Improvement of Mg alloy corrosion resistance and apatite formation ability via PEO/Alginate/Akermanite hybrid coatings for biomedical purposes

Abstract

The rapid degradation of Mg alloys in biodegradable orthopedic applications is a significant concern to be investigated. In this study, a bilayer composite coating based on alginate/akermanite and plasma electrolytic oxidation (PEO/ALG-AK) on the AZ31 Mg alloy was intended to enhance its corrosion resistance. For this purpose, akermanite nanopowder was synthesized using the sol-gel method and characterized by TEM and XRD. The influence of the PEO/ALG-AK coating layer on the physicochemical properties, electrochemical, and immersion behaviors in simulated body fluid (SBF) was compared to that of uncoated and PEO-coated samples. The AFM images revealed that the bilayer coating exhibited the highest average roughness (Ra = 781 ± 49.3 nm) among the other samples. The EIS test was conducted after 2, 72, and 168 h of immersion in SBF. The bilayer coatings on Mg alloy consistently demonstrated the highest corrosion resistance due to the ALG/Ak layer covering the PEO coating pores. The results of the polarization test also confirmed the achieved results. Furthermore, the implementation of the coatings, particularly the PEO/ALG-AK coating, illustrated a significant reduction in the release of Mg2+ ions and hydrogen bubbles, resulting in corrosion resistance improvement. Finally, the ability of samples to induce apatite formation was also assessed. The results displayed that the deposits on the bare Mg alloy, PEO, and PEO/ALG-AK coatings exhibited Ca/P ratios of 0.83, 1.11, and 1.57, respectively, which closely resembled the natural bone in the PEO/ALG-AK sample. Hence, this developed bilayer coating can be a suitable candidate for enhanced corrosion resistance and bioactivity (apatite formation ability) in the field of Mg biodegradable implants.