Heat-resistant coatings and corrosion under insulation (CUI)

Where heat-resistant coatings are needed

Steam and hot-product pipelines, heat exchangers, columns, furnaces, chimneys and hot equipment in power generation, oil refining and chemicals operate at temperatures that ordinary epoxies and polyurethanes cannot withstand: according to manufacturers, their continuous limit is generally 100–150 °C. Besides the temperature itself, the coating must withstand heating and cooling cycles and thermal shock, and on insulated surfaces, constant moisture under the insulation.

Types of material by temperature

Type	Temperature guide (from manufacturers’ technical data sheets)	Features
Epoxy phenolics and novolac epoxies	up to 200–230 °C	high-build, resistant to water and chemicals; also used under insulation
Zinc silicate primers	about 400 °C continuous, up to 540 °C short-term or under a topcoat	the basis of systems for hot surfaces; carbon steel only
Silicone acrylics	up to 600 °C	coloured topcoats for identification marking and appearance
Silicones, including aluminium-pigmented grades	up to 540–600 °C depending on the product	thin film; some products only develop their full properties after heating
Inert multipolymer matrices	from cryogenic temperatures to 649–650 °C continuous, higher short-term	protection against corrosion under insulation and cracking of stainless steel; can be applied to hot surfaces

The figures in the table are a guide, not a norm: the permissible temperature is always taken from the technical data sheet of the specific product, separately for continuous service, peaks and cyclic operation.

Corrosion under insulation

Under thermal insulation, steel often corrodes faster than when it is exposed. Moisture enters through damaged cladding and joints, is retained in the insulation and, when heated, creates a hot, humid environment at the metal surface. Cyclic operation, shutdowns and areas where the wall temperature allows water to stay for a long time without evaporating are particularly dangerous. The defect is hidden under the cladding and is found late.

Protection is provided by coatings tested specifically for these conditions: epoxy phenolics and novolacs at moderate temperatures, and inert multipolymer matrices over a wide range from sub-zero temperatures to 650 °C, including cyclic service. The industry reference is the NACE SP0198 standard, now maintained by AMPP; the technical data sheets of a number of materials refer to it. Austenitic stainless steel under insulation faces an additional risk — external chloride stress corrosion cracking; for it, manufacturers offer separate zinc-free systems.

How to choose a heat-resistant coating

Operating temperature and regime. Continuous, peak or cyclic; whether there are cool-downs below zero and shutdowns.
Whether there is insulation. Under insulation, use only materials designed for corrosion under insulation.
Wall material. Carbon or stainless steel; zinc-containing primers are not applied to stainless steel.
Lower operating limit. Some silicones develop their full properties only after heating in service; if the surface does not reach the required temperature, choose another material.
Whether the equipment can be shut down. If not, you need materials that can be applied to hot surfaces.
Colour requirements. For pipeline identification, use coloured heat-resistant topcoats; many pigments change shade when heated.

Application and commissioning

Silicone heat-resistant coatings are applied in thin coats — 20–30 µm per coat for the materials in this guide — and the thickness is strictly controlled: excessive thickness risks cracking and flaking on heating. Most products require abrasive blast cleaning. The first heat-up of a system with a topcoat is carried out gradually, following the manufacturer’s schedule, so that the solvent can escape from the film; otherwise blistering may occur. Only materials for which this is explicitly permitted are applied to hot surfaces, within the stated temperatures and with special safety measures.

Common mistakes

“Heat-resistant paint” without a technical data sheet and a proven temperature rating.
Ordinary heat-resistant paint under insulation instead of a system against corrosion under insulation.
Zinc silicate or another zinc-containing primer on stainless steel.
Excessive thickness of silicone coatings.
Rapid first heat-up of a freshly applied system.
A coating designed for high temperatures on a surface that only stays warm in service: the film does not develop its properties.
A topcoat that is not compatible with the heat-resistant primer.

Materials from leading manufacturers

Intertherm 50 — International (AkzoNobel) — moisture-curing silicone for protecting steel up to 540 °C.
Intertherm 751CSA — International (AkzoNobel) — inorganic copolymer with aluminium against corrosion under insulation, for cycling from −196 to 650 °C.
Intertherm 228HS — International (AkzoNobel) — epoxy phenolic coating against corrosion under insulation, up to 230 °C.
Solvalitt — Jotun — silicone acrylic up to 600 °C in 14 heat-stable colours.
Heat-Flex Hi-Temp 1200 — Sherwin-Williams — multipolymer matrix against corrosion under insulation, up to 649 °C.
Thermaline 4700 — Carboline — silicone topcoat for surfaces at 204–540 °C.
PPG HI-TEMP 1027 — PPG — multipolymer matrix with ceramic, continuous service up to 650 °C, can be applied to hot steel.
THERMOTECT 600 — Kansai Paint — aluminium-pigmented silicone up to 600 °C that needs no heat curing.
THERMOTECT EP — Kansai Paint — phenolic epoxy coating against corrosion under insulation, continuous service up to 200 °C.
Jotatemp 210 — Jotun — phenolic novolac epoxy coating for hot pipework, including under insulation.
Jotatemp 1000 — Jotun — heat-resistant coating on an inert multi-polymer matrix for high-temperature equipment.
PPG SIGMATHERM 540 — PPG — single-pack silicone aluminium coating for dry heat.
TEKNOHEAT 500 — Teknos — silicone paint for hot surfaces: +200…+400 °C depending on the colour, aluminium RAL 9006 — up to +650 °C.
TEKNOHEAT 500-100 CS — Teknos — single-pack aluminium silicone paint for up to +650 °C that needs no pre-heating.
TEKNOLINE EP 305 — Teknos — epoxy phenolic primer for insulated (CUI) and uninsulated pipework and vessels up to 200 °C; cures from 0 °C.
THERMOTECT 300 — Kansai Paint — coloured modified-silicone heat-resistant paint up to 300 °C, lead and chromium free.
Heat-Flex 750 — Sherwin-Williams — MIO-reinforced epoxy for insulated pipes and vessels, −196 to 204 °C service.
PPG SIGMATHERM 350 — PPG — one-pack heat-resistant silicone acrylic finish for steel up to 350 °C.
PPG SIGMATHERM 230 — PPG — epoxy phenol novolac for insulated steel up to 230 °C, also for cryogenic service.
Intertherm 875 — International (AkzoNobel) — one-pack coloured silicone acrylic finish up to 260 °C over zinc silicate primers.
Heat-Flex ACE — Sherwin-Williams — epoxy novolac coating against corrosion under insulation, from −196 to +232 °C.
Heat-Flex M505 — Sherwin-Williams — one-pack matt silicone heat-resistant finish, aluminium grade up to 600 °C.
PPG SIGMATHERM 500 — PPG — one-pack heat-resistant modified alkyd aluminium paint up to 500 °C.
PPG SIGMATHERM 175 — PPG — high-gloss heat-resistant modified alkyd aluminium finish up to 175 °C.
Intertherm 3350 — International (AkzoNobel) — waterborne acrylic thin-film insulation coating for hot surfaces up to 177 °C.
Jotatemp 250 — Jotun — glass flake epoxy composite for corrosion under insulation, −196 °C to 250 °C.
PPG HI-TEMP 500 — PPG — silicone acrylic heat-resistant topcoat to 260 °C, 426 °C intermittent, hot-surface application.
Epo-Phen FF — Sherwin-Williams — flake-filled epoxy phenolic novolac for CUI service to 230 °C, API 652 lining.
TEKNOHEAT 650 — Teknos — silicone aluminium paint for hot indoor surfaces, heat resistant to +650 °C.
THERMOTECT WR — Kansai Paint — inorganic copolymer coating against CUI, 540 °C continuous, 650 °C peak.
Solvalitt Midtherm — Jotun — one-pack silicone acrylic heat-resistant coating to 260 °C for insulated and bare surfaces.
Jotatemp 1000 HT — Jotun — ceramic copolymer CUI coating for stainless steel, P91 and ceramics, up to 1000 °C.
Heat-Flex 7000 — Sherwin-Williams — waterborne acrylic aerogel insulation coating for personnel protection, −62 °C to +177 °C.

Zinc silicate primers are also suitable for hot surfaces — Resist 86 (Jotun), PPG SIGMAZINC 158 (PPG), ZINCTECT 1500 (Kansai Paint); for moderate temperatures and service under insulation, Phenoline 1205 (Carboline), recommended for a number of NACE SP0198 systems, and the Carbomastic 15 (Carboline) mastic for surfaces up to 150 °C. This list is not exhaustive; the final choice is made on the basis of the manufacturer’s documents and the project requirements.

Frequently asked questions

Can a hot pipeline be painted without a shutdown?

Yes, some materials can be applied to a heated surface, within the limits stated in the technical data sheet. This requires a special procedure and safety measures.

Why did the aluminium heat-resistant paint crack after start-up?

Most often because of excessive thickness, a poorly prepared surface or too rapid a first heat-up.

Is a primer needed under silicone paint?

It depends on the system: some materials are applied directly to steel in several coats, others over a zinc silicate primer. For high temperatures and an aggressive atmosphere, a zinc silicate primer usually extends the service life.

How should stainless steel under insulation be protected?

With special zinc-free systems that the manufacturer specifies for stainless steel; their purpose is to prevent chloride stress corrosion cracking.

We will select a heat-resistant coating or a system against corrosion under insulation — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.