Retouching and reconditioning
Non-galvanised spots and/or damage to hot-dip galvanised steel
During hot-dip galvanising, steel products are submerged in a zinc bath containing liquid zinc at a temperature of 450°C. The zinc that covers the material during this process bonds with the steel, creating an unbreakable layer.
Sometimes the zinc fails to bond locally, resulting in spots where the material is not galvanised. This may be a result of contamination (paint, grease, oils etc.) that could not be removed in the pickling plant. It is also possible for hot-dip galvanised steel to be damaged during storage, transportation, assembly or mechanical processing.
Galvanisation takes place in accordance with NEN-EN-ISO 1461. Non-galvanised spots that are retouched by the galvanisation plant may not exceed a total of 0.5% of the total surface area of a component. Any non-galvanised spot requiring retouching must not exceed 10 cm². If the non-galvanised area is larger, the component in question must be galvanised again unless the client and the galvanisation plant have agreed otherwise.
The galvanisation standard lists three methods for repairs:
- zinc-rich paint;
- zinc spraying;
- zinc alloy bars with a low melting point.
These repair methods can also be used for minor damage. In practice, zinc-rich paints are the most widely used approach. The amount of zinc in the dry matter of these paints should be more than 90% by weight. The coatings applied to the retouched sections must be at least 100 µ thick.
- Remove loose zinc flakes.
- Remove dirt and corrosion products by sanding, filing and/or brushing and then degrease. There should be a smooth transition with the intact zinc coating that is approximately 10 mm wide and the adjoining, intact zinc coating should be cleaned and degreased in the same way.
- Apply at least two coats of zinc compound using a long-haired brush.
- The zinc compound may be covered with a zinc, a zinc/aluminium or an aluminium spray depending on the desired colour. The only reason for doing this is to create an appearance that matches the surrounding galvanised coating as much as possible.
The galvanisation standard states that when there is an organic finishing coat on top of the zinc coating (duplex system), the coater should ensure that the system selected for the finishing coat matches the retouching method used. To prevent irregularities in the organic finishing coat, the minimum thickness of the repair should be the same as the surrounding zinc coating.