Fabrication, characterisation and corrosion of HA coated AZ31B Mg implant material: Effect of electrodeposition current density

Due to its biodegradability, Mg has recently been chosen as a potential temporary load bearing implant material. However the corrosion resistance of Mg must be improved to control its degradation rate matching with the bone healing process. One way to solve the issue is to deposit another bioactive and biocompatible hydroxyapatite (HA) coating on Mg implant via electrodeposition. With this motivation, the current paper explores how the current density, a crucial process parameter, influences the formation, characteristics and degradation of HA-coating. Electrodeposition at three different current densities (3, 6 and 13 mA/cm(2)) was performed on AZ31B Mg substrate followed by an alkali treatment to ensure the complete HA deposition. Surface morphology, chemical composition, crystallinity and texture of the coating were evaluated, followed by potentiodynamic corrosion tests to assess the degradation resistance of the coated samples. Characterisation results show that low current density of 3 mA/cm(2) results in a compact, dense and uniform coating layer as compared to 6 and 13 mA/cm(2). Accelerated H-2 gas evolution at Mg cathode and faster deposition at higher current density (of 13 mA/cm(2)) would cause the lack of integration and bonding of crystals, thus resulting in relatively porous, inhomogeneous and rough HA coating. HA coating deposited at low current density is shown to exhibit the best corrosion protection compared to those at higher current density. The findings clearly indicate that the integrity of the coating layer is a crucial quality indicator. Higher current density can produce the coating thickness of as high as 50 mu m with appropriate application of the current density followed by alkaline treatment. It is also demonstrated that the electrodeposition could be a promising viable method to fabricate HA-coated AZ31B implants with better and prolonged corrosion protection.