---
title: "Electropolishing Spec: Process, Ra, ASTM B912 | OpenSpindle"
description: "Electropolishing spec: how anodic dissolution removes a thin surface layer to smooth, deburr, brighten, and passivate stainless steel per ASTM B912."
canonical: https://openspindle.com/reference/electropolishing-spec
---

# Electropolishing Spec

How electropolishing works, how much material it removes, the surface and corrosion benefits on stainless steel, and how to call it out per ASTM B912.

Electropolishing is reverse electroplating: the part is made the anode in an electrolyte, and a controlled current dissolves metal from the surface rather than depositing it. Dissolution happens fastest at the microscopic peaks, so the surface is smoothed, deburred, brightened, and left with a clean passive layer. It is used most on stainless steel, where it also removes embedded free iron and improves corrosion resistance. Because it removes material, it slightly reduces dimensions and can round sharp edges.

## What electropolishing does

Electropolishing preferentially removes the high points of a surface profile through anodic dissolution. The result is a smoother, cleaner, and more corrosion-resistant surface, which is why it is common in medical, food, pharmaceutical, and semiconductor work where cleanability matters.

| Effect | What happens | Why it matters |
| --- | --- | --- |
| Smoothing | Peaks dissolve faster than valleys, lowering Ra | Can roughly halve Ra (varies with starting finish and alloy) |
| Deburring | Removes fine burrs and loose particles | Cleans up milled and machined edges without hand work |
| Passivation | Strips free iron, enriches the chromium oxide film | Improves corrosion resistance on stainless, per ASTM B912 |
| Brightening | Leaves a bright, reflective surface | Cosmetic finish plus easier cleaning and inspection |

*Typical effects on stainless steel. Actual Ra improvement and appearance depend on the alloy, starting finish, and process control. Confirm on your part.*

## How much material it removes

Electropolishing removes a thin surface layer, so it reduces dimensions and can soften sharp corners. Plan for it on tight-tolerance features and inside/outside edges. Light passes remove less; heavier passes to smooth a rough surface remove more.

| Measure | Typical range | Notes |
| --- | --- | --- |
| Layer removed (total) | ~0.0005-0.001 in (12-25 um) | Typical; varies with time, current, and starting roughness |
| Layer removed (light pass, per surface) | ~0.0001-0.0003 in (2.5-7.5 um) | Controllable; used when dimensions are tight |
| Edges | Slight rounding | Sharp corners round off; note any edges that must stay crisp |

*Ranges are typical and vary by shop and setup. For tight tolerances, tell the shop the finished dimension you need so removal is accounted for.*

## Where it fits vs other finishes

Electropolishing both smooths and passivates in one electrochemical step, which sets it apart from purely chemical passivation and from mechanical polishing.

| Process | Removes material? | Smooths surface? | Passivates stainless? |
| --- | --- | --- | --- |
| Electropolishing | Yes (thin layer) | Yes | Yes (per ASTM B912) |
| Chemical passivation | Negligible | No | Yes (per ASTM A967) |
| Mechanical polishing | Yes (abrasive) | Yes | No (can smear surface) |

*Chemical passivation (ASTM A967) removes free iron without smoothing; electropolishing (ASTM B912) does both. Confirm which standard your part calls for.*

## How to call it out on a drawing

Name the standard and the surface requirement. A common callout is "Electropolish per ASTM B912" for stainless steel where passivation is required. Where finish matters, add a target Ra (for example, "Ra 0.4 um max after electropolish"), and note any masked areas, threads, or edges that must remain unaffected. ASTM B912 covers passivation of 200-, 300-, and 400-series and precipitation-hardened stainless by electropolishing; ASTM E1558 is the related guide for electrolytic polishing of metallographic specimens.

Values reflect ASTM B912 and common practice; ASTM A967 covers chemical passivation and ASTM E1558 covers electrolytic (metallographic) polishing. Material removal, Ra, and appearance vary by alloy, starting finish, and shop. Verify what a shop can hold on your part before release.

## On OpenSpindle

- [Stainless steel machining](https://openspindle.com/materials/stainless-steel.md)
- [Passivation spec](https://openspindle.com/reference/passivation-spec.md)
- [Surface finishes compared](https://openspindle.com/reference/surface-finishes-compared.md)

## Frequently asked questions

### What is electropolishing?

Electropolishing is reverse electroplating. The part is the anode in an electrolyte and a controlled current dissolves metal from the surface, removing the high points first. That smooths, deburrs, brightens, and passivates the surface. It is used most on stainless steel.

### How much material does electropolishing remove?

It removes a thin surface layer, typically on the order of 0.0005 to 0.001 in (about 12 to 25 um) total, though a light, controlled pass can remove as little as 0.0001 to 0.0003 in per surface. Because it removes material, it slightly reduces dimensions and can round sharp edges, so account for it on tight-tolerance features.

### Does electropolishing passivate stainless steel?

Yes. Electropolishing removes embedded free iron and enriches the chromium-rich passive film, improving corrosion resistance. ASTM B912 is the standard for passivation of stainless steels using electropolishing. Purely chemical passivation is covered separately by ASTM A967.

### How do I specify electropolishing on a drawing?

Call out the standard and the surface requirement, for example "Electropolish per ASTM B912." Where finish matters, add a target Ra such as "Ra 0.4 um max," and note any masked areas, threads, or edges that must stay unaffected since electropolishing removes material and rounds edges.