Corrosion Under Insulation (CUI): The Hidden Threat in Insulated Piping and Vessels

Corrosion under insulation is one of the most expensive problems in the process industries, and it's expensive specifically because it's invisible until it isn't. Insulated piping and vessels look fine from the outside — clean jacketing, intact seams, no obvious damage. Underneath, moisture has been working on the steel for years. CUI is consistently cited as a leading cause of unplanned piping failures and integrity incidents in refineries, chemical plants, and process facilities, and by the time it shows up in an inspection, the steel is often already thinned past a safe margin.

The frustrating part is that CUI is almost entirely preventable. It's a coating and inspection discipline problem, not a materials-science mystery. Here's how it actually forms and what stops it.

How CUI Forms

Insulation isn't waterproof, and jacketing isn't either — not permanently. Moisture gets in through seam gaps, penetrations, damaged jacketing, or simple condensation, and once it's inside the insulation system it has nowhere good to go. The insulation holds that moisture directly against the steel surface, and thermal cycling — the pipe or vessel heating and cooling through its operating cycle — pulls moisture in and pushes it around rather than letting it evaporate away.

The result is a corrosion cell that runs continuously in a spot no one is looking at. It's worse than open-air corrosion in some ways: the insulation traps humidity at the steel surface instead of letting it dry between wet cycles, so the corrosion doesn't get the intermittent relief that outdoor steel gets from sun and airflow.

CUI risk peaks in a specific temperature band — roughly -4°F to 350°F for equipment in cyclic or intermittent service. That's the range where water condenses and re-evaporates against the steel repeatedly. Equipment that runs continuously hot enough to keep the insulation dry, or cold enough to stay below freezing consistently, sees somewhat lower CUI risk — but "somewhat lower" is not "safe to ignore," especially at insulation breach points, boot seals, and pipe supports where moisture gets in regardless of the operating temperature.

Why It's So Hard to Catch

Jacketing hides the problem by design — that's what it's there for. Visual inspection from outside tells you almost nothing about what's happening at the steel surface. By the time you see staining, bulging jacketing, or rust streaks at a seam, corrosion has usually been active for a long time.

That's why CUI inspection has shifted toward non-destructive testing methods that don't require stripping insulation off miles of piping: pulsed eddy current (PEC) for wall thickness screening through insulation, computed radiography for imaging pipe walls in place, and targeted insulation removal at statistically likely trouble spots — low points, penetrations, supports, and dead legs — rather than blanket removal across an entire system.

AMPP SP0198: The Systems Approach

AMPP SP0198 (the standard formerly published as NACE SP0198) is the industry reference for CUI control, and it's built around a simple premise: coating selection, insulation design, jacketing integrity, and inspection intervals all have to work together — no single element solves CUI on its own.

The standard's coating guidance is organized by operating temperature:

• Below approximately 120°C (250°F): Epoxy phenolic and modified silicone coating systems are typically specified. These hold up well in the cyclic wet/dry conditions typical of this range.
• Above approximately 120°C (250°F): Inorganic zinc silicate or thermal spray aluminum (TSA) systems are generally required — organic coatings degrade faster at sustained high temperatures under insulation, where the heat and any trapped moisture combine to accelerate breakdown.
• At insulation support and penetration points — the highest-risk locations regardless of general operating temperature — coating specifications are often upgraded beyond the baseline system for the rest of the run.

The standard also addresses jacketing: aluminum jacketing at roughly 0.016–0.024 inch thickness is standard for onshore process service, while stainless steel or PVDF-coated aluminum is specified for coastal or offshore environments where chloride-laden air accelerates jacketing corrosion and breach — which, once breached, is exactly how moisture gets to the steel underneath in the first place.

Coat it before it goes back under insulation, or plan on paying to strip the insulation, blast the steel, and do it right anyway — except now you're also paying for the corrosion damage that happened while nobody could see it.

What Facility Owners Should Be Doing Now

• Audit known breach points first. Insulation supports, penetrations, valve boots, and low points on horizontal runs are where CUI starts. If your last insulation inspection is more than a couple years old, that's the place to start looking.
• Never re-insulate uncoated or under-coated steel. Any time insulation comes off for maintenance, that's the cheapest possible opportunity to inspect the steel and apply or repair the coating system before it goes back under jacketing. Skipping this step to save a day of schedule is the single most common way CUI gets a foothold.
• Check your original coating spec against actual operating temperature. Equipment sometimes runs hotter or colder than it did when originally specified, especially after process changes. A coating selected for the wrong temperature band degrades faster than the spec assumed.
• Build CUI screening into your inspection program rather than treating it as a one-time project. AMPP SP0198's systems approach explicitly includes ongoing risk-based inspection intervals, not a single pass.

Where This Applies at Your Facility

Any insulated line, vessel, or tank exterior with jacketing is a CUI candidate — process piping, steam lines, chilled water systems, storage tanks, and reactor vessels all qualify. We spec coating systems for structural steel and tank exteriors by operating temperature and environment as a standard part of every project assessment, and we're set up to work efficiently during planned insulation removal windows so the steel doesn't sit exposed and unprotected any longer than it has to.