Why does stainless steel rust? Is it due to quality?
Stainless steel precision castings are popular for their beauty, corrosion resistance, and difficulty in damage. But when brown rust spots appear on the surface of stainless steel precision castings, people will be surprised: Why does stainless steel rust? If it rusts, is it stainless steel? ?Is there something wrong with the material?
In fact, this is a one-sided misconception about stainless steel, because stainless steel precision castings will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, that is, it is resistant to corrosion, acid, alkali, and salt media. However, the corrosion resistance of steel varies with its chemical composition, mutual state, usage conditions, and type of environmental media. For example, 304 stainless steel precision castings It has excellent anti-rust ability in a dry and clean environment. However, if it is moved to a coastal area, it will soon rust in the sea fog containing a lot of salt. 316 material has good performance. Therefore, it is not any stainless steel, It is corrosion-resistant and does not rust in any environment.
Stainless steel is a very thin, strong, delicate and stable chromium-rich oxide film (protective film) to prevent oxygen atoms from continuing to penetrate. It continues to oxidize and acquires corrosion resistance. Once the film is continuously damaged for some reason, it will be exposed to air or liquid The oxygen atoms will continue to penetrate, or the iron atoms in the metal will continue to separate, forming loose iron oxide, and the metal surface will continue to corrode. There are many forms of damage to this surface film, which are very common in daily life:
1. Different types of accessories are stored on the surface of stainless steel, containing dust or metal particles from other metal elements. In humid air, the accessories connect with stainless steel condensation water to form a micro-battery electrochemical reaction, damaging the protective film, which is called electrochemical corrosion.
2. Organic juice, phlegm, etc. adhere to the stainless steel surface and form organic acids in the presence of water and oxygen, which will corrode the metal surface for a long time.
3. The surface adhesion of stainless steel contains acids, alkalis, salts and other substances (such as alkaline and lime water splashes on decorative walls), causing local corrosion.
4. In polluted air (such as the atmosphere containing a large amount of sulfides, carbon oxides and nitrogen oxides), condensed water will form sulfuric acid, nitric acid and acetic acid points, causing chemical corrosion.
