A composite coating for corrosion protection of AM60B magnesium alloy

Abstract

Oxide films have been produced on AM60B magnesium alloy by micro-arc anodic oxidation in an environmentally friendly alkaline solution, with and without addition of nanoparticles (TiO2, ZrO2 and Al2O3). Because of the anodic oxide porosity, inherent in the sparking process, organo-functional silanes top coat has been applied to seal pores and cracks, and achieve an efficient protective coating system. The surface and cross-section morphology of samples were analyzed by Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Scratch tests were performed for evaluating the adhesion strength and scratch hardness of the anodic oxides to the AM60B substrate. The corrosion resistance of both anodic oxides and oxide/silane composite coatings was evaluated in 0.6M NaCl solution using potentiodynamic polarization tests. The addition of nanoparticles to the anodizing solution doesn't affect significantly the corrosion resistance in comparison with anodic oxides produced in nanoparticles free solutions. Conversely, the adhesion strength and scratch hardness of the anodic oxides to the substrate is quite scattered, and it is higher for the samples produced in ZrO2 and in Al2O3 rich solutions. For this reason specimens anodized in ZrO2 and Al2O3 containing solutions were chosen for silane deposition. Two silanes were used, namely octyltrimethoxysilane (OSi) and 1, 2-bis [triethoxysilyl] ethane (BTSE). The anodizing treatment carried out in oxides nanoparticles containing solutions (ZrO2 or Al2O3), followed by a silane top coat treatment performed using OSi precursor, is an interesting way, suitable for industrial applications, to synthesize adherent corrosion resistant coatings on magnesium alloy AM60B in a short process time.