Self-Healing Engineered Multilayer Coatings for Corrosion Protection of Magnesium Alloy AZ31B

Nonporous, crack-free hybrid glass coatings have provided excellent corrosion protection to the AZ31B magnesium alloy. However, if a crack develops in the coatings, then corrosion will proliferate at that point. The novelty of this study consists of engineering a bilayer protection system that combines the "barrier" properties of the hybrid glass coatings with the "inhibitor" or "self-healing" effect of an internal layer of mesoporous silica doped with cerium(III) ions. The mesoporous layer was obtained using a sol-gel solution with 1 mol % cerium(III) ions. The inner cerium-doped mesoporous coating has a thickness of 0.25 μm, and the electrochemical characterization through Open circuit potential (OCP) and Electrochemical Impedance Spectroscopy (EIS) indicates a corrosion inhibition process provided by cerium(III) ions triggered by the corrosion. The combination of the Ce-doped and hybrid glass coatings reaches a total thickness of 5.1 μm. The corrosion evaluation through OCP and EIS does not show any evidence of corrosion during the first 575 h of immersion. After this, there are several steps of a sudden drop in potential and subsequent recovery of the previous values, which could be associated with the activation of the corrosion inhibition mechanism provided by the Ce (III) ions. EIS show a maximum impedance module of 10^6.7 Ohm cm², a decrease of impedance values and phase angle fluctuations after the potential drops observed, and, then, a recovery of the previous values of impedance and phase angle. This behavior confirms activation of the corrosion inhibition mechanism. Polarization curves shows that the multilayer coating leads to a low current density (∼10^-11 A cm^-2), around 5 orders of magnitude lower in comparison with the bare substrate. A post-mortem SEM-EDX analysis study, performed on the cracks generated during electrochemical testing, shows the accumulation of cerium as a consequence of the corrosion inhibitory process.