Zinc Coating
During the galvanising process, the zinc coating forms a metallurgical bond with the underlying steel.The molten zinc reacts with the surface of the steel to produce layers of zinc-iron alloys. The adherence of the coating is very high, typically between 20 – 30 M.P.A. The steel must be perfectly clean prior to dipping to avoid bare spots.
The inner layer, comprising a 75:25 zinc-iron alloy, is extremely hard and resistant to damage. The middle layer is a 90:10 zinc-iron alloy and the surface layer is pure unalloyed zinc and quite ductile. If the coating is physically damaged the galvanising will continue to provide cathodic protection to the exposed steel. Zinc corrodes very slowly but preferentially to the iron or steel, preventing the spread of corrosion from the exposed area. This is in contrast to other coatings that experience under-creep corrosion where there is a gap or pinhole in the coating.
The galvanised coating provides a uniform thickness of zinc on flat surfaces, and penetrates recesses to provide complete protection to potential corrosion spots. With painting, the coating thickness on the edges is less than on flat surfaces. This is in contrast with hot-dip galvanising, which provids full corner edge protection. These corners normally have 50% more zinc than the flat surfaces.
The thickness of the hot-dip galvanised coating is relative to the thickness of the steel profile that is to be galvanised. The zinc coating on a 3mm profile will be approximately 85-100 µm; thereafter the coating gets heavier according to the thickness of the steel profile, providing an extra-long lifetime. See table below:
|
Steel Profile Thickness |
Average Zinc Coating |
|
1.6mm – 2.0mm |
50 – 75 um |
|
2.5mm – 3.5 mm |
75 – 100um |
|
4.0mm – 6mm |
100 – 150um |
|
7mm – 9mm |
150 – 200um |
|
10mm – 12mm |
200 – 250um |
|
12mm – 20mm |
250 – 300um |
Cape Galvanising Consolidated carries out work in accordance with the requirements of SABS 7631988 (ISO 1461certification supplied on request. Customers undertaking work to this specification should be fully conversant with its contents, especially welding techniques and design for galvanising.
When steel that contains reactive levels of silicon and/or phosphorous is galvanised, the result is a dark, grey and heavy coating. Excessively thick coatings can be brittle. Customers should always order steel suitable for galvanising, preferably with a maximum silicon and phosphorous content of 0.3%.
