Influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel

The influence of layer microstructure on the corrosion behavior of plasma nitrided cold work tool steel, of commercial name "DC53", in 3.5% NaCl solution is reported. The specimens were nitrided at 520 A degrees C for different treatment times using a constant [N-2 + H-2] gaseous mixture by a DC-pulsed plasma system. The microstructure of the nitrided layers was investigated by optical microscopy and X-ray diffraction. The corrosion behavior was evaluated by potentiodynamic polarization experiments. The plasma nitriding process considerably improves the corrosion resistance of material in NaCl environment as compared to the unnitrided DC53 steel. The modified surface layer consisting mainly of epsilon-nitride (Fe2-3N) and a small amount of gamma'-nitride (Fe4N) confers this outstanding behavior. The corrosion resistance dependence on specific nitriding processes is reported and the role of the epsilon-nitride is discussed. In particular, the correlation of pitting current density, density of pits, and volume fraction of epsilon-nitride with nitriding time is analyzed. The results denote that the most important parameter for controlling the corrosion resistance of the material is the volume fraction of epsilon-nitride and the nitrided layer thickness. It is expected that a nitrided layer would be thicker and rich in epsilon-nitride phase to achieve a high corrosion resistance.