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Performance of silanes in protecting metals from corrosion: Effect of substrate cleaning

Organofunctional silanes are a class of coupling agents which have a functional group on one end and a hydrolyzable Si ester group at the other. Upon hydrolysis, the Si ester group produces silanol which typically bonds to the metal surface through the surface hydroxyl group forming a stable metal-O-Si bond. Silanes are sensitive to the nature of the metal substrate onto which they are adsorbed. Of particular importance are the nature and the concentration of the surface hydroxyl groups. Hydrated hydroxyl groups form the main composition of the surfaces of metals and metal oxides which are in equilibrium with the atmospheric moisture [1]. These hydroxyl groups may vary widely in concentration and nature. A minimum number of hydroxyls are found on freshly fused substrates stored under neutral conditions. Isolated or free hydroxyls react slowly in comparison to hy-

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drogen-bonded vicinal hydroxyls which react more readily with the silane coupling agent [2].