Guides / Technical
Passivation and Electropolish on Machined Medical Parts: What Each One Actually Does
September 26, 2026
Two operations get bundled together on drawings and in casual conversation, and they are not the same thing. Getting them backwards on a quote either pays for a step the part does not need or skips one it does.
What passivation actually does
Machining a stainless part, especially with steel tooling, smears free iron onto the surface and disrupts the thin chromium oxide layer that makes stainless resist corrosion in the first place. Passivation, to ASTM A967 or AMS 2700, is a chemical treatment (typically nitric or citric acid) that removes that free iron and lets the chromium oxide layer reform uniformly.
It does not remove material in any meaningful dimensional sense and it does not change the surface finish you can see or feel. It changes the chemistry of the outermost few nanometres. A part can look identically bright before and after passivation, and be substantially more corrosion resistant afterward.
This is why it applies to 316L, 316LVM and 17-4 PH, the stainless grades that show up across instruments and temporary fixation, and why it is close to universal on those materials in a medical supply chain.
What electropolish actually does
Electropolish is an electrochemical process that removes a thin, controlled layer of material from the surface, on the order of microns, preferentially from the peaks of the surface roughness. The result is a smoother, brighter surface with fewer microscopic crevices where contamination or bacteria can lodge.
ASTM B912 covers electropolishing titanium and titanium alloys specifically, which matters because electropolish parameters that work on stainless will not work correctly on titanium’s oxide chemistry. For stainless parts, electropolish is often specified alongside or instead of chemical passivation, because a properly controlled electropolish leaves a chromium-enriched surface as a byproduct of the process, which is functionally similar to what passivation achieves on its own.
Electropolish also does something passivation cannot: it measurably reduces surface roughness. On a part where Ra matters, an as-machined surface at Ra 0.8 micrometres does not get to Ra 0.2 by passivation alone. Electropolish is one of the few ways to get there without hand polishing.
Deciding which one, or both
| The part needs | Reach for |
|---|---|
| Corrosion resistance restored after machining, no finish change needed | Passivation (ASTM A967 / AMS 2700) |
| A measurably smoother, brighter surface, plus corrosion resistance | Electropolish (ASTM B912 for titanium, or a stainless-qualified process) |
| Titanium surface prep before anodising or a UDI mark | Electropolish, not chemical passivation |
| Cheapest path to a corrosion-resistant stainless part with no cosmetic requirement | Passivation alone |
A common mistake on a drawing is writing “passivate per ASTM A967” on a titanium part. The standard does not apply to titanium the way it does to stainless, and a vendor who does not catch it will either quote the wrong process or ask you a clarifying question that costs a week.
What to ask a vendor before you send the lot
- Which standard is your process qualified against, and does it match the alloy on this drawing.
- How do you verify the result. A water break test, salt spray per ASTM B117, or a copper sulfate test, recorded per lot.
- What is the turnaround, and does it change with lot size or geometry (blind holes and deep pockets trap solution and take longer to verify).
- Do you handle masking, if only part of the surface needs treatment.
Where this sits in the supply chain
Passivation and electropolish are almost always outsourced by the machining shop to a specialist, not run in house. That is one of the five categories of auxiliary vendor a medical machining supplier typically relies on, alongside laser marking, sterilization, heat treatment and testing. If you run this process and want machining suppliers we have matched to find you, list your service here. If you are the buyer trying to work out what belongs on your drawing, the buyer’s guide to medical CNC machining covers where this fits in the wider process.
See also: ASTM F86 on implant surface preparation and marking, for what the practice covers and where it stops.
Sources
- ASTM A967, chemical passivation treatments for stainless steel parts
- ASTM B912, passivation of titanium and titanium alloys by electropolishing
- AMS 2700, passivation of corrosion resistant steels
Standards and regulations change. Check the current text at the source before you rely on it in a submission.
Frequently asked questions
Does electropolish replace passivation?
It can, on the alloys ASTM B912 covers, because the process removes a thin layer of the surface and leaves the remaining chromium enriched, which is what a separate passivation step is trying to achieve on a machined surface. Confirm with your supplier which standard their electropolish is qualified against before you drop a separate passivation step.
Which stainless grades need passivation?
Any austenitic or martensitic stainless implant or instrument part, most commonly 316L, 316LVM and 17-4 PH. Passivation restores the chromium oxide layer that machining, especially with steel tooling, can smear or contaminate with free iron.
Can titanium be passivated?
Titanium forms its own stable oxide layer in air and does not need chemical passivation the way stainless does. What titanium implant parts more often need is electropolish or anodising for surface finish and, in some cases, for marking contrast, which is a different job from passivation.
How is passivation verified?
A common method is a water break test, salt spray exposure per ASTM B117 or a copper sulfate test, recorded per lot against the ASTM A967 or AMS 2700 method used. Ask for the test method and the result, not just a certificate that says passivated.
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