Nonmetallic Inorganic Coatings
These coatings consist of an inorganic film formed over the surface of the material to be protected. They include enamels, ceramics, composites, glasses, porcelains, oxides, carbides, chromates, nitrides, and silicides. Coatings formed over the surface to be protected should be uniform and have good adherence, in order to effectively protect the metallic substrate in aggressive media. The most widely used coating processes of this category include anodizing, chromate conversion coatings, and phosphate conversion coatings.
Anodizing, in particular aluminum anodizing, consists in the thickening of the protective film that forms spontaneously over aluminum. Anodizing is an electrolytic oxidation process, in an appropriate solution, in which the material to be treated acts as an anode, oxidizing and forming a thick protective layer of oxides. Besides aluminum, there are other materials, such as magnesium, titanium, and zirconium, which can be anodized in order to increase their corrosion resistance.
Chromate conversion consists of forming a coating from a solution containing chromates or chromic acid. These coatings can be applied directly onto the metal to be protected or over a layer of oxides and phosphates. In addition to their anticorrosive function, conversion coatings can be used to promote the adherence of organic coatings. Given the high toxicity of the chromate ion, whose use is gradually being prohibited, the application of this type of coating should be severely restricted in the near future.
Phosphate conversion allows a layer of phosphates to be applied over oxides and hydroxides native to metal surfaces, such as iron, aluminum, galvanized steel, cadmium, and magnesium. Further to its protective function, this layer also increases the adherence of paints or other organic coatings, being used as a pretreatment, although it may also be used as it is for decorative purposes.