Application of novel sol–gel composites on magnesium alloy

Abstract

Abstract Application of defect-free, adherent, and corrosion protective sol–gel film on the magnesium alloys is generally difficult. In this study, two novel sol–gel/conversion coating composites were successfully deposited on AM60B magnesium alloy in order to provide sufficient protection against the corrosion. The first composite (Ti–Zr/hybrid) was obtained via combination of a hybrid sol–gel film (synthesized by mixing tetraethoxysilane (TEOS), and 3-glycidyloxypropyl-trimethoxysilane (GPTMS)) as outer layer and Ti–Zr conversion coating as primer. Also, the second composite (Ti–Zr/PTMS) was applied in a similar manner by combination of phenyl-trimethoxysilane (PTMS) sol–gel film with the Ti–Zr conversion coating. The morphology and elemental composition of the Ti–Zr conversion film were assessed by the Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS), respectively. A cracky conversion film was applied on the alloy surface after immersion in the Ti–Zr conversion coating bath which was mainly composed of MgO, TiO2, ZrO2, and MgF2 compounds. Uniform, but not-adherent PTMS and hybrid sol–gel films (pure sol–gel films) with obvious defects were directly deposited onto the magnesium alloy without the Ti–Zr pretreatment which were morphologically characterized by the SEM. However, formation of relatively uniform and completely defect-free Ti–Zr/hybrid and Ti–Zr/PTMS composites after using the Ti–Zr conversion coating as pretreatment was revealed by the SEM observations. In addition, the defects of the Ti–Zr conversion coating were completely filled by the sol–gel layers. The Ti–Zr/PTMS and Ti–Zr/hybrid composite coatings were provided much better corrosion protection capacity than the pure PTMS and hybrid sol–gel films, respectively which was confirmed by the Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP) examinations in 0.05 M NaCl solution.