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Passivation of Machined Stainless Parts: What It Does and How to Specify It

Finishing / 20 Sept 2026

A machined SUS316L part can come off the machine looking perfect and show rust spots two weeks later. The material is not at fault. The surface is.

Passivation is the process that fixes it, and it is one of the cheapest things on a stainless drawing to specify correctly.

What machining does to the surface

Stainless resists corrosion because chromium in the alloy reacts with oxygen to form a thin, self repairing chromium oxide film. That film is what is doing the work, not the bulk material.

Machining damages it in two ways.

It contaminates the surface with free iron. Tooling, fixtures, chucks, deburring media and even the shop’s own swarf transfer iron particles onto the part. Those particles are ordinary steel. They rust, and the rust product then attacks the surface beneath.

It disturbs the film itself. The cutting action smears and deforms the surface layer. The film reforms, but unevenly, and around embedded contamination it cannot form at all.

The visible result is rust spots, usually small, usually appearing days or weeks after machining, often in storage rather than in service. Buyers frequently assume the wrong grade was supplied. Usually the grade was correct and nobody passivated the part.

What passivation does

The part is cleaned, then immersed in an acid solution that dissolves free iron from the surface without meaningfully attacking the chromium rich base material. It is then rinsed, and the chromium oxide film reforms in air over the following hours.

It is a chemical cleaning process. It adds nothing. It deposits no coating. It changes neither the dimensions nor the appearance in any way you would notice.

That last point matters, because passivation is regularly confused with processes that do change the part:

Process	What it does	Material removed	Appearance
Passivation	Removes free iron, restores oxide film	Negligible	Unchanged
Pickling	Removes scale and a base metal layer	Measurable	Dull, etched
Electropolishing	Levels the surface electrochemically	10 to 40 µm typical	Brighter, smoother
Bead blasting	Mechanical texturing	Negligible	Matte, uniform

The two standards, and the two methods

Even on a JIS drawing, passivation is normally specified to an American standard, because JIS has no equivalent in common use.

ASTM A967 is the general specification for chemical passivation of stainless parts. It defines nitric treatments (Nitric 1 through 5) and citric treatments (Citric 1 through 5), plus the tests used to verify the result.

AMS 2700 is the aerospace specification. Method 1 is nitric, Method 2 is citric. Where AMS 2700 is called out, expect more documentation and a higher price.

Nitric acid passivation is the traditional route. Effective across the range, and normally the better choice on the free machining grades such as SUS303, where the sulphide inclusions need the stronger treatment, and on high carbon martensitic grades like SUS440C, which can be attacked by the wrong bath. Nitric with sodium dichromate is specified where the grade is at risk of flash attack.

Citric acid passivation has largely displaced nitric for general work. It is effective on the austenitic grades, considerably safer to handle, far easier to dispose of, and does not produce nitric fumes. Most commercial shops default to it now, and most specifications accept it.

Unless you have a reason to insist, allowing either gives the shop useful flexibility. If you do have a reason, name the method.

Verification

ASTM A967 includes tests that can be specified when the result has to be proven:

Water immersion. Alternate immersion and drying cycles, then inspect for rust. Simple and slow
High humidity. 24 hours in a humidity cabinet, then inspect
Salt spray. Per ASTM B117, a fixed duration then inspection
Copper sulphate. A solution applied to the surface. Copper plates out where free iron is present, showing contamination directly. Fast and visual, and not for use on free machining or martensitic grades
Potassium ferricyanide nitric acid. A more sensitive free iron test

For most commercial parts, no test is needed and the certificate of conformance is sufficient. For medical, pharmaceutical, semiconductor or aerospace parts, name the test in the requirement, because otherwise nobody runs one.

Grade specific notes

Austenitic grades, SUS304, SUS316, SUS316L. Straightforward. Citric works well. This is the bulk of passivation work.

SUS303. The sulphide inclusions that make it machine well are also corrosion initiation sites, and they respond poorly to mild treatments. Nitric is usually specified. Be realistic about the outcome: passivated SUS303 is still less corrosion resistant than unpassivated SUS304.

SUS430 and the ferritics. Passivate normally, but the base corrosion resistance is lower than the 300 series and passivation does not change that.

SUS410, SUS420J2, SUS440C. High carbon martensitic grades can be attacked by aggressive baths, which is called flash attack. Nitric with dichromate, or a carefully controlled citric process, at lower temperature and shorter time. Tell the processor the grade and the hardness. These grades also benefit most from passivation, because their surfaces hold contamination more readily.

SUS630 (17-4 PH). Passivate after aging, not before. Specify the condition on the order.

Sequence

Passivation goes near the end, after everything that could recontaminate the surface:

Machine
Deburr
Heat treat, if any
Grind or finish machine
Clean and degrease thoroughly
Passivate
Rinse, dry, inspect
Package in clean material, not in contact with carbon steel

Step 5 is where passivation most often fails. Oil, coolant residue or cutting fluid on the surface prevents the acid from reaching the metal, so the part comes out looking passivated and is not. Any shop that treats degreasing as optional will produce inconsistent results.

Step 8 matters more than people expect. A passivated part wrapped in a carbon steel shaving or stored on a rusty shelf is recontaminated immediately.

How to write the requirement

Minimum:

Passivate per ASTM A967

Better:

Passivate per ASTM A967, Citric 1 or Nitric 2 at supplier's option
Verify per ASTM A967 water immersion test
Certificate of conformance required
Package individually, no contact with carbon steel

For a regulated part:

Passivate per AMS 2700 Method 2
Copper sulphate test per ASTM A967, results recorded
Certificate of conformance with batch identification

When you do not need it

Passivation is cheap but it is not free, and it is an outside process with a lead time.

Skip it when the part is genuinely dry and indoor, is not cosmetic, is not a wetted surface, and is not covered by a customer or regulatory requirement. A SUS304 bracket inside a sealed enclosure does not need passivating.

Specify it when the part is wetted, outdoors, cosmetic, in food, pharma, medical or semiconductor service, or when a standard requires it.

For the more aggressive surface process and how to choose between them, see passivation vs electropolishing. For the surface finish side of the specification, see JIS surface finish symbols.

Have a part that needs finishing as well as machining? Get a quote. Machining and passivation are quoted as one order, placed with a shop that already has the processor relationship, and the parts are inspected after treatment. Priced quote within 24 business hours.

Questions

Why does stainless need passivation if it is already stainless?

Machining smears iron from tooling and fixtures onto the surface, and the cutting itself disturbs the chromium oxide film. Free iron on the surface rusts, and the rust spreads. Passivation dissolves that iron and lets the oxide film reform.

Nitric or citric?

Citric for most work: it is safer, environmentally easier, and effective on the austenitic grades. Nitric, usually with dichromate, is still preferred on the free machining grades like SUS303 and on high carbon martensitic grades where citric can be less reliable.

Does passivation change dimensions?

Essentially no. It removes free iron, not base metal, so material loss is negligible on any normal tolerance. Electropolishing does remove measurable material, which is a different process.

Is passivation the same as pickling?

No. Pickling is aggressive, removes scale and a layer of base metal, and leaves a dull etched surface. Passivation is mild, removes only surface contamination, and does not change appearance.

Have a drawing for a stainless part?

Send it and an engineer reads the title block, quotes it within 24 business hours, and if you order we place, manage, inspect and ship the parts on one PO and one invoice.

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