Many people take it for granted from the name of stainless steel that stainless steel will not rust, but here I want Zheng Zheng to tell you: stainless steel will rust! Because, in the actual use process, many insulators will damage the passive film of stainless steel, transform the passive state of stainless steel into an active state, cause the corrosion resistance of stainless steel to decrease, and then the rust phenomenon will occur.
Stainless Steel Rust
So what factors will destroy the passive film on stainless steel and cause stainless steel to rust?
1. Chloride Ion
Chloride ions are extremely harmful to stainless steel. During the passivation process, the content of chloride ions in the passivation solution should be strictly controlled, and the chemical materials used for passivation have limited requirements for chloride ions.
The preparation of passivation liquid and cleaning water also have strict water quality requirements for chloride ions to ensure that the passivation product does not adhere to chloride ions, so as to avoid future problems.
2. Surface Cleanliness
For stainless steel alloys, the lower the surface roughness, the smoother the surface, the more difficult it is for foreign bodies to adhere, and the lower the probability of local corrosion in each part. Therefore, stainless steel should use a finished surface as much as possible.
In addition, the cleanliness of the stainless steel surface is also very important. The final cleaning after passivation should be carried out carefully, because the residual acid solution promotes the cathodic reaction and ruptures the film, thereby activating the stainless steel and greatly reducing the corrosion resistance.
3. Use Environmental Media
Stainless steel passivation film belongs to a thermodynamically suppressed metastable structure. Its protective effect is related to the environmental medium. During use, it should be cleaned regularly to remove harmful substances attached to the surface for a long time.
Especially in an environment with chloride ions, avoid the long-term emulsion of chloride ions on the surface and the concentration in water. For example, stainless steel is used for food industry appliances. After each contact with food, it must be washed to avoid the effect of chloride ions and damage the dullness of the film.
Such as containers and equipment used for dairy products, the chloride ions in the dairy products have a destructive effect on the passivation film. For long-term storage of dairy products, it will cause corrosion and perforation of the capacity and equipment. Therefore, it should be cleaned regularly to restore the passivation film.
4. The Internal Factors of Stainless Steel
The martensite content and the chromium-nickel content in the stainless steel have a great influence on the passivation performance of the stainless steel. The lower the nickel content, the lower the passivation performance. The passive film performance of martensitic stainless steel is not as good as that of austenitic stainless steel.
Therefore, in the daily use of stainless steel, it is very important to pay attention to the environment in which stainless steel is used, and to choose the appropriate stainless steel variety according to the environment!