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

Abstract

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 NaCI 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 x 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 S-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 modified silica network action, may be responsible for the good anticorrosive behavior of the VTMS/TEOS/EDTPO films. (C) 2014 Published by Elsevier B.V.