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

• Published in: Surface & coatings technology 2011-09, Vol.205 (23), p.5254-5259
• Main Authors: Zuleta, A.A., Correa, E., Villada, C., Sepúlveda, M., Castaño, J.G., Echeverría, F.
• Format: Article
• Language: English
• Subjects: Anodizing; Applied sciences; Calcium carbonate; Carbonation; Cerium; Chromium-free treatment; Coatings; Conversion coating; Cross-disciplinary physics: materials science; rheology; Electrochemical impedance spectroscopy; Exact sciences and technology; Magnesium; Materials science; Metals. Metallurgy; Morphology; Other surface treatments; Physics; Production techniques; Protective coatings; Surface treatment; Surface treatments
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2011.05.048

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 CeO 2. 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. ► 3 different Chromium-free methods of surface modification on pure Mg. ► All the alternatives, non-toxic methods, improve corrosion behavior of Mg. ► Coating obtained from carbonated treatment exhibits better corrosion protection. ► Compact, stable and adherent layers of CaCO 3 are formed in this method.