White rust, or ‘white storage stain’, is a problem encountered on the surface of metals newly coated with zinc. It’s a particularly common problem with galvanised steel and generally appears as a white, powdery dust that forms quickly on the surface under certain conditions. White rust can deteriorate the zinc coating of a galvanised metal, causing it to flake away and expose the unprotected steel below.
White rust is a problem encountered with zinc coated or galvanised products. To ensure the life of a galvanised material be sure to take the appropriate precautions to prevent white rust from forming.
Galvanisation
Galvanisation is the process of applying one or more layers of zinc coatings to a metal base. In the case of galvanised steel, the base is carbon steel. Zinc is a highly reactive metal, and zinc oxide commonly forms on the zinc layer, which is durable, protective, and prevents iron oxide from forming underneath it. Zinc oxides form progressively as the metal is exposed to air, with carbon dioxide particularly beneficial in the formation of stable oxides.
‘White rust’ forms in certain situations involving water, air and condensation, the bi-product being zinc hydroxide. The extent of white rust can range from a blemish to deep structural damage.
How Bad is White Rust?
White rust isn’t always structural. In a lot of cases, small amounts of white rust that have appeared will eventually “tone-in” to the metal and disappear. Heavier deposits can be more troublesome as zinc hydroxides do not easily stick to other metals and therefore flake off from the base material. In some cases, metal panels may need to be soaked in a bath of a chemical such as sodium dichromate, however, if the damage is severe the metal may need replacing altogether.
Preventing White Rust
The best method to establishing the long-term life of a galvanised product is preventing white rust from appearing in the first place.
The best ways to prevent the growth of white rust are:
• Eliminating or greatly reducing the exposure of the zinc coated material to water on a freshly coated piece of metal. Try increasing airflow around the product, increasing temperature and controlling humidity in the room.
• Allowing the zinc layer to form stable oxides by aging the metal. This will form a protective layer meaning the material will be less likely to form white rust after exposure to water. Decreasing humidity and exposure to carbon dioxide is the best way to promote zinc oxides.
• Another way to prevent white rust is to avoid bringing a cold galvanised product into a warm area, as condensation could form on the material.
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