White rust on zinc-plated parts
White powdery zinc corrosion in storage and transit: passivation, drying, sealer and packaging faults that cause it, and how to judge it against a salt-spray acceptance criterion.
What you see
A white, grey or chalky powder on the zinc surface — sometimes spots and streaks, sometimes covering the part. It usually appears in the box rather than on the line: after a sea container crossing, in a humid store, on parts packed while still warm or damp, or under film where condensation collected. The steel underneath is normally still protected, because white rust is corroded zinc rather than corroded steel, but part of the coating life has been consumed.
How to confirm it
Identify it by location and by what lies underneath: white product on a bright surface with no red staining is zinc corrosion. In a chamber test to ASTM B117 / ISO 9227 NSS the acceptance criterion must state white corrosion separately from red rust — hours to first white product is the passivation and sealer indicator, hours to red rust is the thickness and substrate indicator. Confirm the coating is present and to thickness by XRF to ISO 3497, and inspect the packaging and neighbouring parts in the same box for condensation. Colour and appearance alone do not prove the passivation type; the process record does.
Related calculator: Salt spray hours plannerRoot causes — most common first
- 1Parts packed damp or still warm, so moisture condenses inside sealed packaging.
- 2Passivation missing, exhausted, too thin or damaged — the passivate is the layer that delays white corrosion.
- 3No sealer or topcoat where the required white-rust hours cannot be reached by passivation alone.
- 4Storage and transit conditions: high humidity, temperature swings in a container, coastal salt-laden air, stacked parts holding moisture.
- 5Passivation film damaged after the tank — drying too hot for the film, tumbling, or heavy vibration cracking it before it matured.
- 6Contaminated rinses or drag-out leaving salts on the surface that hold moisture against the zinc.
Process controls that prevent it
- Dry parts fully and let them cool before packing — never bag warm parts.
- Control the passivation stage (concentration, pH, temperature, immersion and rinse) and keep drying below the temperature that dehydrates the film.
- Specify a sealer or topcoat when the requirement is high white-rust hours; that is precisely what the seal is for.
- Package for the journey: moisture-barrier bag with desiccant and VCI for sea freight, no bare parts in unlined cartons.
- Handle passivated parts gently while the film matures, and avoid tumbling after passivation.
- Put the acceptance criterion in the order — white-rust hours, red-rust hours, significant surfaces and test standard — so the coating system is selected to meet it.
What to send us for diagnosis
- Affected parts in their original packaging and, if possible, one unopened package from the same lot.
- The coating call-out from the drawing, the required salt-spray criterion, and how the parts were stored and shipped.
- When the parts were plated and when the white product appeared, plus photographs of the packaging as opened.
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Part types this shows up on
Questions engineers ask
Does white rust make the parts rejectable?+
It depends on the written criterion. Many specifications judge only red rust on significant surfaces and accept light white product in transit after cleaning; others state hours to white corrosion explicitly. Decide against the drawing and the agreed test plan before scrapping a lot.
Can white rust be removed?+
Light white product can be wiped or gently cleaned off, but the passivation film it consumed does not come back and the remaining protection is reduced. Heavy attack means zinc has already been used up locally.
Does thicker zinc solve it?+
Not directly. Zinc thickness mainly controls time to red rust. White-rust resistance comes from the passivation, the sealer and dry storage. Adding thickness without fixing those simply delays a problem you have not addressed.
Guidance based on published standards and general fastener metallurgy. Values are typical ranges that depend on the part, the material and the governing specification — the customer drawing and the applicable standard always prevail. This page does not replace a qualified failure analysis of your own parts.