Above‑ground steel storage tanks (ASTs) play a critical role in storing oil, chemicals, water, and industrial liquids at terminals, refineries and utility facilities. Over time, however, the steel base plates of these tanks are exposed to aggressive corrosion conditions, especially where they interface with soil, sand cushions or concrete foundations.
At Corrpro Europe, we help asset owners mitigate this risk by designing and implementing cathodic protection (CP) systems tailored to the specific tank configuration and environmental conditions. This article outlines the key design considerations for impressed current and galvanic systems, with a focus on base plate protection, monitoring techniques, and long-term system performance.
Why Tank Base Plate Corrosion Requires Focus
Tank base plates are exposed to corrosion risks on both external (soil-side) and internal (product-side) surfaces. Externally, prolonged contact with soil and moisture, combined with low oxygen levels and poor drainage, creates conditions favourable for electrochemical corrosion. Factors such as high soil resistivity, compromised coating adhesion, or synthetic liners can further accelerate deterioration.
Internally, base plates may encounter corrosive water phases in crude oil or other stored liquids, while water tanks and other liquid-containing vessels frequently require cathodic protection at the tank bottom and the shell. Without proper protection, corrosion can result in leaks, operational interruptions, environmental incidents, and costly repairs. Cathodic protection, implemented via impressed current or sacrificial anode systems, effectively mitigates both internal and external corrosion, preserving structural integrity and extending service life.
CP System Options: Impressed Current vs Galvanic
Two primary cathodic protection systems are suitable for tank base plates: galvanic (sacrificial anodes) and impressed current cathodic protection (ICCP).
Galvanic (Sacrificial) Anodes
Galvanic cathodic protection systems are commonly used for the internal corrosion protection of aboveground storage tanks. They use sacrificial anodes made of active metals, typically zinc, aluminium or magnesium, which corrode to protect the steel tank structure. The potential difference between the anodes and the tank steel generates a protective current that prevents corrosion without the need for an external power supply.
For internal applications, anodes are typically installed along the tank shell and bottom. They can be suspended from the roof or mounted on brackets welded or bolted to the tank shell, while bottom anodes are often provided with a suitable stand-off distance to ensure electrical isolation from the tank structure. This ensures even distribution of protective current across the steel surfaces.
The output current of galvanic anodes is limited by the anode-steel potential and the resistivity of the tank contents. For large tanks or environments with high-resistivity liquids, galvanic systems may not be suitable for providing sufficient current to fully protect the entire shell or bottom, potentially leaving localised areas under-protected.
Impressed Current Cathodic Protection (ICCP)
For aboveground tanks, ICCP systems provide reliable corrosion protection for both internal and external surfaces.
External Tank Bottom Protection:
For external protection of the tank bottom, anodes are arranged in a grid pattern beneath the tank. This grid system ensures uniform current distribution across the entire base, accounting for variations in soil resistivity. MMO Ribbon anodes and titanium conductor bars are positioned at calculated spacing and connected to the rectifier system. This arrangement allows the ICCP system to deliver consistent current even under challenging soil conditions or for large-diameter tanks.
Internal Tank Protection:
Inside water tanks, ICCP anodes are typically installed in strings suspended from the roof nozzles. These anode strings provide coverage along the tank shell and across the bottom, ensuring uniform current flow to all internal steel surfaces. The anodes are suspended circumferentially at calculated intervals to provide consistent protection along the tank walls and the bottom.
Corrpro Europe’s engineering team designs each ICCP system to meet site-specific requirements, optimising anode placement, current output, and rectifier settings to ensure complete corrosion protection and full compliance with international standards such as ISO 15589 and API 651.
Key Design Considerations for Storage Tank CP Systems
1. Current Demand Calculation
The required protection current is estimated based on tank surface area, coating condition, and exposure environment
The selected current density figures are multiplied by the area of the tank bottom to calculate total current demand, which determines the number and output of anodes or the size of the ICCP power supply.
2. Resistivity
The soil and water resistivity significantly impacts the CP system design and performance. High-resistivity environment may limit galvanic anode current output and influence anode selection and placement.
3. Anode Location and Spacing
For uniform protection, ICCP external anodes are typically installed in grid configurations beneath the tank bottom or distributed along the tank perimeter for existing tanks. For internal ICCP systems, anodes are suspended close to the shell or positioned over the tank bottom. For sacrificial internal systems, anodes are evenly spaced and attached to the shell and bottom using welded or bolted connections.
4. Coating Compatibility
The presence and condition of any coating on the tank bottom affects the required current density and system efficiency. Coated tanks require less current, but coating defects still must be accounted for.
5. Foundation Design
Tank pad design, whether a sand cushion, concrete ring wall or geotextile base, affects CP strategy. Corrpro Europe engineers evaluate these configurations to optimise anode placement and electrical continuity. For tanks with double bottoms or replacement bases, designs are adjusted to ensure protection of the active bottom plate.
Monitoring and Testing Methods
Cathodic protection effectiveness must be validated through regular testing and potential measurements. Corrpro Europe incorporates reference electrodes and test stations into each system design to support long-term monitoring.
Reference Electrodes
For external systems, silver/silver chloride or copper/copper sulphate reference electrodes are installed near the tank base to monitor tank-to-soil potential. These electrodes provide real-time data to verify that the steel is receiving adequate protection. For internal systems, zinc or silver/silver chloride reference cells are used to monitor the internal tank steel potential.
Instant-Off Testing and IR Drop Correction
In ICCP systems, instant-off potential measurements are used to eliminate voltage drops caused by current flow (IR drop), ensuring accurate assessment of protection levels. These readings are obtained by synchronously interrupting the transformer-rectifier output.
Long-Term Monitoring
Corrpro Europe offers remote monitoring systems to track CP system performance continuously. We also provide scheduled maintenance, system balancing, and performance audits to maintain protection across the asset’s lifecycle.
Corrpro Europe’s Full-Service Approach
Corrpro Europe supports clients through every phase of cathodic protection deployment for above-ground storage tanks:
- Initial Survey and Soil Testing
- Detailed Engineering Design
- Supply of CP Materials and Anodes
- Transformer Rectifiers and Junction Boxes
- Installation Supervision and Site Support
- System Commissioning and Handover
- Ongoing Monitoring and Maintenance
With our UK-based manufacturing and decades of experience, we deliver proven, compliant and cost-effective protection strategies for tank farms, oil terminals, water utilities and industrial facilities across Europe and beyond.
Long-Term Protection for Storage Tanks with Tailored CP Design
Protecting the base plates of above-ground storage tanks is critical for maintaining structural integrity, meeting regulatory expectations, and avoiding costly unplanned maintenance. Corrpro Europe designs site-specific cathodic protection systems, using either ICCP or galvanic cathodic protection, to mitigate corrosion risks in even the most challenging soil, product or foundation conditions.
Our full lifecycle support, from engineering surveys and materials supply through to commissioning and remote monitoring, ensures long-term asset protection. Backed by over 50 years of industry expertise and UK-based manufacturing, Corrpro Europe delivers proven solutions for oil terminals, utilities, and industrial tank operators across the globe.
Explore our Engineering Services and Cathodic Protection Materials pages to find out how we can support your tank corrosion protection strategy.
