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

BRIDGES · THERMAL SPRAY ZINC · DUPLEX COATINGS · METALLIZING · LIFECYCLE PROTECTION

Norway’s bridges face salt, moisture, wind, cold temperatures, remote locations, and difficult access—yet many have required remarkably little coating maintenance over decades of service.

One reason is the country’s long-standing use of thermal spray zinc duplex coating systems on steel highway bridges. Since the 1960s, the Norwegian Public Roads Administration has relied on metallized zinc combined with paint topcoats to protect steel structures in some of Europe’s most demanding environments.

Norway’s Bridges Create a Demanding Corrosion Environment

Norway’s road network includes thousands of bridges crossing fjords, waterways, valleys, and coastal regions.

Many of those structures are exposed to marine salts, persistent moisture, wind, temperature cycling, and remote access conditions that can make conventional bridge maintenance expensive and disruptive.

For an infrastructure owner, that makes coating durability especially valuable. The longer the protective system can remain effective between major interventions, the less often crews have to mobilize access equipment, containment, blasting equipment, and coating operations over roads or waterways.

Norway Began Testing Thermal Spray Zinc in the 1950s

The Norwegian Public Roads Administration first tested thermal spray zinc on the Djupfjord Bridge, a small suspension bridge in the Lofoten archipelago, in 1958.

By 1965, the agency had adopted thermal spray zinc duplex systems for new steel highway bridges.

The approach proved well suited to the country’s demanding bridge environments, and decades of field performance provided the agency with a growing body of evidence about how the systems aged.

What Is Thermal Spray Zinc?

Thermal spray zinc is a metallic coating applied by melting zinc and propelling the molten or semi-molten particles onto a prepared steel surface.

The deposited particles build into a continuous metallic zinc layer that bonds mechanically to the roughened steel substrate.

The process is commonly referred to as metallizing or metallization.

Flame Spray vs. Arc Spray

Thermal spray zinc can be applied using different processes. Two common methods are flame spraying and electric arc spraying.

Why Zinc Protects Steel

Zinc provides steel with more than simple barrier protection.

Because zinc is more active than steel in the galvanic series, it can provide sacrificial protection to exposed steel at small defects where the metallic layer remains electrically connected to the substrate.

That electrochemical behavior is one of the reasons metallic zinc coatings can perform differently from barrier-only coating systems when scratches, holidays, or localized damage occur.

What Makes a Duplex Coating Different?

A thermal spray zinc layer can be used by itself, but applying paint over the metallic zinc creates what is known as a duplex coating system.

The paint provides an additional barrier that slows environmental exposure of the zinc.

At the same time, the underlying metallic zinc can help protect steel at small coating defects and reduce the tendency for corrosion to creep beneath damaged paint.

The result can be a synergistic service life that is longer than would be expected from simply adding the independent life of the zinc coating and paint system.

Maintenance Can Focus on the Paint Before the Zinc Is Consumed

One of the most important lifecycle advantages described in the Norwegian experience is the ability to maintain the paint layer before significant deterioration reaches the thermal spray zinc.

The historical source reported that first maintenance could occur after decades of service and, in favorable cases, might involve renewal of the paint system rather than abrasive blasting back to bare steel.

If the topcoat is renewed while the zinc remains in suitable condition, the owner may preserve the metallized layer and avoid repeating the most intensive parts of the original coating operation.

Could a Bridge Reach a 100-Year Design Life Without Major Coating Rehabilitation?

The long-term performance documented by the Norwegian Public Roads Administration led researchers to suggest that some bridges protected with properly applied thermal spray zinc duplex systems could potentially reach their full design life with relatively limited coating intervention.

That does not mean a bridge requires no inspection or maintenance. Structural condition, coating defects, drainage, joints, fasteners, damage, local exposure, and many other bridge components still require ongoing evaluation.

What it demonstrates is that a well-designed and properly applied corrosion-protection system can significantly reduce one major source of lifecycle maintenance demand.

Application Quality Determines Whether the System Reaches Its Potential

Long theoretical service life does not guarantee long field service life.

The historical research emphasized that coating quality was one of the most important factors affecting actual durability.

Defects such as pinholes, spitting, inadequate film build, poor surface preparation, or inconsistent thermal spray application can create localized weak points that reduce the performance of the overall system.

Inspection Is Essential for Thermal Spray Zinc

Thermal spray application requires close control because the finished metallic layer can vary based on surface preparation, spray distance, equipment settings, wire feed, operator technique, geometry, and access.

Inspection helps verify that the metallized layer and subsequent paint system meet the requirements intended to produce long-term performance.

This is especially important on bridge projects, where discovering widespread application deficiencies years later can result in expensive and difficult repairs.

Thermal Spray Zinc Has Tradeoffs

The durability benefits do not mean thermal spray zinc is the simplest option for every project.

Thermal spray work requires specialized equipment, trained operators, appropriately prepared surfaces, and careful control of application quality.

Mobilization and setup costs can also be higher than for some conventional coating systems, particularly on smaller projects.

The thermal spray process also introduces task-specific occupational hazards that must be evaluated and controlled through the applicable safety program, equipment requirements, ventilation, respiratory protection, PPE, and work procedures.

The Bigger Lesson: Evaluate Coatings Over the Asset Life

Norway’s experience illustrates why protective coating decisions should not be based only on the lowest initial installed cost.

A system that requires less frequent major maintenance can reduce future blasting, containment, access, traffic disruption, labor, waste generation, and environmental exposure.

That can make a more complex initial coating system economically attractive over the full service life of a bridge.

Sources & Additional Reading

• Materials Performance: Duplex Zinc Coatings for Steel Bridges — Martin Gagné and Ole Øystein Knudsen
• Materials Performance: Strategies to Inspect Thermal Spray Zinc Metalizing — Bernardo Duran