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Estudo da influência das condições de síntese nas propriedades de híbridos orgânico-inorgânicos puros e modificados por Ce(IV)

Authors
  • Santos, Fábio Cesar dos
Publication Date
Jun 11, 2018
Source
Repositório Institucional UNESP
Keywords
Language
Portuguese
License
Unknown
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Abstract

Organic-inorganic hybrid coatings for corrosion protection of aluminum alloys are promising alternatives to the current methods based on chromate passivation. From the optimization of polymerization efficiency (Ep) of the organic phase and using the sol-gel process, hybrids with different polymer/silica ratios and cerium concentrations were prepared, being studied in terms of the hybrid nanostructure formed and its effect on the thermal stability and anticorrosive performance of the coatings. Hybrids were prepared by the radical polymerization in tetrahydrofuran (THF) of the methyl methacrylate (MMA) and 3- methacryloxypropyltrimethoxysilane (MPTS) using benzoyl peroxide (BPO) as thermal initiator, followed by hydrolysis and acid polycondensation of tetraethoxy silane (TEOS) and MPTS. The ceric ammonium nitrate (CAN) was added to the hydrolysis/polycondensation step to obtain the doped samples. After mixing the organic and inorganic phases, the hybrid solutions were used to prepare by dip-coating the siloxane-PMMA coatings onto the Al2024- T3 aluminum alloy. In the first step, using a factorial design of experiments, 4 variables were studied in the polymerization reaction of the organic phase (temperature, BPO/MMA ratio, time and MMA/MPTS ratio) and the optimized parameters were fixed for a detailed investigation of the polymer/silica molar ratio (MMA/MPTS and TEOS/MPTS) in the range of 2 to 10. In the second moment, one of the optimized MMA/TEOS conditions was doped with different concentrations of cerium. The nanostructural properties obtained with the small angle X-ray scattering (SAXS) of the unsupported films were correlated with its thermal stability evaluated by thermogravimetry (TG) and with the corrosion protection of the hybrid coatings (2-6 μm thickness), evaluated by electrochemical impedance spectroscopy (EIS) in saline/acid solution (NaCl 3.5 % + HCl pH 3). The results of TG, Raman and X-ray photoelectron spectroscopy showed that higher BPO/MMA ratio induces higher polymerization than prolonged reaction time and both are much more effective than the temperature and MMA/MPTS ratio. Thermogravimetry showed high thermal stability (temperature of 5% mass loss at 287 °C) for the hybrid compositions with lower silica fractions, which presented smaller distance between the siloxane domains present in the polymer matrix. The addition of cerium to the hybrids also increased the distance between the siloxane domains and consequently promoted a decrease in thermal stability. Electrochemical measurements showed that coatings with Ep close to 100 % and MMA/TEOS ratio of 7 provided the best corrosion protection with impedance modulus (|Z|) of 25 GΩ cm2, being about 7 orders of magnitude higher than the |Z| of the uncoated Al2024-T3 substrate. In the case of doped coatings, it was observed that the higher corrosion protection (> |Z|) was obtained for the samples in which the x-ray absorption spectroscopy (XAS) identified a greater abundance of cerium in the oxidation state Ce4+.

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