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A Primer on Corrosion Under Insulation (CUI)

Asset Intelligence Report

We hope this report helps in your pursuit of a higher level of Asset Integrity Intelligence.

Version 2015/January

Important note: Corrosion under insulation (CUI) can often be very difficult to predict and even more challenging to locate without remov-ing the insulation. The purpose of this report is to serve as an intro-ductory primer. For additional information and resources on CUI, we recommend performing your due diligence. The references provided in this report are an excellent place to start, especially API RP 583.

OverviewCorrosion under insulation (CUI) is one of the most well known phenomena in the process industries, and yet it still makes up an inordinately large percentage of global mainte-nance expenditures. CUI is a subject that is well researched and understood; extensive studies have been commissioned to determine the causes, effects, prevention, and mitigation of CUI.

In the simplest terms, CUI is any type of corrosion that occurs due to moisture buildup on the external surface of insulat-ed equipment. The buildup can be caused by one of multiple factors that are detailed below. The corrosion itself is most commonly galvanic, chloride, acidic, or alkaline corrosion. If undetected, the results of CUI can lead to the shutdown of a process unit or an entire facility, and in rare cases it may lead to a process safety incident.

History of CUICorrosion under insulation has been around since insulation started being put on pipes. However, CUI was not general-ly understood until the release of ASTM STP 880 “Corrosion of Metals Under Thermal Insulation” in 1985. This led to the funding of a study by the US Materials Technology Institute to determine the effectiveness of nondestructive evaluation (NDE) methods in dealing with CUI. Not one single NDE tech-nique was identified as being best, but multiple techniques used together were seen to increase confidence levels for de-tecting CUI. Fortunately, NDE technology and techniques have improved significantly since that early study.

In 1998, NACE published RP 0198-98, The Control of Corrosion Under Thermal Insulation and Fireproofing Materials - A Systems Approach. When published, RP 0198-98 was the only standard specifically directed at combating CUI that was available to the public. This recommended practice suggested using protective coatings to prevent CUI.

Significant strides have been made since the release of the RP 0198-98, and there are now several ways in which to detect and prevent CUI.

CausesWhile CUI may be one of the most well known phenomena in the process industries, it is also the most prevalent. It is difficult to prevent because, by and large, no matter what pre-cautions are taken, water eventually gets into the insulation, sometimes unnoticed until process leakage occurs.[1]

According to API 570, there are specific susceptible tempera-ture ranges under which CUI may occur. For carbon steel pip-ing systems, the range is between 25 and 250°F, particularly where operating temperatures cause frequent or continuous condensation and re-evaporation of atmospheric moisture. Carbon steel piping systems that normally operate in-service above 250°F, but that are in intermittent service, are also at risk. CUI has even occurred in process piping operating above 600°F when insulation is soaked during downtimes by deluge systems and rain.

There are several different types of corrosion that can occur, the most common of which are galvanic, chloride, and acidic or alkaline.[2]

• Galvanic Corrosion: this kind of corrosion generally occurs from wet insulation with an electrolyte pres-ent which allows a current to flow between metals of different corrosion potential. The severity of the attack depends on both the difference in potential between the two metals along with their relative areas.[2]

• Chloride Corrosion: this is generally caused by a combi-nation of insulation containing leachable chlorides with the 300 series austenitic stainless steel surfaces, when moisture is present and temperatures are above 140°F. The concentration of chloride is usually from the evapo-ration of rainwater, water used to fight fires, or process water. The speed of crack propagation caused by chloride corrosion is governed by the temperature of the steel and the chloride concentration at the metal surface. Chlorides can also come from the atmosphere, and from exposure to work environments when the insulation was not pres-ent and the material was not cleaned from the surface prior to re-insulating. This explains why one must make sure, for example, that materials, such as dye penetrant components, are certified chloride free. [2]

• Acidic/Alkaline Corrosion: this type of corrosion occurs when either an alkali or an acid are present in certain fibrous or granular insulations, which is inadvertently

Figure 1. CUI damage found on sweating service small bore pipe.

mixed with moisture in the environment. One such cause of alkaline corrosion is from insulating cement, which may contain both alkaline chemicals and water.[2]

DetectionThere are several methods for detecting CUI, including “brute forcing” (i.e. removing insulation, inspecting, mitigating and re-insulating) conventional and unconventional radiography, pulsed eddy current, guided-wave ultrasonics, or ultrason-ic thickness measurements from the internal surface of the equipment. Some operating facilities apply risk and/or critical-ity analyses to prioritize pressure vessels and piping for CUI inspection, versus “brute forcing”. Unfortunately, there is no NDE “silver bullet” for CUI yet.

Prior to selecting one or more inspection methods, one should understand what is or is not likely to be found based on the limitations of the methods selected and the impact on deci-sions that will be made about the anticipated reliability and suitability for service of the component(s) in question, i.e. what risk or probability of failure remains. As with any inspection strategy, it is common to complement approaches.

A partial sampling of the most popularly used detection tech-niques are described as follows. As with anything, they have ca-veats, benefits, and limitations. Check with your NDE special-ist to make sure you understand the availability, limitations, and benefits of the various options.

• Brute forcing: the least complex way to detect CUI, brute forcing involves simply stripping the insulation off of the equipment and examining it for corrosion. This is a comparatively time-consuming, fairly expensive work process, especially if the insulation contains asbestos, so it may not be suitable for all situations.[1]

• Conventional radiography: this is the most common NDE technique used for detecting CUI without insula-tion removal. [1] Conventional radiography involves a pro-cess where radioactive rays are directed at the object to be inspected, passing through it and capturing the image on a silver halide film to be examined. [3] It has numerous advantages, including: it can be used with insulation of any thickness or type, through many types of internal product (some cases might require very high radiation sources), on pipes of varying diameters, and on both thick and thin wall pipes. Check with your NDE specialist to make sure conditions permit a valid “shot”. [3]

• Digital radiography: as opposed to conventional radi-ography, digital radiography relies on exposing reusable storage phosphor screens as an alternative traditional silver halide film. This allows the information to be stored digitally, saving both time and storage space. It also requires less radiation due to the phosphor film and therefore has a reduced impact on the safety compared to conventional radiography. Consult with your radiation safety specialist to make sure you fully understand the safety impact prior to establishing the Safe Zone.[3]

• Low intensity x-ray: the low-intensity x-ray imaging scope is a hand-held, totally portable fluoroscopic device utilizing a low-energy, low-intensity gamma source of Iridium. This can be a very quick way of qualitatively screening pipe for CUI. Iridum-192 is a typical radiation source for this technique.

• Pulsed eddy current (PEC): this method has been used in corrosion detection for several years and is highly useful in situations where an object’s surface is rough or inaccessible. Moreover, this method does not require surface preparation or the removal of insulation, thus it can be a quick and cost-effective solution for corrosion detection. The method works by sending out a pulsed magnetic field via probe coil, which penetrates through the non-magnetic insulation between the probe and the object being inspected. This will induce eddy currents that can be measured to determine whether or not corro-sion is present.[4]

• Guided-wave ultrasonics (GWUT): this method of testing involves sending guided waves out along the axial direction of a pipe and then measuring the reflections for echoes, which might be caused by corrosion. The main advantage of this method is that it is possible to inspect supports that are not directly accessible for visual inspection. The downside of the method though is that the actual accuracy of the results are strongly dependent on how good the inspector and the testing procedures used are; thus an inexperienced inspector can lead to inaccurate results.[5]

• Ultrasonic thickness measurements: this process can be used to determine the external condition of vessels and remaining thickness of piping components. It works by sending ultrasonic waves into the surface of the object and measuring the time taken by the wave to return to the surface. It is a fairly simple technique. However, it does require adequate contact with the material, so it is not viable for every situation.

Other techniques, such as neutron back scatter and infrared thermography, can help to find moisture under insulation, which may then help detect where CUI is occurring as well. These methods infer that there is potential CUI activity, as op-posed to directly detecting metal loss or cracking.

Prevention/MitigationThere are several ways to prevent CUI. In general, it is typically more cost effective to prevent CUI than to repair the damage later, or worse, replace the piece of equipment.

First and foremost, the most effective method of preventing CUI is to keep water or electrolytes from coming into contact with the unprotected metal surface. Inevitably, it is nearly im-possible to guarantee that the insulation or coating will not be breached. Effective protective coatings and weather barriers can help minimize the potential for CUI. Furthermore, effec-tive maintenance practices will help to prevent corrosion dam-

All information found in this report is without any implied warranty of fitness for any purpose or use whatsoever. None of the contributors, sponsors, administrators or anyone else connected with this Asset Intelligence Report, in any way whatsoever, can be held responsible for the inclusion of inaccurate information or for your use of the information contained herein. DO NOT RELY UPON ANY INFORMATION FOUND IN THIS REPORT WITHOUT INDEPENDENT VERIFICATION.

age before it becomes a severe problem. However, maintenance alone is not an effective solution without a well thought out inspection strategy, because none of these mitigation practices guarantee the complete prevention of CUI.[2]

Codes, Standards, and Best Practices• API 510, Pressure Vessel Inspector Program is an inspection code that covers the in-service inspection, repair, alteration, and rerating

activities for pressure vessels and the pressure relieving devices protecting these vessels. It applies to most refining and chemical process vessels that have been placed into service. CUI inspection is covered in section 5.5.6 of the standard (10th Edition released April, 2014).

• API 570, Piping Inspection Code - Inspection, Repair, Alteration and Rerating of In-Service Piping Systems provides guidance on how to determine which piping systems are most susceptible to CUI (section 5.2.1) as well as some of the most common locations to find CUI (section 5.4.2) on those systems that are determined to be susceptible to CUI (3rd Edition released November, 2009).

• API RP 574, Inspection Practices for Piping System Components discusses inspection practices for piping, tubing, valves (other than control valves), and fittings used in petroleum refineries and chemical plants. In order to aid inspectors in fulfilling their role implementing API 570, this document describes common piping components, valve types, pipe joining methods, inspection planning processes, inspection intervals and techniques, and types of records. CUI is covered in section 6.3.3 (3rd Edition released November 2009).

• API RP 583, Corrosion Under Insulation and Fireproofing covers the design, maintenance, inspection, and mitigation practices to ad-dress external CUI as it applies to pressure vessels, piping, storage tanks and spheres. It examines the factors that affect the damage mechanisms, as well as going providing guidelines to prevent external corrosion or cracking under insulation, maintenance practices to avoid damage, inspection practices to detect and assess damage and the guidelines for risk assessment of equipment or structural steel subject to CUI (1st Edition released May 2014).

• ASTM STP 880, Corrosion of Metals Under Thermal Insulation provides information on corrosion problems that can occur on ther-mally insulated plant equipment and piping components if the insulation becomes wet (1st Edition released 1985).

• NACE SP0198-2010, Control of Corrosion Under Thermal Insulation and Fireproofing Materials – A Systems Approach (Published July, 2010). This standard is a replacement for NACE RP0198-08 (March, 2004).

Further Reading• Portable Pipe Wall Thickness Measuring Technique - CUI Exposed, January/February 1999 Inspectioneering Journal.• 99 Diseases of Pressure Equipment: Corrosion Under Insulation, May/June 2004 Inspectioneering Journal.• Detection of Corrosion Under Insulation (CUI) and Blockages on Piping System Using Profiler System, July/August 2010

Inspectioneering Journal.• Corrosion Under Insulation and Best Fit Solutions, July/August 2014 Inspectioneering Journal.

References1. Inspectioneering, 99 Diseases of Pressure Equipment: Corrosion Under Insulation; May/June 20042. Liss, V. Mitchell, 1988. “Preventing Corrosion Under Insulation” National Board BULLETIN.3. Patel, Ramesh J., 2005. “Digital Applications of Radiography”. Qatargas Operating Company Limited, Qatar4. Scottini, Robers R., 2002. “Pulsed Eddy Current in Corrosion Detection,” NDT.net.5. Alleyne, David, 2012. “Guided Wave Testing for touch point corrosion,” NDT.net.

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