During the production of forgings, high temperatures and mechanical forces often lead to the formation of scale, oil stains, and other impurities on the surface. These contaminants not only affect the appearance of the forgings but also reduce their corrosion resistance and coating performance, ultimately impacting their service life. To address this issue, the pickling process has been developed. Pickling not only effectively removes oxides from the surface of forgings but also eliminates oil stains and other impurities, restoring the smoothness of the forgings and enhancing the effectiveness of subsequent processes.
Basic Principles of Forging Pickling
Pickling involves immersing forgings in acidic solutions (such as sulfuric acid or hydrochloric acid) to chemically react with surface contaminants, removing scale, oil stains, and other impurities. The primary goal of pickling is to dissolve or strip away these unwanted substances through chemical reactions, thereby restoring the cleanliness and smoothness of the forging surface. The basic process of pickling includes the following aspects:
1. Removal of Scale
Forgings react with oxygen in the air at high temperatures, easily forming scale on the surface. This layer of oxide not only affects the appearance of the forging but may also interfere with subsequent processing, such as welding or coating. Pickling dissolves the scale through the chemical action of the acid, exposing the metal's original color and restoring surface gloss.
2. Removal of Oil Stains and Other Impurities
During the forging process, lubricants, coolants, and environmental pollutants may adhere to the surface of the forging. These oil stains and impurities not only affect the appearance but may also hinder subsequent coating, plating, and other treatments. Pickling effectively emulsifies or dissolves these contaminants, ensuring the cleanliness of the forging surface.
3. Improvement of Coating Performance
After pickling, the surface of the forging becomes smoother and more uniform, reducing the contact area between corrosive media and the metal, thereby enhancing the forging's corrosion resistance. At the same time, the pickled surface provides a good foundation for subsequent coating processes, improving the adhesion and durability of the coating.
4. Preparation for Subsequent Processes
A clean metal surface is crucial for subsequent heat treatment, plating, or spraying processes. During heat treatment, a clean surface facilitates uniform heat conduction, improving the treatment effect. In plating or spraying processes, a clean surface ensures strong bonding between the coating and the metal substrate, enhancing strength and durability.
Implementation Methods of Forging Pickling
There are several common methods for pickling forgings, including immersion pickling, spray pickling, and acid paste rust removal. Different methods are suitable for different production needs and forging sizes.
Immersion Pickling: This is the most common pickling method, suitable for mass production. Forgings are immersed in a tank containing an acidic solution, where the acid reacts with the oxides on the surface to remove scale and impurities. The immersion method is relatively simple to operate and offers stable results.
Spray Pickling: Spray pickling is also widely used in mass production. Acid is sprayed onto the surface of the forging using high-pressure nozzles to quickly remove oxides. This method is suitable for forgings with irregular shapes or complex surfaces.
Acid Paste Rust Removal: In some special cases, acid paste rust removal is used. The acid paste adheres to the surface of the forging, effectively acting on localized scale and removing rust through chemical reactions.
Key Control Factors in the Pickling Process
Several critical parameters need to be strictly controlled during the pickling process to ensure effectiveness and avoid excessive corrosion of the forging. The following are key factors to pay special attention to during pickling.
Concentration of the Pickling Solution: The concentration of the pickling solution directly affects the removal of scale. Generally, a 10%~20% sulfuric acid solution works well, while hydrochloric acid concentrations typically range from 20%~80%. Excessive concentration may lead to over-corrosion of the forging surface, affecting metal properties.
Temperature Control: The temperature of the pickling solution also affects the reaction rate. Typically, a pickling temperature of around 40°C is ideal. If the temperature is too high, it may cause over-pickling, damaging the metal substrate of the forging.
Pickling Time: The duration of pickling determines the cleanliness of the forging surface. Excessive time may lead to over-corrosion, affecting the mechanical properties and dimensional accuracy of the forging. It is crucial to control the pickling time based on the material, shape, and thickness of the oxide layer of the forging.
Replacement and Cleaning of the Pickling Solution: When the iron content in the pickling solution exceeds 80g/L or ferrous sulfate exceeds 215g/L, the solution should be replaced promptly. Additionally, thorough rinsing with water after pickling is necessary to remove residual acid and prevent corrosion of the forging.
Long-Term Effects of Pickling on Forgings
Pickling not only removes surface contaminants but also has a positive impact on the long-term performance of forgings. After pickling, the surface of the forging becomes smooth and uniform, effectively preventing subsequent corrosion and extending the service life of the forging. At the same time, pickled forgings are more suitable for subsequent coating, plating, and other processes, ensuring overall quality improvement.
Conclusion
Pickling is a crucial process in the surface treatment of forgings. By removing scale, oil stains, and other impurities, it restores the smoothness and original metal appearance of the forging. Pickling not only improves the corrosion resistance and coating performance of forgings but also provides a solid foundation for subsequent heat treatment, plating, and spraying processes. Proper control of pickling parameters ensures the quality and performance of forgings, thereby enhancing the service life and reliability of the final product.
