The properties of stainless steel welded pipes are achieved through their unique alloy composition, with the chromium content playing a decisive role. Chromium combines with oxygen to form an extremely thin, hard chromium oxide film, which protects the underlying stainless steel. When this chromium oxide film exists, the metal is in a passive state, making stainless steel welded pipe resistant to strong acid and alkali corrosion. Therefore, the corrosion resistance of stainless steel is determined by the ability of the oxide layer to resist corrosion.
1. Damage or contamination reduces the corrosion resistance of stainless steel welded pipes. Damage to the film and other forms of contamination hinder the natural re-formation of the passive film, leading to corrosion. All beneficial properties of stainless steel welded pipes can be destroyed during processing, such as heat treatment or mechanical processes like welding, cutting, sawing, drilling, and bending. As a result of these processes, the protective oxide film on the surface of stainless steel welded pipes is often damaged or contaminated, making spontaneous and complete passivation impossible. Consequently, stainless steel welded pipes may experience localized corrosion and rust even under relatively mild corrosive conditions. During use, this can lead to an unsatisfactory final stainless steel welded pipe product or, worse still, a critical system failure.
A: Stainless steel welded pipe experiences accelerated oxidation both inside and outside the weld and adjacent welds. Oxidation is visible as 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 areas is relatively thick and has a modified composition (less chromium), resulting in reduced localized corrosion resistance. Oxidation and discoloration can be minimized on the inside of the stainless steel welded pipe by using an appropriate backflush method. After welding stainless steel welded pipe, post-weld treatments such as pickling and grinding are often necessary to remove the (colored) oxide layer and restore corrosion resistance. A color chart is often used, with color levels used to determine whether the weld requires pickling. However, this determination is subjective; in principle, each color indicates the presence of oxidation and an affected oxide layer, resulting in reduced corrosion resistance.
B: Mechanical treatment typically involves either mechanical or non-mechanical contamination of the surface. Organic contamination can be caused by lubricating oils. Inorganic contaminants, such as foreign iron particles, can be introduced by contact with tools. Generally, all types of surface contamination on stainless steel welded pipe can cause pitting. Furthermore, foreign iron particles can lead to galvanic corrosion. Both pitting and galvanic corrosion are localized forms of corrosion that initially require water treatment. Therefore, surface contamination often reduces the corrosion resistance of stainless steel.
2. Surface Treatment of Stainless Steel Welded Pipe
To prepare the surface, remove discoloration, and restore corrosion resistance, a variety of post-treatment methods are available. A distinction should be made between chemical and mechanical methods. Chemical methods include pickling (by immersion, with pickling pastes, or sprays), auxiliary passivation (after pickling), and electrolytic polishing. Mechanical methods include sandblasting, shot blasting with glass or ceramic particles, immersion, brushing, and polishing.
While all methods can produce welded stainless steel pipe joints, no single mechanical post-treatment provides corrosion resistance suitable for demanding applications. Chemical methods remove surface oxides and other contaminants, while mechanical methods can potentially remove contamination from previously removed, polished, or immersion materials. All types of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should ideally be cleaned under dry conditions.
After pickling, proper rinsing with water is crucial to remove all contaminants and pickling residues. Final rinsing should be performed with demineralized water to prevent calcium stains and contaminants from becoming embedded in the growing oxide layer of welded stainless steel pipe, which is essential for establishing the passive layer. Furthermore, since stainless steel welded pipe is chemically treated to enhance its corrosion resistance (pickling and electropolishing), iron dissolves faster than other metals in the pickling and electrolyte solutions, enriching the surface with chromium and making it more inert. Therefore, chemical methods like pickling and electropolishing are the only post-treatments capable of restoring the corrosion resistance of stainless steel in welds and other surface damage that occurred prior to welding. This is virtually irrelevant to the type of stainless steel; the effectiveness of pickling stainless steel welded pipe by immersion in a tank or using pickling pastes or sprays is virtually identical.
Post time: Sep-04-2025