nC2’s expertise in corrosion testing and prevention is ensuring the UK’s largest university towing tank remains corrosion free.
The Boldrewood towing tank – the largest of its kind in a UK university
The challenge: corrosion prevention in a freshwater environment
We were contacted by colleagues from the team who manage the 138-metre Boldrewood Wave and Towing Tank at the University of Southampton.
The towing tank is a large freshwater tank with submerged galvanised steel components (a lifting platform and an end beach – essentially a passive wave absorber).
The facility’s manager asked for our help after an inspection of the hydraulic rams that raise and lower the underwater platform in the tank revealed corrosion within the metal structure. While freshwater corrosion is typically slower than seawater corrosion, it still occurs due to electrochemical reactions on the steel surface.
Applying nC2’s corrosion protection expertise
Dr Spencer Court carrying out a potential check on the towing tank plaform
After examining the corrosion issue, we designed a cathodic protection (CP) system – an electrochemical technique designed to prevent metal deterioration that would work in the tank’s freshwater environment. CP works by making the steel act as a cathode in an electrochemical cell, preventing oxidation (rusting).
Adapting DNV RP-B401 design standards
The design standards for corrosion protection systems assume a seawater environment – they are typically used for offshore infrastructure such as oil rigs and pipelines – so a bespoke approach was required.
- Using DNV RP-B401 (a recommended practice for CP design), we adapted the current density for freshwater conditions. While seawater environments typically require high current densities (around 100–250 mA/m²), freshwater demands significantly lower values, often in the range of 5–10 mA/m². To ensure robust protection, we adopted a conservative figure within this range, drawing on our specialist experience and expertise to design a system suitable for this relatively unusual scenario.
- We first identified all the steel surfaces and calculated the total wetted surface area (in square meters). This is critical in the design as the protection current depends on the exposed area. Then using the current densities in RPB401 we calculated the area by the current density to give us the total protection current in amperes. From the protection current we were able to calculate the total mass of anode material needed to deliver the required current for the design life.
- We chose magnesium anodes as these are used in freshwater environments, corroding in preference to the structure itself. The anodes are bolted to the structure and can be easily replaced.
- Finally, we made a plan to distribute the anodes evenly along the tank to ensure uniform protection, considering shielding effects and easy access for inspection and replacement. We also positioned the anodes so they would not interfere with any moving parts, maintaining both functionality and safety.
Verifying effectiveness and ongoing corrosion protection performance
To verify the effectiveness of the cathodic protection system, we conducted potential measurements using a silver/silver chloride reference electrode at regular intervals around the structure before installing the anodes. Three months after installation, we repeated the measurements, which allowed sufficient time for the structure to polarise. The results confirmed that the anodes successfully polarised the steel and provided the required protection.
Subsequent inspections, carried out every three months for one year, demonstrated that the system continued to perform effectively over time.
nC2’s added value
The nC2 team brought:
- A wealth of industry experience and knowledge in the field of corrosion protection, enabling us to design a CP system tailored for an atypical environment.
- The ability to follow up and assess protection into the long-term.
The outcome: corrosion protection for a significant research facility
The CP system has been in place since April 2024 and is proving effective in preventing corrosion of the tank’s metal structures. This in turn will increase the tank’s longevity and avoid costly maintenance and down-time.
- By preventing corrosion, the CP system ensures that the tank remains fully operational, supporting uninterrupted research and testing activities.
- Extending the life of the structures significantly lowers long-term maintenance and replacement expenses.
- The system helps maintain a safe environment for staff and equipment.
We will continue to monitor the electric potential of the metal structures to ensure they stay corrosion-free for many years to come.
