Retrofit Galvanic Anodes on a Coated Buried Tank Bottom
A 24 m diameter flat-bottomed carbon steel storage tank sits on a sand pad with a perimeter seal. Soil resistivity averages 1,800 ohm-cm, but a 60 m band along the north side reads 6,000 ohm-cm and is seasonally dry. The tank bottom has a 350 um epoxy coating with 4% coating breakdown assumed by the owner's standard. A close-interval survey with temporary copper/copper sulfate reference electrodes reads -0.62 V (CSE) at the center and -0.78 V at the edge. The owner wants a galvanic retrofit using magnesium ribbon anodes in horizontal trenches around the perimeter, no drilling under the tank. Design current density for bare steel is 1.0 mA/ft2 and the target is -0.85 V CSE at the tank bottom.
1. Is a perimeter-only galvanic retrofit likely to achieve protection at the tank center? Show the reasoning with the attenuation and resistance factors.
Read the worked answer
No, not with perimeter anodes alone at these resistances. Two effects compound:
Current demand. Tank area = pi*(12 m)^2 = 452 m2 = 4,870 ft2. At 4% breakdown and 1.0 mA/ft2 on bare area, demand is about 4,8700.041.0 = 195 mA, plus coating and dielectric losses. That is within galvanic capability, so current supply is not the binding constraint.
Attenuation to center. Current from a perimeter anode ring must travel under the bottom to the center. The center currently reads -0.62 V while the edge reads -0.78 V: a 160 mV drop across 12 m of radius. That gradient tells you the soil path from the perimeter is already resistive even before adding anodes. Adding anodes at the edge raises edge potential first and the center lags; the effective attenuation is a function of soil resistivity, coating quality, and the geometry of the current path under the plate.
Anode resistance. In the 6,000 ohm-cm north band, a single 3 m magnesium ribbon segment has high anode-to-electrolyte resistance; the north side will deliver far less current than the south side, worsening asymmetry.
Conclusion: the center will not reach -0.85 V from perimeter anodes in 1,800-6,000 ohm-cm soil. Recommend distributed anodes (vertical or angled under the tank) or an impressed current system, and treat the high-resistivity north band as a separate design problem.
2. What would you tell the owner about the 6,000 ohm-cm north band, and what design change addresses it?
Read the worked answer
The north band is a design driver, not a detail. At 6,000 ohm-cm, magnesium anode resistance roughly triples versus the 1,800 ohm-cm average, so the north side will underperform and the tank will be protected unevenly. Options:
- Extend anodes deeper or further out on the north side to reach moister, lower-resistivity soil, and increase anode count there to compensate for higher resistance.
- Use a low-resistivity backfill (e.g., bentonite or gypsum-bentonite) around north-side anodes to lower the local anode-to-earth resistance.
- Switch to impressed current if the resistivity cannot be reduced economically; a rectifier with a deep or remote anode bed can push current to the center and the north band uniformly.
Document the asymmetry in the design basis; a uniform perimeter ring sized to the average resistivity is a common and expensive mistake.
3. The owner asks whether the -0.78 V edge reading means the tank is already protected. How do you respond?
Read the worked answer
No. -0.78 V CSE is below the -0.85 V CSE criterion for carbon steel in this environment, and the center at -0.62 V is well short. The edge reading reflects proximity to whatever anodes or stray current exist at the perimeter, not protection of the bottom. The center reading is the controlling one for a flat-bottom tank because it is the most remote point from perimeter current sources. A retrofit must be judged on the center potential, and the design should include permanent reference electrodes or a close-interval survey plan at the center and along the north band after installation.