Corrosion resistance and biodegradability of micro-arc oxidation coatings with the variable sodium fluoride concentration on ZM21 magnesium alloys

As a biodegradable metal implant material with mechanical properties closest to those of natural bone, the degradation rate of magnesium (Mg) alloy is so fast in body fluid environment. Therefore, the preparation of a coating on Mg alloy surface to improve its corrosion resistance has become a research focus. In the study, Mg-2Zn-Mn (ZM21) alloys and micro-arc oxidation (MAO) are used as the substrate and surface modification method, respectively. Then, the influence of sodium fluoride (NaF) concentration in silicate basic electrolyte on the corrosion resistance and biodegradability of MAO coatings are investigated. The results show that (1) When NaF concentration increases to 1.0 g/L, the thickening and hydrophilic effect of MAO coating are more obvious. (2) Adding 1.0 g/L NaF makes excellent corrosion resistance and great adhesion, because F- can enhance discharge and refine sparks to enhance the interface adhesion and form a more uniform surface. (3) Excessive NaF concentration (＞ 1.0 g/L) can decrease the breakdown voltage, produce the abundant discharge channels, and degrade the coating quality. (4) The remarkable reduced weight loss rate and pH value of MAO samples after immersion shows the certain biological safety. (5) Cytotoxicity and cell adhesion test proves the available biocompatibility.