Vasabron paves the way for next-generation corrosion protection

Vasabron has connected Tegelbacken and Riddarholmen since 1878, serving pedestrians, cyclists, cars and buses. Today, cracks have been identified in the bridge’s load-bearing structure, meaning it cannot be preserved in its current form. As a temporary measure, a steel support structure was installed beneath the existing bridge – designed to function for five to six years while a new bridge is developed. The structure consists of seven beams, each approximately 20–25 metres long.

This temporary solution created an unexpected opportunity: to use the beams as a testbed for evaluating alternative corrosion protection systems under realistic conditions, within a time-pressured infrastructure project in an urban environment.

For Frank Axhag at the City of Stockholm Traffic Office, this presented an opportunity to test environmentally improved coating systems at scale, with limited risk. As a representative of the City of Stockholm in RISE’s industrial network MRC Corrosion Protection, he had followed developments in plastic-free systems for several years and now saw a chance to evaluate them in practice.

“We saw that the seven beams in the temporary structure could be used to test different coating systems and compare their performance under similar conditions. This is important knowledge – we manage several large steel bridges in Stockholm where corrosion protection will need to be renewed within the next decade. Working with an independent research institute like RISE gives us a reliable basis for decision-making,” says Frank Axhag.

From research to real-world application

RISE has been working with plastic-free corrosion protection systems for several years. The work began with literature studies showing that a significant share of microplastics in marine environments can be linked to coatings used on ships, offshore structures and infrastructure. Many conventional systems are based on epoxy and polyurethane – thermosetting plastics that can contribute to microplastic emissions as coatings degrade over time.

“We didn’t just want to describe the problem, but also explore which technical alternatives already exist,” says Björn Tidbeck, Senior Researcher at RISE.

The work has included laboratory testing and accelerated corrosion testing on smaller panels, combined with long-term exposure in real environments. Tests have been carried out in Nordic climates as well as in Panama, where heat and high humidity create highly corrosive conditions.

Among the most promising alternatives are zinc silicate systems – a type of coating that does not contain plastic and protects steel through different mechanisms than conventional systems. As part of the research, RISE has also developed new zinc silicate systems together with industry partners, combining practical applicability with environmental benefits.

When the research moved beyond laboratory conditions, a full-scale test environment was needed. Vasabron provided that opportunity. Applying corrosion protection to 25-metre steel beams in an industrial setting involves entirely different practical conditions compared to coating small panels in controlled lab environments.

RISE was responsible for the technical coating specification, quality control during application and final inspection, in collaboration with contractors and the City of Stockholm.

Seven systems – clear differences

The test included both plastic-free alternatives and established reference systems, ranging from water-based zinc silicates and acrylics to alkyd, epoxy and polyurethane systems. To enable a fair comparison, all systems were documented systematically during surface preparation, application and curing. The results highlight clear differences – both technically and practically.

The time from blasting to finished beam ranged from one day to more than ten days, directly affecting capacity and costs in large-scale projects. Solvent emissions varied significantly, from around 37 kilograms per beam at one end of the spectrum to less than 100 grams at the other. Four of the systems were completely plastic-free and therefore associated with a lower risk of microplastic emissions from the binder.

Painters were interviewed throughout the project, and both challenges and effective working methods were documented.

“This was the first time several of these systems were tested in a full-scale industrial environment. We were interested in the practical lessons – what actually happens when you leave the laboratory,” says Björn Tidbeck.

Documented results and insights

The project shows that plastic-free corrosion protection systems can work in full-scale applications – but also that the choice of system has clear implications for implementation, environmental impact and long-term performance.

“Some systems are more sensitive to apply, and that needs to be acknowledged. In practice, it comes down to balancing ease of application with long-term performance and environmental impact. For example, a solvent-based zinc ethyl silicate combined with a silicate topcoat can be applied three times faster, results in one tenth of the solvent emissions and requires 30 percent less labour compared to today’s bridge coating systems. Accelerated corrosion testing also indicates improved performance,” explains Björn Tidbeck.

Several of the systems tested are not covered by the international coating standard ISO 12944-5, despite similar technologies having been used internationally for many years. This means that established guidelines for their use are lacking, making the results from Vasabron particularly relevant for Nordic infrastructure.

Knowledge that extends beyond the project

The results are not only relevant to this specific project – they have attracted interest from both public and industrial stakeholders in Sweden and internationally, as the findings can be directly applied to future material selection and specification.

Through the industrial network MRC Corrosion Protection, members gain access to documentation, test protocols and insights from the full-scale test as part of a broader knowledge-sharing environment.

“We now have a five- to ten-year period during which we can monitor and evaluate the coating systems in terms of corrosion protection, environmental impact, durability and cost – and then select the solutions that best meet our requirements,” says Frank Axhag.

The seven beams will be monitored throughout the lifetime of the temporary structure. RISE plans regular inspections to document how the systems perform in Nordic climate and urban environments. Once the structure is dismantled, parts of the beams will be moved to exposure sites for continued long-term monitoring, enabling data collection over several decades.

A decision basis for future infrastructure

The project demonstrates how full-scale testing, carried out with limited risk, can reduce uncertainty and build practical knowledge for future material decisions in critical infrastructure.

“This testing has the potential to influence how we specify future coating systems, where environmental and sustainability targets will play an increasingly important role,” concludes Frank Axhag.

By testing, documenting and evaluating systems in real environments, sustainability becomes an operational aspect of planning, implementation and long-term asset management.

Vasabron is a clear example of this in practice. When the bridge was built in the late 19th century, it represented the engineering of its time. Over more than a century, it has been rebuilt, reinforced and adapted to new demands. Today, as it reaches the end of its technical lifespan, it takes on a new role – as a testbed for future materials and more sustainable infrastructure management.