Corrosion Under Insulation (CUI) is one of the most well-known phenomena in the process industries, and yet it still makes up an inordinately large percentage of global maintenance expenditures. CUI is a subject that is well-researched and understood; extensive studies have been commissioned to determine the causes, effects, prevention, and mitigation of CUI.
In the simplest terms, CUI is any type of corrosion that occurs due to moisture present on the external surface of insulated equipment. The damage/attack can be caused by one of the multiple factors, and can occur in equipment operating at ambient, low, and heated services, depending upon conditions. Moreover, CUI can occur in equipment that is in service, out of service, or in cyclic service.
The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. If undetected, the results of CUI can lead to leaks and the shutdown of a process unit or an entire facility.
Under the right conditions, Corrosion Under Insulation or CUI is an insidious damage mechanism that can corrode Assets beneath their insulation cladding which, if left undetected can lead to catastrophic failure & costly Asset downtime.
Broken down simply CUI is rapid form of a basic electrical Corrosion Cell consisting of the:
- Anode – The physical item sacrificed (Corroded)
- Cathode – The physical part that is protected
- Conductor – Electrical Connection
- Electrolyte – Conductive Liquid & Contaminants
Following Asset preventive maintenance or repairs a CUI Visual Inspection can ensure the quality of the insulation cladding reinstatement & provide a set of fresh eyes to identify any potential water ingress points to help eliminate future CUI occurrences.
The most common variations of CUI include:
- Galvanic Corrosion – The sacrificial corrosion of the less noble dissimilar metals
- Acidic/Alkaline Corrosion – Jacket Corrosion by dissolved contaminants after evaporation
- Chloride Corrosion – Stress Corrosion Cracking catastrophic failure
A thorough CUI Visual Inspection campaign can provide invaluable data two fold, by providing an complete examination of all potential ingress points for follow up maintenance & to provide necessary diagnostic information to target further specialized Inspection or Testing.
Further follow-up Inspections could include a complete or partial removal of insulation cladding for a Close Visual Inspection which may also require complimentary Ultrasonic Thickness Testing to verify the Assets remaining wall thickness for Fitment for Service calculations.