Passivation of stainless steel
Machining, tooling and handling leave free iron on a stainless steel surface, and it shows up later as rust spots. Passivation is an acid treatment that dissolves that iron without significantly attacking the stainless itself, so the passive film can form again on a clean surface. It is a cleaning step, not a coating, and it is done by outside Houston processors.
Degrease, passivate, rinse, test.
- STEP 01
Degrease
Oil and grease come off first, because the acid will not remove them.
- STEP 02
Descale if needed
Scale or heat tint is removed separately, by pickling or mechanical means. Passivation alone will not take it off.
- STEP 03
Passivate
The parts are immersed in a nitric or citric acid solution, or treated electrochemically, at the concentration, temperature and time set for the alloy.
- STEP 04
Rinse and dry
They are rinsed thoroughly without being allowed to dry between steps, then dried.
- STEP 05
Test
Samples are checked for free iron by the test the order names, such as water immersion, high humidity or copper sulfate.
- It does not remove scale, heat tint, oil or grease; those have to come off first.
- Nitric acid solutions attack carbon steel, so assemblies that include carbon-steel parts are not passivated as one.
- ASTM A967 covers stainless steels; nickel alloys are outside its scope.
- Free-machining and hardened grades need treatments chosen for them.
Finishes are applied by outside coaters and platers in Houston, to the specification on the order.
Nothing added, but the treatment must suit the grade.
Passivation adds no layer, so it does not change thread size. A properly passivated part shows a clean surface with no etching, pitting or frosting.
The treatment must suit the alloy. Free-machining grades such as 303 and 416 can etch in plain nitric acid; inhibited solutions reduce that. Hardened martensitic and precipitation-hardening grades can take up hydrogen in acids that generate it, so treatments are chosen that do not.
For bolting to ASTM A193, A194 and A320, passivation is an option: supplementary requirement S71 of ASTM A962 has stainless bolting passivated to A380 or A967, at the maker's choice, or electropolished.
Designations and classes.
How to call it out.
- Name the specification, the treatment and the test, for example passivate to ASTM A967, nitric acid treatment, verified by Practice A (water immersion).
- For bolting to ASTM A193, A194 or A320, order supplementary requirement A962 S71.
- Give the alloy, so a treatment suited to it is used.
Passivation of stainless steel: common questions.
Is passivation a coating?
No. It removes free iron and other contamination, and the passive film then forms by itself in air on the clean surface. Nothing measurable is added to the part.
Nitric or citric?
Both are recognised in ASTM A967. ASTM A380 describes the citric acid treatment as the least hazardous way to remove free iron and light contamination. The order or your own procedure decides.
How do you know it worked?
By a test on passivated parts, named on the order: water immersion, high humidity, salt spray or copper sulfate, for example, which show up free iron left on the surface.
Do stainless bolts need passivating?
Not by default. For A193, A194 and A320 bolting it is ordered as A962 supplementary requirement S71.
Compare with.
Silver plating
A thin silver layer plated onto the nut thread that acts as a solid lubricant. It stops austenitic and nickel-alloy nuts galling on their studs, including in high-temperature service.
Uniform, hard and corrosion resistant, even inside threadsElectroless nickel
A nickel-phosphorus deposit laid down by chemical reduction rather than current, so it builds evenly on every surface, including thread roots and bores. It is harder than zinc and resists chemicals and wear.
A clean black finish without dimensional changeBlack oxide
A chemical conversion of the steel surface to black iron oxide, sealed with oil or wax. It adds no thickness, so it suits precision parts and socket products, and gives a uniform dark appearance with light corrosion resistance indoors.