Magnesium and its alloys have become a hot research topic due to their properties such as low density, easy recyclability, and high strength-to-weight ratio properties that make them good alternatives for a variety of industries applications such as automotive and aerospace industries. However, thy are extremely susceptible to corrosion, particularly in contact with aqueous solutions, limiting their use. The main via to control the corrosion rate is to use surface modification techniques which act as barriers. In recent years, the use of plasma electrolytic oxidation (PEO) films combined with sol-gel coatings were proposed for protecting metal surfaces. These coatings have demonstrated to improve the corrosion resistance of Mg alloys, but they are no capable to stop the corrosion process in severe corroding environments. For obtaining active coatings, the incorporation of inhibitors to the coatings or to the corrosive medium have been considered. The inhibitors can precipitate as insoluble compounds blocking the corrosion and acting as conversion coatings. However, the amount of inhibitor introduced is quite low, limiting their industrial application.
Under this perspective, the aim of this work was to develop a smart self-healing protective system to provide an active and long-term protection to the Mg alloys. Our objective is the deposition of a first oxide layer using PEO technique followed by a post-sealing process using a Ce-based solution and a final SiO2 hybrid sol to provide good barrier properties. The PEO coating was obtained with a friendly electrolyte based on NaOH, NaH2PO4, and Na2CO3. The Ce-based solution was prepared from cerium acetate and the SiO2 sol from tetraethoxysilane (TEOS) and 3-(glycidyloxypropyl) trimethoxysilane (GPTMS).
The integrated system was characterized by FITR spectroscopy, x-ray diffraction (XRD), spectroscopic ellipsometry, optical profilometry and water contact angle. The electrochemical properties were studied in 3.5%wt NaCl solution by potentiodynamic polarization technique and Electrochemical Impedance Spectroscopy (EIS) (Gamry FAS2 Femtostat), revealing that the appropriate anodization electrolyte together with the post-treatment process result in a promising solution to control the corrosion performance.