Why does stainless steel also rust?

1、 Why does stainless steel also rust?

When brown rust spots (spots) appear on the surface of stainless steel pipes, people are greatly surprised: they believe that "stainless steel does not rust, rust is not stainless steel, it may be a problem with the steel quality". In fact, this is a one-sided misconception about a lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.

Stainless steel has the ability to resist atmospheric oxidation - i.e. rust resistance - and also has the ability to corrode in media containing acids, alkalis, and salts - i.e. corrosion resistance. But the magnitude of its corrosion resistance varies with the chemical composition, processing state, usage conditions, and environmental medium type of the steel itself. For example, 201 steel pipes have absolutely excellent rust resistance in a dry and clean atmosphere, but when moved to coastal areas, they quickly rust in sea mist containing a large amount of salt; And 304 steel pipe performed well. Therefore, not any type of stainless steel can withstand corrosion and rust in any environment.

Rust steel is a stable chromium oxide film (protective film) formed on its surface, which is extremely thin, sturdy, and fine, preventing the continued infiltration and oxidation of oxygen atoms, thus obtaining the ability to resist corrosion. Once for some reason, this thin film 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 metal surface will also be constantly corroded.

This kind of surface facial mask is damaged in many ways, and the following types are common in daily life:

1. Dust containing other metal elements or attachments of dissimilar metal particles accumulate on the surface of stainless steel. In humid air, the condensate between the attachments and the stainless steel forms a micro battery, triggering an electrochemical reaction and damaging the protective film, which is called electrochemical corrosion.

2. The surface of stainless steel adheres to organic juices (such as melons and vegetables, noodle soup, phlegm, etc.), forming organic acids in the presence of water and oxygen. Over time, organic acids corrode the metal surface.

3. The surface of stainless steel adheres to substances containing acids, alkalis, and salts (such as alkaline water and lime water splashing during wall decoration), causing local corrosion.