Stainless steel precision castings are widely popular due to their beauty, corrosion resistance, and low susceptibility to damage. But when brown rust spots appear on the surface of stainless steel precision castings, people will be surprised: why does "stainless steel" still rust? If it rusts, is it "stainless steel"? Is there a problem with the materials?
In fact, this is a mistaken view of the stainless steel surface, as stainless steel precision castings can also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation, namely rust prevention, and also has the corrosion resistance in acid, alkali, and salt media, namely corrosion resistance. However, the corrosion resistance of steel varies with its chemical composition, mutual state, usage conditions, and type of environmental medium. For example, stainless steel precision castings made of 304 have absolutely excellent rust prevention ability in dry and clean environments. However, if it is moved to coastal areas, it will soon rust in sea mist containing a large amount of salt. 316 material has good performance. Therefore, not any type of stainless steel can withstand corrosion and rust in any environment.
The stainless steel protective film can also be damaged
Stainless steel forms an extremely thin, strong, delicate, and stable chromium rich oxide film (protective film) on its surface to prevent oxygen atoms from continuing to penetrate and oxidize, and to obtain corrosion resistance. Once for some reason, the membrane is constantly damaged, oxygen atoms in air or liquid will continuously penetrate, or iron atoms in metal will continuously separate, forming loose iron oxide, and the surface of the metal will continue to be corroded. There are many forms of destruction of this surface facial mask, and the following are common in daily life:
1. The stainless steel surface stores different types of attachments containing dust or metal particles containing other metal elements. In humid air, the attachments and stainless steel condensate are connected to form a micro battery, which is caused by electrochemical reactions. The protective film is damaged, resulting in electrochemical corrosion.
2. Organic juices (such as gourds, noodle soup, sputum, etc.) adhere to the surface of stainless steel, forming organic acids in the presence of water and oxygen, which can corrode the metal surface for a long time.
3. The adhesion of stainless steel surface contains substances such as acid, alkali, and salt (such as alkaline or lime water splashing 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), condensation water can form sulfuric acid, nitric acid, and acetic acid droplets, causing chemical corrosion.