Professional technical knowledge of stainless steel rolling
The performance of stainless steel is achieved through its unique alloy composition, with chromium playing a dominant role. Chromium combines with oxygen to form an extremely thin and hard chromium oxide film, which protects the underlying stainless steel. In the presence of chromium oxide film, we call the metal in a passive state and stainless steel has corrosion resistance. Therefore, the corrosion resistance of stainless steel is attributed to the ability to naturally form a corrosion-resistant oxide layer when in contact with air.
1. Reduced corrosion resistance due to damage or contamination
Corrosion may occur in areas where the passivation film is damaged and other forms of contamination hinder its natural formation. All beneficial properties of stainless steel may be damaged during processing, such as heat treatment or mechanical processing such as welding, cutting, sawing, drilling, and bending. Due to the results of these treatments, the oxidation protection film on the surface of stainless steel 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 in use, it may cause the final product to be unsatisfactory, or even worse, it may cause a critical system to malfunction.
A: Welding on both the inner and outer sides of the weld seam and adjacent areas causes accelerated oxidation. Because there are discolored areas where oxidation can be seen, the color is related to the thickness of the oxide layer. Compared to the oxide layer on stainless steel before welding, the oxide layer in the discoloration zone is relatively thick, and the composition is changed (reduced chromium), resulting in a decrease in local corrosion resistance. For the interior of the pipe, oxidation and discoloration can be minimized by using an appropriate back blowing method. After welding, it is often necessary to perform post weld treatments such as pickling and grinding to remove the oxide layer (colored) and restore corrosion resistance. A color chart is often used to determine whether the weld seam needs to be pickled based on the level of color. However, this decision is subjective, and in principle, each color represents the presence of oxidation and the affected oxide layer, resulting in a decrease in corrosion resistance.
B: Mechanical treatment usually uses mechanical or non mechanical contaminated surfaces. Organic pollutants may be caused by lubricating oil. Inorganic pollutants such as foreign iron particles may be caused by contact with tools. Usually, all types of surface contamination can cause pitting. In addition, foreign iron particle fields may lead to electrochemical corrosion. Spot corrosion and electrochemical corrosion are both localized corrosion forms that initially require water treatment. Therefore, surface pollution usually reduces the corrosion resistance of stainless steel.
2. Surface treatment
In order to treat the surface, remove discoloration, and restore corrosion resistance, there are now many post-processing and methods available. Here we should distinguish between chemical methods and mechanical methods. Chemical methods include: pickling (immersion, pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods include sandblasting, shot blasting with glass or ceramic particles, immersion, brushing, and polishing.
Although all methods can produce welded seams, no mechanical rear treatment provides corrosion performance suitable for demanding applications. Using chemical methods to remove surface oxides and other pollutants, while using mechanical methods to
1. Reduced corrosion resistance due to damage or contamination
Corrosion may occur in areas where the passivation film is damaged and other forms of contamination hinder its natural formation. All beneficial properties of stainless steel may be damaged during processing, such as heat treatment or mechanical processing such as welding, cutting, sawing, drilling, and bending. Due to the results of these treatments, the oxidation protection film on the surface of stainless steel 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 in use, it may cause the final product to be unsatisfactory, or even worse, it may cause a critical system to malfunction.
A: Welding on both the inner and outer sides of the weld seam and adjacent areas causes accelerated oxidation. Because there are discolored areas where oxidation can be seen, the color is related to the thickness of the oxide layer. Compared to the oxide layer on stainless steel before welding, the oxide layer in the discoloration zone is relatively thick, and the composition is changed (reduced chromium), resulting in a decrease in local corrosion resistance. For the interior of the pipe, oxidation and discoloration can be minimized by using an appropriate back blowing method. After welding, it is often necessary to perform post weld treatments such as pickling and grinding to remove the oxide layer (colored) and restore corrosion resistance. A color chart is often used to determine whether the weld seam needs to be pickled based on the level of color. However, this decision is subjective, and in principle, each color represents the presence of oxidation and the affected oxide layer, resulting in a decrease in corrosion resistance.
B: Mechanical treatment usually uses mechanical or non mechanical contaminated surfaces. Organic pollutants may be caused by lubricating oil. Inorganic pollutants such as foreign iron particles may be caused by contact with tools. Usually, all types of surface contamination can cause pitting. In addition, foreign iron particle fields may lead to electrochemical corrosion. Spot corrosion and electrochemical corrosion are both localized corrosion forms that initially require water treatment. Therefore, surface pollution usually reduces the corrosion resistance of stainless steel.
2. Surface treatment
In order to treat the surface, remove discoloration, and restore corrosion resistance, there are now many post-processing and methods available. Here we should distinguish between chemical methods and mechanical methods. Chemical methods include: pickling (immersion, pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods include sandblasting, shot blasting with glass or ceramic particles, immersion, brushing, and polishing.
Although all methods can produce welded seams, no mechanical rear treatment provides corrosion performance suitable for demanding applications. Using chemical methods to remove surface oxides and other pollutants, while using mechanical methods to
