The kinetics and the effects of tempering on the corrosion resistance of AISI 4130 steel were studied. The tempering activation energy was close to that for the diffusion of C in α-iron. This implies that the tempering kinetics in this steel is controlled by carbon diffusion. By increasing the time and temperature of tempering, the corrosion current density (i corr ) in the polarization curves decreased whereas the diameter of the semicircular arc in the Nyquist plots increased, which implies higher corrosion resistance. The i corr was successfully related to hardness, which actually indicates the effect of microstructure.Based on these findings, the quench and tempering treatment was used to adjust both the hardness and corrosion resistance of the AISI 4130 steel.