In this paper, the effect of deep cryogenic treatment on the corrosion resistance of 42CrMo low alloy steel is investigated and compared with conventional heat-treated counterparts. The low-temperature treatments of the cryogenic process are -120 °C, -160 °C, and -190 °C, respectively. Electrochemical corrosion tests show that the self-corrosion current density of -120 °C, -160 °C and -190 deep-cooled specimens is reduced by 38%, 20% and 30% respectively compared to the usual heat-treated specimens. Scanning electron microscope analysis shows that the precipitation of fine carbides on the surface of the samples treated at -120 °C has improved their corrosion resistance. Electrochemical impedance spectroscopy also shows that the samples treated with -120 °C cryogenic treatment have the smallest corrosion tendency. At a -160 °C deep-cooling process, the precipitated carbide aggregation limits the corrosion resistance of the material. The corrosion resistance of the samples in the -190 °C process group is between the two. The simulation results also express a similar trend to the electrochemical corrosion results.