Galvanizing is a common way of providing corrosion protection to carbon steel. The term is often used to describe zinc coatings in general.
Zinc corrodes in preference to steel and hence sacrifices itself to protect the steel. The corrosion products from the zinc are deposited onto the steel, resealing it from the atmosphere and stopping corrosion. Corrosion rates are very low, and are linear. Thus it is possible to predict performance for a given environment. For a typical interior condition with occasional condensation, the corrosion rate is between 0.1 & 0.7 μm/year. This equates to a theoretical life of 120 years for a coating thickness of 85 μm (see below). On the other hand, the corrosion rate in a swimming pool can be 4 μm/year giving a life of only 20 years.
It may be seen then that galvanizing is not a substitute for stainless steel, which has an indefinite life. It is however a way of giving efficient protection to steel, particularly to fabrications where the protection is required to the finished article, and not just the component parts.
 The method most usually specified to achieve this is ‘hot dip’ galvanizing (HDG). The process involves degreasing the work, chemical cleaning and then immersion in a bath of molten zinc at a temperature of 450 °C. As the work is immersed, zinc wets the surface, and a reaction takes place to form a series of zinc/iron alloy layers, bonded to the substrate.
When the work is removed from the bath, some pure molten zinc will remain on the top of the layers of alloy, solidifying as the work cools, and resulting in the familiar light spangle appearance.
 The thickness of the galvanizing is largely determined by the thickness of the base material. BS EN ISO 1461 gives requirements as shown. It is possible to achieve thicker coatings but in most cases this is not necessary.
In any case it involves further pre-treatment of the steel which has a cost implication.
Dipping twice does not increase the thickness since no reaction takes place the second time, and the new zinc simply drips off.
In most cases, unless quantities are very large, it is sufficient to specify a mean coating thickness of 610 g/m2 with a local minimum of 505 g/m2. |
