Structure and Corrosive Wear Resistance of Plasma-Nitrided Alloy Steels in 3% Sodium Chloride Solutions

Type 304 (UNS S30400) stainless steel (SS), type 410 SS (UNS S41000), and type 4140 low-alloy steel (UNS G41400) were plasma nitrided in a commercial furnace at 560°C for 50 h. Microstructure and the composition of the nitrided layer were analyzed. The resistance to corrosive wear was evaluated by a tribotest in which the specimen was held under potentiostatic control at anodic and cathodic potentials in 3% sodium chloride (NaCl) solution (pH 6.8). Electrochemical polarization measurements were made, and the surface morphology and composition after corrosive wear were examined. Wear rates at cathodic potentials were very low, but significant weight losses were observed as the applied potentials were increased anodically. The coefficient of friction varied in a fashion similar to the wear rate. For the untreated alloys, the magnitude of the wear rate and coefficient of friction decreased as follows: type 4140 alloy > type 410 SS > type 304 SS. For the plasma-nitrided alloys, the ranking was: type 304 SS > type 410 SS > type 4140 alloy. Plasma nitriding was shown to be beneficial to the corrosive wear resistance of type 4140 alloy, but an adverse effect was obtained for types 304 and 410 SS. These findings could be interpreted in terms of the electrochemical polarization characteristics of a static specimen and were strongly related to the subtleties of the nitrided microstructures. The stable chromium nitride (CrN) segregated in the y-iron (Fe, type 304 SS) and α-Fe (type 410 SS) matrices and resulted in a pitting and spalling type of corrosive wear mechanism. The phases ϵ-(Fe, Cr)_2-3N and γ'-(Fe, Cr)₄N enriched in the surface layer of nitrided type 4140 alloy provided excellent corrosive wear resistance.