Imparting a better corrosion resistance and bioactivity for magnesium through the formation of zinc - zinc oxide - zinc phosphate (hopeite) - magnesium ammonium phosphate (struvite) composite coatings

Abstract

The present study aims to develop zinc-zinc oxide-zinc phosphate (hopeite)-magnesium ammonium phosphate (struvite) composite coatings to reduce the rate of degradation and to impart a better bioactivity for Mg. Galvanic deposition was employed to form Zn-ZnO coating on Mg, which was subsequently converted to hopeite and struvite by chemical conversion method using diammonium hydrogen phosphate. The structural and morphological characteristics, nature of functional groups, degradation behaviour in Hanks’ balanced salt solution (HBSS) and bioactivity in simulated body fluid (SBF) were evaluated. The findings of the study revealed that Zn-ZnO-hopeite-struvite composite coating has considerably reduced the rate of degradation of Mg in HBSS and promoted the bioactivity of Mg in SBF. The rate of degradation of Mg is decreased from 1.27 ± 0.11 mm/y to 0.13 ± 0.04 mm/y, amounting to ⁓90 % reduction in the corrosion rate. The morphological features reveal that the extent of corrosion attack on Zn-ZnO-hopeite-struvite composite coated Mg is very much limited when compared to that of the uncoated one. The surface of Zn-ZnO-hopeite-struvite composite coated Mg shows many thin needle-like crystals as well as globular-like particles after immersion in SBF when compared to that of uncoated and Zn-ZnO coated Mg. The uniform and compact coating with good adherence, improved corrosion resistance and better bioactivity suggest that Zn-ZnO-hopeite-struvite composite coating is a promising surface modification approach towards the development of Mg-based degradable implants.