First, the ability of metal materials to resist corrosion in corrosive media is called the corrosion resistance of metals. Corrosion resistance in pure metals is usually one of the following three conditions:
(1) Metals with high thermodynamic stability. Usually it can be judged by its standard electrode potential. Its value is more stable than the positive one; it is less stable than the negative one. Precious metals with good corrosion resistance, such as Pt, Au, Ag, Cu, etc. belong to this category.
(2) Metals that are easily passivated. Many metals can form dense oxide films with protective effects in oxidizing media. This phenomenon is called passivation. The most easily passivated metals are Ti, Zr, Ta, Nb, Cr, Al and so on.
(3) Metals whose surface can form insoluble and well-protected corrosion product films. This occurs only when the metal is in a specific corrosive medium, such as Pb and Al in H2SO4 solution, Fe in H3PO4, Mo in hydrochloric acid solution, and Zn in the atmosphere.
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Second, the method of obtaining corrosion-resistant alloys: According to the above principles, the industry uses a method of alloying to obtain a series of corrosion-resistant alloys. There are generally three corresponding methods. Our company Alloy c 22 Round Bar is on sale.
(1) Improving the thermodynamic stability of metals or alloys, that is, adding alloying elements with high thermodynamic stability to the original non-corrosive resistant metals or alloys, so as to form a solid solution and increase the electrode potential of the alloy, and enhance corrosion resistance. Such as Cu added Au, Ni added Cu, Cr, etc., is such. However, this method of adding precious metals in large quantities has limited application in industrial structural materials.
(2) Adding easily passivating alloy elements, such as Cr, Ni, Mo, etc., can improve the corrosion resistance of the base metal. By adding an appropriate amount of Cr to the steel, chromium-based stainless steel can be obtained. The experiment proves that in stainless steel, Cr content should generally be greater than 13% to play the role of corrosion resistance. The higher the Cr content, the better the corrosion resistance. This type of stainless steel has good corrosion resistance in oxidizing media, but has poor corrosion resistance in non-oxidizing media such as sulphuric acid and hydrochloric acid. This is because the non-oxidizing acid is not easy to cause the alloy to form an oxide film, and it also has a dissolving effect on the oxide film.
(3) Adding alloy elements that can promote the formation of a dense corrosion product protection film on the alloy surface is another way to make a corrosion resistant alloy. For example, the steel's resistance to atmospheric corrosion is due to the protective effect of the densely formed compound iron oxyhydroxide (FeOx · (OH) 3-2x) on its surface. Adding Cu and P or P and Cr to the steel can promote the formation of such a protective film, and thus Cu, P, or P, Cr can be used to make a low-alloy steel resistant to atmospheric corrosion.
Metal corrosion is the most spontaneous process in the industry, so the development and application of corrosion resistant alloys have great social significance and economic value.
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