Improved corrosion resistance of 42CrMo4 steel by reconstructing surface integrity using ultrasonic surface rolling process

The ultrasonic surface rolling process (USRP) was adopted to improve the corrosion resistance of 42CrMo4 steel by reconstructing the surface integrity in terms of surface roughness, microhardness, residual stress, and microstructures. The corrosion behaviors of 42CrMo4 steel were studied by salt spray corrosion, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). The results indicated that the surface integrity of 42CrMo4 steel was restructured after USRP with the surface roughness reduced to 0.052 mu m, the surface microhardness improved to 305 HV0.1, the residual compressive stress achieved up to - 274 MPa at 0.6 mm depth, and the average grain diameter minimized to 2.02 mu m in the zone of 5-150 mu m depth. Owing to the modified surface roughness, hardened layer, and induced residual compressive stress, the corrosion resistance of 42CrMo4 steel was enhanced by USRP with the annual corrosion rate of 2.972 mm/a reduced about 5.2 %, the corrosion current density of 2.748 mu A center dot cm(-2) decreased up to 45.2 %, and the charge transfer resistance of 1809 ohm center dot cm(2) increased up to 19.6 %.