Electropolishing
Electrochemical material removal, alloy and starting-surface limits, dimensional allowances, and acceptance requirements for electropolished parts.
Tags: processes, finishing, electropolishing, surface-treatment
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Compare surface-finishing options before specifying the complete route.
Assumed symmetric removal of 8–12 µm per face takes a 10.020 mm pin to 9.996–10.004 mm, opens a 5.000 mm bore to 5.016–5.024 mm, and leaves a 0.500 mm wall at 0.476–0.484 mm when both faces are exposed. Check incoming size, asymmetry, edge effects and uncertainty before accepting the actual part.
Euro Inox's 2010 first edition describes preferential anodic removal at projections. Current, electrolyte efficiency and exposure time affect smoothing and material removal. Euro Inox, principles, printed pages 4–5
Establish material, process, and purpose
Identify the actual alloy, condition, product form, welds, incoming surface, and required function. Stainless-steel processing should not be transferred unchanged to aluminum, titanium, copper, or nickel alloys. Able's material-removal guidance lists different alloy/application combinations and describes alloy-specific chemistry and custom racking. Those are supplier capabilities requiring part review, not universal compatibility or removal guarantees. Able, material-removal control
Distinguish the physical finish from passivation. A chemical passivation route principally addresses surface contamination and formation of a passive stainless surface; electropolishing deliberately removes base metal and changes microgeometry. Proper stainless-steel electropolishing can also passivate. ASTM B912-26 specifically covers passivation using electropolishing for 200-, 300-, and 400-series stainless steels and precipitation-hardening alloys. Its public scope says the outcome depends on alloy, solution formulation, and operating conditions. Do not assume every bright surface is passive or that every alloy needs a separate subsequent passivation dip. ASTM B912-26
ASTM A967/A967M-25 is a separate chemical-passivation specification. Its public abstract includes nitric, citric, and electrochemical treatments; calling it exclusively a nitric-acid dip would be incorrect. Select the actual specified treatment and verification, rather than using “passivate” and “electropolish” as interchangeable purchase instructions. ASTM's catalog dates are June 16, 2026 for B912-26 and January 24, 2025 for A967/A967M-25. ASTM A967/A967M-25, ASTM B912-26
Starting condition limits the result
Electropolishing is not a substitute for correcting every machining or fabrication defect. Able's August 13, 2020 preparation guidance says deep scratches and machining lines may remain visible, and that macro burrs can require another removal method first. It also warns that adhesive residue, lubricants, and heavy scale can produce uneven treatment or resist patterns. Evaluate representative incoming parts before promising a final appearance. Able, preparing parts
Specify the initial and final texture separately. A claim such as “up to 50% Ra improvement” is a supplier-described possibility, not a rule converting every incoming Ra into an acceptable outgoing value. Lower Ra also does not prove absence of an isolated pit, crevice, or unacceptable burr. State the parameter, maximum value, units, measured surfaces, and measurement conditions needed for acceptance. Harrison's specification guide, updated October 2026, explicitly asks for a finish limit and verification rather than a bare “electropolish” note. Harrison, specifying electropolishing
Euro Inox distinguishes micro-smoothing from the condition of the underlying material. Its discussion links reduced micro-roughness with improved cleanability; that is a surface characteristic, not proof of sterile condition, biological safety, or a validated cleaning process. Those require their own applicable requirements and evidence. Euro Inox, introduction
Geometry, fixturing, and removal allowance
The whole part does not necessarily experience identical removal. Edges and projections behave differently from sheltered regions; internal passages need a suitable electrical and electrolyte arrangement. Harrison's guide calls out access for internal cathodes, masking of critical features, and preferential rounding of sharp edges and thread crests. A polished exterior therefore does not establish that a long bore received the specified treatment. Harrison, surfaces and removal
Define electrical-contact/fixture locations and acceptable witness marks. Identify edges that must remain sharp, close-fit diameters, seal lands, threads, and surfaces that must be masked. Able's February 13, 2023 specification guidance emphasizes critical-area identification and fixturing review. Qualify the rack and loading with production-representative parts; changing orientation or shielding can change which surfaces receive treatment. Able, specifying electropolishing
Express stock removal unambiguously: per surface, on diameter, or total thickness change. Specify which dimensions apply after treatment and how much removal is permitted. Do not borrow a supplier's minimum cleaning removal as an allowance for deburring another alloy or geometry. Able publishes separate removal examples by alloy and objective; its table is not a design standard. Able, material-removal control
No bath composition, current density, voltage, temperature, or cycle duration is prescribed here. These depend on the qualified alloy, geometry, electrolyte, equipment, rack, and desired removal. Request a process proposal and sample results that satisfy the drawing instead of specifying a generic time in an unspecified tank.
Original calculation: one surface allowance has several dimensional effects
Use the hypothetical 8–12 µm removal per exposed surface, with symmetric removal and unchanged form:
| Feature before treatment | Geometric change | Predicted finished range |
|---|---|---|
| External pin diameter 10.020 mm | Decreases by twice removal | 9.996–10.004 mm |
| Bore diameter 5.000 mm | Increases by twice removal | 5.016–5.024 mm |
| Wall thickness 0.500 mm, both faces exposed | Decreases by sum of face removals | 0.476–0.484 mm |
For the pin, 10.020 − 2 × 0.012 = 9.996 and 10.020 − 2 × 0.008 = 10.004. For the bore, material removal increases the opening rather than decreasing it. The wall loses 3.2–4.8% of its original thickness: 0.016/0.500 through 0.024/0.500.
A final pin tolerance of 10.000 ±0.005 mm would contain that example range, but leaves only 0.001 mm margin at either extreme. Incoming-size variation, asymmetric attack, edge effects, and measurement uncertainty could consume it. Thus this arithmetic does not establish process capability or part acceptance. Use it to identify the need for an agreed incoming allowance and final measurements.
Cleaning, inspection, and subsequent service
Plan drainage and rinsing as part of the component design and processing route. Able's preparation guidance discusses trapped electrolyte in porous welds or closed tubing and the need to address drain access in the drawing. Do not add holes or alter a welded assembly without design approval; review whether components should be processed before assembly instead. Able, welds and drain access
ASTM B912's public abstract includes preparation, post-treatment/final rinsing, and verification methods such as immersion, humidity, salt spray, copper sulfate, and modified ferroxyl testing. The applicable specification and purchase agreement determine the required tests and acceptance. A corrosion/passivity result does not establish final dimensions, texture, or residue cleanliness; inspect those separately. ASTM B912-26, public abstract
For production acceptance, retain measurements of critical dimensions and texture at defined locations, visual or accessible internal-surface inspection, and the specified passivation/corrosion evidence. Define sampling and reporting before processing. Corrosion resistance remains dependent on alloy and exposure; electropolishing does not establish universal chemical compatibility or convert a laboratory exposure into a guaranteed service life.
Protect the finished surfaces through handling and packaging. Able's 2023 guidance treats packaging needs as part of the finishing specification. If subsequent welding, machining, assembly, or handling can alter the surface, establish where the finishing and verification steps belong in the manufacturing sequence. Able, packaging requirements
Useful production handoff
Provide the part revision/alloy/condition, incoming surface and burr condition, applicable specification issue, treated and masked surfaces, final texture and dimensions, permitted removal, and contact-mark locations. Include internal-access constraints, cleaning/rinsing/drying requirements, verification and sampling, and packaging. Retain the approved sample and processing configuration as a reference when incoming condition, geometry, rack loading, or requirements change.