Sol-gel films of tetraethoxysilane (TEOS), phenyltriethoxysilane (PTES) and phenyltrimethoxysilane (PTMS) were prepared on aluminium and aluminium alloy AA7075 and characterized. The corrosion inhibition properties of uncoated and coated specimens were evaluated with potentiodynamic polarization scan, linear polarisation resistance and open circuit potential measurements. With scanning electron microscopy and optical microscopy effects of the sol-gel coating were shown. Weight loss measurements were performed at different pH-values. More hydrophobic coatings as PTMS show a considerably better corrosion inhibition capacity as compared to the capacity provided by less hydrophobic ones as TEOS and PTES. The pictures show samples of the alloy AA 7075 uncoated (top) and coated with PTMS (bottom) before (left) and after (right) immersion for aqueous solution of 0.05 mol/l NaCl for 24 days with obvious corrosion damage. The inset shows potentiodynamic curves for all alloy samples after immersion in an aqueous solution of 0.05 mol/l NaCl for 24 hr. Results obtained with all employed methods correlate well with the hydrophobicity of the coating with an efficiency growing according to PTMS < PTES < TEOS as visible at the corrosion potentials in the diagram.
More detailed information can be found in: A. A. Younis, W. Ensinger, M. M. B. El-Sabbah, R. Holze; Corrosion protection of pure aluminium and aluminium alloy (AA7075) in salt solution with silane-based sol-gel coatings; Materials and Corrosion 2013, 64, 276.