Parts passivation is the batch treatment of finished stainless steel components, immersed lot by lot, verified lot by lot and certified lot by lot. It is a different job from passivating an installed piping system, where chemistry is circulated through welded tube that cannot be moved. Paul Industries provides both. This page covers the batch service: machined parts, fittings, manifolds, valve bodies, instrument housings, fabricated brackets and assemblies that an OEM, a machine shop or a device maker sends in for passivation to ASTM A967, ASTM A380, AMS 2700 or ASTM F86 and receives back with a certificate of passivation for the lot.
What batch passivation does to a machined part
Machining, forming and handling leave free iron on a stainless surface: particles transferred from cutting tools, steel fixtures, blasting media and shop dust, plus the sulfide inclusions that machining exposes at the surface. Free iron rusts, and a rust site is where pitting starts. Passivation dissolves that free iron and the exposed sulfides in an oxidizing acid bath, leaving a chromium-enriched surface that reforms its own protective oxide. The part is not coated. Nothing is added. What changes is the ratio of chromium to iron in the top few nanometres of the surface, which is why the process is verified by test rather than by appearance.
The sequence for a lot of parts
- Intake and lot control. Each shipment is logged against the purchase order with part number, alloy, quantity and the requested specification. Mixed alloys are separated before they reach a tank because 303, 416 and other free-machining or martensitic grades need different chemistry and time than 304L and 316L.
- Cleaning per ASTM A380. Cutting oil, coolant, marking ink and fingerprints block the acid. Parts are degreased in an alkaline cleaner, rinsed and inspected for a water-break-free surface before passivation. A part that beads water is not clean and goes back.
- Passivation bath. Citric acid or nitric acid per the A967 method the customer specifies, at the concentration, temperature and time the method calls for. Parts sit in baskets or on racks so every surface, including blind holes and threads, is wetted; small parts are agitated, complex parts are oriented so pockets drain.
- Rinse and dry. Deionized water rinse to remove residual acid, then drying so no rinse water is trapped in threads or bores where it would leave deposits.
- Verification per lot. The A967 test the specification names, run on parts from the lot or on witness coupons processed with it.
- Certificate and packaging. A certificate of passivation for the lot, then packaging that keeps the parts from touching carbon steel on the way back.
Citric vs nitric for parts
| Factor | Citric (A967 Citric 1 to 5) | Nitric (A967 Nitric 1 to 5) |
|---|---|---|
| Alloys | All austenitic, ferritic, precipitation-hardening and most martensitic grades | Same range; Nitric 1 and 2 with dichromate historically used for free-machining and martensitic grades |
| Free-machining 303, 416, 420 | Works, with attention to time and temperature to avoid sulfide attack | Traditional choice; sodium dichromate versions are being phased out for environmental reasons |
| Bath handling | Non-fuming, biodegradable, lower disposal burden | Oxidizing acid, fume extraction and neutralization required |
| Typical process | 4 to 10 percent by weight, room temperature to 70 C, 4 to 20 minutes depending on the method number | 20 to 50 percent by volume, 21 to 60 C, 20 to 30 minutes depending on the method number |
| Surface after treatment | Unchanged appearance | Unchanged appearance; over-exposure can etch some grades |
| Documentation | Method number, concentration, temperature, time recorded per lot | Same |
Both routes meet A967 when the verification test passes. Medical device and food equipment customers usually specify citric because it keeps nitric fumes and dichromate out of the process; aerospace and defense drawings that cite AMS 2700 or the older QQ-P-35 often still call a nitric method by name. We run the method the drawing calls for, not a substitute.
Verification tests and what each one proves
| A967 test | What it detects | Where it is typically used |
|---|---|---|
| Water immersion | Free iron, shown as rust after alternating immersion and drying cycles | General parts, low cost, slow (1 hour cycles for 24 hours) |
| High humidity | Free iron, shown as rust after 24 hours at 97 percent humidity and 38 C | General parts, requires a humidity cabinet |
| Salt spray | Free iron and marginal passivation, shown as rust after 2 hours at 5 percent neutral salt fog | Parts going into corrosive service; a stricter screen |
| Copper sulfate | Free iron, shown as copper plating out on the surface within 6 minutes | Quick shop check; not permitted on parts for food or surgical use because copper residue is a contaminant |
| Potassium ferricyanide-nitric acid | Free iron, shown as blue spots within 30 seconds | Sensitive check for machined surfaces; not for parts that will contact food or tissue |
| Damp cloth | Gross free iron, shown as rust stains on a cloth held against the surface | Screening of large simple surfaces |
The specification, not the shop, chooses the test. Where a drawing is silent we recommend water immersion or high humidity for general parts and salt spray where the part will see chlorides, and we never run copper sulfate or ferricyanide on implant, surgical or food-contact parts.
Implant and surgical parts: ASTM F86
Parts made from implant alloys such as 316L per ASTM F138, cobalt-chromium and titanium alloys are governed by ASTM F86 rather than A967. F86 covers surface preparation and marking of metallic surgical implants and sets a passivation step after all machining, polishing and marking are complete, with attention to residues that would sit in a marking or a pocket. For these lots we hold the parts as a separate stream, use fresh chemistry, and record the passivation as a step in the customer’s device history record under 21 CFR 820 and the Quality Management System Regulation.
Parts passivation vs in-place system passivation
A batch of parts is passivated by immersion, every surface in contact with fresh chemistry at a known temperature, verified by pulling a sample from the lot. An installed system is passivated by circulating chemistry through it, where flow, temperature and contact time vary along the loop and verification relies on surface samples, rinse water chemistry and coupons inserted in the flow path. Batch work is cheaper per surface and easier to prove. System work is the only option once the tube is welded in and the part cannot be moved. When an OEM builds a skid, the best sequence is often both: passivate the components in batch before assembly, then passivate the completed skid after welding so the heat-affected zones are treated too.
What to require from a parts passivation service
- A written procedure that names the A967, AMS 2700 or F86 method, with concentration, temperature and time controlled and recorded per lot.
- A380 cleaning with a water-break test before the passivation bath, documented.
- Bath chemistry monitored and changed on a schedule, with titration records available.
- Alloy separation, so free-machining and martensitic grades never share a bath cycle with austenitic parts.
- The verification test your drawing specifies, run on parts or witness coupons from the same lot, with the result on the certificate.
- A certificate of passivation per lot that states the specification, method, parameters, test, result, quantity, part number, date and the person responsible.
- Packaging that separates parts from carbon steel and from each other where finish matters.
- Traceability from receiving to shipping, so a part can be tied to its bath log a year later.
What Paul Industries does
Paul Industries passivates lots of OEM and machine-shop parts to ASTM A967 citric and nitric methods, AMS 2700 and ASTM F86, with A380 cleaning, lot-level verification and a certificate of passivation for every lot. The same crews perform in-place passivation and derouging of installed piping, vessels and water systems, so a customer building equipment can have components passivated before assembly and the finished system passivated after welding under one procedure and one set of records. Send drawings and the applicable specification with your request and we will confirm method, test and turnaround before the parts ship.
Standards referenced: ASTM A967 · ASTM A380 · ASTM F86 · ASME BPE
Frequently asked questions
What is parts passivation?
Parts passivation is the batch chemical treatment of finished stainless steel components in a citric or nitric acid bath to remove free iron and exposed sulfides left by machining and handling, so the surface reforms its protective chromium oxide. Each lot is cleaned, immersed, rinsed, tested and certified as a unit, which is what makes it different from passivating an installed system.
Do you offer a passivation service for parts as well as for installed systems?
Yes. Paul Industries provides batch passivation of OEM parts and assemblies by immersion, and in-place passivation of installed piping, vessels and water systems by circulation. Many equipment builders use both: components passivated before assembly, then the finished welded skid passivated after fabrication so the heat-affected zones are treated.
Which specification should I put on the drawing for OEM parts passivation?
ASTM A967 is the general specification for chemical passivation of stainless parts and lets you name a citric or nitric method. AMS 2700 is common on aerospace and defense drawings. ASTM F86 governs metallic surgical implants. Name the specification, the method number if you have a preference, and the verification test you want on the certificate.
Can free-machining grades like 303 and 416 be batch passivated?
Yes, with the right method and time. Free-machining and martensitic grades are more sensitive to acid attack on their sulfide inclusions, so they are run as separate lots with chemistry chosen for the grade. Mixing them in the same bath cycle as 304L or 316L parts is a common cause of etched or stained parts.
How is a lot of passivated parts verified?
By one of the ASTM A967 practices named on the specification: water immersion, high humidity, salt spray, copper sulfate, potassium ferricyanide-nitric acid or damp cloth. The test is run on parts drawn from the lot or on witness coupons processed with it, and the method and result are recorded on the certificate for that lot.
Why is copper sulfate testing not used on medical or food-contact parts?
The copper sulfate and ferricyanide tests leave chemical residue on the surface they touch. On implant, surgical or food-contact parts that residue is a contaminant, so ASTM A967 restricts those tests and the verification is done by water immersion, humidity or salt spray, or on witness coupons rather than the parts themselves.
What does batch passivation cost compared with passivating a system?
Batch work is priced per lot, driven by part count, size, alloy, the method specified and the verification test required, and it is generally cheaper per surface than circulating chemistry through an installed system because every surface is immersed in fresh chemistry at once. Quotes itemize cleaning, passivation, testing and documentation so the two can be compared.
What should the certificate of passivation for a lot contain?
The specification and method used, bath concentration, temperature and immersion time, the cleaning step, the verification test and its result, part number, quantity, lot or heat identification, date, and the name of the person responsible. A certificate that only says the parts were passivated per A967 without those parameters will not satisfy a device or aerospace auditor.
Does passivation change the appearance or dimensions of a machined part?
Correctly done, no. Passivation removes a few atomic layers of iron and sulfide from the surface and does not measurably change dimensions or finish. Etching, dulling or staining indicates the wrong method for the alloy, contaminated chemistry or excessive time, and is a defect rather than a normal result.
How should parts be prepared before they are sent for passivation?
Deburr and complete all machining, grinding and marking first, since passivation must be the last step. Remove heavy oil where practical, keep parts from contacting carbon steel, and ship them in clean containers with the drawing, alloy and specification. Parts arriving with heat tint from welding may need pickling or mechanical cleaning before passivation, which we will flag at intake.
Can a welded assembly be batch passivated, or does it need in-place treatment?
If the assembly fits the tank it can be immersed, which is the better treatment because every surface sees fresh chemistry. Assemblies too large to immerse, or already installed, are passivated by circulation or by spray and soak. Weld heat tint is not removed by passivation and needs pickling or mechanical removal first in either case.
Which standard governs the cleaning step before a lot is passivated?
ASTM A380 covers the cleaning and descaling practice that must precede passivation, including the water-break test used to confirm a surface is free of oils and residues. Parts that fail it go back for recleaning, because an acid bath cannot reach a surface that is still carrying coolant or fingerprints. The passivation itself and its acceptance testing are then performed to ASTM A967.
How are implant parts handled differently in batch passivation?
Implant alloys are passivated per ASTM F86 after all machining, polishing and marking are complete, as a separate stream with fresh chemistry, and the step is recorded so it can be entered in the device history record under 21 CFR 820. Verification avoids residue-leaving tests, and packaging and handling are controlled to prevent recontamination.
Is titanium or aluminum passivated the same way as stainless parts?
No. ASTM A967 and the acid baths described here are for stainless steel. Titanium implant alloys covered by ASTM F86 use their own cleaning and passivation practice, and aluminum is treated by chromate or non-chromate conversion coating, which is a different process. Send the alloy with the drawing so the correct process is quoted.
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