Passivation is a method that converts the metal surface into a state that is not easily oxidized and delays the corrosion rate of the metal. In addition, the phenomenon of an active metal or alloy in which the chemical activity is greatly reduced and becomes a noble metal state is also called passivation.
Phenomenon: If the corrosion product generated by the metal due to the action of the medium has a dense structure and forms a thin film (often invisible), which closely covers the surface of the metal, the surface state of the metal is changed and the electrode potential of the metal is changed. Greatly jumps to the positive direction, and becomes a passive state of corrosion resistance. For example, when Fe→Fe++, the standard potential is -0.44V, and it jumps to +0.5~1V after passivation, and shows noble metal properties of corrosion resistance. This thin film is called a passivation film.
The purpose of passivation of stainless steel is to use strong oxidants or electrochemical methods to turn the surface of stainless steel into an inactive state, that is, to turn the surface of stainless steel into a passive state and have certain anti-rust properties.
The difference between passivation treatment and anti-rust oil
The main difference between passivation and anti-rust oil is that the products produced are different; anti-rust oil uses the oil film to close the pores on the metal surface to isolate and prevent rust from contacting with oxygen. It is a veritable physical reaction. The oil film is relatively easy to be removed and destroyed as the production progresses; passivation is the use of oxidizing substances in the passivation solution to produce redox reactions with metals, to promote the formation of a layer of metal oxide compounds on the metal surface to achieve effective The purpose of protecting metal. This process is a chemical reaction. The generated passivation film is dense and complete, and is not easy to be damaged.
Advantages of stainless steel passivation:
- ①Improve the corrosion resistance of stainless steel surface
- ②Improve the smoothness and clean appearance of the stainless steel surface
- ③Remove burrs on stainless steel surface and edges (polished stainless steel surface)
- ④Improve and extend the service life of stainless steel
Disadvantages of stainless steel passivation:
① Some alloys cannot be passivated, such as some stainless steel alloys with low chromium and nickel content that will actually be destroyed. Therefore, they cannot be passivated.
②Stainless steel welding parts may not be suitable for passivation. Because arc welding exposes stainless steel to short-cycle high-temperature thermal cycling, it destroys its corrosion resistance.
③ The temperature and type of acid used in the passivation process must be adjusted according to the alloy to be passivated. This adds cost and complexity compared to electropolishing.
Conclusion
In the process of stainless steel processing, pickling and passivation are generally carried out at the same time. Pickling is to remove rust, rust and oxide scale on the surface of stainless steel, and passivation is to form a passivation film in a state where the surface of stainless steel is rust-free and clean to protect the surface of stainless steel from being rusted. It is generally used in the aspects of anti-rust requirements such as copper bars.