The properties of stainless steel welded pipes are obtained through their unique alloy composition, with chromium content playing a decisive role. Chromium combines with oxygen to form an extremely thin and incredibly hard chromium oxide film, which protects the underlying stainless steel. In the presence of this chromium oxide film, the metal is in a passive state, giving the welded pipe resistance to strong acid and alkali corrosion. Therefore, the corrosion resistance of stainless steel is determined by the ability of this oxide layer to resist corrosion.
1. What factors reduce the corrosion resistance of stainless steel welded pipes?
Where the film is damaged or other forms of contamination exist, the natural reformation of the passivation film is hindered, thus corrosion may occur. All the beneficial properties of stainless steel welded pipes can be damaged during processing, such as heat treatment or machining processes like welding, cutting, sawing, drilling, and bending. As a result of these processes, the protective oxide film on the surface of the stainless steel welded pipe is often damaged or contaminated, making spontaneous and complete passivation impossible. Therefore, stainless steel welded pipes may experience localized corrosion and may even rust under relatively weak corrosive conditions. During use, unsatisfactory final products of stainless steel welded pipes may occur, or worse, cause the failure of a critical system.
A: Accelerated oxidation occurs on both the inner and outer sides of the weld and the areas near the weld in stainless steel welded pipes. Oxidation is visible in discolored areas, and the color is related to the thickness of the oxide layer. Compared to the oxide layer on the stainless steel before welding, the oxide layer in the discolored area is relatively thick, and its composition is altered (chromium reduction), resulting in reduced resistance to localized corrosion. For the interior of stainless steel welded pipes, oxidation and discoloration can be minimized by using an appropriate backflushing method. After welding, post-weld treatments such as pickling and grinding are often necessary to remove the oxide layer (color) and restore corrosion resistance. A color chart is often used, with color grades determining whether pickling is required for the weld. However, this decision is subjective; in principle, each color indicates the presence of oxidation and an affected oxide layer, thus reducing the corrosion resistance of the stainless steel welded pipe.
B: Mechanical treatment typically involves mechanically or non-mechanically contaminated surfaces. Organic contaminants may be introduced by lubricating oil. Inorganic contaminants like foreign iron particles can be caused by contact with tools. Generally, all types of surface contamination on stainless steel welded pipes can lead to pitting. Furthermore, foreign iron particles can also cause electrochemical corrosion. Both pitting and electrochemical corrosion are localized forms of corrosion and initially require water treatment. Therefore, surface contamination generally reduces the corrosion resistance of stainless steel.
2. What are the surface treatments for stainless steel welded pipes?
To treat the surface, remove discoloration, and restore corrosion resistance, there are now many post-treatments and methods. Here, we should distinguish between chemical and mechanical methods.
A: Chemical methods include: pickling (by immersion, with pickling paste or spray), assisted passivation (after pickling), and electrolytic polishing.
B: Mechanical methods include: sandblasting, shot peening with glass or ceramic microparticles, burring, brushing, and polishing.
While all methods can produce weld seams on stainless steel welded pipes, no single mechanical post-treatment provides corrosion resistance suitable for harsh applications. Chemical methods remove oxides and other contaminants from surfaces, while mechanical methods can potentially remove contaminants from previously removed materials, polishing agents, or burrs. All kinds of contaminants, especially foreign iron particles, can become sources of corrosion, particularly in humid environments. Therefore, mechanical surface cleaning should ideally be performed under dry conditions.
After pickling, proper rinsing with water is crucial to remove all contaminants and pickling residues. The final rinse should be performed with softened water to prevent calcium stains and contaminants from embedding into the ever-growing oxide layer of the stainless steel welded pipe, which is essential for the formation of a passivation layer. Furthermore, because the corrosion resistance of stainless steel welded pipes is improved through chemical methods (pickling and electropolishing), iron dissolves faster than other metals in the pickling and electrolyte solutions, leading to chromium enrichment and increased inertness on the surface. Therefore, these chemical methods of pickling and electropolishing are the only post-treatment methods capable of restoring the corrosion resistance of stainless steel at weld seams and other surface damage that occurred before welding. This is actually unrelated to the type of stainless steel; there is no difference in the effect between immersing stainless steel welded pipes in a tank for pickling and using pickling paste or spray.
Post time: Jan-21-2026