Why do stainless steel castings also rust?

Why do stainless steel castings also rust?
Stainless steel has the advantages of beautiful appearance, corrosion resistance, and not easy to be damaged, and is deeply loved by people. But when brown rust spots appear on the surface of stainless steel castings, people will be surprised: Why? Can "stainless steel" also rust? If it rusts, then Is it still considered "stainless steel"? Is there something wrong with the material?

In fact, this is a one-sided misconception about stainless steel, because stainless steel can also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation - that is, stainless steel, but it also has the ability to corrode acid, alkali, and salt media - that is, corrosion resistance. However, its corrosion resistance depends on the chemical composition, mutual state, and use of the steel itself. Changes with conditions and environmental media types. For example, stainless steel castings of 304 material have excellent corrosion resistance in a dry and clean atmosphere, but it will soon rust in sea fog containing a large amount of salt; 316 material has good performance .Therefore, not any stainless steel can resist corrosion and rust in any environment.

The stainless steel protective film will also be damaged

Stainless steel is a very thin, strong and stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to penetrate and oxidize, and to gain corrosion resistance. Once for some reason, the film is continuously damaged, Oxygen atoms in the air or liquid will continue to penetrate, or iron atoms in the metal will continue to separate, forming loose iron oxide, and the metal surface will continue to rust. This surface film has many forms and is very common in daily life:

1. Dust containing other metal elements or attachments of different metal particles are stored on the surface of stainless steel. In humid air, the condensation water between the attachments and stainless steel connects the two into micro-batteries, resulting in electrochemical reactions and protective films. Damage is called electrochemical corrosion.

2. The surface of stainless steel (such as vegetables, noodle soup, phlegm, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, organic acid is formed, and the organic acid corrodes the metal surface over a long period of time.

3. The surface of stainless steel contains acid, alkali and salt substances (such as alkaline water and lime water splash on the wall decoration), causing local corrosion.

4. In polluted air (such as an atmosphere containing a large amount of sulfides, carbon oxides, and nitrogen oxides), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid liquid points are formed, causing chemical corrosion.

All these situations will cause damage to the protective film on the surface of stainless steel and lead to corrosion. Therefore, in order to ensure that the metal surface is bright and rust-free for a long time, it is recommended to:

1. Decorative stainless steel surfaces need to be cleaned and scrubbed frequently to remove attachments and eliminate external factors that cause decoration.

2. 316 stainless steel should be used in seaside areas. 316 material can resist seawater corrosion.

3. The chemical composition of some stainless steel pipes on the market cannot meet the corresponding standards and cannot meet the material requirements of 304. Therefore, it will also cause rust, which requires users to carefully choose products from reputable manufacturers.