Plasma surface treatment of AISI 4140 steel for improved corrosion resistance

Alloy AISI 4140 steel was plasma-nitrided in a low-pressure abnormal glow discharge in 75% N-2+25% H-2 atmosphere (P=3 torr and T=540 degrees C for 2h) using a home-made laboratory reactor. The corrosion behaviour was evaluated in a chloride environment by potentiostatic and potentiodynamic polarization as a function of thermal treatment and cooling cycle. Results from potentiodynamic polarization in NaCl (3%) indicated the presence of a low anodic dissolution current of about 1 mu Acm(-2) for ionitrided steel having layers thickness >5 mu m as compared to 0.1 Acm(-2) for untreated steel. In addition, the open circuit potential shifted to noble potential clearly indicating that the ionitridation process is a very efficient process for corrosion protection of AISI 4140 steel. Typical results obtained from samples oxidized during the cooling cycle showed an improvement in the corrosion resistance. The potential for pitting initiation and propagation as well as for pitting protection has been determined in a chloride environment. The use of electrochemical techniques as a control of standard parameters has been suggested.