Comparative study of conversion treated magnesium alloy in the presence of Mn/adipic acid additives before and after application of epoxy coating: Anti-corrosion, surface, and adhesion properties

The use of magnesium (Mg) has been limited due to its poor corrosion resistance. In the present study, the AZ31 Mg alloy was coated by a one-step zirconium-based conversion coating (ZrCC). In order to study the change in the microstructure and the corrosion behavior of the coating, adipic acid (AA) and manganese (Mn) as additives were added to the conversion solution. The morphology and composition of the samples were analyzed by field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The electrochemical behavior of conversion coatings was evaluated by the DC polarization technique and electrochemical impedance spectroscopy (EIS) measurement. The results revealed that the Zr-Mn sample possesses the highest polarization resistance (Rp = 9837 omega cm2) and the lowest current density (0.25 mu A/cm2). Also, Rp recorded for this sample was almost twice as much as the ZrCC sample (4055 omega cm2). The Zr-Mn-AA sample compared to other samples showed better adhesion strength, the minimum delamination of coating, and the highest anti-corrosion resistance after the application of epoxy coating.