White Rust on Galvanized Coatings

Introduction
One of the commonly encountered problems with galvanized coatings of all kinds is ‘white rust’ or ‘white storage stain’. It is manifested as a bulky, white, powdery deposit that forms rapidly on the surface of the galvanized coating under certain specific conditions.

White rust can cause considerable damage to the coating and is always detrimental to the galvanized coating’s appearance.

The surface of galvanized coatings is almost 100% zinc. It is the durability of the zinc that provides the outstanding anti-corrosion performance for steel, yet zinc is a relatively ‘reactive’ metal. It is the stable oxides that form on the zinc’s surface that determine its durability, and these oxides are formed progressively as the zinc is exposed to the atmosphere. Carbon dioxide in particular is a contributor to the formation of these stable oxides.

With newly galvanized steelwork, the zinc’s surface has been subjected to little oxidation and is at its most vulnerable. For this reason, galvanizers use a chromate passivation in conjunction with their galvanizing operations to provide protection to the galvanized coating during the ‘youth’ period of the coating. This passivation coating provides short term protection to the zinc to give the stable oxides time to form on the surface.

White rust formation
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 galvanized 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 galvanized 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:

1. Keep the packed work dry
2. Pack the items to permit air circulation between the surfaces
   
3. Stack the packed items on an angle to allow water to drain out
   
4. Treat the surface with proprietary water repellent or barrier coatings to prevent moisture contact with galvanized surface.

Treating galvanized surfaces affected by white rust
Once the galvanized surface has been attacked and the zinc hydroxide compounds have formed, it is desirable to remove the oxide products from the surface because:

a.their presence inhibits the formation of stable carbonate based oxides and
b.they are unsightly

The effect on the galvanized 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 galvanized products.

1. Light white rusting.  This is characterised by the formation of a light film of white powdery residue and frequently occurs on freshly galvanized 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 at this level.
2. Moderate white rusting.  This is characterised by a noticeable darkening and apparent etching of the galvanized coating under the affected area, with the white rust formation appearing bulky.  The galvanized coating thickness should be checked to determine the extent of attack on the coating. In the majority of cases, less than 5% of the galvanized 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:
                      a.  Wire brush the affected area to remove all white corrosion products
                      b.  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                    galvanized surfaces.
3. Severe white rusting. This is characterised by very heavy oxide deposits. Items may be stuck together. Areas under the oxidised area may be almost black of show signs of red rust. A coating thickness check will determine the extent to which the galvanized coating has been damaged. Remedial treatment to reinstate the coating should be undertaken as follows:
                      a.Wire brush or buff the affected area to remove all oxidation products and rust if any.
                      b.Apply one or two coats of approved epoxy zinc-rich paint (see section “Repairs to Hot                    Dip Galvanized Coatings) to achieve required dry film thickness of 100 microns                    minimum.

Chemical removal of white rust
Zinc hydroxide dissolves readily in chromic acid. Bulky white rust deposits can be quickly removed by washing with a 5% solution of chromic acid. Precautions must be taken in the handling and containment of this solution and its residues. This treatment has the advantage of re-passivating the zinc surface and is well suited to treating batches of small parts. Other chemical treatments based on phosphoric acid can be used, however, these will darken the coating in the process.

Re-passivating the galvanized 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% sulfuric acid, brushing with a stiff wire brush for 30 seconds before thorough rinsing of the surface.

Conclusion
White rust is a post-galvanizing phenomenon. Responsibility for its prevention lies in the manner it is packed, handled and stored prior to the galvanized product’s installation and use. The presence of white rust is not a reflection on the galvanized 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 minimised on newly galvanized steel.

Back to FAQ index