To improve the corrosion resistance of metal, according to the mechanism of corrosion, there are three types of aspects to achieve this purpose: One is to make the surface of the metal form a stable, dense, and solid film with the matrix metal, because this layer of film makes the metal matrix and the corrosive medium separated, can prevent further corrosion of the metal, play a protective role, such a film is often called passivation film, such as blue, black treatment, phosphating and other surface treatment process is this category. The other is alloying, so that the alloy is a single uniform phase or organization at room temperature, so that no battery or microbattery can be formed in the corrosive medium, or by alloying to improve the electrode potential of the alloy itself, so that the corrosive medium does not corroded to the alloy itself, thereby avoiding electrochemical corrosion. The alloy elements are added to the steel, and the stainless steel series formed belongs to this category. Another type of protection is electrochemical protection, which uses direct current to polarize the protected metal, so that corrosion is slowed or stopped.
The common form of corrosion is oxidation, the formation of a certain oxide film on the surface of the metal, with the continuous generation of oxide film will form two situations. In one case, the resulting oxide film is loose, unstable, and not firmly bound to the matrix, and oxygen continues to diffuse through the oxide film, making the oxide film thicker, and also continuously corrodes the matrix metal, resulting in continuous corrosion. In the other case, the generated oxide film is dense, stable, and firmly bound to the matrix, and the oxygen is isolated by the oxide film to prevent further corrosion of the matrix, and this layer of oxide film has a protective effect to prevent further corrosion.
Rust is iron trioxide, has the characteristics of loose, and the matrix is not firmly combined, so it can not play a protective role, and the iron trioxide oxide film has the characteristics of dense, stable, and the matrix is firmly combined, so it can have a certain protective effect. The surface treatment technology of blue and black treatment is to form a thin film of ferric oxide on the surface of steel to achieve a certain protective effect.
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The reason why stainless steel fasteners resist corrosion in some environments is related to their passivation performance. When the steel is in the passivation state, a dense oxide film can be formed on the surface, which can prevent oxygen ions from infiltrating and oxidation reaction leading to rust, if this film is destroyed, it is easy to rust. Stainless steel is an alloy. An alloy is a metallic property composed of two or more chemical elements (at least one of which is a metal). Stainless steel is composed of iron, chromium, nickel, manganese and other elements. These alloying elements make the internal structure and properties of the steel change, and generate a dense oxide film on the surface of the steel, making it resistant to acid, alkali, oxidation and corrosion. Because of its excellent corrosion resistance, stainless steel is widely used in modern industrial and agricultural production and daily life.