Electropolishing of Inside Surfaces of Stainless-Steel Tubing
Authors: Chaghazardi Zahra , Wuthrich Rolf 
Published: May 2020

Electropolishing has proved to be an affordable and flexible method for improving the surface quality of metal parts. It improves surface roughness and eliminates the minor scratches and deformed layer present on the surface from previous manufacturing processes. Therefore, the final metal finish is smooth, bright and shiny. In addition to enhancement of the visual appearance of the piece, during electropolishing, a thin but dense and compact passive layer is formed on the surface that makes it hydrophobic and prevents moisture accumulation. Also, by removing rust, debris and nonmetallic inclusions that can act as potential sites for surface contamination, electropolishing yields ultra-clean metal parts with greatly improved corrosion resistance.[1–4]

Many industries can benefit from the merits of electropolishing. The mirror-like surface finish can be quite appealing for numerous decorative purposes such as appliances and automotive. Meanwhile, the cleanliness of electropolished parts has always been a reason for many industries to incorporate electropolishing. For example, the semiconductor industry demands ultra-clean distribution equipment that supplies highly pure gases and chemicals. Therefore electropolishing is commonly used for obtaining ultra-clean metal parts and preventing process contamination.

Also, nowadays, to comply with regulations for contamination control such as FDA, pharmaceuticals and food and beverage industries utilize electropolishing treatment to ensure that the inside surfaces of their piping systems, mixers, vessels, etc. are ultra-clean.

Many studies are published on the effect of different electropolishing parameters such as potential, polishing time, temperature and bath composition, on surface roughness of the workpiece. [5–9] However, only a few studies have thoroughly investigated the effect of operating conditions on the brightness of the surface. Considering the broad range of decorative applications of electropolishing, more research on this topic is justified.

Results from several studies indicate that to achieve a uniform electropolishing effect, a proper cathode to anode distance should be maintained.[10,11] This can make uniform electropolishing of internal spaces practically challenging. While many industrial equipments have internal surfaces to be electropolished, there seems to be a gap in our knowledge in this area that calls for further investigation.

In the present study, the inside walls of stainless-steel tubing were electropolished in sulfuric and phosphoric acid electrolyte using internal cylindrical copper electrodes. Effect of various parameters including the outer diameter and position of the cathodes as well as the length of the tubing, on the final quality of the surface, were systematically investigated.

It was observed that under the proper combination of electropolishing voltage and duration, irrespective of the initial surface condition of stainless-steel samples, they were all significantly brightened, and their surface roughness was decreased to almost similar final values.

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