Guides / Finishes
Passivation vs Electropolish: Two Different Jobs on Stainless Steel
October 12, 2026
Passivation and electropolish are the two finishes buyers reach for most often on stainless steel, and they are frequently confused because both are chemical processes, both improve corrosion resistance, and neither changes the part’s color the way plating or powder coat would. They solve different problems, and knowing which one your part actually needs avoids paying for a step that does not fix what is wrong.
What passivation actually does
Machining stainless steel can smear free iron from the tooling onto the surface, or leave iron particles embedded from the cutting process. That free iron is not stainless, and it can rust, creating surface staining on a part that is otherwise corrosion resistant. Passivation removes this free iron with a nitric or citric acid bath, leaving the chromium-rich passive oxide layer that gives stainless steel its corrosion resistance intact and exposed.
Passivation removes no meaningful base material and adds no thickness. It fixes surface contamination from machining; it does not change the underlying metallurgy or fix a poor alloy choice for the environment the part will see.
Specify it for: stainless parts straight from machining, especially parts destined for medical, food contact or high-corrosion-risk service, where surface contamination from tooling is a real concern.
Write on the drawing: ASTM A967 or AMS 2700, the acid type if your application restricts it, and the test method (water immersion, high humidity, salt spray or copper sulfate) your spec requires.
What electropolish actually does
Electropolish is an electrochemical process that removes a thin layer of material from the surface, working faster on peaks and slower in valleys, which smooths and brightens the surface overall. It reduces surface roughness, removes embedded contaminants along with a thin layer of base metal, and rounds sharp edges slightly. The result is a smoother, brighter, more chemically clean surface than passivation alone produces.
Because electropolish removes actual material, it changes dimensions slightly, typically a small fraction of a millimeter depending on the process and target finish, and it rounds edges in a way passivation does not. For parts with tight tolerances or sharp edges that must stay sharp, this matters.
Specify it for: parts where surface smoothness, cleanability or a bright cosmetic finish matters as much as corrosion resistance, such as medical instruments, food processing equipment and pharmaceutical components.
Write on the drawing: the target surface roughness (Ra), which dimensions are critical and must account for material removal, and whether edges need to stay sharp or rounding is acceptable.
Side by side
| Passivation | Electropolish | |
|---|---|---|
| What it removes | Free iron and surface contamination | A thin layer of base material |
| Dimensional change | None | Small, process dependent |
| Edge effect | None | Slightly rounds sharp edges |
| Surface brightness | Unchanged | Brighter, smoother |
| Typical spec | ASTM A967, AMS 2700 | Customer spec, often referencing ASTM B912 |
| Common use | General corrosion resistance after machining | Cosmetic, cleanability, medical and food contact |
When to use both
Electropolish followed by passivation is common where both smoothness and a verified passive layer matter, such as medical devices and pharmaceutical process equipment. Electropolish handles the material removal and surface finish; a follow-up passivation step confirms and documents the passive layer against a named spec, which electropolish alone does not always certify to.
If your part only needs corrosion resistance after machining and cosmetic finish is not a requirement, passivation alone is usually the lower-cost, faster answer. If the surface finish or a rounded edge condition matters as much as corrosion resistance, electropolish is doing work passivation cannot.
Nitric acid vs citric acid passivation
ASTM A967 accepts several passivation chemistries, and the two a buyer is most likely to see quoted are nitric acid and citric acid. Nitric acid is the older, longer-established method and works reliably across most stainless grades. Citric acid runs at lower concentration, generates less hazardous waste and has become the default at many shops for that reason, with performance considered equivalent to nitric acid for most common grades.
Some free-machining grades and certain martensitic stainless alloys respond differently to the two chemistries. If your part uses an alloy the shop does not run often, ask which acid they plan to use and whether they have data for that specific grade, rather than assuming either method is a universal default.
How passivation is verified
A passivation pass is normally confirmed with one of a small number of standard tests named in ASTM A967: water immersion, high humidity exposure, salt spray, or a copper sulfate test that reacts visibly with any remaining free iron. Which test applies is usually set by your customer’s spec or by the class of stainless and the service environment.
The copper sulfate test is fast and common for incoming inspection, since a failure shows up as visible copper deposits on the surface within minutes. Salt spray and humidity tests take longer but better represent actual field exposure. Ask the shop which test they run by default and whether it matches what your own spec requires, since a shop’s standard test and your customer’s required test are not always the same one.
A note on alloys
Both processes assume a genuinely stainless alloy. Free-machining stainless grades with added sulfur, chosen for easier machining, passivate less reliably because the sulfide inclusions that make them easy to cut also resist the acid treatment. If your part is a free-machining grade and passivation performance is critical, discuss the alloy with the shop before finishing, not after a test fails.
How we help
We are a matching service, not a plating shop. Tell us which problem you are solving, corrosion resistance, surface finish or both, and we pass your RFQ to up to three shops that run passivation, electropolish or both to the spec your part needs. See which finish fits your part for the broader comparison across substrates, or go straight to the RFQ form.
See also the specification reference for ASTM A967 and AMS 2700 passivation.
Frequently asked questions
Do I need both passivation and electropolish on the same part?
Often yes, in that order. Electropolish removes material and can leave the surface chemically ready for passivation, but many shops still run a separate passivation step afterward to confirm the passive layer meets a named spec's test requirements.
Does passivation change the dimensions of my part?
No. Passivation is a chemical treatment that removes free iron from the surface without meaningfully removing base material, so dimensions are unaffected within normal tolerances.
Will electropolish remove machine marks and tool lines?
It reduces them and gives a brighter, smoother surface, but it will not fully erase deep tool marks or scratches. Surfaces need reasonable machining or grinding quality going in; electropolish refines a surface, it does not rescue a poor one.
Can passivation or electropolish be done on parts that are already assembled?
Rarely for a full assembly, since both processes are chemical baths the part is immersed in, and dissimilar materials, seals, bearings or electronics usually cannot go through them. They are almost always done on individual stainless components before assembly.
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