Technical challenges in assessing corrosion under insulation

In the oil and gas industry, corrosion under insulation (CUI) represents one of the most critical threats to asset integrity. This form of degradation, which is hidden and progresses rapidly, affects pipes, vessels, and tanks subjected to thermal cycling or exposed to ambient humidity. According to the AMPP (formerly NACE), between 40% and 60% of industrial maintenance costs are related to CUI; therefore, detection and mitigation are a priority for process plants, refineries, and offshore platforms.

The objective of this article is to analyze the mechanisms, causes, and methods of detecting Corrosion Under Insulation (CUI) in industrial systems, integrating technological advances, international standards, and real-life cases. The article seeks to offer technical criteria for the assessment, prevention, and management of CUI risk, based on AMPP, API, and NACE guidelines, and on the operational experience of refineries, petrochemical plants, and transportation systems.

What is Corrosion Under Insulation (CUI)?

Corrosion Under Insulation (CUI) is a type of localized corrosion caused by leaks, precipitation, or condensation. It arises as a result of the interaction of water present between the insulation and the metal surface of pipes (See image), tanks, and other equipment.

It is common in the chemical/petrochemical, refining, offshore, and maritime industries. It is most commonly found in insulated carbon steel, low-alloy steel, and stainless steel equipment operating at high temperatures of 175 °F or less.

In the oil and gas industry, safety and operational efficiency are critical to ensuring a steady flow of hydrocarbons in the supply chain. However, Corrosion Under Insulation (CUI) causes leaks, equipment failures, prolonged downtime for repairs or replacements, and safety and environmental issues. In this context, predictive techniques offer solutions for risk detection and mitigation.

Types of Corrosion Under Insulation (CUI) and forms of degradation

Type of Corrosion	Promoting Condition	Technical Description
Galvanic Corrosion	Insulations containing conductive particles	Potential difference between dissimilar metals and moist contaminants.
Chloride-Induced Corrosion (ECSCC)	Presence of chlorides and temperatures above 60 °C	Stress corrosion cracking in stainless steels.
Alkaline Corrosion	Materials containing cement or hydroxides	Formation of alkaline hydroxides that attack protective coatings.
Salt-Saturation Corrosion	Coastal areas or industrial brines	Deposition of hygroscopic salts that retain moisture.

Causes of Corrosion Under Insulation (CUI)

The main causes of this type of damage include the following:

Thermodynamic factors and critical temperature range

From a thermodynamic point of view, Corrosion Under Insulation (CUI) develops when three essential factors converge: retained moisture, the presence of oxygen, and a temperature within the range susceptible to corrosion.

According to API 570 and API RP 583, carbon steel pipes are at greatest risk of CUI in the range of –4°C to 121°C (25°F to 250°F). In this range, atmospheric moisture repeatedly condenses on the metal surface, causing cycles of wetting and drying that accelerate the mechanisms of uniform corrosion and localized pitting corrosion.

In stainless steels, exposure to chlorinated contaminants can trigger chloride-induced stress cracking (ECSCC), affecting the structural integrity of the material and compromising the service life of assets.

Effect of thermal cycles and intermittent operation

Variable or intermittent operating conditions (thermal cycling) are one of the main aggravating factors of CUI. The repetitive expansion and contraction of the insulation generates microcracks in the coatings and seals, allowing water to enter during each thermal cycle.

During periods of shutdown, maintenance, or equipment shutdown, insulation can become saturated by rain, deluge systems, condensation, or drift from cooling towers, accumulating moisture that remains trapped.

When the system returns to operation, that moisture is thermally activated, creating an electrochemical environment conducive to accelerated corrosion, even in lines operating at temperatures above 315°C (600°F), where moisture would not be expected to have an effect.

Chemical influence and compatibility of insulating materials

In addition to water, the chemical composition of the environment plays a decisive role in the severity of CUI. The presence of chloride ions, sulfate ions, or hygroscopic salts (common in coastal atmospheres or industrial processes) increases electrical conductivity and activates combined mechanisms of galvanic, alkaline, and pitting corrosion.

Insulating materials that do not meet ASTM C795 or NACE SP0198 specifications can release aggressive chemical compounds, such as alkali silicates or fluorosilicates, which accelerate the degradation of the base metal. In complex industrial environments, these chemical interactions can be intensified by the presence of process contaminants, minor vapor leaks, or exposure to saline cleaning products.

Weather barrier system failures

The weather barrier system is the first line of defense against moisture intrusion, and its performance depends on both design and installation quality. Low thermal resistance coatings, poorly sealed joints or seams, unprotected pipe penetrations, and poorly sealed anchor points are the most common routes for water intrusion.

In older installations, where mineral or fiberglass insulation without a hydrophobic coating was used, capillary water absorption multiplies moisture retention and prolongs the time the metal is exposed to the saturated environment. This phenomenon is particularly critical in horizontal lines or low-slope areas, where natural water drainage is insufficient.

Inspection methods for detecting CUI

Detecting corrosion under insulation (CUI) represents one of the greatest challenges in asset integrity management, as deterioration occurs hidden from view and without visible external signs until the damage is advanced. Direct visual inspection, the most basic method, requires removing the insulation, which involves high operating costs, exposure risks, and downtime. Therefore, modern inspection strategies combine Non-Destructive Testing (NDT) methods, risk-based prioritization (RBI), and continuous monitoring technologies that allow CUI to be detected without completely removing the insulation.

1.Risk-Based Inspection (RBI) as a starting point

Before performing any physical inspection, the Risk-Based Inspection (RBI) methodology is essential for prioritizing critical areas. According to API 580/581, the areas most prone to CUI are determined by considering variables such as:

• Operating temperature range (25–350 °F).
• Insulation material and type of weather barrier.
• Frequency of exposure to water or condensation.
• Maintenance history and years in service.
• Equipment location (indoor, coastal, offshore, etc.).

The RBI approach allows resources to be optimized by directing inspections towards areas where the probability and consequence of damage are greatest, such as pipe supports, flanges, elbows, valves, drain pans, and penetrations.

2. Inspection: How to detect CUI?

Non-Destructive Testing (NDT) Inspection Methods

Once the critical areas have been defined, NDT methods are used to enable detection without removing the insulation, or by making minimal openings at strategic points. The most commonly used methods are listed below.

• Ultrasonic thickness measurement (conventional UT or A-scan): Applied through windows or removed insulation sections. It allows for high-precision determination of metal thickness loss, although it requires direct contact with the surface.
• Guided wave ultrasound (GWUT or LRUT): Uses longitudinal elastic waves that propagate along the pipe, detecting discontinuities and corrosion up to 30 m in both directions from a single measurement point. It is especially useful on long or hard-to-access lines.
• X-ray or gamma ray radiography (RT): Allows corrosion, pitting, or material loss to be visualized without removing the insulation, provided the coating thickness is moderate. However, it requires radiological safety control and does not distinguish well between moisture and degraded metal.
• Pulsed Eddy Currents (PEC): An emerging electromagnetic technique that allows thickness loss to be measured through insulation and metal coatings. Its advantage is that it does not require direct contact with the metal or removal of the insulation, making it widely used in systems in service.
• Infrared Thermography (IR): Detects surface temperature differences associated with the presence of moisture or metal loss under insulation. It is ideal for wide-ranging inspections and early detection of water infiltration.
• Electrochemical and Digital Radiography Techniques: Applied to critical equipment. They provide corrosion maps located in inaccessible areas.

Each method has advantages and limitations depending on system conditions, insulation type, and the level of accuracy required. Therefore, the most effective strategies combine advanced techniques with a hybrid approach, integrating data into digital platforms to generate a more reliable condition map.

3. Monitoring and predictive maintenance

In highly critical plants, corrosion and humidity sensors embedded in the insulation are being implemented, allowing real-time monitoring of conditions conducive to CUI. These sensors can measure conductivity, relative humidity, temperature, and corrosion potential, sending early warnings when parameters exceed set thresholds.

In addition, advanced non-destructive testing techniques, Digital Twin systems, and AI-powered robotic inspection (using drones or crawlers) are revolutionizing detection by integrating thermal imaging, remote ultrasound, and predictive modeling, reducing the need for intrusive inspections and maximizing asset availability¹_. Using 3D photogrammetry and advanced thermal analysis, it is possible to detect temperature anomalies associated with moisture under insulation, even on complex or coated surfaces.

How to prevent corrosion under insulation (CUI)?

Preventing CUI is one of the biggest challenges in asset integrity management, especially in industries where continuous operation and harsh environmental conditions increase the risk of moisture infiltration in insulated systems. According to AMPP and API RP 583 guidelines, the most effective strategy is not damage correction, but proactive prevention through design, material selection, installation, and maintenance.

1.Systemic approach to prevention

CUI must be addressed as a systemic problem, not as an isolated phenomenon. This involves simultaneously applying three integrated lines of defense:

• Protective pipe coatings,
• Well-designed weather barriers or weather jackets, and
• Certified thermal insulation for CUI environments.

The fundamental principle is to prevent water from entering and remaining on the metal surface. When the system is correctly specified, sealed, and maintained, the risk of corrosion can be substantially reduced, even in coastal environments or petrochemical plants with recurrent condensation.

2. Design and material selection

The design should be geared toward minimizing the possibility of moisture accumulation and facilitating its natural drainage or evaporation. Hydrophobic insulators, such as silica aerogels, polyisocyanurate (PIR) foams, or silicone-treated mineral wool, are preferred for their low capillary absorption and superior thermal resistance.

In terms of coatings, NACE SP0198 and ISO 19277 recommend the use of immersion-grade coatings, especially heat-activated epoxies and Ceramic-Bonded Phosphate Coatings (CBPC), which are capable of maintaining their integrity under cyclic conditions of temperature and high humidity. These materials form a dense, adherent barrier that resists the penetration of water and chlorides.

External weather barriers must be designed with a focus on complete airtightness: metal joints must be crimped or sealed with industrial silicones, avoiding points of penetration without drainage or with dissimilar joints that generate galvanic couples. The use of hybrid jackets with polymer reinforcement offers a lightweight, resistant, and easily inspectable solution.

3. Control during installation

A well-designed system can fail if its installation does not follow quality control protocols. It is essential to ensure a continuous seal free of discontinuities, especially at flanges, supports, valves, and joints.

During the application of coatings, surface preparation in accordance with NACE No. 2 / SSPC-SP10 (near white metal blast cleaning) and complete drying prior to closure of the insulation must be ensured. Installers must verify that materials are certified as “low-chloride,” as chloride or sulfate residues in the insulation can induce localized corrosion or stress cracking (SCC).

4. Risk-based inspection and maintenance (RBI)

Effective prevention requires a continuous, risk-based inspection strategy. The RBI (Risk-Based Inspection) approach, recommended by API 580/581, allows critical areas to be prioritized based on operating temperature, insulation type, environmental exposure, and historical failure records.

The points most prone to deterioration include pipe supports, flanged joints, insulation penetrations, and horizontal areas without drainage. Proactive maintenance should include non-destructive testing (NDT) using guided ultrasound (GUL), infrared thermography, and digital radiography, which allow trapped moisture and thickness loss to be detected without removing the entire insulation.

5. Emerging mitigation technologies

The development of new technological solutions has improved the ability to mitigate CUI from design and operation. Among the most notable are:

• Heat-Activated Epoxy Coatings: high-temperature curable epoxy coatings that create a dense film resistant to moisture and chlorides.
• Hybrid Jackets and Aerogel Blankets: closed-cell insulation that reduces capillary absorption and prevents condensation infiltration.
• Digital Twin and IoT Monitoring: digital monitoring systems that integrate humidity and temperature sensors, generating predictive models of degradation in real time.
• Smart Coatings: capable of changing color or emitting electrochemical signals when detecting incipient corrosion under the insulation.

These technologies, combined with predictive analysis based on historical data, enable the detection of risk patterns before failure, optimizing maintenance strategies and extending the useful life of assets.

6. Preventive maintenance and operational integrity

Maintenance should focus on preventive and predictive actions rather than reactive corrections. This includes:

• Periodic review of the condition of the sealing and drainage of roofs.
• Replacement of contaminated or degraded insulation.
• Monitoring of residual moisture after interventions or cleaning.
• Reapplication of coatings in areas with repeated mechanical or thermal exposure.

In this way, comprehensive CUI management becomes an ongoing practice involving design, operation, and inspection, allowing the integrity of the system to be maintained throughout its useful life cycle.

Codes, standards, and best practices

Effective control of Corrosion Under Insulation (CUI) depends not only on the application of coatings or advanced inspections, but also on strict compliance with international mechanical integrity standards. The guidelines published by API, AMPP (formerly NACE), and ASTM International provide the technical framework for standardizing inspection, maintenance, repair, and risk assessment procedures, ensuring that industrial plants maintain optimal levels of safety and operational reliability.

The following are the most relevant codes and standards for comprehensive CUI management:

API Standards Inspection of Corrosion Under Insulation (CUI)

1.API 510: Pressure Vessel Inspection Code

API 510 is one of the fundamental codes for the inspection and management of pressure vessels in service. Its scope covers the evaluation, repair, alteration, and recertification of pressure equipment, along with associated relief devices.

Section 5.5.6 specifically addresses CUI inspection, establishing frequency criteria and recommended methods for evaluating critical areas where thermal insulation can retain moisture. This standard emphasizes the importance of correlating operating conditions (temperature, insulation type, environment) with corrosion risk, and promotes the use of Non-Destructive Testing (NDT) such as ultrasound, digital radiography, or pulsed eddy current (PEC) for early detection. It applies to refineries, petrochemical plants, and liquefied natural gas (LNG) plants, where pressure vessels operate under variable thermal cycles and aggressive environments.

2. API 570: Piping Inspection Code

API 570 regulates the inspection, repair, and alteration of in-service piping systems, providing guidelines for determining areas most susceptible to CUI. In Section 5.8, this standard identifies the most common locations where corrosion under insulation occurs:

• Welded and flanged joints.
• Pipe supports and low points without drainage.
• Horizontal sections with frequent condensation.

API 570 emphasizes the need to apply a risk-based inspection (RBI) approach and to record inspection results historically. In addition, it requires that inspections be performed by inspectors certified under API 570 or API 510, ensuring traceability and consistency in asset evaluation.

3. API 574: Inspection Practices for Piping System Components

API 574 complements API 570 by providing best practices for inspecting pipeline system components, including valves, joints, supports, and fittings.

Section 6.3.3 addresses corrosion under insulation and details inspection planning, recommended intervals, and the most effective methods for evaluating thickness loss. Its value lies in offering practical criteria for inspectors to correlate visual findings, NDT data, and operating conditions in complex systems. This standard is widely used in Mechanical Integrity (MI) programs within the OSHA PSM (Process Safety Management) framework.

4. API RP 583 – Corrosion Under Insulation and Fireproofing

API RP 583 is the most specific and modern guide focused directly on CUI and CUF (Corrosion Under Fireproofing). First published in 2014, its objective is to provide a systematic approach to the design, maintenance, inspection, and mitigation of CUI in metallic equipment covered with thermal insulation or fireproofing systems. It includes guidelines on:

• Environmental factors that accelerate damage (moisture, salts, temperature).
• Proper selection of insulating materials and coatings.
• Non-destructive testing (NDT) methods.
• Risk-based mitigation and repair strategies.

NACE Corrosion Under Insulation Standards

1. NACE SP0198-2010 Control of Corrosion Under Thermal Insulation

The NACE SP0198 standard, currently administered by AMPP, establishes a systemic approach to the prevention and control of CUI, defining best practices from insulation selection to periodic in-service inspection. Its recommendations include:

• Verification of halide-free thermal insulation (low-chloride insulation).
• Chemical compatibility requirements between insulation and base metal.
• Control of moisture content and permeability of insulation.
• Sealing and drainage methods in weather barriers.

NACE SP0198 replaced the former RP198 and is used globally as a reference for integrity programs in refineries and cryogenic plants.

ASTM: Testing and Evaluation of Insulation Materials

The ASTM standards complement the API and NACE guidelines by providing standardized methods for evaluating the chemical and physical behavior of insulation materials. Among the most relevant are:

1.ASTM C795-08 (2018): Standard specification for thermal insulation compatible with austenitic stainless steels. It evaluates the tendency of insulation to release chloride or fluoride ions that can induce stress corrosion cracking (ECSCC).

2. ASTM C871-18: describes chemical methods for analyzing the leaching of chloride, fluoride, silicate, and sodium ions in thermal insulation.

3. ASTM C1617-19: defines accelerated laboratory tests to determine the influence of thermal insulation on the aqueous corrosion of steels and alloys.

4. ASTM C1763-20: measures water absorption by immersion, a key parameter for evaluating the suitability of insulation in conditions of high humidity or direct exposure to water.

5. ASTM STP 880: provides a technical compendium on the mechanisms of corrosion under thermal insulation in metals and industrial equipment, and has been a fundamental reference since its first edition in 1985.

Best practices and cross-references

Industry best practices recommend integrating these standards into an Asset Integrity Management System that combines predictive inspection, digital monitoring, and planned maintenance, in accordance with API RP 583, API 570, and NACE SP0198 guidelines. For example:

• Inspectioneering (2023) documents cases where the joint application of API RP 583 and NACE SP0198 reduced CUI failures in process lines at a Gulf of Mexico refinery by 60%, demonstrating the importance of risk-based inspection and coordinated preventive maintenance. See also the Inspenet article on Risk-Based Inspection (RBI) and Fitness-for-Service (FFS) Assessment.

• Belzona (2022) reports the successful use of CBPC (Ceramic-Bonded Phosphate Coatings) combined with aerogel insulation, extending the service life of heat exchangers by more than 10 years with no evidence of corrosion. Related: Advanced protective coatings for severe environments.

• Predictiva (2024) highlights the integration of IoT sensors and moisture analysis using artificial intelligence (AI), capable of predicting the progression of CUI in real time and optimizing inspection intervals. See also: Digital transformation in structural integrity monitoring.

These experiences reinforce the need to combine international standards, emerging technologies, and a culture of maintenance as pillars for effective corrosion mitigation under insulation.

Conclusions

Effective detection and mitigation of CUI is important in the oil and gas industry. Investment in online detection technology has a significant impact on improving safety, reducing costs, and minimizing adverse effects on the environment.

The combination of different inspection techniques is essential to obtain a complete assessment of asset integrity. The lack of an accurate detection technique and the need for risk assessment and prediction tools are challenges that must be addressed in the industry to ensure safe and efficient operations in a highly competitive environment.

Compliance with standards such as API 510, 570, 574, and RP 583, along with NACE SP0198 and ASTM standards, provides a solid technical framework for proactive CUI management. These guidelines not only define inspection and material selection procedures, but also promote a comprehensive approach based on risk, compatibility, and durability, which is key to protecting industrial assets from one of the most costly and difficult-to-detect damage mechanisms.

References

1. Javaherdashti, R. (2014). Corrosion under insulation (CUI): A review of essential knowledge and practice. International Journal of Computational Materials Science and Surface Engineering, 5(1), 3–24.
2. Contreras, J. (2022, octubre 10). Técnicas avanzadas de END para la detección de corrosión bajo aislamiento (CUI). Inspenet. https://inspenet.com/articulo/deteccion-corrosion-bajo-aislamiento-parte-1/