How the two layers work together
Zinc protects steel in two ways: as a barrier, and as a sacrificial metal that corrodes in preference to steel at scratches and cut edges. Left bare, zinc slowly forms corrosion products and is consumed, faster in aggressive atmospheres. A paint layer slows that consumption by isolating the zinc. In return, the zinc limits undercutting at defects in the paint, because rust cannot spread from the steel underneath. This mutual protection is why duplex systems are chosen for long service in severe corrosivity categories, for appearance and colour coding, and for galvanized steel that has started to weather.
Typical configurations
| Metallic layer | Applied by | Topcoat options | Reference |
|---|---|---|---|
| Hot-dip galvanizing | Immersion in molten zinc, per ASTM A123 or ISO 1461 | Liquid paint system or powder coating | ISO 12944-5 duplex systems, ASTM D6386, ASTM D7803 |
| Thermal spray zinc or zinc-aluminum | Metallizing | Sealer, then paint system if required | SSPC-CS 23.00/AWS C2.23M/NACE No. 12, ISO 2063 |
Preparing the zinc surface
Most duplex failures are adhesion failures between paint and zinc. Preparation therefore matters as much as the paint itself:
- remove oil, grease and zinc corrosion products ("white rust") by appropriate cleaning;
- roughen the surface by a light sweep blast per SSPC-SP 16 or by another method allowed by ASTM D6386;
- for powder coating, follow ASTM D7803, which also addresses outgassing of galvanized steel;
- apply the first coat within the time allowed after preparation.
Common pitfalls
- Wrong primer. Some alkyd and oil-based paints react with zinc and lose adhesion. The primer must be approved by its manufacturer for galvanized surfaces.
- Over-blasting. Aggressive blasting removes the zinc layer that the system relies on; see brush-off blast.
- Contamination from the galvanizing process. Quench residues or passivation treatments can prevent adhesion if not identified and removed.
- Damaged areas. Repairs to the zinc layer follow ASTM A780 before painting.
Where galvanized steel has weathered for some time, the zinc is assessed first: remaining thickness, white rust and contamination decide the preparation and whether a duplex system is still the right choice.
Duplex systems are part of our protective coatings service, including thermal spray zinc through our metallizing service.