Design and Characterization of Phosphatizing Coatings for Magnesium Implants

Magnesium alloys are promising biodegradable implant materials, but their rapid corrosion in physiological environments limits their clinical applications. This work is focused on the development of cementitious coatings inducing magnesium phosphate formation on magnesium AZ31 alloys. First, the alloy surfaces immersed in orthophosphoric acid (OPA) solutions with six additives of various functions (sodium chloride, magnesium chloride, calcium nitrate, magnesium nitrate, trisodium citrate, and hydroxyethyl cellulose (HEC)) were comparatively analyzed to understand the effect of solution chemistry on surface evolution. OPA solutions were also saturated with respect to magnesium ions, which effectively limited surface degradation. Sample mass and solution pH were monitored for 21 days, and depositions were characterized using SEM, EDX, and electrochemical methods to identify the surface composition and investigate its effectiveness against Mg degradation. In the next stage, alloy plates were dip-coated with the multicomponent suspension of the most effective composition (OPA, MgCl₂, HEC, and Mg-saturated deionized water). The phase evolution of the dried samples in 3.5 wt % NaCl solution was monitored with regular gravimetric, pH, quantitative XRD, SEM, EDX, and electrochemical Tafel analyses. Samples passivated despite the high chlorine concentration, as initially formed newberyite crystals, were replaced by Mg oxychlorides, Mg phosphates, and Mg hydroxide in order, in response to the shift in solution pH from acidic to alkaline values that is driven by the dissolution and transformation of the alloy and coating phases. Thermally cross-linking HEC improved the stability of the coatings, which slightly retarded the degradation kinetics. In vitro cell culture tests validated the coated AZ31 as both being biocompatible and potentially bioactive. Thus, the phosphatizing coating approach offers a promising strategy for controlled biodegradation of magnesium implants in physiological environments.