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How the Sundsvall Bridge is prevented from rusting away

As a result of bacterial attack, the Sundsvall Bridge us rusting more rapidly than intended and is at risk of falling short of its planned 120-year working life. RISE has looked at possible methods for addressing the problem.

The Sundsvall Bridge opened at the end of 2014 to improve travel between the northern and southern districts of Sundsvall, bypassing the city centre and instead carrying traffic across the Bay of Sundsvall.

Rusting faster than anticipated

Only a few months after the bridge opened, the Swedish Transport Administration discovered that the speed of corrosion on the support structure above the bridge’s foundations was significantly higher than anticipated. While the conclusion was that there was no immediate danger. it was apparent that, at that rate of corrosion, the bridge would not last for the intended 120 years.

The Swedish Transport Administration therefore turned to RISE for help in analysing the problem and identifying a solution. RISE spent the whole of 2015 first taking stock of all conceivable anticorrosion methods for the bridge support and then testing the most promising alternatives.

“With the help of RISE, we identified the technically and financially optimal solution to the problem,” says Magnus Borgström, project manager for the bridge’s construction.

Cathodic protection halts corrosion

Currently, the preferred method is cathodic protection, which involves running a voltage between anodes placed in the water and on the rusting bridge supports. Electrical current is then supplied that renders the metal immune to rusting.

“This is a proven method that is mainly used in marine waters for ships and offshore facilities. In this case, the challenge has been to make the system work reliably in the brackish water of Sundsvall Bay, says RISE corrosion specialist Bertil Sandberg.