The corrosion resistance of stainless steel mainly depends on the surface passivation film. If the film is incomplete or defective, the stainless steel will still be corroded. In engineering, pickling and passivation treatment is usually carried out, so that the corrosion resistance of stainless steel can be exerted even more. In engineering, pickling and passivation treatment is usually carried out, so that the corrosion resistance of stainless steel can be exerted even more. Stainless steel surface cleaning, pickling, and passivation not only maximize corrosion resistance but also prevent product contamination and obtain a beautiful appearance.
The corrosion resistance of stainless steel is mainly because the surface is covered with a very thin and dense passivation film, which is the basic barrier for stainless steel protection. Stainless steel passivation has dynamic characteristics and should not be regarded as a complete stop of corrosion but as a barrier to diffusion, which greatly reduces the anode reaction speed. Generally, it tends to damage the membrane in the presence of a reducing agent, while maintaining or repairing the membrane in the presence of an oxidizing agent.
The stainless steel workpiece will form an oxide film when placed in the air, but the protection of this film is not perfect. Usually, thorough cleaning, including alkali cleaning and acid cleaning, and then passivation with an oxidizing agent are required to ensure the integrity and stability of the passivation film. One of the purposes of pickling is to create favorable conditions for passivation treatment to ensure the formation of high-quality passivation films. Because the surface of the stainless steel is corroded by an average of 10μm thick by pickling, the chemical activity of the acid makes the dissolution rate of the defect parts higher than other parts on the surface. Therefore, pickling can make the entire surface tend to be evenly balanced, and some hidden dangers that are easy to cause corrosion are removed. But more importantly, through pickling passivation, the iron and iron oxides are dissolved preferentially than the chromium and chromium oxides, and the chromium-poor layer is removed, resulting in the enrichment of chromium on the surface of the stainless steel. This chromium-rich passivation film The potential is close to that of precious metals, which improves the stability of corrosion resistance. Different passivation treatments will also affect the composition and structure of the film, thereby affecting the rust resistance. For example, the electrochemical modification treatment can make the passivation film have a multi-layer structure, so that the stainless steel can exert maximum corrosion resistance.
Comparison of stainless steel pickling passivation methods
| Method | Scope of application | Pros and cons |
| Dipping method | Used for parts that can be put into the pickling tank or passivation tank, but not suitable for large equipment | Pickling liquid can be used for a long time, with high production efficiency and low cost; large-volume equipment is filled with pickling liquid and immersion consumes too much liquid |
| Brushing method | Suitable for internal surface and local treatment of large equipment | Manual operation, poor working conditions, acid can not be recovered |
| Ointment method | For installation or maintenance site, especially for welding part treatment | Manual operation, poor working conditions, high production cost |
| Spray method | For the installation site, the inner wall of the large container | Low liquid consumption, low cost, fast speed, but need to be equipped with spray gun and skewer ring system |
| Round robin | Used for large-scale equipment, such as heat exchanger, tube and shell treatment | Convenient construction, acid liquid can be reused, but pipes and pumps are required to connect the circulation system |
| Electrochemical method | Not only can be used for parts but also can be used to brush the surface treatment of field equipment | The technology is more complicated and requires a DC power supply or a potentiostat |
