Pickling

Pickling classification

Classification by type of acid

1.Sulfuric acid pickling

Features: Sulfuric acid is a strong acid, with strong oxidation and dehydration. Sulfuric acid pickling can effectively remove the oxide and rust on the surface of steel, but the corrosion of metal is also large.

Application: It is often used for pickling of iron and steel materials, especially for thick oxide skin and severely rusted workpieces. However, attention should be paid to controlling the concentration and temperature of sulfuric acid in the pickling process to avoid excessive corrosion.

2.Hydrochloric acid pickling

Features: Hydrochloric acid is also a strong acid, with good solubility and volatility. Hydrochloric acid pickling speed is fast, the corrosion of metal is relatively small, but it is easy to produce acid fog, which has a certain impact on the environment and human health.

Application: Suitable for pickling all kinds of metal materials, especially for stainless steel, copper, aluminum and other non-ferrous metals pickling effect is good. When using hydrochloric acid pickling, it is necessary to take effective ventilation measures to reduce the production of acid fog.

3.Nitric acid pickling

Features: Nitric acid is a strong oxidizing acid, has a strong corrosive. Nitric acid pickling can quickly remove the oxide and rust on the metal surface, but it is more corrosive to the metal, and it is easy to cause excessive corrosion on the metal surface.

Application: Generally used for pickling of special materials such as stainless steel and titanium alloy, or in the case of rapid removal of oxide skin. Due to the greater danger of nitric acid, special care should be taken when using it.

4.Phosphate pickling

Features: Phosphoric acid is a medium strong acid, with weak corrosion and better corrosion inhibition properties. Phosphoric acid pickling can remove the oxide and rust on the surface of the metal, while the corrosion of the metal is less, suitable for precision parts and high surface quality requirements of the workpiece pickling.

Application: Often used in electronic, aerospace and other fields of metal parts pickling.

5.Mixed acid acid wash

Features: The mixed use of two or more kinds of acids can comprehensively utilize the advantages of various acids to improve the pickling effect. For example, the mixed acid of sulfuric and hydrochloric acid can improve the pickling speed and the ability to remove the oxide skin; The mixed acid of nitric acid and hydrofluoric acid can be used for pickling stainless steel to improve the pickling effect.

Application: According to different pickling requirements and metal materials, choose the right mixed acid for pickling.

Passivation advantage

1. Improve corrosion resistance

Forming a protective film: Passivation treatment forms a dense oxide film or other type of protective film on the surface of the metal. This film can effectively prevent oxygen, moisture and other corrosive media from contacting the metal matrix, thus slowing down the occurrence of corrosion reactions. For example, after passivation treatment of stainless steel, the passivation film on the surface can resist a variety of corrosive environments, such as atmosphere, seawater, acid, alkali, etc., which greatly improves its corrosion resistance in different environments.
Reduced corrosion rate: The passivation film has high chemical stability and can significantly reduce the corrosion rate of the metal. Even under harsh corrosion conditions, the passivated metal can maintain a relatively low corrosion rate and extend its service life. For example, in the chemical industry, some metal equipment can operate stably in corrosive media for a long time after passivation treatment, reducing equipment maintenance and replacement costs.
2. Enhances surface hardness and wear resistance

Increase the surface hardness: During the passivation process, some microstructure changes may occur on the metal surface, so that the surface hardness is improved. This makes the passivated metal more resistant to wear and scratches during use. For example, after some tool steels are passivated, the surface hardness increases, which can better maintain the sharpness of the cutting edge and improve the service life of the tool.
Improved wear resistance: Passivation film can reduce the coefficient of friction between the metal surface and other objects, reducing the degree of wear. At the same time, the passivation film itself also has a certain wear resistance, which can protect the metal matrix from wear to a certain extent. For example, the piston rings of automotive engines are usually made of materials that have been passivated to improve their wear resistance and reduce engine wear and fuel consumption.
3. Improve appearance quality

Uniform surface color: Passivation treatment can make the color of the metal surface more uniform and beautiful. For some metal products with high appearance requirements, such as decorations, furniture hardware, etc., passivation treatment can improve the overall appearance quality. For example, stainless steel jewelry after passivation treatment, the surface shows a bright, uniform color, increasing the attractiveness of the product.
Removal of surface defects: In the passivation process, some minor surface defects, such as scratches, oxidation spots, etc., can be repaired to a certain extent. The formation of passivation film can cover these defects, making the metal surface more smooth and flat. For example, some aluminum alloy profiles after passivation treatment, the surface scratches and defects are significantly reduced, improving the quality of the product.
4. Improve cleanliness and hygiene

Easy to clean: The passivated metal surface is smoother, and it is not easy to absorb dust, dirt and other impurities. This makes the passivated metal products easier to clean and maintain good appearance and hygiene. For example, after passivation treatment, kitchen utensils, medical equipment, etc., are easy to clean and disinfect, and meet the requirements of health standards.
Prevent bacterial growth: Some passivation membranes have antibacterial properties that can inhibit the growth and reproduction of bacteria on metal surfaces. This is very important for some metal products used in health care, food processing and other fields. For example, stainless steel medical devices are passivated to reduce the risk of bacterial infection and improve medical safety.
5. Enhance the adhesion of paints and coatings

Provide a good base: The metal surface after passivation treatment is cleaner and more uniform, providing a good basis for the adhesion of paint and coating. The paint and coating can better bond with the passivated metal surface, improving the adhesion and durability of the coating. For example, in automobile manufacturing, the body of the car is usually passivated before painting is applied to ensure that the paint can firmly adhere to the body surface, improving the appearance quality of the car and corrosion resistance.
Prevent the coating from falling off: Because the passivation film is closely combined with the metal matrix, it can effectively prevent the paint and coating from falling off and foaming during use. This extends the service life of the coating and reduces maintenance costs. For example, passivation treatment of steel structure buildings before anti-corrosion coating can improve the adhesion and anti-corrosion performance of the coating and extend the service life of the building.