Electropolishing is not a cosmetic finish. On a vacuum chamber it removes the mechanically deformed surface layer, strips embedded iron and sulphur, and cuts the real surface area that traps water vapour. A chamber that outgasses will never hold UHV no matter how good the welds are.
Why electropolishing matters for vacuum, not just for looks
Mechanical polishing smears metal rather than removing it, leaving a Beilby layer of deformed material with embedded contamination. That layer is a reservoir of hydrogen and water vapour. Electropolishing dissolves it anodically, typically removing 20–40 µm, and leaves a chromium-enriched passive surface. The result is a lower real-to-apparent surface area ratio, faster pump-down, and a lower ultimate pressure.
What we control in-house
Electropolishing and passivation are done at our own facility rather than subcontracted. That matters for two reasons: the finish is part of the dimensional and cleanliness chain, so it cannot be handed to a third party without losing traceability; and when a chamber needs rework after leak testing, the turnaround is days rather than weeks.
Typical specification we work to
Ra < 0.4 µm on internal wetted surfaces, 20–40 µm material removal, uniform current density on complex geometry, and post-treatment rinse and dry to prevent residue staining. Internal weld caps are ground flush before polishing so the electropolish does not exaggerate a step at the seam.
Materials
304, 304L, 316, 316L and 316LN stainless steel, plus aluminium alloys where the customer specifies a chem-film or anodised route instead. 316L is the default for UHV semiconductor chambers; 316LN is specified when the application needs low magnetic permeability.
Surface finish against typical vacuum performance
| Internal surface treatment | Typical Ra | Typical ultimate pressure | Outgassing behaviour |
|---|---|---|---|
| As-welded, no treatment | 3–6 µm | High vacuum only | High — weld oxide and embedded contamination |
| Mechanical polish | 0.8–1.6 µm | High vacuum (HV) | Moderate — smeared layer retains water vapour |
| Glass-bead blast + clean | 1.0–2.0 µm | HV to UHV with bake-out | Moderate — depends on bead grade and rinse |
| Electropolished | < 0.4 µm | UHV with bake-out | Low — deformed layer removed, Cr-enriched surface |
| Electropolished + 400 °C bake | < 0.4 µm | UHV, lowest achievable | Lowest — surface water driven off before sealing |
What we control in-house
- 304 / 316L stainless steel and 6061 / 5083 aluminium fabrication
- Clean-environment TIG welding with full penetration and no micro-porosity
- Helium mass-spectrometer leak testing to 1 × 10⁻⁹ mbar·l/s
- In-house electropolishing and passivation — not subcontracted
- Chamber bake-out to 150–400 °C for UHV outgassing control
- Precision-machined ISO-KF, ISO-K and CF (ConFlat) flanges
- Zeiss CMM verification to ±0.02 mm across large work envelopes
- 12 m gantry CNC machining — 12 m × 4 m × 2 m envelope
- SEMI-referenced documentation and inspection protocols
Frequently asked questions
What does electropolishing do to a vacuum chamber?
It anodically dissolves 20–40 µm of the surface, removing the deformed layer left by machining and mechanical polishing. That layer stores water vapour and hydrogen; removing it lowers outgassing and lets the chamber reach UHV after bake-out.
How much does electropolishing reduce outgassing?
It is the surface layer, not the bulk material, that drives outgassing. Removing the deformed layer and enriching the surface in chromium typically reduces the water-vapour desorption rate by an order of magnitude — which is the difference between pumping down in hours and pumping down in days.
Do you electropolish in-house?
Yes. Electropolishing and passivation are performed at our own facility, so finish, dimensional verification and leak testing stay in one traceable chain and rework does not add a subcontractor lead time.
Can you electropolish large chambers?
Yes — our gantry machining envelope is 12 m × 4 m × 2 m, and electropolishing is sized to match, so large weldments do not have to be split for finishing.
Is electropolishing the same as passivation?
No. Passivation removes free iron from the surface chemically but does not remove the deformed layer. Electropolishing removes material and levels the surface. For UHV work you normally want electropolishing, optionally followed by a passivation rinse.
Related capabilities
Need this made to spec?
Send the drawing or the specification and we will come back with a DDP price, lead time and inspection plan.