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  • Writer's pictureEagle Insulation Company

(CUI) Part II – Selection of Insulation Material

Updated: Oct 9, 2021

In the first installment of our blog post series on Corrosion Under Insulation we discussed the ingredients necessary for corrosion under insulation (CUI) to occur:

  • Water

  • Oxygen

  • Corrosive chemicals

  • Operating temperatures in the range of 100°F – 300°F (38°C – 149°C)

The article also noted four general areas in which the insulation industry and specifiers typically direct their efforts in order to control the presence of water:

  • Selection of the insulation material

  • Protective jacketing design

  • Protective coatings

  • Insulation system maintenance

As we continue our series, this post will focus on the selection of the insulation materials.

In industrial applications, the most commonly-used insulation materials for piping or equipment above ambient temperature include mineral fiber, calcium silicate, expanded perlite, and thin, flexible felts/blankets, e.g., aerogel and fumed silica.

Certain components contained in the binders used in the manufacture of mineral fiber insulations add a certain degree of water repellency to the final product. However, the material is still able to absorb a small amount of water through wicking between the fibers. In addition, at operating temperatures above 350°F (177°C), the organic binder undergoes oxidation, and the water repellency is greatly reduced in the areas of the material that experience the elevated temperatures. Mineral fiber insulations also have low compressive strengths, so damage to the protective jacket from foot traffic, ladders, etc. compromises the jacket seal and will allow for the intrusion of water (rain, etc.).

Calcium silicate insulations are cementicious (cement-like) and are, by their nature, wicking materials. Calcium silicate’s advantage is its compressive strength (100 psi, 690 kPa) meaning it can withstand day-to-day abuse much better than a fibrous / felt material and allows the protective jacketing to maintain its integrity under most conditions. (See our blog post “The Importance of Compressive Strength for Industrial Insulation” January 9, 2014)

There are also calcium silicates on the market (for the past seven years) that are inhibited, i.e., contain an additive in the formulation that acts to inhibit corrosion. This chemical inhibitor is infused throughout the material (as opposed to a surface coating), is non-organic and will not oxidize at elevated temperatures. Such a product will inhibit corrosion throughout its temperature-use range, but it is important to note that not all calcium silicate manufacturers add corrosion-inhibiting components to their formulations.

Expanded perlite pipe and equipment insulations are hydrophobic (they repel water), and while offering a higher level of compressive strength than mineral fiber materials (but lower than calcium silicate), they contain a small level of organics in their chemistry. At operating temperatures above 600°F (315°C), the organic compound will begin to oxidize, and the material’s ability to wick water may increase.

Thin, flexible felt/blankets (aerogel and microporous) are materials currently being used in certain industrial applications for insulating pipe and equipment. Aerogel blankets are fairly new materials (<10 years) whereas microporous materials have been used for over 25 years in the industrial market. These materials are manufactured with a hydrophobic component in their formulations, so they are embedded within the material (as opposed to being a surface coating).

However, this component is an organic-based material, and as it oxidizes at temperatures above 600°F (315°C), the material’s propensity to wick water also may increase. In addition, the felt materials exhibit low compressive strengths, so damage to their protective jacket from foot traffic, ladders, etc., tends to compromise the protective jacketing seal, facilitating the intrusion of water (rain, etc).

There is more to come in our Corrosion Under Insulation series; next we will be discussing the additional methods used to control the incursion of water in industrial insulations. There are also several articles related to CUI contained in the publication archives of the National Insulation Association

– See more at:

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