Stainless steel is widely used in various fields such as construction, medical equipment, and food processing due to its excellent corrosion resistance. However, the corrosion resistance of stainless steel mainly depends on the passive film formed on its surface. This article will analyze the key factors affecting the passive film on the surface of stainless steel from multiple perspectives to help readers gain a deeper understanding of this important characteristic.
Influence of Material Composition
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.
The chemical composition of stainless steel directly determines the formation and stability of the passivation film. High chromium content is a prerequisite for the spontaneous formation of the passivation film. The following is a detailed description of the key components:
Chromium (Cr): The chromium content must be higher than 10.5% to form a stable chromium oxide film. For example, 304 and 316 stainless steels rely on the presence of chromium to improve their corrosion resistance.
Nickel (Ni): The addition of nickel not only enhances the corrosion resistance of stainless steel but also improves the toughness of the passivation film, especially in strong acid and alkaline environments.
Molybdenum (Mo): The addition of molybdenum can significantly improve the pitting resistance of the passivation film and is suitable for chlorine-containing environments.
Influence of Surface Treatment on Passivation Film
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. Therefore, stainless steel should use a finished surface as much as possible. In addition, the cleanliness of the stainless 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.
The surface treatment process of stainless steel is directly related to the quality and stability of the passivation film:
Mechanical polishing and chemical polishing: Smooth surfaces are more likely to form a uniform passivation film and reduce impurity adhesion.
Pickling and passivation treatment: Pickling can remove surface oxides, while passivation treatment enhances the integrity and stability of the chromium oxide film.
Electrolytic polishing: This process can further improve the finish and corrosion resistance of stainless steel, especially in the food and medical industries.
Influence of Environmental Factors
Use environmental media. The passive film of stainless steel 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. 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.
The temperature, humidity, and chemical composition of the use environment have a significant impact on the formation and durability of the passivation film:
Oxygen concentration: When there is sufficient oxygen, the passivation film can self-repair, thereby improving corrosion resistance.
Chloride concentration: In the ocean or other high-chlorine environments, chlorides will damage the passivation film, so it is wise to choose 316 stainless steel containing molybdenum.
Acidity and alkalinity: Acidic or strongly alkaline environments may accelerate the degradation of the passivation film, and materials need to be reasonably selected according to actual working conditions.
Influence of Temperature and Time
Temperature: High-temperature environment can accelerate the formation of the passivation film, but too high temperature may also cause the film layer to be unstable.
Use time: As the use time increases, the performance of the passivation film may decline, so regular maintenance and inspection are particularly important.
Impact of Improper Operation
Improper operation can cause serious damage to the passivation film:
Mechanical damage: Welding, cutting, and other processes may cause damage to the passivation film, and subsequent treatment is required promptly.
Contaminant adhesion: Contaminants such as oil and dust will affect the formation of the passivation film, and it is essential to keep the surface clean.
Maintenance and Care Measures
Regular maintenance can significantly extend the service life of stainless steel:
Regular cleaning: Especially in heavily polluted environments, regular cleaning can prevent the accumulation of chlorides and other impurities.
Repassivation treatment: When the performance of the passivation film is found to have deteriorated, passivation treatment can be performed to restore its corrosion resistance.
Conclusion
The corrosion resistance of stainless steel depends on the quality and stability of the surface passivation film. By understanding the various factors that affect the passivation film, companies can choose suitable stainless steel materials and treatment processes to ensure its corrosion resistance in various environments. Through regular maintenance and correct operation, the service life of stainless steel equipment can be effectively extended, and maintenance costs can be reduced.
If you have any questions about the selection or surface treatment of stainless steel, please contact us for professional advice and quotes.