White Rust on Galvanized Steel Coils
White rust is the corrosion product that forms when zinc meets water with no free air circulation, between the laps of a coil or between stacked sheets. This guide covers what it is, how condensation inside a closed coil produces it, what it costs the zinc coating, what passivation, oiling and packing can and cannot do about it, and what to photograph and record if a container opens on a stained coil.
Zinc hydroxide
What the deposit is before it dries
About six weeks
How long treatment holds once water is between laps
Two months
Beyond this, extended storage is best avoided
Dark grey to black
The colour at which iron has joined the reaction
White rust between the laps of a coil: trapped moisture, no air movement, and the white deposit spreading along the wrap.
What White Rust Is
White rust is a corrosion stain on the zinc coating, and on galvanized sheet it is the same finding the industry also calls storage stain. The GalvInfo Center puts it from that side: storage stain on galvanized sheet products is a corrosion stain that is typically white, but which can also take the form of a grey or black deposit on the surface, and since the most common form of discoloration is white in appearance, storage stain is often called white rust. The discoloration is due to the corrosion products that form after zinc reacts with moisture in the absence of free air circulation.
The deposit changes chemistry as it dries. White rust on storage stain-damaged galvanize is initially zinc hydroxide, and turns to zinc oxide when left to dry in the open air. That is one reason a coil can look different on the day the wrap comes off than in photographs taken a week later, and a reason appearance alone is a poor guide to how much zinc has gone.
The terms are not settled between sources, and this page says so rather than choosing quietly. The American Galvanizers Association defines wet storage stain as the bulky white or gray deposit formed by accelerated corrosion of the zinc coating when closely packed, newly galvanized articles are stored or shipped under damp and poorly ventilated conditions. That association treats wet storage stain and white rust as two different things, reserving white rust for what it associates with cooling towers, where the GalvInfo Center says plainly that storage stain on sheet is often called white rust. This page follows the sheet-industry source and uses the two names for one finding, while flagging the disagreement wherever a specification turns on the word itself.
How It Forms
Water plus a closed gap is the whole mechanism. Each condition below has to be present; remove any one of them and the stain does not develop.
Zinc needs a drying cycle to protect itself
Atmospheric wet-dry cycles are necessary for zinc to develop passivity: it is during the drying part of a rain cycle that the zinc carbonate passive film develops. A wound coil gives the surface no drying part of any cycle, which is why fresh, bright zinc in a closed coil is the most vulnerable zinc there is.
Water reaches the surfaces in contact
Storage stain can occur when sheets of galvanized steel that are in close contact, in a coil or stacked in lifts and bundles, get wet, either by direct water intrusion or by condensation between the surfaces. Direct intrusion is the obvious route and the rarer one in a sealed container.
Condensation needs no rain at all
Condensation occurs when the coil or stacked sheets are below the dew point of the local atmosphere, for example when coils shipped in cool or cold weather are placed into a warmer warehouse where the humidity is not at a controlled low level. A container that loads in a cold month and discharges into a warm, humid port reproduces exactly that, without a drop of rain ever touching the steel.
What gets in does not get out
Once moisture gets between laps it will not evaporate, because air movement is absent. From that point the reaction runs at its own pace, and nothing the coil is wrapped in changes the conditions inside it.
What It Does to the Coating
White rust is not a film sitting on an intact coating. When white rust occurs there is an actual loss of zinc coating, and the extent of the damage is primarily dependent on the exposure time to moisture, the temperature experienced during storage, and the presence of accelerating corrosive agents such as chloride-containing salts. Those three variables are what separate a coil that is marked from a coil that has lost service life, and none of them can be read off a photograph of the stain.
The appearance does not come back. The surface of the zinc coating in the area that experienced white rust is etched and no longer has the bright, reflective appearance of as-produced galvanized sheet, and removing the white rust will not eliminate the etched appearance. For material going to paint that may be acceptable. For material going out as a visible finish it usually is not, and the finding is then about the surface rather than about the remaining zinc.
Colour is the one reading that carries a threshold. When the stains on galvanize turn black it usually means that iron has become part of the corrosion product, that is, enough zinc has been consumed that iron from the steel substrate is involved. One exception is worth knowing before anyone reports a container: it has been observed that initial, relatively light storage stain on some water-damaged galvanized sheet surfaces is not white but dark grey to black, and this has been seen with product that has been temper rolled, or skin passed. Dark stain on skin-passed material is therefore not automatically the late stage it resembles, and the report should say which material it is.
Prevent It at Order Stage
Two mills describe what a surface treatment is actually for, and both use the same word. SSAB states that surface treatments provide only temporary surface protection during transportation and storage, and that white rust tends to form easily on bright, newly coated coils or between tightly packed sheets if condensed water or rainwater collects on the surface and cannot evaporate quickly. ArcelorMittal states that galvanised strips are usually supplied passivated or oiled to temporarily limit any risk of white rust formation, and that during transport and storage all necessary precautions must be taken to keep the material dry and to prevent the formation of condensation.
Temporary is the operative word in both. What is decided before the coil is made is the condition it starts in, not the condition it arrives in. Each choice below buys something and costs something further down the line.
- Passivation is the strongest single controlThe best way of minimizing the chance of white rust forming during shipment and storage of coils, lifts of sheared blanks, or bundles of roll-formed panels is the application on the galvanizing line of a very thin and invisible chemical treatment by the coated sheet manufacturer. It is applied where the coil is made, so it is an order decision and not a later one.
- It minimizes; it does not eliminateMill-applied passivation treatments minimize the tendency for storage stain; they do not eliminate its occurrence if the product is subjected to very adverse conditions. Chromate passivate films in particular do not prevent the eventual formation of storage stain if water is allowed to remain between contacting surfaces.
- Use the word the mill usesSteel sheet manufacturers use the terms passivation treatment and chemical treatment for one and the same thing, and when an order is placed it is necessary to specify whether chemical treatment is or is not required. An order that says nothing about it has not made the choice; it has passed the choice to somebody else.
- Oil is the alternative, and it runs on a clockAn alternative to passivation treatments is to apply specially formulated oils containing rust inhibitors; they delay the onset of staining from humidity by preventing moisture from condensing between the laps of a coil or sheets in a bundle. SSAB adds that the temporary corrosion protection provided by oil is especially dependent on storage time, so long storage times should be avoided with oiled products.
- Both together, where the voyage is hardIt is possible to order a product with both chemical treatment and oil; typically this combination provides better white-rust protection than either used separately, and it should be considered when harsh storage conditions are expected.
- Untreated is sometimes the correct orderAny type of chromium passivation interferes negatively with the spot weldability of galvanized sheet, so almost all galvanized sheet intended to be fabricated using automated spot welding equipment is ordered and produced as non-chemically treated. When the end use involves spot welding, or coil prepainting, it may be necessary to order the product as unpassivated, and there must then be absolute certainty that it will not get wet before it is used.
- Untreated material does not go outdoorsUnpassivated galvanize should not be applied to applications where the product will be placed outside, because unpassivated zinc has a very active surface and dew or other condensate can darken it quickly. If the part is stored in the open at any point, that rules the choice out on its own.
- Passivation and paint pull against each otherIt is very difficult to obtain good paint adherence on new zinc-coated surfaces passivated with chromate solutions; where possible, the sheet should be allowed to weather for at least 12 months before field painting, or be pretreated. A coil ordered for how it survives the voyage can therefore arrive wrong for the line that has to coat it, which is a conversation to have before the order and not after it.
- Chromium is being written out of the processDue to health, safety and environmental concerns, the use of hexavalent chromium passivation treatments is being discontinued; they are being replaced by treatments in which the chrome is only in the trivalent state, or by products entirely free of chrome. Where a specification excludes hexavalent chromium, that exclusion belongs in the order line rather than in a covering note.
- The letter that carries all of thisIn an EN 10346 order the decision is a letter: the order states a letter for the surface treatment (C, O, CO, P, PO or S), alongside a letter for the coating finish (N or M) and one for the surface quality (A, B or C). SSAB describes those surface treatment options as chemical passivation (C), oiling (O), chemical passivation and oiling (CO) and unprotected (U). Under JIS the same decisions are written differently again: C is chromate treatment, NC is chromate-free treatment and M is no treatment, with O for oiled and X for non-oiled. Each system has its own letters, and neither set is read against the other's list.
Packaging and Shipment
Packing has two jobs on this product: keep water out, and let anything that does get in dry out again. Wrapping that only does the first can make the second impossible.
- Ship covered, without exceptionEven if the coils and bundles are wrapped, ship only in covered, watertight conveyances. If an uncovered conveyance must be used, wrap the load completely with a tarp, and avoid tearing the wrapping paper.
- Do not wrap tightly in plasticSheets should not be wrapped tightly in plastic; the product should be allowed to breathe by providing air circulation, and a bundle should not be wrapped if the sheets are wet, because this traps moisture and prevents drying. Tight plastic is the packing that looks the most protective and behaves the worst.
- If a bundle gets wet, open itIf bundles of galvanized sheets or blanks get wet, the only way of preventing or minimizing storage stain is to immediately separate all the pieces so they can individually dry. If the material is unpassivated it may already be too late, because storage staining will occur immediately after becoming wet.
- A torn wrap is a schedule problemOnce water is between the laps, even the best chemical treatment cannot protect against storage stain for longer than about six weeks. That is the clock a damaged wrap on the quay starts, which is why the packing condition is worth photographing at every handover rather than only at the end of one.
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Storage After Arrival
Once the coil is inside a building you control, most of the remaining risk is yours to manage, and almost all of it is about temperature and time.
- Check the package before the coilJFE Steel advises that when receiving product you check for moisture in the package and dry it immediately if it is wet. Storage areas should be as dry as possible, because storage in damp conditions and with broken packaging can result in discoloration or rusting.
- Let cold steel warm up before unwrappingSteel that is cold after transit should be allowed to acclimatize, and where the coils or bundles are wrapped with paper the best practice is not to remove the wrapping until the steel temperature is above the dew point. Unwrapping a cold coil in a warm shed is the fastest way to condense water onto it.
- Climate control, and a short stayStore the coils in a climate-controlled warehouse, use the material promptly and, wherever possible, avoid storing the product for extended periods of time, in excess of two months.
- Unheated winter storage is its own hazardStoring galvanized coils in unheated warehouses over the winter carries the risk of condensation forming between the laps due to sudden temperature drops after a warm period, even when the coils are paper wrapped and encased inside metal shipping shrouds. White rust can form after a few weeks in that circumstance, even on well-passivated sheet.
- Dry is the whole answerIf galvanize is kept dry, storage stain will never be an issue. Every other line on this page is a way of making that sentence hold through a voyage, a quay and a warehouse.
On Arrival: Inspect and Document
A container that opens on stained coil becomes a documentation exercise within the hour, and the evidence that is easiest to collect is the evidence that disappears first. The steps below are in the order the material allows.
They are read against records made earlier. Before shipment ZIVO checks thickness, width, surface, weight, packing and labels; photos of coil labels can be provided; and each shipment travels with a mill test certificate and a packing list.
Photograph the packaging before it is opened
JFE Steel's advice on receiving product is to check for moisture in the package. Photograph that condition before the wrap comes off, because the packaging is what shows where water entered and a bare coil no longer can.
Dry anything wet, straight away
Where the package is wet, dry it immediately rather than restacking or rewrapping. Storage in damp conditions and with broken packaging can result in discoloration or rusting, so a wet wrap left on is an ongoing cause and not just a piece of evidence.
Record what identifies the material
Coil numbers, the labels, the position of each affected coil in the container and the packing condition of each one. Photos of the coil labels can be provided for comparison, and the packing list and the mill test certificate that travel with the shipment are what the coil numbers are matched against.
Read the colour, not the area
White or light grey and dark grey to black are different findings, not degrees of the same one. When the stains turn black it usually means iron has become part of the corrosion product, that is, enough zinc has been consumed for iron from the steel substrate to be involved. The exception is temper rolled, skin passed material, where initial light stain has been observed as dark grey to black, so the report should state which material it is.
Judge it against what the coil is for
Wherever white rust has occurred there is an actual loss of zinc coating, and the extent depends on the exposure time to moisture, the temperature experienced during storage and the presence of accelerating corrosive agents such as chloride-containing salts. Separately, the surface is etched and will not recover the bright, reflective appearance of as-produced galvanized sheet once the stain is removed. A coil going to paint and a coil going out as a visible finish fail different tests, so the report should name the end use.
Send it in writing, with the photographs
Quality problems found after delivery are handled from written feedback with photos, followed by a resolution. A report carrying the container, the coil numbers, the packing condition and dated photographs is one that can be acted on; a verbal description of a colour is not.
White Rust, Wet Storage Stain and Fretting
Not every mark on a galvanized coil is the same finding, and the words for them are not agreed between sources. The table sets out only what has a source behind it, with the source named beside each description.
| Term | How the source describes it | Whose description it is |
|---|---|---|
| Storage stain | A corrosion stain that is typically white, but which can also take the form of a grey or black deposit on the surface; because the commonest form of discoloration is white in appearance, storage stain is often called white rust | GalvInfo Center, written for galvanized sheet in coil, lifts or bundles |
| White rust | Treated as a different thing from wet storage stain, and associated specifically with cooling towers | American Galvanizers Association |
| Wet storage stain | The bulky white or gray deposit formed by accelerated corrosion of the zinc coating when closely packed, newly galvanized articles are stored or shipped under damp and poorly ventilated conditions | American Galvanizers Association, written for batch hot-dip galvanized articles, naming galvanized sheets among the items affected |
| Black staining | When the stains on galvanize turn black it usually means that iron has become part of the corrosion product, that is, enough zinc has been consumed that iron from the steel substrate is involved | GalvInfo Center |
| Fretting marks | A surface oxide phenomenon that can be a major aesthetic issue, but with no evidence that it has a negative effect on corrosion resistance | GalvInfo Center, galvanized sheet in coil or bundles |
Two things follow for a specification. A clause built on the words white rust will be read differently depending on which of those two sources the reader has in mind, so a clause is safer written around the condition — stain on the coated surface after transit or storage — than around the name. And fretting is worth separating out at the point of inspection, because it is an appearance finding rather than a coating-loss finding, and the two are settled in different ways.
If It Has Already Happened
What follows is what the sheet-industry source publishes about treating stain. It is published guidance, not a repair service ZIVO performs.
Appearance first: galvanized sheet damaged by storage stain generally cannot be restored to its original high lustre appearance, although there are treatments that improve the appearance depending on the severity of the stain. So the realistic question is how far a coil can be brought back, not whether it can be made to look new.
Where the stain is not too severe, the published method is to remove it by washing with white vinegar, followed immediately by a thorough rinsing with water to neutralize the surface, with the sheets dry before they are restacked. The immediately and the dry are both load-bearing. An acid left on zinc keeps working, and sheets restacked damp reproduce the conditions that caused the stain in the first place.
Where the stain has progressed to dark grey or black in colour, removal is probably not possible. At that point the question stops being how to clean the coil and becomes what the coil is still fit for, which is a conversation about the part rather than about the surface.
Ordering Galvanized Coil With This in Mind
Three decisions carry most of the storage risk, and all three are made before the coil exists.
Surface protection
Passivated, oiled, both, or neither. Ordering a product with both chemical treatment and oil typically provides better white-rust protection than either used separately, and it should be considered where harsh storage conditions are expected.
What the line does next
Where the end use involves spot welding or coil prepainting it may be necessary to order the product as unpassivated, and there must then be absolute certainty that it will not get wet before it is used. Unpassivated galvanize should not go to applications where the product will be placed outside, because unpassivated zinc has a very active surface and dew or other condensate can darken it quickly.
The specification on file
The specification is confirmed by ZIVO sales together with you before the order and kept on file, so what was agreed about surface treatment and packing can be read back against what arrived.
Where the order is written
The grades, the coating masses and the fields an inquiry has to carry are set out on the galvanized coil page, together with how the surface treatment and the packing are agreed before an order is placed.
Frequently Asked Questions
It is a real loss of coating, and it is not automatically a rejection. When white rust occurs there is an actual loss of zinc coating, and the extent of the damage is primarily dependent on the exposure time to moisture, the temperature experienced during storage, and the presence of accelerating corrosive agents such as chloride-containing salts. The surface is also etched and no longer has the bright, reflective appearance of as-produced galvanized sheet, and removing the stain will not eliminate that. What the finding is not, on its own, is proof that the coil left the mill wrong: mill-applied passivation treatments minimize the tendency for storage stain but do not eliminate its occurrence if the product is subjected to very adverse conditions.
It depends on the colour and on what the coil is for. Light stain may be removed by washing with white vinegar, followed immediately by a thorough rinsing with water to neutralize the surface, with the sheets dry before restacking, but the etched surface will not come back to its as-produced brightness and the sheet generally cannot be restored to its original high lustre appearance. Where the stain has progressed to dark grey or black, removal is probably not possible, and black usually means that iron has become part of the corrosion product.
Keeping the steel dry, and ordering the coil so that the dry period lasts. If galvanize is kept dry, storage stain will never be an issue. Where that cannot be guaranteed end to end, the best way of minimizing the chance of white rust forming during shipment and storage is the application on the galvanizing line of a very thin and invisible chemical treatment by the coated sheet manufacturer, and where harsh storage conditions are expected it is possible to order a product with both chemical treatment and oil, which typically provides better protection than either used separately.
No. Mill-applied passivation treatments minimize the tendency for storage stain; they do not eliminate its occurrence if the product is subjected to very adverse conditions, and chromate passivate films do not prevent the eventual formation of storage stain if water is allowed to remain between contacting surfaces. Once moisture gets between laps it will not evaporate, because air movement is absent, and from that point even the best chemical treatment cannot protect against storage stain for longer than about six weeks.
Photographs, then drying, then a written report. Photograph the packaging before the wrap comes off, because that is what shows where water entered. Dry anything wet immediately, since storage in damp conditions and with broken packaging can result in discoloration or rusting, and if bundles of sheets or blanks are wet the only way of preventing or minimizing further stain is to separate all the pieces immediately so they can individually dry. Then send written feedback with photos, which is how quality problems found after delivery are handled, followed by a resolution.
Sources
- GalvInfoNote 2.10: Imparting Resistance to Storage Stain (Rev 2.0, Jan 2019)
- GalvInfoNote 2.11: Preparing Galvanize for Field Painting (Rev 1.3, Dec 2017)
- GalvInfoNote 3.2: Protecting Galvanized Steel Sheet Products from Storage Stain (Rev 3.0, Oct 2020)
- GalvInfoNote 6.1: Successfully Shipping and Storing Galvanized Steel Sheet (Rev 1.0, Dec 2017)
- GalvInfoNote 6.3: Successfully Manufacturing Products made with Galvanized Sheet (Rev 1.0, Dec 2017)
- EN 10346:2015 Continuously hot-dip coated steel flat products for cold forming – Technical delivery conditions (public sample: foreword, scope, terms, classification, ordering information)
- Industry catalogue E20 – Hot dip galvanised steels
- Metal coated formable steels, EN10346:2015 – DX51D, DX52D, DX53D, DX54D, DX56D, DX57D
- Hot Dip Galvanized Steel Sheet catalog (Cat. No. B1E-004-11)
- DURGRIP™ hot-dip galvannealed steel sheets catalog (U019en)
- Wet Storage Stain (knowledge base article, 2010)
- White Rust: Causes and Prevention (knowledge base article, updated July 2026)
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