Causes of Rust Spots on the Surface of Spiral Welded Steel Pipes

Release date:

2021-05-13

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Spiral welded pipe is produced by feeding strip steel into the welding unit and passing it through a multi-roll forming process. Subsequently, the strip is gradually rolled into a circular tube blank with an open gap. The diameter of the pressure rolls is adjusted to maintain the weld gap within 1 to 3 mm and to ensure that both ends of the weld are flush.

　　 Spiral steel pipe The strip steel is fed into the welded pipe unit and subjected to multi-roll rolling. Subsequently, the strip is gradually rolled into a circular tube blank with an open gap. The contraction diameter of the pressure rolls is adjusted to maintain the weld gap within the range of 1 to 3 mm and to ensure that both ends of the weld are flush.

　　How should brown rust on the surface of spiral pipes be handled? In fact, this is a misconception stemming from a lack of understanding about spiral pipes. Spiral steel pipes can indeed rust under certain conditions.

　　Spiral tubes possess resistance to atmospheric oxidation—meaning they do not rust—but they are also susceptible to corrosion in acidic, alkaline, and saline environments—i.e., they exhibit corrosion resistance.

　　The spiral tube forms an extremely thin, robust, dense, and stable chromium-rich oxide film (a protective layer) that prevents the penetration of oxygen atoms and oxidation, thereby providing corrosion resistance. Once this film is continuously damaged, oxygen atoms from the air or liquid will continue to penetrate, or iron atoms within the metal will continue to separate, forming loose iron oxide; as a result, the metal surface will keep rusting. There are many ways in which this surface film can be compromised. In everyday life, the following situations are more common:

　　1. Dust accumulates on the surface of spiral steel pipes, often containing deposits of other metallic elements or heterogeneous metal particles. In a humid atmosphere, the contact between the spiral steel pipe and condensation water creates a micro‑battery, generating an electric current. Following this chemical reaction, the protective film is compromised—this phenomenon is known as electrochemical corrosion.

　　2. Organic juices (such as those from melons, vegetables, noodle soup, or phlegm) adhering to the surface of spiral steel pipes can, in the presence of water and oxygen, form organic acids that corrode the metal surface over an extended period.

　　3. In polluted air—such as atmospheres containing high concentrations of sulfides, carbon oxides, and nitrogen oxides—it will come into contact with condensed moisture, forming liquid deposits of sulfuric acid, nitric acid, and acetic acid, thereby causing chemical corrosion.

　　4. The surface of spiral steel pipes contains acidic, alkaline, and saline substances (for example, alkaline and lime water splashed onto decorative walls), leading to localized corrosion.

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