Localized corrosion of magnesium alloys potentially applicable for medical implants: fundamental aspects


A comparative analysis of the corrosion activity of bioresorbable MA8 magnesium alloy (Mg– Mn – Ce system) in a medium for cultivation of mammalian cells (minimum essential medium, MEM) and 0.83 wt. % aqueous NaCl solution was performed. The development of the corrosion process on the surface of a magnesium alloy in two media was established using local scanning electrochemical methods (scanning vibrating electrode technique and scanning ion-selective electrode technique), traditional electrochemical methods (potentiodynamic polarization, electrochemical impedance spectroscopy) and hydrogen evolution measurements. The protective properties of the corrosion film formed on the alloy in MEM, as well as during exposure to NaCl solution, were established. The composition of the layer of corrosion products formed on the magnesium alloy in MEM was studied using confocal Raman spectroscopy, X-ray diffraction analysis, scanning electron microscopy, and energy dispersive X-Ray analysis. Calcium-phosphate compounds are the main products formed on the surface of a magnesium alloy during immersion in MEM. The model of corrosion mechanism of magnesium alloy in MEM, which includes three stages of the development of surface film, is proposed. The formation on the magnesium alloy in MEM of corrosion product layer, including magnesium-substituted hydroxyapatite, stabilizes the local pH below 9.0, which along with the presence of organic acids, does not allow increasing the pH during corrosion. A diagram that allows one to determine the possible reactions occurring on the surface of a magnesium alloy in MEM and the thermodynamic probability of the formation of chemical compounds based on the values of the local pH is designed. The obtained results indicate the prospect of using bioresorbable magnesium implants in surgery.
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