Failure Analysis of Chromium Plating Layer on the Surface of the Piston Rod of the Hydraulic Jack

The piston rod of the hydraulic jack was a kind of artillery alloy structural steel of 40Cr and its surface was the chromium plating layer. The piston rod worked for a while; the defects of corrosion pit and peeling appeared on the chromium layer. A stereo-microscope, metallographic microscope, and scanning electron microscope (SEM) were used to observe the macromorphology, micromorphology, and microstructure of the failed parts. The composition analysis was performed by the energy dispersive spectroscopy (EDS), and a Vickers hardness tester was used to measure the hardness. The results showed that pores and penetrating cracks existed in the chromium layer, leading to the corrosion medium invading the interface and forming a corrosion source. Then, the corrosion intensified, resulting in the bubbling, cracking, and peeling of the chromium layer. More O and minor S elements were detected in the corrosion pit. Finally, the fracture of the chromium layer was a 45° angle destruction mode. The peeling of the chromium layer was caused by the pores and microcracks, working medium, and poor working environment. Some suggestions were put forward to prevent the peeling of the chromium layer.