Why Are Metals Electrically Polished?
Metals are electrically polished to streamline the material. There are generally two reasons why the process is applied to work pieces:
- Preparation:The process significantly reduced surface roughness by removing atoms. This prepares work pieces for further treatments, such as boronizing. The diffusion coatings can be layered more firmly and smoothly on the surface, preventing an imperfect surface finish. We recommend, for instance, that austenitic metals (stainless steels such as 204 and 316, nickel-based alloys) are electrically polished before our BoroCoat® treatment to ensure the best efficiency. A smoother surface is key for the process.
- Aesthetics: As its name already suggests, electropolishing polishes metals. Electropolished surfaces are shining and sleek.
How does Electropolishing work?
The mechanism of electropolishing is based on electrochemical electrolysis: An electrical current forces a redox reaction of two electrodes. They are placed in a liquid electrolyte in which ions are detached and placed on the cathode and/or anode.
Electropolishing works on the same principle. The workpiece (anode) is placed in a liquid electrolyte along with a cathode, with both electrodes being connected to a power supply. Then, the workpiece is connected to the positive port while the cathode is connected to the negative port.
The anode is then charged. As the electrical current passes the work piece, the surface oxidizes. The rusted, negative particles are then emitted into the electrolyte where they are attracted to the cathode, leaving the metal smooth.
The process steps in electropolishing stainless steel
Electropolishing stainless steel typically requires three steps to achieve the best possible results.
Step 1: Preparation of the surface
In the first step, the material surface is cleaned and degreased. This is done with an alkaline solution. After this step, any contact with the metal should be avoided. After an intermediate rinse, light oxide layers are removed with the help of pickling or an acid treatment. This is followed by another intermediate rinse with the aim of removing the last residues.
Step 2: Electropolishing of the workpiece
In electropolishing, the uppermost layer of material surface is removed with the aid of anodic dissolution. As a result, the surface is polished and cleaned. The process is also used for deburring. The workpiece is now metallically pure, shiny and features the optimum properties of the material.
The electropolishing bath is normally composed of 96% sulfuric acid and 85% orthophosphoric acid mixed in equal proportions.
Step 3: Post-treatment
After the actual electropolishing process, the workpiece is subjected to post-treatment to remove possible residues. This often involves heavy metal sulfates and phosphates. Since these usually cannot be dissolved with water, post-treatment is mostly carried out with nitric acid. The removal of these residues is important, as this is the only way to guarantee hygienic storage.
What are typical applications for electropolishing?
The electrochemical process is frequently used for pipelines. In particular, when it is required that the inner and outer surfaces of the pipes remain free of foreign matter and particles and corrosion resistant, or when a clean surface is the goal. This applies to the petroleum, pharmaceutical and the food and beverage industries.
Because electropolishing results in surfaces that repel microorganisms, it is regularly used in the manufacture of medical devices. This includes surgical equipment, such as scalpels and clamps, as well as joint implants. Other hospital equipment, which typically needs to be decontaminated, are also widely processed with the procedure.
In the pharmaceutical industry, particle-free and clean surfaces are of great importance. For this reason, the insides of tanks, filters, sieves and other important utility items are usually electropolished.
Food and beverage industry
The process is also used in the food and beverage industry. On the one hand, electropolished application objects are easy to clean and have an aesthetic surface, while on the other hand they offer a high level of hygiene. This ensures excellent cleanliness and a high level of protection against bacteria.
The Benefits of Electropolishing
- It can be used on any shape and form. Parts with intricate designs are unfit for traditional abrasive polishing. For instance, washing machine drums are best to be polished by electropolishing.
- By removing metal particles, stresses are relieved from the surface
- The layer that is removed from the surface is relatively thin (2.5 µm to 65 µm).
- Reduction of scaling and removal of corrosion.
- Electropolished stainless steel show better corrosion resistance than passivated steel.
- Unlike mechanical polishing, the process does not deform surface layers.