All beneficial properties of stainless steel welded pipes may be damaged during processing
The corrosion resistance of stainless steel pipes may decrease during heat treatment or mechanical processing such as welding, cutting, sawing, drilling and bending. Because of these treatments, the oxidation protection film on the surface of stainless steel welded pipes is usually damaged or stained.
As a result, stainless steel welded pipes may experience localized corrosion and rust even under relatively weak corrosion conditions.
Stainless steel welding causes accelerated oxidation on the inside and outside of the weld and near the weld.
Compared with the oxide layer on the stainless steel before welding, the oxide layer in the discolored area after welding is relatively thick, and the composition is changed (the chromium is reduced), so that the local corrosion resistance is reduced. For the interior of stainless steel welded steel pipe, oxidation and discoloration can be minimized by using an appropriate backflush method.
After welding of stainless steel welded pipes, it is often necessary to perform post-weld treatments such as pickling and grinding to remove the oxide layer (colored) and restore the corrosion resistance. Here is a color chart that determines whether the weld requires pickling based on the color level.
In principle, each color indicates the presence of oxidized and affected oxide layers, so the corrosion resistance of stainless steel welded pipes is reduced.In order to remove the discoloration of the welded pipe surface and restore the corrosion resistance, there are many ways:
- Chemical methods include: pickling (by immersion, pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing.
- Mechanical methods include: blast cleaning, blast cleaning with glass or ceramic particles, annihilation, brushing and polishing.