Bridge
Cross-sea cable-stayed bridge — steel box girder and tower
Marine atmosphere at C5-M. Cold spray zinc was applied as the base layer on the steel box girder and tower segments that could not be sent to a galvanizing bath, then sealed and finished with an aliphatic polyurethane topcoat. Application was carried out in the fabrication yard on completed segments, avoiding post-assembly touch-up over the water.
Why hot-dip galvanizing was not an option
The steel box girder segments and tower anchorages were far beyond the size any galvanizing bath accepts, and the tower joints carried a large share of site welds that only exist after erection. Once the segments were lifted over water there was no way to build a shelter, blast, or thermally spray. The coating therefore had to be completed in the fabrication yard and then survive salt spray, UV and thermal cycling for the full design period without maintenance access.
System selection
ZD96-1 at 80 µm dry film as the base: the zinc content keeps a continuous conductive path so the layer still acts as a sacrificial anode where the film is mechanically damaged. A sealer closes the porosity between zinc particles, which is what would otherwise draw moisture through the layer. ZD6430 aliphatic polyurethane at 160 µm carries the weather side — colour retention and gloss under UV being the reason for aliphatic rather than aromatic. The 240 µm total corresponds to a high-durability system for C5-M under ISO 12944.
How it was applied
Application was done in the fabrication yard on completed segments, after weld inspection and before shipment. Blasting to Sa 2.5 with Rz 40–75 µm, primer within four hours of blasting — beyond that window the surface flash-rusts and the profile has to be re-established, not painted over. Two shifts worked the segment in sequence: underside and webs first, top face after turning, so no scaffold had to be moved mid-coat. Work stopped below 5 °C steel temperature or above 85 % relative humidity.
Acceptance and follow-up
Film thickness was measured per segment on a grid, with the 80 % rule applied (no reading below 80 % of nominal) plus a minimum-value floor, and pull-off adhesion tested at representative locations. Coating damaged by lifting points and post-erection welding was repaired with material from the same batch, feathered well beyond the damaged zone so the repair edge does not become a stress riser for the next coat failure.
Products used
Other projects
Offshore wind towers — internal and external protection
Salt spray and constant humidity at the coast. Tower sections received cold spray zinc on the external surface followed by an epoxy intermediate and a polyurethane topcoat; the internal surface used the same base with a lighter build. The single-component format removed mixing errors across multiple application crews working in parallel.
Transmission line lattice towers — field repair of galvanized steel
Galvanized lattice towers in a mountainous corridor had developed white rust and local red rust at bolt heads, weld seams and cut edges. Cold spray zinc was applied by brush and roller for in-place remediation without dismantling the towers or interrupting the line, restoring sacrificial protection at the damaged points.
Not sure which grade fits your environment?
Send us the structure, the steel condition and the corrosivity category (C2 to CX). We reply with a coating system, film builds and a delivered price.