Will 304 Stainless Steel Welded Pipe Rust?

In addition to iron, the composition of 304 stainless steel welded pipe also includes chromium, nickel, aluminum, silicon, etc. The chromium content of general 304 stainless steel welded pipes is generally not less than 12%, and the highest can even reach 18%. Adding elements such as chromium to steel can change the properties of the steel. For example, the molecular structure of the steel will be more uniform, and it will be easier to form a dense oxide protective film on the surface of the steel, thereby greatly improving the corrosion resistance of stainless steel. Therefore, stainless steel can resist the corrosion of fire, water, acid, alkali and various solutions and is not rusty.
 
Scientists have discovered that the more uniform the internal structure of steel, the more closely connected the various components are, making it more difficult for corrosive substances to invade. In addition, a layer of protective oxide film adheres to the surface, just like putting armor on the steel, making it naturally less likely to rust.
 
Why does the stainless steel welded pipe also rust?
 
When brown rust spots (dots) appear on the surface of stainless steel pipes, people are surprised: they think that “stainless steel does not rust, and if it is rusted, it is not stainless steel, so there may be a problem with the steel quality”. In fact, this is a one-sided misperception that stainless steel is short in understanding. Stainless steel also can rust under certain conditions. Stainless steel has the ability to resist atmospheric oxidation - that is, it is rust-proof, and it also has the ability to resist corrosion in media containing acids, alkalis, and salts - that is, it is corrosion-resistant.
However, its corrosion resistance varies with the chemical composition of the steel itself, its corrosion resistance, its usage conditions and the type of environmental media. Such as 304 stainless steel pipe, in dry and clean atmosphere, has excellent corrosion resistance, but it will be moved to the coastal areas, in contains a lot of salt fog, will soon rust; The 316 stainless steel pipe performs well. Therefore, not all types of stainless steel are corrosion-resistant and rust-free in any environment. Stainless steel gains its corrosion resistance by forming an extremely thin, strong, fine and stable chromium-rich oxide film (protective film) on its surface, which prevents further infiltration and oxidation of oxygen atoms.
Once for some reason, this film is continuously damaged, oxygen atoms from the air or liquid will continue to penetrate or iron atoms from the metal will continue to separate out, forming loose iron oxide, and the metal surface will continue to rust.
 
There are many forms of damage to this surface film, and the following are common in daily life:
 
1. The surface of the stainless steel welded pipe is accumulated with dust containing other metal elements or foreign metal particles. In the humid air, the condensed water between the attachments and the stainless steel connects the two into a micro-battery, triggering an electrochemical reaction, and the protective film is damaged, which is called electrochemical corrosion.
2. Organic juice (such as vegetables, noodle soup, sputum, etc.) adheres to the surface of stainless steel. In the presence of water and oxygen, organic acid is formed. Over a long period of time, the organic acid will corrode the metal surface.
3. The surface of stainless steel adheres to substances containing acid, alkali, and salt (such as alkali water and lime water splashed from decorating the walls) and causes local corrosion.
4. In polluted air (such as atmosphere containing a large amount of sulfide, carbon oxide, and nitrogen oxide), when encountering condensed water, sulfuric acid, nitric acid, and acetic acid liquid points are formed, causing chemical corrosion.
 
How to avoid or delay rust?
 
1. Choose regular materials: confirm that they comply with standards such as GB/T 14976 (stainless steel fluid pipe standard) or ASTM A312, and require suppliers to provide material test reports.
 
2. Surface treatment:
After welding, the welded pipe is subjected to solution treatment (high temperature annealing) to eliminate welding stress and restore the passivation film. If necessary, pickling passivation or electrolytic polishing should be performed to enhance the surface corrosion resistance.
 
3. Environmental Control:
Avoid contact with highly corrosive media. If contact is necessary, assess the corrosion level in advance and take protective measures (such as coating, lining).
Clean regularly: Wipe the surface with a mild cleaner (such as soapy water) to remove dust and contaminants.
 
4. Special scene upgrade material:
For high-salt and high-acid environments, it is recommended to use 316L stainless steel (containing molybdenum, which has stronger corrosion resistance) or duplex stainless steel.
 
304 stainless steel welded pipe is "rust-resistant" rather than "stainless". It performs well in normal environments, but may still rust under strong corrosion, high stress or improper maintenance. The risk of rust can be significantly reduced by selecting qualified products, optimizing processes and reasonable maintenance. If used in harsh environments, a higher grade of stainless steel should be selected according to specific working conditions.

Read more: Is 304 Stainless Steel Welded Pipe Corrosion Resistant?