The outside of a tank can be repainted. The inside, practically speaking, cannot. To reach the internal surface the vessel must be emptied, cleaned, gas-freed and ventilated, and the whole unit taken out of service for the duration. That is why an internal lining is not specified to last “as long as possible”, but to cover the entire maintenance interval in one go. Below: what destroys steel from the inside, how it is prevented, and how such work is accepted.

Inside is a different problem, not “the same paint”

An external coating works in the atmosphere: moisture appears and dries off, and sunlight adds ultraviolet. An internal lining lives in permanent contact with the medium — that is, in immersion. ISO 12944 treats these as separate categories: Im1 (fresh water), Im2 (sea and brackish water), Im3 (soil). Materials that perform perfectly in atmospheric category C4 may not survive a single season in immersion.

There are three differences, and each changes the requirements:

No respite. The medium acts continuously; the coating never gets to dry out and recover.
No local repair. A defect on an external wall is touched up within the hour. A defect inside waits for the next full shutdown.
The cost of an error exceeds the error itself. Corrosion products enter the stored product. For fuel that means particulate contamination — a rejected batch, not merely lost steel.

Three zones of one tank — three different duties

The mistake we see most often: the tank is treated as a single surface and coated identically throughout. In reality three zones stand out clearly inside, and they degrade differently.

Bottom and lower shell courses

This is where bottom water collects — heavier than the petroleum product, it settles and carries salts and particulates with it. This is the most aggressive zone in the tank. Bottom corrosion progresses as pitting rather than uniformly, and it is the usual cause of through-wall failures.

Shell within the product

Permanent immersion, but a more uniform and generally less aggressive medium than bottom water. The key requirements are chemical resistance to the product itself and no leaching of coating components into it.

Vapour space and roof

The zone that gets underestimated. With daily temperature swings — and ours are wide — condensation forms on the roof and upper courses. It runs down the shell, becoming saturated with product vapours and their oxidation products. The result is a moist, acidic film on steel that formally “never contacts the liquid”. In tanks holding sour products the upper courses often degrade faster than the lower ones.

The practical conclusion: an internal protection specification is written zone by zone. Coating the whole volume with a single system is acceptable, but it must be a deliberate decision, not the result of never having considered the zones.

How the material is matched to the medium

There is no universal internal lining — the choice follows from what is actually stored:

Crude oil, petroleum products, diesel, gasoline — two-component epoxy coatings. The main working class for fuel storage.
Acids, alkalis, solvents — epoxy novolac and vinyl ester systems; at high concentration and temperature, sheet lining or chemically resistant tile.
Potable water — only coatings holding potable water contact approval. Here technical suitability without the certificate means nothing.
Food media — likewise: food contact approval is required.

★ Suitability for a medium must be verified from the documentation for the specific product, not from the type of binder. “Epoxy” is not a resistance rating; the spread within that class is enormous.

Surface preparation: the requirements are stricter inside

For atmospheric exposure, cleanliness grade Sa 2½ to ISO 8501-1 is usually sufficient. For immersion, Sa 2½ is the minimum and Sa 3 is often required — that is, blast cleaning to visually clean steel.

Three further points are critical inside a tank that are forgiven outside:

Surface profile. Roughness after blast cleaning must fall within the range specified by the coating manufacturer. Too smooth means poor adhesion; too coarse means the profile peaks are not covered to the required thickness and become failure points.
Welds and edges. Weld spatter, undercuts and sharp edges are dressed mechanically before coating. Coating on a sharp edge is always thinner, and the first breakdown will start there.
Salts on the surface. Chlorides invisible to the eye remain on the steel after cleaning and cause osmotic blistering beneath the coating. They are measured by a separate soluble salt test — often neglected outdoors, mandatory inside a tank.

A separate article in this series covers surface preparation — the subject deserves it, because that is where most of the service life is lost.

Thickness and continuity: two numbers that decide everything

Internal linings are accepted on two measurable parameters.

Dry film thickness — measured with a gauge over a grid of points, not “in a couple of places”. A 20 % shortfall does not mean losing 20 % of the service life: the relationship is non-linear, and in immersion a shortfall can cost several times more.

Continuity — verified by holiday detection: an electrode under voltage is passed over the coating and a spark jumps at any pore or missed area. For internal tank linings this is a mandatory procedure. One undetected pore in immersion is the point from which underfilm corrosion starts, and within a year the delaminated area will be out of all proportion to the size of that pore.

If a contract for internal tank coating does not specify holiday detection, there is nothing on which to accept the work.

What to read in the product data sheet

When selecting an internal lining, look at the technical data sheet, not the sales leaflet. Six lines matter — and here is what each means in practice.

Intended use and list of media. The first thing to check: is your specific product listed? “For petroleum products” and “for aviation fuel” are not the same thing; the latter is more demanding.
Volume solids. Shows what proportion of the applied wet film remains once the solvent has evaporated. The higher it is, the less the film shrinks.
Dry film thickness per coat. A key parameter for confined spaces — see below.
Theoretical spreading rate. Always theoretical: actual consumption is higher by the losses, which depend on application method and vessel geometry. Budgeting from the data sheet figure with no allowance is a mistake.
Pot life. How long the crew has after mixing. Inside a tank, where movement and material transfer take longer, that margin is consumed faster than on an open site.
Overcoating intervals and application conditions. Temperature range, permissible humidity, intervals between coats.

Why volume solids matter more than they appear to. A material with high volume solids reaches the specified thickness in a single pass. With low volume solids the same thickness must be built up in several coats — and every additional coat in a confined space means another overcoating interval and another ventilation cycle. On a tank that is not “slightly longer”, it is an extra day of shutdown.

★ And above all: the material must hold documentation for your specific duty. For potable water, a potable contact approval; for food media, food contact approval; for aviation fuel, confirmation of suitability from the manufacturer. Technical properties without that document do not close the requirement, however suitable the figures look.

Work inside a vessel is confined space work

The technical side is half the job. The other half is organisation:

gas-freeing and confirmation that no explosive vapour concentration remains before work starts;
forced supply and extract ventilation throughout application and curing, not only while spraying;
explosion-protected lighting and equipment;
a standby attendant outside the manway for the whole shift.

Overcoating intervals stretch in a confined volume: the solvent must escape, otherwise the next coat traps it in the film and the lining never develops its stated properties. An internal coating programme is always longer than an external one for the same area — that is normal, and it must be allowed for in the budget.

How to accept the work

Cleanliness grade — before coating starts, with a record. After the first coat there is nothing left to inspect.
Surface profile and soluble salt test — at the same stage.
Application conditions: steel, air and dew point temperatures. The steel must be at least 3 °C above the dew point — otherwise there is condensation on it that cannot be seen.
Dry film thickness over a grid of points, not selectively.
Holiday detection — over the entire surface.
Full cure before filling. Putting product into an under-cured lining is a reliable way to lose it entirely.

In brief

Internal tank protection differs from external protection not in the material but in the absence of any right to error: access to the surface comes round once every few years. Three rules follow. The specification is written zone by zone, not for “the tank” as a whole. The material is selected from documentation for the specific medium, not from the type of binder. The work is accepted on two numbers — thickness and continuity — and both are measured with instruments, not by eye.

The next article in this series covers surface preparation and abrasive blast cleaning: where corners are most often cut, and what that leads to.

Internal corrosion protection of tanks: choosing a lining for an aggressive medium