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Non-Destructive Testing: Managing Corrosion Under Insulation

By Ana Benz, Fadi Basma and Mike Townsend, IRISNDT |

Corrosion under insulation (CUI) creates a pervasive and versatile challenge to the integrity of insulated equipment, but non-destructive inspection can help to avoid undesirable CUI surprises

Corrosion under insulation (CUI) has been occurring ever since industry started insulating equipment. However, most of the CUI reported until the 1970s developed in stainless steel due to stress-corrosion cracking. At that time, with limited insulation, carbon-steel equipment did not develop significant CUI [ 1]. Since then, with higher energy costs, carbon-steel piping, tanks and vessels have been more intensively insulated to save on energy costs, and as a result, they have begun to develop CUI.

CUI is a common problem for petroleum refineries, petrochemical facilities, power plants and chemical and fertilizer plants. One’s predisposition could be to consider CUI as a problem that is isolated only to sites in close proximity to a marine environment, but this is not always the case. CUI can happen in dry industrial sites as far away from the sea as the Canadian prairies (Figure 1). Once equipment is insulated, the proximity to cooling towers, leaking heat tracing, rain, mist from melting snow that saturates insulation and other leaks or steam can have an impact on CUI.


Figure 1. This horizontal, coated 0.375-in.-thick pipe leaked due to CUI after only four years of service

CUI can remain undetected until a leak develops or until the insulation and cladding or jacketing are removed or damaged, making CUI exceedingly challenging to find (Figure 2). Earlier reports stated that one single non-destructive examination (NDE) could not be used to identify CUI [ 2]. This remains the case today as engineers combine several NDE techniques to perform CUI assessments. In such assessments, corrosion rates are usually presented in mils per year (mpy), where one mil is equal to 1/1,000 of an inch. The techniques and strategies now used are summarized throughout this article, and several images are provided to illustrate CUI in piping and tanks and the detection methods applied in industry today to improve overall reliability.


Figure 2. This pipe with a corrosion-monitoring location (CML) showed minimal losses near the CML (left). Approximately 2.5 ft below the CML (middle), the NPS 2 pipe had experienced losses from 0.055 to 0.154 in. Computed radiography inspection (right) confirmed the presence and severity of this corrosion

Risk-based inspection

Prior to performing NDE, a facility needing CUI inspections must have a thorough understanding of its CUI history and risk-management needs. This helps to select the circuits or equipment on which the inspections should concentrate. Inspection for CUI should be considered for insulated pressure vessels and piping in intermittent service or when operating temperatures fall within the known and accepted ranges for CUI occurrence for the material in use. The facility’s staff must determine probability and consequence of failure in order to establish the magnitude and scope of inspection that will be required. The facility must also determine which codes or practices will be used to determine and define that scope. These could include, but are not limited to, the following standards developed by the National Association of Corrosion Engineers International (NACE; Houston; www.nace.org) and the American Petroleum Institute (API; Washington, D.C.; www.api.org):

  1. NACE SP0198 [3] details the variables to consider. A key variable is the potential source of water ingress, including: rainfall; drift from cooling towers; condensate falling from cold service equipment; steam discharge; process liquids spillage; spray from fire sprinklers; deluge systems and washdowns; leaking heat tracing; groundwater; and condensation on cold surfaces after vapor-barrier damage
  2. API 571 Section [4] lists the following critical factors:
    -The physical location (industrial, marine or rural); moisture (humidity), particularly designs that trap moisture or when present in a cooling tower mist; temperature; and the presence of salts, sulfur compounds and soil
    -Marine environments, which can be very corrosive (20 mpy), as are industrial environments that contain acids or sulfur compounds that can form acids (5–10 mpy)
    -Inland locations exposed to a moderate amount of precipitation or humidity, which are considered moderately corrosive environments (around 1–3 mpy)
    -Dry rural environments, which usually have very low corrosion rates (less than 1 mpy)
    -Designs that trap water or moisture in crevices that are more prone
    to attack
    -Operating temperatures up to around 250ºF (121ºC) experience increased corrosion rates. Above 250ºF, surfaces are usually too dry for corrosion to occur except under insulation (see Section 4.3.3 of API 571)

With all this information, walkdowns can commence. Site and NDE personnel will benefit from meticulously planning around the access points and techniques to be used line by line and component by component. Choosing how many locations per line should have spot NDE is important. Using modern digital technologies, the walkdown information, the static equipment information and drawings can all be pre-loaded for NDE personnel to develop reports with acceptance and rejection comments as soon as the NDE is completed in the field in real time.


General NDE selection

To minimize, if not eliminate, insulation removal, realtime radiography (RTR) is a commonly used pre-screening examination. After the RTR results, digital or computed radiographic (CR) testing is commonly performed on lines of NPS 8 and smaller. Nominal Pipe Size (NPS) is a North American set of standard sizes for pipes used for high or low pressures and temperatures. “Nominal” refers to pipe in non-specific terms and identifies the diameter of the hole with a non-dimensional number, loosely based on the inner diameter (ID) of the pipe in inches. The flowchart in Figure 3 illustrates a common NDE selection process for examining piping for CUI [ 5].


Figure 3. This project flowchart illustrates considerations for ferrous piping NDE selection, including realtime radiography (RTR), computed radiography (CR), pulsed eddy-current method (PEC), electromagnetic acoustic testing (EMAT), ultrasonic thickness testing (UTT) and visual testing (VT)

Realtime radiography (RTR)

Realtime radiography (RTR) is also called fluoroscopy [ 6]. Inspection personnel view the silhouette of the pipe or vessel outer diameter (OD) on a monitor as the location is inspected (Figure 4). Corrosion products form buildups on top of the OD. Due to size limitations, this technique is used more often on piping rather than vessels. The technique is defined by its radiation source and image intensifier or detector. It does not require film. RTR results in images such as those shown in Figure 5. The maximum opening of the arms on the equipment determines the largest diameter of the equipment and insulation that can be inspected. This has changed with time and will likely continue to do so.


Figure 4. Realtime radiography (RTR) is performed on piping to show the degree of CUI damage that has occurred


Figure 5. RTR Images show minimal OD corrosion (left) and significant OD corrosion (right)


Profile radiography with film; with computed radiography (CR); and with digital detector array (DDA) radiography

Profile radiography (also referred to as a shadow shot), as its name implies, refers to a profile view of the wall thickness. Profile radiography has been used to measure wall thickness values since the 1950s [ 7]. Today, it typically follows RTR for piping of NPS 8 and smaller.


Figure 6. Computed radiography (CR) imagery is the preferred method for visualizing CUI damage to smaller-diameter pipes

With the advent of CR and DDA, profile radiography is the technology of choice for inspecting smaller-diameter piping (Figure 6). Though occasionally, film radiography is used in CUI studies, CR and DDA have largely displaced traditional profile radiography with film. With these techniques:

  1. The piping is typically exposed to gamma rays from an Iridium-192, Selenium-75 or Cobalt-60 gamma source. This point is common with traditional radiography
    with film
  2. The remaining thickness can be assessed along pipe edges
  3. Shorter exposures are needed to obtain images than for traditional radiography with film
  4. Digital radiographs and images can be stored and shared quickly
  5. Multiple insulated components of various diameters and with some thickness variations can be viewed in one radiograph

CR requires imaging cassettes to be placed in a laser reader, where they are scanned to obtain a digital image. DDA images develop directly on the detector and need not be processed in a reader. They are obtained in significantly less time than CR (seconds versus minutes). They also require less radiation than is needed for radiography with traditional film and for CR. On the other hand, durability of the cassettes and other factors are more favorable for CR than for DDA (Figure 7).


Figure 7. Digital detector array (DDA) radiography images are a quick way to obtain inspection data


Pulsed eddy-current (PEC) method

For CUI examinations, PEC (Figure 8) is often used in piping with NPS 8 or greater. This method is also used when inspecting vessels for CUI, because this allows for a decrease in insulation removal. A PEC probe scans the insulated equipment and identifies general (averaged) losses, but isolated pits can be difficult to detect. The detection sensitivity of this electromagnetic inspection technology depends on the footprint of the probe, liftoff or insulation thickness and on the steel thickness [ 2]. As with other electromagnetic techniques, edges (for example, nozzles, flanges or the end of a structure) pose a challenge for inspection.


Figure 8. A tank with heavy deposits is shown on the top image. The lower image shows a pulsed eddy-current inspection that was performed without removing the deposits. The inspection identified that the areas beneath ladder rungs experienced the
greatest losses


Guided wave testing (GWT)

GWT, an ultrasonic inspection method (Figures 9 and 10), can be used to screen for potential CUI damage in piping. The inspection is performed with minimal insulation removal, and it has been used in piping with asbestos insulation. A special tool (transducer ring) is clamped around the pipe that transmits guided waves in both directions along the pipe. Reflected signals from defects and pipe features, such as welds, are received by the transducer ring and sent to the main unit. The tested sections must be carefully selected, since the length inspected in one scan is limited by the number of elbows, fittings or welded shoes. Losses due to corrosion must be greater than 4 to 10% of the pipe cross-section to be detectable [2].

Ultrasonic thickness measurement

In CUI studies, direct ultrasonic thickness measurements can be made on pressure equipment with diameters larger than NPS 1 where the insulation has been removed or in locations with insulation plugs. Since removing insulation is expensive, these inspections are not performed routinely during CUI pre-screening.


Figure 9. A guided-wave testing (GWT) transducer ring is placed on insulated piping to screen for potential CUI damage



Figure 10. Here, GWT was used to identify CUI pits

Thermal or infrared imaging examination

Using this technique, one aims to identify where the insulation is wet as opposed to dry. Based on thermal or infrared imaging, a heat picture of the surface is obtained. This technique detects areas where the insulation has been breached; it is not used during surveys where remaining thickness values are needed. Once the insulation is dry, the suspect area is not flagged.

Neutron backscatter examination

This technique uses a radioactive source to identify wet insulation. It does not assess remaining thickness and has specific applications, such as screening for locations where stress-corrosion cracking can develop. Once the insulation is dry, the suspect area is not flagged.

Other NDE techniques

NDE technologies are evolving rapidly and those less commonly used have not been described in this article. Updates on applications of the technologies are often available from the American Society for Nondestructive Testing (Columbus, Ohio; www.asnt.org), NACE International, API and in journals such as Inspectioneering.

Rope access services 

This innovative technology is a backbone for today’s CUI programs. Specialized harnesses and ropes are used to reach difficult-to-access areas safely and with minimum process disruptions. An inspection project comprising more than 1,400 piping circuits [ 4] was completed using rope access, aerial work platforms and existing scaffolding. Rope access was used for NDE and to refurbish damaged insulation and piping (Figure 11). Without rope access, the cost and time needed to complete the project would have been prohibitive.


Figure 11. Rope access is a modern method for inspecting and refurbishing insulation in difficult-to-access areas

Digital technologies

Today’s digital information is a backbone for CUI projects. Digital information is available to all key personnel in these projects — the equipment owner, the inspection crew and management personnel — while the projects progress. The findings and reports provide the most revealing images of equipment damage that have been available to date. The prompt communication allows for technical and cost-control discrepancies to be addressed and managed promptly.

Digitalization and other modern advances in NDE allow engineers to look into the integrity of insulated equipment with minimal insulation removal. Rope access allows us not only to inspect, but also to refurbish insulated equipment within affordable times and budgets. Digital technologies for NDE and for reporting, such as intrinsically safe tablets, allow prompt communications that were not possible a decade ago. Nevertheless, multiple NDE techniques are still needed to assess CUI. As well, detailed planning prior to inspection allows for effective and efficient CUI inspection projects

Inspecting for CUI is necessary, since moisture finds its way into insulation via many mechanisms, leading to sometimes severe damage, as illustrated in the photos shown in this article. There is certainly more than meets the eye when considering the challenges of CUI. CUI damage is full of surprises, but the NDE guidelines described here should help engineers to better tackle the always-changing CUI environment at their facilities. ■

Edited by Mary Page Bailey



1. Delahunt, J.F., Corrosion Under Thermal Insulation and Fireproofing, an Overview, Paper No. 03022, NACE Corrosion Conference, 2003.

2. Fitzgerald, B.J., Lazar, P., Kay, R.M., Winnik, S., Strategies to Prevent Corrosion Under Insulation in Petrochemical Industry Piping, Paper 03029, NACE Corrosion Conference, 2003.

3. NACE SP0198, Control of Corrosion Under Thermal Insulation and Fireproofing Materials — A Systems Approach, 2017.

4. API 571 Section 4.3, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, 2 nd Ed., April 2011.

5. Nichols, M.O., Townsend, M., Partnering to Zero in on Corrosion Under Insulation with NDE and In-Touch Software, RMC-18-Paper #RMC-18-240, 2018 AFPM Reliability & Maintenance Conference.

6. API Recommended Practice (RP) 583, Corrosion Under Insulation and Fireproofing, 1 st Ed., May 2014.

7. Hecht, A., Bauer, R., On-Line Radiographic Wall Thickness-Measurement of Insulated Piping in the Chemical and Petrochemical Industry, ECNDT Conference Paper, May 1998.


Ana Benz is the chief engineer at IRISNDT (5311 86 th Street, Edmonton, Alberta, Canada T6E 5T8; Phone: 780-577-4481; Email: ana.benz@irisndt.com). She has worked for 21 years as a corrosion, failure and inspection specialist. Her expertise includes inspections and organizing plant inspection projects using advanced inspection technologies. Benz has worked extensively for the chemical process industries, in petrochemical plants, fertilizer plants and nickel refineries around the world, as well as oil-and-gas production sites. She holds a degree in materials engineering from the University Simon Bolivar in Venezuela and also holds an M.S. in materials engineering from the University of British Columbia. She has several Canadian General Standards Board (CGSB) NDT certificates, as well API 510 certifications and Level 3 certification from the CWB Group. Benz was a member of the NACE Edmonton Executive chapter for 15 years, and before that, participated in various capacities for the Edmonton Chapter of the Canadian Welding Institute.

Fadi Basma is the senior operations director of IRISNDT (4649 S. Sam Houston Parkway E., Houston, TX 77048; Phone: 713-209-2701; Email: fbasma@irisndt.com). He is known for advocating the use of innovation and technology in the NDE industry. He holds a Level 3 certification from the American Society for Nondestructive Testing. Basma received his M.B.A. from Lebanese American University in 2006 and his M.S. in electrical and computer engineering from the University of Oklahoma in 2008.



Mike Townsend is currently operations director overseeing specialty projects at IRISNDT (5101 Taravella Road, Marrero, LA 70072; Phone: 504-328-0070; Email: michael.townsend@irisndt.com). Previously, Townsend inspected F/A-18 aircraft for four years as part of the United States Marine Corps. After returning to the civilian sector, he transitioned into the industrial inspection field starting with remote-access technologies. He has been in the inspection field for over 14 years working for chemical plants, refineries, bridges, towers, stacks, nuclear facilities and more. The inspections have encompassed basic non-destructive examinations, structural bridge inspection, API inspection, automated ultrasonic testing (AUT) and advanced UT inspections, as well as a wide array of maintenance functions predominantly via rope access. He has been with IRISNDT for over three years.




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