1.
What is Corrosion Under Insulation?
1.1.
What Components Are Prone to Corrosion Under Insulation?
1.2.
What Causes Corrosion Under Insulation?
2.
Corrosion Under Insulation Testing Methods
2.1.
Pulsed Eddy Current
2.2.
Infrared Thermography
2.3.
Real-Time Radiography
2.4.
Neutron backscattering
3.
Corrosion Under Insulation Mitigation Strategies
4.
Conclusion
Insulation protects assets from environmental pollutants, prevents energy loss, and shields personnel from hazardous exposures. While necessary by all means, insulation can also cause some nuisances: Trapped moisture can act as a perfect catalyst for corrosion.
Corrosion under insulation (CUI) is a major problem. It accounts for 40% to 60% of pipe maintenance costs in the oil and gas industry and causes major losses in other industries.Why such high rates? Early CUI detection and prevention are difficult — yet not altogether impossible.
Learn what causes CUI in the first place, which detection methods work best, and what you can do to prevent it.
Corrosion under insulation occurs when moisture accumulates between metal surfaces and insulation. Depending on the material, CUI can manifest as localized corrosion (on carbon steels), pitting (on aluminum alloys), or stress corrosion cracking (on austenitic stainless steels (SS).
The challenge with CUI is that the insulation layer may appear visually intact, yet hide rust or corrosion underneath. A microcrack or minute thinning allows the water to seep in and ruin the underlying surface.
Because of this, CUI may go undetected for years on end until it results in devastating consequences. CUI caused 50% of hydrocarbon leaks at onshore plants over the last two decades.
Corrosion under insulation usually affects carbon steel, low-alloy steel, austenitic stainless steel, and aluminum assets the most. That includes storage tanks, pipelines, and pressure vessels used in oil and gas, chemical, nuclear, and fertilizer industries, among others.
Given high enough temperatures and long enough moisture exposure, CUI can affect any insulated asset. However, some components are more prone to CUI due to constant vibrations, temperature changes, acid vapors, and tensile stresses. Moreover, pieces with irregular shapes carry greater risks.
| Assets | Susceptible components | Standards |
| Pressure vessel | Stiffening rings Fireproofed skirts Leg support on smaller vessels Ladder clips, pipe supports Anchor bolts Top and bottom heads Davit arm supports, lifting lugs, body flanges | API 510API RP 583 |
| Piping systems | Dead legs, vents, and drains Pipe hangers Valves and fittings Insulation plugs Steam-tracing tubing penetrations Branch connections overlapped with larger diameter lines | API 570, API RP 574 |
| Cold piping | Pipe supports Pipe-to-flange connections Flanges with stud bolts Vapor retarder or jacket | API RP 583 |
| Storage tanks and spheres | Areas above chime Stairway tread attachments Insulation support rings Fireproofed legs Nozzles and brackets on shell and roof | API RP 583 |
Corrosion under insulation needs just two conditions to thrive: moisture presence and temperature exposure.
Moisture can come from seawater (for offshore structures), poor weather (rain, snow, sleet), condensation, leaks from deluge systems and steam tracings, or high humidity in general.
For damage to occur, the temperature must reach a certain level, which varies from metal to metal. According to the API RP 583, the risk of CUI increases for carbon and low-alloy steels at 77-110 °C (170-230°F), for austenitic SS – at 60-177°C (140-350°F), and duplex SS – 140°C and higher. Meanwhile, as per CINI and NACE, the risk of CUI becomes high between 50-175°C.
CUI can progress faster if the insulation is worn, insufficiently sealed, or has other structural flaws (like missing jacketing or damaged insulation plugs). Plus, there are many additional factors that you should be on the lookout for:
Although considered a type of external damage, CUI is difficult to locate with routine visual inspections. The usual way is removing the protective layer, cleaning the surface, and examining it. This is expensive, time-consuming, and disruptive, so many companies postponed this.
Soundly, new methods don’t require such costly preparations, lengthy downtimes, and insulation removals. Modern non-destructive testing tools can estimate the exact location and extent of corrosion without damaging the protective layers.
The best approaches for CUI detection are pulsed eddy current testing, infrared thermography, neutron backscatter analysis, and radiography.
Pulsed eddy current (PEC) testing leverages short, high-energy electromagnetic pulses that can pass through any non-conductive material (including insulation and coatings). The pulse penetrates the entire thickness of the object and generates a magnetic field, measured by the probe. Measurement discrepancies indicate wall thinning and metal loss.
The core benefit of pulsed eddy current testing is that it doesn’t require direct surface contact and can cover large areas in one go. PEC inspections are feasible even at temperatures between -100-500° C (-150-932°F), so no asset shutdown is needed. The method shows great result repeatability, with up to 10% accuracy for corrosion detection and a 0.2% accuracy rate for ongoing corrosion monitoring.
Voliro’s newest Pulsed Eddy Current payload lets you detect the early stages of CUI on aluminum jackets, low-carbon steel pipes, and storage tanks. With a lift-off distance of 100 mm – literally on the fly – the probe measures material thicknesses with an accuracy down to 3-18 mm. Thanks to embedded noise shielding, noise canceling, and specialized algorithms, our PEC payload can penetrate through thick insulation (up to 100 mm) without losing sensitivity.
The caveat: PEC only works for carbon and low-alloy steels. Also, it detects general metal thinning but not isolated corrosion pits.
Infrared thermography (IRT) measures the infrared radiation emissions and generates a surface heat map. Since dry and wet insulation has various temperature profiles, IRM can reveal CUI-prone areas. Conducting IRT 2-3 hours after sundown is best since wet insulation retains temperatures longer than dry one.
It’s important to understand that IRT identifies wet spots, not necessarily corrosion. We can only assume corrosion exists or is likely to occur at the highlighted locations. This way, IRT is best for preventive maintenance.
Like PEC, IRT requires no direct surface contact. Also, it provides very accurate measurements, pinpointing the smallest temperature fluctuations (down to several degrees).
However, environmental factors can affect IRT readings. Some surfaces may reflect radiation from other sources, like sunlight, creating artificial hot spots. Air humidity makes the surface temperature appear higher than it actually is. Also, IRT loses its accuracy when the insulation is too thick.
Modern inspection drones with IR cameras streamline inspections of large industrial sites and elevated assets. You can capture accurate footage of hotspots before dispatching an inspector to verify the issue with extra means.
Real-time radiography (RTR), or fluoroscopy, is a digital radiography method that sends x-rays or gamma rays to the tested object and converts these signals into high-resolution images. Unlike traditional radiography, RTR produces images electronically rather than on film, enabling faster results.
The X-ray RTR equipment uses low-frequency radiation that penetrates through insulation, rather than walls. This lets you envision the asset’s outer wall and see corrosion patterns. According to API RP 583, to detect CUI on insulated pipe, your inspectors might need to maneuver the RTR instrument 360°around a pipe.
The core benefit of RTR is that it operates with multiple radiation arrays and covers a wide area at once. Images are often accessible in real-time, on an embedded video display. The limitations are that RTR might produce inconclusive results when operating on wet insulation. Also, it requires clearance of up to 30 cm (12in) for pipelines in crowded areas.
Neutron backscattering uses radioactive power (usually americium 24) to emit high-energy neutrons into the insulation. When these neutrons collide with hydrogen atoms, they release energy and lose speed. By counting these low-energy neutrons, the spectrometer can determine the amount of moisture in the insulation. The more neutrons – the more water. Sometimes, infrared thermography is used to validate the results of neutron backscattering.
Neutron backscattering is great at finding moisture under insulation, which can help you timely dry it out, remove the source, and reduce risks of potential CUI. On the other hand, the method is prone to false positives: wet insulation does not necessarily mean corrosion. Dry insulation isn’t a guarantee of immunity to corrosion (as water might have been evaporated during high-temp operations.
Proper design, installation, inspection, and maintenance increase assets’ resistance to damage and water ingress, making them less prone to CUI.
Generally, the more sophisticated your asset’s design and installation, the more complicated its insulation and CUI inspection will be. Place assets in areas where other objects do not obstruct your access if you cannot control this aspect to accommodate more frequent inspections.
Other aids for CUI prevention and mitigation include:
Without the right inspection equipment, CUI can remain undetected for years until it manifests as a leak or severe asset degradation. Take a lesson from Marathon Oil, whose piping ruptured due to a concealed CUI. The incident caused a release of high-pressure methane gas and subjected the company to $1.16 million in fines. According to Britain’s regulator, frequent inspections and maintenance would have prevented this incident.
Voliro T can help you detect CUI at early stages with more frequent inspections. Avoid scaffolding construction or rope-climbing to access elevated assets, reliably testing angled or curved elements. Using our range of NDT payloads, you can test everything from the early stages of corrosion to metal degradation and wall thinning.
