At first glance electrolytic cleaning and electropolishing look to be identical procedures. They both involve creating an electrical circuit between the stainless steel being processed and the tool being passed over it. Furthermore, both processes use the same electrolytic cleaning solution to aid in removing the contaminants. However, the two procedures not only have different purposes, they have correspondingly different results.
Both procedures are primarily performed on austenitic stainless steel, primarily the 300 series metals such as 304, 316 as well as AL6XN. However, they can also be performed on other types ranging from 201 on the low end to Hastelloy on the high end. It is fair to say that electropolishing is a maximal version of electrolytic cleaning in that the former goes beyond what the latter can do. But, that “beyond” is not always desired in terms of the time or the additional effects.
Simply put, electrolytic cleaning, also known as “electrocleaning” or “electrochemical cleaning”, is used to remove metallic contaminants along with some of the non-metallic ones from the surface of the piece being worked on. The most common purpose is to remove rouge, the discoloration caused by iron oxide deposits on the surface of stainless steel. Primarily orange or red, but ranging to brown or black. Rouge rests on or in the chemically-inert “passive” layer of Chromium Oxide (Cr2O3) that protects the underlying metal. Most often rouge comes from free iron introduced through the water or degradation of non-stainless parts elsewhere in the system.
Because of its location, rouge is not as tightly bonded to the surface as deeper contamination and it can be quickly removed by electrolytic cleaning. The tool is passed over the metal’s surface much faster than during an electropolishing service. In some cases, electrolytic cleaning It is a very effective method for removing heat discoloration caused by welds or other applications of heat to the surface. This tinting of the metal is usually limited to only the surface of the metal and does not require deeper processing afforded by electropolishing.
By virtue of its speed, electrolytic cleaning does not alter the actual surface of the material. For that reason, if there is an area of roughness on the surface of the stainless steel that would cause rouge to accumulate, this process will not change that profile and rouge may re-form in that area if free iron is again introduced within the system.
As was referenced earlier, electrolytic cleaning does not alter the composition of the metal. By contrast, electropolishing dissolves the actual surface of the metal. It reduces surface roughness on a microscopic level. Between two and four ten-thousandths (2/10,000 -4/10,000) of an inch of material is removed to produce an extremely smooth and extremely pure profile. This material removal affects the higher “peaks” of the surface more so than the lower “valleys” and it is not sufficient to remove all but the most microscopic of gouges from the surface. That would require mechanical polishing to occur beforehand. Hence, electropolishing is usually performed after every mechanical polishing service to remove any remaining residue as well as the small amounts of unevenness left on the surface by even the finest abrasives.
For many applications a totally smooth surface is critical. The common industrial measurement of surface smoothness is “Ra” or roughness average. It is an arithmetic average of the surface heights over an evaluation length. For biopharmaceutical applications, an Ra of 15 or less is required and it should be noted that Astro Pak’s technicians are regularly able to surpass this with Ra measurements in the low single digits. This smoothness makes it harder for biofilms to stick to the surface where they can grow into colonies that can contaminate entire batches of product. Similarly, the smoothness prevents the materials being processed from embedding in the surface of the vessel. Embedded material damages the passive layer and exposes the underlying alloy to system contents triggering a potential chemical reaction as the metal is directly exposed to any oxygen in the water or atmosphere within the structure. Should the oxygen interact with the iron ions within the metal, then it will form iron oxide, or rouge (rust). Not only does the presence of rouge affect the purity of the contents, but it can affect the surrounding metal as well, leading an unplanned shutdown of the unit as well as the potential loss of one or more batches of product.
In addition to smoothing the stainless steel to prevent corrosion, electropolishing also restores the passive by removing iron atoms from the surface, leaving a layer of alloy a few microns thick that is predominantly chrome and nickel. This layer serves to protect the deeper bulk alloy. It is important to remember that while this level of passivation for 200, 300 and 400 series stainless steels is sufficient to meet ASTM B912 standards, it is not as corrosion resistant as a full passivation treatment where the preferred chrome to iron ratio is 1.5 to 2.0 as achieved with Astro Pak’s Ultra Pass® passivation. As a result, electropolishing is most often the step performed right before passivation to ensure the “best” surface possible for any high purity application.
The Final Word
Electrolytic Cleaning is the proper procedure for removing iron oxide surface contamination quickly yet thoroughly and returning the serviced unit back into production in a timely manner. Electropolishing goes deeper into the metal itself, smoothing and cleaning it to enhance its resistance to corrosion.