100 Years Maintenance-Free: Zinc is Key to Norwegian Bridge Protection

To maintain the 5,800 bridges that cross fjords, link remote islands, and expedite travel throughout the “Land of the Midnight Sun,” the Norwegian Public Roads Administration (NPRA) performs routine bridge inspections and uses continuous real-time monitoring through tools such as digital twin technology and movement sensors to determine what kind of repairs are required. 

For many of those bridges, there’s no maintenance needed. 

Since 1965, the use of thermal spray zinc (TSZ) duplex coatings has shown to be a highly successful method for protecting steel highway bridges in Norway. Despite harsh environmental conditions, coating performance indicators show that many of these bridges may realize their full 100-year design life without maintenance.

The NPRA first tested TSZ duplex coatings on the Djupfjord Bridge, a small suspension bridge in the Lofoten archipelago, in 1958. In 1965, inspired by Scotland’s Forth Road Bridge, the NPRA mandated that all new steel highway bridges be protected with TSZ duplex coatings, as no other protection has proven close in terms of durability.

Since it was first patented in Germany and Switzerland by Max Schoop in 1909, the use of TSZ duplex coatings has shown to be highly effective, offering a coating life of 35 years before first maintenance, which is often only a renewal of the paint topcoat — no blast cleaning to base metal, only power washing of the paint layer. By taking preventative action to repair only the paint, the TSZ base layer will not corrode. 

TSZ can be applied by either flame spraying or electric arc spraying, following a surface prep process of grit blasting in order to provide adequate mechanical adhesion. This process is also known as metallization or metallizing. 

The flame spray technique uses an acetylene-oxygen flame to melt a zinc wire or powder that is continuously fed into this flame. A stream of compressed air then propels the molten metal droplets onto the steel surface. For the arc spraying technique, two zinc wires are brought together at an angle, and an imposed voltage difference between them creates an electric arc. A gas flow then atomizes and propels the molten metal droplets toward the steel substrate.

Paint on top of metallic zinc-coated steel is called a “duplex” coating. The combination of the zinc coating plus a paint topcoat system creates a synergistic service life extension that exceeds the sum of its parts. The paint topcoat provides extra barrier protection for the zinc, but the main reason for the synergy is that the metallic zinc coating protects the paint from underfilm corrosion creep at scratches or holes. 

The most important factor in determining coating life is the quality of the coating. Application errors like spitting, pinholes, and low paint film thickness have been found to result in coating failures.

There are a few disadvantages to using a thermal spray zinc (TSZ) duplex coating system. High initial setup costs for thermal spray applications may deter some asset owners. Arc spraying can create hazardous working conditions and require additional personal protective equipment (PPE). Additionally, zinc sprays can be harder to control than other zinc-based coating systems, requiring specialty operator knowledge. 

Sources:

Gagné, Martin, and Ole Øystein Knudsen. “Duplex Zinc Coatings for Steel Bridges.” Materials Performance Magazine, Oct. 2021. 

Duran, Bernardo. “Strategies to Inspect Thermal Spray Zinc Metalizing.” Materials Performance Magazine, March 2022