Stainless steel performance in all aspects

1. Reduced corrosion resistance due to damage or contamination.

Corrosion may occur where the film has been damaged, and other forms of contamination are present that prevent the natural re-formation of the passivated film. All the beneficial properties of stainless steel can be destroyed during processing, such as heat treatment or machining, such as welding, cutting, sawing, drilling and bending. As a result of these treatments, the oxide protective film on the stainless steel surface is usually damaged or contaminated, making it impossible to achieve spontaneous and complete passivation. As a result, localized corrosion may occur, even under conditions where corrosion is relatively weak. When used, it can result in an unsatisfactory end product, or even worse, the failure of critical systems.

A: Welding causes accelerated oxidation on the inside, outside and near the weld. Oxidation is visible because there are areas of discoloration, 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 zone is relatively thick, and the composition changes (containing less chromium), which reduces the local corrosion resistance. For the inside of the tube, oxidation and discoloration can be minimized by using proper backwashing methods. After welding, post-welding treatments such as pickling and grinding are usually required to remove oxides (color) and restore corrosion resistance. Color charts are often used to determine whether a weld needs pickling based on color grade. However, this decision is subjective, and in principle each color indicates the presence of oxidation and the affected oxide layer, thus reducing corrosion resistance.

B: Mechanical treatment usually uses mechanical or non-mechanical contaminated surfaces. Organic pollutants can be caused by lubricating oil. Contact with tools can lead to inorganic contaminants such as foreign iron particles. Plaque is often caused by surface contamination of all kinds. In addition, foreign iron particles may cause galvanic corrosion. Pitting and galvanic corrosion are local forms of corrosion that initially require water treatment. Therefore, surface contamination usually reduces the corrosion resistance of stainless steel.

2. Surface treatment.

In order to prepare the surface, remove discoloration and restore corrosion resistance, there are many post-treatments and methods. Here again we should distinguish between chemical and mechanical methods. Chemical methods are: pickling (soaking, with pickling paste or pickling spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods are: sandblasting, glass or ceramic pellet shot blasting, painting, polishing.

While all methods produce welded joints, no mechanical post-treatment will provide corrosion properties suitable for harsh applications. Use chemical methods to remove oxides and other contaminants from surfaces, while using mechanical methods to potentially remove contaminants from previously removed, polished, or erased materials. All kinds of pollution, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces are best cleaned regularly under dry conditions.

After pickling, it is important to properly rinse with water to remove all contaminants and pickling residues. The final rinsing should be done with softened water to avoid calcium stains and contaminants embedded in the growing oxide layer, which is necessary to build the passivation layer. In addition, due to the use of chemical methods (pickling and electrolytic polishing) to improve corrosion resistance, iron dissolves faster in pickling solutions and electrolytes than other metals. According to this, the surface is enriched with chromium and becomes more durable. Therefore, chemical methods of pickling and electropolishing are the only post-treatment methods that can restore the corrosion resistance of stainless steel in the weld and other surface damage that occurs before welding. This is actually independent of the type of stainless steel, there is no difference between the effect of pickling by soaking in the tank and using pickling paste or spray.