Hybrid nanocomposite coating by sol-gel method: a review

The development of environmentally friendly process for pretreatments of metallic substrates is a field of growing research due to the ban against chromates used as protective pretreatments. Among the possible candidates for environmentally friendly pretreatments of aluminum alloys are the silica-based sol-gel coatings. Such coatings are able to form an Si-O-Al conversion layer providing a stable alumina/sol-gel film interface, which inhibits the onset of corrosion. Sol-gel technology offers a wide range of chemical mechanisms and exhibits high potential substitutes for the environmentally unfriendly chromate metal-surface pretreatment. Sol-gel derived organo-silicate hybrid coatings, preloaded with organic corrosion inhibitors, have been developed to provide active corrosion protection when integrity of the coating is compromised. The incorporation of organic corrosion inhibitors into hybrid coatings has been achieved as a result of physical entrapment of the inhibitor within the coating material at the stage of film formation and cross-linking. Sol-gel derived coatings, especially the hybrid films, provide a dense barrier against electrolyte uptake, and offer a wide range of applications as corrosion protective, hydrophilic coatings, hydrophobic anti-reflective coatings, migration barriers against liquid and volatile compounds, antibacterial modification of textiles and water-repellent antistatic textiles. In this paper, the novel applications of the sol-gel derived coatings are presented and discussed.