Protective PMMA-silica coatings for aluminum alloys: Nanostructural control of elevated thermal stability and anticorrosive performance

Abstract

Organic-inorganic hybrid coatings for corrosion protection of aluminum alloys are promising alternatives to the current methods based on chromate passivation. This study examined the role of the polymer/silica ratio in terms of the hybrid nanostructure formed and its effect on the thermal stability and anticorrosive performance of the polymethyl methacrylate (PMMA)-siloxane-silica hybrid coatings. The chemical and nanostructural properties of the hybrid coatings assessed using Fourier transform infrared (FTIR) and small angle X-ray scattering (SAXS) were correlated with the thermal stability evaluated by thermogravimetry (TGA) and corrosion protection evaluated by electrochemical impedance spectroscopy (EIS) in saline/acid solution (NaCl 3.5 % + HCl pH 3). TGA showed that the high thermal stability (up to 287 °C) of the hybrid formulations with lower silica fractions is related to the adequate size and spacing of the siloxane nodes in the embedding polymer matrix. Correlation of SAXS and EIS measurements allowed to identify the specific molar ratios and nanostructural configuration in which the polymer and siloxane-silica nodes ideally combine forming thin PMMA-silica coatings (2−5 μm) that present long-term stability (> 6 months) with a corrosion resistance of up to 25 GΩ cm2, being approximately 7 orders of magnitude higher than that of the uncoated Al2024-T3 substrate.