Corrosion performance of electrochemically anodized and calcium phosphate coated AZ31 magnesium alloy

Abstract

The present study explores duplex coating approach, wherein an anodic oxide layer is first formed on AZ31 Mg alloy via electrochemical anodization, followed by the electrodeposition of a calcium phosphate layer, with the objective of improving the surface properties and degradation resistance for biomedical applications. The AZ31 Mg alloy was initially anodized in an alkaline medium at different voltages to generate a magnesium silicate-based oxide layers. Although formation of porous oxide layer was confirmed on both conditions, samples anodized at 20 V exhibited a thick uniform anodized layer. Subsequently, a layer of calcium phosphate (CaP) was electrodeposited on anodized magnesium surface (20V), to have a duplex coating. Surface morphological studies evidenced that the formation of CaP layer effectively sealed the anodized surface. The Ca2p and P2p peaks obtained from high-resolution spectra of x-ray photoelectron spectroscopy confirmed the presence of CaP on anodized magnesium surface. Electrochemical impedance spectroscopy (EIS) results showed increase in the capacitance semicircle diameter and absolute impedance value for duplex coated samples. The potentiodynamic polarization studies, immersion and hydrogen evolution tests in simulated body fluid environment were assessed to ascertain the suitability of the duplex coated samples in controlling the degradation rate of magnesium. The results revealed that the duplex coated sample outperformed the direct CaP coated Mg sample possibly due to the sealing of the porous anodized layer with the CaP. Furthermore, cytotoxicity evaluation of MG63 cells demonstrated higher cell viability with the duplex coated sample.