Effect of Li Content on the Surface Film Formed on the Binary Mg–Li Alloys in NaCl Solution

The surface film formed on the Mg–Li alloys with different Li content in 0.1 M NaCl solution were investigated via electrochemical testing, morphologies observation and chemical composition analysis in this work. The results revealed that the surface film of Mg–14Li alloy (L14: body centred cubic (BCC)) possessed higher electrical resistivity and remained a higher tolerating over-potential (0.1 V vs. open circuit potential) than those of Mg–4Li (L4: hexagonal closed-packed (HCP)) and Mg–7.5Li (L7: HCP+BCC), resulting in a better corrosion performance. After 24 h immersion in NaCl solution, the whole surface film of L14 remained undamaged and displayed weave-like and dense characteristic, while both L4 and L7 present a severely damaged film. The cross-sectional details illustrated typical two layers of surface film formed on L14 with the outer layer of ~ 1.5 μm and inner layer of ~ 2.5 μm in thickness, whilst only one layer of film with less than 2.5 μm in thickness was observed on L4 and L7. At last, combining analysis of X-ray diffraction, energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy and transmission electron microscope confirmed the composition of outer layer of surface film on the L14 contains much more Mg(OH) 2 and Li-containing compounds, and the porous Mg(OH) 2 film can be sealed by the Li-containing compounds, which is very different the single Mg(OH) 2 film on the traditional magnesium and L4/L7 alloys. As a result, the best protective function of the surface film on the L14 alloy contribute to its high corrosion resistance. Graphical Abstract