The effects of alloying elements on the passivity of sputter-deposited amorphous Al-Cr-Mo alloys in 1M HCl

The corrosion behavior of sputter-deposited amorphous Al-Cr-Mo alloys in 1 M HCl solution was investigated by electrochemical measurements. Particular attention was paid to the effects of alloying elements on the surface film composition related to the corrosion behavior. The amorphous Al-Cr-Mo alloys are passivated spontaneously even in 1 M HCl, while the open circuit potential of Al-Cr binary alloys are in the active region even if a large amount of chromium is added. The addition of molybdenum to Al-Cr alloys leads to an enrichment of chromium in the surface film formed by anodic polarization with a consequent decrease in the passive current density and prevention of pitting corrosion, though the excess addition of molybdenum raises the anodic current density because the protective ability of the surface film is lowered by transpassive dissolution of molybdenum. The formation of the chromium enriched passive film suppresses the deterioration of the passive film due to transpassive dissolution of molybdenum. Thus, additions of chromium and molybdenum to aluminum are synergistically effective in improving the passivity of aluminum in a wide potential region. The cationic fraction of molybdenum in the spontaneously passivated film is almost the same as the atomic fraction of molybdenum in the bulk alloy, and molybdenum becomes deficient in the passive film formed by anodic polarization. It is therefore considered that the addition of molybdenum improves the passivating ability not by the formation of the molybdenum enriched passive film, but by assisting the formation of the chromium enriched passive film. Copyright (C) 1996 Elsevier Science Ltd.