In situ observation on filiform corrosion propagation in Mg-Al-Y alloy: the effects of continuous and discontinuous Mg17Al12 precipitates

Abstract

The filiform corrosion behavior of an aged Mg-9Al-0.2Y (wt.%) alloy in a chloride solution was systematically investigated by in situ observations and advanced characterization. The results show that filiform corrosion preferentially propagates along the lamellar discontinuous Mg17Al12 precipitates (DPs) regions, whereas the dense continuous precipitates (CPs) regions exhibit a barrier effect against corrosion propagation. The stronger micro-galvanic effect and the higher α-Mg fraction in the DPs regions facilitate the preferential propagation of filiform corrosion. Due to the weaker micro-galvanic interaction and compact cross-linked microstructure, a dense and well-adhered corrosion product layer forms at the CPs-region interfaces, effectively suppressing the anodic dissolution tendency of the Mg matrix. It was further demonstrated that corrosion preferentially propagated along grain boundary regions with higher electrochemical activity, particularly when its propagation was hindered by CPs-rich regions.