Effect of Different Surface Treatments of 42CrMo Steel Piston Rod on its Corrosion Resistance in Sodium Chloride Solution

42CrMo has good hardenability, high temperature strength and creep resistance, and is widely used in the manufacture of piston rod and other parts. Piston rod is the core moving part of automobile shock absorber, the failure of shock absorber will affect the performance of the entire automobile suspension system, and has a great threat to the safety of automobile driving. Therefore, improving the surface corrosion resistance of piston rod has important practical significance. In this paper, the piston rod surface was strengthened by trivalent chromium electroplating technology and gas oxy-nitriding composite treatment technology, and 42CrMo steel was used as the matrix material of the piston rod to study the corrosion resistance of the sample with electrochromium plating and gas oxy-nitriding composite treatment. The modified layer was observed and characterized by IM3000 inverted metallographic microscope. The potentiodynamic polarization curves and electrochemical impedance spectrum of the samples were measured by GAMRY ESA410 electrochemical workstation to analyze theircorrosion resistance. The results show that the total thickness of oxide layer and compound layer is 20.89 μm for GNO sample and 18.56 μm for GNPO sample. The compound layer has good anticorrosion effect. The corrosion resistance of the samples after the different processes in order of strength to weakness are GNPO sample, GNO sample, chrome-plated sample, nitriding sample and substrate. The Ecorr of corrosion potential and Icorr of corrosion current density of GNPO sample are -363 mV and 0.89 μA/cm2 respectively, showing the best corrosion resistance.