Study on Corrosion Resistance of AZ31 Magnesium Alloy Coated with Graphene Modified Epoxy and Polyurethane Coatings

Graphene-modified epoxy resin coating can impart better corrosion resistance to magnesium alloys. In this regard, the present work focuses on the preparation of graphene-modified oil-based epoxy resin/polyurethane composite coating (G/OEP/OPU) and waterborne epoxy resin/polyurethane composite coating (G/WEP/WPU) and is coated over the AZ31 magnesium alloy to elucidate the corrosion protection behavior. Scanning electron microscopic observation shows that the surface quality of graphene flakes is good. When graphene is added to the oil-based and waterborne epoxy primers, the surface quality of the oil-based polyurethane topcoat is improved, while the surface quality of the waterborne polyurethane topcoat is reduced. Fourier transform infrared spectroscopic testing shows that the coating and the curing agents are the major structural components of the coating. The electrochemical test results show that the composite coatings can significantly improve the corrosion resistance of magnesium alloys. In the oily composite coatings, G/OEP/OPU-0.3wt% shows the highest corrosion resistance where the corrosion current density is 1.81×10-12A/cm2. For waterborne composite coatings, G/WEP/WPU-0wt% has the least corrosion current density value of 5.07×10-12 A/cm2. The comprehensive analysis suggests that the graphene improves the bonding performance of the primer and topcoat in the case of oily composite coating leading to the improvement in the surface quality of the topcoat and increase the coating's ability to resist the penetration of corrosive media. For waterborne composite coatings, the hydrophobic graphene improves the hydrophobicity of the primer coat, and therefore, the curing quality of the waterborne polyurethane topcoat is deprived that weakens the corrosion resistance of the waterborne composite coating.