The Electropolishing Process
Commercial applications for electropolishing have been in use since the early 1950s. Most formal research on the process occurred at that time, motivated by the growth and sophistication in electroplating technology. Today there are about 500 industrial installations nationwide, and perhaps several dozen electropolishing job shops.
Electropolishing is often referred to as a “reverse plating” process. Electrochemical in nature, electropolishing uses a combination of rectified current and a blended chemical electrolyte bath to remove flaws from the surface of a metal part.
The typicalinstallation is deceptively similar to a plating line. A power source converts AC current to DC at low voltages. A tank typically fabricated from steel and rubber-lined is used to hold the chemical bath. A series of lead, copper or stainless steel cathode plates are lowered into the bath and installed to the negative (-) side of the power source. A part or group of parts are fixtured to a rack made of titanium, copper or bronze. That rack in turn is fixtured to the positive (+) side of the power source.
As the adjoining illustration depicts, the metal part to be electropolished is charged positive (anodic) and immersed into the chemical bath. When current is applied, the electrolyte acts as a conductor to allow metal ions to be removed from the part. While the ions are drawn towards the cathode, the electrolyte maintains the dissolved metals in solution. Gassing in the form of oxygen occurs at the metal surface, furthering the cleaning process.
Once the process is completed, the part is run through a series of cleaning and drying steps to remove clinging electrolyte. The resultant surface is clean and bright. In fact, the bright surface is the most identifiable trait, and the one that helped coin the name “electropolishing.”
Benefits of Electropolishing
While the process is best known for the bright polish left on a surface, there are some important, often overlooked benefits of this metal removal method. These benefits include deburring, size control, microfinish improvement and others. Deburring and other metal improvement benefits offer great promise to design and production engineers for cost savings and product improvement
Although the process is roughly 65 years old, substantial refinements have taken place. Many electrolytes have been developed to allow for electropolishing of a broad range of metals. These newer electrolytes, together with advanced parts handling techniques, have combined to improve production yields on a wide range of metal products.
Electropolishing Superior to Passivation
In the area of metal finishing there is a lot of confusion about the difference of electropolishing and passivation. The major difference between the two operations is that electropolishing removes material from a part and passivation does not. Electropolishing removes the outer skin of metal along with the imbedded contaminants. Passivation only removes the free iron and surface contaminants. When the outer skin is dissolved, not only are the imbedded contaminants removed but the surface becomes smoother eliminating defects where bacteria can harbor and corrosion can initiate. The end result is the electropolished part is 30 times more corrosion resistant when subjected to salt spray testing. In addition, the surface is much more suited for food, medical and pharmaceutical.
Because so many industries, including medical, dental, pharmaceutical, aerospace and more, utilize electropolishing for their products, it has become a common finishing procedure for many small metal parts, including:
- Fasteners – Ideal for deburring threads, reducing galling, and improving corrosion resistance; it is especially useful for marine applications.
- Springs – Ideal for reducing or removing cracks and fissures created during forming, as well as preventing premature product failure.
- Wire racks – Ideal for removing weld discoloration and creating a clean, attractive, sanitary finish, as well as for improving corrosion resistance at the weld joints.
- Blades – Ideal for creating a sharp, smooth surface. By removing, stamping, and grinding burrs, it eliminates the chance that flecks of metal will dislodge from the part.