In order to explain the corrosion inside 20 th century power lines, the galvanic corrosion of steel and aluminum from 1949 was examined. In this investigation, 0.1 M Na2SO4 + 1 mM NaCl was used as moderately corrosive medium. Various electrochemical methods were applied to determine the corrosion potential and the different reactions involved in the corrosion process. The thickness of the oxide layer that regulates the corrosion of aluminum was obtained from impedance measurement and it was shown that the diffusion of oxygen through a porous oxide layer is the governing stage for the reaction occurring at the steel electrode. Finally, the corrosion of steel regulates the corrosion of both metals when they are in electrical contact.