Guide
Is white rust on zinc-plated parts a reject, and how do you prevent it in storage and transit?
White rust is the white or gray powdery corrosion product, mostly zinc hydroxide, that forms on zinc-plated and galvanized steel when the zinc stays wet without free air, typically inside tightly packed boxes, bags and crates during storage or sea freight. Light white rust is usually a cosmetic finding rather than a functional reject, but it is a reject when the drawing calls for a cosmetic surface, when it has consumed the zinc down to below the specified coating thickness, or when the stain has turned dark gray or black.
By Saola Manufacturing engineering team · Published · Updated
The situation this guide is written for is a familiar one at receiving: a carton of zinc-plated brackets, fasteners or turned parts arrives after several weeks in a container, and the parts in the middle of the box are covered in a white bloom. Quality wants to reject the lot, purchasing wants the parts on the line, and the supplier says it is “only white rust.” All three can be right, depending on how much zinc is left, what the surface does in service, and what the drawing actually says. The sections below cover how to recognize white rust, what causes it, how to grade it before you decide, which finish and packing choices prevent it, and the wording that keeps the next lot from arriving the same way.
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How do you recognize white rust, and how is it different from red rust or a bad passivate?
White rust is a bulky, chalky white or gray residue on the zinc that wipes or brushes off and leaves a dull, etched surface underneath. Red rust is orange-brown iron oxide from the steel substrate and means the zinc is already gone at that spot.
The chemistry explains the look. In open air, fresh zinc reacts with oxygen and moisture to form zinc hydroxide, which then reacts with carbon dioxide in the air to become a thin, compact zinc carbonate film that slows further corrosion. The International Zinc Association’s GalvInfoNote 3.2 (Rev 2.1, 2017) explains that when zinc surfaces stay wet while packed tightly together, the hydroxide never gets the air it needs to convert, so it keeps growing into the loose, voluminous residue buyers call white rust. The galvanizing industry calls the same thing wet storage stain.
Three things are easy to confuse with it. A cloudy, streaky or iridescent passivate that looks uneven but has no powder on it is a finishing appearance issue, not corrosion. Dried rinse water or salts leave a white film that dissolves in water and does not etch the zinc. Dark gray or black staining that sits under or around a heavy white residue is the serious case: the same note says that a stain turning black usually means enough zinc has been consumed that iron from the steel is taking part in the corrosion. Any orange-brown spots on top of the white residue confirm bare steel.
Also note where it appears. White rust that sits between parts, under the contact points of nested parts, in the middle of a bag and on the faces that touched each other is the classic transit pattern. White rust that appears on every part on the same feature, or on parts that never touched, points to the finish itself.
What causes white rust on zinc-plated parts in storage and sea freight?
Trapped moisture with no air circulation causes it: condensation inside the container or package, parts packed while still damp from rinsing, and wet packing materials. A thin, fresh or missing passivate, chloride contamination and long dwell times make the same moisture do much more damage.
Ranked by how often they explain a white-rusted shipment, the causes are:
- Condensation in transit. The German Insurance Association’s Container Handbook (section 10.3.3, water vapor in the container) attributes relatively large quantities of sweat in a closed container to the cargo or its packaging, or a wooden container floor, and describes that vapor condensing on walls and a ceiling cooled by night-time radiation. Zinc parts packed face to face in a polyethylene bag are exactly the kind of tight, airless pack where that water stays.
- Parts packed wet or warm. Plated parts that leave the rinse and dryer with water in blind holes, threads or between nested parts start the reaction before the box is closed. Parts bagged while still warm also release moisture that condenses as the bag cools.
- Damp or unsuitable packing. The American Galvanizers Association’s prevention guidance says wood used in transport should be dry and untreated with preservatives or fire retardant chemicals, and that resinous wood should not be used for spacers or packing.
- A thin, missing or under-cured passivate. A clear (colorless) passivate gives the shortest protection before white corrosion appears, and a passivate that is handled or packed before it has dried and hardened loses part of its protection.
- Temperature swings at destination. Cold parts moved into a warm, humid warehouse collect condensation, and GalvInfoNote 3.2 recommends letting the steel warm above the dew point before taking off its wrapping.
- Salts. Chlorides from sea air, fingerprints or contaminated rinse water accelerate the attack, which is why the same note lists accelerating agents such as chloride salts alongside exposure time and temperature as the factors that decide how much zinc is lost.
Is white rust a reject, and how do you grade it at receiving?
Grade it before you decide: light white rust that brushes off and leaves continuous zinc is usually acceptable on non-cosmetic surfaces, while heavy residue, dark or black staining, red rust, or coating thickness below the drawing minimum are rejects. The drawing decides for cosmetic and functional surfaces, because general finish standards do not give a white rust acceptance limit for parts that were stored and shipped.
The galvanizing industry’s guidance is the clearest public reference. The American Galvanizers Association’s wet storage stain article says most cases are light and do not necessarily imply any likely reduction in the expected life of the product, while medium and heavy stain left untreated can damage the coating and may require stripping and regalvanizing. Its cleaning article adds that after medium or heavy stain is removed, the coating thickness should be measured to confirm enough zinc remains. The same logic applies to electroplated parts, with one difference that matters: electroplated zinc is much thinner than a hot-dip coating, so the same amount of corrosion uses up a far larger share of the protection. ASTM B633 sets four electroplated thickness classes, Fe/Zn 5, 8, 12 and 25, and the number is the minimum zinc thickness in micrometers per ASTM B633.
A practical grading sequence at receiving:
- Quarantine and photograph the lot before unpacking further: box positions, condition of bags, water marks, and any desiccant or humidity indicator.
- Sample across the shipment, not just the top box: center of bags and contact faces are where it is worst.
- Stop it getting worse. Let cold cartons reach room temperature before opening them, then take the parts out of wet bags, spread them out and dry them with air moving over them. Wet storage stain keeps growing only while the surfaces stay wet without air, so the rest of the lot should not be left sealed in the bags while the dispute is settled.
- Brush a spot with a stiff nylon brush and look at what is left: bright or dull continuous zinc, dark staining, or orange-brown steel.
- Measure zinc thickness on cleaned spots with a magnetic gauge per ASTM B499, or by X-ray fluorescence per ASTM B568, which suits thin electroplated layers better (or by the method the drawing names), and compare with the drawing minimum. Measure an unaffected part from the same lot too, so you can see how much zinc the white rust actually took.
- Check for salt: wet a cotton swab with clean water and wipe a white spot. A film that dissolves and leaves sound, unetched zinc is residue from rinsing or handling, not corrosion.
- Check function: thread fit with go/no-go gauges, since bulky residue in threads can stop a gauge, and any surface that seals, conducts or will be painted.
- Decide and document: accept, rework under a deviation, or reject, and record which grade drove the decision.
| What you see | What it usually means | Usual disposition |
|---|---|---|
| Light white bloom that brushes off; continuous zinc underneath, slightly dull | Little zinc consumed; surface etched, not breached | Accept on non-cosmetic surfaces after drying and brushing; reject only if the drawing calls for a cosmetic finish |
| Thick, crusty white residue, often between contact faces or inside bags | Measurable zinc loss; residue looks worse than the loss may be | Clean, measure coating thickness; accept only if at or above the drawing minimum and threads gauge; otherwise reject or replate |
| Dark gray or black stain under or around the residue | Zinc heavily consumed; iron involved in the corrosion product | Reject; strip and recoat, or replace |
| Orange-brown spots in or next to the white residue | Zinc gone at that spot; steel corroding | Reject the affected parts; investigate the whole lot |
| White film that dissolves in water, no etching | Dried rinse water or salt residue, not corrosion | Clean; investigate rinse quality at the finisher |
How do you prevent white rust: which passivate, sealer and packing fit which shipment?
Specify a heavier passivate or a passivate plus sealer when parts travel by sea, require parts to be fully dry and cool before packing, and keep the package either ventilated or sealed with enough desiccant. Passivation slows white rust but does not make wet packing safe.
The finish type sets how long the zinc resists white corrosion. ASTM B633 judges its salt spray requirement on white corrosion, and the ASTM B633-23 supplementary finish types rank clear chromate lowest and colored finishes highest, with the chromium(VI)-free Types V and VI following the same order. Trivalent passivates are often combined with a sealer or topcoat to close the gap to the old hexavalent yellow. The hours themselves are covered in the zinc plating salt spray hours guide.
Packing is the other half. GalvInfoNote 3.2 notes that moisture caught between sheets does not evaporate because there is no air movement, and that in that state even the best chemical treatment cannot protect against storage stain for longer than about six weeks. The same note warns against wrapping bundles that are wet and against wrapping them tightly in plastic. For bulk parts in a container that leaves two workable strategies: keep air moving between parts with spacers and breathable inner packing, or close the package in a real water-vapor barrier with desiccant sized for the voyage and check a humidity indicator on arrival. A thin polyethylene bag with a token desiccant sachet is neither.
| Situation | Finish to specify | Packing to specify |
|---|---|---|
| Short, dry route; parts used soon after arrival; appearance not critical | Clear trivalent passivate (Type V) acceptable | Dry parts, closed carton, no wood contact |
| Sea freight through humid ports; bulk parts in bags or bins | Colored trivalent passivate (Type VI) or Type V plus sealer | Parts dry and at room temperature before bagging; layer separators; barrier bag with sized desiccant, or ventilated packing; dry, untreated pallet |
| Cosmetic or visible surfaces; long storage after arrival | Passivate plus sealer or topcoat; consider zinc-nickel | Individual wrap or partitioned trays so parts do not touch; heat-sealed barrier bag; humidity indicator card |
| Parts to be painted or powder coated after arrival | Passivate and sealer compatible with the coating, confirmed with the coater | Barrier bag with desiccant; no oil unless the coater accepts it |
| Threaded parts with gauge fit requirements | Plating thickness and passivate chosen with the thread allowance in mind | Bulk bags with desiccant; no water left in tapped holes before packing |
What should the drawing, RFQ and purchase order say so white rust does not recur?
Name the finish by standard, thickness class and supplementary type, state the acceptance rule for white rust on arrival, and write packing as numbered requirements. “Zinc plated, clear” and “export packing” leave every decision to the supplier.
Clauses a supplier can follow and a receiving inspector can check:
- Finish. “ZINC PLATE PER ASTM B633, FE/ZN 8, TYPE VI” (or the class and type your service needs), plus “SEALER REQUIRED” if you want one. The zinc plating versus galvanizing guide helps if the real question is whether plating is thick enough at all.
- Surface classes. Mark which surfaces are significant or cosmetic. White rust on a hidden face and on a visible face should not get the same answer.
- Drying and cure. “PARTS FULLY DRY, INCLUDING BLIND HOLES AND THREADS, AND AT ROOM TEMPERATURE BEFORE PACKING. PASSIVATE CURED BEFORE HANDLING IN BULK.”
- Packing. “NO PART-TO-PART CONTACT ON COSMETIC SURFACES. INNER PACK: HEAT-SEALED WATER-VAPOR BARRIER BAG WITH DESICCANT SIZED FOR TRANSIT AND STORAGE, HUMIDITY INDICATOR CARD VISIBLE. DRY, UNTREATED, NON-RESINOUS WOOD OR PLASTIC PALLET.”
- Acceptance on arrival. “LIGHT WHITE CORROSION PRODUCT THAT REMOVES WITH A NYLON BRUSH IS ACCEPTABLE ON NON-COSMETIC SURFACES PROVIDED ZINC THICKNESS MEETS THE SPECIFIED CLASS. DARK OR BLACK STAIN, RED RUST, OR THICKNESS BELOW CLASS IS REJECTABLE.”
- Evidence. “SUPPLY PLATING THICKNESS READINGS AND PHOTOS OF THE CLOSED INNER PACK BEFORE SHIPMENT.”
Put the acceptance rule in the same place as your receiving inspection sampling plan, so the inspector applies it the same way every lot. Without it, each shipment turns into a negotiation.
Who does what on a Saola order for zinc-plated parts?
The steel parts are machined by CNC turning and CNC milling at our lead CNC plant in Vietnam. Zinc plating, passivation and sealing are done by partner plants we manage, and the quote names the production site and the country of origin.
Plating sits under the surface finishing menu, and it is quoted against the finish callout on your drawing: standard, thickness class, supplementary type and any sealer. Where a callout is missing or ambiguous, the quote flags it instead of guessing. When the RFQ carries a packing and acceptance clause like the one above, the quote confirms each item, including the inner pack, the desiccant and the evidence that comes with the lot, and lists anything that cannot be met as written.
If a shipment does arrive with white rust, the photos, indicator readings, thickness measurements and sample parts collected at receiving are what lets the cause be traced to plating, drying, packing or the voyage, and the corrective action addressed at the step that failed. Send the drawing with the finish and packing requirement through the quote form to get a price on the parts, the finish and the packing together.
Frequently asked
Can white rust be cleaned off zinc-plated parts?
Light white rust can usually be removed by drying the parts and brushing with a stiff nylon brush. For heavier stain on galvanized sheet, GalvInfoNote 3.2 describes a white vinegar wash, and for more severe stain a dilute phosphoric acid solution, each followed at once by a thorough water rinse and drying; it adds that stain that has turned dark gray or black probably cannot be removed. Cleaning does not restore the original bright appearance, because the zinc underneath is etched, and any chemical cleaning on plated parts should be agreed with the finisher so it does not strip the passivate.
Does white rust mean the parts will rust red soon?
Not necessarily. White rust consumes some zinc, and on thin electroplated coatings that loss can shorten the time to red rust, which is why the thickness check matters. If the zinc meets the specified class after cleaning and no dark staining is present, the remaining zinc still protects the steel.
Is white rust covered by the plater’s salt spray result?
No. A salt spray result shows the finish met its hours to white corrosion in a test cabinet when it was tested. It says nothing about parts that were packed damp or sat in a wet bag for weeks, so passing salt spray and arriving with white rust are not contradictory.
Should zinc-plated parts be oiled for shipping?
A rust-inhibitive oil helps against humid air but, as GalvInfoNote 3.2 notes for galvanized sheet, is less effective against bulk water getting between parts. Oil also has to be removed before painting or bonding, so specify it only if the next process accepts it.
Does VCI packaging protect zinc from white rust?
Only VCI products that their maker states are suitable for zinc and zinc-coated steel; many VCI papers and films are formulated for ferrous metals only. Ask for the maker’s metal compatibility statement before using VCI on plated parts.
Sources
- GalvInfoNote 3.2: Protecting Galvanized Steel Sheet Products from Storage Stain, International Zinc Association, Rev 2.1, December 2017
- Wet Storage Stain, American Galvanizers Association, 2010
- How to Clean Wet Storage Stain, American Galvanizers Association, 2013
- How to Prevent Wet Storage Stain, American Galvanizers Association, 2011
- ASTM B633-23, Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel, ASTM International, 2023
- Container Handbook, section 10.3.3: Water vapor in the container, German Insurance Association (GDV), accessed 2026
- ASTM B568-98(2021), Standard Test Method for Measurement of Coating Thickness by X-Ray Spectrometry, ASTM International, 2021