Why will the surface of stainless steel welded pipe rust？

Stainless steel welded pipe part of the surface appears "rust" phenomenon is normal, with small make up to understand.

"Although the welded pipe is not rusty in theory, once it is rusty, it is not the welded pipe. It may be that the steel has a problem.". This kind of view belongs to the lack of understanding or one-sided understanding of welded pipe. In fact, welded pipe will rust under certain conditions. Theoretically, welded pipe has the ability to resist atmospheric oxidation, that is, rust resistance, and also has the ability to resist corrosion in the medium containing acid, alkali and salt, that is, corrosion resistance. However, the corrosion resistance of the steel varies with its chemical composition, additive state, service condition and environmental medium type. For example, 304 steel pipe has excellent corrosion resistance in dry and clean atmosphere, but when it is moved to the coastal area, it will rust in the sea fog containing a lot of salt for a long time, while 316 steel pipe performs well. Therefore, not any kind of welded pipe can resist corrosion and rust in any environment. Stainless steel welded pipe rust is mainly reflected in the appearance of brown rust spots (spots) on the surface.

The antirust of welded pipe mainly depends on a very thin, firm, fine and stable chromium rich oxide film (protective film) formed on its surface to prevent the continuous infiltration and oxidation of oxygen atoms, so as to obtain the ability of anti-corrosion. Once there is some reason, such as transportation, collision, contact with other objects, the film is constantly damaged, oxygen atoms in the air or liquid will continue to penetrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be constantly corroded. There are many forms of surface film damage. In addition to the above reasons, there are several kinds of daily use.

There are dust containing other metal elements or attachments of dissimilar metal particles on the surface of welded pipe. In humid air, the condensation water between the attachments and welded pipe connects them into a micro battery, which causes electrochemical reaction and damages the protective film, which is called electrochemical corrosion

2, welded pipe surface adhesion organic juice (such as gourd, noodle soup, sputum, etc.), in the case of water and oxygen, constitute organic acid, long time organic acid corrosion on the metal surface.

3, welded pipe surface adhesion contains acid, alkali, salt substances (such as the decoration of the wall of alkali water, lime water splashing), causing local corrosion.

4. In the polluted air (such as the atmosphere containing a large amount of sulfide, carbon oxide and nitrogen oxide), condensed water will form liquid points of sulfuric acid, nitric acid and acetic acid, causing chemical corrosion.

The above conditions can cause the damage of welded pipe surface protection film and lead to corrosion, resulting in color rust, spots (spots) and other "rust" on the surface of the steel pipe.