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Passivation: the certificate without the nitric

The citric route to AMS 2700 and ASTM A967 conformance — NASA-validated, ultrasonically assisted, and cheaper to run than the nitric line it replaces.

6 min readBy Misonics application engineers

Stainless steel is only stainless when its chromium-oxide layer is intact — and machining, welding and handling leave free iron that corrodes exactly where the certificate says it will not. This study documents the citric passivation route: NASA-validated as equivalent or better than nitric, conforming to AMS 2700 and ASTM A967, without the nitric acid handling burden — and the operating-cost workbook that quantified the switch.

Industry
Aerospace · medical · food · marine
Components
Machined and welded stainless assemblies
Contamination
Free iron · shop dust · weld discolouration
Process
Clean → citric passivate → rinse → verify
Chemistry
Brulin AquaVantage Passivation — citric
Standards
AMS 2700 · ASTM A967 · ASTM A380

The problem citric solves twice

Free iron contamination

Tooling and handling embed iron in the stainless surface; it rusts, stains and seeds pitting. Passivation dissolves it and lets the chrome-oxide layer reform — the whole point of the process.

The nitric burden

Traditional nitric passivation works — at the price of fuming acid handling, NOx exposure risk, neutralisation, disposal and the WHS file that follows it everywhere.

The evidence base

NASA’s published comparative studies found citric passivation equal or superior on the alloys tested — the reference that lets a quality system change without argument.

The process

Process diagram
LOADparts in basketSTAGE 1 · CLEANUltrasonic pre-cleanoils & shop soil off first —passivation is not cleaningASTM A380 disciplineSTAGE 2 · PASSIVATECitric immersioncitric solutionfree iron dissolvedSTAGE 3 · RINSEDI cascade rinseacid clearedspot-free surfacesSTAGE 4 · VERIFYTest & certifywater-break / ferroxyl /copper sulphate per speccertificate issuedRESULTverified & loggedUltrasonics improves both wet stages: cavitation cleans the pre-clean and refreshes solution at the surface through the passivation soak.
ParameterValue
Pre-cleanUltrasonic alkaline clean — passivation only works on a clean surface (ASTM A380)
PassivationCitric acid solution
RinseDI water cascade
VerificationPer specification: water-break, ferroxyl or copper-sulphate test
ConformanceAMS 2700 (citric methods) · ASTM A967 · ASTM A380 practice

The results

Specification conformance without nitric. AMS 2700 and ASTM A967 both recognise citric methods — the certificate reads the same, the hazard file does not.
NASA-validated equivalence. The published studies on file found citric equal or better across the tested alloys — the evidence a quality manager needs to approve the change.
Operating cost down. The BAE-pattern cost workbook on file quantifies the saving across chemistry, neutralisation, disposal and handling.
Safer line, same throughput. No fuming acid, no NOx controls, simpler waste stream — the WHS argument writes itself.

Hardware and chemistry

Multi-tank ultrasonic line with rinse cascade

Passivation line equipment

Heated ultrasonic pre-clean, citric immersion and cascade rinse tanks — bench scale to production line.

Ask about passivation lines →
Brulin AquaVantage Passivation citric passivation chemistry

Passivation chemistry

Brulin AquaVantage Passivation — a dilutable citric chemistry made for passivating stainless, with the conformance documentation to match.

The passivation method →
The compliance short-cut that isn’t a short-cut: citric is not the lenient option — it is the modern method the specifications themselves recognise, with a federal research agency’s test program behind it.
Switch the passivation line

Passivation to your specification

Book a passivation trialParts passivated, tested and certified against your specThe chemical rangeOrganised by soil familyPassivation to specificationQuoted per job — talk to our engineer
FAQ

Questions about this process

Does citric passivation meet the standard?
Yes. The process conforms to AMS 2700 and ASTM A967, and NASA's published comparative studies found citric equal or better than nitric on the alloys tested.
Why move away from nitric?
Nitric works, at the price of fuming acid handling, NOx exposure, neutralisation, disposal and the WHS file that follows it. Citric removes that burden and costs less to run.
Is cleaning part of passivation?
It comes first. Oils and shop soil come off in an ultrasonic pre-clean to ASTM A380 discipline, then the citric immersion dissolves free iron and lets the chromium-oxide layer reform.

Related: MSX floor-standing tanks · Cleaning before plating and coating · All case studies

What are you cleaning?

Tell us what the part is, what comes off it, and what it has to pass. Technical sales will tell you whether ultrasonics is the right answer for that part and what it takes to run it on your own floor.

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Keep Reading

Next in the series

The chemistry guide

Acids, alkalines and where passivation fits.

Multistage cleaning

Clean-treat-rinse-verify as a line.

Life sciences industry page

Hygiene-critical stainless applications.

Planning a passivation line in a new plant? See the worked example on engineered industrial cleaning systems, with renders and the general arrangement.
The full method, with the ASTM A967 treatments and tests, is in the citric acid passivation guide.