Evolution of metastable phases during Mg metal corrosion: An in situ cryogenic x-ray photoelectron spectroscopy study

Magnesium and its alloys are potential structural material candidates for a wide variety of applications due to their high strength-to-weight ratio. However, ductility and poor corrosion resistance under ambient environmental conditions are the bottlenecks for industrial deployment. Designing passivation layers and/or corrosion-resistant alloys requires fundamental understanding of the corrosion process. The traditional ex situ spectroscopic measurements of a polycrystalline metal surface with ubiquitous surface impurities and grain boundaries only provided an indistinct view of the corrosion process. To clearly distinguish the mechanism and the sequence of the corrosion process, we employed in situ cryo-based x-ray photoelectron spectroscopy (XPS) measurements on the Mg single crystal surface exposed to aqueous salt solution. Clean Mg (0001) surfaces were exposed to pure D2O and NaCl aqueous solution (5 wt. % NaCl + 95 wt. % D2O). The interfacial reactions were studied using a multimodal analysis including XPS, x-ray diffraction, and scanning electron microscopy. In contrast to previous studies, our experiments demonstrated the formation of magnesium chloride hydroxide hydrate during aqueous salt corrosion processes. Evidence of metastable ClO* radicals were also found during the initial aqueous salt solution exposure.