Improving corrosion resistance of Mg–Li alloys by Sn microalloying

It is well known that the corrosion resistance of ultra-light Mg–Li alloys is inferior to traditional Mg alloys due to the rapid oxidation of reactive Li on the surface. In this study, microalloying Sn has been found to improve the corrosion resistance of Mg–Li alloys due to the formation of Mg₂Sn on the corrosion pits and reduction of electro-potential difference. Indeed, the addition of Sn not only reduced the corrosion rate to the minimum value of 4.27 mmpy but also promote the formation of dense corrosion oxide films. The volt potential difference between the matrix (α-Mg and β-Li) and Al-X (Gd, Y, Mn) phases has also been measured to reduce from 260 mV (α-Mg) and 343 mV (β-Li) to 220 mV (α-Mg) and 220 mV (β-Li). After homogenization, pitting was significantly reduced, as shown by the in-situ observation of the corrosion process using X-ray tomography. By quantifying the secondary phases, it has been found that the corrosion expansion was restricted by the Mg₂Sn and Al-X phases. The corrosion products are primarily MgO, SnO, and Li₂O oxides and the corrosion mechanism follows three stages: I. Breakthrough of MgO protective layer by corrosive fluid; II. Hydrogen evolution reaction (HER) between matrix phase and water immersion; III. Segregation of secondary phases and corrosion products. It has been found that all three processes are highly dependent on the Sn solutes and Mg₂Sn precipitates, showing the bright future development of corrosion-resistant Mg–Li alloys by Sn microalloying.