Examples of the Affect of Electropolishing
20 Ra w/ EP. That’s a pretty common surface finish spec for a piece of stainless steel equipment to be used in a pharmaceutical application. But what does it mean? Well, the 20 Ra is the roughness average of the product surface. It’s a measurement of texture. As discussed in previous posts, a Ra value is the arithmetic mean of the vertical deviations of the roughness profile from a mean line. Basically, it measures the smoothness of a surface. Surface roughness plays an important role in determining how a surface interacts with its environment.
Rougher surfaces create more friction and wear quicker. Surface irregularities are nucleation sites for not only cracks and corrosion, but also bacteria. Do you know what surface roughness is not? It is not electropolish alone. While our polishers are good and they can create close to mirror surface finishes, mechanical polishing cannot create the surface attributes that electropolishing does. And no amount of EP will replace a well-trained craftsman, as we like to think of our polishers. This post will focus on what electropolishing is, and isn’t and why we use it in the high purity process world.
Electropolishing is the effective removal of particulate and contaminants on a surface by means of an electro-chemical reverse plating process. As alluded to above, to achieve a low Ra value, components are mechanically polished and buffed. In most cases electropolishing will marginally improve a surface that that has a good mechanical polish but it does several other important things. Polishing leaves compounds and some foreign matter embedded deep in the surface of a piece of metal. Electropolishing removes the top layer of metal along with the contaminants in the base metal. This not only removes impurities and surface material, but creates an oxygen rich environment that allows for the formation of an excellent chrome oxide layer and renders the material passive.
Electropolishing is essentially a reverse plating process. The set up for EP actually looks quite a bit like a plating line. The first step in electropolishing is to take the sinusoidal AC current coming out of the wall and rectify to a low current DC voltage. A series of lead and copper cathodes are then lowered into a rubber lined tank and connected to the negative side of the power source. The part to be polished is fixed to a rack and connected the positive side of the power source. This charges the part positive.
Once we have that positive charge, the part is immersed in a chemical bath. The chemical bath, made up of several different electrolytes, conducts the charge from the DC power source and leads to ion removal from the stainless surface.
The electropolishing process removes material in the peaks of a surface faster than in the valleys, thereby reducing microscopic peaks and causing a leveling action. This improves the surface finish valves if the material has been properly mechanically polished.
While the ions are drawn to the cathode, the electrolyte solution helps keep the dissolved metals in solution, preventing them from falling out of solution and recoating the parts. Because this is also a chemical reaction, oxygen bubbles or gassing occurs at the metal surface, furthering the cleaning process.
After the chemical bath, the parts are run through a series of cleaning steps to remove any excess electrolyte. The end product is a bright, clean surface. This finished surface reduces product adhesion, increasing cleanability and preventing product build up.
While there are many different applications for electropolishing, it is used in the pharmaceutical industry to improve surface uniformity, cleanability, and prevent attack from aggressive chemicals by rendering parts passive. While small surface finish improvements are achieved, electropolishing is meant to complement, not replace, mechanical polishing.
Electropolishing is as much an art as it is science. Because it does involve surface material removal, electropolishers must be careful not to remove too much product. This can lead to burn through and discoloration which will require the reworking of parts. At Holland, we work with subcontractors for our electropolishing that polish in accordance with ASTM B912 and ASME BPE specifications.
In conclusion, mechanical polishing is not a substitute for electropolish and vice versa. To obtain the optimal pharmaceutical grade surface finish you really need both. Holland has been having parts electropolished for decades and we make daily trips to subcontractors to pick up and drop off parts to be EP’ed. If you have a question about your products or process surface finish, contact a Holland Sales engineer today.