The properties of stainless steel are obtained through the unique alloy composition, in which chromium plays a leading role. Chromium combines with oxygen to form an extremely thin and extremely hard chromium oxide film, which protects the underlying stainless steel. In the presence of a chromium oxide film, we say that the metal is in a passive state, and stainless steel has corrosion resistance. Therefore, the corrosion resistance of stainless steel is due to the ability to naturally form a corrosion-resistant oxide layer when in contact with air.
1. Reduced corrosion resistance due to damage or pollution:
Corrosion may occur where the film has been damaged and other forms of contamination are present that prevent the natural re-formation of the passivating film. All the beneficial properties of stainless steel can 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 oxidation protective film on the stainless steel surface is often damaged or contaminated, making it impossible to achieve spontaneous and complete passivation. Therefore, local corrosion may occur and even rust may occur under relatively weak corrosion conditions. When used, it can result in an unsatisfactory end product, or even worse, the failure of a critical system.
A: Welding causes accelerated oxidation on both the inner and outer sides of the weld and the area close to the weld. Oxidation is visible because there are discolored areas, and the color is related to the thickness of the oxide layer. Compared with the oxide layer on the stainless steel before welding, the oxide layer in the discolored area is relatively thick, and the 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 backflush method. After welding, post-weld treatments like pickling and grinding are often necessary to remove the oxide layer (colored) and restore corrosion resistance. A color diagram is often used to determine whether the weld requires 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 and therefore reduced corrosion resistance.
B: Mechanical treatment usually uses mechanical or non-mechanical contamination of the surface. Organic contaminants can be caused by lubricating oil. Inorganic contaminants such as foreign iron particles may be caused by contact with the tool. Usually, all kinds of surface contamination can cause plaque. Additionally, foreign iron particles may cause galvanic corrosion. Pitting and galvanic corrosion are both forms of localized corrosion that initially require water treatment. Surface contamination therefore usually reduces the corrosion resistance of stainless steel.
2. Surface treatment
There are now many post-processing treatments and tools available to treat surfaces, 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), assisted passivation (after pickling), and electrolytic polishing. Mechanical methods include: sandblasting, shot blasting with glass or ceramic particles, obliteration, brushing, and polishing. While all methods produce welded joints, no mechanical post-treatment will provide corrosion performance suitable for severe applications. Chemical methods are used to remove oxides and other contaminants from the surface, while mechanical methods may be used to wipe away contamination from previously removed materials, polished materials, or obliterated materials. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should preferably be cleaned regularly under dry conditions. After pickling, it is important to perform a proper rinse with water to remove all contaminants and pickling residue. The final rinse should be done with demineralized water to avoid calcium stains and contaminants becoming embedded in the growing oxide layer which is necessary to establish the passivation layer. In addition, due to the use of chemical methods (pickling and electrolytic polishing) to improve corrosion resistance, iron dissolves faster than other metals in pickling solutions and electrolytes. According to this, the surface is enriched with chromium and becomes more durable. Inertia. Therefore, chemical methods such as pickling and electropolishing are the only post-treatment methods capable of restoring the corrosion resistance of stainless steel at welds and other surface damage that occurred before welding. This really has nothing to do with the type of stainless steel, there is no difference in the effect between pickling by immersion in a tank or using a pickling paste or spray.
Post time: Jan-11-2024