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 em ployed in car industries to protect steel against corrosion. Because of the presence of zinc and chromated layers on the substrate, the polarization m ust be cathodic to avoid oxidation. Reversal of the silica surface charge i s obtained using different methods, such as cation adsorption, cationic pol ymer (polyvinyl imidazole) adsorption or oxide (cerium oxide) adsorption. T he production of deposits using cations is quite difficult because it is ne cessary to add a high concentration of salt to reverse the silica charge, p rovoking high conductivity and gas release at the cathode. With polyvinylim idazole, the instability of the polymer is also a restrictive factor for th e application of electrophoresis. The most promising results are obtained w ith ceriacovered silica. In fact the system is stable under the electric fi eld and allows homogeneous coatings to be produced. The polarization resist ance measurements indicate the good protective effect compared with the unt reated substrate. (C) 1998 Kluwer Academic Publishers.