Plasma electrolytic oxidation (PEO) coating to enhance in vitro corrosion resistance of AZ91 magnesium alloy coated with polydimethylsiloxane (PDMS)

To improve the corrosion behavior of a magnesium alloy, two-step coatings have been fabricated through plasma electrolytic oxidation coating (PEO) under constant voltage conditions applied to the AZ91 substrate surface in the silicate electrolyte. A polymeric layer [Polydimethylsiloxane (PDMS)] has been successfully implemented by the immersion method on both bare AZ91 alloy and pre-treated samples by the PEO process. PEO coating contains MgO, MgSiO3, and Mg2SiO4 phases detected by X-ray diffraction patterns (XRD). The surface morphology observation investigated using the field emission scanning electron microscopy (FESEM) showed a porous and non-porous layer on the substrate for PEO and PEO/PDMS coatings, respectively. The topography and roughness of the PEO coating evaluated by atomic force microscopy (AFM) revealed a high roughness for the PEO coating. The chemical composition of coatings was determined using X-ray energy distribution spectroscopy (EDS), and the corrosion behavior of the AZ91 substrates with/without coating was studied by the potentiodynamic polarization test in the Ringer solution. The results showed that the corrosion (E-corr) potential of the AZ91 substrate increased compared to a single-layer PDMS coating from - 1.48 to - 1.47 V, and their corrosion current density (i(corr)) reduced from 4.3 to 0.486 mu A center dot cm(-2) in the order given. However, by applying PEO coating as a pre-treatment utilizing the PEO/PDMS composite coating, these values were obtained - 0.75 V and 0.9 nA center dot cm(-2), respectively. So, applying PEO/PDMS hybrid coating may effectively reduce the high corrosion rates of magnesium alloys as the main limitations in the physiological environment.