I . Definition
Electrolytic polishing is a process in which the part to be polished acts as the anode, an insoluble metal serves as the cathode and both are simultaneously immersed in an electrolytic bath.
Direct current is applied to selectively dissolve the anode, thereby increasing the surface brightness of the workpiece.
II . Principle
There is much debate around the world about the underlying principle of electropolishing, with the commonly recognized explanation being the Membrane Theory.
According to this theory, the metal ions that come off the part form a layer of phosphate film on the surface of the part, combining with the phosphoric acid in the polishing liquid.
This membrane is thinner in the protruding areas and thicker in the recessed areas. Since the current density is higher in the protrusions, they dissolve quickly. As the membrane flows, the uneven surface is gradually smoothed out.
III . Advantages of Electrolytic Polishing
(1) Uniform color and luster inside and out, which is durable; even inaccessible recesses can be smoothed out.
(2) High production efficiency and low cost.
(3) Higher corrosion resistance of the workpiece surface, suitable for all stainless steel materials.
4 . Required conditions and equipment for electrochemical polishing
1. Power Source:
Two-phase 220 V or three-phase 380 V power supply can be chosen.
2. Rectifier:
The requirements for the power source waveform in electrolytic polishing are not very strict. Silicon-controlled rectifiers or high-frequency rectifiers can be used.
- Rectifier no-load voltage: 0-20V
- Charge voltage (working voltage): 8-10V
If the working voltage drops below 6V, the polishing speed decreases and the gloss is insufficient.
Rectifier current: Determined based on customer part size.
3. Electrolytic cell and support facilities (anode rod)
The cell can be made of rigid welded polyvinyl chloride (PVC) plates.
It is equipped with three electrode rods, with the movable anode rod in the center, connected to the power supply anode (or positive pole), and the cathode rods on each side, connected to the power supply cathode (negative pole).
4. Heating installations and cooling equipment
① Quartz heating tubes or titanium heating tubes can be used for heating.
② Serpentine tubes can be used for cooling, which can also facilitate heating.
5. Light fixtures
Titanium fixtures are preferred because of their high corrosion resistance and long service life, and they do not affect the bath solution.
It is advisable to avoid using copper fixtures, as copper ions can deposit a poorly adhering layer of copper on the surface of the stainless steel, which can affect the quality of the polish.
The exposed copper parts can be made into a film with PVC glue and the insulating film can be scraped off at the contact points.
6. Anode and cathode materials
Lead sheets can be used for the cathode material while copper can be used for the anode material. The anode-cathode ratio must be between 1:2. The ideal distance between the cathode and the anode should be 10 to 30 centimeters.
Currently, electrolytic polishing is mainly used for surface brightening of stainless steel parts. Stainless steel parts can be divided into 200, 300 and 400 series materials, each requiring a specific electropolishing solution.
For example, 200 series stainless steel must use the 200 series formula, which cannot be adapted to 300 or 400 series of stainless steel materials.
This has always been a major problem domestically, as some manufacturers use composite parts that include 200, 300, and 400 series stainless steel materials.
In December 2007, a senior engineer at Weihai Yunqing Chemical Development Institute developed a universal electrolyte for stainless steel.
This electrolyte is suitable for all types of stainless steel. It integrates all the advantages of the original electrolyte, with the ideal specific gravity, and achieves a mirror-like shine.
It also has new advantages such as increasing the original brightness, reducing the current density by half, saving 50% on electricity costs during production, and extending the service life by 40%. This electrolyte has always been at the forefront of national technology.
Electrolytic Polishing Process: Degreasing — Water Washing — Rust Removal — Water Washing — Electrolytic Polishing — Water Washing — Neutralization — Water Washing — Packaging
V. Types of Electrolytic Polishing
The main types of electropolishing solutions currently used in production include:
1. Polishing solution composed of sulfuric acid, phosphoric acid and chromic anhydride;
2. Polishing solution composed of sulfuric acid and citric acid;
3. Mixed polishing solution composed of sulfuric acid, phosphoric acid, hydrofluoric acid and glycerol or similar compounds.
Electrochemical Polishing of Steel Parts
1. Influence of material type: There are many types of steel materials, and different polishing solutions should be used for different steels.
2. Influence of various factors: Phosphoric acid is the main component of the polishing solution. The phosphate it forms adheres to the anode surface and plays an important role during the polishing process. Sulfuric acid can increase the polishing speed, but the content should not be too high to avoid causing corrosion. Chromic anhydride can improve the polishing effect and make the surface shiny.
The current density has a great impact on the polishing quality. Different current densities must be used for different solutions. If the current density is too low, the leveling effect will be weak, and if it is too high, it will cause excessive corrosion. Temperature has some impact on the quality of polishing, but it is not the main factor.
3. Operation precautions:
(1) The freshly prepared solution must be subjected to electrification treatment under the condition of a large cathode area (the cathode area is several times larger than the anode area), so that part of the hexavalent chromium is reduced. If the amount of trivalent chromium increases greatly during use, the opposite occurs, that is, the electrification treatment is carried out in the condition of large anode and small cathode.
(2) Frequently measure the density of the solution and add water or concentrate the solution by heating in a timely manner. The content of phosphoric acid, sulfuric acid, chromic anhydride and trivalent chromium in the solution must be analyzed and adjusted regularly.
(3) During use, the dissolved iron content in the anode gradually increases. When the iron content (calculated as Fe2O3) reaches 7-8%, the solution must be partially or completely replaced.
(4) Preparation: First, mix phosphoric acid and sulfuric acid, dissolve the chromic anhydride in water, then pour the mixed acid solution into the chromic anhydride aqueous solution and heat it to 80°C. Slowly add the gelatin while stirring constantly (the reaction is intense at this point). After the reaction is complete (about 1 hour later), the solution turns a uniform green.
Electrolytic Metal Polishing
Electrolytic polishing of copper and its alloys generally uses phosphoric acid electrolytes. For aluminum and its alloys, a solution with phosphoric acid, sulfuric acid and chromic acid is used for electrolytic polishing.
Electrolytic polishing of aluminum and its alloys is widely applied in production. If polished parts are subjected to a brief subsequent treatment, not only can they obtain a smooth and shiny appearance, but a complete oxidation film can also be formed, increasing their corrosion resistance and maintaining the surface shine for a long time.
For the solution preparation method, you can refer to the relevant content on electrolytic polishing of steel parts.
When the aluminum content in the solution exceeds 5%, partial or complete replacement of the solution should be considered. If the chloride ion content exceeds 1%, pinpoint corrosion can easily occur on the surface of the parts. The chloride ion content in the water used to prepare the solution must be less than 80 mg/L.
