Plasma nitriding experiments were carried out with DC-pulsed plasma in 25 % N-2 + 75 % H-2 atmosphere at low temperature (350 degrees C) and normal temperature (550 degrees C) for 15 h. The composition, microstructure, microhardness profiles, residual stress profiles and electrochemical impedance spectrum analyses of the nitrided samples were examined. The influence of plasma nitriding on the erosion and erosion-corrosion resistance of AISI 420 martensitic stainless steel was investigated using a jet solid particle erosion tester and a slurry erosion-corrosion tester. Results showed that the 350 degrees C nitriding layer was dominated by epsilon-Fe3N and alpha(N) phase, a supersaturated nitrogen solid solution. However, nitrogen would react with Cr in the steel to form CrN precipitates directly during 550 degrees C nitriding, which would lead to the depletion of Cr in the solid solution phase of the nitrided layer. Both 350 and 550 degrees C plasma nitriding could improve the erosion resistance of AISI420 stainless steel under dry erosion, but the former showed better results. In both neutral and acid environment, while the erosion-corrosion resistance of AISI 420 was improved by means of 350 degrees C nitriding, it was decreased through 550 degrees C nitriding.