Passivation & Phosphatisation
Chromatization / Chemical passivation
The non-noble zinc surface covers itself in contact with atmospheric oxygen with a thin passive layer, which then slows down the further oxidation. Unlike other metals such as e.g. Chrome or aluminum does not protect this self generated passive layer from zinc as it would be technically desirable.
Chromating, which are applied after the actual galvanizing process, produce an oxidic barrier layer, which significantly hinder the corrosive attack on the substrate (Zn layer / white rust formation) and thus considerably increase the corrosion protection of the galvanized steel. This is also referred to as passivation, because the base metal zinc is hindered by this protective layer to further oxidation.
It must be distinguished between different chromating types, which have different passivating power and thus lead to differences in the corrosion resistance of the galvanized steel.
Today only chromium (VI) -free passivations are in use.
Another form of protecting the zinc layer is phosphating, that means that the application of a zinc phosphate layer to the galvanized surface. The corrosion resistance of phosphated surfaces corresponds to the chromated zinc layers, especially if additional oil or wax layers are applied.
Phosphate layers are mainly used:
- As corrosion protection
- As a primer for organic layers (for example lacquers)
- To improve the friction characteristics and the formability
Comparison of the methods and their applications
|Improvement of friction characteristics||1||3|
|1 = suitable 2 = conditionally suitable 3 = not suitable|
Electrolytically galvanized strip or sheet is a composite material made of cold-rolled quality steel and a zinc coating, which is applied by an electrolytic process and firmly adheres to the base steel material.
The zinc coating protects the steel sheet against corrosion and together with the usual surface treatments (chromating or phosphating), is an excellent primer for a subsequent coating.
Edge protection through "all-round galvanizing".
Corrosion-related component edges are always more problematic than smooth surfaces of a construction. Part edges can attack corrosive media more intensively. They are always at greater risk from mechanical influences than other areas.
With HBV, even narrow strip dimensions can be galvanized.
- No uncoated cut edges
- For physical reasons, the zinc coating layer thickness is significantly higher in the edge area than on the flat side (+ 2-3 μm), thus further improving corrosion protection.