Corrosion Under Insulation (CUI)

Corrosion Under Insulation (CUI) is one of the leading causes of unplanned shutdown in refineries, petrochemical plants, and offshore facilities. Pipe fittings — elbows, tees, reducers — are disproportionately affected because their complex geometry traps water, makes coating difficult, and creates pockets that hold moisture longer than straight pipe. Identifying the risk zone, specifying the correct coating system, and choosing CUI-resistant grades where appropriate prevents failures that often go undetected until leakage or wall thinning is found during a turnaround.

The CUI Mechanism

CUI occurs when water penetrates insulation and reaches the pipe or fitting surface. At operating temperatures between 0°C and 175°C (particularly 60–150°C), the wet-dry cycling beneath the insulation creates an aggressive corrosion environment — ionic contaminants from the insulation material (chlorides from mineral wool, sulphates from calcium silicate) concentrate at the metal surface during drying cycles. The result is localised pitting and general corrosion that can penetrate a fitting wall in 2–5 years if unprotected.

The critical temperature range is 60–150°C for carbon and low-alloy steel. Below 60°C the rate is lower; above 150°C the surface is too hot for sustained water presence. Fittings cycling through this range (e.g. on steam tracing, intermittent service, or startup/shutdown cycles) are at highest risk because every thermal cycle can draw moisture into the insulation.

Why Fittings Are Higher Risk Than Straight Pipe

Complex geometry means coating application is difficult — inside radius of elbows, behind flange backs, and tee crotches are commonly under-coated
Insulation joints at fitting ends create ingress points that straight-pipe insulation doesn't have
Butt welds at fitting-to-pipe joints are a common initiation site — weld profile irregularities trap moisture
Fittings on low-point legs and drain points accumulate water by gravity

Coating Systems for CUI Service

For carbon steel fittings in CUI risk service, the standard coating approach is: blast to Sa 2.5 (white metal or near-white), then apply a thermal-resistant epoxy phenolic or polysiloxane coating system to DFT ≥250 µm — with special attention to the inside elbow radius and behind weld beads. Thermal-spray aluminium (TSA) at 100–200 µm is the premium solution for very aggressive CUI risk above 100°C — TSA provides cathodic protection and mechanical abrasion resistance, and is preferred on offshore facilities. Never use bituminous coatings on hot-service fittings — they soften and disbond above 80°C, trapping water between the coating and the steel.

Alloy Upgrades for CUI Risk Areas

In areas of very high CUI risk where inspection access is difficult (buried sections, insulated supports, heavily congested areas), specifying 316L stainless fittings instead of carbon steel eliminates the CUI risk entirely — at the cost of a higher initial material price that is usually recovered within the first inspection cycle. This approach is particularly well-suited to: small-bore nozzle connections (NPS ≤2) on carbon steel vessels; fitting clusters at flanged equipment connections; and fittings in sleeved or buried service.

Procurement Checklist for CUI Risk Fittings

State CUI risk zone on the PO — allows the coating applicator to use appropriate procedures
Specify blast standard Sa 2.5 minimum (not Sa 2) for CUI coatings
Require coating holiday test (wet sponge or high-voltage) on complex geometry fittings
Specify DFT measurement on the inside radius of elbows as a named check point
For TSA: specify HVOF or arc-spray with a sealer; confirm curing temperature matches service range
Consider alloy upgrade for NPS ≤2 fittings in congested, hard-to-inspect areas