Abstract: The corrosion behavior of a chromium-plated 30CrMnSiNi2A ultra-high strength steel piston rod treated with diamond tool finishing and fine polishing was studied by electrochemical test in a NaCl solution with 3.5% mass fraction, 48 h SO₂ salt spray corrosion test, and microstructural characterization. The results show that chromium plating improved the corrosion resistance of the piston rod in NaCl medium, which was manifested as the increase in the self-corrosion potential and the decrease in the self-corrosion current density. After 48 h of SO₂ salt spray corrosion, obvious corrosion was observed on the surface of the chromium-plated piston rod, mainly manifested as pitting accompanied by localized discoloration; reddish-brown corrosion products adhered to the pitting areas. The pitting corrosion originated from the initial corrosion of the chromium plating in the SO₂ salt spray environment, exposing the inherent microcracks of the chromium plating. After the corrosive medium penetrated through these microcracks, the chromium plating and the exposed steel matrix formed a large cathode-small anode corrosion couple, which accelerated the corrosion of the steel matrix, resulting in the formation of pitting pits and causing the crack of chromium plating. The localized discoloration was attributed to the roughening of the coating surface due to slight corrosion. The microcracks in the plating were identified as the main cause of corrosion of the steel matrix.