Operators spend a lot of time on tank shells and roofs, because that is what inspectors can see and touch. But the part that most often ends a tank's service life is the one nobody looks at: the bottom plate, sitting on the foundation, quietly corroding from the soil side up.
A question we get again and again sounds something like this: "The shell looks great and the roof is fine — so why did we find a wet spot in the containment area?" Nine times out of ten, the answer is under-tank corrosion. This note walks through why AST bottoms corrode and the practical options used to protect them.
Why AST Bottoms Corrode
An AST bottom faces corrosion from two directions at once — the product side above and the soil side below — and each has its own drivers.
Soil-side (external) corrosion
- Trapped moisture: rainwater and condensation migrate under the tank and cannot dry out.
- Differential aeration cells: uneven oxygen access across the plate creates active corrosion sites.
- Poor pad drainage: foundations that hold water accelerate attack, especially at the edge.
- Aggressive soils & stray current: low-resistivity soils, chlorides, and stray DC current all increase corrosion rate.
Product-side (internal) corrosion
- Water bottoms: free water and sediment settle at the low point and sit on the plate.
- Microbiologically influenced corrosion (MIC): bacteria under deposits cause localized pitting.
- Sediment traps: solids shield the surface and set up under-deposit corrosion.
The Cost of an Unprotected Tank Bottom
Bottom corrosion is dangerous precisely because it is hidden. By the time a leak reaches secondary containment, the damage is usually advanced. Consequences include:
- Loss of containment and potential environmental release.
- Emergency out-of-service, cleaning, and gas-freeing costs.
- Extensive bottom-plate replacement instead of a planned repair.
- Regulatory and API 653 out-of-service implications.
Methods to Protect the Bottom
1. Cathodic Protection (CP)
CP is the primary defense for the soil side. Sacrificial (galvanic) anodes suit smaller tanks or lower current demand, while impressed-current CP (ICCP) suits large tanks or high-resistivity soils. CP is most effective when the foundation allows current to reach the whole plate — which is why CP and foundation design go together.
2. Protective Coatings & Linings
Internal linings protect the product side, particularly where water bottoms collect. External coatings can protect the soil side where CP alone is not enough. Whatever the system, surface preparation drives performance — most coating failures trace back to poor prep, not the coating.
3. Release Prevention Barriers (RPB) & Secondary Containment
An RPB — such as an HDPE liner or a secondary steel bottom — sits under the tank to contain any release and channel it to a leak-detection point. It does not stop corrosion, but it buys detection time and limits consequence.
4. Foundation & Drainage Design
Much bottom corrosion is really a drainage problem. A well-graded pad, an oiled-sand or asphalt cushion, and a slope that sheds water keep the plate dry and make CP work better. Fixing drainage is often the cheapest long-term win.
Comparing the Options
| Method | Protects | Typical Use | Notes |
|---|---|---|---|
| Cathodic Protection | Soil side | Most in-service ASTs | Needs periodic potential surveys |
| Internal Lining | Product side | Water-bottom / corrosive service | Surface prep is critical |
| External Coating | Soil side | New builds / re-bottoms | Often paired with CP |
| RPB / Secondary Bottom | Containment | Leak detection & consequence control | Not a corrosion barrier by itself |
Inspection & Standards
Bottom protection is only as good as the inspection program behind it. Key references and activities:
- API 653 — in-service inspection, repair, and reconstruction of ASTs.
- API RP 651 — cathodic protection of AST bottoms.
- API RP 652 — lining of AST bottoms.
- Bottom scanning (MFL/UT) during internal inspection to map metal loss.
- Settlement surveys to catch foundation problems before they distort the plate.
A Practical Best-Practice Checklist
- Confirm and maintain foundation drainage; keep water away from the bottom.
- Install and survey cathodic protection on the soil side.
- Line the product side where water bottoms or corrosive service exist.
- Include an RPB with leak detection on new or re-bottomed tanks.
- Inspect the chime and bottom on the API 653 interval — do not stretch it.
- Trend bottom-scan data so you repair on a plan, not on a leak.
The Bottom Line
Tank-bottom corrosion is preventable, but only if it is treated as a system: drainage, cathodic protection, coatings or linings, and a disciplined inspection interval working together. The cheapest bottom repair is the one you plan years ahead — not the one you discover in the containment dike.