Abstract: The microstructure, micro area composition, electrochemical corrosion property and corrosion product composition of the corroded Al-Mg-Cu alloy drainage plate were investigated and compared with those of the new drainage plate. The reason for corrosion failure was analyzed. The results show that the corrosion of the Al-Mg-Cu alloy drainage plate was caused by acid rain containing sulphur element; the thermal effect of electric current led to the aged structure of the drainage plate, and the precipitation and coarsening of θ phase decreased the corrosion resistance of the drainage plate further.