Titania-zinc phosphate/nanocrystalline zinc composite coatings for corrosion protection of biomedical WE43 magnesium alloy

Biodegradable medical implants made of magnesium (Mg) alloys must have sufficient corrosion resistance in clinical applications. Herein, a nanocrystalline zinc (Zn) coating is electrodeposited on the WE43 Mg alloy followed by a chemical conversion treatment to produce a titania-Zn phosphate layer. The Zn coating has a rice-like nanostructure and the conversion layer is composed of ZnO, Zn₃(PO₄)₂, and TiO₂. The Zn coating reduces the corrosion current density of the WE43 Mg alloy from 151.1 ± 13.8 μA cm⁻² to 29.4 ± 7.4 μA cm⁻² in the simulated body fluid (SBF), while the composite coating decreases it to 4.1 ± 0.8 μA cm⁻². The surface morphology, pH variation, and average corrosion rates after immersion for 7 days in SBF reveal that the Zn coating in fact accelerates dissolution of the Mg substrate due to the formation of galvanic couples between the Zn coating and Mg alloy substrate. In comparison, only slight corrosion is observed from limited areas on the sample with the composite coating. The titania-Zn phosphate/nanocrystalline Zn composite coating provides long-term protection in the physiological environment and the protection mechanism is discussed.