THE MECHANISM OF WHITE RUST FORMATION

Zinc is a relatively reactive metal and it will react vigorously with both acids and alkalis. Its delivers its best anti-corrosion performance in neutral pH conditions, and is thus well suited as a protective coating in most atmospheric exposure classifications, other than severe marine. However, zinc, like aluminium, relies on the formation of an oxide film on its surface for its durability. Once this oxide film is formed, the rate of corrosion of zinc (galvanized) coatings is very slow – typically 2 microns or less in thickness per year in normal environments.

When steel is freshly galvanized, the zinc has no significant oxide film on its surface. The chemical reactions that occur to form this film take some time. They are:
1. The oxidation phase 2Zn + O2 = 2ZnO
2. The hydration phase 2Zn = 2H2O + O2 = 2Zn(OH)2
3. Carbonation 5Zn(OH)2 = 2CO2 + 2ZnCO3.3Zn(OH)2 + 2H2O
It is the formation of the zinc carbonate oxide film, that is highly water insoluble, that provides the underlying zinc with its good anti-corrosion performance. Other reactions can occur in the presence of chlorides, sulfates and other corrodents that may accelerate the degradation of the zinc-based coating at a rapid rate. It is the exposure of `young’ zinc-coated surfaces to pure water that is the principal mechanism associated with 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 where galvanized products are nested together, tightly packed, or when water can penetrate between the items and remain for extended periods. In favourable (for white rust)conditions, very rapid consumption of the zinc can occur and corrosion rates 20-50 times higher than those normally experienced. While this type of corrosion is called ‘white rust’, it may have a dark gray or even black appearance on the galvanized
surface.

It is standard galvanizing practice in hot dip galvanizing facilities, to cool the work by quenching it in water. In most operations, the quench water contains a low concentration of sodium dichromate (usually less than 0.5%). The quenching of the hot steel in this weak dichromate solution creates a passivating film on the galvanized coating’s surface that provides some initial protection for the zinc, and gives it time to develop its own protective oxide film. Some proprietary coatings are applied to continuously galvanized products to perform the same function. These treatments must be considered temporary. In periods of heavy rain, the dichromate passivation film, which is slightly soluble in water, can be washed off the surface and can increase the propensity of the zinc surface to white rust when exposed to pure water. Short-term exposure to rain water is not necessarily a problem, and wetting and drying cycles may in fact assist in the development of the protective oxide film.
Carbon dioxide is required to initiate the development of the stable carbonate – based oxide film, thus good access to air is an essential part of the process. Poorly ventilated, damp conditions are conversely very detrimental in white rust formation.

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 at 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
5. Provide adequate ventilation when transporting galvanized items for extended periods

* An extract from 'White Rust on Zinc Coatings' article 06 - Industrial Galvanizers Corporation Pty Ltd