Hot-dip galvanised steel for purely technical surfaces
For hot-dip galvanizing steel as the basic material is dipped into a zinc bath (about 450°C ≈ 842°F). After draining a more or less thin zinc layer remains as a coating. This layer forms an active as well as a passive corrosion protection: The thin layer protects the basic metal against humidity (passive) and also works as a sacrificial anode (active). The steel underneath the zinc coating is protected from corrosion until the zinc is completely consumed. In the fresh condition the zinc layer is shining brightly. As time goes by a patina develops as a result of this corrosion which makes it appear darker and matter.
Hot-dip galvanising is a cost-effective way to achieve a durable but purely technical surface because the main purpose of hot-dip galvanising is purely to protect against corrosion. Requirements for the aesthetics of the surfaces are not defined. In order to avoid complaints, we therefore strongly recommend that metal objects that have to meet visual requirements are additionally or alternatively finished with a colour-coated surface.
Layer thickness according to ISO standard
All our hot-dip galvanized metal garden objects are treated according to the standards of the DIN EN ISO 1461. This guarantees a minimum coating thickness of 55 μm for material thicknesses between 3 mm and 6 mm and even a coating thickness of 70 μm for sheet thicknesses greater than 6 mm. With these thicknesses – depending on the environmental conditions – a service life of more than 50 years can be achieved through the passive and active protective effect of the zinc coating without maintenance.
Problem: White rust on hot-dip galvanised surfaces
White rust is the term for streaky corrosion phenomena on the hot-dip galvanised surface. The white rust has no effect on the weather resistance, but makes the garden object look bad. In order to reduce the development of white rust some companies in the galvanizing industry offer additional protective layers. These are relatively expensive and above all useless if the freshly zinc-galvanized objects are treated and stored appropriately. That is why an additional protective coating is no standard and has to be agreed on and ordered separately.
Formation of white rust
White rust can form if the freshly galvanised objects are stored on the construction site under unfavourable weather conditions or temporary air exclusion and at the same time come into contact with water, especially with condensation water with a low mineral content. Also, lower than dew point and low temperatures promote the formation of white rust. The classic case is: the objects stand in the rain under a protective cover in spring or autumn, waiting to be installed. Then the ugly white rust can form after just a few hours!
Removal of white rust
The white rust can be removed under certain circumstances. The rule is: the quicker the reaction after the formation, the easier the removal. White rust can be removed mechanically by using hard nylon brushes. The treated surfaces have to be rinsed with lots of water. Wire brushes are not suitable, as they leave scratches in the soft zinc coating.
Colour coating for hot-dip galvanised objects?
Hot-dip galvanised objects can also be coated with paint. To ensure that the paint adheres well, an adhesion improver (primer) is usually applied beforehand. However, it is difficult to check whether the primer has actually been applied to the entire hot-dip galvanised surface after the paint has been applied and this often only becomes apparent after years due to rust spots. We therefore recommend that garden objects which are to be colour-coated are made of the corrosion-resistant base material stainless steel (V2A). This may not be the cheapest option but it is certainly the most durable.
Environmental aspects speak in favour of hot-dip galvanising
Corrosion protection by the way of hot-dip galvanizing saves recources and is sustainable. In direct comparison with other procedures a big amount of CO2 is saved.
Tender text for GaLaBau, for landscape architects and planners
“Rolled flat steel with corrosion protection by hot-dip galvanizing (piece galvanizing) in accordance with DIN EN ISO 1461: 2009-10. For load-bearing, hot-dip galvanized metal and steel components in the area regulated by building authorities, DASt Guideline 022 “Hot-dip galvanizing of load-bearing steel components” shall also be applied.”
The thickness of the zinc coating does not have to be specified separately, as this is regulated in DIN EN ISO 1461.
For the best possible protection against white rust, we recommend the additional sealing of the hot-dip galvanised surface directly ex works during hot-dip galvanising. This is a separate service outside of DIN EN ISO 1461 and must therefore be arranged additionally. Text suggestion: “With additional protective coating for further protection against white rust after the hot-dip galvanizing process.”
for basic material