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Inspection after hot-dip galvanising

Our galvanisation plants specialise in the hot-dip galvanisation of steel in accordance with NEN-EN-ISO 1461. To check whether the requirements of this standard have been met, it is possible to conduct an inspection after hot-dip galvanising that focuses primarily on:

  • the thickness of the zinc coating; and
  • the appearance of the zinc coating.

The thickness of the zinc coating determines the duration of protection. In general: the thicker the zinc coating, the longer the expected life of the product!

Thickness of zinc coating

The thicknesses are always related to the wall thickness of the product being treated. NEN-EN-ISO 1461 defines local and mean coating thicknesses as follows:

  • local coating thickness: the mean value of the zinc coating obtained from the specified number of measurements within a reference area.
  • mean coating thickness: the average value of the local thicknesses either on one large article or on all the articles in the control sample.

Coating thickness measurements are always conducted using the only non-destructive method, the "magnetic method" as defined in NEN-EN-ISO 2178.

Table 1

This table lists the minimum local mean coating thicknesses required by NEN-EN-ISO-1461.
 

Articles
Thickness article
Local coating thickness in
micrometres (μm)
Mean coating thickness in
micrometres (μm)
Steel > 6mm 70 85
Steel > 3mm tot ≤ 6mm 55 70
Steel ≥ 1,5mm tot ≤ 3mm 45 55
Steel < 1,5mm 35 45
Cast items ≥ 6mm 70 80
Cast items < 6mm 60 70

Table 2

This table lists the minimum local mean coating thicknesses required by NEN-EN-ISO 1461 for centrifugally galvanised objects.

Article with threading Local coating thickness in
micrometres (μm)
Mean coating thickness in
micrometres (μm)
> 6mm tot ≤ 20mm 40 50
≤ 6mm middellijn 20 25
Other objects
(including
cast items)
   
> 3mm 45 55
< 3mm 35 45

Appearance of the zinc coating

The second requirement relates to the appearance of the coating. NEN-EN-ISO 1461 has the following to say in this respect:

  • At normal visual inspection, the zinc coating must be free of sharp points and drops. Thicker sections or irregularities do not constitute grounds for rejection unless they interfere with the use or application, for example on assembly surfaces.
  • There must not be any uncoated areas.
  • The presence of darker or lighter grey spots does not constitute grounds for rejection. White stains resulting from storage (see the document Wet Storage Stain) do not constitute grounds for rejection as long as the thickness of the coating exceeds the stated minimum value.
  • Flux traces are not permitted.
  • Zinc smelting ash is not permitted where it may affect the intended use of the hot-dipped galvanised objects or corrosion resistance.