Sol–Gel Protective Coatings for Metals

Abstract

This chapter presents a review of the most recent and relevant works developed by a wide range of researchers on different types of protective and functional coatings deposited on metals and alloys to improve their corrosion resistance. The review is focused on the study of coatings produced by sol–gel trying to identify the perspectives for further research. Sol–gel process is a powerful tool to produce environmentally friendly coatings to replace CCC and CAA chromium-containing films. In this chapter, the corrosion behavior of different sol–gel coatings is discussed considering the nature of the coating. Three groups of films have been considered: inorganic coatings, organic–inorganic hybrid coatings, and inorganic and hybrid coatings that incorporate corrosion inhibitors. In general, the adequate performance of the different films requires considering different aspects such as the composition, sol processing conditions, deposition methods, thermal treatments, and final structure. Inorganic sol–gel coatings cannot provide adequate corrosion protection due to the presence of micro-cracks or defects. On the other hand, the introduction of organic components to an inorganic sol–gel system leads to the formation of thicker and more flexible films that enhances the electrochemical behavior and improves the compatibility with different organic top coatings. However, corrosive ions can still diffuse through micro-pores and attack the metallic substrates, producing their degradation when they are exposed to aggressive media for a long time. In all these cases, the protection of the alloys is only sought by creating a dense physical barrier.The most recent trends in corrosion protection focus on sol–gel coatings doped with environmentally friendly inhibitors. In this case, the barrier effect is combined with the active protection provided by the inhibitor. However, the amount of inhibitor (organic or inorganic) able to be incorporated to the coating is limited. Higher concentrations of inhibitor lead to the formation of defects and to porous structure reducing the barrier properties of the films. The most recent alternatives are related with the preparation of pure cerium glass-like coatings or with the incorporation of nano-reservoirs or nano-containers to sol–gel coatings. However, and despite the high number of published articles, there is still no industrially suitable alternative treatment for aircraft aluminum alloys able to provide the required corrosion protection. Alternatively, new sol–gel deposition methods are under development to allow the upscaling of coating processes to be implemented at industrial scale.