There are various forms of corrosion failure of mold steel in industrial production. Different steel materials under different loads and different media environments, the main types of corrosion are summarized as follows:
① General corrosion
A large area of relatively uniform corrosion occurs on the exposed surface of the steel material. Although the effective area and the service life of the component are reduced, it is less harmful than local corrosion.
② Intergranular corrosion
Refers to the corrosion along the product boundary, causing the connection of the grains to be damaged. This kind of corrosion is the most harmful. It can become brittle or lose its strength, lose the sound of metal when struck, and easily cause sudden accidents. Product corrosion is the main form of corrosion of austenitic stainless steel. This is due to the difference in electrode potential between the product boundary region and the intragranular composition or stress, which causes a significant decrease in electrode potential in the product boundary region.
③ Stress corrosion
The cracking of metal occurs under the combined action of corrosive medium and tensile stress (external or internal stress). The fracture mode is mainly crystalline and also penetrating. This is a dangerous low-stress brittle fracture. Stress corrosion often occurs in chlorinated and alkaline atmosphere oxides or other water-soluble media. It accounts for many equipment accidents. Sizable proportion.
④ Pitting corrosion
Pitting corrosion is a form of corrosion damage that occurs in a local area of a metal surface. After pitting corrosion is formed, it can rapidly develop into the depth and finally penetrate the metal. Pitting corrosion is very dangerous, especially for various containers. After pitting occurs, it should be polished or painted in time to avoid deepening of the corrosion.
Pitting corrosion is caused by the local damage to the passivation film on the metal surface under the action of the medium. Or in the medium containing chloride ions, loose material surface defects and non-metallic inclusions can cause pitting corrosion.
⑤ Corrosion fatigue
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. Significantly reduces the fatigue strength of steel, leading to premature fracture. Corrosion fatigue is different from mechanical fatigue. It does not have a certain fatigue limit. As the number of cycles increases, the fatigue strength always decreases.
In addition to the various forms of corrosion described above, there is also corrosion due to macroscopic cell effects. For example, the rivets and riveting materials in metal components are different, the welding of dissimilar metals, the hull and propeller materials are different, and the corrosion is caused by the difference in electrode potential. It can be seen from the above-mentioned corrosion mechanism that the focus of preventing corrosion should be to reduce the number of primary cells as much as possible, so that the surface of the steel forms a stable, complete passivation film that is firmly bonded to the steel substrate. Reduce the electrode potential difference between the two electrodes as much as possible.