Lithium salt post-treatment of anodized 7075-T6 alloy for enhancing corrosion resistance in neutral and acidified NaCl environment

An anodized 7075-T6 aluminum alloy was post-treated in three different lithium salt solutions (Li2CO3, Li2C2O4 or LiNO3) under optimized conditions, respectively, in order to improve corrosion resistance of the alloy in both neutral NaCl (0.05 M, pH = 6.5) and acidified NaCl (0.6 M, pH = 3.5) solutions. The results indicate that the porous anodic film can be effectively sealed by the post-treatment and the sealing products are a mixture of boehmite, Li-Al amorphous hydroxides and Li-Al-LDH, with the relative proportion of each component varying with the sealing conditions. All lithium salt treated alloy specimens passed 1100 h of neutral salt spray test, and the Li2C2O4-based alloy specimen exhibited higher corrosion resistance than others in the neutral NaCl environment. Although the corrosion resistance of the coating deteriorated in the acidified NaCl solution, the lithium salt treated alloy specimens still showed distinct advantage over the hot water sealed counterpart, especially for the LiNO3-based process.