Protection and surface modification of metals with sol–gel coatings

The preparation and properties of protective sol–gel coatings on metallic substrates are reviewed. Control of the coating microstructure is critical to obtain the desired properties and this requires optimisation of processing parameters that include sol composition, synthesis and processing conditions, deposition method and post-deposition sintering or curing. Current knowledge in these areas, e.g. structure–property–processing relationships, and the characterisation of the coatings produced are discussed. SiO2 layers have been found to confer good oxidation resistance, but for electrochemical corrosion resistance, mixed oxide layers or hybrid organic–inorganic layers are required. Incorporation of alkylalkoxides with polymerisable groups attached has allowed synthesis of hybrid nanostructured coatings consisting of interpenetrating, chemically bonded organic and inorganic networks. Doping of hybrid sol–gel coatings with environmentally friendly inhibitors is also a promising approach combining barrier properties with active inhibition of corrosion. The potential to control functional properties such as biocompatibility is also briefly considered. It is concluded that both fundamental work on new coating compositions and the development of improved industrial technologies are priorities for future research.