Why does the surface of stainless steel welded pipe rust

"Although welded pipes are theoretically non-rusting, once they get rusty, they are no longer welded pipes, and there may be problems with the quality of the steel." Such viewpoints lack understanding or one-sided understanding of butt welded pipes. In fact, welded pipes will also rust under certain conditions. Theoretically, welded pipes have the ability to resist atmospheric oxidation -- that is, rust resistance, and also have the ability to resist corrosion in media containing acids, alkalis, and salts -- that is, corrosion resistance. But the size of its resistance to corrosion with the chemical constitution of its steel itself, add mutual state, working conditions and surrounding medium type and change. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to tarnishing ability is arranged, but it is moved on to riviera, in containing the sea fog of a large amount of salts, will get rusty for a long time like this; good. Therefore, not any kind of welded pipe is corrosion-resistant and rust-free in any environment. The rust of stainless steel welded pipe is mainly reflected in the appearance of brown rust spots (spots) on the surface.
The anti-rust of welded pipe mainly depends on the formation of a layer of extremely thin, firm, fine and stable chromium-rich oxide film (protective film) on its surface, which prevents the continuous infiltration and oxidation of oxygen atoms, and obtains the ability of anti-corrosion. Once there is some reason such as transportation, collision, and contact with other items, the film is continuously damaged, oxygen atoms in the air or liquid will continuously infiltrate or iron atoms in the metal will continuously separate out to form loose iron oxide. The metal surface is also subject to constant corrosion. There are many forms of damage to this surface film. In addition to the above reasons, there are the following types in daily use:

Dust containing other metal elements or attachments of heterogeneous metal particles accumulate on the surface of the welded pipe. In humid air, the condensed water between the attachment and the welded pipe connects the two into a micro-battery, triggering an electrochemical reaction, protecting The membrane is damaged, which is called electrochemical corrosion.

2. Organic juices (such as vegetables, noodle soup, sputum, etc.) adhere to the surface of the welded pipe. In the presence of water and oxygen, organic acids will be formed, and the organic acids will corrode the metal surface for a long time.
3. The surface of the welded pipe contains acids, alkalis, and salts (such as alkaline water and lime water splashed on the wall for decoration), causing local corrosion.
4. In polluted air (such as an atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), when it encounters condensed water, it will form liquid points of sulfuric acid, nitric acid, and acetic acid, causing chemical corrosion.
The above conditions can cause damage to the protective film on the surface of the welded pipe and cause corrosion, resulting in color rust, spots (spots) and other "rust" phenomena on the surface of the steel pipe.