Unlike electroplating, electropolishing is a process that uses an electrochemical method to remove material from the surface of a workpiece. This process gives metal parts a clean and shiny surface.
| Processing Cost | Typical Application | Suitability |
| * No mold cost, but requires fixtures * Low unit cost | * Buildings and structures * Food processing and storage * Pharmaceutical industry and hospitals | * Single piece to mass production |
| Quality | Related Processes | Cycle Time |
| * Bright, glossy, hygienic, high-quality surface | * Electroplating * Grinding, sanding, and polishing | * Moderate cycle |
Process Introduction
Electropolishing gives metal surfaces a bright shine. It’s an electrochemical process that can precisely remove material from the surface when the metal is in an electrolyte solution. Just like regular polishing, it cleans metal surfaces, removes grease, gets rid of burrs, passivates the metal, and makes it more resistant to corrosion. Compared to other processes, it has less impact on the environment, which is why it’s widely used.
Electropolished stainless steel looks a lot like chrome-plated metal. The electropolishing process is simpler than chrome electroplating and uses less water and chemicals, so it’s cheaper. As more people use electropolishing, the cost keeps going down.
Typical Applications
Electropolishing has become a popular way to treat metal surfaces. In construction and structural metal parts, it’s used for both looks and functionality (corrosion resistance and reducing stress concentration). In the pharmaceutical and food industries, it’s mainly used for functionality (hygiene and corrosion resistance), with aesthetic uses making up over 90% of all applications.
Related Processes
Electropolishing is replacing many processes that harm the environment, like chrome plating (see Electroplating, page 364). This change comes from the industry’s goal to use less chrome and other heavy metals (since toxic chromic acid is used in chrome plating). Electropolishing does produce some chemical waste, which needs to be purified and have its pH neutralized. But overall, it’s less harmful than other processes.
Similar to mechanical polishing, electropolishing removes material. It can polish complex shapes that are hard to handle with mechanical methods.
Processing Quality
Electropolishing improves metal products’ surface quality both in looks and function. Aesthetically, it makes the metal surface brighter and more reflective. Under a microscope, the high spots on the metal dissolve faster than the low spots. This makes the surface smoother and smaller in area. But the final result depends a lot on how rough the surface was before electropolishing. Usually, no more than 50μm of material is removed, so rough surfaces get smoother but not perfectly flat. Still, this process is great for removing burrs because burrs dissolve quickly.
Functionally, electropolished surfaces are clean, reduce stress concentration, and resist corrosion better. The electrochemical reaction cleans away dirt, grease, and other contaminants. Since it reveals tiny bacteria and dirt, it’s easier to sterilize the surface thoroughly. Iron dissolves more easily than other metals during electropolishing, leaving a chromium-rich layer on stainless steel. This layer reacts with oxygen to form chromium oxide, passivating the surface and protecting the steel from corrosion. Besides protection, this layer looks shiny, making it seem like the metal is chrome-plated.
Design Opportunities
Since the workpiece stays in a solution during electropolishing, it has several advantages over mechanical polishing. It works well for both small technical parts and large structures. The processing time doesn’t depend on the size or complexity of the part. It doesn’t put any mechanical stress on the part, so it’s perfect for cleaning and polishing delicate parts that can’t handle mechanical methods.
This process is also great for parts where surfaces need to touch each other. It removes burrs and reduces friction between mating surfaces, like in threaded components.
Design Considerations
The quality of the surface treatment depends on the metal’s surface condition before electropolishing, so preparation is key. Marks from dull tools in forming or cutting equipment, deep scratches, and other flaws won’t go away during electropolishing. In fact, they may stand out more as the surface gets smoother. For metal products like castings, sandblasting can create an even surface that’s easier to work with.
Suitable Materials
Most metals can be electropolished, but this process is often used on stainless steel, especially austenitic stainless steel. Special electropolishing setups are usually needed for different materials because you can’t polish them together or even in the same electrolyte solution. Aluminum and copper can also be treated this way.
Processing Costs
There are no mold costs, but you need fixtures for electrical contact.
The processing time depends on how much material needs to be removed and how clean the part is.
Since the process is often fully or partly automated, labor costs are relatively low. Electropolishing adds about 5% to the base material cost, while metal electroplating adds 20%.
Environmental Impact
This process has three positive effects on the environment. First, it’s replacing chrome-plated steel because electropolishing uses fewer harmful chemicals, less water, and is simpler. Second, it boosts the material’s natural corrosion resistance and extends the life of stainless steel. Third, since it removes material instead of covering it, it uses less material and avoids problems like delamination.
About 25% of the chemical solvents need to be replaced each year, but chemical costs are a small part of the total expense. Compared to other processes, electropolishing is less harmful to workers, and the waste is easier to deal with.
Electropolishing Process
Technical Explanation
The electropolishing process has three stages: pre-cleaning (if needed), polishing (as shown in the figure), and a final cleaning to remove chemical contaminants.
This process takes place in a tank filled with an electropolishing solution made of phosphoric acid and sulfuric acid. The temperature of the tank is kept between 50°C and 90°C, depending on how fast the reaction needs to be. The higher the temperature of the solution, the faster the reaction. The workpiece is hung on a charged fixture and acts as the anode. The cathode, usually made of the same material as the workpiece, is also placed in the electropolishing solution. In electroplating, the workpiece would be the cathode.
When an electric current passes between the cathode and the workpiece, the electropolishing solution dissolves metal particles from the workpiece’s surface. The surface dissolves faster at the peaks because that’s where the power density is highest. Areas with lower power density take longer to dissolve. This gradually makes the material’s surface smoother.
Electropolishing has become a widely accepted way to treat metal surfaces. In construction and structural metal parts, it’s used for both its appearance and functionality (corrosion resistance and reducing stress concentration). In the pharmaceutical and food industries, it’s mainly used for functionality (hygiene and corrosion resistance). Aesthetic applications account for over 90% of all uses.
After electropolishing, the parts are neutralized, rinsed, and cleaned.
Case Study: Electropolishing Steel for Construction
This case shows the latest developments in electropolishing technology. The equipment has been improved to reduce wastewater and increase environmental friendliness.
During the process, first, test pieces are inspected, and the electropolishing speed is measured accurately (Figure 1). The removal depth in electropolishing is usually around 40 μm, but in this sample, it was 30 μm less. This also shows how long it takes to remove material in the electrolyte.
These are stainless-steel spider-shaped parts used in buildings to protect window glass (Figure 2). They are fixed with fixtures that make electrical contact (Figure 3). This special process only needs a few tanks to effectively electropolish, neutralize, and rinse the parts. They are mounted on a rotating frame and hung above the tank (Figure 4).
They soak in the electrolyte for about 10 minutes. When they are taken out, the acidic mixture flows back into the tank (Figure 5). After the polishing is done, all that’s left is neutralizing and rinsing.
After rinsing, the spider-shaped parts are hung to drain (Figure 6). Then, they are cleaned one last time to remove any remaining contaminants. Once the parts are dried by heating (Figure 7), they can be safely handled, packaged, and shipped.
About the Author: Gavin Xia
This article was written by engineers from the RAPID PROTOS team. Gavin Xia is a professional engineer and technical expert with 20 years of experience in rapid prototyping, metal parts, and plastic parts manufacturing.
