The phenomenon that the metal is gradually damaged under the action of external media is called corrosion. There are basically two forms of corrosion, chemical corrosion and electrochemical corrosion. The corrosion encountered in production is mainly electrochemical corrosion. No current is generated in chemical corrosion, and some corrosion products are formed during the corrosion process. This corrosion product is usually covered on the metal surface to form a film to isolate the metal from the medium. If this layer of chemical products is stable, dense, complete and firmly combined with the metal surface layer, it will greatly reduce or even prevent the further development of corrosion and protect the metal. The process of forming a protective film is called passivation. For example, SiO2, Al2O3, Cr2O3 and other oxide films are formed. These oxide film structures are dense, complete, non-loose, non-cracked and not easy to peel off, which can protect the base metal and prevent further oxidation. For example, Fe2O3 is formed when iron is oxidized at high temperature. On the contrary, some oxide films are discontinuous or porous, and do not protect the base metal. Some metal oxides, such as Mo2O3 and WO3, are volatile at high temperatures and do not have the protective effect of covering the substrate at all.
It can be seen that the generation of the oxide film and the structure and properties of the oxide film are important characteristics of chemical corrosion. Therefore, the ability to improve the chemical resistance of metals is mainly formed by alloying or other methods, forming a stable, complete, dense, and firmly bonded oxide film on the surface of the metal, also known as a passivation film. Electrochemical corrosion is an important and universal form of metal corrosion. It is caused by the difference in electrode potential between different metals or different grooves of metal. This galvanic cell corrosion is generated between microstructures, so it is also called microcell corrosion. Electrochemical corrosion is characterized by the presence of a dielectric, potential differences in communication or contact between different metals, metal micro-regions, or phases, and a corrosion current is generated.