Improving Corrosion Protection of Magnesium Alloys by Plasma Electrolytic Oxidation Based Coatings

The high susceptibility to corrosion limits the broad application of magnesium alloys, and therefore, the corrosion protection of magnesium is of major concern in practical conditions. A great effort has been made in the last few decades to solve this problem. Various types of surface coatings have been developed to provide corrosion protection for magnesium alloys, among which plasma electrolytic oxidation (PEO) is one of the most promising techniques. The PEO treatment can produce a hard ceramic-like oxide coating on magnesium and its alloys, leading to significantly enhanced wear and corrosion resistance. However, the intrinsic porous morphology of the PEO coatings still limits their effect of corrosion protection. The objective of the present work is to overcome this microstructural drawback, and further improve the corrosion protection ability of PEO coatings on magnesium and its alloys. Different approaches have been adopted to reduce the degradation rate of PEO coatings, including optimisation of the PEO treatment itself, sequential processing combining PEO coating with various post-treatments and formation of smart self-healing PEO coatings inspired by biological systems. The PEO process was investigated by analysing the current/voltage transients, and the PEO coatings were systematically characterised by means of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) was also used to study the chemical composition of the plasma enhanced chemical vapour deposition (PECVD) coatings on the PEO coated magnesium alloy. The corrosion resistance of the PEO coatings in 3.5 wt.% NaCl solution was investigated by the electrochemical methods, including open circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation scans (PDP). In addition, the mechanical behaviour of PEO coatings was also examined by scratch testing. It was found that both voltage and frequency have significant effect on the properties of PEO coatings, and a more compact coating was produced by pulsed bipolar voltage mode. The PECVD post-treatment was proven to be an effective method of improving the corrosion protection ability if appropriate precursors were used, as this method could cause both positive and negative effects. Coating degradation could also occur during immersion post-treatments, although the corrosion resistance of the PEO coating was also improved by the Ce deposition and benzotriazole (BTA) adsorption. In this case, a better corrosion protection was achieved by combining the PEO coating with Ce-based immersion post-treatment, as the insoluble Ce-containing compounds provided both sealing effect and the inhibition of cathodic reaction. Finally, the self-healing PEO coating incorporated with inhibitor loaded nanocontainers was developed and shown a good potential for providing a long-term corrosion protection for magnesium alloys, even though the corrosion resistance was not significantly increased compared with conventional PEO coating. However, none of the above approaches was perfect, indicating that there is still plenty of work to be done in the future.