Abstract: After one week of installation, the surface of 304 stainless steel pipes of a anti-theft cover suffered serious tarnishing. In order to find out the tarnishing causes and mechanism, the pipe were analyzed by methods of macroscopic observation, metallographic examination, scanning electron microscope analysis, energy spectrum analysis, chemical composition analysis, electrochemical corrosion test and simulated contrast test. The results show that the surface of the pipe was corroded by chlorinated acid, which caused the passivation layer of material to be damaged and pitting corrosion, and it gradually developed to serious tarnishing.