Hot-dip galvanising involves immersing steel in molten zinc. During powder coating, powder is applied to the material in an electrostatic process, after which the powder-coated material enters the furnace and the powder is cured.
During the curing phase, both the zinc and the powder coating form as small an area as possible: a curved shape. This results in the zinc and the powder coating pulling away from the edge and thickening next to the edge. The surface covering the edges becomes too thin and the material is not sufficiently protected against corrosion.
To guarantee good adhesion to the surface, therefore, it is very important for sharp edges to be rounded off to a mechanically-applied radius of 2 mm. If the radius is less than 2 mm, the thickness of the layer decreases visibly.
Layer thickness and protection against corrosion
The drawings below show clearly what happens with the layer thickness in practice in different situations.
1 Flat sheets
Flat material does not have any sharp edges. Depending on the thickness and properties of the material, the thickness of the zinc layer is at least 60 mμ and it therefore meets the requirements of NEN-EN-ISO 1461. The layer thickness of the powder coating (where appropriate in two layers) will be approximately 120 mμ and it therefore meets the requirements of EN 15773.
Rounding off R < 2
When the sharp edges of materials are not rounded off (radius less than 2 mm), the layer thickness will not be adequate. This applies to both the thickness of the zinc layer and the thickness of the powder coating (where appropriate in two layers).
The maximum zinc coating thickness at the corners will be only about 30 mμ, a fraction of the standard. The same applies to the twin powder coating, where a layer thickness of only 20 mμ is achieved. The total layer thickness of approximately 50 mμ is insufficient as protection against corrosion, certainly in the long term.
Rounding off R = 2
In materials where the sharp edges are rounded off (radius equal to 2), it is possible to build up the zinc layer and the powder coating (where appropriate into layers) to an adequate thickness. The total layer thickness of approximately 120 mμ provides adequate protection against corrosion for the base material.
When working with materials with sharp edges (R < 2 mm) that are hot-dip galvanised and/or powder coated, the layer thickness will not be adequate and these materials will therefore not be adequately protected against corrosion, certainly in the long term.
The layer thickness of materials with sharp edges that have been rounded off (R = 2) will be adequate after galvanisation and/or powder coating. These materials will therefore be adequately protected against corrosion in both the short and long terms.
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