White rust on galvanised steel sheet -
how it forms and how to assess the material
White rust is zinc hydroxide, not a product of steel corrosion. It forms when the surface of freshly galvanised sheet stays wet while having no free access to air containing oxygen and carbon dioxide. Under normal conditions zinc oxidises to oxide, then turns into hydroxide, which in the presence of carbon dioxide forms protective zinc carbonate, the patina. When sheets or coil wraps lie tightly against each other and are damp, the reaction stops at the hydroxide stage, which has no protective properties and remains powdery.
The practical consequence is less dramatic than the appearance suggests. Zinc hydroxide is light and fluffy, so it builds up a volume out of proportion to the amount of metal corroded. A batch that looks seriously damaged has often lost only a fraction of its coating thickness.
Three conditions that must occur together
The extent of staining is determined by three factors: the time the surface is in contact with moisture, the storage temperature and the presence of substances that accelerate corrosion, above all chlorides.
The most common source of moisture is not rain but condensation. When a cold coil is brought into a warmer warehouse, the air temperature right at the steel surface falls below the dew point and water settles on the sheet. There is no air circulation between coil wraps or between sheets in a pack, so moisture stays there for a long time and corrosion continues for as long as the surface is wet.
EN 10346 explicitly identifies moisture, and in particular condensation between sheets and coil wraps, as the cause of corrosion products and recommends dry transport and storage. The rule formulated by GalvInfo is even simpler: if a galvanised product stays dry, storage stain will not occur at all.
Colour as an indication, not a measure
The basic form of the stain is white and powdery. A change in colour, however, carries specific diagnostic information.
| Stain colour | What it means |
|---|---|
| white, powdery | zinc corrosion product, steel substrate intact |
| grey | iron from the substrate starts to take part in the corrosion products |
| dark grey or black | the zinc coating has been largely consumed at that spot |
Colour alone is not a precise measure of zinc loss, and assessment requires experience. One technological detail is also worth knowing: sheet that is temper-rolled after galvanising darkens faster than non-temper-rolled sheet, because its surface has a higher concentration of aluminium-based oxides. A darker shade on such a product therefore does not necessarily mean advanced coating corrosion.
When staining does not determine fitness for use
Whether the material is fit for use depends on two criteria: the remaining coating thickness and the aesthetic requirements of the end product.
EN ISO 1461, covering hot-dip galvanised articles, states the rule directly: the presence of storage stain is not a reason for rejection as long as the coating thickness remains above the minimum value specified for the given steel thickness. Measurement, not appearance, is decisive. The reference method is magnetic measurement to ISO 2178, and in case of dispute the gravimetric method to ISO 1460 decides, in which the coating is stripped and weighed, using a density of 7.2 g/cm3 for the calculation.
Industry practice for batch galvanised products distinguishes three degrees of severity. Light and moderate staining weathers away in service over time and is acceptable in most technical applications. Heavy staining must be removed and, in extreme cases, the part regalvanised, because if left untreated it can lead to coating failure.
Continuously galvanised sheet is more demanding for two reasons. The coating is much thinner than on batch galvanised products, so the same absolute zinc loss is a larger share of the total. In addition, many applications of this material, from housings to facades, have visual requirements. The stained area becomes etched and permanently loses its original bright, reflective appearance. Cleaning removes the stain but does not restore the sheen.
For products painted after fabrication this is of secondary importance. For visible products supplied without further finishing it can decide the use of an entire batch.
Protection on the production line
EN 10346 provides several surface treatment options intended to prevent this.
| Designation | Treatment |
|---|---|
| C | chemical passivation |
| O | oiling |
| CO | passivation and oiling |
| P | phosphating |
| PO | phosphating and oiling |
| S | sealing with an organic coating |
Chemical passivation protects the surface against moisture and limits the formation of corrosion products during storage and transport, and the standard permits related variations in surface colour. Oiling means applying an even, two-sided layer of neutral, non-drying oil, and the standard indicates that under normal conditions the protection lasts around three months from production. Producers declare similar periods for passivated products.
The limitations of these treatments are, however, well documented. The passivation layer is less than a micrometre thick and does not protect effectively if water remains between the surfaces. Corrosion-inhibiting oils limit the effects of condensation between coil wraps but do not protect against larger amounts of water that have got between the layers. GalvInfo states plainly that even the best chemical treatment does not protect against staining for longer than about six weeks under adverse conditions, i.e. with no air circulation and moisture trapped between sheets.
Factory protection therefore buys time, not resistance.
Storage and transport
The recommendations of industry institutes are consistent and apply both to coiled sheet and to batch galvanised products.
Transport only in covered, watertight vehicles, and where this is not possible, with the load fully protected against water ingress. Storage in a climate-controlled room and use of the material as soon as possible, with the recommendation to avoid storage longer than about two months. No tight wrapping in film, because the material needs air circulation, and condensate collects under non-breathable covers. Coils and packs placed at a slight incline so that water can drain freely, with no places where it can stand. Dry, seasoned timber spacers ensuring airflow between packs, and the material raised above floor level, by at least several tens of centimetres when stored under a roof.
For open-section products one more rule is added: place them opening downwards so that water does not collect inside. Autumn and winter are periods of increased risk because of rain, fog and thermal condensation.
Removing the stain
The method depends on the degree of staining and follows industry institute guidelines.
| Degree of staining | Treatment |
|---|---|
| light | leave to weather naturally outdoors or clean gently with a mixture of mineral oil and sawdust |
| moderate | wash with a solution of about 10% acetic acid using a nylon brush, rinsing thoroughly with water |
| heavy | solution of about 5% phosphoric acid with a wetting agent, applied with a brush |
| advanced, dark grey or black | removal is usually no longer possible, painting with zinc-rich paint after cleaning is recommended |
In all cases cleaning restores an acceptable appearance, but not full metallic sheen. After any acid treatment the surface must be rinsed thoroughly and neutralised.
Commercial significance
For batch galvanised products, light and moderate staining is treated as a cosmetic defect, acceptable in most technical applications, and the standard explicitly excludes it as a sole reason for rejection. For coiled sheet, where buyers often order a specific surface class, visible staining may mean that the batch does not meet the visual requirements of a particular application, even though the protective properties of the coating are retained.
In market practice this leads to technically sound material leaving the main sales channel. For a buyer whose product is painted, built in or not visible in service, this is an opportunity: mechanical properties, dimensions and coating mass remain in line with the standard, and the price difference can be significant. The condition is a reliable assessment of the batch and a clear agreement on what the deviation concerns. More on the classification of surface deviations in the article on surface defects of second-grade steel sheet.
Liability is a separate issue. Staining that forms in the buyer’s warehouse after delivery, as a result of storage conditions, has a different status from staining that formed before dispatch. This is another argument for receiving, checking and protecting the material in line with the recommendations immediately after delivery.
Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL helps select the material for the application and indicates where the surface condition does not matter for the product and where visual requirements are important. Send us the specification and a description of the application, and we will check availability and propose the most favourable option.
Frequently asked questions
Is white rust corrosion of the steel?
No. It is a zinc corrosion product, mainly zinc hydroxide. As long as the stain remains white, the steel substrate is intact. Only a grey or black colour means that iron from the substrate is taking part in the corrosion products.
Is material with white rust suitable for production?
In most technical applications yes, as long as the coating thickness remains above the required minimum. EN ISO 1461 states explicitly that staining alone is not a reason for rejection when coating thickness is retained. The limitation is applications with high visual requirements.
How can the remaining coating be checked?
The reference method is magnetic measurement of coating thickness to ISO 2178. In case of dispute the gravimetric method to ISO 1460 decides, in which the coating is stripped and the sample weighed.
Does passivation protect against storage stain?
It reduces the risk and extends the safe storage period, but does not replace proper conditions. Under adverse conditions, with dampness and no air circulation, chemical protection loses its effectiveness within a few weeks.
Can the stain be removed?
Light staining weathers away by itself or can be removed mechanically. Moderate staining is removed with diluted acetic acid, heavy staining with diluted phosphoric acid, always with thorough rinsing. The original sheen cannot be restored, because the surface remains etched where the stain was.
How can white rust be prevented in the warehouse?
By keeping the material dry: storage under a roof above floor level, dry spacers, air circulation, placement at an incline so that water drains away, no tight film wrapping and the shortest possible storage time before use.
Sources
- GalvInfo Center, GalvInfoNote 3.2, Protecting Galvanized Steel Sheet Products, Storage Stain
- GalvInfo Center, GalvInfoNote 6.1, Successfully Shipping and Storing Galvanized Steel Sheet
- GalvInfo Center, GalvInfoNote 2.10, Imparting Resistance to Storage Stain
- EN ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles - specifications and test methods
- EN 10346, Continuously hot-dip coated steel flat products for cold forming - technical delivery conditions
- American Galvanizers Association, Wet Storage Stain
- Galvanizers Association, HDG Datasheet 11, Wet Storage Stain