Corrosion Under Insulation

Learn how to mitigate the effects of CUI

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Protect your investment and people

Corrosion Under Insulation (CUI) is one of the industrial sector’s greatest challenges with respect to materials. It’s a major issue that needs to be addressed from the design phase until decommissioning of equipment. Insulation is part of this system challenge. By selecting the best available solution, the effects of CUI could be mitigated. But first this:

What are the effects of CUI?

In the industrial sector and especially when you’re working with hazardous, explosive or flammable materials, the potential consequences of uncontrolled CUI can be catastrophic. CUI on pipes or equipment may result in leaks, which again could catch fire, explode or cause pollution, all with significant impact on safety and the environment. So, CUI deserves your highest attention!


total costs of corrosion (NACE)


of pipe maintenance costs are due to CUI


Of the CUI incidents occurs on pipes (ExxonMobil)

How big is the issue and how does CUI occur? Here are some of the facts:
  • Corrosion of unprotected metal occurs in the presence of water and oxygen, and elevated temperatures (i.e. hot pipe). Where the metal is hidden by insulation and cladding, the process may proceed unnoticed for years – maybe until leaking occur (which can be too late).
  • Even when the metal is protected by coatings and the insulation is made weatherproof by cladding, corrosion can still occur. The installation may not have been perfect, or the system was damaged during service or simply by aging.
  • The total annual estimated direct costs of corrosion are estimated by NACE at 276 bio USD. [7]
  • CUI studies from a petrochemical facility have shown that 40-60% of pipe maintenance costs are due to CUI and approximately 10% of the total maintenance budget is spent repairing damage from CUI. [2]
  • ExxonMobil reports that 80% of their CUI events are on pipes.[1]
The risk of corrosion under insulation is considered high in the temperature range 50 – 175°C (122 – 347°F) and extreme when in cyclic temperature service between -20 and 320°C (-4 – 608 °F)
SHELL specification: DEP September 2011

So, what’s the solution?

There’s not just 1 answer to this question. CUI is a systemic challenge and should be addressed as such from design to decommissioning. You have to take into account the probability of corrosion and the consequence in case of corrosion.

  • Probability:
    What is the corrosive potential of the environment? Is it an indoor installation or outdoor? If outdoor, is it a dry rural environment or a humid/wet coastal environment? At what temperatures will the equipment operate? And will it be cyclic or continuous operation?
  • Consequence:
    Does the pipe transport flammable liquids or is it a cooling water – the consequence in case of a leak may be significantly different, and for safety, environmental and cost reasons the approach to CUI mitigation should therefore also be different.

If there is a high risk (probability x consequence), CUI should be a critical design parameter, and CUI should be monitored and handled throughout the lifetime of the equipment. Step by step.

  • Equipment design. The design must prevent water penetration and water traps inside the system, but also ensure there is sufficient space to apply a correct surface treatment and install the insulation and cladding.
  • Choice and installation of coating system. The coating system/metallization is the primary corrosion protection and must be chosen according to the temperature – bearing in mind the system may be exposed to very hot water. Installation of the coating, including surface preparation, is critical for service life and performance.
  • Design, choice and installation of the insulation system. The insulation material must be optimized so that it does not accelerate corrosion. Cladding must be selected based on considerations on the environment and service conditions. It needs to be installed correctly so that water drains away from the surfaces, assemblies must be sealed, and drain holes provided if feasible for the design criteria. A moisture barrier on the interior surface of the cladding shall be considered in the material selection/design to help prevent. Galvanic (bimetallic) corrosion shall be considered in the material selection.
  • Quality control. Quality control is important throughout the process.
  • Actual operation. Lastly, inspection routines must be drawn up and cladding, insulation and coating systems under the insulation must be maintained after commissioning
Insulation material is an important element in this system challenge.

Investigation shows that CUI can occur under all insulation materials, but the corrosion rate can be affected by the insulation material. Accelerating factors are:

  • the materials content of water-soluble salts (especially chloride)
  • the capacity of the material to absorb/retain water and dry again
  • the propensity of some organic insulation materials to produce acid during ageing. [3, 6]

Our next generation ProRox mandrel wound pipe section with WR-Tech™ ticks all the boxes and effectively helps you mitigate the effect of CUI in industrial installations.

[1] B.J. Fitzgerald, P. Lazar III, R. M. Kay and S. Winnik. “Strategies to prevent corrosion under insulation on petrochemical industry piping” NACE CORROSION 2003, paper no. 03029 (San Diego, CA: NACE 2003)

[2] B. J Fitzgerald, S. Winnik, “A strategy for Preventing Corrosion Under Insulation on Pipeline in the Petrochemical Industry”, JPCL, April 2005.

[3] NACE SP0198-2016, “Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systematic Approach. NACE International.

[4] C. Zwaag and S.N. Rasmussen, “Mineral wool and water repellency” NACE CORROSION 2018, paper no. 10929 (Phoenix, AZ: NACE, 2018). 

[5] SHELL specification: DEP September 2011

[6] C. Zwaag and S.N. Rasmussen, “Cyclic CUI testing of Insulation materials” NACE CORROSION 2017, paper no. 8877 (New Orleans, LA: NACE, 2017).

[7] Showpad insight number 9: Koch, G. H., Brongers, M. P., Thompson, N. G., Virmani, Y. P., & Payer, J. H. (2002). Corrosion cost and preventive strategies in the United States (No. FHWA-RD-01-156,).