Corrosion is a phenomenon in which a material undergoes a chemical or electrochemical reaction with the surrounding material, resulting in disintegration. Whether in our daily life, or in industrial production, metal “rust” can be seen everywhere, from small screw corrosion, large cars, airplanes, bridges and other corrosion. Corrosion will not only cause economic losses, and even lead to safety accidents, the importance of anti-corrosion should not be underestimated.
In the boundary layer of the substrate, the first reaction layer will be generated. Due to the presence of oxygen in the atmosphere, the reaction layer usually exists in the form of an oxide and is therefore also known as a primary oxide film (POF). This layer is usually thin and initially prevents further corrosion.
On top of the reaction layer, substances accumulate in adsorbed layers. Usually the first one is water, which, due to the amphoteric character of most metal oxides, reacts with the primary oxide film in an acid-base reaction, forming free hydroxide groups on the surface, in which other reactive substances can also be embedded. This layer is a chemisorption layer, which is strongly bound and difficult to re-dissolve. The chemisorption layer is closely followed by the physical adsorption layer, which has poor molecular binding and is easily replaced.
The primary oxide film is the most important layer of corrosion resistance, the thicker the film, the stronger the adhesion, the more corrosion resistance. In other words, corrosion protection should be started during the formation and stabilization of the primary oxide film (POF). Depending on the metal material, additives (e.g. surfactants, redox agents) are required. Corrosion usually starts with the decomposition of the primary oxide film, which is highly likely to occur in unalloyed steel materials, but in stainless steel the primary oxide film is more stable due to the presence of alloying components (especially chromium).
Common corrosion in life has a variety of different forms of expression, let's take a look at the following seven important types of corrosion.
1. Erosion corrosion: the metal is subjected to erosion almost parallel to the surface. It is the most common form of corrosion and is usually caused by water or dirty air.
2. crevice corrosion: crevices between metals or structural members can lead to severe corrosion because the electrolyte is retained by capillary action and may produce large concentration differences. This can be effectively prevented by design optimization measures.
3. Contact corrosion: Electrochemical corrosion resulting from two dissimilar metals being in contact with each other while simultaneously in an electrolyte, with one of the metals corroding at a significantly faster rate. It can be prevented by selecting appropriate materials or interrupting the conductivity between materials.
4. Pitting: Pitting results in pitting, cratering or pinpointing. It is usually caused by pitting damage to the protective layer, such as pores in the coating or chloride erosion on the passivation layer.
5. Intergranular corrosion: mainly ferrite Cr and CrNi austenitic steel in the grain boundaries are eroded, this corrosion will make the bond between the grains greatly weakened. Serious intergranular corrosion can make the metal lose strength and ductility, crumbling under normal load, appropriate heat treatment is to prevent intergranular corrosion of the premise.
6. dew-point corrosion: dew-point corrosion refers to the saturated steam due to cooling and condensation into a liquid on the material caused by corrosion, low-alloy steel, non-alloy steel, and CrNi stainless steel is susceptible to strong erosion, must be protected by a suitable protective layer.
7. Stress corrosion cracking: in corrosive media, while under mechanical stress the material will form cracks, especially in chlorine and strong alkali solutions, will lead to CrNi austenitic steel within the stress corrosion cracking.
Post time: May-21-2024