Active corrosion protection of magnesium alloy
• School of Materials Science and Engineering (CN)
Plasma electrolytic oxidation (PEO) is an advanced anodizing process which leads to formation of ceramic-like oxide coatings on the surface of light metals. The oxide layers developed by PEO are usually hard, strongly-adherent to the substrate and confer both corrosion and wear resistance. In spite of many advantages, the layers are generally composed of high porosity as a result of discharge breakdowns and gas evolution during the coating growth process. Moreover, the size of the pores increases with the coating thickness in many cases. Such an intrinsic porosity often compromises and even deteriorates the barrier properties of the layer. From another hand, the high porosity could be advantageous and considered as natural reservoir to load inhibitor-containing nanocontainers, which release inhibitors on demand.
Recently it was shown that layered double hydroxides (LDHs) can act as such nanocontainers, releasing suitable corrosion inhibitors in the presence of Cl- or OH- anions. The general formula of the most common LDHs can be represented as [MII1-xMIIIx(OH)2]x+(Ay-)x/y·zH2O. Following their structure, the protective LDH action can be explained via the anion-exchange reaction induced by mentioned triggers. In other words, when corrosion conditions occur, the LDH nanocontainers release inhibiting anions and absorb corrosion-active ions such as Cl-.
Moreover, formation of LDH based nanocontainers in the pores of PEO layer will improve the barrier properties of the resulting system via sealing. In spite of significant recent progress, achieved in LDH formation on PEO treated Mg substrates. Two main drawbacks, limiting their further industrial applications, remain: (1) LDH formation occurs under autoclave conditions, (2) LDH are formed in the presence of carbonate anions, leading to the formation “dead” non-functionalizable LDH, which cannot be loaded with corrosion inhibitors. In the present study, we will focus not only on incorporation of inhibitor-loaded nanocontainers (both in- and ex-situ) into PEO coatings but also on direct formation of LDH on PEO treated surfaces in order to provide a controlled and prolonged release of the functional species to achieve active long-lasting corrosion protection for Mg alloys.