---
type: process
name: "Electropolishing"
category: "Finishing"
subcategory: "Subtractive"
materials: ["Metal"]
tolerances: "Removes 0.0002–0.001 in (5–25 µm) per surface, so bores grow and shafts shrink by twice that on diameter; sharp external edges break by 0.002–0.003 in (50–75 µm)"
volumes: "1 part to high-volume barrel work; small parts can be run in bulk baskets"
lead_time: "3–7 business days at a job shop; 1–2 days expedited"
url: https://manufacturingprocesses.org/processes/finishing/electropolishing
---

# Electropolishing

Electropolishing reverses plating: current dissolves the surface fastest at the peaks, leaving a bright, deburred, passive finish.

- **Category**: [Finishing](https://manufacturingprocesses.org/processes/finishing.md)
- **Family**: Subtractive
- **Materials**: Metal
- **Typical tolerances**: Removes 0.0002–0.001 in (5–25 µm) per surface, so bores grow and shafts shrink by twice that on diameter; sharp external edges break by 0.002–0.003 in (50–75 µm)
- **Surface finish**: Roughly halves incoming Ra — 32 µin (0.8 µm) in typically yields about 16 µin (0.4 µm) out; mirror results require mechanical polishing first
- **Typical volumes**: 1 part to high-volume barrel work; small parts can be run in bulk baskets
- **Lead time**: 3–7 business days at a job shop; 1–2 days expedited

## Overview

Electropolishing is electroplating run backwards. The part is the anode in a viscous acid electrolyte, and current dissolves metal preferentially from the high points of the surface profile, leaving a bright, micro-smooth, deburred and chemically passive finish. It is a removal process, not a coating: expect 0.0002–0.001 in (5–25 µm) off each surface in a typical cycle, and up to 0.0015 in (38 µm) on heavier work.

The characteristic result is a roughly 50% reduction in Ra — a 32 µin (0.8 µm) machined surface commonly finishes near 16 µin (0.4 µm) — combined with removal of the mechanically disturbed surface layer and enrichment of chromium at the surface of stainless steel. ASTM B912 recognizes electropolishing as a method of passivating stainless. It is the standard finish for pharmaceutical, semiconductor, food and vacuum components, where cleanability and low particle entrapment matter more than appearance.

## How it works

1. **Clean and rack.** Oil, drawing compound and shop soil must be gone before the part enters the bath. Racking is critical: current has to enter the part somewhere, and that contact point does not polish.

2. **Immerse and energize.** The part hangs as the anode in a phosphoric/sulfuric acid electrolyte typically run at 100–190 °F (38–88 °C), with cathodes positioned around it. Current density is usually in the 50–500 A/ft² range depending on alloy and bath.

3. **Anodic dissolution.** A viscous, ion-rich boundary layer forms at the surface. It is thinner over peaks than over valleys, so peaks see lower resistance, higher local current density and faster dissolution. The profile flattens from the top down. Removal rate is roughly proportional to current density and time, so the operator controls stock loss by the amp-minutes applied.

4. **Rinse and neutralize.** Multi-stage rinsing followed by a neutralizing dip removes entrained acid, especially from blind features and threads.

5. **Passivate (inherent).** On austenitic stainless, the iron dissolves preferentially, leaving a surface enriched in chromium that immediately forms a passive chromium oxide film. This is why an electropolished part usually needs no separate passivation step.

### What it does and does not fix

Electropolishing levels micro-roughness — the fine peaks left by machining, grinding or blasting. It does not level waviness or long-wavelength form error, and it will not remove a deep scratch, a tool witness line, a weld undercut or a pit. Those defects come out of the tank brighter and often more visible, because the surrounding surface is now mirror-like. Grinding, sanding or polishing to a uniform starting Ra is the prerequisite; see the [surface finish chart](/charts/surface-finish-chart) for the bands each prep leaves.

## Design guidelines

### Start from a uniform surface

The finish is only as good as what goes in. Electropolishing roughly halves Ra, so a 63 µin (1.6 µm) as-machined surface comes out near 32 µin (0.8 µm), and a 16 µin ground surface comes out near 8 µin. If a mirror finish is the requirement, mechanical polishing precedes the tank; the electropolish then brightens and passivates it.

### Expect edges and corners to round

Current density is highest at edges, external corners and thin sections, so those areas dissolve fastest. A sharp corner will visibly break, and thin knife edges can lose 0.002–0.003 in (50–75 µm). Where a sharp edge is functional — a sealing land, a cutting edge, a mating datum — mask it or move the operation earlier in the routing.

### Budget the stock loss on toleranced features

Removal is 0.0002–0.001 in (5–25 µm) per surface, so a bore grows and a shaft shrinks by twice that on diameter. Machine to the pre-polish dimension and state on the drawing which condition the tolerance applies to.

### Threads and blind features are problem areas

External thread crests polish aggressively while roots polish little, and internal threads may barely polish at all because current cannot reach them. Blind holes, deep bores and internal passages trap electrolyte and need designed drainage — a cross-drilled vent or a through-hole. Anything that traps acid becomes a corrosion site later.

### Choose the alloy deliberately

300-series austenitic stainless electropolishes exceptionally well and is the default. 400-series martensitic and precipitation-hardening grades polish acceptably but less brightly. Free-machining grades with sulfur (303) leave sulfide inclusions that pit and streak. Cast and sintered parts reveal their porosity. Copper alloys, nickel alloys, aluminum and titanium can be electropolished in specialized baths.

### Provide a rack contact location

The contact point does not polish and often carries a burn mark. Designate an acceptable location — an internal thread, a mounting hole, an end face that gets machined later.

| Feature | Recommended | Limit | Why |
| --- | --- | --- | --- |
| Stock removal per surface | 0.0002–0.001 in (5–25 µm) | 0.0015 in (38 µm) | Removal scales with amp-minutes |
| Ra improvement | About 50% of the incoming value | — | Micro-peaks dissolve first; waviness does not |
| Incoming Ra for a bright result | ≤ 32 µin (0.8 µm) | — | The process brightens, it does not level |
| Sharp external edges | Mask, or accept 0.002–0.003 in break | — | Current density peaks at edges |
| Blind features | Provide a drain or vent path | — | Trapped electrolyte causes later corrosion |
| Best alloys | 304, 316, 316L | Avoid 303 and free-machining grades | Sulfide inclusions pit and streak |

## Cost drivers

Electropolishing is quoted per part or per rack over a lot minimum, and is materially more expensive per unit area than passivation because it consumes electricity, bath chemistry and rack time.

- **Rack density and fixturing.** Each part needs its own electrical contact and a clear path to a cathode. Complex internal geometry may need dedicated internal cathodes, which is tooling.
- **Incoming surface condition.** A part that must reach a specified Ra requires mechanical prep first, and that prep usually costs more than the electropolishing.
- **Alloy and thickness of removal.** Heavier removal means more amp-minutes and more bath consumption.
- **Verification.** Ra measurement, passivation testing to ASTM A967 practices, and documentation for pharmaceutical or semiconductor work are real line items.
- **Masking.** Sealing lands, threads and mating datums that must not be attacked all require masking labor.

Cost-reduction tactics:

1. Specify the Ra you need, not the brightest available — every step down the finish scale multiplies the prep cost.
2. Design a rack contact point in a non-critical area so the shop does not choose one for you.
3. Give internal passages a through path so they can be flushed rather than individually cathode-fixtured.
4. Avoid 303 and other free-machining grades on parts that will be electropolished; 304 and 316 cost less to finish.
5. Where the requirement is only corrosion resistance and cleanliness, price passivation against electropolishing — passivation is far cheaper and adds no dimensional change.

## FAQ

### How much material does electropolishing remove?

A typical cycle removes 0.0002–0.001 in (5–25 µm) per surface, and heavy work can reach 0.0015 in (38 µm). A bore therefore grows and a shaft shrinks by twice that amount on diameter, so toleranced features must be machined to a pre-polish dimension.

### How much does electropolishing improve surface finish?

It reduces Ra by roughly 50%. A 32 µin (0.8 µm) machined surface commonly finishes near 16 µin (0.4 µm). It levels micro-roughness only — waviness, deep scratches, tool witness lines and weld undercut survive and often become more visible against the brightened background.

### Does electropolishing passivate stainless steel?

Yes. Iron dissolves preferentially, leaving a chromium-enriched surface that immediately forms a passive oxide film. ASTM B912 recognizes electropolishing as a method of passivating stainless steel, which is why an electropolished part normally needs no separate passivation step.

### Why do sharp edges round off during electropolishing?

Current density concentrates at edges, external corners and thin sections, so metal dissolves fastest there. Sharp corners visibly break and knife edges can lose 0.002–0.003 in (50–75 µm). Mask any edge that has to stay sharp, such as a sealing land or a mating datum.

### Which stainless grades electropolish best?

304, 316 and 316L give the brightest, most consistent results and are the default choices. 400-series and precipitation-hardening grades polish acceptably but duller. Free-machining 303 contains sulfide inclusions that pit and streak, and cast or sintered parts reveal their porosity.

### Is electropolishing the same as passivation?

No, though electropolishing accomplishes passivation as a side effect. Passivation is a chemical dip that removes free iron and adds nothing and removes essentially nothing dimensionally. Electropolishing removes 0.0002–0.001 in of metal, brightens the surface, deburrs it, and costs considerably more.

## Alternative processes

- [Passivation](https://manufacturingprocesses.org/processes/finishing/passivation.md): Passivation removes free iron from a stainless steel surface with an acid bath so the chromium oxide layer can reform unbroken.
- [Grinding, Sanding and Polishing](https://manufacturingprocesses.org/processes/finishing/grinding-sanding-and-polishing.md): Grinding, sanding and polishing remove a thin layer of material with bonded or coated abrasive to correct form or improve surface finish.
- [Vibratory Tumbling and Mass Finishing](https://manufacturingprocesses.org/processes/finishing/vibratory-tumbling-and-mass-finishing.md): Vibratory tumbling agitates parts with abrasive media in a bowl so edges are broken and surfaces smoothed in bulk, without hand work.
- [Abrasive Blasting](https://manufacturingprocesses.org/processes/finishing/abrasive-blasting.md): Abrasive blasting propels media at a surface with compressed air to clean it, strip scale, or produce a uniform matte texture.
- [Bead Blasting](https://manufacturingprocesses.org/processes/finishing/bead-blasting.md): Bead blasting propels glass or ceramic beads at a surface to produce an even satin matte texture without cutting into it aggressively.

## Related processes

- [Passivation](https://manufacturingprocesses.org/processes/finishing/passivation.md): Passivation removes free iron from a stainless steel surface with an acid bath so the chromium oxide layer can reform unbroken.
- [Electroplating](https://manufacturingprocesses.org/processes/finishing/electroplating.md): Electroplating deposits a metal coating from solution onto a conductive part by passing current through an electrolyte bath.
- [Grinding, Sanding and Polishing](https://manufacturingprocesses.org/processes/finishing/grinding-sanding-and-polishing.md): Grinding, sanding and polishing remove a thin layer of material with bonded or coated abrasive to correct form or improve surface finish.
- [Vibratory Tumbling and Mass Finishing](https://manufacturingprocesses.org/processes/finishing/vibratory-tumbling-and-mass-finishing.md): Vibratory tumbling agitates parts with abrasive media in a bowl so edges are broken and surfaces smoothed in bulk, without hand work.
- [Photo Etching](https://manufacturingprocesses.org/processes/finishing/photo-etching.md): Photo etching masks a surface photographically and etches shallow detail into it for decoration, marking or texture.

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*Source: [Manufacturing Processes](https://manufacturingprocesses.org/processes/finishing/electropolishing)*

*Last updated: August 11, 2026*