Electropolishing vs. Pulsed Electrochemical Machining (PECM)
What is the difference between electropolishing and electrochemical machining? Electropolishing and pulsed electrochemical machining (PECM) are similar but distinct processes. While they are both advanced manufacturing processes utilizing electrochemistry, electropolishing is largely limited to surface finishing, while electrochemical machining can create more precise geometries with high repeatability.
In certain applications, PECM may act as a replacement finishing process to electropolishing, such as surface polishing to metal additive manufacturing projects. In other cases, incorporating PECM may involve making it the sole machining process to the part, as the machining and surface finishing are all included in this singular process.
In this educational article, we'll compare the similarities and differences of these distinct processes, and highlight the best applications of both.
· Set-up contains a cathode and an anode, where the workpiece operated on is the anode.
· Use of specific electrolyte solutions, for their conductive properties and influencing interactions with the workpiece surface.
· Use of electric current to dissolve workpiece surface material through oxidation.
· Material removal is performed without addition of mechanical or thermal stress.
· Creates a smooth, shiny, ultra-clean surface.
· Can only be used on conductive metals.
In electropolishing, the cathode is generically shaped, using a universal tool for almost all jobs. Conversely, PECM uses a cathode that is the inverse shape of the desired part. This works to dissolve the macro-level irregularities on the part’s surface as well as micro-level. Thus, PECM functions not only as a finishing process but also a shaping process.
Inter-electrode Gap Size
Electropolishing utilizes a large gap (> 10 mm) and its selectivity for material removal is based upon micro-scale concepts of mass transport and diffusion layer resistance. PECM utilizes a narrow gap (<0.1 mm), orders of magnitude smaller than electropolishing, which leads to selectivity based upon the resistance properties of the bulk electrolyte solution.
As material is removed, the cathode is dynamically moved closer to the workpiece, maintaining the same narrow space between the tool and the workpiece at all times. Conversely, the cathode remains stationary in electropolishing, and given the small amount of material removal and large inter-electrode gaps, movement is unnecessary. This makes electropolishing a much simpler operation than PECM.
Electrochemical material removal is linearly correlated to the rate of current you apply. PECM often operates with current densities that are 100-1000x greater than electropolishing, which leads directly to a 100-1000x increase in speed of material removal. The low current density of electropolishing means that the process is primarily diffusion controlled in terms of mass transport. In contrast, PECM operates by convection controlled mass transport via high speed electrolyte flow, which means that the rate of achievable material removal is primarily limited by how quickly the electrolyte flushes away the waste material.
The electrolytes in each process, while both conductive, are dramatically different between electropolishing and PECM. Electropolishing electrolytes are generally viscous, highly acidic, and toxic, often including a number of additives to manage the diffusion controlled processes. In contrast, PECM utilizes pH neutral, non-toxic electrolytes.
Electropolishing has a significantly lower price point than PECM, accounting for the generic tool, the comparative simplicity of its operation given the stationary cathode, its exclusive use for removing small amounts of material, and its popularity as a finishing process. Therefore, it could be a more cost-effective choice for a part that does not require strict tolerances.
However, PECM can achieve a much more precise, repeatable shape than electropolishing. This includes not only creating the smooth, shiny finish that electropolishing is known for, but also machining the metal into its final shape. This allows PECM to simultaneously achieve form tolerance while also resolving surface defects such as seams, waviness, or pits in the final part. In contrast, electropolishing is strictly a finishing process, and one that does not handle macro-level surface defects well.
The precision of PECM suits it to machining of high value parts found in aerospace, medical, defense, and industrial applications. While electropolishing may be a cost-effective method for smoothing some surfaces, PECM is more adept at creating features and surfaces directly while also delivering many of the surface finish benefits associated with electropolishing.
While electropolishing and PECM have inherent similarities, the cathode’s shape, movement, and distance from the anode suit each process for different purposes. If you would like help figuring out which one is right for you, or if you would like to discuss using PECM to shape your part, contact us.
This article is part of our ongoing “PECM vs. Competing Processes” series that compares PECM to other, popular machining processes. Find other articles in the series below:
CNC Milling vs. ECM
EDM vs. ECM