Non-VOC Water-Based Nanocomposite Sol-Gel Thin Films for Corrosion Protection of Commercial Magnesium Alloys

Novel non-VOC and non-chromate water-based organic-inorganic hybrid thin films doped with ZrO₂ nanoparticles have been successfully prepared for corrosion protection of AZ31 and AZ61 magnesium alloys. These nanocomposite thin films were synthesized by sol-gel process. The organosiloxane sols were obtained by mixing of 3-glycidoxypropyltrimethoxysilane (GPTMS) and methyltriethoxysilane (MTEOS). Additionally these hybrid sols were doped with ZrO₂ nanoparticles prepared from hydrolyzed zirconium tetra-tert-butoxide (ZTB). Different concentrations ZrO₂ have been tested in order to obtain several experimental materials. For properly preparing the organosilane coating, hexamethoxymethylmelamine (HMMM) has been added to the solution during the sol stage. Small aliquots of p-toluenesulphonic (p-TSA) were also added to coating formulation just before coating. HMMM acts as a crosslinking agent and p-TSA as a blocked acid catalyst. In a second stage of the study a set of these ZrO₂ nanocomposite thin films has been modified by adding to the sols variable amounts of corrosion inhibitors (benzotriazole, L-cysteine). The thermal stability of the prepared hybrids was investigated by using thermal analysis (TG/DTG). Attenuated total reflectance Fourier transformer infrared spectroscopy (FTIR-ATR) has been used to determine which functional groups are present in coatings. The hydrophobic behaviour of coatings was evaluated by contact angle measurements. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray (EDX) microanalysis have been applied to study the surface morphology and composition of coated samples. Thickness of the sol-gel coatings was determined using cross-section SEM images. Atomic-force microscope (AFM) has been used for obtaining 3D topography images and for calculating the root-mean-squared roughness (RMS roughness) of both bare and sol–gel coated surfaces. The corrosion behaviour of the resulting metal/coating systems was studied by immersion tests in NaCl aqueous solutions at different concentrations and variable exposure time. The obtained results show that the combination of XPS, SEM/EDX and AFM with global and local electrochemical impedance spectroscopies (EIS and LEIS) allow not only to assess the protective ability of these new water-based sol-gel thin films but also the influence of inhibitors added to the films on the kinetics and mechanisms of corrosion of the metal substrates. The ZrO₂ nanoparticles act as active nanoreservoirs providing a prolonged and smart release of corrosion inhibitors during contact of the sol-gel thin film with NaCl aqueous solutions. The organic inhibitors provide self-healing properties to the protective system suppressing the corrosion processes as expected. This self-healing mechanism can be ascribed to sealing or absorption phenomena which take place on the bare metal surface into the micropores and defects developed on the sol-gel coatings during the immersion tests in the NaCl aqueous solutions. The water-based sol-gel films modified with benzotriazole were the most effective but controlling appropriately the formulation of films, L-cysteine could be used as an environmentally friendly inhibitor for potential replacement to benzotriazole.