A storage tank bottom corrodes on both sides: on the product side, from settled water, sediment and bacteria that collect at low points; on the soil side, from moisture, salts and oxygen under the plate. Internal lining protects the top, cathodic protection and a sound foundation protect the underside, and API 653 inspection measures the remaining steel to decide between repair, lining and replacement.
The bottom is the part of a tank that is seen least and leaks most often. A perforation there goes unnoticed until product reaches the soil, with clean-up costs that far exceed maintenance costs. This guide helps maintenance managers and project managers understand the mechanisms and prepare an outage. It builds on our tank lining page and our reference page on protective coatings.
Why does a tank bottom corrode faster than the shell?
Because it combines two exposures the shell does not have: water and deposits that gather on the floor on the product side, and contact with the soil underneath. It is also hard to inspect without emptying and cleaning the tank, so corrosion there often progresses unmeasured.
| Side | Main causes | Corrosion form | Protection |
|---|---|---|---|
| Product (top) | Settled water, sediment, sludge, bacteria, acidic products | Localized pitting at low points, around sumps and under deposits | Internal bottom lining, regular water draw-off |
| Soil (underside) | Moisture, salts, oxygen, poorly drained foundation, uneven contact | General or patchy corrosion, worse at the periphery | Cathodic protection, drained foundation, sealed chime |
| Periphery (annular ring) | Rainwater entering between plate and foundation | Corrosion of the annular ring and shell toe | Sealing of the plate-to-foundation joint, maintenance |
What happens on the product side?
On the product side, water, heavier than most hydrocarbons and stored products, settles to the bottom with sediment; under those deposits, the steel forms local cells and deep pits. Sulfate-reducing bacteria often make it worse in crude oil, fuel and water tanks.
- Pitting. It perforates the plate well before average thickness drops; see our article on pitting corrosion.
- Microbial corrosion. It develops under sludge and in stagnant water; see our article on microbial corrosion.
- Sensitive points. Sumps, welds, column supports and areas under inlet pipes where product strikes the floor.
Operating practice matters as much as the lining. Drawing off settled water regularly, keeping sumps clear and avoiding long periods with a water layer on the floor all slow product-side corrosion, whether the bottom is lined or not.
What happens on the soil side?
On the soil side, the plate rests on a sand, gravel or concrete foundation; wherever moisture and oxygen reach the steel unevenly, corrosion forms under the floor, invisible from inside. A settled, poorly drained or salt-contaminated foundation speeds up the attack, especially near the periphery.
- Cathodic protection. By sacrificial anodes or impressed current, it protects the underside of the plate; API RP 651 is the reference for aboveground tanks. See our article on cathodic protection.
- Double bottom or liner. When a floor is replaced, a liner or second bottom makes leak detection and soil protection easier.
- Plate-to-foundation joint. A maintained seal stops rainwater from getting under the annular ring.
How does API 653 inspection guide the decision?
Inspection under API 653 measures the remaining bottom thickness and pit depth on both sides, then compares them with the standard's acceptance criteria to decide on return to service, repair or replacement and to set the interval to the next internal inspection. The criteria and calculations belong to the tank inspector and the standard; they do not reduce to a simple rule.
Usual inspection methods:
- Magnetic flux leakage floor scanning. It locates metal loss on the soil side and product side over large areas.
- Ultrasonic measurement. It confirms thickness and the depth of the defects found.
- Weld examination. Vacuum box testing of bottom welds, visual inspection of sumps and supports.
- Foundation review. Settlement, drainage and condition of the perimeter seal.
Inspection also tells you whether the existing lining, if there is one, can be kept. A lining that is still adherent and continuous may only need local repair; a lining with widespread blistering or disbondment is removed, because corrosion may be spreading underneath.
These results set the scope of the tank and water tower painting work and whether repairs come before lining.
For buyers comparing quotes, ask each bidder to state which inspection data their scope relies on. A lining quote prepared without the latest floor scan assumes a condition nobody has measured, and the real repair quantities will only appear once the tank is open and clean.
Which internal lining suits the bottom?
For the bottom, choose a thick internal lining resistant to the product and to settled water: immersion-grade epoxy or novolac epoxy, sometimes glass-fibre reinforced where the plate is pitted or an extra barrier is wanted. API RP 652 is the reference for linings of aboveground petroleum storage tank bottoms.
- Thin-film lining. Several coats of immersion epoxy over blast-cleaned steel; suited to a floor that is still sound.
- Reinforced lining. Laminated or glass-flake filled, applied at high build; it bridges repaired pits and adds margin on a thinned floor, without replacing a structural repair.
- Preparation. High-grade blasting, soluble salt control and a suitable profile; see our NACE 2 / SSPC-SP 10 guide.
- Continuity. Checked by holiday test over the whole surface.
To compare lining families by stored product, see our guide on industrial tank lining selection.
How is a corroded bottom repaired?
Repair depends on the extent of damage found: isolated pits are filled or covered with welded patch plates, thinned areas receive lap plates or partial replacement, and a badly degraded floor is replaced. Cold-bonded composite repairs suit some cases, within the framework set by API 653.
| Condition found | Usual repair |
|---|---|
| Isolated pits, sound plate around them | Pit filling, then internal lining |
| Localized thinned areas | Welded lap plates, then lining |
| Widespread soil-side corrosion | Floor replacement, often with cathodic protection or a liner |
| Leak or area where welding is difficult | Composite repair within the standard's framework |
Our article on API 653 composite tank repair explains when that option applies, and our guide to tank lining shutdown planning shows how to sequence cleaning, inspection, repair and lining.
Whatever the repair, the final record should include the inspection report, the repair drawings, the lining test results and the date of return to service. That file is what the next API 653 inspection will start from, and what supports any warranty claim on the lining.
Frequently asked questions
Can a tank bottom be lined without inspection?
No. Without thickness and pit depth measurements, the lining may cover plate that is already too thin. Inspection comes before lining.
Does the internal lining also protect the underside?
No. It only protects the product side. The underside depends on the foundation and cathodic protection.
Is cathodic protection enough without a lining?
On the soil side, it is often the main protection. On the product side, settled water and deposits make an internal lining necessary in most services.
Does a reinforced lining replace a plate repair?
No. It adds a barrier and bridges repaired pits, but mechanical integrity and tightness depend on the repairs set out in API 653.
How do you get a quote?
A quote starts from the inspection report, the stored product and the outage time available. Our tank and water tower painting service covers cleaning, preparation, internal bottom lining and coordination of repairs; our crews and qualified subcontractors work from LaSalle, including in Bécancour and the Côte-Nord. Send the inspection report, photos and dimensions through the request a quote form: pricing on consultation, reply within 24 business hours.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.