Effect of Cr Plating and Plasma Nitriding on Hardness and Corrosion Resistance of H13 Steel

H13 steel has been widely used in several metal working industries. Plasma nitriding is employed for improving surface hardness, but it significantly decreases corrosion resistance of H13 steel. To improve corrosion resistance, Cr plating before and after plasma nitriding is studied in the research. Surface characterization, phase analysis, hardness and corrosion test were done to evaluate the appropriate method to improve corrosion resistance. It was found that Cr plating after plasma nitriding can improve the corrosion resistance close to as-heat treated sample. The formation of Cr can increase the corrosion potential to-310.66 mV(Ag-AgCl) comparing to-349.54 mV(Ag-AgCl) of as-heat treated sample. For the corrosion resistance at constant applied voltage tested by potentiostatic technique, the sample with plasma nitriding prior to Cr plating shows the lowest corrosion current which implies the lowest corrosion rate. For the process in which Cr plating is applied before plasma nitriding, CrN layer can be formed on the surface, but this layer is very thin (about 100 nm thick). This thin layer retards nitrogen diffusion; hence, nitrided layer beneath CrN layer cannot be formed. With only thin layer of CrN, both corrosion resistance and micro-scale surface hardness cannot be improved.