Self-degradation of micro-arc oxidation/chitosan composite coating on Mg-4Li-1Ca alloy

Regulating degradation rate and moderate pH micro-environment for biodegradable magnesium alloys face huge challenge. The chemical and morphological characteristics of micro-arc oxidation (MAO) and chitosan (CS) composite coatings, fabricated on Mg-4Li-1Ca alloy, are analyzed through field-emission scanning electronic microcopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy. Corrosion resistance of the samples is evaluated via hydrogen evolution, potentiodynamic polarization and electrochemical impedance spectroscopy in Hank's solution. Results indicated that the MAO and CS coating enhances the corrosion resistance and antibacterial growth activity. With increasing immersion time, the degradation of the MAO/CS coatings gives rise to a decrease in pH value and leads to a rapid increase in hydrogen evolution rate after an immersion in Hank's solution after 100 h. The MAO/CS coatings retain the solution pH at a moderate level (<= 8.25). A novel self-degradation mechanism of the MAO/CS coating on Mg-Li-Ca alloy is proposed due to the fact that MAO/CS coating is cathodic relative to the substrate.