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Combined coatings – duplex systems

Combined (duplex) coatings are combination of zinc coatings with coating from coating material or plastic.

Material protection consists in connection of barrier and cathode principle of protection. Corrosion resistance is up to 2,5 multiple of resistance sum of single coatings.. . Spreading of organic paint systems on the zinc coating is demanding, both for logistic and technological reasons. It is necessary to be aware of the fact that zinc coating surface builds mostly flat crystal areas (Eta-phases). This can be well evident, if there were other alloying elements, e.g. tin, bismuth and others in a molten zinc bath. After removal of a zinc-coated object from the bath cooling it gradually, these elements (because they are insoluble in solid zinc phase) are driven out on the boarders of these crystals and so they make them „visible“ – they build so called zinc flowers. However, these crystal areas are very „smooth” and do not build a suitable base for following adhesion of other organic coatings. Zinc itself is on the surface of material very tightly imbedded (ca. up to 28MPa) by means of transient diffusion phases Gama1 and 2; however, following organic layer can be imbedded only mechanically. Generally, they are considered to be suitable, if they have value during separation at least 3 respectively 4 MPa. It is evident and logical that a suitable anchor profile has to be ensured in the appropriate manner. This can be done e.g. chemically – e.g. by pickling, or mechanically by slight abrasive blasting (so called sweeping). In order to create coating with good mechanical properties other operations are necessary. Then it is possible to create coatings with values at tearing off up to ca. 14-18 MPa (adhesion fracture), or even ones that have cohesive fracture at tearing off (overcoming of material strength). You can see a result of such wrong choice of preliminary treatment (respectively its absence) in combination with unsuitably chosen paint system very well on illustrative photographs. A special chapter are so called steel-zinc coatings, or coatings spread on already passivated surface of a zinc-coated layer. .

Consequentiality is also important in zinc preliminary treatment. We offer you a complex delivery of combined coatings from our long-standing and proven partners. We guarantee perfect technological processing.
edited by n.e.s.p.i.