The process is often used for deburring and passivation of workpieces made of metals such as stainless steel. Since the process does not cause any mechanical or thermal stress, it is suitable for parts of any shape and size.
In electropolishing, the workpiece is placed in a suitable electrolyte. A thin layer of material is removed by applying an electric current. Thus, the process represents the exact reverse of galvanizing.
First, in electropolishing, the workpiece is positively charged in a plastic bath. The electrolytes filled into the bath act as conductors and remove metal ions as soon as current flows. The oxygen released during the process enhances the effect.
When treating stainless steel by electropolishing, it should be noted that the alloying elements dissolve at different rates. Thus, a chromium-rich surface often results because iron and nickel atoms dissolve from the crystal structure faster than chromium atoms. This leads to higher corrosion resistance because passivation is accelerated.
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 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?
Electropolishing 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, 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 often 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 often 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.