Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox PS
- Max. temp.
- 650 °C
- Density
- 120–150 kg/m³
- Thermal conductivity (λ)
- 0.037–0.038 W/m·K (50 °C)
Industrial and high temperature
What is corrosion under insulation (CUI)?
In short
Corrosion under insulation (CUI) is rusting or cracking of a metal pipe or vessel beneath its insulation, caused by water held between the insulation and the metal. It cannot be seen from outside until the insulation is removed or a leak appears. Prevention has to work as a system: a coating on the metal, insulation that absorbs little water and dries quickly, jacketing that really sheds water, and a plan for inspection and maintenance. No single material solves it alone.
How does CUI happen?
Carbon steel rusts when it is in contact with water and oxygen. An insulated pipe usually has a small gap between insulation and metal; once water gets in, it stays there. The insulation slows evaporation, so the metal stays wet for longer than a bare pipe would. The longer the surface is wet, the greater the damage.
The US National Insulation Association (NIA) sums up the part insulation plays in three points: it creates a space that can hold water, it may absorb or carry water to the metal, and some materials may contribute contaminants that speed up corrosion.
Where does the water come from?
The UK Health and Safety Executive (HSE) and NIA list very similar routes:
- Cladding or jacketing that is poorly designed or installed, with joints that do not shed water
- Joint sealant that has aged
- Jacketing that is dented, torn or walked on
- Places where insulation was removed for maintenance and not put back properly, such as valve boxes
- Penetrations through the insulation: pipe supports, nozzles, instrument connections
- Moisture from the air condensing on cold surfaces, or systems that cycle between temperatures
Water that gets in tends to collect at low points and discontinuities: vessel bases, pipe supports, the underside of horizontal runs and elbows, insulation support rings and drain legs. These are the places to inspect first.
ROCKWOOL's own literature states plainly that water ingress cannot be fully avoided and occurs under every type of insulation. Design therefore has to cover both keeping water out and letting water that does get in drain and dry quickly.
Which temperatures are of concern?
The temperature bands quoted in different documents do not match exactly, so use only the values in the standard your project refers to. Examples from documents we opened:
- Carbon steel — an NIA article reports that, from equipment inspections, the greatest risk lies at roughly −4 to 150 °C (25–300 °F)
- ROCKWOOL (WR-Tech) factsheet — states that CUI typically occurs below 175 °C and in cyclic plant operation; its diagram marks 50–175 °C as the high-risk band
- Austenitic stainless steel — the NIA article cites NACE RP0198 as saying external chloride cracking generally occurs at about 50–150 °C
What all the documents agree on: systems whose temperature cycles, or which are shut down from time to time, are at higher risk than systems that stay steadily hot, because the metal is wetted again and again. A pipe that is always very hot drives water off, but it still passes through the risky band at every start-up and shutdown.
Chlorides and stainless steel
Austenitic stainless steel (grades such as 304/316) does not rust like ordinary steel, but it is vulnerable to external chloride stress corrosion cracking. Water seeping in brings chlorides with it; as the water evaporates on the hot surface the chlorides concentrate until they can crack metal that is under stress.
Chlorides come from two places: the insulation itself, and outside — rain, sea air, wash water or fire-water. So:
- Choose insulation documented to ASTM C795, the specification for insulation in contact with austenitic stainless steel
- Low-chloride insulation deals only with the insulation's own contribution. Water from outside still has to be kept out, and coatings considered as the designer specifies
How is the risk reduced?
API RP 583 and NACE SP0198 treat this as a system with several layers of defence, not a material choice alone.
1. A coating on the metal
The NIA article describes protective coatings as the basic prevention method, because they are the last barrier between water and metal. The coating type must suit the pipe material and service temperature — a task for the designer and the coating manufacturer.
2. Insulation that absorbs little water and dries quickly
For hot work, stone wool made with a water-repellent binder is worth considering:
- ROCKWOOL ProRox PS 960 SA / PS 970 SA — pre-formed stone-wool pipe sections made with the water-repellent WR-Tech binder. The manufacturer's data sheets give water absorption of no more than 0.2 kg/m² (EN 13472), also after 24 hours at 250 °C, and leachable chloride of no more than 10 ppm with ASTM C795
- ROCKWOOL ProRox WM — wired mats for large pipes, vessels and curved surfaces. The data sheets give water absorption below 1 kg/m² (EN 1609) and chloride below 10 ppm with ASTM C795
- ASK CS 220 calcium silicate — rigid insulation with good compressive strength, suited to places where insulation tends to be stepped on or knocked until the jacketing fails. It is a porous material, however, so it relies on good jacketing and coating just the same
Water absorption is a test result for the material; it does not mean the system cannot suffer CUI. See insulation, moisture and water repellency.
3. Jacketing and sealing
- Lap joints so that water runs off, not in
- Seal around penetrations and close off insulation ends neatly
- Give water that has entered a way to drain at low points
- Use insulation strong enough where people step or lean, so the jacketing is not dented
See sealing insulation joints.
4. Inspection and maintenance
Jacketing and sealants age, so an inspection plan is needed. API RP 583 recommends prioritising by risk (risk-based inspection), looking at equipment age, coating type, location, operating temperature and the consequence of a leak. Visual inspection by trained people remains the foundation, supported by non-destructive methods chosen by specialists.
This is specialist work
CUI affects the safety of piping and pressure equipment. The coating system, insulation, jacketing and inspection plan should be selected by qualified engineers and inspectors to the standards your plant uses. We can provide product information and manufacturers' documents to support the decision, but we do not design corrosion-protection systems.
Common misunderstandings
- "Water-repellent insulation means no coating is needed" — low-absorption insulation shortens the time the metal is wet; it does not replace a coating
- "Hot pipes do not rust" — start-up, shutdown and cyclic service still pass through the risky temperature band
- "The jacketing looks fine, so it is dry inside" — water enters at small defects and collects somewhere else
- "Stainless steel is not affected" — stainless is vulnerable to chloride cracking, which is harder to see than rust
Read next: choosing high-temperature insulation · insulating hot-water and steam pipes
Sources
- National Insulation Association (NIA), Insulation Outlook — An Industry Perspective: Revisiting Recommendations (summary of NACE RP0198)
- UK Health and Safety Executive (HSE) — SPC/TECH/GEN/18 Corrosion under insulation of plant and pipework (read via search summary)
- American Petroleum Institute — API RP 583 Corrosion Under Insulation and Fireproofing (scope and contents leaflet, first edition)
- AMPP (NACE) — SP0198 Control of Corrosion Under Thermal Insulation and Fireproofing Materials — A Systems Approach (title and secondary summaries)
- ROCKWOOL Technical Insulation — ProRox WR-Tech factsheet (RW-TI/06.21)
- ROCKWOOL Thailand — ProRox PS 960 SA / PS 970 SA and ProRox WM 950 / 960 / 970 SA Product Data Sheets (08/25)
- Zhejiang ASK — Calcium Silicate CS 220 product introduction (distributor copy)
Related products
Categories and example products for this topic
- ROCKWOOL ProRox PS — WR-Tech pipe sections: water absorption ≤ 0.2 kg/m², chloride ≤ 10 ppm (ASTM C795)
- ROCKWOOL ProRox WM — Wired mat for large pipes, vessels: water absorption < 1 kg/m², low chloride (ASTM C795)
- ASK CS 220 — Rigid, high compressive strength — for spots where foot traffic damages jacketing
Industrial and high-temperature insulation · Stone wool
ROCKWOOL ProRox WM
- Max. temp.
- 650–750 °C
- Density
- 80–128 kg/m³
- Thermal conductivity (λ)
- 0.037–0.038 W/m·K (50 °C)
Industrial and high-temperature insulation · Calcium silicate
ASK CS 220
- Max. temp.
- 650 / 1,000 °C
- Thermal conductivity (λ)
- ≤ 0.060 W/m·K (93 °C)
- Density
- ≤ 220 kg/m³