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Why does stainless steel also rust? What conditions can cause stainless steel to rust?
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When brown rust spots appear on the surface of Stainless Steel, people are greatly surprised: they believe that "stainless steel will 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, namely rust resistance, and also has the ability to resist corrosion in media containing acids, alkalis, and salts, namely corrosion resistance. But the magnitude of its corrosion resistance varies with the chemical composition, mutual state, usage conditions, and type of environmental medium of the steel itself. Materials such as 304 have absolutely excellent corrosion resistance in a dry and clean atmosphere, but when moved to coastal areas, they quickly rust in sea fog containing a large amount of salt. Therefore, not any type of stainless steel can resist corrosion and rust at all times.
Stainless steel is a stable chromium rich oxide film (protective film) formed on its surface, which is extremely thin, sturdy, and fine, preventing oxygen atoms from continuing to penetrate and oxidize, thus obtaining the ability to resist corrosion. However, for some reason, this thin film is constantly damaged, and oxygen atoms in air or liquid will continuously penetrate or iron atoms in the 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. Organic juices adhere to the surface of stainless steel, forming organic acids in the presence of water and oxygen. Over time, organic acids corrode the metal surface.
3. Local corrosion is caused by the adhesion of acid, alkali, and salt substances on the surface of stainless steel (such as alkaline water and lime water spraying for wall decoration).
4. In polluted air (containing a large amount of sulfides, oxides, and hydrogen oxide in the atmosphere), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid droplets form, causing chemical corrosion.
The above situations can cause damage to the protective film on the surface of stainless steel and cause corrosion. So, to ensure that the metal surface is permanently bright and not rusted, we recommend:
① It is necessary to regularly clean and scrub the surface of decorative stainless steel to remove attachments and eliminate external factors that can cause corrosion.
② There is currently a type of stainless steel made of 201 and 202 materials on the market that is prone to rusting in coastal areas and suitable for use in environments without industrial pollution and air corrosion.
③ In coastal areas, 316L Stainless Steel Pipe should be used, which can resist seawater corrosion.
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