Passivation

Chemical treatment that removes free iron from the surface of stainless steel and enhances the naturally occurring chromium oxide passive layer. It is not a coating -- nothing is deposited on the surface. The process dissolves contaminants (primarily free iron particles deposited during machining, forming, or handling) and allows the chromium in the stainless steel to react with oxygen, forming a thin, self-healing oxide film that provides corrosion resistance. This guide covers when passivation is needed, which method to use, and how to verify the result.

Does This Part Need Passivation?

Not every stainless steel part needs passivation. The process only makes sense when the part has been exposed to conditions that deposit free iron on the surface. Use this table to decide.

Material / Application	Passivate?	Reason
Stainless steel (304, 316, 303, 17-4PH) after CNC machining	Yes	Tooling, fixturing, and handling deposit free iron particles. Machining also heats the surface and can damage the existing passive layer.
Stainless steel after forming, bending, or stamping	Yes	Die contact and mechanical deformation disturb the passive film and embed iron particles from tooling.
Stainless steel after welding	Yes	Heat-affected zone loses chromium from the matrix, and weld spatter and slag deposit iron contamination. Pickling before passivation is usually required.
Stainless steel after grinding or polishing with steel media	Yes	Steel abrasives embed ferrous particles. These cause rust spots if not removed.
Stainless steel parts for medical, food, or pharmaceutical use	Yes (mandatory)	Regulatory requirements (FDA, USP, ISO 13485) mandate passivation for biocompatibility and corrosion resistance.
Stainless steel for marine or outdoor environments	Yes	Chloride exposure accelerates corrosion on any contaminated surface. Passivation provides maximum inherent corrosion resistance.
Stainless steel parts with no machining or handling (as-cast, as-forged)	Depends	If the surface has not been mechanically disturbed or contaminated with iron, the natural passive layer may be sufficient. Testing (copper sulfate or salt spray) confirms.
Stainless steel that will be painted or coated immediately	No	The coating isolates the surface from the environment. Passivation is unnecessary and adds cost. The coating is your corrosion barrier.
Carbon steel, aluminum, copper, titanium, or plastics	No	Passivation is specific to stainless steel and some nickel-based alloys. It does not apply to non-stainless materials.
Stainless steel already electropolished	Usually no	Electropolishing removes the surface layer and leaves a chromium-enriched surface. In most cases, electropolishing provides equal or better passivation. Some specs still require a post-EP passivation rinse.

Rule of Thumb If the part is stainless steel, has been machined or handled in a shop environment, and will be exposed to moisture, chemicals, or corrosive atmosphere in service -- passivate it. The cost is low and the consequence of skipping it (rust spots on "stainless" parts) is a customer complaint and potential rejection. Only skip passivation when the part is coated, or when the surface has not been contaminated.

Passivation Methods at a Glance

Property	Nitric Acid Passivation	Citric Acid Passivation	Electropolishing
Process	Immersion in nitric acid (HNO₃) solution, 20–50% concentration, room temp or elevated, 20–60 min	Immersion in citric acid solution, 4–50% concentration, elevated temp (50–70 °C), 10–30 min	Electrochemical process: part is anode in phosphoric/sulfuric acid bath, 10–30 VDC, material is removed from surface
How It Works	Dissolves free iron and other contaminants from surface. Enhances existing chromium oxide layer.	Chelates and dissolves free iron from surface. Promotes chromium oxide formation.	Removes surface material electrochemically, preferentially dissolving iron and leaving a chromium-enriched surface.
Surface Removal	Minimal (0.0001–0.0005 in / 2.5–12 μm)	Minimal (0.0001–0.0003 in / 2.5–7.5 μm)	Significant (0.0005–0.002 in / 12.5–50 μm per surface)
Surface Finish Effect	No change -- does not alter Ra	No change -- does not alter Ra	Improves Ra by 30–50%. Leaves a smooth, bright surface.
Safety	Hazardous	Safe	Moderate
Environmental	Nitric oxide (NOx) fumes, hazardous waste disposal required	Biodegradable, organic, low hazard waste	Acidic waste, requires neutralization
Cost Factor	1x (baseline)	0.8–1x	2–4x
Lead Time Impact	+1–3 working days	+1–2 working days	+3–5 working days
Standards	ASTM A967, AMS 2700, QQ-P-35	ASTM A967, AMS 2700	ASTM B912, AMS 2514
Best For	General-purpose passivation, aerospace and military specs, all stainless grades	General-purpose passivation where safety and environmental compliance are priorities, large parts	Parts that need both passivation and surface smoothing, medical devices, food equipment, decorative stainless
Limitations	NOx fumes require ventilation and scrubbing. Not suitable for shops without proper fume handling.	Less established track record for some aerospace specs. Some older MIL specs reference nitric acid only.	Changes dimensions. Cannot access deep blind holes uniformly. Expensive.

How Passivation Works

Stainless steel resists corrosion because of its chromium content. When chromium is exposed to oxygen, it forms a thin (1–3 nanometer), transparent, self-healing oxide layer on the surface. This chromium oxide layer is what makes stainless steel "stainless." Passivation is the process of maximizing this natural protection.

The Problem: Free Iron Contamination

During CNC machining, the cutting tools (typically carbide or high-speed steel), fixtures, vises, and even handling deposit microscopic iron particles on the stainless steel surface. These free iron particles sit on top of or embedded in the passive chromium oxide layer. They do not have the chromium content to form their own protective oxide, so they rust. A single rust spot on a "stainless" part is enough to trigger a customer rejection.

The Solution: Dissolve the Contaminants

Passivation uses an acid solution to selectively dissolve free iron and other contaminants from the surface. The acid is chosen to be aggressive enough to remove iron, but mild enough to not significantly attack the stainless steel matrix itself. Once the iron is removed, the underlying chromium reacts with oxygen (in the air or the solution) to reform a continuous, uniform chromium oxide layer.

What Passivation Is NOT

Common Misconceptions Passivation is NOT a coating. Nothing is deposited on the surface. It is NOT a cleaning process -- it does not remove oil, grease, or shop soils (those must be removed before passivation). It is NOT a pickling process -- it does not remove weld scale, heat tint, or oxides (pickling with stronger acids does that). It is NOT a substitute for electropolishing -- it does not smooth the surface or improve Ra. It is NOT a rust converter -- it does not "fix" already-rusted stainless steel. It is a chemical treatment that removes free iron and maximizes the natural chromium oxide passive film.

Typical Process Flow

Step	Process	What Happens
1. Degreasing / Cleaning	Alkaline cleaner or solvent wipe	Removes cutting oil, grease, fingerprints, and shop contamination. This step is mandatory -- if the surface is oily, the acid cannot contact the metal.
2. Water Rinse	Deionized or clean tap water	Removes cleaning residue. Any alkaline cleaner left on the surface will neutralize the passivation acid.
3. Pickling (if required)	Mixed acid (HNO₃ + HF) or proprietary pickle	Removes weld scale, heat tint, and surface oxides. Only needed after welding, heat treatment, or severe surface oxidation. Skipped for most CNC-machined parts.
4. Passivation	Nitric acid or citric acid immersion	Dissolves free iron from the surface. Promotes chromium oxide formation. Temperature, concentration, and time are controlled per the specification.
5. Water Rinse	Deionized water (multi-stage)	Removes all acid residue. Incomplete rinsing leaves acid traces that continue to attack the surface.
6. Drying	Hot air or compressed dry air	Prevents water spots and flash rusting during storage.

Nitric Acid vs Citric Acid

Both methods achieve the same result: removal of free iron and enhancement of the chromium oxide layer. The choice between them depends on safety requirements, environmental regulations, cost, and specification compliance. Citric acid has been gaining market share rapidly because it is safer and environmentally benign, but nitric acid remains the traditional choice for many aerospace and military applications.

Parameter	Nitric Acid (HNO₃)	Citric Acid (C₆H₈O₇)
Typical Concentration	20–50% by weight	4–50% by weight (most common: 10–20%)
Temperature	Room temperature (20–25 °C) or elevated (50–65 °C) depending on spec	Elevated: 50–70 °C (most common: 60–65 °C)
Immersion Time	20–60 min (per ASTM A967 Method 1 or 2)	10–30 min (per ASTM A967 Method 4)
Safety Classification	Corrosive, oxidizer. Causes severe burns. NOx fumes are toxic.	Mild organic acid. Low toxicity. No hazardous fumes. Safe to handle with basic PPE.
Ventilation Required	Yes -- fume hood or scrubber system mandatory. NOx fumes are regulated.	Standard shop ventilation is sufficient.
PPE Required	Acid-resistant gloves, face shield, chemical apron, respiratory protection	Standard gloves and safety glasses. No special respiratory equipment.
Waste Disposal	Hazardous waste. Requires neutralization and disposal by licensed handler. High disposal cost.	Biodegradable. Can often be neutralized to pH 6–8 and discharged to sewer with permits. Low disposal cost.
Bath Life	Long -- nitric acid does not degrade quickly. Iron buildup is the limiting factor.	Moderate -- citric acid chelates iron effectively but bath must be monitored and replaced when iron concentration exceeds limits.
Effect on Welded Parts	Good, but does not remove heat tint. Requires separate pickling step before passivation.	Good, same limitation. Does not replace pickling for heat tint removal.
Effect on 303 Stainless	Adequate but marginal. Sulfur in 303 creates persistent black spots. Longer immersion or higher concentration may be needed.	Generally better on 303 due to chelating action. Some studies show more consistent results on sulfur-bearing grades.
Specification Acceptance	All major specs: ASTM A967 (Methods 1–3), AMS 2700, QQ-P-35, MIL-S-5002	ASTM A967 (Method 4), AMS 2700. Some older military specs reference nitric acid only -- check your specific callout.
Cost Per Part (typical)	$1–5 (batch dependent)	$0.80–4 (batch dependent)
Overall Cost (including waste handling)	Higher -- hazardous waste disposal, ventilation, PPE, and compliance overhead	Lower -- simpler waste handling, less PPE, lower compliance cost
Industry Trend	Declining in commercial use. Still dominant in aerospace/military due to legacy specs.	Growing rapidly. Now the default for most commercial applications.

Citric Acid: The Modern Default For most CNC machined stainless parts, citric acid passivation per ASTM A967 Method 4 is the right choice. It is safer for workers, cheaper to dispose of, equally effective, and accepted by all current standards. Only specify nitric acid when your customer or spec explicitly requires it (some legacy MIL or aerospace specs). When in doubt, ask your customer which method they accept.

Which Stainless Grades Can Be Passivated?

All austenitic, ferritic, martensitic, and precipitation-hardening stainless steels can be passivated. However, some grades present challenges that require modified procedures or closer process control.

Grade	Type	Passivation Result	Watch Out For	Recommendation
304 / 304L	Austenitic	Excellent	None significant. The most commonly passivated grade. Consistent, reliable results.	Standard nitric or citric acid passivation. No special measures needed.
316 / 316L	Austenitic	Excellent	Molybdenum content does not cause issues. Same process as 304.	Standard nitric or citric acid passivation. No special measures needed.
321	Austenitic (stabilized)	Excellent	Titanium stabilization does not affect passivation.	Standard passivation. No special measures needed.
347	Austenitic (stabilized)	Excellent	Niobium stabilization does not affect passivation.	Standard passivation. No special measures needed.
303	Austenitic (free-machining)	Adequate, with caveats	Sulfur (0.15% min) creates manganese sulfide inclusions. These appear as dark spots after passivation that do not wipe off. The spots are not rust -- they are sulfide inclusions exposed by the acid.	Citric acid generally performs better on 303 than nitric acid (chelating action helps). Accept that dark spots may appear -- they are cosmetic, not a corrosion concern. If cosmetic appearance is critical, switch to 304.
17-4PH (Condition A)	Precipitation hardening	Do NOT passivate in Condition A	Condition A is solution-treated but not aged. The microstructure is not stable. Passivation can cause surface etching and poor corrosion resistance.	Age to the specified condition (H900, H1025, H1150, etc.) before passivation. Then standard nitric or citric acid passivation works well.
17-4PH (H900, H1025, H1150)	Precipitation hardening	Good to excellent	Must be fully aged before passivation. Oversized passivation (too long or too concentrated) can etch the surface and reduce dimensional accuracy.	Standard passivation after aging. Use the milder end of the concentration/time range to minimize dimensional effect.
410 / 420	Martensitic	Good, with caution	Lower chromium content (11.5–14%) means less chromium available for the passive film. More sensitive to over-passivation and etching.	Use milder concentrations and shorter times. Nitric acid at 20–25% for 20–30 min. Avoid high concentrations.
430	Ferritic	Good	Similar to martensitic grades -- lower chromium than 300-series. Sensitive to over-passivation.	Standard passivation with shorter time. Monitor for etching.
2205 / 2507 (Duplex)	Duplex	Good	High chromium and molybdenum content provide excellent passive film formation. No special issues.	Standard passivation. Same parameters as 316.
904L	Austenitic (superaustenitic)	Excellent	Very high chromium (20%) and molybdenum (4.5%) content. Outstanding passive film.	Standard passivation. No special measures needed.

303 Warning: Sulfide Inclusions Are Not Rust After passivating 303 stainless, you may see dark spots or streaks that look like rust. These are manganese sulfide inclusions (intentionally added for machinability) that have been exposed by the acid. They are not corrosion and they will not propagate. If a customer rejects parts for "rust" that is actually sulfide staining, you need to either (a) switch to 304 and accept the higher machining cost, or (b) educate the customer and provide a test report showing the spots are sulfide, not iron oxide.

17-4PH Warning: Always Age Before Passivation Never passivate 17-4PH in Condition A (solution-treated, unaged). The unstable microstructure produces inconsistent and unreliable passivation results. Always complete the specified aging heat treatment before passivation. This is a hard requirement, not a recommendation.

Common Failures

Passivation is chemically simple, but several things can go wrong that compromise the result. Most failures are preventable with proper cleaning, process control, and testing.

Failure	Appearance	Root Cause	Prevention
Flash attack (over-passivation)	Dark, etched, rough surface. Heavily pitted or frosted appearance. Dimensional changes visible to the naked eye.	Acid concentration too high, temperature too high, or immersion time too long. The acid attacks the stainless steel matrix itself, not just the free iron. Common with nitric acid above 50% or at elevated temperature.	Follow spec parameters exactly. Use the lower end of concentration/time ranges for martensitic grades. Monitor bath temperature. Never exceed the maximum values in ASTM A967.
Incomplete cleaning before passivation	Spotty passivation -- some areas are passive, others rust. Oil or grease patterns visible under inspection.	Cutting oil, coolant, or handling contamination blocks the acid from contacting the metal surface in those areas.	Thorough degreasing before passivation. Use alkaline cleaner for oils, solvent wipe for fingerprints. Inspect after cleaning: water should form a uniform film (no beading) on a clean surface.
Embedded iron from tooling	Random rust spots appearing days or weeks after passivation. Spots are typically at machining marks, vise marks, or fixture contact points.	Free iron was driven into the surface (smearing/embedding) by tooling pressure, grinding, or abrasive blasting with contaminated media. The acid cannot reach deep enough to dissolve it.	Avoid using steel tools, fixtures, or abrasives on stainless. Use stainless fixtures or plastic-tipped jaws. If iron is deeply embedded, pickling or light etching before passivation may be needed. In severe cases, electropolishing is the only solution.
Mixed-grade contamination	Random rust spots on parts that should be fully passive. Often appears in mixed batches.	Carbon steel or low-chromium stainless parts were processed in the same basket, rack, or tank as the stainless parts. Iron transferred between parts.	Never process carbon steel and stainless steel in the same passivation run. Use dedicated racks and baskets for stainless. Clean tanks thoroughly between different material types.
Insufficient rinse after passivation	Surface rusting, acid staining, or discoloration appearing within hours. White or yellow residue on the surface.	Acid residue left on the surface continues to attack the metal. Chlorides or other contaminants in rinse water deposit on the surface.	Multi-stage deionized water rinse. Final rinse in deionized water with resistivity >1 megaohm-cm. Dry immediately after rinsing.
Heat tint not removed before passivation	Discolored (blue, brown, gold) areas near welds that continue to show discoloration after passivation. May rust preferentially at heat-affected zones.	Welding creates a thick chromium-depleted oxide scale (heat tint). Passivation acid is not strong enough to remove it.	Pickle welded parts before passivation (HNO₃ + HF or proprietary pickle paste). Pickling dissolves the heat tint and restores the surface chromium level. Then passivate normally.

Testing: Salt Spray and Copper Sulfate

Verification is critical. Passivation that was not done correctly is worse than no passivation at all -- you think the part is protected, but it is not. Several standard test methods verify passivation quality.

Test Methods Overview

Test Method	Standard	What It Tests	Procedure	Result	Cost	Speed
Water Immersion	ASTM A967 Practice C	Basic passivity -- detects gross contamination	Immerse in deionized water at ambient for 24 hours. Inspect for rust.	No rust = pass. Rust = fail.	Very low	24 hours
High Humidity	ASTM A967 Practice D	Passivity under humid conditions	Expose to 95–100% relative humidity at 38 °C for 24 hours.	No rust = pass.	Low	24 hours
Salt Spray (Fog)	ASTM B117 / ASTM A967 Practice A	Corrosion resistance under aggressive conditions	Expose to 5% NaCl fog at 35 °C for specified duration (typically 2–24 hours for passivation verification; up to hundreds of hours for coating qualification).	No rust spots = pass. Number of hours to first rust is the metric.	Moderate (chamber time)	2–24+ hours
Copper Sulfate	ASTM A967 Practice F / ASTM A380	Presence of free iron on surface	Swab or immerse surface in 6–10% copper sulfate (CuSO₄) solution for 6 minutes. Rinse and inspect.	No copper plating (no reddish deposit) = pass. Copper deposition indicates free iron present = fail.	Very low	10 minutes
Ferroxyl Test	ASTM A380	Free iron detection (sensitive)	Apply ferroxyl indicator solution (potassium ferricyanide + nitric acid). Blue spots indicate free iron.	No blue spots = pass. Blue spots = free iron present = fail.	Low	10 minutes
Potentiostatic / Electrochemical	ASTM G61	Quantitative pitting resistance	Measure electrochemical behavior in chloride solution. Determines pitting potential.	Numerical result -- pitting potential above threshold = pass.	High (lab equipment)	1–2 hours (lab)

Acceptance Criteria

Application	Typical Test	Acceptance Criteria	Notes
General industrial	Copper sulfate or water immersion	No rust or copper deposition	Lowest cost verification. Suitable for most commercial applications.
Medical / pharmaceutical	Salt spray (ASTM B117) or humidity	No rust after 2–24 hours salt spray	More stringent than industrial. Often specified on purchase orders and drawings.
Food processing	Salt spray + visual inspection	No rust after 24 hours salt spray, no surface discoloration	USDA and FDA expectations. Combined with cleanability assessment.
Aerospace / military	Per the specific callout (AMS 2700, etc.)	Varies by spec. Often salt spray 24–96 hours or humidity 24 hours.	Follow the exact spec referenced on the drawing. Do not substitute tests without engineering approval.
Marine / offshore	Salt spray (extended)	No rust after 72–240 hours salt spray (depending on severity)	Chloride-rich service demands the most rigorous testing. Extended salt spray is standard.

Copper Sulfate Test: Quick and Practical The copper sulfate spot test is the most practical field verification method. It takes 10 minutes, costs almost nothing, and gives a clear pass/fail result. For incoming inspection or shop-floor verification, it is the go-to test. Apply the solution to a clean area of the part. If the surface is properly passivated, no copper deposits (no reddish color) appear within 6 minutes. If copper deposits form, free iron is present and the part needs re-passivation. Note: copper sulfate testing is destructive to the passivation layer in the tested area -- re-passivate after testing if the part is to be shipped.

Salt Spray Duration Matters "Pass salt spray" means nothing without a specified duration. A part that passes 2 hours may fail at 24 hours. Always specify the test duration on the drawing or PO: "PASSIVATION PER ASTM A967, VERIFY PER ASTM B117 SALT SPRAY, 24 HR MINIMUM, NO RUST." Without the duration, the test is meaningless.

Cost Impact

Passivation is one of the least expensive surface treatments available for stainless steel. The cost structure is dominated by lot charges and handling, not per-part material cost.

Cost Factor	Impact	Detail
Lot Charge / Setup	Dominant for small orders	Most shops charge $30–100 minimum per lot. On an order of 10 small parts, the setup dominates. Larger batches amortize this cost.
Per-Part Cost (nitric acid)	$1–5 typical	For small to medium CNC parts (under 1 kg). Larger or heavier parts cost more due to tank space and acid volume.
Per-Part Cost (citric acid)	$0.80–4 typical	Slightly cheaper than nitric acid due to lower waste disposal cost and simpler handling.
Per-Part Cost (electropolishing)	$5–30 typical	Significantly more expensive. Provides both passivation and surface smoothing. Justified when both are needed.
Testing (copper sulfate)	Included or $0.10–0.50 per part	Most passivation shops include copper sulfate testing as part of the service. If not, it is very cheap to perform.
Testing (salt spray)	$50–200 per batch	Requires chamber time. Cost is per batch, not per part. 2–24 hour tests are typical. Extended tests (72+ hours) cost more.
Pickling (if required)	+50–100%	Only needed for welded parts or parts with heat tint. Adds a separate acid process step.
Rush / Expedite	+25–100%	Standard lead time is 1–3 working days. Rushing disrupts batch scheduling.
Certification / Documentation	$25–75 per lot	Certificate of conformance (C of C) with test results. Required for aerospace, medical, and military.

Cost by Batch Size (Typical Small CNC Part, Citric Acid)

Quantity	Lot Charge	Per-Part Acid	Per-Part Total	Notes
1–5 pcs	$50	$1–2	$11–12	Lot charge dominates. Not much cheaper than plating at this volume.
10–25 pcs	$50	$1–2	$3–7	Starting to become economical. Most common prototype quantity.
50–100 pcs	$50–80	$1–2	$1.50–2.80	Cost per part approaches the acid cost. Very economical.
500+ pcs	$80–100	$0.80–1.50	$0.96–1.70	Near the floor cost. Passivation is essentially free at this volume relative to machining cost.

Passivation Is Cheap Insurance At $1–5 per part (and under $2 at production volume), passivation is one of the cheapest ways to prevent a customer rejection. A single rust spot on a stainless part that was supposed to be "stainless" is enough to reject the entire lot. The cost of rework, re-passivation, and delayed shipment far exceeds the cost of doing it right the first time.

Common Mistakes

Mistake	Consequence	Fix
Not passivating machined stainless steel parts	Free iron from tooling causes rust spots on "stainless" parts. Customer rejects at incoming inspection.	Always passivate stainless steel after machining, unless the part will be coated or the customer explicitly waives the requirement.
Skip cleaning before passivation	Oil and grease block acid contact. Passivation is incomplete -- some areas remain active and will rust.	Degrease with alkaline cleaner or solvent. Verify by water-break test: clean surface holds a continuous water film with no beading.
Using steel abrasives or tools on stainless before passivation	Iron particles embed in the surface. Standard passivation cannot reach deeply embedded iron. Rust spots appear later.	Use stainless steel or ceramic tools and abrasives. Use glass bead or ceramic bead for blasting. Never use steel shot or steel grit on stainless parts.
Over-passivating (too strong, too hot, too long)	Flash attack -- the acid etches the stainless steel matrix. Surface becomes rough, dark, and pitted. Dimensions change.	Follow ASTM A967 parameters. Use the lowest concentration and time that the spec allows. Monitor bath temperature. Never "give it extra" thinking more is better.
Passivating 17-4PH in Condition A (unaged)	Unreliable passivation, potential surface etching, unpredictable corrosion resistance.	Always complete the specified aging heat treatment (H900, H1025, etc.) before passivation.
Not specifying a test method on the drawing	No objective way to verify passivation quality. Shop may or may not test. If rust appears, there is no baseline for rejection.	Specify test method: "PASSIVATE PER ASTM A967, METHOD 4 (CITRIC ACID), VERIFY PER PRACTICE F (COPPER SULFATE)" or equivalent.
Processing carbon steel and stainless in the same passivation batch	Iron transfers from carbon steel parts to stainless parts. Stainless parts rust after passivation.	Always process stainless steel in dedicated batches. Use separate racks, baskets, and tanks for stainless.
Not specifying the passivation standard on the drawing	Shop uses their own default process, which may not meet your requirements. No traceability if issues arise.	Call out the standard: "PASSIVATE PER ASTM A967" or "PASSIVATE PER AMS 2700, METHOD 4." Include the test method and acceptance criteria.
Expecting passivation to fix weld discoloration	Heat tint and weld scale remain after passivation. Discolored areas may rust preferentially.	Pickle welded parts (HNO₃ + HF or proprietary pickle) to remove heat tint before passivation. Passivation alone does not remove heat tint.
Storing passivated parts without protection	Atmospheric contaminants, fingerprints, and moisture degrade the passive layer over time. Parts may show light surface rust after weeks of storage.	Package passivated parts in VCI (vapor corrosion inhibitor) paper or bags. Avoid bare-handling -- use gloves. Ship in sealed bags.

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