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Passivation vs Electropolishing: Choosing the Right Stainless Finish

Finishing / 21 Sept 2026

Both processes make stainless more corrosion resistant. They do it differently, they cost very differently, and one of them changes your dimensions.

Side by side

	Passivation	Electropolishing
Mechanism	Chemical, acid immersion	Electrochemical, part is the anode
Removes	Free iron and surface contamination	A layer of the base material
Material loss	Negligible	10 to 40 µm typical
Surface roughness	Unchanged	Reduced, often by around half
Appearance	Unchanged	Brighter, smoother, reflective
Deburring effect	None	Removes micro burrs
Chromium at surface	Restored	Enriched above bulk level
Relative cost	Low	Several times higher
Typical lead time	Short	Longer
Geometry sensitivity	Low	High, needs current path to every surface

How each one works

Passivation immerses the cleaned part in nitric or citric acid. The acid dissolves free iron that machining smeared onto the surface, without meaningfully attacking the chromium rich base metal. The part is rinsed, and the chromium oxide film reforms in air. Nothing is added, and the dimensions do not move. The standard is normally ASTM A967 or AMS 2700, covered in detail in passivation of machined stainless parts.

Electropolishing makes the part the anode in an electrolyte, usually phosphoric and sulphuric acid, and passes current through it. Metal dissolves from the surface, and it dissolves fastest from the peaks, because current density is highest there. The result is a levelled surface. The process also preferentially removes iron over chromium, leaving a surface layer richer in chromium than the material underneath.

That last effect is the real reason electropolishing outperforms passivation on corrosion. The surface is not merely cleaned, it is compositionally better than the bulk.

What electropolishing gives you that passivation does not

A smoother surface. Roughness typically drops by around half. Ra 0.8 becomes something in the region of Ra 0.4. It is not a substitute for grinding, because it levels micro peaks rather than correcting form, but on a surface already in the right range it is a real improvement.

No micro crevices. Machined surfaces have tears, folds and smeared metal at a microscopic scale. Those are where contamination lodges and where corrosion starts. Electropolishing removes that layer entirely.

Micro deburring. Current concentrates on edges and burrs, so they dissolve first. This is genuinely useful on parts with small intersecting holes where mechanical deburring cannot reach. It does not remove a real burr, only a small one.

A cleanable surface. This is why pharmaceutical and semiconductor specifications call for it. A surface with no crevices can be cleaned to a validated standard. A machined surface cannot.

Appearance. Bright and uniform, which is sometimes the actual requirement.

What it costs you

Money. Several times the cost of passivation, and it is an outside process with a minimum lot charge. On a small quantity the minimum often dominates.

Material. 10 to 40 micrometres off every exposed surface. On a 20 mm diameter that is up to 80 micrometres off the diameter, because it comes off both sides. That is four times an h7 band. The part has to be machined to allow for it, which means the shop needs to know at quoting time and the drawing needs to say so.

Geometry limits. Current has to reach the surface. Deep bores, blind holes and internal corners electropolish unevenly or not at all, and may need a dedicated cathode inside the feature. Sharp external edges dissolve faster than flat faces, so a part with a critical sharp edge will lose it.

Grade limits. It works best on austenitic grades. SUS303 electropolishes poorly, because the sulphide inclusions pull out and leave pits. High carbon martensitic grades are difficult. If the part is SUS303 and somebody has specified electropolishing, one of the two needs to change.

Choosing

Passivation is enough when:

The part is a general engineered component
Service is normal industrial, outdoor or mildly corrosive
Appearance does not matter
The surface only needs to be clean, not cleanable to a validated standard
Tolerances are tight and you do not want another variable

Electropolishing earns its cost when:

The part is a pharmaceutical, biotech or semiconductor wetted surface
The part must be cleaned to a validated standard, or is CIP or SIP cleaned in place
It is a vacuum component and outgassing matters
Appearance is a requirement
The part has small internal features that cannot be deburred mechanically
Maximum corrosion resistance is worth the money, for example in a marine or chemical application where a failure is expensive

Both is usually unnecessary. Electropolishing leaves a chromium enriched passive surface, so a separate passivation step afterwards adds cost and little else. Some specifications require it anyway, and in that case follow the specification rather than the logic.

Designing for electropolishing

If the part will be electropolished, the drawing has to account for it:

State it clearly and give the allowance.

Electropolish per ASTM B912
Machine to allow 0.025 mm per surface removal
Dimensions shown are after electropolishing

That last line is the one that prevents the argument. A drawing that gives final dimensions and specifies electropolishing without saying which comes first is ambiguous.

Give a target Ra, before and after if you can. Without it, every shop assumes a different starting point.

Flag features that must not be affected. Threads, sealing edges, sharp corners. Masking is possible and costs money, so identify what needs it rather than letting the processor decide.

Consider the starting finish. Electropolishing improves a surface proportionally, it does not fix a bad one. Starting at Ra 1.6 gets you to roughly Ra 0.8. If you need Ra 0.2, the machined surface has to be good before it goes in the tank.

Mind the sharp edges. If the part has a functional sharp edge, electropolishing will round it. Say so on the drawing and expect masking.

A short decision path

Does the part need to be cleanable to a standard, or merely clean? Cleanable means electropolishing
Is appearance a requirement? If yes, electropolishing
Are there internal features that cannot be deburred mechanically? If yes, electropolishing helps
Is the grade austenitic? If not, check whether electropolishing is even suitable
Are tolerances tight? If yes, either allow for the removal explicitly or stay with passivation
Otherwise, passivate

For the standards and methods behind the cheaper option, see passivation of machined stainless parts. For the grade side of the decision, see SUS316 vs SUS316L.

Have a part with a finishing requirement attached? Get a quote. Machining and finishing are quoted as one order and placed with a shop that has the processor relationship rather than one that will find it later. Priced quote within 24 business hours.

Questions

What is the basic difference?

Passivation is a chemical clean that removes free iron and lets the oxide film reform. Electropolishing is an electrochemical process that removes a measurable layer of the surface itself, levelling it and leaving a chromium enriched, brighter finish.

How much material does electropolishing remove?

Typically 10 to 40 micrometres, depending on time and current. On a tight tolerance feature that is significant, so the part has to be machined undersize or oversize to allow for it.

Does electropolishing include passivation?

Effectively yes. The process removes the contaminated surface layer entirely and leaves a chromium enriched surface, so a separately specified passivation step afterwards is usually redundant.

Which one improves corrosion resistance more?

Electropolishing, clearly. It removes the damaged layer rather than just cleaning it, eliminates the crevices that hold contamination, and leaves a surface richer in chromium than the bulk material.

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