Epoxy is resistant to acids is a sentence that has ruined tanks. Why concentration and temperature matter more than the generic resin family on the label.
A stainless steel tank is being replaced with carbon steel plus a lining, and the specification says epoxy, because the medium is dilute sulfuric acid. The lining is applied, the tank is filled, and eleven months later the lining is blistered across the lower third. The product was an epoxy. It was not the epoxy that had been tested in that acid at that concentration at that temperature.
Resistance is a matrix, not a property
There is no such thing as a coating that is resistant to acids. There is a coating that is resistant to a specific concentration of a specific acid at a specific temperature for a stated period.
- Concentration changes the mechanism. Dilute sulfuric acid and concentrated sulfuric acid attack a resin by different routes. A product rated for 20 percent may be unsuitable at 70 percent, and the reverse is also true for some systems.
- Temperature accelerates permeation. Diffusion through a polymer increases steeply with temperature. A rating at 20 °C does not extend to 60 °C, and a tank that is cleaned with hot caustic is a different service from the same tank holding cold product.
The families and where they actually belong
- Amine-cured epoxy. Good for alkaline environments, water, salt solutions and many dilute acids. Moderate resistance to some solvents. It is the general-purpose lining for water and neutral service, and it is frequently over-specified into acid service where a novolac would be correct.
- Novolac epoxy. Higher crosslink density and better resistance to acids and some solvents than standard epoxy. The usual choice for acidic and mixed chemical service at moderate temperature.
- Vinyl ester. The workhorse for aggressive acid and hypochlorite service, including many pulp and paper and mineral processing applications. Excellent resistance to acids and oxidizing media, with good tolerance to higher temperatures.
- Polyurethane and polyaspartic. Good where flexibility and abrasion resistance matter, less suitable for strong acids or hot caustic.
- Thermoplastics and rubber linings. Where the chemistry or the temperature is beyond what a thermoset can hold, sheet lining or flake lining takes over. That is an engineering decision, not a coating selection.
The four parameters that must be in the enquiry
A supplier cannot select a lining from the name of the chemical. The data required is the same list every time.
- Concentration, including the range. A tank that is filled with 30 percent and washed with 5 percent sees two different services.
- Temperature, including cleaning and upset conditions. The maximum, the minimum, and how long the maximum lasts.
- Co-existing substances. Solvents, chlorides, dissolved gases, and anything that might be present as a contaminant.
- Mechanical duty. Will the tank be entered, scraped, or does it hold an abrasive slurry? A lining that resists the chemistry and fails in abrasion is a failed lining.
Permeation is the failure mode to design against
A lining in immersion service does not usually fail by being dissolved. It fails because the medium passes through the film, reaches the steel, and the corrosion products lift the coating from below.
- Permeation rate is thickness-dependent. Doubling the film thickness roughly halves the permeation rate, which is why immersion linings are specified at 400 to 600 microns rather than the 150 microns used for atmospheric service.
- Pinholes are the shortcut. A 500-micron film with one pinhole behaves like a 500-micron film with a hole in it. Holiday testing at the specified voltage is not optional here.
- Blistering is evidence, not a defect. A blister tells you that something got through. The useful question is what, and at what point, and whether the adhesion under the surrounding film is still intact.
- Steel temperature matters. A cold wall in contact with a warm chemical can produce condensation at the interface, which is a permeation problem with water rather than the chemical.
How to reduce the uncertainty
- Use the manufacturer's chart, then confirm in writing. Ask specifically for the concentration, temperature and duty cycle, and keep the answer. A chart read at arm's length is a guess with a document attached.
- Run a coupon test for anything unfamiliar. A coated steel coupon immersed in the actual medium at the actual temperature, inspected at intervals, answers more than any table. It costs a few weeks and a small panel.
- Look at the cleaning regime. In many tanks the most aggressive exposure is the clean-in-place cycle, not the product. Hot caustic, hypochlorite or acid wash can be the real service condition.
- Consider the repair path. A lining system that can be repaired locally in service is worth more than one that requires a full reblast, even if the initial cost is higher.
Application rules that override the selection
- Surface preparation to Sa 2½ with a defined profile, and salt testing before the first coat. In a chemical lining, contamination at the interface becomes the initiation site for undercutting.
- Stripe coats on weld seams, nozzle necks and the shell-to-bottom junction. These are the areas where the first failure appears, and they are the areas least amenable to a full coat.
- Thickness measured on a grid, plus holiday testing at the specified voltage. Both records should be in the handover pack, because the warranty conversation two years later will be about numbers.
- Cure verified before filling. A lining that is 90 percent cured will pass a visual inspection and can fail its first immersion test.
The honest summary
For every failed chemical lining there is a sentence that starts with the product being suitable. The sentence is usually true for a concentration, a temperature and a time that is not the one in the tank. The work of selecting is not finding a resistant coating; it is narrowing the operating envelope until it matches something that has actually been tested.