Comparative study of different environmentally friendly (Chromium-free) methods for surface modification of pure magnesium

In this work, three different chromium-free methods of surface modification of pure magnesium were studied; namely, a cerium conversion coating, a carbonated coating and an anodic film. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), micro Raman spectroscopy (MRS) and X-ray diffraction (XRD) were employed to study the morphology and composition of the surface after treatments. The corrosion resistance of the treated surfaces was evaluated by means of electrochemical impedance spectroscopy (EIS). XRD and MRS showed that conversion coatings obtained from cerium salts consisted of cerium oxides and hydroxides and the major component of the layer was CeO₂. The morphology of the cerium conversion coating consisted of a fiber-like structure that exhibited several network-like cracks. The carbonated coating was mainly composed of aragonitic CaCO_3, whereas the oxide layer formed in the anodizing process was a porous film made of MgO and some phosphate species compounds. The results of EIS tests indicated that calcium carbonated coating provided higher corrosion protection.