Considerations for electropolishingShawn Martin | July 24, 2019
Electropolishing is an anodic surface treatment of metallic parts. Commonly referred to as "reverse plating," a rectified current and a blended chemical electrolyte bath collectively act to polish and deburr metallic surfaces. While the process offers several advantages over mechanical treatments, there are considerations that must be made before treatment.
Electropolishing is basically controlled chemical erosion of a metallic surface. However, with proper set up, the resultant surface is clean and bright. It is not labor intensive and it provides stress relief and acts to remove oxides and other impurities. It also acts to deburr and polish parts simultaneously and a wide range of electrolyte solutions are available to address surface treatments of different metals and metal alloys.
Considerations that should be taken into account before electropolishing have to do with process limitations and part requirements. Establishment of proper engineering controls and thorough process development allows for high precision with material removal ranging between 0.0005 in and .001 in and tolerances ranging between +/- 0.0001 in to 0.0002 in.
The first consideration that should be taken into account with electropolishing has to do with the degree by which surface roughness can be improved. The smoothness of a surface is a function of the average distance between microscopic peaks and valleys and the unit of surface roughness, Ra, is the mean deviation from the assessed profile and is measured either in micrometers or microinches.
The electrochemical behavior of electropolishing has preferential stock removal across peaks, while valleys experience lower activity, however, there is a case of diminishing returns. Activity across the valleys increases as material is removed and there is a general 50% guideline, meaning that the best possible reduction in surface roughness is 50% and a surface machined to 32 Ra can only be reduced to 16 Ra through electropolishing.
Another consideration that must be evaluated with electropolishing is that the electrochemical process is indiscriminate and material is removed across the entire part. When high precision parts are being treated, the amount of material being removed needs to be taken into account to help ensure critical features aren’t undersized post-treatment.
One last consideration to take into account with electropolishing has to do with the electrolyte bath and racking of parts. In electropolishing, work pieces are positively charged and act as the anode. Stock materials are dissolved in an electrolyte solution and then collected by a cathode. Part geometry, racking of the parts and placement of the cathode can all affect the uniformity of the current density. This can lead to non-uniform material removal between parts or across the geometry of the part to be treated. For this reason, pre-production sampling is almost always required to ensure desired surface profiles are achieved, that parts are racked adequately and that the cathode has been properly located.