Reversal of the surface charge of a mineral powder: application to electrophoretic deposition of silica for anticorrosion coatings

Electrophoretic deposition is used to produce a silica layer on zinc-coated steel and chromated zinc-coated steel. This type of coating is commonly employed in car industries to protect steel against corrosion. Because of the presence of zinc and chromated layers on the substrate, the polarization must be cathodic to avoid oxidation. Reversal of the silica surface charge is obtained using different methods, such as cation adsorption, cationic polymer (polyvinyl imidazole) adsorption or oxide (cerium oxide) adsorption. The production of deposits using cations is quite difficult because it is necessary to add a high concentration of salt to reverse the silica charge, provoking high conductivity and gas release at the cathode. With polyvinylimidazole, the instability of the polymer is also a restrictive factor for the application of electrophoresis. The most promising results are obtained with ceriacovered silica. In fact the system is stable under the electric field and allows homogeneous coatings to be produced. The polarization resistance measurements indicate the good protective effect compared with the untreated substrate. (C) 1998 Kluwer Academic Publishers.