Electrochemical polishing: the secret to a brand new metal surface

In modern industrial production, the surface quality of metal materials has a vital impact on the performance and service life of products. As an advanced metal surface treatment technology, electrochemical polishing has been widely used in many fields with its unique advantages. This article will deeply explore the principles, process flow and application effects of electrochemical polishing on different metal materials, and compare it with traditional mechanical polishing to reveal its significant advantages.

Principle of electrochemical polishing

Electrochemical polishing is a process that uses electrochemical reactions to selectively dissolve specific areas of the metal surface to achieve the purpose of polishing. During the electrochemical polishing process, the metal workpiece serves as the anode and the insoluble metal serves as the cathode. Both electrodes are immersed in the electrolyte at the same time and a direct current is passed through. When the current passes through, a layer of tiny protrusions and depressions will form on the surface of the workpiece. Through electrolysis, the metal atoms in the protruding parts are preferentially dissolved, so that the surface gradually becomes smooth and flat.

The principle of electrochemical polishing is mainly based on the following two aspects:

Electrochemical dissolution

• During the electrolysis process, the protruding parts of the metal surface will preferentially undergo dissolution reactions due to their large current density. This is because the electric field strength of the protruding parts is higher, making it easier for metal atoms to lose electrons and enter the solution. As the electrolysis proceeds, the protruding parts of the metal surface are gradually dissolved, while the concave parts are relatively retained, making the surface smoother.

Formation and dissolution of oxide film

• During the electrochemical polishing process, a thin oxide film will form on the metal surface. The formation and dissolution of this oxide film is a dynamic equilibrium process. The formation of the oxide film can prevent the further dissolution of the metal, while the dissolution of the oxide film can expose the new metal surface, allowing it to continue to participate in the electrolytic reaction. By controlling the electrolytic conditions, the balance between the formation and dissolution of the oxide film can be achieved, thereby achieving the best polishing effect.

Electrochemical polishing process

The electrochemical polishing process generally includes the following steps:

Pretreatment

• Pre-polishing or polishing
  • Electrochemical polishing belongs to the category of fine polishing, which can reduce the surface roughness of the substrate by several levels from the original roughness. Therefore, the lower the original roughness of the substrate, the brighter the surface after electrochemical polishing. In order to obtain a high-brightness surface, some workpieces with relatively rough surfaces are best polished, rolled or pre-polished, and then electrochemically polished. For substrates with relatively bright surfaces or workpieces that do not require high brightness, pre-polishing or polishing is not necessary.
• Degreasing
  • Most of the grease used in the processing of the substrate is mineral oil. Unlike vegetable oil, it cannot be removed by alkaline soap. It cannot be removed by ordinary caustic soda, sodium carbonate and sodium cyanide solutions. In particular, parts that have been pre-polished with polishing paste in advance often contain high-viscosity grease. After washing with gasoline, an oil film will remain. Usually, special wax removal water or polishing paste cleaning agent is used for degreasing. For general parts, just use a degreasing liquid containing an appropriate surfactant.
• Rust removal
  • If there is rust on the surface of the workpiece, rust removal is required. Generally, the pickling method is used. The workpiece is immersed in dilute hydrochloric acid or sulfuric acid, and then taken out after soaking for a certain period of time and rinsed with clean water.

Electrochemical polishing

• The pre-treated workpiece is mounted on the anode, and the insoluble metal is used as the cathode. The two electrodes are immersed in the electrolyte at the same time and a direct current is passed. The composition, temperature, voltage, current and other parameters of the electrolyte need to be adjusted according to the material and requirements of the workpiece. During the electrolysis process, the raised parts on the surface of the workpiece are dissolved first, and the surface gradually becomes smooth and flat.

Post-treatment

• Cleaning
  • After the electrochemical polishing is completed, the workpiece is taken out of the electrolyte and washed in a flowing cold water tank to wash away the polishing liquid remaining on the surface to prevent the surface from being corroded.
• Neutralization
  • Put the washed workpiece into an alkali bath for neutralization to further remove the traces of the acid.
• Drying
  • The neutralized workpiece is washed in a cold water tank to remove the residual alkali on the surface, so that the surface of the workpiece is neutral, and then dried.

Effects of electrochemical polishing on different metal materials

Stainless steel

• It can form a chromium-rich oxide film on the surface of stainless steel to improve its surface corrosion resistance.
• Microscopically flatten the surface of the workpiece, reduce surface roughness, and make the stainless steel surface show a uniform, smooth and highly bright mirror effect.
• Remove burrs and scratches from the surface of stainless steel to improve its surface quality and aesthetics.
• For example, in the fields of pharmaceutical machinery, medical equipment, food machinery, etc., electrochemically polished stainless steel workpieces can meet the requirements of hygiene and corrosion resistance.

Copper

• Effectively removes the oxide film and dirt on the copper surface, leaving the copper surface with a bright metallic luster.
• Improve the electrical and thermal conductivity of copper and improve its processing performance.
• In the fields of electronics and electrical engineering, electrochemically polished copper materials can meet the requirements of high precision and high performance.

Nickel

• It can form a dense oxide film on the nickel surface to improve its corrosion resistance.
• Reduce the roughness of the nickel surface to make it smoother and flatter.
• In the fields of aerospace, chemical industry, etc., electrochemically polished nickel materials can meet the requirements of corrosion resistance and high precision.

Tungsten

• Remove oxides and impurities on the surface of tungsten to make the surface of tungsten present a bright metallic luster.
• Improve the hardness and wear resistance of tungsten and improve its processing performance.
• In the fields of cutting tools, molds, etc., electrochemically polished tungsten materials can meet the requirements of high hardness and high wear resistance.

Comparison between electrochemical polishing and traditional mechanical polishing

Surface quality

• Electrochemical polishing can flatten the workpiece surface at a microscopic level, reduce the surface roughness, and make the metal surface present a uniform, smooth and highly bright mirror effect.
• Traditional mechanical polishing can only flatten the workpiece surface at a macroscopic level. Although it can also reduce the surface roughness, it is difficult to achieve the surface quality of electrochemical polishing.

Corrosion resistance

• Electrochemical polishing forms an oxide film on the metal surface, which improves the corrosion resistance of the metal.
• Traditional mechanical polishing will form a cold-work hardened deformation layer on the metal surface, and the corrosion resistance will not be significantly improved.

Surface stress

• The metal surface after electrochemical polishing is stress-free.
• Traditional mechanical polishing will cause stress on the metal surface and will contain polishing abrasives.

Scope of application

• Electrochemical polishing is applicable to any metal material, especially for hard metal materials, where electrochemical polishing has unique advantages.
• Traditional mechanical polishing is difficult to effectively polish hard metal materials.

Production efficiency

• Electrochemical polishing has a fast processing speed, high productivity, can be mass-produced, and is easy to automate.
• Traditional mechanical polishing has slow processing speed and low productivity, making it difficult to achieve mass production and automation.

Environmental performance

• The electrochemical polishing process will produce wastewater and waste gas, but compared with the dust generated by traditional mechanical polishing, electrochemical polishing has better environmental performance.

Conclusion

As an advanced metal surface treatment technology, electrochemical polishing has many significant advantages. It can micro-level the metal surface, reduce surface roughness, improve the metal's corrosion resistance, and make the metal surface present a uniform, smooth and highly bright mirror effect. Compared with traditional mechanical polishing, electrochemical polishing has obvious advantages in surface quality, corrosion resistance, production efficiency, etc. In the future development, electrochemical polishing technology will continue to be improved and innovated, and its application field will continue to expand, making greater contributions to the development of industrial production.