Stainless steel performance knowledge
1. Reduce corrosion resistance due to damage or contamination
Where the film is damaged and there are other forms of contamination, the passivation film is hindered from being re-formed naturally and therefore corrosion may occur. All the beneficial properties of stainless steel 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 oxide protective film on the surface of the stainless steel is often damaged or contaminated, making it impossible to achieve spontaneous and complete passivation. As a result, localized corrosion may occur, and rust may develop even under relatively weak corrosion conditions. When used, it may cause the final product to be unsatisfactory, or worse, cause a critical system to fail. welding pipe
A: The acceleration of oxidation is caused by both welding inside and outside of the weld and near the weld. Because there is visible oxidation in discolored areas, the color is related to the thickness of the oxide layer. As 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 reduction), so that the resistance to local corrosion is reduced. For the interior of the tube, oxidation and discoloration can be minimized by using an appropriate backflush method. After welding, post-weld treatments such as pickling and grinding are often necessary to remove the oxide layer (colored) and restore corrosion resistance. A colored drawing is often used to determine if the weld needs pickling based on the color rating. However, this decision is subjective. In principle, each color indicates the presence of oxidation and the affected oxide layer, and therefore the corrosion resistance is reduced. pipe reducer
B: Mechanical treatment usually uses mechanical or non-mechanically contaminated surfaces. Organic pollutants may be caused by lubricating oils. Inorganic contaminants such as foreign iron particles may be caused by contact with the tool. Usually all kinds of surface contamination can cause plaque. In addition, foreign iron particles may cause galvanic corrosion. Both spotting and galvanic corrosion are localized forms of corrosion and require initial treatment with water. Therefore, surface contamination usually reduces the corrosion resistance of stainless steel.
2, surface treatment
In order to treat the surface, remove the discoloration and regain corrosion resistance, there are now many back-end treatments and measures. Here again we should distinguish between chemical methods and mechanical methods. Chemical methods include: pickling (by immersion, pickling or spraying), assisted passivation (after pickling) and electrolytic polishing. Mechanical methods include: blast cleaning, shot blasting with glass or ceramic particles, annihilation, brushing and polishing.
Although all methods produce weld joints, none of the mechanical rear treatments provide corrosion performance for demanding applications. Chemical methods are used to remove oxides and other contaminants from the surface while mechanically removing the contamination from previously removed materials, polishing materials, or annihilation materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, the mechanical cleaning surface should preferably be cleaned in a dry condition.
After pickling, it is very important to rinse properly with water in order to remove all contaminants and pickling residues. The final rinse should be softened with water to prevent calcium stains and contaminants from intercalating into the increasing oxide layer, which is essential for establishing a passivation layer. In addition, since chemical resistance is improved by chemical methods (pickling and electropolishing), iron will dissolve rapidly in the acid and the electrolyte, which will enrich the surface with chromium. Inert. Therefore, pickling and electrolytic polishing are the only chemical treatments that can restore the corrosion resistance of stainless steel at welds and other surface damage. These surface damages occur before welding. This is virtually independent of the type of stainless steel, and there is no difference between the effects of pickling by immersion in the tank or using a pickling cream or spray.