Wet storage stain or white rust presents a challenge in the metal cladding industry

With the start of the rainy season, the problem of wet storage stain or white rust presents a challenge in the metal cladding industry. Galvanized products are particularly vulnerable to this phenomenon. However what is not generally known is that aluminium/zinc and painted coating, as well as non-anodized aluminium are equally vulnerable.

As part of their natural weathering process metals react with the atmosphere to form an impervious weather-resistant layer or patina. Iron is the exception in that it forms a porous layer, rust.

In the case of zinc the weather-resistant layer is formed in three stages;
Initially the zinc surface reacts with the surrounding air to form zinc oxide.
This surface, in the presence of free flowing air, reacts with moisture in the atmosphere to form zinc hydroxide. During drying the zinc hydroxide reacts with carbon dioxide to form zinc carbonate and it is this layer that gives zinc its durability.

When galvanized surfaces are closely packed and exposed to moisture in the absence of free flowing air the above process is halted at the second stage which will continue until all the zinc is consumed. Zinc hydroxide is colourless when wet but changes to a powdery white/grey deposit on drying, i.e. white rust. This is the reason that when bundles of wet cladding are first separated the white rust is not apparent. The extent of the damage is dependent on time, temperature and contaminants in the moisture. In hot conditions staining can occur within twelve hours. Under the right conditions the coating can be destroyed in seven to ten days.

It is, therefore, imperative that bundles of cladding are exposed to free moving air and kept dry from the time of packing into bundles through transportation and storage to fixing into position.