Abstract
LZ91 alloy is an ultra-lightweight Mg alloy for weight-reduction applications. However, its dual-phase structure (HCP alpha and BCC beta) complicates the corrosion behavior. This study investigated the localized corrosion behavior of a commercial LZ91 alloy in 3.5 wt% NaCl solution and its relationship to the microstructure of the surface corrosion films on the two phases. Microstructure characterizations showed that the surface corrosion films on the alpha and beta phases both consisted of an outer Li2CO3/Mg(OH)2 layer, a middle Mg(OH)2 layer, and a thin Zn-enriched interfacial MgO layer. Nevertheless, the overall thickness of the surface corrosion film on beta was thicker than that on alpha due to the lower Volta potential of beta and the resultant microgalvanic effect. Localized corrosion initiates on the alloy after 8-12 h of immersion, accelerating the corrosion rate. Noteworthily, localized corrosion preferentially propagates in alpha because of the thinner and less protective surface corrosion film.