Characterization and corrosion property of nano-rod-like HA on fluoride coating supported on Mg-Zn-Ca alloy

The poor corrosion resistance of biodegradable magnesium alloys is the dominant factor that limits their clinical application. In this study, to deal with this challenge, fluoride coating was prepared on Mg–Zn–Ca alloy as the inner coating and then hydroxyapatite (HA) coating as the outer coating was deposited on fluoride coating by pulse reverse current electrodeposition (PRC-HA/MgF(2)). As a comparative study, the microstructure and corrosion properties of the composite coating with the outer coating fabricated by traditional constant current electrodeposition (TED-HA/MgF(2)) were also investigated. Scanning electron microscopy (SEM) images of the coatings show that the morphology of PRC-HA/MgF(2) coating is dense and uniform, and presents nano-rod-like structure. Compared with that of TED-HA/MgF(2), the corrosion current density of Mg alloy coated with PRC-HA/MgF(2) coatings decreases from 5.72 × 10(−5) A/cm(2) to 4.32 × 10(−7) A/cm(2), and the corrosion resistance increases by almost two orders of magnitude. In immersion tests, samples coated with PRC-HA/MgF(2) coating always show the lowest hydrogen evolution amount, and could induce deposition of the hexagonal structure-apatite on the surface rapidly. The results show that the corrosion resistance and the bioactivity of the coatings have been improved by adopting double-pulse current mode in the process of preparing HA on fluoride coating, and the PRC-HA/MgF(2) coating is worth of further investigation.