Understanding the chemistry, process, and regulatory requirements that prevent costly FDA audit failures
The Hidden Cost of Improper Passivation
Passivation failures in FDA-regulated facilities are expensive. Beyond the immediate costs of halted production and emergency remediation, failed audits trigger cascading problems: customer contract penalties, re-validation expenses, regulatory scrutiny, and damaged reputation.
The most common pattern? Companies cut corners on passivation chemistry to save small amounts of money—then pay exponentially more when things go wrong.
⚠ What Happens When Stainless Steel Isn't Properly Passivated
- Surface corrosion begins within days despite the "stainless" name
- Product contamination risk in pharmaceutical and food applications
- FDA 483 observations and warning letters for medical device manufacturers
- Failed USDA inspections for food processing equipment
- Premature equipment failure costing 10-20x the passivation cost
- Production halts ranging from days to months for remediation
The chemistry is inexpensive next to a failed audit. Whether the line uses phosphoric acid for cleaning or nitric or citric acid for passivation, run it as a controlled process step with recorded concentration, temperature, time and test results, not as a cost center.
What Is Passivation (And Why Does It Matter)?
Proper passivation creates a stable chromium oxide layer that protects against corrosion
Stainless steel isn't inherently "stainless." Its corrosion resistance comes from a thin, invisible chromium oxide layer (Cr₂O₃) that forms naturally on the surface when exposed to oxygen. This passive layer is what protects the underlying iron from rust.
But here's the problem: manufacturing processes contaminate and damage this protective layer.
What Destroys the Passive Layer
- Welding: Heat tint, scale, and discoloration from oxidation
- Machining: Embedded iron particles and tool marks
- Forming/bending: Surface stress and micro-cracks
- Grinding: Embedded carbon steel particles from grinding wheels
- Handling: Fingerprints, oils, and surface contaminants
- Storage: Surface oxidation and environmental contamination
Without proper passivation, these contaminants create initiation sites for corrosion—often invisible to the naked eye but detectable through testing and, eventually, through product failures.
The Role of Phosphoric Acid
Phosphoric acid is not one of the passivation treatments ASTM A967 defines (those are nitric acid immersion, citric acid immersion and electrochemical treatment). Where it earns its place is cleaning and descaling, which works in two stages:
- Cleaning stage: Removes surface contaminants, free iron, and embedded particles
- Passive film stage: On the cleaned surface the chromium oxide film re-forms on its own in air; ASTM A380 (2017 edition) describes this film as forming spontaneously once the surface is thoroughly clean
Unlike nitric acid passivation (which is more aggressive and generates hazardous fumes), phosphoric acid provides:
- Better safety profile: Lower fume generation, safer handling
- Same verification tests: Cleaned parts can be checked with the ASTM A967 test practices, though passing a test does not make phosphoric acid an A967 treatment
- Lower disposal costs: Less hazardous waste stream
- Equipment compatibility: Less aggressive to tanks and fixtures
✓ Check the Drawing Before You Choose the Acid
If a drawing, purchase order or validation protocol calls out ASTM A967, use one of the treatments the standard sets out: nitric acid, citric acid or electrochemical. Its definition of passivation also covers other chemical solutions capable of producing similar results, and any chemistry used has to pass the tests the specification names. A phosphoric acid clean can come before the treatment but is not one of the treatments A967 lists. If the requirement only says "passivate," ASTM A380 (2017 edition) reads that as removing exogenous iron with an acid that does not significantly affect the stainless steel; check that your acid and conditions fall within A380 section 6.4 and its acid-cleaning annex, and confirm the result with the tests your specification names.
The Chemistry of Passivation: What's Actually Happening
Understanding the chemistry helps explain why concentration, temperature, and time all matter—and why cutting corners leads to failures.
Stage 1: Surface Cleaning & Iron Removal
Phosphoric acid dissolves free iron and iron oxides on the surface:
3Fe + 2H₃PO₄ → Fe₃(PO₄)₂ + 3H₂↑
Fe₂O₃ + 2H₃PO₄ → 2FePO₄ + 3H₂O
This reaction removes:
- Embedded iron particles from machining/grinding
- Surface rust and oxides
- Heat tint from welding
- Contamination from carbon steel tooling
Stage 2: Chromium Oxide Layer Formation
With free iron removed, the chromium in the stainless steel reacts with oxygen (from air or the acid solution) to form a stable passive layer:
4Cr + 3O₂ → 2Cr₂O₃
This chromium oxide layer is:
- Self-healing: Reforms if scratched (in the presence of oxygen)
- Chemically stable: Resists further oxidation
- Biocompatible: Critical for medical and pharmaceutical applications
- Thickness: A few nanometers. One study measured about 4.85 nm on stainless steel in water by atomic force microscopy and 3.8 nm in vacuum by electron microscopy (Scientific Reports, 2019)
⚠ Critical Parameters for Success
Concentration: Hold the concentration your specification sets for the bath. Too weak won't clean effectively; too strong can over-etch the surface.
Temperature: Hold the temperature your specification sets. Lower temperatures slow the reaction; higher temperatures can damage the substrate.
Time: Hold the contact time your specification sets. Rushing the process leaves contaminants behind.
Rinsing: Thorough DI water rinse removes acid residues that could interfere with passive layer formation.
💡 Two Failures That Are Easy to Miss
Two failures are easy to miss: skipping the pre-cleaning step, and rinsing with tap water instead of DI water. Both leave contaminants that prevent proper passive layer formation—and both are easy to fix once you know to look for them.
Does Stainless Steel Need to Be Passivated?
Not always. ASTM A380 (2017 edition) states that stainless steel forms its passive film spontaneously in air once the surface is thoroughly clean, and that an oxidizing chemical step is generally not necessary for the film to form. Passivation is needed when fabrication has left free iron or other contamination on the surface, or when a drawing or customer calls for it.
Do you passivate before or after machining or welding?
After. ASTM A967 lists iron-tool marks, embedded iron, iron dust and iron deposits in welds among the forms of free iron passivation removes, so machine, weld and grind first, degrease, then passivate, and keep carbon-steel tools off the part afterward.
Will vinegar passivate stainless steel? Can you passivate at home?
Vinegar is dilute acetic acid, which is not one of the treatments ASTM A967 lists (nitric acid, citric acid, electrochemical). Citric acid is on that list, which is why small shops use it. Read the SDS first: nitric acid has an OSHA PEL of 2 ppm and an IDLH of 25 ppm (NIOSH Pocket Guide), so it belongs under ventilation, not on a kitchen counter.
What Does ASTM A967 Actually Specify?
ASTM A967 is the specification for chemical passivation treatments of stainless steel parts. Its published abstract names three treatment families: nitric acid immersion, citric acid immersion and electrochemical treatment, plus seven tests that confirm free iron has been removed. Phosphoric acid is not on the treatment list.
- Parameters: concentrations, temperatures and times for each treatment are in the standard, which ASTM sells; we do not reproduce them.
- Acceptance: a chemically clean surface with no etching, pitting or frosting on visual inspection.
- Selection: A967 makes no recommendation on which grade, treatment or acceptance criteria suit an application; the purchaser's specification decides.
- Editions: current is A967/A967M-25; earlier ones include A967-05, -13 and -17. Check which one your drawing cites.
Which tests does ASTM A967 use to confirm passivation?
Seven qualitative practices: A water immersion, B high humidity, C salt spray, D copper sulfate, E potassium ferricyanide-nitric acid, F damp cloth and G boiling water immersion. Your specification picks which apply; exposure conditions and pass criteria, including for the high humidity test, are in the purchased standard.
Which standard covers surgical implants?
ASTM F86, the practice for surface preparation and marking of metallic surgical implants, written to improve the corrosion resistance of implant surfaces and markings.
The Passivation Process, Step by Step (and Where ASTM A967 Fits)
Properly passivated stainless steel maintains its corrosion resistance for years
ASTM A967 is the standard specification for chemical passivation treatments for stainless steel parts. The steps below follow its sequence: clean, passivate, rinse, dry, test. Phosphoric acid, which A967 does not list as a treatment, belongs in the cleaning step; the passivating bath is the treatment your specification names, run at the parameters the standard gives.
Materials Required
- Passivating acid your specification names - for ASTM A967, nitric or citric acid (or electrochemical); phosphoric acid only for the cleaning step
- Deionized or distilled water - for rinsing
- Immersion tank - corrosion-resistant (plastic, rubber-lined, or 316L stainless)
- Temperature control - heaters and thermometers
- Timing equipment - for process control
- Test solutions - copper sulfate or salt spray for verification
Step-by-Step Process
Pre-Cleaning
Remove oils, greases, and organic contaminants using alkaline cleaner or solvent. This step is critical—oils and greases will interfere with acid contact and prevent proper passivation.
Rinse thoroughly with DI water before proceeding to passivation.
Passivation Bath Preparation
Prepare the passivating bath your specification names (for ASTM A967, a nitric or citric acid treatment) at the concentration and temperature the standard gives. Use DI or distilled water for dilution to avoid introducing contaminants.
Verify concentration with hydrometer or titration before use.
Immersion
Fully submerge parts in the passivating bath. Ensure complete coverage—trapped air pockets will leave areas unpassivated.
Maintain temperature throughout the process. Use agitation if possible to ensure uniform treatment.
Dwell Time
Hold parts in the bath for the time your specification sets for the chosen treatment, and record the actual time for each batch.
Do not rush this step. Insufficient dwell time is a common cause of passivation failure.
Rinse
Remove parts and immediately rinse with flowing DI or distilled water for at least 2-3 minutes. Rinse until pH neutral (test with pH paper).
Critical: Acid residues left on the surface will prevent proper passive layer formation.
Drying
Air dry in a clean environment or use forced air. Avoid wiping with cloths that could recontaminate the surface.
Protect from contamination during and after drying.
Verification Testing
Perform acceptance testing per ASTM A967, using the practices your specification calls for:
- Water immersion test (Practice A): no rust spots after immersion
- High humidity test (Practice B): no rust after humidity exposure
- Copper sulfate test (Practice D): no copper deposit, which would indicate free iron
- Salt spray test (Practice C): duration per specification
Exposure times, humidity levels and pass criteria for each practice are in the purchased standard.
Documentation
Record batch number, concentration, temperature, time, and test results. This documentation is essential for FDA, ISO, and customer audits.
✓ Signs of Successful Passivation
- Uniform matte or bright finish (depending on base finish)
- No visible discoloration or staining
- Passes copper sulfate test with no copper plating
- No rust spots after the water immersion test
- No etching, pitting or frosting on visual inspection
- Clean, non-reactive surface
What Goes Into a Stainless Steel Passivation Line?
A passivation line is a row of tanks run in the order the standards describe: precleaning, descaling if needed, the passivating bath, rinsing, drying and testing. For ASTM A967 work the passivating tank holds one of the treatments the standard sets out, usually nitric or citric acid; the rest of the line gets parts into it clean and out of it without new iron.
- Precleaning: alkaline, solvent or detergent cleaners, vapor degreasing or ultrasonics (ASTM A380 lists these and more). Check with a water-break test.
- Descaling: chemical descaling, acid pickling or mechanical methods for heat tint and scale. A380 (2017 edition) notes pickling can itself provide passivation in the iron-removal sense.
- Passivating tank: the treatment your spec names, in a tank material rated for that acid.
- Rinse tanks: stagnant, countercurrent or spray rinses, with or without a neutralizing step (the A967 abstract allows each). The rinse stops the acid reaction. Finish with DI water.
- Test and record: the A967 practices your spec calls for, with concentration, temperature, time and lot number.
A380 asks that parts and equipment be designed so cleaning solutions cannot get trapped and can circulate and drain; blind holes and poorly drained racks are where rinse failures start. For tanks and piping passivated in place, A380 covers installed systems (not equipment already in service), with the same sequence circulated or sprayed through.
Common Passivation Failures (And How to Avoid Them)
What causes passivation to fail? Most failures trace to contamination the acid never reached or iron put back afterward: oil left by skipped precleaning, a bath outside its specified concentration, temperature or time, a tap-water final rinse, or bare hands and carbon-steel tools touching the part after the bath. A failed copper sulfate or water immersion test is usually how it shows up.
1. Inadequate Cleaning Before Passivation
Problem: Oils and greases prevent acid contact, leaving contaminated areas unpassivated.
Solution: Use alkaline cleaner or vapor degreaser before passivation. Verify with water break test—water should sheet uniformly on clean surfaces.
2. Wrong Acid Concentration
Problem: Too weak doesn't remove contaminants; too strong can over-etch and damage the surface.
Solution: Test and verify concentration before each batch, and hold the value your specification sets for the bath.
3. Insufficient Temperature
Problem: A bath below its specified temperature works too slowly and may not fully clean the surface.
Solution: Hold the specified temperature throughout the process. Monitor with a calibrated thermometer.
4. Too Short Dwell Time
Problem: Rushing the process leaves iron contamination that will rust.
Solution: Hold the full contact time your specification sets, and record it for each batch.
5. Poor Rinsing
Problem: Acid residues prevent proper passive layer formation and can cause staining.
Solution: Use flowing DI or distilled water. Rinse until pH neutral (test with pH strips). Minimum 2-3 minutes flowing rinse.
6. Contamination After Passivation
Problem: Touching with bare hands, dirty gloves, or contaminated tools reintroduces iron.
Solution: Handle only with clean nylon gloves or stainless tools. Store in clean, dry environment.
7. Using "Generic" or Off-Spec Phosphoric Acid
Problem: Industrial-grade phosphoric acid may contain contaminants (iron, heavy metals) that defeat the purpose of passivation.
Solution: Use a grade whose Certificate of Analysis shows low metal content, and check it against your process limits.
⚠ Common Cost-Cutting Mistakes
The most expensive passivation failures come from three cost-cutting decisions:
- Switching to "technical grade" phosphoric acid (often contains excess iron contamination)
- Reducing dwell time to "increase throughput" (leaves insufficient time for iron removal)
- Skipping verification testing to "save costs" (failures discovered only during audits or customer complaints)
These shortcuts might save a few thousand dollars annually in operating costs—but result in audit failures, production shutdowns, and remediation expenses that can easily exceed $100,000-500,000.
💡 Rust Spots Weeks After Passivation?
Check the final rinse water first. City tap water carries dissolved minerals and chlorides onto the freshly cleaned surface; a final rinse in DI water removes that variable.
Industry Applications: Where Passivation Is Critical
Medical Device Manufacturing
Why it matters: FDA 21 CFR Part 820, now the Quality Management System Regulation, requires device manufacturers to document a quality management system that complies with ISO 13485 (21 CFR 820.10), so a passivation step on device parts runs under that documented system with its records. Surgical instruments, implants, and manufacturing equipment must be corrosion-free.
Standards: ASTM A967, ISO 13485, FDA guidance documents
Typical parts: Surgical instruments, implantable devices, sterilization equipment, cleanroom fixtures
Pharmaceutical Processing
Why it matters: Product contact surfaces must not shed particles or react with formulations. cGMP requirements mandate validated cleaning procedures.
Standards: ASTM A967, ASME BPE, 3-A Sanitary Standards
Typical equipment: Reaction vessels, storage tanks, transfer lines, filling equipment, CIP systems
Food & Beverage Processing
Why it matters: USDA and FDA regulations require corrosion-resistant, sanitary surfaces. Failed passivation leads to rust contamination in product.
Standards: 3-A Sanitary Standards, NSF/ANSI 51, ASTM A967
Typical equipment: Processing tanks, piping systems, heat exchangers, conveyors, filling lines
Aerospace Components
Why it matters: Corrosion causes structural failures. Aerospace requires documented passivation with lot traceability.
Standards: AMS 2700, ASTM A967, customer specifications
Typical parts: Fasteners, fittings, hydraulic components, structural assemblies
Semiconductor & Cleanroom
Why it matters: Contamination from corroding surfaces causes defects. Ultra-high purity requirements demand pristine surfaces.
Standards: SEMI standards, ASTM A967, customer specifications
Typical equipment: Gas delivery systems, chemical distribution, process tools, cleanroom fixtures
Pharmaceutical and food processing facilities depend on properly passivated equipment
Phosphoric Acid vs. Nitric Acid Passivation
Is phosphoric acid an ASTM A967 passivation treatment?
No. The ASTM A967 abstract names nitric acid, citric acid and electrochemical treatments, and phosphoric acid is not among them. The standard's definition also covers other chemical solutions capable of similar results, which still have to pass the tests the specification names. Phosphoric acid is a cleaner and descaler that can come before an A967 treatment.
| Property | Nitric acid | Citric acid | Phosphoric acid |
|---|---|---|---|
| Formula | HNO₃ | C₆H₈O₇ | H₃PO₄ |
| CAS | 7697-37-2 | 77-92-9 | 7664-38-2 |
| Named in ASTM A967? | Yes, nitric acid immersion | Yes, citric acid immersion | No |
| OSHA PEL (8-hr TWA) | 2 ppm (5 mg/m³) | None listed (OSHA Z-1) | 1 mg/m³ |
| Typical role | Passivating bath; A380 (2017) calls a nitric acid solution a mild oxidant | Passivating bath | Cleaning, descaling, pretreatment |
Sources: ASTM A967/A967M-25; PubChem: nitric acid; PubChem: citric acid; NIOSH: nitric acid; NIOSH: phosphoric acid; 29 CFR 1910.1000, Table Z-1.
| Factor | Phosphoric Acid (cleaning step) | Nitric Acid (ASTM A967 treatment) |
|---|---|---|
| Safety | Lower fume generation Less corrosive to handle Safer for workers |
High fume generation (NOx) Highly corrosive Requires extensive ventilation |
| Effectiveness | Excellent for cleaning Removes free iron effectively Good passive layer formation |
Very aggressive Fast iron removal Excellent passive layer |
| Environmental | Easier waste treatment Lower disposal costs Less hazardous waste |
Nitrogen oxides (air emission) Higher disposal costs Hazardous waste stream |
| Equipment | Less demanding on tanks Longer equipment life Plastic tanks acceptable |
Aggressive to equipment Requires special materials Higher maintenance costs |
| Cost | Moderate chemical cost Lower PPE/ventilation costs Lower disposal costs |
Moderate chemical cost High ventilation requirements High disposal costs |
| Standards | Not an ASTM A967 treatment Cleaning and descaling (ASTM A380) Check your spec before substituting |
Named in ASTM A967 (nitric acid immersion) Traditional standard Widely specified |
💡 Where Phosphoric Acid Fits
Where a spec calls out ASTM A967, the passivating bath itself still has to be one of the treatments the standard lists; phosphoric acid can do the cleaning and descaling before it.
Safety and Regulatory Compliance
Personal Protective Equipment (PPE)
⚠ Required Safety Equipment
- Eye protection: Chemical splash goggles and face shield for concentrated solutions
- Hand protection: Neoprene or nitrile gloves (check chemical compatibility)
- Body protection: Chemical-resistant apron or suit
- Respiratory: Adequate ventilation; respirator if fumes present
- Emergency equipment: Eyewash station and safety shower within 10 seconds travel time
Regulatory Documentation Requirements
For FDA-regulated industries, proper documentation is as important as the process itself:
- Validated procedure: Written SOP with critical parameters defined
- Batch records: Solution concentration, temperature, time for each run
- Test results: Copper sulfate or other verification test results
- Certificate of Analysis: For phosphoric acid used (metal content, assay)
- Equipment calibration: Thermometers, timers, concentration testing equipment
- Personnel training: Documentation that operators are trained and qualified
Waste Disposal
Spent phosphoric acid passivation solutions contain dissolved metals and must be disposed of properly:
- Neutralization: Raise pH to 6-9 using sodium hydroxide or calcium hydroxide
- Metal precipitation: Heavy metals precipitate as hydroxides
- Separation: Settle and filter precipitated solids
- Disposal: Solid metal sludge to hazardous waste facility; treated liquid per local regulations
Note: Check local, state, and federal regulations. Some jurisdictions have specific requirements for metal-bearing waste streams.
Get Technical Support for Your Passivation Process
Questions about phosphoric acid passivation, ASTM A967 compliance, or selecting the right chemistry? Our team has practical experience helping regulated manufacturers solve real problems.
How We Can Help:
- Product Selection: Which concentration and grade for your application?
- Process Troubleshooting: Why did your passivation fail verification testing?
- Documentation: What COA information do you need for audits?
- Regulatory Questions: How to interpret ASTM A967 requirements
- Pricing & Availability: Quote for your volume requirements
📞 Direct Contact
Phone: (512) 365-6838
Email: sales@alliancechemical.com
Response Time: We respond to technical inquiries within one business day
Ask for Andre Taki or our technical team with questions about passivation applications.
About Alliance Chemical
Chemical Supplier to Regulated Industries
We stock the acids used on cleaning and passivation lines: nitric acid, citric acid and phosphoric acid.
Our Customers Trust Us For:
- COA Documentation: Certificate of Analysis on request, no charge, with metal content verification
- Regulatory Compliance Support: We help navigate ASTM A967, FDA, and industry requirements
- Technical Problem-Solving: Real expertise in troubleshooting passivation failures
- Reliable Supply: Consistent quality, on-time delivery for production schedules
- Grade Options: each product page lists the grades we stock
- Bulk and Drum Quantities: From 5-gallon pails to bulk tanker delivery
Technical Support from Real Experts
Andre Taki, Chief Commercial Officer brings deep knowledge of chemical applications in regulated manufacturing. Our team can help quality managers and engineers:
- Select the right passivation chemistry for their specific alloys and applications
- Troubleshoot failed passivation processes (wrong concentration, temperature, time)
- Understand ASTM A967 requirements and verification testing
- Source proper documentation for FDA and customer audits
- Calculate dilution ratios and determine batch quantities
- Establish proper safety protocols and waste disposal procedures
Direct Access: Call or email with technical questions. We respond to inquiries within one business day.
What We Don't Claim
We're a chemical supplier, not a certification body or consulting firm. We don't hold ISO certifications ourselves, and we don't provide formal process validation services. What we do provide: quality chemicals with the COA on request at no charge, accurate technical information, and practical troubleshooting guidance.
Setting Up a Cleaning and Passivation Line?
Phosphoric acid for pre-passivation cleaning and descaling. Need the Certificate of Analysis? Just ask, we send it at no charge.
View Products & Get QuoteAvailable in multiple concentrations and packaging sizes | Technical support included
References & Authoritative Sources
Chemical identity, properties, and safety data sourced from the U.S. National Library of Medicine's PubChem database — the authoritative open-chemistry data resource maintained by the National Institutes of Health.
- PubChem CID 962: Deionized Water — National Center for Biotechnology Information, U.S. National Library of Medicine. CAS 7732-18-5.
- PubChem CID 14798: Sodium Hydroxide 50% Membrane Grade (Caustic Soda, Lye) — National Center for Biotechnology Information, U.S. National Library of Medicine. CAS 1310-73-2.
- PubChem CID 1004: Phosphoric Acid 75% — National Center for Biotechnology Information, U.S. National Library of Medicine. CAS 7664-38-2.
- ASTM A967/A967M-25 (abstract and scope), ASTM International. Accessed 2026-09-28.
- ASTM A967/A967M-17, scope, Note 1 (forms of free iron), ASTM International. Accessed 2026-09-28.
- ASTM A380/A380M-25 (abstract and scope), ASTM International. Accessed 2026-09-28.
- ASTM A380/A380M-17, scope 1.1.1 (meanings of passivation), ASTM International. Accessed 2026-09-28.
- ASTM F86-21 (scope), ASTM International. Accessed 2026-09-28.
- NIOSH Pocket Guide to Chemical Hazards: Phosphoric acid, CDC/NIOSH. Accessed 2026-09-28.
- NIOSH Pocket Guide to Chemical Hazards: Nitric acid, CDC/NIOSH. Accessed 2026-09-28.
- PubChem Compound Summary: Citric Acid (CID 311), National Center for Biotechnology Information, U.S. National Library of Medicine. Accessed 2026-09-28.
- 21 CFR 820.10, Requirements for a quality management system, eCFR, U.S. Government Publishing Office. Accessed 2026-09-28.
- Wang et al., AFM measurement of passive film thickness on stainless steel, Scientific Reports 9, 13094 (2019), PubMed Central. Accessed 2026-09-28.
Related: Phosphoric Acid: From Cola to EV Batteries — Uses, Grades & Rust Removal — the same acid-passivation principle that leaves a protective metal finish.
Sourcing for this job? See our citric acid supplier page — every grade and strength we stock, with live pricing from quarts through drums and totes.
Key numbers and sources
| Fact | Value | Source |
|---|---|---|
| Passivation treatments named in ASTM A967 | Nitric acid immersion, citric acid immersion, electrochemical (phosphoric acid not listed) | store.astm.org |
| ASTM A967 test practices | A water immersion, B high humidity, C salt spray, D copper sulfate, E potassium ferricyanide-nitric acid, F damp cloth, G boiling water | store.astm.org |
| Current edition of ASTM A967 | A967/A967M-25 | store.astm.org |
| Phosphoric acid formula and CAS | H₃PO₄, CAS 7664-38-2 | pubchem.ncbi.nlm.nih.gov |
| Phosphoric acid OSHA PEL / IDLH | 1 mg/m³ TWA / 1,000 mg/m³ | cdc.gov/niosh |
| Nitric acid OSHA PEL / IDLH | 2 ppm (5 mg/m³) TWA / 25 ppm | cdc.gov/niosh |
| Citric acid formula and CAS | C₆H₈O₇, CAS 77-92-9 | pubchem.ncbi.nlm.nih.gov |
| Measured passive film thickness on stainless steel in water (AFM) | about 4.85 nm in water (AFM); 3.8 nm in vacuum | pmc.ncbi.nlm.nih.gov |
Frequently asked questions
What is stainless steel passivation?
Passivation is a chemical process that removes free iron and other contamination from a stainless steel surface so the chromium oxide passive film can form evenly. ASTM A967 sets out three kinds of treatment for it: nitric acid immersion, citric acid immersion and electrochemical treatment. The result is confirmed with tests such as the water immersion, high humidity or copper sulfate test.
Does stainless steel need to be passivated?
Not always. ASTM A380 (2017 edition) states that stainless steel forms its passive film spontaneously in air once the surface is thoroughly clean, and that an oxidizing chemical step is generally not necessary. Passivation is needed when machining, welding, grinding or handling has left free iron on the surface, or when a drawing, customer or validation protocol calls for it.
Is phosphoric acid an ASTM A967 passivation treatment?
No. The ASTM A967 abstract names nitric acid immersion, citric acid immersion and electrochemical treatment; phosphoric acid is not on that list. Phosphoric acid is used to clean and descale stainless steel before passivation. If a drawing or purchase order calls out A967, use one of the listed treatments; the standard's definition also covers other chemical solutions capable of similar results, which must pass the tests the specification names.
What tests does ASTM A967 use to confirm passivation?
A967 lists seven qualitative practices: A water immersion, B high humidity, C salt spray, D copper sulfate, E potassium ferricyanide-nitric acid, F damp cloth and G boiling water immersion. The purchaser's specification picks which ones apply. Exposure times and pass criteria for each are in the standard itself, which ASTM sells.
Do you passivate before or after machining or welding?
After. Passivation removes the iron that fabrication leaves behind, and ASTM A967 lists iron-tool marks, embedded iron, iron dust and iron deposits in welds among the forms of free iron it targets. Machine, form, weld and grind first, degrease the part, then passivate, and keep carbon-steel tools off it afterward.
Will vinegar passivate stainless steel?
Not to ASTM A967. Vinegar is dilute acetic acid, and acetic acid is not one of the treatments A967 lists (nitric acid, citric acid, electrochemical). Citric acid is on the list, which is why small shops use it. A part meets A967 only if it goes through one of the standard's treatments and passes the tests its specification names.
What causes passivation to fail?
Most failures trace to contamination the acid never reached or iron put back afterward: oil left by skipped precleaning, a bath outside its specified concentration, temperature or time, a tap-water final rinse, or bare hands and carbon-steel tools touching the part after the bath. A failed copper sulfate or water immersion test is usually how it shows up.
What is a stainless steel passivation line?
A row of tanks run in order: precleaning, descaling if needed, the passivating bath, rinsing, drying and testing. For ASTM A967 work the passivating tank holds one of the treatments the standard sets out, usually nitric or citric acid. ASTM A380 asks that parts and equipment be designed so cleaning solutions cannot get trapped, which is where most rinse failures start.
What is the difference between ASTM A380 and ASTM A967?
A380 is a practice covering cleaning, descaling, pickling and passivation of stainless steel parts, equipment and installed systems, including new tanks and piping passivated in place. A967 is a specification that defines the passivation treatments (nitric, citric, electrochemical) and the tests that confirm them. A380 does not cover equipment that has already been in service.
What are the exposure limits for nitric and phosphoric acid at a passivation line?
Per the NIOSH Pocket Guide, nitric acid has an OSHA PEL of 2 ppm (5 mg/m3) as an 8-hour TWA, a NIOSH short-term limit of 4 ppm and an IDLH of 25 ppm. Phosphoric acid has an OSHA PEL of 1 mg/m3, a NIOSH short-term limit of 3 mg/m3 and an IDLH of 1,000 mg/m3. Keep the SDS for each bath at the line.
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About the author
Andre Taki
This article is for informational purposes only.
