# Bio-Corrosion Behavior of Ceramic Coatings Containing Hydroxyapatite on Mg-Zn-Ca Magnesium Alloy
## 3 Results and Discussion
### Effect of HA on the Corrosion Resistance of the Coatings  
Figure 3 illustrates the mass loss curves of the coatings and Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy immersed in [PERSON]'s solution at 37 \({}^{\circ}\)C for 20 d. When the bare Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy was soaked in [PERSON]'s solution, corrosion occurred immediately on the surface due to the rapid dissolution of active Mg and Ca. However, the coated alloys both present a relatively slow mass loss rate compared with bare Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy. This result indicates that the coatings can effectively protect the substrate to avoid further corrosion. The mass loss of the coatings tended to linearly increase at the early immersion stage (approximately 3 d), but subsequently slowed down. It is well known that there is a loose and porous layer on the surface of coatings [29]. The corrosion medium can easily permeate into the loose superficial layer and spread out, resulting in severe corrosion. With the increase of immersion time, the dense inner layer would prevent corrosive medium further permeation, which leads to a slow corrosion rate; on the other hand, the coating may achieve a self-protection by the corrosion products obtained via a spontaneous mineralization in [PERSON]'s solution [45], resulting in a slow corrosion rate  
Figure 3: Mass loss curves of the coatings immersed in [PERSON]’s solution at 37 \({}^{\circ}\)C for 20 d.  
Figure 2: X-ray diffraction patterns of the coatings formed at HA concentration of 0.4 g/L. as well. The mass loss rate was much lower in the HA-containing coating than that in MAO coating. As previous analysis, few cracks and pores were found in the HA-containing coatings, which inhibited the permeation of the corrosion medium, leading to good corrosion resistance. The measurement of the OCP changes with time (OCP-\(t\)) can be employed to understand the chemical stability and corrosion process that occurs on the sample surface. The recorded OCP-\(t\) curves of Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloys after soaking in [PERSON]'s solution for up to 3 h are plotted in Figure 4. All of the samples exhibited a similar trend in the evolution of OCP: the potentials moved rapidly toward the positive direction at the initial stage, and then OCP was slightly increased (bare alloy) or maintained constant (HA-containing coating, MAO coatings). The rapid increase in OCP at the initial stage of immersion indicates the formation of a certain kind passivation layer on the sample surfaces. The bare alloy, especially, requires more time to reach a stable potential, indicating a slow rate of establishing equilibrium between corrosion and protection. Commonly, more positive OCP implies that the alloy is comparatively more passive. Therefore, the electrochemical activity of different samples follows the order bare alloy > MAO > MAO + HA. Figure 5 shows the potentiodynamic polarization curves of HA-containing coating, MAO coating, and bare Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy in [PERSON]'s solution at 37 \(\pm\) 0.5 \({}^{\circ}\)C. The corrosion potential (\(E_{\rm corr}\)) and corrosion current density (\(I_{\rm corr}\)) can be calculated from polarization curves through a software using the _Tafel extrapolation method_. The fitting results are listed in Table 1. Both coatings in [PERSON]'s solution show good anti-corrosion property. The \(E_{\rm corr}\) of the coatings, e.g., HA-containing coating (\(-1.215\) V) and MAO coating (\(-1.263\) V), significantly increased in comparison with that of bare Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy (\(-1.422\) V); the \(I_{\rm corr}\) of the coatings reduced remarkably with values of \(6.138\times 10^{-8}\) A\(\cdot\)cm\({}^{-2}\) and \(8.831\times 10^{-7}\) A\(\cdot\)cm\({}^{-2}\), respectively, manifesting better corrosion resistance than Mg\({}_{66}\)Zn\({}_{29}\)Ca\({}_{5}\) alloy.