Accelerated Low Water Corrosion: Microbes and a New Ferry Dock

A large expansion and repair project is underway at the Washington State Ferry Seattle Terminal. Planning and design began in 2012, construction started in 2017, and the scheduled completion date is 2023. Key upgrades include replacement of a timber trestle with a new concrete trestle. Steel pipe piles support the concrete trestle. At completion, this project will improve safety and access by reducing bicycle, pedestrian, and vehicle conflicts. The project will provide 1,300 m² of shallow fish water habitat area and remove over 4,500 metric t of creosote-treated timber piles from Puget Sound. In 2018, a bright orange corrosion product appeared on the newly installed steel piling underneath the concrete trestle. Testing revealed the presence of sulfate-reducing bacteria as well as other corrosion-influencing microbes on the steel surface. This paper details the steps taken to identify the microbes, measure the extent of their effects on the piling, and the corrosion plan developed to mitigate the accelerated corrosion.