There are thousands of alloys used in industry, and only a few of them are briefly introduced.
The ability of metal materials to resist medium corrosion in corrosive medium is called corrosion resistance of metal. Pure metals with high corrosion resistance usually meet one of the following three conditions:
① Metals with high thermodynamic stability. Usually, the standard electrode potential can be used to judge, and the one with a positive value is more stable; Those who are less stable are less stable. Precious metals with good corrosion resistance, such as Pt, Au, Ag and Cu, belong to this category.
② Metals that are easily passivated. Many metals can form dense oxide film with protective effect in oxidizing medium, which is called passivation. The most easily passivated metals are Ti, Zr, Ta, Nb, Cr and Al.
③ A metal whose surface can form a corrosion product film that is insoluble and has good protection performance. This situation only occurs when the metal is in a specific corrosion medium. For example, Pb and Al are in H2SO4 solution, Fe is in H3PO4 solution, Mo is in hydrochloric acid, and Zn is in the atmosphere.
Therefore, according to the above principles, a series of corrosion-resistant alloys are obtained by alloying in industry. Generally, there are three corresponding methods:
① Improve the thermodynamic stability of metal or alloy, that is, add alloy elements with high thermodynamic stability to the original non corrosion resistant metal or alloy, so as to form a solid solution, improve the electrode potential of the alloy, and enhance its corrosion resistance. For example, adding Au to Cu and Cu, Cr, etc. to Ni belong to this category. However, this method of adding a large number of precious metals has limited application in industrial structural materials.
② Adding easily passivated alloy elements, such as Cr, Ni, Mo, etc., can improve the corrosion resistance of the base metal. Chromium stainless steel can be made by adding an appropriate amount of Cr into the steel.
The experiment shows that the corrosion resistance of stainless steel can only be achieved when the Cr content is more than 13%. The higher the Cr content, the better the corrosion resistance. This type of stainless steel has good corrosion resistance in oxidizing medium, but poor corrosion resistance in non oxidizing medium such as dilute sulfuric acid and hydrochloric acid. This is because the non oxidizing acid is not easy to make the alloy form oxide film, and also has dissolution effect on the oxide film.
③ Adding alloy elements that can promote the formation of dense corrosion product protective film on the alloy surface is another way to prepare corrosion resistant alloys. For example, steel can resist atmospheric corrosion due to the formation of a compact compound of ferric hydroxide [FeOx · (OH) 23-2x] on its surface, which can play a protective role. The addition of Cu and P or P and Cr in the steel can promote the formation of such protective film, thus the low alloy steel with atmospheric corrosion resistance can be made of Cu, P or P and Cr.
Metal corrosion is the most harmful spontaneous process in industry, so the development and application of corrosion resistant alloys have great social significance and economic value
