Transition of interface oxide layer from porous Mg(OH)2 to dense MgO induced by polyaniline and corrosion resistance of Mg alloy therefrom

•Polyaniline/epoxy coating significantly enhanced corrosion resistance of Mg alloy.•The improved protection of Mg alloy is due to an effective interface oxide layer.•The effective interface layer under polyaniline/epoxy coating is dense and MgO rich.•The driving force to form the oxide layer is the interaction between polyaniline and Mg alloy.•The compactness and MgO content can be enhanced by increasing polyaniline in coating. The feasibility of polyaniline emeraldine base (EB) for enhancing long-term corrosion resistance of magnesium alloy (AZ91D Mg alloy) was confirmed, since the complex impedance of Mg alloy protected by EB/epoxy resin (ER) composite coating with 10wt% EB loading maintained around 2GΩcm2 even after 80 day exposure in 0.5M NaCl solution, while that of pure ER coated analogue decreased to 0.17MΩcm2 only after 31 days. The improvement in corrosion resistance was attributed to the transition of interface layer from porous Mg(OH)2 dominated one underneath pure ER coating to dense MgO dominated one underneath EB/ER coating, induced by the redox interaction of EB with Mg alloy. When the EB loading in EB/ER coating increased from 0 to 10wt%, the relative XPS peak area ratio of MgO to Mg(OH)2 increased from 0.78 to 1.18, indicating that EB behaved as effective corrosion inhibitor causing the transformation of oxide layer from porous Mg(OH)2 to dense MgO.