Coating Thickness
Steel was zinc-coated for many years by dipping short lengths of flat or profiled sheet metal in a bath of molten zinc, and the steel was then hung to cool while the excess zinc coating drained off.
More than sixty years ago manufacturers developed a continuous hot dipping method. During the continuous hot dipping process, the steel coil is run through a bath of molten metal. The thickness is controlled by blowing off the excess coating with air jets applied to both sides of the strip as it leaves the molten metal bath.
Continuous hot dipping, as opposed to the batch immersion process, is more cost-effective and allows for greater control of the consistency, thickness, and surface condition of the metallic coating.
It is a similar process to that for continuous paint coating, shown in The Paintline Process, with priming, coating, and ovens replaced by the molten metal tank and blow-off section.
The atmospheric corrosion performance of a hot-dipped zinc coating is closely proportional to its thickness.
The thickness of coatings in micrometres (µm) can be measured with a non-destructive magnetic induction meter or similar device which can then be converted into grams per square metre (g/m²).
There is confusion about the method of describing the coating thickness of coil-coated sheet and strip products in g/m², compared to products that were hot-dipped after fabrication. The coating thickness of sheet and strip refers to the collective amount of coating on both sides of the sheet, effectively dividing the coating weight by half. It is invalid to equate the coating weight in g/m² of hot-dipped zinc coatings on fabricated products, such as nails and screws, with that of metallic coatings on sheet and coil; the coating thickness of the fabricated products relates to one surface only.
A micron (µm) is one-thousandth of a millimetre.
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