demonstrated that the corrosion characteristics of the nitrided compound layer were further improved by post-oxidation treatment. (C) 2010 Elsevier B.V. All rights reserved.