Colour variation

The colour variation is one of the most common queries. There is nothing wrong with the coating.

The galvanized coating consists of many layers. To keep it simple, there are two distinct areas – the alloy layers and the free zinc layer. The free zinc layer is pure zinc and is a bright silver colour. This silver layer is a layer of zinc that has frozen on the surface of the article when it gets extracted from the bath – much like the layer of water left on your skin when you get out the pool. The alloy layer is a mixture of steel and zinc and so is a dull grey colour. The alloy layer is formed by the diffusion of zinc into the steel. All hot dip galvanised articles under the Australian Standard AS4680 develop the alloy layer. There are two main causes of colour variation:

1. Silicon content of steel – steels with a distinct composition range of silicon are prone to developing dull grey coatings as the silicon allows the reaction to continue and use up the free layer of zinc on the surface.

2. Thickness of the steel – temperature is a catalyst of the reaction and so thicker pieces of steel stay hot for longer and so allow the reaction to continue and consume the silver layer of zinc. Thinner sections cool rapidly and so the reaction stops before the silver layer can be consumed.

colour variation dull inconsistent different

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