Corrosion Behavior of Fluorine-Doped Calcium Phosphorus Coating on AZ31 Alloy with Various Chemical Conversion Parameters

To improve the corrosion resistance of biomedical magnesium alloys, fluoride ions (0.3 M, 0.5 M, and 0.7 M) were added to a solution of water containing 0.042 mol/L Ca(NO₃)₂·4H₂O and 0.025 mol/L NH₄H₂PO₄. Calcium Phosphorus (Ca-Ph) coatings with different morphology and corrosion resistance were obtained by changing the temperature and fluorine ion content to explore the growth mechanism of fluorine-doped Calcium Phosphorus (F-CaP) coating. The surface morphologies, and chemical compositions of these coatings were characterized by x-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy, and the corrosion resistances of the coatings were investigated by electrochemical tests and immersion tests. The results show that temperature has a great effect on the morphology of F-CaP coating. An increase in the temperature accelerated the reaction rate of the ions during the deposition process, and the low amount of fluoride prevented the F-CaP coating from fully covering the surface of the matrix. When the amount of doped fluorine was 0.5 M at 60 °C, the complete and compact coating morphology effectively protected the AZ31 magnesium alloy from the corrosion of simulated body fluids.