Guide
Electropolishing vs passivation: when should you electropolish stainless steel parts?
Electropolishing is an electrochemical process that dissolves a thin, controlled layer of metal from a stainless steel part, leaving a smoother, brighter and cleaner surface; passivation, by contrast, is a chemical cleaning step that removes free iron without removing measurable metal. When you electropolish stainless steel you are changing the part, not just cleaning it, and that decides what the drawing has to say about dimensions, roughness and verification.
By Saola Manufacturing engineering team · Published · Updated
This guide is for design engineers and buyers who specify the last treatment on machined stainless parts for medical, food, bioprocess and semiconductor service. It works from the published scopes of ASTM B912, ASTM A967 and ASTM A380, the SAE AMS 2700 passivation specification and the ASME BPE surface finish designations. Shop practice is labeled as typical industry guidance. Check the revision your drawing cites before relying on any detail.
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What does each process actually remove from a stainless surface?
Passivation removes free iron and other foreign matter from the surface and leaves the stainless steel itself essentially untouched. Electropolishing dissolves a layer of the alloy, and takes free iron, micro-burrs, heat tint and the tops of the roughness peaks away with it.
ASTM A380 is direct about what passivation means: for the purposes of that practice, it is the removal of exogenous iron or iron compounds from the surface by chemical dissolution, without significantly affecting the stainless steel. The protective chromium-rich oxide then forms on its own in air. On a machined part the iron comes from steel chips, shared coolant, steel fixtures, wire brushes and tumbling media. Nothing is added and nothing measurable is taken off, so a passivated part looks and measures the same as it did before the bath.
Electropolishing works the other way round. The part is made the anode in a concentrated acid electrolyte, as typical industry guidance usually a phosphoric and sulfuric acid blend, and direct current dissolves metal from its surface. A viscous film forms over the part; the film is thinner over microscopic peaks than in the valleys, so the peaks carry more current and dissolve faster. The result is a leveled, bright surface with fewer sites for dirt, bacteria or particles to hold on to.
The published scope of ASTM B912 spells out what that achieves: surface passivation occurs at the same time as electropolishing under proper operating conditions, free iron is removed, the smoothing itself improves corrosion resistance, and the process also removes heat tint and oxide scale. That last point matters on welded assemblies, where passivation alone does not touch the discoloration and ASTM A380 points to pickling or descaling first. In short, passivation is a cleaning step and electropolishing is closer to a controlled, very light machining operation done with current.
How do ASTM B912, ASTM A967 and AMS 2700 differ?
ASTM A967 and SAE AMS 2700 specify chemical passivation treatments, mainly nitric and citric acid, while ASTM B912 specifies passivation achieved by electropolishing. All three judge the result by the absence of free iron, so roughness and stock removal have to be called out separately.
The current ASTM A967/A967M-25 covers immersion in nitric acid solutions, immersion in citric acid solutions and electrochemical treatments, with seven test practices, A to G, that range from water immersion and salt spray to the copper sulfate and ferricyanide spot tests. It leaves the choice of treatment and test to the purchase order. SAE AMS 2700 is the aerospace counterpart; a United Space Alliance study filed with NASA summarizes the nitric and citric Methods of revision AMS 2700B and the Classes that set how often parts are tested (NASA NTRS, 2009). The current revision is AMS2700G (2025), so check those details against it. Note that the electrochemical option inside A967 is a passivation treatment, not a smoothing one; a note reading “passivate per A967” never obliges anyone to electropolish.
ASTM B912-26, the 2026 edition, covers the 200, 300 and 400 series and the precipitation-hardened grades. It requires basis material free of clearly visible defects, preparatory cleaning where needed, a post dip and final rinsing, and evaluation by one or more of five tests: water immersion, humidity, salt spray, copper sulfate and a modified ferroxyl test for free iron. What its published summary does not set is a roughness value, a brightness grade or a stock removal limit. “Electropolish per ASTM B912” on its own therefore proves that the part is passive, not that it is as smooth as the designer pictured.
| Point | Passivation (ASTM A967, AMS 2700) | Electropolishing (ASTM B912) |
|---|---|---|
| What it removes | Free iron, embedded iron particles and surface contamination | A layer of base metal and everything on it: free iron, micro-burrs, heat tint, oxide scale |
| Mechanism | Chemical immersion, usually nitric or citric acid | Part as anode in an acid electrolyte under direct current |
| Stock removal per surface | None measurable | About 2.5 to 25 µm (0.0001 to 0.001 in), set by time and current |
| Effect on Ra | None | Lower; often by a third to a half from a fine machined or polished start |
| Weld heat tint | Not removed; pickle or descale first per ASTM A380 | Removed |
| Burrs | Not removed | Micro-burrs removed; visible burrs need mechanical deburring first |
| Verification tests | Practices A to G: water immersion, humidity, salt spray, copper sulfate, ferricyanide, damp cloth, boiling water | Water immersion, humidity, salt spray, copper sulfate, modified ferroxyl |
| Handling | Baskets or racks; small parts can run in bulk | Each part racked with a firm electrical contact, which leaves a contact mark |
| Relative cost | Lower | Higher, driven by racking, masking and current-density control |
How much does electropolishing change dimensions and surface roughness?
Electropolishing removes metal, so every finished dimension moves, and edges and outside corners lose more than flat faces or bores. It lowers roughness in proportion to the starting finish: it refines a good machined or polished surface, but it does not erase tool marks.
Typical stock removal is 0.0001 to 0.001 in (about 2.5 to 25 µm) per surface, set by time and current density. Because a diameter has two surfaces, a turned outside diameter typically shrinks by twice the per-surface removal and a bore grows by the same amount. Current concentrates on edges, thread crests and outside corners, so those lose more and come out slightly rounded; recesses, slots, blind holes and long bores receive less current and, as typical industry guidance, finish poorly unless the processor inserts an auxiliary cathode. A tight fit dimension therefore needs either an allowance cut into the machined size or a mask.
Electropolishing levels short-wavelength roughness, not waviness. Turning feed marks, chatter and grinding scratches stay visible, and often look sharper on a bright surface. As typical guidance, the roughness average drops by roughly a third to a half from a fine starting surface, which is why the industries that need very smooth wetted surfaces specify a mechanical polish first and electropolish after. The ASME BPE standard for bioprocessing equipment builds this into its surface finish designations, summarized below. Measure roughness before and after, with the cutoff and traverse stated on the drawing per ASME B46.1 conventions, and say on which surfaces the limit applies.
| Designation | Ra max (µin) | Ra max (µm) | Process |
|---|---|---|---|
| SF1 | 20 | 0.51 | Mechanically polished |
| SF2 | 25 | 0.64 | Mechanically polished |
| SF3 | 30 | 0.76 | Mechanically polished |
| SF4 | 15 | 0.38 | Mechanically polished, then electropolished |
| SF5 | 20 | 0.51 | Mechanically polished, then electropolished |
| SF6 | 25 | 0.64 | Mechanically polished, then electropolished |
When should you electropolish stainless steel instead of passivating it?
Require electropolishing when the surface itself is a functional requirement: cleanability, low particle shedding, no micro-burrs, removal of weld heat tint, or a roughness limit that machining alone cannot meet economically. If the only goal is to stop rust from shop contamination, passivation per ASTM A967 does the job without moving a dimension.
Use these as starting rules, then check them against the end customer’s own specification:
- Bioprocess and pharmaceutical wetted parts. Where the specification calls out ASME BPE SF4, SF5 or SF6 on product-contact surfaces, electropolishing is part of the designation; SF1 to SF3 are mechanically polished only. Non-contact faces usually only need passivation.
- Semiconductor gas and chemical wetted parts. High-purity components are commonly electropolished and characterized to SEMI F19, which covers the surface condition of stainless steel wetted surfaces, including surface defects and surface chemistry as well as roughness. A part that is only passivated is not what those buyers expect.
- Food and dairy equipment. The 3-A Sanitary Standards commonly call for product-contact surfaces at least as smooth as 0.8 µm (32 µin) roughness average, usually reached by machining or mechanical polishing. Electropolishing earns its cost on crevices, cross holes and welded joints that are hard to clean.
- Medical instruments and implants. ASTM F86 covers surface preparation and marking of metallic implants and includes passivation. Instrument makers often add electropolishing to remove micro-burrs in serrations, slots and cannulas that a passivation bath cannot reach.
- Vacuum and cleanroom hardware. A leveled surface has less true area to hold moisture and particles, so electropolishing is typical industry guidance for chamber parts and fittings.
- General industrial machined parts. Brackets, shafts, fasteners and housings rarely gain anything from electropolishing that justifies the dimensional change and cost. Passivate them.
One more test is worth applying: if the requirement can be checked only with a profilometer, a particle count or a surface chemistry analysis, it is an electropolishing requirement. If it can be checked with a copper sulfate drop or a humidity cabinet, it is a passivation requirement.
Which stainless grades and part features cause trouble?
Free-machining grades such as 303 and 416 electropolish poorly because their sulfide inclusions dissolve and leave pits, and threads, sharp edges and blind holes do not finish evenly. Choose the grade and the features with the finish in mind before the part is quoted.
The austenitic grades 316L, 316 and 304 respond best. The free-machining bars covered by ASTM A582 machine well because of added sulfur in most grades, but the manganese sulfide stringers that break the chip are exactly what an electropolishing bath attacks first; the result is a frosted or pitted surface that no rework will clear. Martensitic and precipitation-hardening grades fall within the scope of ASTM B912 and can be electropolished, but as typical guidance they come out less bright than 316L. Coarse grain or heavy segregation in the bar shows up as an orange-peel texture, so material condition counts as much as the grade name.
Electropolishing reveals defects rather than hiding them, which is why B912 asks for basis material free of clearly visible defects. Scratches from handling, porosity in a weld and inclusions in the bar all become more visible after the bath. Large burrs must be removed mechanically first; the process rounds and removes micro-burrs, not a rolled-over edge.
Threads deserve a decision on the drawing. Crests lose more metal than flanks and roots, so a thread gaged before the bath can gage differently after it, particularly a class 3A or 3B fit. The options are to mask the thread, to cut it to allow for removal and gage after finishing, or to accept a looser class. Rack contact points also leave a small unfinished mark; tell the processor where a contact is acceptable, such as a non-wetted face or a hidden shoulder.
What should the drawing callout and the RFQ say?
The callout should name the specification and revision, the roughness limit and where it applies, whether dimensions apply before or after the finish, and the verification test. The RFQ should add the grade, masking, rack-contact zones and certificate content so the finisher quotes the part you designed.
Generic wording to adapt, not any supplier’s standard:
- ELECTROPOLISH PER ASTM B912-26 ALL OVER EXCEPT THREADS (MASK). SURFACES MARKED ◇ ROUGHNESS [VALUE] MAX AFTER ELECTROPOLISH. ALL DIMENSIONS APPLY AFTER ELECTROPOLISH. VERIFY PER B912 [TEST], EACH LOT. Wetted or cleanable parts with a roughness requirement.
- PASSIVATE PER ASTM A967/A967M-25, NITRIC OR CITRIC AT PROCESSOR’S OPTION. VERIFY PER PRACTICE [LETTER], EACH LOT. NO DIMENSIONAL CHANGE PERMITTED. General machined stainless parts.
- PRODUCT-CONTACT SURFACES ASME BPE [SF DESIGNATION]. ALL OTHER SURFACES PASSIVATE PER ASTM A967/A967M-25. Bioprocess components with mixed surfaces.
The mistakes that cause rejections are predictable. A roughness target with no starting finish leaves the machinist guessing how fine to cut. Tolerances that do not say before or after finishing invite a dispute over who owns the removal. Calling both B912 and A967 without saying which one governs acceptance doubles the testing for no gain, and calling only A967 when the real need is smoothness gets you a clean but rough part. A free-machining grade chosen for cycle time, then sent to electropolishing, is the most expensive mistake of all, because it shows up only after the parts are made.
For the RFQ, send the grade and bar specification, the callout as written above, the starting and final roughness and the surfaces they apply to, the stock removal you can accept, the masked features, acceptable rack-contact zones, the verification test and sampling, and what the certificate of conformance must report: specification and revision, lot, test result, and the name of the shop that ran the bath if it is not the machining supplier. Ask also how parts are packed, since contact with carbon steel bins can put iron straight back on a finished surface.
On Saola’s side, the surface finishing page lists the eight finishes our partner plants apply, and the quote names where each step is done; neither electropolishing nor passivation is on that menu today. If your drawing calls for either, say so in the RFQ so the finishing route is settled before tooling rather than after, and send the drawing for a quote with the callout written exactly as you intend it.
Frequently asked
Do parts need passivating after electropolishing?
Usually not as a separate treatment. ASTM B912 notes that passivation occurs at the same time as electropolishing under proper operating conditions, and its process includes a post dip and final rinse. Some medical and bioprocess drawings still add an ASTM A967 step afterward; if yours does, state which specification governs acceptance so the parts are not tested twice against two different criteria.
Can electropolishing fix a rough machined surface?
No. It removes micro-peaks and lowers the roughness average, but feed marks, chatter and scratches survive and often become more visible. Machine or mechanically polish close to the target first, and treat electropolishing as the final refinement.
Will electropolishing change thread fit?
It can. Thread crests carry more current and lose more metal than the rest of the part, so either mask the thread, cut it to allow for removal and gage it after finishing, or accept a looser fit class. Put that decision on the drawing rather than leaving it to the processor.
Is an electropolished part more corrosion resistant than a passivated one?
Generally yes, for the same grade. Both remove free iron, but ASTM B912 notes that the smoothing itself improves corrosion resistance, and a leveled surface gives fewer crevices for chlorides and soil to sit in. Grade still matters more than finish: an electropolished 303 will not outperform a passivated 316L in a chloride environment.
Does electropolishing remove weld discoloration?
Yes. ASTM B912 states that electropolishing removes heat tint and oxide scale. Passivation alone does not; ASTM A380 treats descaling and pickling as separate steps that have to come before it.
Sources
- ASTM B912-26, Standard Specification for Passivation of Stainless Steels Using Electropolishing, ASTM International, 2026
- ASTM A967/A967M-25, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, ASTM International, 2025
- ASTM A380/A380M-25, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems, ASTM International, 2025
- SAE AMS2700G, Passivation of Corrosion Resistant Steels, SAE International, 2025
- Citric Acid Passivation of Stainless Steel, United Space Alliance and NASA, 2009 (summaries of ASTM A967 and AMS 2700B requirements)
- ASME BPE-2026, Bioprocessing Equipment, ASME, 2026
- ASTM F86-21, Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants, ASTM International, 2021
- ASTM A582/A582M-22, Standard Specification for Free-Machining Stainless Steel Bars, ASTM International, 2022
- Surface Finish and Polishing for Hygienic Food Processing Equipment, 3-A Sanitary Standards, Inc., accessed October 2026
- ASME B46.1-2019, Surface Texture (Surface Roughness, Waviness, and Lay), ASME, 2019