Effect of Ga Content on Localized Corrosion of Al-9 mass%Mg Alloys in H<SUB>2</SUB>SO<SUB>4</SUB>–NaCl Solution

Abstract

The effects of Ga content and aging treatment on localized corrosion characteristics of Al-9 mass%Mg alloys which contain a small amount of Ga were investigated by the measurements of polarization curves, potentiostatic corrosion tests and immersion corrosion tests. The scanning electron micrographs of precipitates of the secondary phase with aging treatment and the surface of the specimens corroded in NaCI containing 1 x 10 -2 kmol/m 3 H 2 SO 4 solution at 303 K were also observed. The principal conclusions are as follows: (1) The Al 3 Mg 2 compounds of the secondary phase of Al-9 mass%Mg alloys aged at 453 K for 6.0 ks precipitate along the grain boundary irrespective of the specimens used. However, rodlet precipitates are observed even within the grain, and precipitate-free zones are formed along the grain boundary of the specimens aged at 453 K for 307.2 ks. (2) The pitting potential E pit of Al-9 mass%Mg alloys is shifted to a more basic potential with increasing NaCI concentration, Ga content in the alloys and aging time intervals, and thus the degradation of localized corrosion resistance were observed. (3) The apparent anodic dissolution rates under the potentiostatic condition are enhanced with increasing Ga content in the alloys and aging time. The rates are increased in the order of super-saturated solid solution<the specimens aged at 453 K for 6.0 ks<the specimens aged at 453 K for 307.2 ks. (4) The corrosion of super-saturated Al-9 mass%Mg alloys in H 2 SO 4 -NaCl solution is mainly pitting attack. However, the localized types of corrosion of the aged specimens are observed as intergranular and pitting attacks.