Corrosion under insulation is the quiet one. The pipe looks fine from outside, the jacket looks fine, and underneath it the steel is rusting or cracking where nobody can see. By the time it shows up as a leak or a failed inspection, it has usually been going a while. In the Permian, where a lot of hot lines run outdoors through weather and blowing dust, it is a chronic maintenance headache for every operator. Here is what drives it and what keeps it from starting.
CUI is corrosion of the pipe or vessel wall underneath the insulation, driven by water trapped against the metal. On carbon steel it is general corrosion and pitting. On austenitic stainless it is chloride stress-corrosion cracking, which is more dangerous because it can crack a line that still looks solid. The temperature band matters: the worst zone is roughly 140F to 350F, where trapped water stays liquid against the pipe instead of drying off. Lines that cycle through or sit in that range are the ones to watch.
The same insulation system that saves energy seals the evidence in. Water gets under the jacket through a damaged spot, an open seam, failed sealant, or a penetration, and then it has nowhere to go. It sits in the insulation against the pipe. The jacket keeps the rain off, so from outside everything looks maintained. Meanwhile the wet insulation holds moisture right where it does the most damage. This is why CUI inspection programs exist and why the damage is so often further along than expected when someone finally pulls the jacket.
Every prevention strategy comes down to two ideas: keep water out, and give any water that does get in a way to leave. In practice several things work together.
The front line and the cheapest insurance. Intact jacketing, seams lapped and sealed, a factory moisture barrier like polysurlyn inside the jacket, and careful detailing at every termination, elbow, and penetration. Most CUI water gets in through sloppy jacket work at exactly those transition points. Good detailing costs labor, not much material, and prevents the expensive problem.
Protective coatings on the pipe surface under the insulation give a second line of defense, so if moisture reaches the steel it meets a barrier instead of bare metal. Immersion-grade coating systems are common in CUI-prone service.
Where CUI is a known risk, closed-cell insulation that does not absorb or wick water changes the equation, because there is no wet blanket against the pipe. Foamglas is the classic pick. On stainless, C795-qualified insulation is specified to limit leachable chlorides that would feed stress-corrosion cracking.
No system is sealed forever. Jackets get dented, sealant ages, someone cuts in a tap and does not close it up right. So prevention is partly design and partly upkeep: fix damaged jacketing when you see it, reseal penetrations after work, and do not leave insulation open to weather. On existing lines, the highest-value move is usually re-jacketing the sections that have lost their weather protection before water has time to work.
For CUI-resistant work you want the jacket system sorted: aluminum or stainless with a moisture barrier, banding, and the sealants and mastics to close it up properly, plus closed-cell insulation where the service calls for it. The combination that keeps water off the pipe is not exotic. It is standard material installed with attention to the details that let water in.
Water. When moisture gets under the jacket and sits against the pipe, usually between about 140F and 350F where it stays wet rather than boiling off, it drives corrosion on carbon steel and chloride stress-corrosion cracking on stainless. The insulation hides it, so it often runs a long time before anyone catches it. Water gets in through damaged jacketing, open seams, failed sealant, or penetrations.
Keep water out and give any water that gets in a way to leave. That means intact jacketing with sealed seams and proper overlaps, a moisture barrier on the jacket, protective coatings on the pipe under the insulation, correct handling of penetrations and terminations, and using insulation that does not hold water where CUI is a known risk. Good jacketing detail is the cheapest CUI insurance there is.
Closed-cell materials that do not absorb or wick water, like Foamglas, are often specified in CUI-prone service for that reason. On stainless, ASTM C795-qualified insulation is used because it limits leachable chlorides that would otherwise contribute to stress-corrosion cracking. The jacket and moisture barrier still matter regardless of the core material.
Yes. We stock aluminum and stainless jacketing with polysurlyn moisture barrier, banding, sealants, mastics, and closed-cell insulation. Tell us the service and we help put together a system that keeps water out.
We stock the material and price it the same business day.