Degradation rate modulation of resorbable zinc implants via conversion coatings and plasma electrolytic oxidation

The degradation rate of Zn was tunned using different coatings: four phosphate conversion coatings (PCC) as well as a plasma electrolytic oxidation (PEO) one. The incorporation of Mg, Ca, and Fe into PCC systems altered morphology, thickness (thinner ∼ 1 µm vs. ∼ 4 µm), and roughness (specially with Fe: S a ∼ 0.2 µm vs ∼ 0.6 µm). X-ray diffraction (XRD) revealed hopeite (Zn 3 (PO 4 ) 2 ·4(H 2 O)) in all PCC coatings and ZnO (n-type semiconductor, band gap 3.1–3.2 eV) in the PEO coating (the highest thickness and roughness: ∼ 20 µm and S a ∼ 2.5 µm). Corrosion behaviour was evaluated in SBF at 37 °C using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), complemented by gravimetric measurements following immersion tests under CO 2 -controlled conditions (pH 7.4, 168 h). The results indicate that Zn degradation is governed primarily by coating composition and morphology, rather than thickness alone. Ca- and Mg-enriched PCCs exhibited up to twofold higher long-term polarization resistance, whereas Fe incorporation accelerated corrosion, likely due to Fe 2+ reduction and Zn-Fe microgalvanic coupling (R p ∼ 10% lower than Zn substrate). The PEO coating provided limited initial protection (R p ∼ 2.4% vs Zn substrate), which can be attributed to its stratified structure and solubility of ZnO. These features promoted localized (crevice-type) corrosion, resulting in 0.23–0.25% weight loss after 168 h. Despite this, the higher Ca content facilitated the progressive formation of a compact corrosion product layer composed of hydroxyapatite and zinc carbonate hydroxide, which may enhance bioactivity and support bone mineralization.