Stainless steel precision castings are widely popular for their advantages such as good appearance, 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 that "stainless steel"? Is there anything wrong with the materials?

In fact, this is a one-sided and wrong view of stainless steel, because stainless steel precision castings can also rust under certain conditions.

           Stainless steel has the ability to resist atmospheric oxidation - that is, rust prevention, and also has the ability to resist corrosion in acidic, alkaline and salt media - that is, corrosion resistance. However, the corrosion resistance of steel varies with its chemical composition, mutual state, usage conditions and the type of environmental medium. For instance, stainless steel precision castings made of 304 have absolutely excellent anti-rust ability in dry and clean environments. However, if it is moved to coastal areas, it will soon rust in the sea fog containing a large amount of salt. Material 316 has good performance. Therefore, not all types of stainless steel can resist corrosion and remain rust-free in any environment.

The stainless steel protective film can also be damaged

Stainless steel forms a very thin, strong, fine and stable chromium-rich oxide film (protective film) on its surface to prevent oxygen atoms from further penetrating and oxidizing, and to obtain corrosion resistance. Once for some reason the membrane is constantly damaged, oxygen atoms in the air or liquid will continuously penetrate or iron atoms in the metal will continuously separate out, forming loose iron oxide, and the surface of the metal will be constantly corroded. There are many forms of damage to this surface film. The common ones in daily life are as follows:

         Electrochemical corrosion occurs when different types of accessories containing dust or metal particles of other metal elements are stored on the surface of stainless steel. In a humid environment, the condensation water between the accessories and the stainless steel forms a micro-battery and undergoes an electrochemical reaction, causing damage to the protective film.

2. Organic fruit juices (such as gourd, noodle soup, phlegm, etc.) adhering to the surface of stainless steel, in the presence of water and oxygen, form organic acids, which corrode the metal surface over a long period of time.

3. The surface adhesion of stainless steel contains substances such as acids, alkalis, and salts (such as alkaline substances used for wall decoration and lime water splashing), causing local corrosion.

4. In polluted air (such as when the atmosphere contains a large amount of sulfides, carbon oxides, and nitrogen oxides), condensation water can form liquid spots of sulfuric acid, nitric acid, and acetic acid, causing chemical corrosion.