Duplex coating system
A duplex system combines a metallic zinc layer with an organic topcoat. The zinc layer protects the steel cathodically, the topcoat seals the zinc surface. This extends corrosion protection far beyond the sum of both individual layers.
Physicochemical mechanism of action
The first layer is usually hot-dip galvanizing. Zinc sacrifices itself during corrosion because it is electrochemically less noble than steel. The second layer is an organic coating, for example powder coating or wet paint. This top layer blocks water and oxygen from the zinc surface. This causes the zinc layer to wear down only very slowly. At scratches or edges, bare steel is exposed. Here, the zinc continues to act cathodically and prevents rust creep under the paint layer.
Metallurgical relevance & risks when welding
If you weld a coated part, zinc evaporates in the arc. Zinc vapor causes pores and cracks in the weld. The organic top layer burns off and releases toxic gases. Carbon residues from the burned paint can harden the weld and promote cracking. Therefore, remove both layers completely in the weld area before welding. After welding, clean the weld and fully restore the duplex system.
Workshop practice & instructions
Grind off zinc and paint at least 50 mm on both sides of the weld joint. Use extraction and respiratory protection. Weld only on bare steel. After welding, deslag the weld. First apply a zinc dust primer or thermal spray galvanizing. Then apply the top coat according to the manufacturer's specifications. The dry film thickness of the top coat is typically 80–120 µm, the total system often over 120 µm.