Pure water (H2O) contains no dissolved salts or minerals and zinc will react quickly with pure water to form zinc hydroxide, a bulky white and relatively unstable oxide of zinc. Where freshly galvanised steel is exposed to pure water (rain, dew or condensation), in an oxygen deficient environment, the water will continue to react with the zinc and progressively consume the coating. The most common condition in which white rust occurs is with galvanised products that are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods.
Avoiding White Rust Formation
There are a number of simple steps that can greatly reduce or eliminate the formation of white rust. These are:
• Keep the packed work dry
• Pack the items to permit air circulation between the surfaces
• Stack the packed items to allow water to drain out
• Treat the surface with proprietary water repellent or barrier coatings to prevent moisture contact with galvanised surface
Treating Galvanised Surfaces Affected by White Rust
Once the galvanised surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because:
• Their presence inhibits the formation of stable carbonate based oxides and
• They are unsightly
The effect on the galvanised coating can range from very minor to extremely severe and various levels of remedial treatment are available to deal with white rust problems at the various levels at which they are likely to occur.
The following treatments are recommended to deal with white rust on galvanised products:
Light White Rusting
This is characterized by the formation of a light film of white powdery residue and frequently occurs on galvanised products during periods of heavy rain. It is particularly evident on areas that have been buffed or filed during quality assurance operations. These treatments remove the passivated surface from the galvanizing and expose unoxidised zinc to attack from rainwater. Provided the items are well ventilated and well drained, white rust rarely progresses past this superficial stage. It can be brushed off if required but will generally wash off in service with normal weather. No remedial treatment is generally required for this level.
Moderate White Rusting
This is characterized by a noticeable darkening and apparent etching of the galvanised coating under the affected area, with the white rust formation appearing bulky. The galvanised coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanised coating will have been removed and thus no remedial work should be required as long as the appearance of the affected area is not detrimental to the use of the product and the zinc hydroxide residues are removed by wire brushing. If appearance is unacceptable, the white rust affected area can be treated as follows:
• Wire brush the affected area to remove all white corrosion products
• Using a cloth pad wet with aluminium paint, rub the surface with the pad to apply a thin film of aluminium paint to the affected area to blend it with the adjacent unaffected galvanised surfaces.
Severe White Rusting
This is characterized by very heavy oxide deposits. Items may be stuck together. Areas under the oxidized area may be almost black and show signs of red rust. A coating thickness check will determine the extent to which the galvanised coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows:
• Wire brush or buff the affected area to remove all oxidation products and rust if any.
• Apply one or two coats of approved epoxy zinc-rich paint to achieve required dry film thickness of 100 microns minimum.
Re-Passivating the Galvanised Surface
Where white rusting has occurred and the item may be subject to continuing exposure that may propagate similar corrosion, re-passivating of the surface can be done by treating the surface with a solution of 5% sodium dichromate 0.1% sulphuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.
White rust is a post-galvanizing phenomenon. Responsibility for its prevention lies in the manner it is packed, handled and stored prior to the galvanised product’s installation and use. The presence of white rust is not a reflection on the galvanised coating's performance, but rather the responsibility of all those involved in the supply chain to ensure that the causes of white rust are recognised and the risks of its occurrence minimized on newly galvanised steel.