Main corrosion types of mold steel

The destruction of metals under the action of corrosive media and alternating stress is characterized by the generation of corrosion pits and a large number of cracks, which significantly reduces the fatigue strength of steel, which leads to premature fracture of steel. Corrosion fatigue is different from mechanical fatigue in that it does not have a certain fatigue limit.

The phenomenon of gradually destroying metals 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, and no current is generated in chemical corrosion, and a certain corrosion product is formed during the corrosion process. Such corrosion products generally cover the metal surface and 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, oxide films such as SiO2, Al2O3, Cr2O3 are formed. These oxide films have a dense, complete structure, no looseness, no cracks, and are not easy to peel off. They can protect the base metal and prevent further oxidation. For example, Fe2O3 generated when iron is oxidized at high temperature. Conversely, some oxide films are discontinuous or porous, and have no protective effect on the base metal. The oxides of some metals, such as Mo2O3 and WO3, are volatile at high temperatures and have no protective effect covering the substrate at all.

It can be seen that the generation of oxide film and the structure and properties of the oxide film are important features of chemical corrosion. Therefore, to improve the metal's resistance to chemical corrosion is mainly through alloying or other methods to form a stable, complete and dense oxide film on the metal surface, which is firmly combined with the substrate, also known as a passivation film. Electrochemical corrosion is an important and common form of metal corrosion. It is produced by different metals or electrode potentials between different grooves of metals to form primary batteries. This kind of galvanic cell corrosion is generated between microstructures, so it is also called microbattery corrosion. Electrochemical corrosion is characterized by the presence of a dielectric. There are potential differences in connection or contact between different metals, metal micro-regions or phases, and corrosion currents are also generated.

The corrosion failure forms of die steel in industrial production are various. Different steel materials under different loads and different media environments, Desong Tool Steel summarizes the following types of corrosion:

General corrosion

A large area of ??relatively uniform corrosion occurs on the exposed surface of the steel material. Although the effective area and service life of the component are reduced, it is less harmful than local corrosion.

Intergranular corrosion

Refers to the corrosion along the product boundary, so that the connection of the grain is destroyed. This kind of corrosion is the most harmful. It can make gold become brittle or lose strength, and lose the sound of metal when struck, which is easy to cause sudden accidents. The product corrosion is the main form of corrosion of austenitic stainless steel. This is due to the difference in composition or stress between the product boundary region and the intragranular region, which causes the electrode potential of the product boundary region to be significantly reduced, resulting in the difference in electrode potential.

Stress corrosion

The metal cracks under the combined action of corrosive medium and tensile stress (external stress or internal stress). The fracture mode is mainly intergranular and transgranular. This is a dangerous low-stress brittle fracture. Stress corrosion often occurs in chlorinated and alkaline atmosphere oxides or other water-soluble media, accounting for many equipment accidents. Quite a large proportion.

Pitting

Pitting corrosion is a form of corrosion damage that occurs in the local area of ??the metal surface. After the formation of pitting corrosion, it can quickly develop deeper and eventually penetrate the metal. The pitting corrosion is very mediocre, especially for all kinds of containers. After pitting corrosion should be polished or painted in time to avoid deepening corrosion.
The cause of pitting corrosion is caused by the local damage of the passive film on the metal surface under the action of the medium. Or in a medium containing chloride ions, loose surface defects and non-metallic inclusions can cause pitting corrosion.

Corrosion fatigue

The destruction of metals under the action of corrosive media and alternating stress is characterized by corrosion pits and a large number of cracks. Significantly reduce the fatigue strength of steel, leading to premature fracture. Corrosion fatigue is different from mechanical fatigue in that it does not have a certain fatigue limit. As the number of cycles increases, the fatigue strength has always decreased.

In addition to the above-mentioned various forms of corrosion, there are also corrosion caused by the action of the macro battery. For example, corrosion of metal components due to differences in rivet and riveting materials, welding of dissimilar metals, and differences in hull and propeller materials due to differences in electrode potential. It can be seen from the above corrosion mechanism that the focus of preventing corrosion should be to reduce the number of primary batteries as much as possible, so that a stable and complete passivation film is formed on the surface of the steel, which is firmly combined with the steel substrate. Lower the electrode potential difference between the two poles as much as possible.