Finishes / Passivation of stainless steel

Stainless steel passivation services

Passivation is a chemical treatment for stainless steel in nitric or citric acid. It dissolves free iron and other contamination left on the surface by machining, tooling and handling, and lets the chromium oxide film that protects stainless reform evenly. It adds no coating and does not change dimensions.

It is standard on machined stainless parts for medical, food, semiconductor, aerospace and marine use. Grade and test method decide which bath a shop uses, so put both on the RFQ.

Send an RFQ for this finish What to put on the drawing

Stainless steel substratebase
Chromium-rich passive film, reformeda few nm

The passive film is a few nanometers thick and is drawn far out of scale. Passivation removes surface iron, it does not add a layer.

Section view, not to scale. Thickness values are typical, not promised.

Typical values

Typical ranges, not promised. The spec on your drawing and the process control of the shop set the real numbers.

Build-up: None. No dimensional change
Methods: Nitric acid, with or without dichromate, or citric acid
Bath time: About 20 to 60 minutes, by grade and method
Appearance: No visible change on a clean part
Tests: Water immersion, humidity, salt spray or copper sulfate, by spec and grade
Prerequisite: Parts must be clean of oil, scale and heat tint first
Weld tint: Not removed by passivation alone. Needs pickling or electropolish

Specs buyers call out

Put the spec and its type, class or grade on the drawing. Shops quote against it, and approvals narrow the list.

ASTM A967: Chemical passivation of stainless steel partsNumbered nitric and citric treatments, electrochemical treatment and test methods. Check the edition for the list. The most common callout
AMS 2700: Passivation of corrosion resistant steels, aerospaceMethod 1 nitric, Method 2 citric, with types and classes
ASTM A380: Cleaning, descaling and passivation of stainlessPractice for parts, equipment and systems
ASTM F86: Surface treatment of metallic surgical implantsUsed for medical implants

Base materials

Name the alloy and hardness on the RFQ. Preparation changes with the base metal, and some shops only run one family.

Suits

300 series austenitic (304, 316)
17-4 PH and other PH grades
400 series ferritic and martensitic, with the right bath

Needs care

Free-machining grades 303 and 416 (sulfur, risk of flash attack)
Hardened martensitic grades like 440C
Mixed grades in one lot

Not typical

Carbon or alloy steel
Aluminum, copper or titanium in the same bath
Parts with weld heat tint that has not been removed

Hardened steel over 39 HRC

Martensitic and PH stainless above 39 HRC, such as 440C or 17-4 in H900, can be sensitive to hydrogen from acid activation or pickling. Ask the shop whether their process for that grade calls for a relief bake, and have the answer on the cert. Name the condition and hardness on the RFQ.

When to specify it, and when to look elsewhere

A good fit for

Machined, ground or formed stainless parts before assembly
Medical, food, semiconductor and marine parts
Parts that show rust spots after machining or storage
When the drawing calls for ASTM A967 or AMS 2700

Look elsewhere for

Removing heat tint, scale or embedded steel shot: pickle or electropolish first
Making stainless look bright: use electropolish
Improving a poor grade choice for the environment
Any steel that is not stainless

What to put on the drawing and RFQ

Six lines that decide whether quotes compare. Missing items come back as questions and add days.

Spec and methodASTM A967 or AMS 2700, and nitric or citric if you have a preference or a restriction.
Grade and conditionAlloy, heat treat condition and hardness. The bath depends on it.
TestWhich verification test and how many parts per lot. Ask for the result on the cert.
CleanlinessMedical and semiconductor parts may need precision cleaning and clean packaging. Say so.
WeldsWhether parts have welds and whether tint removal is part of the job.
PackagingBagging, gloves and labeling if the part goes into a clean assembly.

What moves the price

Shops price by load, lot and labor more than by the chemistry itself.

Lot charge dominates on small lots
Tests and certs per lot add a fixed cost
Free-machining and martensitic grades need more care and sometimes a separate bath
Precision cleaning and individual bagging add labor

Plating and coating cost: what drives the price per part

Questions buyers ask

About passivation of stainless steel specifically. For how matching works, see the home page.

Does passivation change the size of a part?

No. Passivation removes contamination and adds no coating, so dimensions do not change. Electropolish does remove a few microns.

Nitric or citric acid passivation?

Both meet ASTM A967 and AMS 2700 when run correctly. Citric is easier on the environment and on free-machining grades. Nitric has a longer track record on some drawings. Follow your spec or ask the shops.

Will passivation remove weld discoloration?

No. Heat tint and scale need pickling paste, pickling or electropolish first. Passivation then finishes the surface.

How do I know a part was passivated correctly?

By the verification test named in the spec, such as water immersion, humidity or salt spray, run on parts or samples from the lot. Ask for the result on the certificate.

Can carbon steel be passivated?

Not in the stainless sense. Carbon steel needs a coating such as zinc plating, phosphate or black oxide with oil.

Get quotes for passivation of stainless steel

Send one RFQ with the drawing, base material, spec and lot size. We match it with up to 3 shops that run this finish on a line that fits your part, and reply within 24 business hours.

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