Effect of the fluoride concentration in electrolyte on adhesion of an apatite layer formed on an anodized zirconium surface

Zirconium is used clinically as a material for hard-tissue repair because it has excellent corrosion resistance, low magnetic susceptibility, and low cytotoxicity. Bone-bonding ability is required to stably fix zirconium implants in bone, and anodization in a fluoride solution has been proposed as a method to achieve this. Because the corrosion reaction rate and surface crystalline phase change according to the fluoride concentration, it is assumed that the fluoride concentration in the electrolyte will affect bone bonding and mechanical properties. However, few studies have investigated this. In this study, zirconium was anodized in an electrolyte containing 0–1 M NH₄F, and apatite formation in a simulated body fluid was investigated for in vitro evaluation of bone bonding. Adhesion of the apatite layer to the substrate was also investigated. Differences in the NH₄F concentration had a negligible effect on the apatite formation ability of zirconium. However, adhesion was greatly affected by the fluorine concentration, with an intermediate concentration of 0.1 M resulting in the best adhesion. Adhesion was correlated with surface roughness of the anodized surface.