Improving the corrosion performance of hybrid sol-gel matrix by modification with phosphonic acid

Sol–gel films were prepared using different ratios of two silica precursors, vinyltrimethoxysilane (VTMS) and tetraethylorthosilicate (TEOS), targeting the anticorrosion treatment of aluminum alloy 2024-T3. The electrochemical behavior of the alloy coated with such films, which was studied in a 0.05 mol L -1 NaCl solution, revealed that the VTMS/TEOS ratio drastically affects the anticorrosion performance. Electrochemical impedance spectroscopy (EIS) results provided evidences that the best anticorrosive response occurs with the films prepared from 50% v/v ethanol, 46% v/v water, 3% v/v VTMS and 1% v/v TEOS. Moreover, the effects of the addition of ethylenediamine tetra(methylene phosphonic acid) (EDTPO) to a hybrid sol–gel matrix were examined. The corrosion resistance was considerably improved when the concentration of added phosphonic acid was 3.7510 -5 mol·L -1 . Scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses revealed that EDTPO-containing films provide a more uniform coverage. Results evidenced the formation of Si–O–Al linkages in the VTMS/TEOS and VTMS/TEOS/EDTPO systems and that the VTMS/TEOS/EDTPO system contained P–O–Al linkages. The P–O–Al bonds, combined with the silica network action, may be responsible for the good anticorrosive behavior of the VTMS/TEOS/EDTPO films.