Corrosion under Insulation (CUI): Understanding Risks, Detection, and Prevention 

One of the most expensive and persistent forms of corrosion, Corrosion under Insulation (CUI) affects millions of industrial facilities across the world. It targets structural components, tanks, vessels, and pipes, striking silently beneath layers of thermal insulation. Most significantly, it often remains unnoticed until significant damage takes place. For ensuring long-term asset integrity and safety, industries reliant on insulated process systems must find a way to understand and mitigate the risks of CUI.  

What is Corrosion under Insulation (CUI)? 

A form of localized external corrosion, Corrosion under Insulation (CUI) occurs when water gets trapped between the insulation and the metal surface of equipment. Insulation systems can be infiltrated by moisture because of factors such as condensation, weather exposure, and damaged cladding. Once inside, it creates a corrosive environment on the metal surface, often leading to severe pitting or stress corrosion cracking over time. 

Though stainless steels are also vulnerable under specific conditions, CUI is a huge concern for carbon and low-alloy steels. The insidious nature of CUI makes it a major threat in sectors like marine infrastructure, power generation, chemical processing, and oil and gas. 

Risk Factors 

The likelihood and severity of corrosion under insulation depends on several factors.  

  • The temperature range between -4°C and 175°C encourages condensation within the insulation system, particularly during fluctuating operating conditions and shutdowns.  
  • Mineral wool, calcium silicate, and other absorbent insulation materials accelerate corrosion by retaining water.  
  • The risk is higher in tropical, marine, and coastal climates because of salt-laden air and high humidity.  
  • Insulation and cladding integrity can be broken down by frequent thermal cycling, design flaws, and poor sealing. 
  • Though stainless steels are less vulnerable to corrosion compared to carbon steel, they are not immune, especially in the presence of chlorides.  

Uncovering Hidden CUI 

Detection is one of the greatest challenges with CUI. It often progresses unnoticed until significant structural compromise occurs because corrosion remains concealed beneath insulation and jacketing. However, it is possible to identify CUI early with the help of several inspection techniques and technologies.  

Visual inspections are performed during shutdowns or scheduled maintenance, and involve removing insulation at certain areas to inspect metal surfaces. Though this method is cost-effective, it requires downtime and offers limited coverage.  

Temperature variations in insulated systems can be detected by infrared thermography. Presence of cold spots may indicate corrosion or wet insulation. It can be used best as a screening tool in conjunction with other methods. 

The remaining wall thickness of a component can be measured by Ultrasonic Thickness Testing (UTT).  Most importantly, if access points are provided, this test doesn’t require removing insulation. It is effective for assessing the extent of wall loss caused by corrosion. 

Radiography and Computed Tomography (CT) is more time-consuming and expensive, but is useful for critical areas. It offers a non-intrusive view of internal and external corrosion patterns.  

Finally, a non-destructive method, Pulsed Eddy Current (PEC) is useful for screening large areas quickly with minimal disruption.  

Prevention Strategies 

To prevent CUI, it is important to implement an integrated approach involving material selection, insulation design, and protective coatings. 

  • Apply high-performance coatings specifically designed for insulated environments along with proper surface preparation.  
  • Always use hydrophobic, non-absorbent materials and ensure that insulation is installed with vapour barriers to prevent moisture ingress.  
  • Incorporate drainage points and weep holes so that water can escape and avoid horizontal runs where water can pool. 
  • Implement a CUI risk-based inspection (RBI) strategy that monitors high-risk areas more frequently.  
  • Use stainless steel or aluminium jacketing with proper sealing and overlapping joints. 

Corrosion under insulation may be hidden, but its effects are far-reaching. At Coating Management Solutions, we specialize in identifying, managing, and mitigating CUI across complex industrial environments.