Corrosion under insulation (CUI)

Thermal insulation corrosion is an external degradation phenomenon that can affect many industries using thermal insulation to optimize the energy efficiency of their installations. This is external corrosion of low-alloy steel or austenitic stainless steel pipelines and reactors. It can occur between – 4 and + 175°C when water penetrates or condenses up to the interface between the thermal insulation and the external wall of the equipment. This water may contain contaminants (chloride ions, oxygen, etc.) which can initiate or accelerate corrosion. The presence of mechanical stresses can lead to wall cracking through stress corrosion phenomena, particularly in the case of austenitic stainless steels. Such corrosion is not visually detectable, and more often than not leads to leaks, or even explosions and fires. The economic consequences, as well as the impact on safety and the environment, can be considerable. Since thermal protection systems are installed for long periods of time (25 to 30 years or more), it is essential to design high-performance, reliable systems that take into account the entire life cycle, as well as regular inspection plans. For the latter in particular, the use of appropriate non-destructive testing systems is an essential element in maintaining the integrity of equipment throughout its life cycle.

The aim of this article is to provide the reader with a practical view of the problem of lagging corrosion encountered in the chemical and process industries, and in refineries, and thus contribute to enhancing the safety of these installations and extending their service life. The equipment in question is mainly piping and reactors, which may be pressurized. The materials used are mostly unalloyed or low-alloy steels, or stainless steels. Insulation systems can be used for a variety of purposes: preventing heat loss, protecting personnel, protecting installations in the event of fire... The types or materials of thermal insulation covered in this article are therefore extremely varied, and strongly depend on the intended application. The most frequently used systems are 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.).

In the first section of the article, the phenomenon of lagging corrosion is described, detailing its causes and manifestations, for non- or low-alloy steels and for stainless steels. Remedies are dealt with in a second section. They may relate to the material used for the equipment, the application of coatings, or the design of the thermal insulation system. Finally, methods for monitoring the health of installations throughout their operating life are described.