CUI Double Whammy: Selection of Coating Systems for CUI Service
An advanced understanding will help corrosion experts mitigate corrosion under insulation (CUI) by minimizing the duration of insulation wetness.
For those with no real-world experience in oil & gas refining environments or with corrosion under insulation (CUI), the white paper, “Selection of Coating Systems for CUI Service” by Christian Zupicich and Michael MeLampy, is a good introduction to this complex problem area for corrosion professionals.
The paper covers the environments to which CUI coatings must be resistant and the various types of CUI coatings systems from which a specifier can choose. It also covers often-overlooked characteristics that can add or detract significantly from the true value of a coating system.
And while a single paper can only go so far, there are still some crucial gaps in explaining the mechanisms that come into play regarding thermal insulation and CUI.
Characteristics of various types of insulation materials—such as pH, water absorbency, dry-out kinetics and damage resistance—play a critical role in the overall functionality of the insulation, the environment to which the coating will be exposed, and the risk level of CUI occurrence.
Considerations in Coatings Selection
With respect to coatings, the paper’s explanation of the suitability of the various types of coatings used for CUI service, now and in the past, is excellent. But when choosing a coating system for any long-term use, including CUI protection, there are other important aspects that a specifier must consider, such as:
- Shelf life and storage requirements: This may play a role in how readily available some coatings may be in some locations.
- Installation/curing environment: Is there a need for climate control during application, which may occur during winter months or a rainy season?
- Availability of skilled applicators: Some coatings take practice to "get it right." In locations with "local content" requirements regarding a workforce, will additional training need to be provided/budgeted?
- Availability of coatings inspectors: Without third-party inspection, quality assurance almost never happens. Proper expertise and background is essential. (To learn more about inspections and quality, read What Does Quality Control Mean in the Corrosion and Coatings Industry?)
The paper covers the general background on insulation and cladding so that the reader can see that the insulation system (either by damage, poor design, installation error or general aging) will sooner or later allow water in—and water creates an opportunity for corrosion of a steel substrate to occur.
Wet Insulation is No Insulation
It is important for the novice to understand that corrosion of unprotected or improperly protected steel resulting from wet insulation is a "side effect" of something more important.
You might ask who the importance of certain underlying issues depends on. Everyone agrees that in an oil & gas or chemical refining environment, unseen corrosion can cause a catastrophic and deadly calamity. So in cases of severe corrosion, where some equipment has been overlooked and not inspected for decades, CUI is the most important problem.
In most cases, the metal loss of CUI is expensive to repair. It is a sign that the insulation has been wet for long periods of time, and an indication that the process equipment has not been operating energy efficiently. Other root causes include process-related issues, such as product quality (meets specifications) and stability of the operation, especially during and after rain events, etc.
When the insulation is wet enough to cause corrosion of the steel substrate, it has nearly zero insulation value. So, even when the moisture exists only in the outer reaches of the insulation thickness, the insulation value has been severely deteriorated. Even small amounts of moisture have a severe impact on the thermal conductivity of insulation. (This concept is discussed in more detail in The Detrimental Effects of Wet Insulation in the CUI Range.)
Since the temperature range for CUI is on the low end of the hot service operating temperature spectrum (25–300°F, or -3.9–150°C), moisture remains inside the insulation itself, not just under it, when the insulation is the water-absorbent type.
The higher the volume percentage of water inside the insulation, the more process heat energy is needed to increase that water temperature to the point of vaporization. But the accumulation of that heat energy (robbed from the equipment) is counteracted by the insulation's thermal conductivity, which has been severely deteriorated by the moisture. This creates an unfortunate “double whammy.”
As an excellent beginning for a reader who is getting started in understanding CUI, this paper covers the basics of selecting CUI coatings very well. Of course, there is much more to CUI.
An understanding of the dynamics of what is happening inside the insulation and how it is affecting the bottom line of operating facilities is crucial to achieving the energy conservation intended by the insulation in the first place. An advanced understanding will help corrosion experts mitigate CUI by minimizing the duration of insulation wetness.
We hope that this excellent introductory white paper opens the door to further study by interested readers, because our industry is truly in need of this type of expertise.