What Is Corrosion Under Insulation (CUI)? How Plant Pipes and Tanks Fail Silently and How to Prevent It
What Is Corrosion Under Insulation (CUI)? How Plant Pipes and Tanks Fail Silently and How to Prevent It
A steam pipe or tank that looks neatly insulated from the outside can be corroding constantly underneath, unseen. This problem is called Corrosion Under Insulation, or CUI, and it is a leading cause of unexpected pipe leaks and production shutdowns in industrial plants.
CUI happens when moisture seeps in and collects between the insulation and the metal surface. Wet insulation holds water against the pipe, so steel can rust faster than bare pipe left open to the air.
How Moisture Gets Into Insulation
Rain and washdown water entering through loose cladding joints
Failed or missing seals around valves, flanges, and wall penetrations
Chilled-water lines without a vapor barrier, letting humid air condense inside the insulation
Physical damage such as dents from foot traffic or impact
High-Risk Spots to Inspect First
Pipes and tanks whose surface temperature keeps water in liquid form, where rust develops fastest
Areas around valves, flanges, elbows, and other joints with many seams
The underside of horizontal pipes and tank bottoms, where water tends to collect
Lines that cycle between operating and idle, causing repeated condensation
How to Prevent CUI
Install aluminum or stainless cladding with overlaps oriented to shed water, keeping out rain and washdown
Seal joints and penetrations with materials suited to the service temperature, and re-inspect periodically
Use a vapor barrier on chilled-water lines to keep moisture out of the insulation
Choose insulation that resists water absorption and holds its shape, such as rock wool designed and installed with proper cladding
Use removable insulation jackets on valves and flanges that need frequent access, so permanent cladding does not have to be torn out each time
Inspect from the outside regularly; if you see dented sheeting, open seams, or rust stains, open it up and check underneath
Why Installation Matters as Much as Material
Good insulation installed poorly can still lead to CUI, because water enters through seams and unfinished details. Properly detailed cladding, with correct overlaps, sealing, and support, is the first line of defense that extends the life of both the insulation and the pipe or tank beneath it.
Conclusion
CUI is preventable when planned from the start: choose the insulation, design the joints, and install the cladding correctly. Centrotec has an installation team with more than 20 years of insulation and cladding experience, ready to survey your site and advise. Contact us for a quote.
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