Potential bioactivity of coatings formed on AZ91D magnesium alloy by plasma electrolytic anodizing

Abstract

Magnesium is a potential candidate as biodegradable orthopedic implant thanks to its remarkable mechanical properties closed to those of natural bone. But its low corrosion resistance in the body fluid is a limiting factor. By using electrolytic plasma (or microarc) anodizing process, 10-20 μm protective coatings were formed on AZ91D magnesium alloys at a constant current density of 20 mA cm-2. The effect of a small addition of CaO (1-3 g L-1) to a classical electrolytic bath (made of KOH 3 mol L -1, KF 0.5 mol L-1, and K2HPO4 1 mol L-1) was studied. The anodic films, investigated by X-ray diffraction and scanning electron microscope-energy dispersive spectroscopy, show a porous structure containing crystallized Mg(OH)2, MgO, MgAl2O4, as well as Ca and P elements. The 1 g L -1 CaO-containing electrolyte leads to morphology and composition permitting optimum corrosion behavior evaluated by stationary electrochemical measurements and impedance spectroscopy. This improvement is associated with both the appearing of a "sealing" layer and the precipitation of an outermost Ca/P-containing layer (hydroxyapatite like) in simulated body fluid. The control of the corrosion behavior of coated magnesium alloys is therefore required to enhance their potential bioactivity. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2012. Copyright © 2012 Wiley Periodicals, Inc.