EFFECT OF GA CONTENT ON DEGRADATION OF CO RROSION-RESISTANCE OF SUPER-SATURATED AL-9 MASS-PERCENT-MG ALLOY

The degradation of corrosion resistance for Al-9 mass % Mg alloys with Ga is investigated by the measurements of anodic polarization curves, potentiostatic dissolution tests, immersed corrosion tests and scanni ng electron microscopic observation of corroded surfaces of the alloys with a supersaturated solid solution phase in dilute HClO4 solutions. The main results obtained from the experiments are summarized as foll ows: (1) The activation potentials (E(active)) of Al-9 mass%Mg alloys with a supersaturated solid solution phase in a dilute HClO4 solution decreases with increasing Ga content in the alloys and the solution te mperature. Therefore, the corrosion resistance of the alloy decreases with increasing Ga content in the alloys and solution temperature. The corrosion of the alloys is uniform despite of the specimens used at t he supersaturated solid solution phase. (2) The corrosion behavior of Al-9 mass%Mg alloys in dilute HClO4 solutions may be divided into two potential regions: The oxide film is formed on the corroded surfaces o f the alloys, and the anodic dissolution rates decreased at the potent ials below E(active). On the other hand, the anodic dissolution rates increase with increasing concentration of HClO4, solution temperature and Ga content in the alloys at the potentials higher than E(active). The corrosion of the alloys containing Ga is enhanced by the selective dissolution of Mg or Al from the matrix of the alloys.