Corrosion resistant alloy
The ability of a metal material to resist erosion of a medium in a corrosive medium is referred to as the corrosion resistance of the metal. High corrosion resistance in pure metals usually has one of the following three conditions:
1 Thermodynamically stable metal. Usually it can be judged by its standard electrode potential, its value is more stable than the person with a higher stability, and less stable with a lower value. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc. belong to this category.glenflange
2 easily passivated metal. Many metals can form a dense oxide film with a protective effect in an oxidizing medium. This phenomenon is called passivation. The most easily passivated metals are Ti, Zr, Ta, Nb, Cr, and Al.
3 The surface energy can produce metal that is insoluble and has good corrosion protection film. This situation occurs only when the metal is in a particular corrosive medium, for example, Pb and Al in H2SO4 solution, Fe in H3PO4 solution, Mo in hydrochloric acid, and Zn in atmospheric medium.
Therefore, industrially based on the above principles, a series of corrosion-resistant alloys are obtained by alloying methods. Generally, there are corresponding three methods:glenflange
1) To improve the thermodynamic stability of metals or alloys by adding alloy elements with high thermodynamic stability to metals or alloys that are not originally resistant to corrosion, thereby forming solid solutions and increasing the electrode potential of the alloy and enhancing its corrosion resistance. For example, Au is added to Cu, and Cu, Cr, etc. are added to Ni. However, this method of adding a large amount of precious metals has limited application in industrial structural materials.
2) Adding easily passivated alloying elements, such as Cr, Ni, Mo, etc., can improve the corrosion resistance of the base metal. Chromium stainless steel can be prepared by adding appropriate amount of Cr to the steel. Experiments show that in stainless steel, Cr content should generally be greater than 13% to play a role in corrosion resistance, the higher the Cr content, the better the corrosion resistance. Such stainless steels have good corrosion resistance in oxidizing media, but have poor corrosion resistance in non-oxidizing media such as dilute sulfuric acid and hydrochloric acid. This is because non-oxidizing acids do not easily form oxide films on the alloys, and they also have dissolution effects on the oxide films.glenflange
3) The addition of alloying elements that can promote the formation of a dense protective film of corrosion products on the alloy surface is another way to prepare corrosion resistant alloys. For example, steel is resistant to atmospheric corrosion due to the formation of a dense structure on its surface, iron oxyhydroxide [FeOx(OH)23-2x], which protects it. The addition of Cu and P or P and Cr to the steel can promote the formation of such a protective film, and thus can be made of Cu, P or P, Cr, low alloy steels resistant to atmospheric corrosion.
Metal corrosion is the most harmful spontaneous process in industry. Therefore, the development and application of corrosion resistant alloys have great social and economic value.