What Is Wet Storage Stain?

Galvanized steel is protected from corrosion by the formation of a protective zinc carbonate film over the zinc surface. In order to form, exposure is required to free flowing air. Freshly galvanized articles that are stacked in contact with one another so preventing the free circulation of air may, in wet or humid conditions, start to show "white rust". This is a white powdery covering of a mixture of zinc oxide and zinc hydroxide. As this is unable to stabilise, in poorly ventilated packing, the protective zinc carbonate film does not form and the reaction on the zinc surface continues. White rust is voluminous and, as a result, often looks worse than it actually is. However, conditions which encourage its formation should not be allowed to persist.

Cure

Normally, light staining is not serious and is arrested after affected areas are dried and exposed to the atmosphere. In time, the colour of the affected area will age to match that of normally weathered galvanized steel.

Where more stubborn deposits are evident, these are easily removed using a stiff bristle (not wire) brush and, if necessary, a cleaning solution can be used. Typical solutions would be ammonia (2%) or acidified sodium dichromate (2% sodium dichromate with 0.1% sulphuric acid). In both instances, the cleaning solutions should be thoroughly rinsed off after treatment and the article allowed to dry.

1 - Zinc dissolves to Zn+2 2 - Precipitation of Zn(OH)2 WHITE RUST 3 - Oxygen reduced to OH-