Treatment Methods of 304 Stainless Steel Pipes after Welding

Jane

The excellent properties of 304 stainless steel pipes may be destroyed during processing, such as heat treatment or mechanical processing such as welding, cutting, sawing, drilling and bending. As a result of these treatments, the protective oxide film on the surface of the steel pipe is usually damaged or contaminated, making it impossible to achieve spontaneous and complete passivation. Therefore, 304 stainless steel pipes may produce local corrosion and even rust under relatively weak corrosion conditions.

Stainless steel pipes cause accelerated oxidation at the weld and on both the inside and outside sides near the weld. Oxidation is visible because there are areas of discoloration, and the color is related to the thickness of the oxide layer. Compared with the oxide layer of the pipe before welding, the oxide layer in the discolored area is relatively thick and its composition is changed (chromium is reduced), which reduces the local corrosion resistance. For the interior of the tube, oxidation and discoloration can be minimized by using an appropriate backflushing method.

Treatment Methods of 304 Stainless Steel Pipes after Welding

After welding, post-weld treatments such as pickling and grinding are necessary to remove the oxide layer (colored) and restore corrosion resistance. A color chart is often used to determine whether the weld needs pickling based on the color grade. However, this decision is subjective, and in principle each color indicates the presence of oxidation and an affected oxide layer, so the corrosion resistance of the 304 stainless steel pipe is reduced.

Contaminated surfaces are usually cleaned using mechanical or chemical methods. Organic contamination may be caused by lubricating oil, while inorganic contamination such as foreign iron particles may be caused by contact with the tool. Generally, all kinds of surface contamination on stainless steel welded pipes can cause pitting. In addition, foreign iron particles may cause galvanic corrosion. Both pitting and galvanic corrosion are localized forms of corrosion that initially require water treatment. Therefore, surface contamination usually reduces the corrosion resistance of steel pipes.

There are many post-processing methods and means to treat the surface, remove discoloration and restore corrosion resistance. Here we should distinguish between chemical and mechanical methods. Chemical methods are: pickling (by immersion, with pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods are: sandblasting, shot blasting with glass or ceramic particles, immersion, brushing and polishing.

No subsequent treatment, whether mechanical or chemical, provides corrosion performance suitable for demanding applications. For example, chemical pickling and passivation can remove oxides and other pollutants on the surface, but it cannot meet the appearance requirements and can easily pollute the clean room due to the volatilization of chemicals. Therefore, it is necessary to use mechanical methods to process the outer surface. At the same time, mechanical methods may cause contamination by wiping away previously removed material, polishing material, or annihilating material.

The above is the treatment method of 304 stainless steel pipe after welding. After welding, post-weld treatment such as pickling and grinding is required. The mechanical method can ensure the corrosion resistance of the outer surface of the welded pipe and obtain good appearance quality, while avoiding the residue on the surface of the 304 stainless steel pipe due to the use of pickling paste.