Effects of conversion solution pH value on the surface coating characteristics and biodegradation behaviors of Zn-Li-Ce alloys

The corrosion resistance of medical zinc alloy lies between that of an inert Fe substrate and a degradable Mg alloy. Combined with its excellent biocompatibility, zinc alloy shows great potential for clinical implantation. However, its non-uniform corrosion dissolution can easily trigger unpredictable fractures and abrupt failures, posing scientific and practical challenges. In this study, the effect of conversion liquid pH value on the surface coating characteristics and biodegradation behaviors was analyzed using SEM, XRD, 3D/CLSM and EIS tests. The results show that as the pH value of the conversion solution increases (2.25, 2.50, 3.00, 3.25), the coating morphology of the zinc alloy evolves from the initial flake to the globular cluster, while the coating thickness and density initially increase and then decrease. Correspondingly, the corrosion resistance of the alloy under simulated fluid environment is 0.475, 0.303, 0.206 and 0.235 mm/a, respectively. This trend can be attributed to two primary factors. First, the increase in pH value reduces the concentration of H⁺, bringing the electrolysis-generated ions closer to equilibrium and favoring the formation of the coating over substrate dissolution. Second, moderately acidic condition promotes surface activation of the alloy, generating numerous active sites for ion adsorption and deposition. The resulting fine, cluster-like crystals reduce the effective contact area between the solution and the matrix, thus slowing the dissolution rate under Hank’s electrolyte. However, at higher pH value levels, an elevated OH^- concentration enhances the deposition of Zn (PO₄)₂·2H₂O while suppressing the formation of SrZn₂(PO₄)₂, leading to a significant change in coating morphology, and deteriorating the corrosion resistance of the alloy in a body fluid environment.