Last time, we discussed under what circumstances stainless steel rust (What would cause stainless steel sphere to rust? (Ⅰ)). Today, we continue to discuss what cause stainless steel rust.
The reason why stainless steel sphere obtains the ability of anti-corrosion and anti-rust is that it produces a thin, strong and stable chromium-rich oxide film (protective film) on its surface, which prevents the oxygen atoms from infiltrating and continuing to oxidize. Once for some reason, this film is continuously destroyed, the oxygen atoms in the air or liquid will continue to infiltrate or the iron atoms in the metal will continue to separate out, which will form iron oxide, and the surface of the metal alloy will be constantly rust. There are many forms that could damage this surface film. The following are common in daily life:
- On the surface of stainless steel, there are attachment of dust or other metal particles containing other metal elements. In humid air, the condensation between the attachment and the stainless steel connects the two into a microbattery, which triggers an electrochemical reaction. The film is damaged and is called electrochemical corrosion.
- Organic matter juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of stainless steel. In the presence of water and oxygen, they organic acids is formed. For a long time, organic acids corrode metal surfaces.
- There are acids, alkalis, and salts (such as splashes of alkali water and lime water for wall decoration)in the surface of the stainless steel contains, causing local corrosion.
- In the contaminated air (such as the atmosphere containing a large amount of sulfide, carbon oxide, nitrogen oxide)with condensed water, sulfuric acid, nitric acid, and acetic acid are formed, causing chemical corrosion.
The above conditions can cause corrosion for the damage of the stainless steel surface protective film. In order to make sure the surface can kept anti-rusted, there are some suggestions.