Finishing
Electropolishing vs Passivation
Both treat stainless steel for corrosion resistance, but they are different animals. Passivation chemically strips free iron and builds the chromium oxide layer with no dimensional change. Electropolishing removes 10 to 40 µm of surface material electrochemically, smoothing the part and passivating it at the same time.
| Electropolishing | Passivation | |
|---|---|---|
| Material removal | 10 to 40 µm per surface | None, under 0.1 µm |
| Surface finish | Cuts Ra up to 50 percent | No Ra change |
| Cost per part | $2 to $15 racked | $0.25 to $2 batch basket |
| Governing spec | ASTM B912 | ASTM A967, AMS 2700 |
| Process time | 5 to 20 min at 5 to 25 A/dm2 | 20 to 120 min citric or nitric bath |
| Deburring | Removes burrs under 50 µm | None |
| Dimensional risk | Real on ±0.025 mm features | Zero |
Choose Electropolishing when
- Microfinish or Ra reduction on the print
- Deburring plus corrosion protection in one step
- Sanitary, pharma, or semiconductor surfaces
Choose Passivation when
- Tolerances that cannot give up 20 µm
- Low cost bulk lots of machined stainless
- Print calls ASTM A967 and nothing more
The verdict
Passivation is the default for machined stainless: cheap, no dimensional change, meets ASTM A967. Step up to electropolishing when the print calls a microfinish, when edge burrs matter, or for sanitary and high purity service. Never electropolish features toleranced tighter than ±0.025 mm without budgeting the stock removal.
Cost comparison
Citric passivation runs $0.25 to $2 per part in batch baskets with lot minimums of $150 to $400. Electropolishing runs $2 to $15 per part racked, minimums of $250 to $500, plus fixturing charges on odd geometry. There is no volume crossover because they solve different problems. The cost trap is specifying electropolish when passivation meets the corrosion requirement: that is a 5x to 10x premium paid for finish improvement nobody asked for.
Common questions
Does electropolishing replace passivation?
Functionally yes. Electropolishing preferentially dissolves iron and leaves a chromium rich surface, so an electropolished part is also passivated, and ASTM B912 recognizes it as a passivation method. The reverse is not true: passivation does nothing for finish or burrs. If the drawing lists both, confirm with the customer whether electropolish alone satisfies the callout.