Development and assessment of a multifunctional chitosan-based coating applied on AZ31 magnesium alloy: Corrosion resistance and antibacterial performance against Klebsiella Pneumoniae

This work presents a simple method to functionalise the surface of AZ31 magnesium alloy by applying a duplex MgF2/chitosan coating, which improves its corrosion resistance and provides it with some antibacterial performance. First, the effect of three chitosan solutions with different concentrations on the growth of the bacteria Klebsiella pneumoniae in nutritive medium (TSB) was evaluated by absorbance kinetics experiments, where the chitosan solution at 2% (m/V) was selected for the coating preparation. Before coating application, the AZ31 substrate was pretreated with hydrofluoric acid for 48 hours in order to form a MgF2 conversion layer. Subsequently, the coating was applied to the pretreated substrate through the dry-casting method. Samples of the alloy in each surface condition (bare, pretreated, and pretreated + coated with chitosan) were exposed to simulated body fluid (SBF) for 21 days at 37 °C, with the solution renewed every 24 hours and the wastes stored. The surfaces were characterised by SEM-EDS, and XPS after the immersion tests, whereas the stored solutions were employed to measure the change in the Mg-ions concentration. Electrochemical impedance spectroscopy and potentiodynamic polarisation were performed in each surface condition to compare their corrosion resistance in SBF. The antibacterial activity of the functionalised surfaces was evaluated by the plate counting method and compared with bare samples. All results were correlated and demonstrate that the modified surface of AZ31 achieved a higher corrosion resistance when it was exposed to SBF, as well as a reduction of the bacterial growth during in vitro tests.