PLASMA SURFACE-TREATMENT OF AISI-4140 STEEL FOR IMPROVED CORROSION-RESISTANCE

Alloy AISI 4140 steel was plasma-nitrided in a low-pressure abnormal g low discharge in 75% N-2+25% H-2 atmosphere (P=3 torr and T=540 degree s C for 2h) using a home-made laboratory reactor. The corrosion behavi our was evaluated in a chloride environment by potentiostatic and pote ntiodynamic polarization as a function of thermal treatment and coolin g cycle. Results from potentiodynamic polarization in NaCl (3%) indica ted 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 pote ntial shifted to noble potential clearly indicating that the ionitrida tion process is a very efficient process for corrosion protection of A ISI 4140 steel. Typical results obtained from samples oxidized during the cooling cycle showed an improvement in the corrosion resistance. T he potential for pitting initiation and propagation as well as for pit ting protection has been determined in a chloride environment. The use of electrochemical techniques as a control of standard parameters has been suggested.