\"Rust\" its essence is corrosion or rust, is due to the surface of steel and atmospheric oxygen, water and its acid, alkali, salt and other substances chemical or electrochemical action caused by discoloration or corrosion called rust.
The substance produced on the surface is \"rust\
In oxidizing corrosive media, chromium can quickly form a dense passivation film on the steel surface to prevent the metal matrix from being destroyed.
When the chromium content is more than 12.5%, a dense and stable passivation film is formed, and the anti-rust performance takes a leap forward, and the corrosion resistance is greatly enhanced. This is why the chromium content in stainless steel should be more than 12%. Stainless steel, as the name suggests, is stainless steel, here \"stainless\" is relative, not absolute, is relative to carbon steel. Stainless steel is not easy to rust, but not absolutely not rust, but in the same conditions and environment, it is not easy to be corroded and rusted than carbon steel.
Stainless steel in the addition of chromium at the same time, and then add an appropriate amount of Ni, Mo, V and other alloying elements, rust resistance is stronger.
Therefore, in terms of rust resistance, Austenitic stainless steel is stronger than Martensitic stainless steel.
Secondly, the corrosion resistance grade of stainless steel is also related to the amount of carbon, chromium content and surface treatment state.
With the increase of carbon content, the anti-rust performance of stainless steel decreases, and the anti-rust performance of 3Cr13 is less than that of 2Cr13. With the increase of chromium content in steel, the anti-rust performance increases. The surface treatment method and state affect the anti-rust ability simultaneously.
The antirust properties of chrome-plated cutter surface, electrolytic polishing, mirror polishing, sand belt and manual polishing decrease successively.
When the surface of the stainless steel pipe appears brown rust spots (spots), people are surprised: think that \"stainless steel is not rusty, rust is not stainless steel, there may be a problem with the steel.\"
In fact, this is a one-sided wrong view of the lack of understanding of stainless steel.
Stainless steel will rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation - that is, rust, but also has the ability to resist corrosion in the medium containing acid, alkali, salt - that is, corrosion resistance.
However, the size of its corrosion resistance is changed with the chemical composition of the steel itself, the mixing state, the use conditions and the type of environmental media.
Such as 304 steel pipe, in a dry and clean atmosphere, has absolutely excellent corrosion resistance, but it is moved to the seaside area, in the sea fog containing a lot of salt, it will soon rust; The 316 steel pipe performed well.
Therefore, not any kind of stainless steel, in any environment can resist corrosion, does not rust.
Stainless steel is formed on its surface by a layer of extremely thin and solid fine stable chromium-rich oxide film (protective film), to prevent the continued infiltration of oxygen atoms, continue to oxidize, and obtain the ability to resist corrosion.
Once for some reason, this film is constantly destroyed, the oxygen atoms in the air or liquid will continue to penetrate or the iron atoms in the metal will continue to be resolved out, forming loose iron oxide, and the metal surface will be constantly corroded.
There are many forms of damage to this surface film, which are commonly seen in daily life as follows:
1, the surface of stainless steel contains other metal elements of dust or exotic metal particles of the attachment, in the humid air, the attachment and stainless steel condensate between the two into a microbattery, triggered an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.
2, the surface of stainless steel adhesion organic juice (such as melons and vegetables, noodle soup, sputum, etc.), in the case of water oxygen, constitute organic acids, long-term corrosion of organic acids on the metal surface.
3, stainless steel surface adhesion containing acid, alkali, salt substances (such as the decoration of the wall of soda, lime water splash), causing local corrosion.
4, in the polluted air (such as containing a large number of sulfide, carbon oxide, nitrogen oxide atmosphere), in the case of condensate, the formation of sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion. The above conditions can cause the damage of the stainless steel surface protective film and lead to corrosion. Therefore, in order to ensure that the metal surface is permanently bright and not corroded, we recommend:
1, it is necessary to clean and scrub the decorative stainless steel surface frequently, remove attachments, and eliminate external factors that cause modification.
[Principle of steel rust] The process of steel rust. In fact, it is the process of chemical reaction (slow oxidation) of iron with oxygen and water vapor in the air. The reaction process is quite complex, and the final product, rust, is a mixture. Rust (the main component is fe2o3·h2o) is red, loose and porous, and cannot prevent the inner layer of iron from continuing to react with oxygen, water, etc., so steel products can all rust.
[Steel rust prevention method] The main principle of rust prevention is to isolate oxygen and water, the specific methods are:
1. Paint, spray, dip, etc., on its surface;
2, plated with other metals, such as zinc, chromium, etc., as a covering protective film;
3, change the composition of steel, smelting with 17 ~ 22% chromium element, and finally become stainless steel. Stainless steel's rustlessness and corrosion resistance are due to the formation of chromium-rich oxide film (passivation film) on its surface.