Influence of Chromium Content on Corrosion of Plasma-Nitrided Steels

Abstract Commercial steels with varying chromium contents were plasma nitrided for 25 h in a 70% nitrogen (N2) + 30% hydrogen (H2) gas atmosphere at 520°C under a pressure of 670 Pa. Steels tested included AISI types 4140, with 1% Cr (UNS G41400); H-13, with 5% Cr (UNS T20813); D-2, with 12% Cr (UNS T30402); and 431, with 16% Cr (UNS S43100). Surface layers obtained were characterized using optical microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and scanning electron microscopy with energy dispersive x-ray spectroscopy. Potentiodynamic polarization curves were obtained at room temperature (RT) on samples immersed in an aerated 0.05 M sodium sulfate (Na2SO4) solution of pH 3.0. Polarization tests were repeated at various depths beneath the surface after grinding successively with abrasives and finishing with 3-μm diamond paste. Potential-vs-time graphs were recorded. Nitrided steels with up to and including 12% Cr showed an increase in corrosion resistance, whereas a decrease was observed ...