Screening of fluoride-free PEO coatings on cast Mg3Zn0.4Ca alloy for orthopaedic implants

Abstract

In order to improve the corrosion behaviour of Mg3Zn0.4Ca alloy, fluorine-free PEO coatings, developed at different treatment times using a novel transparent electrolyte and a conventional suspension electrolyte, were compared. The surface morphology and chemical composition of the PEO coatings were evaluated by scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The corrosion behaviour of the PEO coatings was carried out by potentiodynamic polarization, electrochemical impedance spectroscopy and hydrogen evolution test in modified α-MEM solution. The coatings formed in both electrolytes showed very similar morphologies and compositions that were independent of the nature of the electrolyte. At short immersion times, all PEO coatings showed an improvement in corrosion resistance by PDP measurements, while by EIS measurements only the coatings at short treatment times showed a significant improvement. After >24 h of immersion, the PEO coating formed in transparent electrolyte suffered a drastic acceleration of the degradation rate that exceeded that of the uncoated substrate. The acceleration was attributed to the presence of a crack at the PEO/substrate interface, and the formation of ZnO from the oxidation of Zn-rich secondary phases of the Mg3Zn0.4Ca alloy. The PEO coating formed in the suspended electrolyte on the Mg3Zn0.4Ca alloy showed great corrosion protection, which was attributed to the precipitation of hydroxyapatite that remains trapped in the pores and cracks of the PEO coatings by the hydrolysed silica, impeding the progress of corrosive species.