Why Corrosion Under Insulation (CUI) Continues to be a Menace For the Energy Industry?

CUI 101

Corrosion under insulation (CUI) is a common phenomenon and a long-standing challenge across various industrial sectors such as power generation, refining, and chemical.

Let us discuss in a few bullet points relevant information of the many factors involved in CUI occurrence, effects, and consequences.

• CUI occurs because of a combination of factors including:
  • Moisture: Either internally from asset leakage, or externally due to breaks in the insulation cladding/jacketing.
  • Contaminants: Either chlorides and sulphides such as environmental-borne sources or produced by leaching from the insulation itself.
  • Temperature: This factor determines the rate of corrosion and can be classified into four categories: Low temperature, Below dew point (sweating service), High temperature, and Cyclic temperature. It is well-accepted that most CUI occurrences are within the range of −4℃ to 175℃. However, CUI occurrences have been reported outside this range.
• CUI results in localized asset’s metal loss, which depends predominantly on its material composition. For instance, in the case of carbon steel, the corrosion will be typically general and localised, while for the case of austenitic stainless and duplex stainless, the corrosion will take the form of pitting corrosion and stress corrosion cracking (SCC).
• CUI causes one of the most significant cost threats to asset owners. Industry data[1] suggests that 60% of pipe leaks are caused by CUI, and that its damage affects the cost of maintenance programmes by significantly increasing outage, scaffolding and insulation replacements.

In the next sections, we will discuss the mitigation approaches and management strategies to tackle CUI.

[1] Asset Integrity theme landscaping study. Final report. Oils & Gas UK. Technology Leadership Board. May 2016.

Is it possible to avoid CUI?

Eliminating water traps and unnecessary insulation are critical measures to avoid CUI. Additionally, these can be complemented with careful selection of equipment design, insulation system, coatings and considering environment-related issues.

For instance, the concentration of water between an asset surface and the insulation system can be minimised by reducing any complexity in the surface to be insulated and by considering the support and connections of materials from different components such as flanges, tees, elbows, or reducers Also, by bearing in mind the consequences for CUI during any modification to the original design plan.

Additionally, following guidance about insulation specifications is fundamental to avoid CUI problems. It is widely accepted that insulations such as glass fibre, calcium silicate, and to some degree cellular plastic foams absorb and retain liquids and vapours, which helps in the of CUI from occurring.

Furthermore, special coatings can offer a protective barrier for assets to corrosion. As an example, thermally sprayed aluminium (TSA) coatings correctly applied to carbon and stainless steels have proven to be low-maintenance, long-term barrier systems for CUI mitigation, and are thus considered cost-effective from a total life cycle.

However, it must be said that is nearly impossible to guarantee that any (or all) the elements mentioned above will not fail at some point in time. Thus, a well-thought-out strategy to minimize the potential for CUI is just the first step. The second step is covered in the following section.

Strategies to tackle CUI

A dedicated assessment team with knowledge of the asset normally is tasked to conduct a thorough, unbiased risk assessment. Outputs from this process are known as Risk Based Inspection (RBI) plans and they constitute the work for inspection schedules. Below we will briefly discuss two approaches that asset owners historically have followed.

Some inspection plans make use of visual inspection to locate CUI, which requires partial or complete removal of the insulation. However, collateral costs associated with scaffolding, insulation removal, and reinstallation can be huge[2].

Others prefer to include in their plans Non-destructive testing (NDT) methods, which allow assessment without causing a major impact on the operation and that have a lower cost associated with their utilization. Commonly considered NDT methods include Radiography, Ultrasound, Pulsed Eddy Current (PEC) and Infrared Thermography.

These NDT methods are required to reliably detect and, in some cases, size the presence of CUI in a variety of geometries and conditions. However, the current experience with NDT methods is mixed. Mainly, because each method has its own set of capabilities and limitations making them suitable for application only to a certain set of components under specified conditions. In other words, there is still no silver bullet amongst the NDT methods.

In the following section, we will talk about a different tactic that has become available recently and that some asset owners see as the smartest approach.

CUI risk monitoring

A recent addition to the toolkit in this constant battle against CUI is a risk monitoring system that remotely detects and predicts CUI. The above-mentioned system is a high-performance patented solution from CorrosionRADAR, capable of continuously monitoring CUI in the most inaccessible locations, in assets with the most complex geometries and extreme/cyclic operating temperatures (-190C to +300C).

The system continuously assesses the potential risks due to CUI by combining existing knowledge of industry standards and live data from two probes, one for corrosion and the other for moisture. The data output is presented in the form of CUI Risk levels to guide inspection planning and help asset owners to improve the effectiveness of their inspection strategies.

This pragmatic approach to CUI management is data-driven and has already been adopted by of the world’s largest oil, gas and petrochemical companies to ensure a successful CUI management process. To learn more about this, click here to view the case studies available on our website.

[2] Guidance for in-situ inspection of Corrosion Under Insulation (CUI). HOIS-G-Issue 3, Jan 2023.

This article was written by Francisco Hernandez-Valle, Product Innovation Leader at CorrosionRADAR