Is electrolytic polishing suitable for complex internal structures?

一, The main idea behind electrolytic polishing is a leveling device that doesn't touch anything.
Anodic dissolution is what makes electrolytic polishing work. The key to its success is the difference in current density distribution. As an anode, the workpiece is submerged in electrolyte. The micro protrusions on the surface dissolve first because the current density is higher, while the depressions dissolve more slowly because the current density is lower. The "mucosal theory" is the main idea behind this process. It says that phosphate ions in the electrolyte form a thick phosphate film with dissolved metal ions. The film is thinner at the protrusions and dissolves faster, and it is thicker in the depression and dissolves more slowly. The dynamic movement of the mucosa keeps leveling out the micro roughness of the surface, which eventually makes it smooth like a mirror.
For example, the interior mesh structure of a 316L stainless steel cardiovascular stent is only 0.1mm wide, and traditional mechanical polishing can easily cause the mesh to break or distort. Electrolytic polishing can make the surface of the internal mesh less rough by controlling the current density (15–50A/dm²) and electrolyte temperature (60–70 °C) very carefully. It can lower the roughness from Ra3.2 μm to Ra0.05 μm or lower without changing the size of the stent. It also gets rid of any residual stresses that were caused by mechanical processing, which makes the stent last longer and be more compatible with the body.
二, The three main technological benefits of processing complicated interior structures
1. Global coverage with no gaps
Electrolytic polishing can work in places where there isn't enough room because it doesn't touch anything. The plasma etching reaction chamber used in the semiconductor industry has tens of thousands of micropores that are 0.5mm in diameter and long channels that are up to 500mm long. To do traditional mechanical polishing, you have to take apart the cavities and use special equipment to work on each part. This takes a lot of time and is easy to get dirty. With a circulating electrolyte system, electrolytic polishing can be done. This lets the current evenly reach all microstructure surfaces and polish them all at the same time. A semiconductor equipment manufacturer has provided practical data showing that electrolytic polishing can lower the surface roughness inside the reaction chamber from Ra1.6 μm to Ra0.02 μm. It can also lower the number of metal particles to less than 5 per square centimeter, which meets the cleanliness standards for 5nm process chips.
2. Fixing microscopic defects and making things work better
During the production process, complex interior structures are likely to have problems such microcracks and porosity. Electrolytic polishing can preferentially eliminate materials from defective regions via a selective dissolution process. For instance, titanium alloy aviation fasteners still have micro holes of 0.01–0.05mm in the internal threads after hot isostatic pressing (HIP) treatment. Electrolytic polishing makes the surface of threads smoother while adjusting the current density (20–30A/dm ²) to progressively dissolve the material at the edges of micropores, which helps close the pores. After being processed, the fasteners' fatigue strength went up by 35%, and their corrosion resistance met ASTM G48 standard grade A.
3. Group processing and cutting costs
Electrolytic polishing is a far more efficient way to polish huge numbers of complex pieces. For example, the fuel injector in a car's fuel injection system has dozens of 0.2mm diameter injection holes and complicated flow pathways inside. It takes more than 2 hours to polish a single piece of metal using traditional mechanical polishing, and it needs to be clamped and positioned several times. Electrolytic polishing uses special equipment and can polish 50 to 100 gasoline injectors at once. This cuts the processing time for a single item down to 8 minutes and makes sure that the surface roughness is the same every time, unlike mechanical polishing. According to data from a certain company that makes car parts, electrolytic polishing has raised the yield rate of fuel injectors from 82% to 98%, which saves the company more than 2 million yuan a year in rework expenses.
三, Examples and data from the industry that support it
1. Field of medical devices: making orthopedic implants more biocompatible
The interior porosity structure of artificial joint prostheses must satisfy the proliferation requirements of osteocytes while inhibiting bacterial adhesion. By carefully adjusting the amount of phosphoric acid and sulfuric acid in the mixed electrolyte (65–75% phosphoric acid and 10–15% sulfuric acid), electrolytic polishing can make a passivation film that is evenly thick on porous surfaces. Experimental data from a multinational medical company shows that electrolytic polishing makes titanium alloy hip joint prostheses smoother, with internal pores going from Ra2.5 μ m to Ra0.3 μ m, a 92% decrease in bacterial adhesion, and a decrease in postoperative infection rate from 1.2% to 0.15%.
2. Aerospace field: Improving the heat resistance of turbine blades
The internal cooling channel diameter of aircraft engine turbine blades is only 0.8mm, and traditional mechanical polishing can readily change the shape of the channel, which makes cooling less effective. Electrolytic polishing uses pulse current technology (30% duty cycle, frequency 1kHz) to make the surface smoother without increasing the size of the channel. It can go from Ra1.6 μ m to Ra0.1 μ m. A test done by a certain aircraft engine maker indicated that the heat transfer coefficient of the treated blades' interior cooling channels went up by 18% at a high temperature of 1200 °C. The engine's overall efficiency went up by 2.3%.
四, Problems and Solutions in Technology
Electrolytic polishing has a lot of benefits when it comes to working with complicated interior structures, but it still has two big problems to deal with:
Controlling the homogeneity of the electrolyte: Structures like deep blind holes might make the electrolyte flow poorly, which can lead to variances in concentration in different areas. The answer is to use ultrasonic-assisted stirring, make unique circulation systems, and make new electrolytes with low viscosity and high conductivity (for example, adding ethylene glycol to make the fluid flow better).
Accurate control of current density: The shape of the workpiece can readily change the current density distribution of structures at the micrometer level. By making a digital twin model and using finite element analysis (FEA) to simulate the current field distribution, the cathode design (like using 3D printed shaped cathodes) and process parameters (like using gradient current density technology) can be improved to get even polishing of complex structures.