Corrosion is one of the main factors causing deterioration of industrial assets worldwide, with an estimated economic impact of $2.2 trillion annually. Approximately 45% of these losses occur in the oil, gas, and petrochemical sectors, in both onshore and offshore facilities. Within this context, corrosion under insulation (CUI) and corrosion under fire protection (CUF) are critical mechanisms due to their hidden nature and the difficulty of early detection.
This technical document aims to describe the mechanisms of CUI and CUF, identify the areas most prone to this type of deterioration, analyze the associated risks, and present criteria for selecting appropriate protective coatings for maintenance and new construction projects, in accordance with the guidelines of NACE SP0198.
1. What is corrosion under insulation (CUI/CUF) and what are its mechanisms?
Corrosion under insulation (CUI) is a phenomenon that occurs in pipes, tanks, and equipment operating with some type of thermal insulation or fireproof coating when water or moisture becomes trapped between the metal substrate and the insulation system. This process can occur under low-temperature conditions, sweating service, high temperatures, or cyclical temperature regimes.
Moisture ingress is usually caused by failures in the insulation's waterproofing system, faulty installations, mechanical damage during operation, or natural deterioration due to aging. Water sources include rain, deluge systems, steam leaks, process spills, and ambient condensation on the metal surface.
From an electrochemical standpoint, CUI develops when the metal remains exposed to moisture for extended periods within a specific temperature range. The water that comes into contact with the heated substrate evaporates, condenses again, and restarts the cycle, generating an accumulation of contaminants and progressive degradation of the coating. This phenomenon not only affects the steel or base material but also deteriorates the insulation itself.
The CUF, for its part, is considered analogous to CUI in terms of damage mechanism and morphology, occurring under passive fire protection systems where significant thermal gradients exist.
2. Characteristic Damage and Temperature Ranges Associated with CUI
The corrosion rate depends on multiple factors, including the duration and frequency of moisture exposure cycles, the aggressiveness of the aqueous environment, and the failure of protective barriers, such as paint or the external coating of the insulation.
3. Most Common Affected Areas CUI and CUF
primarily affect equipment and structures such as pressure vessels, piping, piping systems, flanges, valves, storage tanks, and structural supports. The most susceptible points include: Equipment with damaged or poorly sealed insulation. Protrusions and penetrations through the insulation. Exposed external sheathing seams exposed to weathering, fog, or wind.
Areas with mechanical vibration that degrade the insulation system. Regions exposed to leaks or frequent vapor discharges. Areas subjected to rain, deluge systems, or sprinklers. Joints, flanges, support rings, pipe shoes, and anchor bolts. Components with irregular geometries that hinder waterproofing. Fireproof skirts and anchors covered by thermal insulation (CUF cases). Equipment design plays a determining role in the severity of CUI. Horizontal surfaces, support rings, structural reinforcements, and elements that are difficult to seal significantly increase the likelihood of water ingress and retention in the system.
4. Risks Associated with CUI
Failures Corrosion under insulation failures represent severe risks in terms of safety, health, the environment, and operational continuity. Catastrophic events have been documented in which process lines have failed due to CUI after decades of service, even in systems that transported non-corrosive fluids.
Sudden temperature changes during operation, such as cooling and regeneration cycles, promote the condensation and re-evaporation of atmospheric moisture on the metal surface. This type of scenario often goes unnoticed if the actual and changing operating conditions are not considered.
5. Coating Selection for CUI and CUF Control
Conventional organic paint systems have a variable service life (5-13 years). However, modern coatings based on inert multipolymer matrices or inorganic copolymers have demonstrated better performance and greater damage tolerance in CUI environments, as well as resistance to wider temperature ranges than traditional epoxies.The NACE SP0198 standard recognizes protective coatings as one of the most effective methods for protecting insulated steels.
5.1 Insulated Austenitic and Duplex Stainless Steels
These steels can be susceptible to stress corrosion cracking (SCCC) or liquid metal cracking (AML). To mitigate these risks, the systems must:
Not contain free soluble chlorides or other halides (according to ASTM C795 and ASTM C871). Not incorporate metallic zinc in their formulation. Be compatible with the service temperature and thermal cycling.
Supplement with aluminum foil wrapping as an additional barrier.
5.2 Carbon Steel Under Thermal Insulation and Heat Insulation
For carbon steels below 175 °C, systems formulated for tank service are recommended. The use of zinc-rich inorganic coatings is not considered a preferred option for prolonged or cyclic service in the typical CUI range due to the risk of galvanic inversion at elevated temperatures.
6. Aplika Solution for Maintenance and New Construction
Industrial maintenance demands solutions with high operational tolerance. Traditional epoxy phenolic coatings have curing limitations at low temperatures and low tolerance to over-application.
Within this context, the Interbond 1202UPC coating stands out. This two-component system provides CUI protection over a wide temperature range, from -196 °C to 650 °C. This product offers short minimum recoating intervals and requires no priming or heat curing, thanks to its inorganic copolymer chemistry.
