Study on a facile method of in-situ fabricating enhanced corrosion protection LDH/PEO composites coating on Mg alloy and its growth behavior

In order to achieve in-situ growth of layered double hydroxides (LDH) film on plasma electrolytic oxidation (PEO) coating of AZ31B magnesium alloy under mild conditions, a facile method was used in this work, that is ethylenediaminetetraacetic acid (EDTA)-assisted growth method. Compared to the conventional hydrothermal method, the fabrication of LDH/PEO composite coating by this method does not require a high temperature and high pressure environment and is not constrained by the size of the reactor. Furthermore, the obtained LDH/PEO composite coating exhibits enhanced corrosion protection, which will be widely applied in corrosion protection field of surface engineering. In this work, an effective strategy to control the dissolution of PEO coating and the growth of LDH film by regulating the concentration of EDTA was proposed, after revealing the growth behavior of LDH on PEO coating during this facile fabrication process. The growth behavior of LDH film can be briefly summarized in three stages: PEO coating dissolution, LDH nucleation, and LDH growth. Interestingly, the growth mechanism of LDH was changed with EDTA assistance. LDH shifted from preferential growth within the micropores of the PEO coating to uniform growth. With the assistance of 0.06 M EDTA, which is optimal, the dissolution of original PEO coating provided a stable and sufficient Mg source for the growth of LDH film, resulting in a dense LDH network structure on the PEO coating. This uniformly grown LDH film could not only seal the inherent defects of the PEO coating but also exhibited the ability to adsorb corrosive anions, which increases the tortuosity and difficulty in penetration of corrosive media, significantly improving the corrosion resistance of the LDH/PEO composite coating.