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‘White rust’ or ‘white storage stain’ is one of the most common issues faced in galvanized coatings. It manifests as a white, bulky, powdery deposit that develops quickly on the surface of the galvanized coating under certain conditions.
White rust can lead to significant damage to the galvanized coating and is always harmful to its appearance.
The surface of galvanized coatings is almost entirely zinc. The exceptional anti-corrosion performance of steel is attributed to the durability of zinc; however, zinc is a comparatively ‘reactive’ metal. The durability of zinc is determined by the stable oxides that develop on this element’s surface and these oxides are formed gradually as zinc is exposed to the atmosphere. Particularly, carbon dioxide plays a role in the development of these stable oxides.
The newly galvanized steelwork makes zinc’s surface highly vulnerable since it has been subjected to little oxidation. Therefore, a chromate passivation should be used together with galvanizing operations to offer protection to the galvanized coating during its ‘youth’ period. This passivation coating offers short-term protection to zinc, providing time for the stable oxides to form on the surface.
White Rust Formation
Pure water (H2O) does not include any dissolved minerals or salts and will react rapidly with zinc to produce zinc hydroxide, a bulky white and comparatively unstable oxide of zinc. When freshly galvanized steel is subjected to pure water (for example dew, rain or condensation) in an oxygen-deficient environment, the water will continue to react with zinc and gradually corrode the coating.
The most common circumstance where white rust is produced is with galvanized products that are tightly packed, compactly arranged, or when water can seep through the items and remain for prolonged periods.
Avoiding White Rust Formation
The formation of white rust can be significantly minimized or avoided completely by following these simple steps:
- Ensure that the packed work is dry
- Pack the items to allow air circulation between the surfaces
- Pile up the packed items to enable water to drain out
- Treat the surface with barrier or proprietary water-repellent coatings to avoid moisture contact with galvanized surface
Treating Galvanized Surfaces Affected by White Rust
After the galvanized surface has been attacked and the zinc hydroxide compounds have formed, it is better to eliminate the oxide products from the surface since:
- Their occurrence prevents the formation of stable carbonate-based oxides
- They are unattractive
The impact on the galvanized coating can vary from very trivial to extremely serious, and different levels of remedial treatment are available to tackle different white rust problems.
Light White Rusting
This is characterized by the development of a light film of white powdery residue and often takes place on galvanized products during heavy rain seasons. It is specifically obvious in regions that have been filed or buffed during quality assurance operations. In these treatments, the passivated surface is removed and un-oxidized zinc is exposed to attack from rainwater. Additionally, if the items are well-ventilated and well-drained, white rust hardly ever develops beyond this superficial phase.
If needed, it can be brushed off; however, it will normally wash off in service with normal weather. This level usually does not need any remedial treatment.
Moderate White Rusting
This is characterized by an obvious darkening and clear etching of the galvanized coating under the affected region, with the white rust formation looking large. It is important to examine the thickness of the galvanized coating to estimate the extent of the attack on the coating. In most cases, below 5% of the galvanized coating will have been removed and there is no need for remedial work as long as the appearance of the affected area is favorable to the use of the product and residues of zinc hydroxide are eliminated by wire brushing. In case the appearance is undesirable, the white rust affected area can be treated as follows:
- With the help of a cloth pad wet with aluminum paint, rub the surface with the pad to apply a thin film of aluminum paint to the affected area to merge it with the adjacent unaffected galvanized surfaces
- Eliminate all white corrosion products from the affected area using a wire brush
Severe White Rusting
This is characterized by extremely heavy oxide deposits. Items may be fixed together. Regions under the oxidized area may be nearly black and exhibit signs of red rust. The extent to which the galvanized coating has been damaged can be determined by checking the coating thickness. Remedial treatment to restore the coating should be carried out as follows:
- Buff or wire brush the affected region to eliminate all oxidation products and rust
- Apply one or two coatings of approved epoxy zinc-rich paint to obtain the required dry film thickness of at least 100 µm
Re-Passivating the Galvanized Surface
The surface can be re-passivated where white rusting has taken place and the item may be subject to continuous exposure that may spread similar corrosion. Re-passivation of a surface can be done by treating the surface with a solution of 5% sodium dichromate and 0.1% sulfuric acid and brushing with a stiff wire brush for 30 seconds followed by a thorough rinse of the surface.
White rust is a phenomenon that occurs after galvanization. The way it is packed, handled and stored before the installation and use of the galvanized product is important for its prevention.
The presence of white rust is not an indication of the galvanized coating's performance but indicates the responsibility of all those involved in the supply chain to ensure that the causes of white rust are known and the risks of its occurrence are reduced on the newly galvanized steel.