Corrosion under insulation (CUI)

Corrosion under thermal insulation is a form of external degradation that can affect many industries using thermal insulation to optimize the energy efficiency of their facilities. It involves external corrosion of pipelines and reactors made of low-alloy steel or austenitic stainless steel. It can occur between – 4 and +175 °C when water penetrates or condenses at the interface between the insulation and the outer wall of the equipment. This water may contain contaminants (chloride ions, oxygen, etc.) that can initiate or accelerate corrosion. The presence of mechanical stresses can lead to cracking of the walls due to stress corrosion cracking, particularly in austenitic stainless steels. This type of corrosion is not visually detectable and most often leads to leaks or even explosions and fires. The economic consequences, as well as the impacts on safety and the environment, can be significant. Since these thermal insulation systems are installed for long periods (25 to 30 years or even longer), it is necessary to design high-performance, reliable systems that take the entire life cycle into account, as well as to establish regular inspection plans. Particularly with regard to the latter, the use of appropriate non-destructive testing methods is an essential element in maintaining the integrity of the equipment throughout its life cycle.

The objective of this article is to present the reader with a practical perspective on the problem of corrosion under insulation encountered in the chemical and process industries and in refineries, thereby contributing to enhancing the safety of these facilities and extending their service life. The equipment in question consists primarily of piping and reactors, which may be under pressure. The materials used are overwhelmingly unalloyed or low-alloy steels or stainless steels. Several types of requirements may justify the use of thermal insulation systems: prevention of heat loss, protection of personnel, protection of facilities in the event of a fire, and so on. The types and materials of thermal insulation covered in this article are therefore extremely varied and depend heavily on the intended application. The most commonly used systems consist of mineral materials (glass, calcium silicate, perlite, etc.) in block or fibrous form, or cellular materials with a glass or organic matrix (polyurethane, polyisocyanurate, elastomer, etc.).

The first section of this article describes the phenomenon of corrosion under thermal insulation, detailing its causes and manifestations for unalloyed or low-alloy steels and for stainless steels. Remedies are discussed in a second section. These may involve the material used for the equipment, the application of coatings, or the design of the thermal insulation system. Finally, methods for...