Wet storage stain on aluminium, aluminium-zinc and pre-painted materials

Following on our previous article relating to wet storage stain or white rust on galvanized roofing products this month we address wet storage stain on aluminium, aluminium-zinc and pre-painted materials.

When tightly packed aluminium alloyed surfaces are exposed to moisture under the same conditions that produce white rust on galvanized surfaces they are also subject to accelerated corrosion which manifests in the form of a black stain (≥50% coating loss).

Whilst most aluminium-zinc coatings are provided with a thin transparent resin film this film will only provide limited protection when exposed to wet storage conditions as the coating itself is softened by the moisture and becomes porous. Initial damage will result in the film fogging but if arrested early enough (within a few days of becoming wet) will dry out without any detrimental effect. Prolonged exposure will however facilitate the formation of black staining.

With regard to paint coatings there is a misconception that these coatings are impervious to moisture. With very few exceptions the paints used for coating cladding materials soften and become permeable when exposed to trapped moisture for extended periods as in the case of closely packed bundles of cladding. It must be remembered that colour one side material has only a nominal coating on the reverse side which is particularly vulnerable to attack. Even the top coatings can be irreparably damaged within two weeks of exposure.

It is, therefore, imperative that bundles of cladding materials, including ancillary items, are STORED UNDER COVER, ELEVATED ABOVE A DRY SURFACE AND EXPOSED TO FREE FLOWING AIR. Tightly wrapped bundles are not a foolproof solution as they are prone to sweating.