Effect of electrochemical cathodic hydrogen charging on the surface film of an ultralight Mg–Li–Zn alloy

Abstract

Effect of electrochemical cathodic hydrogen charging on the microstructure and surface films of Mg–11Li-xZn alloys (x = 0, 1, 3, 6 wt%, designated as L11, LZ111, LZ113, LZ116) were investigated in this work. The results demonstrated that hydrogen charging induced the formation of a dual-layered corrosion product film on the alloys. The outer layer was primarily composed of lithium-compounds (Li₂CO₃, LiOH·H₂O, Li₂O), while the inner layer was mainly composed of magnesium compounds (Mg(OH)₂, MgCO₃, MgO, and MgH₂), with a higher MgH₂ concentration in the alloys with lower Zn content. As the Zn content increased, the overall thickness of the corrosion product film significantly decreases, particularly when Zn reached 1 wt%, which was ascribed to the suppressed formation of MgH₂. Additionally, hydrogen charging induced varying hydrogen damage of the four alloys. The L11 alloy with no Zn exhibited surface bulging and cracking due to extensive MgH₂ formation after hydrogen charging. The LZ111 alloy with minimal Zn content displayed the lowest hydrogen concentration, characterized by fewer hydrogen-induced microcracks. This was attributed to its denser surface film and fewer secondary phases. With further increasing Zn content, intergranular cracking occurred at the grain boundaries of the LZ113 and LZ116 alloys, primarily due to the precipitation of secondary phases and thinner surface films. The excessive precipitation of secondary phases acted as hydrogen traps, elevating hydrogen levels at the grain boundaries. Concurrently, the thinner surface films of the alloy with high Zn content reduced the resistance to the hydrogen permeation.