Formation of amorphous precipitates in a corroded over-aged Al-Mg-Si alloy

Abstract Coarse intermetallics and fine precipitates play an important role in the corrosion of Al alloys. So far, there is no systematic experimental investigation about the corrosion of nano-precipitates. In the present work, the morphology and chemical composition of the corroded MgSi nano-precipitates in an Al-Mg-Si alloy with different aging times have been studied by transmission electron microscopy. Coarsened βʹ precipitates occurring in the over-aged state were found to be corroded through dissolution of Mg and oxidation of the remnant Si during twin-jet electropolishing. They finally transformed to amorphous SiO2 precipitates, showing an almost unetched interface with the Al matrix. The proportion of amorphous precipitates increased with prolonging aging time, as larger precipitates were preferentially corroded. This newly discovered crystalline-to-amorphous phase transformation depletes acid electrolyte but shows no damage. Such an intragranular corrosion behavior partly accounts for why over-aged Al-Mg-Si alloys are less susceptible to pitting and intergranular corrosion.