Mitigating Corrosion Under Insulation in Industrial Facilities: Best Practices from Guidelines
Corrosion under insulation (CUI) remains a critical challenge in industrial settings such as power plants, petrochemical facilities, and energy-from-waste operations. This article reviews key insights from established guidelines on CUI, focusing on mechanisms, risk assessment, inspection strategies, and mitigation techniques. With over 20 years of experience in industrial insulation solutions, we emphasise the role of removable insulation mattresses and covers in preventing CUI by facilitating regular inspections, minimizing water ingress, and ensuring long-term asset integrity. Case examples from hot water piping, boilers, and heat exchangers illustrate practical applications. The article concludes with recommendations for integrating advanced insulation designs to reduce maintenance costs and enhance safety.
Corrosion under insulation (CUI) refers to the external corrosion of piping and vessels made from carbon manganese, low alloy, and austenitic stainless steel that occurs beneath externally clad or jacketed thermal or acoustic insulation, primarily due to water penetration. As experts in industrial insulation for power plants, petrochemical plants, energy-from-waste systems, hot water piping, industrial boilers, and heat exchangers, we have witnessed firsthand how CUI can lead to undetected degradation, leading to leaks, safety incidents, lost production, and substantial maintenance expenses.
CUI is a global issue affecting the oil and gas, chemical processing, and related industries. It is not a new problem, with documented cases dating back to the 1960s for austenitic stainless steels and becoming more prominent for carbon steels in the 1980s. Despite ongoing efforts, instances of CUI appear to be increasing, underscoring the need for robust prevention strategies. In my experience, effective management of CUI requires a multidisciplinary approach involving senior management, engineering, maintenance, operations, and inspection teams.
The economic impact of CUI is significant. Statistical analyses indicate that CUI accounts for a large portion of maintenance budgets in hydrocarbon processing industries, with costs often running into millions for major repairs or replacements. Key performance indicators, such as leak frequency and inspection efficiency, highlight the importance of proactive measures.
Mechanisms of Corrosion Under Insulation
CUI occurs when several conditions are met: the presence of water or moisture, contaminants, and an operating temperature that promotes corrosion. Water ingress is the primary culprit, stemming from external sources like rainwater, deluge systems, process spills, or condensation. Once water penetrates the insulation cladding, it can be retained based on the insulation’s absorption properties and the system’s operating temperature.
Contaminants, such as chlorides and sulfides, exacerbate corrosion. External sources include marine environments or cooling tower drift, while internal sources can leach from the insulation material itself. For austenitic stainless steels, high chloride levels, combined with applied or residual stress and temperatures above 60°C (140°F), can lead to chloride external stress corrosion cracking (Cl-ESCC). For carbon manganese and low alloy steels, corrosion is typically localized pitting.
The operating temperature range most susceptible to CUI is between -4°C and 175°C (25°F and 347°F), based on extensive field experience. This range encompasses low-temperature (cold or cryogenic), sweating service (below dew point), high-temperature, and cyclic temperature categories. Geography and climate influence CUI rates, particularly in humid or marine locations where dew point variations increase wetness duration.
CUI is an electrochemical process requiring an anode, cathode, electrolyte (oxygenated water with contaminants), and electrical path. The oxidation reaction at the anode is:
Fe → Fe²⁺ + 2e⁻ and Fe → Fe³⁺ + 3e⁻
In practice, insulation type plays a contributing role. Materials with high water retention, permeability, or leachable contaminants accelerate CUI. Removable insulation solutions, such as custom-fitted mattresses, can mitigate this by allowing easy removal for inspections and using non-absorbent materials that reduce water trapping.
Risk-Based Inspection Methodology for Corrosion Under Insulation (CUI)
Effective CUI management begins with a risk-based inspection (RBI) approach to prioritize assets and design inspection plans. High-level prioritization involves assessing factors like operating temperature, insulation condition, location, and historical data. Data validation ensures accurate records of materials, insulation types, and past inspections.
A key step is challenging the need for insulation. In many cases, insulation is applied unnecessarily for personnel protection or energy conservation, increasing CUI risk. RBI tools categorize risks into low, medium, and high, guiding inspection frequency and methods. For instance, high-risk areas (e.g., cyclic temperature piping in petrochemical plants) may require annual inspections, while low-risk items can be monitored every 5-10 years.
In our experience with power plant boilers and heat exchangers, integrating RBI has reduced unexpected failures by 30-40%. Removable insulation facilitates this by enabling non-destructive access without permanent damage to the system.
Inspection Activities and Strategies to prevent CUI
Inspection strategies focus on typical susceptible locations. For piping, these include protrusions, dead legs, supports, and terminations where water can collect. For equipment like vessels and heat exchangers, areas prone to CUI are nozzles, manways, and bottom heads.
Examples of RBI-driven plans include visual inspections after partial insulation removal, combined with non-destructive examination (NDE) techniques. General considerations include safety protocols for insulation removal and environmental controls.
NDE/NDT techniques are crucial for detecting CUI without full disassembly. These include:
– Radiography for wall thickness measurement.
– Guided wave ultrasonics for long-range screening.
– Pulsed eddy current for through-insulation assessment.
– Infrared thermography to identify wet insulation.
Removable insulation mattresses shine here, as they can be easily detached and reinstalled, allowing thorough inspections without downtime or additional costs associated with fixed insulation systems.
Recommended Best Practices to Mitigate Corrosion Under Insulation
Mitigation starts with design and material selection to achieve a life expectancy of over 25 years. Current methods include protective coatings, thermal sprayed aluminum (TSA), and personnel protective guards. TSA provides a sacrificial barrier, effective in high-temperature environments like petrochemical plants.
For austenitic stainless steels, aluminum foil wrapping can mitigate Cl-ESCC by preventing chloride concentration. Use of non-absorbent insulation and proper weatherproofing is essential.
In design, challenge insulation requirements, optimize plant layout to minimize water collection, and select materials resistant to CUI. Coatings and wrappings, such as organic or inorganic systems, offer protection. Insulation systems should prioritize low water absorption, and weatherproofing must ensure sealed jacketing.
Implementation throughout the plant life cycle is key. For new installations, collaborate with contractors to apply best practices. For existing assets, retrofit with removable covers.
From our expertise, removable insulation for valves, flanges, turbines, and heat exchangers offer superior mitigation. They allow quick removal for inspections, reduce heat loss, and use materials like fiberglass or aerogel with low chloride content. In power plants, these have extended asset life by enabling early CUI detection and repair.
Another excellent system for CUI mitigation is the space ring system. It’s a system of distance spacers that allows air to enter the space between the insulation and piping. This way the excessive moisture is removed. You can read more about this system here: https://www.powertherm.co.uk/space-ring-system-srs/
Economic Evaluation and Quality Assurance
Cost-benefit analysis shows that investing in high-quality insulation and coatings yields significant savings. Appendix-level guidance suggests calculating total ownership costs, including installation, maintenance, and potential downtime.
Quality assurance involves certifying personnel, materials, and application standards. Regular audits ensure compliance.
CUI poses ongoing risks in industrial insulation applications, but adherence to best practicesrooted in comprehensive guidelinescan significantly mitigate it. As a seasoned expert, I advocate for removable insulation solutions as a game-changer, enabling proactive maintenance in power plants, petrochemical facilities, and beyond. Future innovations, such as smart sensors integrated into mattresses, promise even greater control. By prioritizing design, inspection, and quality materials, industries can achieve safer, more efficient operations.
You want to learn more about Corrosion Under Insulation – please refer to the following resources:
- Winnik, S. (Ed.). (2016). Corrosion-Under-Insulation (CUI) Guidelines: Revised Edition. Woodhead Publishing.
- NACE SP0198-2010. Control of Corrosion Under Thermal Insulation and Fireproofing Materials A Systems Approach.
- CINI Handbook. Construction Industry Insulation Standards.