Synergistic chelating agents for in-situ synthesis of Mg-Al LDH films on PEO treated Mg alloy

In-situ growth of layered double hydroxide (LDH) has been considered a feasible method to further improve the corrosion protection of PEO coated Mg alloys, but the process is basically carried out in an autoclave. In this study, LDH was in-situ synthesized on PEO treated AM50 Mg by hydrothermal treatment at ambient pressure in the presence of chelating agents. Results indicated that the synergistic effect of sulfosalicylic acid (SSA) and ethylenediaminetetraacetic acid tetrasodium salt (EDTA) chelating agents is appropriate for Mg-Al LDH formation due to the sufficient quantity of highly stable Mg and Al complexes in the treatment bath. An appropriate hydrothermal treatment time was also revealed in this work, which can efficiently seal the pores and defects of PEO coating with a uniform LDH film. As a result, the corrosion protection of the coating was improved dramatically. The prolonged hydrothermal treatment is detrimental to the integrity of PEO layers. The influence of chelating agents on the LDH formation processes is supported by thermodynamic calculation of the equilibrium composition, and the formation mechanism of LDH film is discussed in depth.