Enhanced corrosion resistance of magnesium alloy by plasma electrolytic oxidation plus hydrothermal treatment

Plasma electrolytic oxidation (PEO) is a promising surface treatment technique to enhance the corrosion resistance of Mg alloy. However, the micro-pores and micro-cracks of the PEO coating limit its anti-corrosion protection capability. In this study, a dense Ca3Al2(OH)(12) layer was fabricated on the PEO coating surface by hydrothermal treatment (HT), forming a PEO/HT composite coating. Surface morphologies, chemical composition, phase composition and growth process of the PEO/HT composite coating were characterized using Field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectrophotometer (FT-IR). Corrosion resistance of the samples was studied through immersion test, hydrogen evolution test, and electrochemical test. Results demonstrated that the PEO/HT coating possessed a polyhedral structure and the micro-pores and micro-cracks of the PEO coating were sealed. The PEO/HT composite coating showed improved bioactivity in comparison with PEO coating and Mg alloy after immersion in Hanks' solution. Moreover, the Ca3Al2(OH)(12) crystals layer improved the corrosion resistance of PEO coating significantly. Furthermore, the growth process of Ca3Al2(OH)(12) layer was discussed.