High-temperature coatings: how to protect hot equipment

A high-temperature coating is chosen first by the actual service temperature and whether the equipment cycles. Silicone (often aluminum-pigmented) covers bare steel exposed to high heat, inorganic zinc serves as a primer on uninsulated hot steel, and insulated steel needs a system designed for moisture and cycling, or thermal spray aluminum. The limit of each product is read in its data sheet, never in a generic catalogue.

Stacks, steam lines, headers, furnaces, dryers, heat exchangers and pressure vessels run at temperatures that destroy an ordinary paint within a few cycles: the resin chars, the film cracks and the steel rusts under a coating that still looks intact. This guide is for maintenance managers and project managers in Quebec and Ontario who have to repaint or protect hot equipment. It complements our industrial painting page and our protective coatings offering.

Why does ordinary paint fail on hot equipment?

Ordinary paint fails because its organic resin breaks down with heat and because steel expands and contracts with every cycle, faster than the film can follow. The result is the same either way: loss of adhesion, cracking, flaking, then corrosion of the exposed steel.

Three mechanisms combine on hot equipment:

• Thermal degradation. Beyond its limit, an epoxy or alkyd resin yellows, hardens, becomes brittle and then decomposes. The limit depends on the product and is stated in its data sheet.
• Thermal cycling. Equipment that heats and cools several times a day, or at every shutdown, works the steel-coating interface. A film that is too thick or too rigid cracks.
• Intermittent moisture. During shutdowns, condensation forms on cold steel. That is often when corrosion advances fastest, not in service.

A high-temperature system answers all three at once: a heat-stable resin, a thin and flexible film, and protection of the steel through cold, wet periods.

Which coatings withstand heat, and when to choose them?

You choose among four families, based on temperature range, insulation and cycling. The table below summarizes generally recognized uses; the exact range of each product is the one in its data sheet.

Inorganic zinc-rich primer is an excellent choice on hot steel exposed to air, often topcoated with silicone. It does not belong under insulation, where trapped hot water can consume the zinc quickly.

What about steel under insulation?

Under insulation, the rule is to protect the steel as if it will spend its life wet, hot and cycling. Corrosion under insulation (CUI) advances unseen, in the temperature range where water that gets into the insulation stays liquid, evaporates and condenses again.

Recognized practices, described in NACE SP0198 (AMPP) and, for system qualification, in ISO 19277, point the same way:

• A system qualified for hot immersion and cycling. Epoxy phenolic, novolac epoxy or inert copolymer depending on the range, applied at controlled thickness.
• No zinc under insulation in the range where CUI is active.
• Thermal spray aluminum as a durable option on new work or during long shutdowns. See the thermal spray aluminum entry and our thermal spray metallizing service.
• Careful detailing at supports, nozzles, flanges and low points, where water collects.

Our article on corrosion under insulation covers inspection and how to prioritize high-risk areas.

What surface preparation should be required?

Require the preparation grade written in the data sheet of the selected system, which is almost always abrasive blasting. Inorganic zinc and thermal spray aluminum need high-grade blasting (SSPC-SP 10 or SP 5 depending on the specification) with an angular profile; some silicones accept lighter preparation on sound steel, but not on mill-scaled or pitted steel.

Checkpoints specific to hot equipment:

• Residues of old coatings. An old silicone or degraded paint is not a base. It is removed to the required grade.
• Salts and contaminants. Process equipment collects soluble deposits that make the new film blister. A soluble salt test is done before application when the specification calls for it.
• Profile. Too low a profile makes inorganic zinc disbond; too high a profile leaves peaks poorly covered by a thin silicone.

Cleanliness grades are explained in our NACE 2 / SSPC-SP 10 surface preparation guide. Abrasive blasting inside a plant is planned together with containment and isolation of nearby equipment.

Can running equipment be painted, or does it have to be shut down?

Some equipment can be painted while running, with products formulated for hot-surface application, but cold application during a shutdown remains the safest route for preparation, thickness and inspection. The choice depends on the shutdown window, surface temperature during the work and risks to crews.

On hot steel, surface temperature is monitored continuously and has to stay within the data sheet range: too hot, the solvent boils and the film blisters; too low for a heat-cured product, it does not cure. Our article on in-place coating of in-service equipment explains how to work around running equipment.

How is a high-temperature system inspected?

A high-temperature system is inspected like any industrial coating, with two particularities: thickness has to stay in a narrow range, and some products only finish curing at start-up. Inspection is therefore done in two stages.

• At application. Ambient conditions, surface temperature, cleanliness and profile, then dry film thickness measured per SSPC-PA 2 or the method in the specification. A silicone that is too thick cracks; an inorganic zinc that is too thick mud-cracks as it dries.
• At heat-up. The temperature rise follows the cure procedure in the data sheet: heating too fast blisters a film still holding solvent. The date and heating profile are recorded.
• In operation. Visual inspection at shutdowns, with particular attention to areas of frequent cycling, supports and welds.

Our article on coating failure modes helps tell thermal cracking apart from an adhesion failure.

How to plan the work and get a quote?

Plan from four inputs: service temperature and its cycles, insulation, the available shutdown window and access. With those inputs, a system, a preparation grade and a heat-up sequence are selected.

Our crews and qualified subcontractors work from LaSalle, including the industrial parks of Bécancour and across Ontario, on plant equipment during shutdowns or, where possible, while running. Send photos, dimensions, service temperature, insulation type and shutdown window through the request a quote form: pricing on consultation, reply within 24 business hours.

Frequently asked questions

Does a high-temperature coating protect against corrosion?

Not always on its own. A straight silicone resists heat but does little for the steel during wet shutdowns. On steel exposed to air, it is often paired with an inorganic zinc primer; under insulation, a system designed for moisture and cycling is chosen.

Why does silicone paint stay soft after application?

Because many silicones only finish curing with heat, at the first start-up. Before that cure, the film remains fragile and marks easily. The data sheet gives the procedure to follow.

Is inorganic zinc suitable under insulation?

No, not in the temperature range where corrosion under insulation is active. Trapped hot water can consume the zinc quickly. A system qualified for insulated steel or thermal spray aluminum is chosen instead.

What is the maximum temperature a coating can take?

It depends entirely on the product. Each manufacturer publishes a continuous service range and sometimes a peak range. We select the system from the temperature measured in operation and the data sheet, never from a generic value.