Multilayer sol–gel coatings for corrosion protection of magnesium

Magnesium, although valuable, is reactive and requires protection before it can be used in many applications. This study evaluates the corrosion protection of Mg by multilayer coatings. The coatings consist of an anodised layer and one or two sol–gel layers. Potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion properties of these coatings. A porous anodised Mg oxide layer is employed as an intermediate layer, which enhances the adhesion of sol–gel layers. Sol–gel layers provide corrosion protection by physically sealing pores in the anodised layer and acting as a barrier. Defects and porosity in the coatings are the main causes of corrosion, allowing diffusion paths for corrosive species to reach the metal surface, where they can initiate corrosion. The multilayer approach is found to significantly improve the corrosion resistance of Mg by reducing levels of porosity.