WET STORAGE TASKS
More commonly known as “white rust”
Zinc exposed to the atmosphere reacts with the ambient elements: humidity, oxygen, carbon dioxide … to form a “patina”. This “patina” is reflected in appearance by a gradual decrease in the metallic luster of the surface. This insoluble, adherent and protective layer consists mainly of zinc carbonate
However, in the presence of stagnant water, the constitution of this layer is thwarted and there is rapid formation of whitish spots consisting in particular of hydroxide and zinc oxide, powdery products generally poorly adherent and non-protective.
The phenomenon is most often seen when water from rain or condensation of moisture has been retained, trapped between pieces or sheets piled or packed for storage or transport.
These wet storage stains have the disadvantage of poor presentation by their appearance.
The NF EN ISO 1461 standard dealing with molten zinc dipping (hot-dip galvanizing) states that the presence of wet storage stains does not generally cause a decrease in corrosion resistance and, therefore, should not be considered as a reason for rejecting the products, as long as the thickness of the coating remains within the minimum value requested.
Stains on products will be eliminated over time as a result of bad weather when these products are put into service outdoors. The protective film of basic zinc carbonate can then be constituted gradually.
On the other hand, it is essential to eliminate stains from products with “white rust” to ensure adherence of the paint.
Some tips for eliminating wet storage stains
- Dry brushing with hard nylon brush. Do not use a wire brush
- Or brushing with soft nylon brush with citric acid solution at 25 to 50 g / liter of water. Rinsing with water and drying.
- Or brushing with soft nylon brush with ammonia solution at 5 to 10% by volume in water. Rinsing with water and drying.
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How to prevent their formation?
To prevent their formation, the products must be stored and transported in such a way that stagnant water can not be trapped between them.
Do not store in direct contact with soil, but on holds. Avoid contact between products, ensure that air can circulate * and tilt to prevent water retention.
Preventive treatments can be applied to the products before storage or transport, namely conversion treatment (passivation), application of clear lacquer or temporary protection film.
The use of these treatments does not exclude the respect of the storage and transport conditions described above.
Source: Galvazinc, The wet storage stains, 25/02/2013
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