The present work is concerned with atmospheric corrosion of Al-1wt%Si-0.5wt%Cu alloy under humid and corrosive atmospheres. The atmosperic corrosion has been investigated in terms of Volta potential and corrosion rate under various relative humidities and also in terms of changes of electrode potential and galvanic current during the transition from a chloride solution to a dry state. The Volta potential difference was measured between the vibrating reference electrode (platinum) and the thin film of water vapour covering the surface of the Al alloy by the Kelvin Probe method. The corrosion rates were determined from polarization curves obtained by using the atmospheric microcell which consists of a flat and compact configuration of working electrode (Al alloy) / reference electrode (hydrogen charged palladium) / counter electrode (platinum). The measured Volta potential difference was found to change from positive to negative value with increasing relative humidity, indicating the existence of a critical humidity. The corrosion rates increased with increasing relative humidity. During the transition from the chloride solution to the dry state, the electrode potential increased in the anodic direction more slowly with increasing chloride concentration and the galvanic current for Al alloy / Pd atmospheric corrosion monitor couple showed a peak value which was diminished with decreasing chloride ion concentration. As against the current peak was not observed for the distilled water. The appearance of the current peak was discussed in view of oxygen reduction and anodic reaction.
