Intergranular Corrosion Mechanism in Al-Mn Alloys

Abstract

To investigate the intergranular corrosion mechanism in Al-Mn alloys, galvanostatic anodic dissolution in chloride solutions of Al-Mn alloys containing 0.4 to 1.2 mass percent of manganese and electrochemical polarization measurements in AlCl₃ solution on the alloys and on Al-Mn compounds have been carried out. Pitting types of localized dissolution were observed after galvano-static anodic dissolution in the alloys quenched after solution treatment at 873 K, while, distinct intergranular dissolution occurred after reheating at 673 K. Iron and other impurities in the alloys accelerated the intergranular attack. According to EDXS on an Al-Mn alloy thin foil the amount of manganese in solid solution was much lower along grain boundaries than in the grain bodies in the alloys susceptible to the attack, while pitting potential increased with increasing manganese content. With these results it could be concluded that intergranular corrosion of Al-Mn alloys occurred due to selective dissolution of a manganese depleted zone along a grain boundary which could be formed with precipitation on the grain boundary. Surface observation after galvanostatic anodic dissolution in acidified chloride solution was usefull also in Al-Mn alloys as a rapid estimation of the intergranular corrosion susceptibility.