# Bio-Corrosion Behavior of Ceramic Coatings Containing Hydroxyapatite on Mg-Zn-Ca Magnesium Alloy
## 1 Introduction  
Traditional permanent implants consisting of stainless steel, titanium and its alloys are important in the field of metallic implant materials for their maintaining mechanical integrity and good biocompatibility during the bone healing period [1]. However, metallic materials may cause the release of toxic corrosion products [2; 3] and allergens [4; 5]; it need a second surgery for implant removal after the healing of damaged tissues. As a result, biodegradable implants for biomedical application have attracted great attention in recent years [6; 7; 8]. Common biodegradable implants are made of polymers with unsatisfactory mechanical strength [7; 9; 10]. Magnesium and its alloys are now becoming a priority due to their low density, high specific strength and good biocompatibility, especially their biodegradability and non-toxicity to the human body [11; 12; 13]. The elastic modulus of magnesium is approximately 45 GPa [14], which is similar to that of human bone (3-20 GPa); this avoidsstress shielding effects when implanted into the human body [15; 16]. However, magnesium and its alloys are considerably active metals in chloride containing physiological environments including human body fluid or blood plasma [13]. Therefore, the severe corrosion and accelerated degradation rate of magnesium and its alloys have limited their biomedical application. In general, the corrosion resistance of magnesium and its alloys can be improved by (i) tailoring the grain size [17; 18] or the microstructure and texture [19] of the base material by alloying and (ii) forming coatings via surface treatments on the substrates to obtain protective ceramics, polymers or composite layers [20]. Considering the requirements of biocompatibility, Mg-Zn-Ca alloys are promising for implant applications due to the fact that elements of Mg, Zn, and Ca can not only improve the mechanical properties but also participate in metabolic processes. As the low solubility of many elements in magnesium, the alloying of magnesium has a limitation for application. Therefore, the surface treatments are of high significance. Chemical conversion coatings are common protective coatings on magnesium and its alloys, such as calcium phosphate-containing layer [21; 22] and fluoride phosphate-containing layer [23; 24], which are biodegradable coatings for biomedical applications. Ion implantation has also been conducted as a technique to improve the corrosion resistance of magnesium alloy. However, the improvements were not significant measured in NaCl solution or Hank's solution, e.g., Al, Zr, or Ti ions implanted in AZ91 [25] and Ta ion in AZ31 alloy [26]. Micro-arc oxidation (MAO), also known as plasma electrolytic oxidation or anodic spark deposition, is a novel, promising surface treatment, which has been developed and applied to magnesium and its alloys [27; 28; 29; 30; 31]. This technique has become one of the most prospective surface treatments due to the good adhesion and corrosion resistance of ceramic coatings. However, the application of these coatings on biodegradable implant materials must be further confirmed. Hydroxapatite (HA, Ca\({}_{10}\)(PO\({}_{4}\))\({}_{6}\)(OH)\({}_{2}\)) is a naturally occurring mineral of calcium apatite and is used as a biomedical material. HA exhibits excellent biocompatibility and bioactivity due to its structural and chemical similarities to bone minerals [32; 33; 34]. HA coatings are degradable in physiological environment and show good early interaction between the implant and the tissue [35]. Much research has been done on the fabrication of HA coatings on magnesium and its alloys; preparation methods include electrodeposition [36; 37; 38], aerosol deposition [39], chemical solution deposition [40], sol-gel method [41], and other processes [42; 43]. It should be pointed that the poor bonding strength, corrosion resistance, and inhomogeneity of simplex HA coatings still restrict their application.