Localized Corrosion Susceptibility of Mg Alloy Extrusions for Automotive Applications

Abstract

Magnesium (Mg) alloys hold significant promise for automotive lightweighting, offering benefits such as improved fuel efficiency for internal combustion engines and extended driving range for electric vehicles. However, their broader application is limited by poor room-temperature formability due to Mg's hexagonal close-packed (HCP) crystal structure and high corrosion susceptibility. Alloying presents a viable solution to enhance both the formability and corrosion performance of Mg alloys. This study evaluates the localized corrosion susceptibility of three novel Mg-Zn-Al-Ca-Mn-Ce (ZAXME) Mg alloys to inform the selection of materials for a lightweight multi-material bumper system, featuring a friction stir welded joint between a hollow Mg beam and AA6061-T6 crush cans. This include three sub-objectives: (1) identifying the most suitable ZAXME alloy version for application in the multi-material bumper structure by balancing corrosion performance, energy absorption, and extrudability; (2) comparing the corrosion behavior of the selected ZAXME alloy to conventional Mg extrusion alloys Mg–3Al–0.4Mn (AM30), Mg–2Zn–0.2Ce (ZE20) and pure Mg to assess the influence of key alloying elements; and (3) refining the mechanistic understanding of filament-like corrosion in Mg alloys based on insights gained from the experimental findings. Microstructural analysis using light optical (LOM) and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) identified three metallurgical regions in the extruded samples: (i) a thin fine-grained outer layer (removed prior to corrosion testing), (ii) a coarse-grained sub-layer (CGSL), and (iii) a fine-grained core. The corrosion susceptibility of each region was assessed using electrochemical polarization, bulk immersion and corrosion visualization techniques during immersion in room temperature 0.1 M NaCl (aq). Post-exposure analysis employed LOM, SEM+EDX, and focused ion beam (FIB-SEM) milling to prepare site-specific samples for scanning transmission electron microscopy (STEM), enabling detailed investigation of the fundamental corrosion filament propagation mechanism. The results underscore the critical role of alloying elements in modulating both anodic and cathodic kinetics, thereby influencing the underlying filament-like corrosion mechanisms. Key findings from this study include: (i) Alloy-3 (Mg–0.4Zn–0.4Al–0.2Ca–0.75Mn–0.2Ce) demonstrated the lowest corrosion rate among the ZAXME versions while maintaining moderate energy absorption; (ii) when compared to the benchmark alloys AM30 and ZE20, Alloy-3 exhibited superior extrudability at lower extrusion temperatures but lower corrosion resistance, attributed to the presence of secondary particles with higher cathodic activity; and (iii) the corrosion kinetics of ZAXME alloys showed an initially high rate within the first 2–6 hours, followed by a transition to a steady-state regime with linear corrosion kinetics. Additionally, filament-like corrosion propagation occurred in multiple passes for all of the experimental Mg alloys, contrasting with the single-pass propagation observed in pure Mg.