Wet storage stain

Causes

Products that have just been galvanised usually have a shiny appearance (depending on the material). After a time in contact with air, this shine becomes matt as a thin zinc patina forms on the surface. The patina is an indissoluble, strongly bonding and protective layer consisting mainly of basic zinc carbonate.

This protective patina fails to form when rain or condensation remains on the galvanised material in the absence of free-flowing air. The moisture results in the formation of white spots (zinc oxides) that are commonly known as white rust. This wet storage stain is particularly common during storage or transport, when moisture can be retained between stacked galvanised parts.

Wet storage stain does not affect the quality or the useful life of galvanised materials and it disappears spontaneously after a while with exposure to air. It does tarnish the material.

Under NEN-EN-ISO 1461, wet storage stain does not constitute grounds for rejection. The standard states: the presence of dark or light grey spots, for example a lacework pattern of dark grey spots or any surface irregularities, does not constitute grounds for rejection. White spots resulting from storage, mainly basic zinc oxide resulting from storage in damp conditions after galvanisation, do not constitute grounds for rejection either. The coating thickness must nevertheless exceed the specified minimum value.

When the material is exposed to air, the protective sink patina forms gradually, and most or all of the wet storage stain will disappear in time. However, if the galvanised material is destined for duplex processing, it is imperative for the wet storage stain to be removed meticulously to ensure proper bonding of the powder coating or liquid paint. Rotocoat can remove wet storage stains using both mechanical and chemical approaches.

You can remove small spots yourself for aesthetic purposes but this is most definitely not appropriate in preparation for duplex processing.

Removing wet storage stain by:

• using a dry, hard nylon (NOT a metal!) brush.
• brush with a soft nylon brush and a solution of citric acid (25 - 50 g/litre of water) or an ammonia solution (5 - 10% ammonia dissolved in water).

Rotocoat is very conscious of the need to avoid contact between galvanised materials and moisture. To prevent the formation of a film of moisture, we take the appropriate preventive measures during storage and transport.

The materials:

• are never left in direct contact with the ground but always placed on dry and clean wood or plastic;
• are not stacked ‘cold’;
• are stacked, when at all possible, so that there is a free flow of air.