Biodegradability and Surface Chemistry of AZ31D Compared with AZ91 Magnesium Alloy in a Modified Simulated Body Fluid

In a modified simulated body fluid (mSBF), biodegradability and surface chemistry of AZ31D compared with AZ91 were studied using electrochemical techniques and immersion test. A protective surface film made the corrosion rate(CR) decreased rapidly and then reach a stable stage with immersion time increasing from 1d to 24d. CR of AZ91 was significantly lower than that of AZ31D and Ca, P, Mg, Al and O were homogeneously distributed in the corrosion layers of AZ31D, while, inhomogeneously in AZ91due to much more beta-phase inhomogeneously distributed in AZ91 and only a little beta-phase distributed in AZ31D. The micro-galvanic couple formed at the boundaries between the beta-phase and Mg matrix to accelerate the alpha phase corrosion. With immersion time increased, the remaining beta phases of AZ91 gradually formed a tight barrier while a loosen one were in AZ31D. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) analysis verified that amorphous calcium phosphate or magnesium calcium apatite has formed on the surfaces of AZ91 and AZ31D.