Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges

The development of biodegradable implants is indeed fascinating and among the various types of materials used in this regard, magnesium and its alloys assume significance. However, the rapid corrosion, generation of a large volume of hydrogen gas, accumulation of the hydrogen bubbles in gas pockets adjacent to the implant, increase in local pH of the body fluid, are the major impediments in using them as an implant material. Hence, development of Mg/Mg alloy based degradable implants requires that (i) they should maintain sufficient mechanical strength and integrity until the affected part of body is healed; (ii) they should exhibit good resistance to corrosion in the body fluid during the initial periods of implantation and subsequently corrode in a controlled and uniform fashion; and (iii) the corrosion products should not exceed the acceptable absorption level of the human body. Reducing the rate of corrosion of Mg is the most appropriate strategy and this can be achieved with the use of alloying, surface treatment/coating and mechanical processing. Surface treatment/coating is a viable approach as it not only enables improvement in corrosion resistance but also provides a suitable surface for better bone bonding and cell growth. Among the various surface modification processes, microarc oxidation (MAO) has received considerable attention since the protective oxide layer would delay the rate of corrosion attack during the initial period of implantation and, the decrease in the extent of hydrogen evolution would enhance the primary neo-formation of bone around the implant. The presence of micropores and cracks on the surface of MAO coatings can be considered as an opportunity or a limitation. The presence of a porous outer layer in MAO coatings would significantly improve the mechanical interlocking effect, the bonding area and stress distribution across the adhesive-substrate interface of the joins, resulting in higher bond strength. However, the presence of a higher pore density on the surface of the MAO coatings increases the effective surface area and thus the tendency of the corrosive medium to adsorb and concentrate into these pores. This would facilitate quicker infiltration of the corrosive medium into the inner regions of the coating and subsequently down to the substrate, thus deteriorating the corrosion resistance of the coating by changing its local pH. The pore density, distribution of pores and interconnectivity of the pores with the substrate are the important factors that decide its corrosion protective ability. In spite of the limitation in corrosion rate, MAO coatings exhibit a slow rate of degradation during the first few weeks and an accelerated degradation in later stages of implantation. Nevertheless, the difficulty in achieving a control over the rate of degradation is still a matter of concern in fabricating implant devices with a desired lifetime. Hence, it is not only essential but also mandatory to increase the corrosion resistance of MAO coatings. In this perspective, this review aims to address the various strategies explored to improve the corrosion resistance of MAO coatings on Mg/Mg alloys. This review provides a detailed outline on how the choice of electrolytes, process parameters, pretreatment, additives, incorporation of ceramic particles and, sealing and post-treatment, influence the porosity and corrosion resistance of MAO coatings on Mg/Mg alloys. In addition, the implications of such modifications/choices on the suitability of the resultant coatings for biomedical applications are discussed. The importance of multifunctional approaches in improving the corrosion resistance as well as imparting a controlled drug delivery, better apatite growth, improved bioactivity, etc. is addressed. The important strategies to improve the corrosion resistance and future prospects are summarized. (C) 2013 Elsevier Ltd. All rights reserved.