Why does stainless steel centrifugal pump also rust?

Posted by angroup on March 25, 2022
multistage-pump

When the surface of the stainless steel centrifugal pump brown rust spots (points), people are surprised: think "stainless steel is not rust, rust is not stainless steel, maybe a problem with the quality of steel". In fact, this is a one-sided misconception of the lack of understanding of the stainless steel material. The actual work of stainless steel in certain conditions is also rusted

Stainless steel has the ability to resist atmospheric oxidation - that is, stainless, but also has the ability to resist corrosion in media containing acids, alkalis, and salts - that is corrosion resistance. But the size of its corrosion resistance is with its steel itself chemical composition, use conditions, and environmental media type and change. Such as ss304, in a dry and clean atmosphere, there is absolutely excellent corrosion resistance, but it will be moved to the seaside area, in the sea spray containing a large amount of salt, which will soon rust; and ss316 is a good performance. Therefore, not any kind of stainless steel, in any environment can be resistant to corrosion, not rust.

Stainless steel material rust phenomenon, there may be several reasons.

The presence of chloride ions in the use environment
Chlorine ions are widely present, such as salt / sweat / seawater / sea breeze / soil, etc.. Stainless steel in the presence of chloride ions in the environment, corrosion quickly, even more than ordinary low carbon steel. So there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry

No solid solution treatment
Alloying elements are not dissolved into the matrix, resulting in low alloy content of the matrix organization, poor corrosion resistance

Inherent intergranular corrosion
Used in applications where welding is required, the lower carbon content minimizes the precipitation of carbide in the heat-affected zone close to the weld, which can lead to intergranular corrosion (weld erosion) of stainless steel in certain environments. Because it will damage the surface of stainless steel and the attachment of iron powder and the formation of the cause of rust. For example, stainless steel centrifugal pump outlet flange and pipeline welding

Stainless steel is formed on its surface by a layer of very thin and strong and fine stable chromium-rich oxide film (protective film), to prevent the continued penetration of oxygen atoms, continued oxidation, and to obtain the ability to resist rust and corrosion. Once there is some reason, this film is continuously destroyed, oxygen atoms in the air or liquid will continuously infiltrate or iron atoms in the metal will continuously precipitate out, forming loose iron oxide, and the metal surface will be subject to continuous rusting.

There are many forms of damage to this surface film, and the following are mostly seen in daily work.

1. Stainless steel surface stored with other metal elements containing dust or foreign metal particles of adhesion, in the humid air, adhesion and stainless steel between the condensate, the two connections into a microcell, triggered an electrochemical reaction, the protective film is damaged, called electrochemical corrosion.
2. Stainless steel surface adhesion of organic juice, in the presence of water and oxygen, constitutes an organic acid, a long time then the organic acid corrosion of the metal surface.
3. Stainless steel surface adhesion containing acid, alkali, salt substances cause local corrosion
4. In the polluted air (such as containing a large number of sulfides, carbon dioxide, nitrogen oxide atmosphere), condensation, the formation of sulfuric acid, nitric acid, acetic acid liquid point, causing chemical corrosion

All of the above can cause the destruction of the protective film on the surface of stainless steel triggering rust and corrosion. Therefore, to ensure that the metal surface is permanently bright and free from rust and corrosion, we recommend that

1. The stainless steel surface must be cleaned and scrubbed frequently to remove adherents and eliminate external factors that cause rust and corrosion
2. According to the centrifugal pump use environment and media to choose the appropriate grade of stainless steel material, such as seaside areas to use 316 stainless steel, 316 material can resist seawater corrosion
3. Some stainless steel centrifugal pumps on the market cannot meet the chemical composition of the corresponding national standards and fail to meet the stainless steel material requirements, thus also causing rust, which requires users to carefully select a reputable stainless steel centrifugal pump manufacturer.

Our client feedback for ss316 material seawater high-pressure multistage pump after running 3 years

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