Passivation FAQs - Electropolishing FAQs

How large of a part can you polish?

Harrison Electropolishing has four electropolishing lines operating on a daily basis. Components which can fit within our operating tanks can be processed and returned on a standard production basis (typically 3 to 6 working days). While most process equipment may fit within our operating tanks, Harrison Electropolishing has developed proprietary techniques in order to electropolish larger components. These proprietary techniques allow us to process equipment of any size. We are not limited by size, size simply dictates how we will process a part.

How smooth of a surface can electropolishing produce?

The electropolishing process may improve a surface finish by up to 50%. The electropolishing reaction removes material while it improves surface roughness. Because of the material removal, process runtimes are often limited to maintain dimensional tolerances. Limited runtimes result in real world surface roughness improvements from 10 to 35%. Remember, electropolishing improves a surface on the microscopic level. If a material has a texture or surface scratch, electropolishing will only result in a lustrous texture or lustrous scratch. Mechanical polishing is utilized to remove macroscopic texture or blemishes.

How durable is an electropolished surface?

Electropolishing is a surface treatment, not a surface coating. Thus, an electropolished surface can be physically damaged or degraded in the same way as its base material can be damaged or degraded. An electropolished 316 stainless steel surface has the same strength and hardness properties as those published for 316 material. Note however, because electropolishing may produce such a lustrous finish, even the finest surface scratch may be visible. Electropolished surfaces possess increased corrosion resistance and thus are more resilient in many corrosive environments.

What is the difference between electropolishing and mechanical polishing?

Electropolishing is an electrochemical process while mechanical polishing is a mechanical process. Electropolishing is a surface treatment that can improve surface finish as it dissolves material from the surface. Mechanical polishing, like machining, alters a surface by cutting material away from the surface. Electropolishing can improve a surface finish on a microscopic level while mechanical polishing can improve a surface finish on a macroscopic level.

What is a #4 (2B, #8 etc.) finish?

#4 finish is one of the many generally accepted, mechanical finishes which are often used in the metal finishing industry. Both the NiDi No. 9012 and previous revisions of ASME BPE specifications include a list that describes various mechanical surface finishes for stainless steel. A few common finishes include:

#4 - A widely used, general purpose finish produced by finishing with a 120-150 mesh abrasive. This finish is commonly found on architectural panels and stainless steel kitchen appliances and sinks.

#2B - A bright, cold-rolled finish. This finish is a standard finish for raw sheet metal and is produced at the mill.

#8 - A highly polished, highly reflective, "mirror like" surface. This finish is produced by polishing with successively finer abrasives followed with extensive buffing.

What does Ra mean?

As described in ASME B46.1, Ra is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys.

L = evaluation length
Z(x) = the profile height function
The digital approximation is:

What does RMS mean?

As described in ASME B46.1, RMS is the root mean square average of the profile height deviations from the mean line, recorded within the evaluation length.

L = evaluation length
Z(x) = the profile height function
The digital approximation is:

What is the difference between Ra and RMS?

Ra and RMS are both representations of surface roughness, but each is calculated differently. Ra is calculated as the Roughness Average of a surfaces measured microscopic peaks and valleys. RMS is calculated as the Root Mean Square of a surfaces measured microscopic peaks and valleys. Each value uses the same individual height measurements of the surfaces peaks and valleys, but uses the measurements in a different formula. The formulas are shown above. One can infer from examination of the formulas, that a single large peak or flaw within the microscopic surface texture will effect (raise) the RMS value more than the Ra value.

What is the difference between electropolishing and electroplating?

Electropolishing removes material while electroplating adds material. Electropolishing and electroplating have similar setups, but they involve opposite reactions. Both are electrochemical reactions, however, in electropolishing the work-piece is charged anodically (thus loses ions) and in electroplating the work-piece is charged cathodically (gains ions).

How much material is removed by electropolishing?

The amount of material that is removed by electropolishing is directly proportional to the runtime and the amp-minutes that pass through the piece. Given this, the amount of material removed by electropolishing may be controlled. Often, during a standard run, material removal is only to 0.0003" to 0.0007". If the goal of an electropolishing run requires a longer runtime, as much as 0.003" of material may be removed. It is always important to review the dimensional tolerances of a component before it is electropolished.

What is the equivalent Ra value for surfaces finish Grade Numbers?

Older drawings may use roughness grade numbers to indicate Ra values. The following table is given in ISO 1302:1992.

View Full Table
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