When the surface of a stainless steel pump shows brown rust, people often get surprised and say, "This 'stainless steel centrifugal pump' is rusting — that's not possible. If it's rusty, then it's not really stainless steel. Maybe there's a problem with the material." In reality, this is a one-sided and incorrect understanding of stainless steel. Stainless steel can indeed rust under certain conditions.
Stainless steel is known for its ability to resist atmospheric oxidation — that's why it's called "stainless." But it also has resistance to corrosion from acids, alkalis, and salts — which is referred to as "corrosion resistance." However, the extent of its corrosion resistance depends on its chemical composition, the surrounding environment, usage conditions, and the type of medium it comes into contact with.
For example, 304 stainless steel performs exceptionally well in dry and clean air, but if it's moved to a coastal area where salty sea mist is common, it will start to rust quickly. On the other hand, 316 stainless steel offers much better performance in such environments. Therefore, no matter what kind of stainless steel pump you use, it cannot be completely rust-free in every environment.
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Stainless steel achieves its corrosion resistance through a very thin, strong, and stable chromium-rich oxide layer on its surface — a protective film that prevents oxygen atoms from penetrating further and causing continued oxidation or corrosion. However, if this protective layer is damaged due to various reasons, oxygen or water can penetrate, and iron atoms may begin to separate, forming loose iron oxide and leading to rust.
The protective film can be damaged in many ways. Common causes include:
1. **Electrochemical Corrosion**: When dust containing other metal elements or foreign particles settles on the stainless steel surface in humid air, it can form a micro-cell with condensation, triggering electrochemical reactions that damage the protective layer.
2. **Organic Acid Corrosion**: If organic substances like vegetable juice, noodle soup, or mucus stick to the surface, they can react with water and oxygen over time, producing organic acids that corrode the metal.
3. **Chemical Corrosion**: Contact with acidic, alkaline, or saline substances — such as wall cleaning solutions or lime splashes — can cause localized corrosion.
4. **Atmospheric Corrosion**: In polluted environments with high levels of sulfur dioxide, carbon dioxide, or nitrogen oxides, condensation can lead to the formation of sulfuric acid, nitric acid, or acetic acid, which cause chemical corrosion.
To maintain the long-term bright and rust-free appearance of stainless steel, we recommend:
1. Regularly cleaning and scrubbing the surface to remove any deposits and eliminate external factors that could trigger corrosion.
2. Using 316 stainless steel in coastal or marine environments, as it is more resistant to seawater corrosion.
3. Being cautious when purchasing stainless steel tubes, as some products on the market may not meet national standards. Some materials labeled as 304 may not actually comply with the required specifications, which can lead to rusting. Always choose products from reputable manufacturers.
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