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Obtenção e caracterização de revestimentos híbridos à base de precursores alcoóxidos sobre o aço galvanizado
Sandra Raquel Kunst
DATA DE PUBLICAÇÃO
Galvanized steel presents a protective layer of Zn and Zn-Fe, obtained by hot dipping, which promotes the steel protection. However, the presence of this layer decreases the adhesion of paint systems and reduces the corrosion resistance. A very efficient method used to protect against the substrate corrosion is chromate layers, which provide excellent anticorrosive and good adhesion properties. However, the processes employed to obtain this type of coating have been abandoned due to the use of hexavalent chromium, which is carcinogenic. In this context, organic-inorganic hybrid coatings obtained by sol-gel process have been presented as environmentally friendly alternative. The addition of corrosion inhibitors or other compounds, to the hybrid films can modify the properties of the barrier layer consequently improving the corrosion performance. This research aimed to evaluate the electrochemical behavior of galvanized steel coated with a hybrid film obtained from a sol composed by the alkoxides precursors 3 - (trimethoxysilylpropyl) methacrylate (TMSM) and tetraethoxysilane (TEOS), with the addition of cerium nitrate in a concentration of 0.01 mol L-1. Different cure mechanisms (thermal and ultraviolet processes and in air) were used. It was also evaluated the immersion time in the solution (2, 5 and 10 minutes), the substrate pretreatment process,and the solution pH. Finally the TEOS/TMSM ratio was varied in 5 levels maintaining the MMA/TMSM ratio. Morphological characterization and film thickness measurements were performed by scanning electron microscopy (SEM). Moreover, the films were evaluated concerning the wettability (contact angle determination), the chemical structures (infrared spectroscopy) and the electrochemical behavior (open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy). From this study it was possible to verify the influence of the sol chemical composition and pH solution on the film formation. Besides the physical and electrochemical properties are influenced by these parameters. It was also evidenced the influence of the substrate immersion time in the solution, as well as the substrate pretreatment and the cure process effect on the coatings corrosion resistance.