October 2022

Special Focus: Plant Safety and Environment

A system approach to mitigating corrosion under insulation

This article will consider how a system approach can help mitigate the risk of CUI and contribute to the performance and longevity of piping and equipment.

Cusick, A., Owens Corning

Industrial facilities employ a variety of metal piping and equipment that require insulation to operate efficiently. When corrosion occurs underneath insulation on this equipment, it can remain hidden for years, potentially leading to serious consequences. As moisture is the de facto contributor to corrosion for pipes running from –4°C–149°C, or that cycle in temperature within this range, protecting against water infiltration and the damage it poses to covered pipes is essential.1

Safeguarding piping and equipment against corrosion under insulation (CUI) may include multiple lines of defense against moisture intrusion. These defense measures include:

  • Coating the pipe with a high-quality, water impermeable coating like paint, mastic, gel or vapor retarder
  • Specifying an impermeable, non-absorptive insulating material
  • Using proper insulation accessories to protect the system from moisture ingress, including joint sealers, adhesives and metal jacketing.

While these strategies comprise the infrastructure of an effective system, they are contingent upon correct installation to perform properly. In addition, an inspection and maintenance process should be established for the longevity of the system. This article will consider how a system approach can help mitigate the risk of CUI and contribute to the performance and longevity of piping and equipment.

Calculating the cost of CUI

A study by NACE International estimated that the annual global cost of corrosion in 2013 was about $2.5 T, or 3.4% of the global GDP.1 However, the same report also found that savings of 15%–35% (or $375 B–$875 B) could be made by a more comprehensive employment of already known corrosion mitigation practices. Similarly, a study from 2019 indicated that 40%–60% of pipeline repair costs are caused by CUI.2

Beyond the costs of unexpected downtime and unintended repairs, corrosion can present a safety hazard. In recent years, industrial incidents around the world have occurred where pipes or equipment failed following the development of corrosion (FIG. 1).3,4,5

FIG. 1. Corrosion under insulation can be a hidden threat that develops out of sight when moisture penetrates an insulating system.
FIG. 1. Corrosion under insulation can be a hidden threat that develops out of sight when moisture penetrates an insulating system.

Understanding CUI

Corrosion refers to the deterioration of a material (typically a metal) into a more chemically stable form of itself via chemical or electrochemical reactions with its environment. One of the most common forms of corrosion specifically affects carbon steel, where oxygen reacts with iron to form iron oxide. In this example, one area of a metal surface (the anode) gives up electrons to another area of a metal surface (the cathode). This transfer of electrons is enabled by the presence of a conducting solution (an electrolyte) in the form of water. Oxygen that is present will chemically react with the anode as it gives up its electrons, leaving behind iron oxide (rust) on the surface of the anode.

CUI develops when moisture penetrates an insulation system and encounters the surface of metal piping or equipment, leading to corrosion. The consequences of CUI include unexpected and costly repairs and downtime, damage to expensive equipment and an increased potential for leaks. Depending on the extent of corrosion and the nature of the facility, CUI can lead to unexpected process system failures, piping failures, fires, explosions and employee injury or death.

Three risk factors that can cause or exacerbate the development of CUI are the presence of moisture, the chemical nature of the moisture present and the process temperature of the piping or equipment. Oxygen is also a key ingredient for corrosion; however, it is almost always readily found in any environment, whether in the air or dissolved in liquid moisture.

Moisture contacting the outer surface of a pipe is a key ingredient for corrosion to develop, as it serves as an electrolyte for electrochemical processes to occur. In an industrial setting, moisture can come from a range of sources, including natural precipitation, cleaning activities in the form of wash downs, and facility elements such as process leaks, deluge systems, cooling tower drift and sprinklers. Often, insulation systems rely on external vapor barriers or jacketings to help prevent moisture ingress from occurring. However, these barriers can become compromised due to poor installation and/or mechanical damage.6

Once present, the chemical nature of the liquid becomes important. Moisture that contains dissolved ions from salt, pollutants or insulation materials can lead to more aggressive rates of corrosion. This is because strengthening the acidity, alkalinity or salt content of water can increase the strength of the electrolyte in the corrosive reactions taking place.

The operating temperature of piping or equipment can also influence the rate of corrosion. In general, the rate of corrosion will increase with the process temperature, as electrochemical reactions typically occur faster at higher temperatures. Pipes are considered at risk for corrosion at temperatures where liquid water can contact the metal surface, which can be roughly viewed as –4°C–149°C. Above these temperatures, moisture that penetrates the insulation system tends to evaporate before reaching the surface of the pipe. However, even these pipes can be at risk for CUI during temperature cycling periods or during any shutdown and restart procedures when moisture that has been forced away from the pipe is allowed to settle and rest against it.

Pipes operating at below ambient temperatures also face risk of moisture intrusion due to different mechanisms. In these systems, vapor drive from ambient air can force water vapor towards cold pipes, which then condenses into liquid moisture. If permeable insulations are used and existing vapor barriers become compromised, water can quickly build up along the cold pipe surface in the form of condensation, creating an environment for CUI to take place. Below roughly –4°C, a pipe may be considered out of this range, as liquid moisture will freeze into solid ice. However, like hot systems, these pipes can become at risk for corrosion if they experience frequent temperature cycling or system shutdowns as part of routine maintenance activities, where ice can thaw and once again act as an electrolyte to the system.

Signs of a problem

A Houston, Texas-based engineering consultant who formerly served as CUI track chair for NACE has seen a variety of corrosion situations in a career spanning more than four decades. According to the engineer, an early sign of moisture ingress for cold and cryogenic insulation is excessive condensation on the system jacketing’s surface. Excessive condensation is the first fail test. Surface condensation caused by a reduced surface temperature is indicative of increased heat transfer due to wet insulation. Eventually, an ice ball will form on the metal jacketing, leading to total insulation failure. The potential presence of moisture within the system can also be a concern because it may indicate that CUI is potentially developing hidden from plain view.

System strategies for mitigating CUI

Early attention to warning signs is essential to address the problem and reduce further damage. Three primary strategies for mitigating CUI are keeping water out of the insulation, changing the chemistry of any moisture that enters the system and providing a dedicated way out for any moisture that does get into a system.

Thoughtful selection and proper installation of an insulation system can help mitigate the development of corrosion under insulation and the problems it presents. Because moisture poses a damage risk to an insulated system, vapor retarders are commonly used alongside insulation materials to help impede moisture infiltration. While effective when executed correctly, vapor barriers can be difficult to properly install and are vulnerable to incidental damage and perforation, especially if applied on top of a permeable insulation.

Pipe coatings are another solution that can be used to help prevent moisture from encountering a metal pipe. These coatings may be applied along the surface of the pipe underneath the insulation to provide an extra line of defense against any moisture that makes its way into the system.

Insulation use in mitigating CUI

As noted earlier, mitigating CUI requires a system approach that goes beyond insulation to consider the right pipe material, and appropriate and properly designed claddings, coatings and non-permeable insulation. Establishing a regular inspection and maintenance program is also essential. Beyond the insulating material, the insulation system includes adhesives and sealants, external coatings or claddings, and accessories.

Beyond ensuring a material will not absorb or retain moisture, specifying engineers should consider an insulation material’s performance and dimensional stability in the needed temperature range. The material should also lack any chemistry that could increase acidity and the shedding of chloride ions (FIG. 2).

FIG. 2. The proprietary systema combines cellular glass insulation with a low-viscosity sealant and utilizes the impermeability of the insulation to establish a vapor barrier that moisture cannot pass through.
FIG. 2. The proprietary systema combines cellular glass insulation with a low-viscosity sealant and utilizes the impermeability of the insulation to establish a vapor barrier that moisture cannot pass through.

A material like closed-cell cellular glass insulation can address each performance aspect. Impermeable to water vapor and liquids, cellular glass insulation does not wick, retain or absorb moisture, including hydrocarbon-based liquids. The insulating material is dimensionally stable across a wide temperature range and will not warp or compress. Its glass composition provides an inert chemistry that is also low on leachable chlorides, much like a beaker used in a laboratory. The insulation can also be used with a range of materials and multiple types of accessories in a larger system. This type of insulating material has been incorporated into sealed systems designed to keep moisture from entering the insulation or reaching the pipe.

Detailing a sealed system

An example of a system designed to keep water outa combines cellular glass insulation with a low-viscosity sealant that has a wide service temperature range and cures at ambient temperatures. This system utilizes the impermeability of the insulation to establish a vapor barrier that moisture cannot pass through. The paired sealant is then used at all joints, penetrations and terminations to ensure all gaps between the insulation are sealed against moisture ingress, protecting the pipe underneath from encountering the liquid water needed for corrosion (FIG. 3).

FIG. 3. A correctly applied insulating system using impermeable cellular glass insulation and a low-viscosity sealant can help mitigate CUI by combating moisture ingress.
FIG. 3. A correctly applied insulating system using impermeable cellular glass insulation and a low-viscosity sealant can help mitigate CUI by combating moisture ingress.

Sealed insulation systems of this nature can be used in a range of industrial applications where CUI is a concern. They are commonly used on below-ambient and cycling systems where insulated pipes are cold and face a high vapor drive. However, they are also relevant on above-ambient systems where physical moisture ingress is anticipated, as the elevated temperatures associated with these lines can lead to faster rates of corrosion development should water penetration occur.

Testing insulating systems

A series of tests were conducted to examine how well the proprietary technologya can be used to protect pipes in extreme conditions. The experiments evaluated a sealed system comprised of cellular glass insulation and a low-viscosity, neutral cure sealant. In the testing, pipes were protected by this system for 28 d in accordance with a modified ASTM B117 Standard Practice for Operating Salt Spray (Fog) Apparatus.7,8 Pipes faced one of two situational challenges: salt rain, or salt fog and rain.

In the first arrangement, pipes experienced an ambient temperature of 21°C with a rain solution of 5% salt sprayed every 20 min. In the second, an ambient temperature of 35°C was combined with a constant fog containing 5% salt along with a 5% salt solution sprayed every 20 min.

Unprotected pipes were visibly and extensively corroded by the end of the testing. However, piping with the sealed insulation system applied were removed at the conclusion of the test with no visible signs of corrosion.

Performance in the field: Comparing two insulating approaches

The use of a closed-cell material can support an insulating system’s longevity. Not all closed-cell materials perform the same and the type of closed-cell material used can also make a difference. The author’s colleague described two insulation systems near a busy shipping channel in Houston’s humid climate. One piping system was insulated with polyisocyanurate insulation (PIR) and the other was insulated with cellular glass insulation. According to the engineer, it was possible after 5 yr to see white ice forming on the PIR surface and the system had to be replaced after 7 yr. After 15 yr, the cellular glass showed no signs of sweating. The engineer has worked on projects where the cellular glass insulation was still performing after 40 yr.

When it comes to insulating pipes in hydrocarbon processing applications, there is no silver bullet, and a system approach is required to safeguard against the hazards moisture presents. Facilities should consider the risks CUI poses and take steps to mitigate its occurrence by correctly installing, designing and maintaining an insulation system. Using an impermeable insulation, like cellular glass insulation, as one part of the insulation system can help control the development of CUI. HP


a FOAMGLAS® Insulation Sealed System


  1. Koch, G., J. Varney,N. Thompson, O. Moghissi, M. Gould, J. Payer, et al., “International measures of prevention, application, and economics of corrosion technologies study,” NACE International, 2016, online: http://impact.nace.org/executive-summary.aspx
  2. Eltai, E., F. Musharavati, E., Mahdi, et al., “Severity of corrosion under insulation (CUI) to structures and strategies to detect it,” Corrosion Reviews, 37(6), 553-564,” September 2019, online, https://doi.org/10.1515/corrrev-2018-0102
  3. Sampaio, A., L. Leite, et al., “More lessons re-learned from corrosion under insulation,” Dow Chemical, 2018, online: www.penderlo.com/doc/Dow_CUI.pdf
  4. U.S. Chemical Safety and Hazard Investigation Board., “E.I. DuPont de Nemours & Co. Inc. Investigation Report: Final Report (Report No. 2010-6-I-WV),” CSB, September 2011, online: https://www.csb.gov/assets/1/20/csb%20final%20report.pdf?13966
  5. Wood, M., A. Vetere Arellano, L. Van Wijk, et al., “Corrosion-related accidents in petroleum refineries: Lessons learned from accidents in EU and OECD countries,” JRC Scientific and Policy reports, 2013, online: https://minerva.jrc.ec.europa.eu/EN/content/minerva/51beddd7-1149-4230-928d-a225bf39471a/tr01corrosionrefineriespdf
  6.  “NACE Standard Practice control of corrosion under thermal insulation and fireproofing materials—A systems approach,” 2010.
  7. RJ Lee Group, “FOAMGLAS® insulation sealed system for prevention of corrosion under insulation (CUI) testing, TMH1048815,” prepared for Owens Corning, March 2020.
  8. ASTM International, “Standard practice for operating salt spray (fog) apparatus (ASTM B117-19),” 2019, online: https://www.astm.org/b0117-19.html
  9. ISO., “Acoustic insulation for pipes, valves, and flanges (ISO 15665:2003),” 2003, online: https://www.iso.org/standard/28629.html

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