Duplex systems – galvanizing plus paint

What a duplex system is

A duplex system combines hot-dip galvanizing with an organic coating – liquid paint or powder coat – on top. The two layers protect each other: the paint shields the zinc from the atmosphere and slows its consumption, while the zinc stops rust creeping under the paint at scratches and edges, so the paint does not blister and flake from the steel. The result is a longer life to first maintenance than either the galvanizing or the paint system would give alone, plus a full colour choice. The benefit is well established in ISO 12944, ISO 14713 and AS/NZS 2312 guidance; the actual life depends on the environment, the zinc thickness and the paint system.

Duplex is therefore the usual answer where steel is exposed, highly visible and hard to maintain. For the comparison with other systems see the corrosion protection guide.

Typical uses

Typical products are steel canopies, architectural balustrades, ball-stanchion handrails and bollards. On PEB projects duplex is often used for canopies and fascias exposed to weather at the building edge.

Preparing the zinc surface

Fresh galvanizing is smooth and may carry passivation residues, oils or zinc salts, so paint will not adhere reliably without preparation. World Steel coordinates the sequence so that the galvanized steel is prepared and painted under control:

• Inspection of the galvanized surface; removal of zinc runs, spikes and ash so they do not show through the paint.
• Degreasing and washing to remove oil and zinc salts (white rust).
• Sweep blasting with a fine, non-metallic abrasive at low pressure to roughen the zinc lightly without removing significant coating thickness, or a chemical pre-treatment, per ISO 12944-4, ASTM D6386 or the paint manufacturer's instructions.
• Application of a primer or tie coat approved for zinc – typically an epoxy – within the time the data sheet allows, followed by the intermediate and topcoat.
• For powder coating, preparation and degassing per EN 15773 (Europe) or ASTM D7803 (US) to avoid pinholes from gas released by the zinc during curing.

Alkyd paint must never be applied directly to zinc – the binder saponifies and loses adhesion. See alkyd systems.

Fabrication, inspection and site care

Steel for a duplex system is detailed for galvanizing first – vent and drain holes, sealed or gapped overlaps, and smooth weld profiles – because every defect in the zinc shows through the topcoat. After galvanizing and painting, World Steel records the zinc thickness from the galvanizer, the surface preparation, the paint DFT to ISO 19840 or SSPC-PA 2 and adhesion results where specified. Finished members are wrapped or padded for export, since repairs to duplex systems on site require both zinc-rich paint and the topcoat colour. A touch-up kit procedure is supplied with the package; background on shop painting is in the PEB article on paint and coating systems.

Selecting and specifying

• State the galvanizing standard (ASTM A123, AS/NZS 4680 or EN ISO 1461) and the paint or powder system with colour.
• Specify the method of preparing the zinc surface or leave it to the coating manufacturer's recommendation.
• Define appearance requirements for visible architectural surfaces before fabrication starts.
• Agree handling, packing and touch-up responsibilities for site damage.