Asset Intelligence Report
A PRIMER ON
CORROSION UNDER INSULATION
SEPTEMBER 2022
A Higher Level of Asset Integrity Intelligence
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A Primer on Corrosion Under Insulation
CONTENTS
Overview
4
History of CUI
4
Causes
5
Detection
6
Prevention and Mitigation
7
Codes, Standards, and Best Practices
7
References
8
A Primer on Corrosion Under Insulation
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Corrosion under insulation (CUI) and corrosion under
reproo ng (CUF) are some of the most well-known phenomena
in the process industries, yet they continue to constitute a large
percentage of global maintenance expenditures. CUI is a subject
that is well researched and understood based on extensive
studies to determine its causes, effects, prevention, and
mitigation.
In simple terms, CUI is any type of corrosion that occurs due to
the presence of moisture on the external surface of insulated or
reproofed equipment. It is the presence of improperly speci ed
insulation (i.e., wicking characteristics, chloride content, etc.)
that makes this class of corrosion so aggressive and hard to
detect and locate. The buildup of moisture can be caused by
Figure 1. Example of CUI after insulation was removed
several factors detailed in the “Causes” section of this primer.
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. In some
cases, it may even lead to a process safety incident.
History of CUI
Corrosion under insulation has been around since insulation
started being used on pipes and vessels. The frequency and
severity of damage and related failures increased markedly after
the 1973 oil shock when re nery operators started insulating
low-temperature hot service piping and equipment. Despite the
prevalence, CUI was not generally 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 technique was identi ed as being the best,
but complementary NDE methods used together were seen to
increase con dence levels for detecting CUI. Fortunately, NDE
technology and techniques have improved signi cantly since
that preliminary study.
In 1998, NACE published RP 0198-98, The Control of Corrosion
Under Thermal Insulation and Fireproo ng Materials - A Systems
Approach. When published, RP 0198-98 was the only standard
speci cally directed at combating CUI that was available to the
public. This recommended practice suggested using protective
coatings in lieu of insulation where applicable.
Figure 2. Example of CUI after insulation was removed
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A Primer on Corrosion Under Insulation
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Overview
Signi cant strides have been made since the release of the RP
There are several different types of corrosion that can occur, the
0198-98, and there are now several standards, best practices, and
most common of which are galvanic, acidic, or alkaline, and
studies that have led to a deeper understanding of ways to
chloride stress corrosion cracking of austenitic stainless steels
prevent and detect CUI, such as API RP 583, Corrosion Under
[2].
Insulation and Fireproo ng, Second Edition, March 2021.
Galvanic corrosion generally results from wet insulation with
Causes
an electrolyte or salt present that allows a current ow between
While CUI may be one of the most well-known phenomena in
dissimilar metals, such as the insulated metal pipe surface and
the process industries, it is also the most prevalent. It is dif cult
the outer jacket or accessories. The extent and severity of the
to prevent because, by and large, no matter what precautions are
attack on the less noble metal depend not only on the difference
taken, water eventually bypasses the weather barrier (jacketing),
in potential of the two metals but also on their relative areas. The
gets through the insulation, and comes in contact with process
complete galvanic series and the voltage potential for each metal
surfaces. In more severe cases, this may even go unnoticed until
or alloy appear in handbooks and other standard references [2].
process leakage occurs [1].
Alkaline or acidic corrosion results when an alkali or acid and
According to API 570, there are speci c susceptible temperature
moisture are present in certain brous or granular insulations.
ranges under which CUI may occur. For carbon steel piping
For hot service above 250°F, most of the water is driven off. This
systems, the range is between 25°F and 250°F, particularly where
water vapor may condense at the edge of the insulation and
operating temperatures cause frequent or continuous
dissolve the alkaline or acidic chemicals, resulting in corrosion
condensation and re-evaporation of atmospheric moisture.
of the aluminum or steel jacketing [2].
Carbon-steel piping systems that normally operate in-service
Chloride (halide) corrosion and stress corrosion cracking
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 becomes soaked during intermittent
downtimes by events such as drift from cooling towers, water
from deluge systems, water from cleaning equipment, and rain.
can be caused by the combination 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 the chloride ion usually results from
the evaporation of rainwater, or of water used to ght res, rain
or condensation, or of process water. Stress-corrosion cracking
of piping and vessels and insulating jackets often results from
airborne salts in coastal regions [2].
Figure 3. CUI leak discovered on hot service vessel
Figure 4. CUI damage found on sweating service small bore
operating at 180°F, 25 years in service
pipe, 5 years in service
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A Primer on Corrosion Under Insulation
Detection
Conventional radiography involves a process where radioactive
There are several methods for detecting CUI, including “brute
rays are directed at the object to be inspected, passing through it,
forcing,” which is the practice of physically removing insulation,
and capturing the image on
visually inspecting, mitigating, repairing, re-coating, and re-
numerous advantages, including use with insulation of any
insulating, where suitable. Other methods may be used with an
thickness or type; through many types of internal product (some
appropriate strategy without removing insulation such as
cases might require very high radiation sources); on pipes of
radiography, pulsed eddy current, guided-wave ultrasonics, and
varying diameters; and on both thick and thin wall pipes. Check
ultrasonic thickness measurements from the internal surface of
with your NDE specialist to make sure conditions permit valid
the equipment, where practicable. Some operating facilities
usage [3].
apply risk and/or criticality analysis to prioritize pressure vessels
Digital radiography: As opposed to conventional radiography,
and piping for CUI inspection as opposed to “brute forcing.”
Unfortunately, there is still no NDE “silver bullet” for CUI yet.
lm to be examined [3]. It has
digital radiography relies on exposing reusable storage phosphor
screens as an alternative to traditional silver halide
lm. This
Prior to selecting one or more inspection methods, one should
allows the information to be stored digitally, saving both time
understand what is or is not likely to be found based on the
and storage space. It also requires less radiation due to the
limitations of the methods selected and the impact on decisions
phosphor lm, and therefore has a reduced impact on the safety
that will be made about the anticipated reliability and suitability
compared to conventional radiography. Consult with your
for service of the component(s) in question (i.e., what risk or
radiation safety specialist to make sure you fully understand the
probability of failure remains). As with any inspection strategy,
safety impact prior to establishing the Safe Zone [3].
it is common to complement approaches.
Low intensity x-ray: A low-intensity x-ray imaging scope is a
A partial sampling of the most popularly used detection
hand-held, totally portable uoroscopic device utilizing a low-
techniques is described as follows. As with anything, they have
energy, low-intensity gamma source. This can be a very quick
caveats, bene ts, and limitations. Check with your NDE
way of qualitatively screening pipe for CUI. Iridium-192 is a
specialist to make sure you understand the availability,
typical radiation source for this technique.
limitations, and bene ts of the various options.
Pulsed eddy current (PEC): This method has been used in
Brute forcing: As the least complex way to detect CUI, brute
corrosion detection for several years and is highly useful in
forcing involves simply stripping the insulation off the
situations where an object’s surface is rough or inaccessible.
equipment and examining it for corrosion. This is a
Moreover, this method does not require surface preparation or
comparatively time-consuming, expensive work process,
the removal of insulation; thus, it can be a quick and cost-
especially if the insulation contains asbestos, so it may not be
effective solution for corrosion detection. The method works by
suitable for all situations [1]. Targeted inspections in areas with a
sending out a pulsed magnetic
high probability of CUI can lower the overall disruptiveness and
penetrates through the non-magnetic insulation between the
cost of this detection strategy. The utilization of insulating
probe and the object being inspected. This will induce eddy
materials which can be removed and reused is often preferred.
currents that can be measured to determine whether corrosion
Brute forcing may or may not be preceded by visual inspection
eld via a probe coil which
is present [4].
of the insulated vessels or piping looking for signs of moisture
Guided-wave ultrasonics (GWUT): This method of testing
ingress such as rust discoloration of the weather barrier,
involves sending guided ultrasonic waves out along the axial
insulation damage, etc. It should also consider areas where
direction of a pipe and then measuring the re ections for echoes
moisture could collect such as low points, supports, insulation
which might be caused by corrosion. The main advantage of this
support rings, etc. Consideration may also include insulation
method is that it is possible to inspect supports that are not
type and their propensity to absorb or hold moisture.
directly accessible for visual inspection. The downside of the
Conventional radiography: This is the most common NDE
technique used for detecting CUI without insulation removal [1].
on the capabilities of the inspector and the testing procedures
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A Primer on Corrosion Under Insulation
method is that the accuracy of the results is strongly dependent
used; thus, an inexperienced inspector can lead to inaccurate
effective maintenance practices will help to prevent corrosion
results [5]. While insulation removal between the transducer
damage before it becomes a severe problem. However,
rings is not needed, access to the pipe surface is required at each
maintenance alone is not an effective solution without a well-
end. Refer to the manufacturer for the maximum distance
thought-out inspection strategy as none of these mitigation
between transducer rings required to achieve the desired
practices guarantee the complete prevention of CUI [2].
sensitivity.
Insulation should be certi ed as chloride free to avoid related
Ultrasonic thickness measurements: This process can be
used to determine the external condition of vessels and
Codes, Standards, and Best Practices
remaining thickness of piping components. It works by sending
API 510, Pressure Vessel Inspector Program, is an inspection
ultrasonic waves into the surface of the object and measuring
code that covers the in-service inspection, repair, alteration, and
the time taken by the wave to return to the surface. It is a simple
rerating activities for pressure vessels and the pressure relieving
technique; however, it does require adequate contact with the
devices protecting these vessels. It applies to most re ning and
material, so it is not viable for every situation and requires
chemical process vessels that have been placed into service. CUI
insulation removal for access to the physical surface.
inspection is covered in Section 5.5.6 of the standard (Tenth
Other techniques (i.e., neutron backscatter, infrared thermography,
Edition released April 2014).
etc.) can help to nd moisture under insulation, which may then
API 570, Piping Inspection Code - Inspection, Repair, Alteration
help indicate where CUI is potentially occurring as well. These
and Rerating of In-Service Piping Systems, provides guidance on
methods infer that there is potential CUI activity, as opposed to
how to determine which piping systems are most susceptible to
directly detecting metal loss or cracking.
CUI as well as some of the most common locations to nd CUI
Dye penetrant testing (PT): PT is often used to sample areas
on those systems (Section 5.8, Fourth Edition released February
for chloride stress corrosion cracking of austenitic stainless
2016).
steels. This does require insulation removal to access the
API RP 574, Inspection Practices for Piping System Components,
chloride exposed surface of the component.
discusses inspection practices for piping, tubing, valves (other
Prevention and Mitigation
than control valves), and
There are several ways to prevent CUI. In general, it is typically
far more cost-effective to prevent CUI than to repair or even
replace damaged equipment later.
ttings used in petroleum re neries
and chemical plants. To aid inspectors in ful lling their role in
implementing API 570, this document describes common piping
components, valve types, pipe joining methods, inspection
planning processes, inspection intervals and techniques, and
It is also important to understand the system approach for the
types of records. CUI is covered in Section 6.3.3 (Fourth Edition
prevention of CUI. First and foremost, the most effective method
released November 2016).
of preventing CUI is to keep water or electrolytes from
contacting the unprotected metal surface. To design a system
that has the best odds of mitigating CUI, a specifying engineer
should use a three-pronged approach that utilizes (1) effective
protective pipe coatings, (2) well-designed and installed weather
barriers (jacketing), and (3) thermal insulation that is designed to
mitigate CUI and is tight
tting, durable, and water resistant.
There are several useful test standards that can inform the
speci er as to which thermal insulations are most appropriate
for use in CUI temperature range services. It is important to
note most standardized tests evaluate insulation materials “out
API RP 583, Corrosion Under Insulation and Fireproofing, covers
design, maintenance, inspection, and mitigation practices to
address 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 provides 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 (First
Edition released May 2014).
of the box,” so understanding how materials will perform “in
NACE SP0198-2010, Control of Corrosion Under Thermal
use” is key in selecting the proper insulation [9]. Furthermore,
Insulation and Fireproo ng Materials – A Systems Approach,
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A Primer on Corrosion Under Insulation
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damage.
(Published July 2010). This standard is a replacement for NACE
6. Alleyne, D., 2012, "Guided Wave Testing for touch point
RP 0198-08 (March 2004).
corrosion," 18th WCNDT, Durban, South Africa.
ASTM STP 880, Corrosion of Metals Under Thermal Insulation,
7.
Wikipedia, 2022, “Ultrasonic thickness measurement,”
provides information on corrosion problems that can occur on
https://en.wikipedia.org/wiki/
thermally-insulated plant equipment and piping components if
Ultrasonic_thickness_measurement.
the insulation becomes wet (First Edition released in 1985).
ASTM C795-08 (2018), Standard Speci cation for Thermal
Insulation for Use in Contact with Austenitic Stainless Steel, is
used to assess the
tness for use of insulation on stressed
austenitic stainless-steel pipe and equipment which is
susceptible to corrosion cracking.
8. Corrosion Doctors, (n.d.) “Corrosion under Insulation (CUI),”
http://corrosion-doctors.org/Forms-crevice/CUI.htm.
9. Williams, J., Evans, O., 2010, “The In uence of Insulation
Material on Corrosion Under Insulation,” NACE Northern
Area Western Conference, Calgary, Alberta.
ASTM C871-18, Standard Test Methods for Chemical Analysis of
Thermal Insulation Materials for Leachable Chloride, Fluoride,
Silicate, and Sodium Ions, provides an analytical technique to
measure the amount of halides and protective ions that can
leach from thermal insulation.
ASTM C1617-19, Standard Practice for Quantitative Accelerated
Laboratory Evaluation of Extraction Solutions Containing Ions
Leached from Thermal Insulation on Aqueous Corrosion of
Metals, provides a test method for assessing the in uence of
different thermal insulations (in their pristine condition) on
corrosion rates of carbon steel and other metallic substrates.
ASTM C1763-20, Standard Test Method for Water Absorption
by Immersion of Thermal Insulation Materials, is used to assess
the water absorption rate of thermal insulation materials.
References
1.
Reynolds, J., 2004, “99 Diseases of Pressure Equipment:
Corrosion Under Insulation,” Inspectioneering Journal, 10(3),
pp. 1-6.
2. Liss, V., 1988, “Preventing Corrosion Under Insulation,”
National Board BULLETIN, The National Board of Boiler and
Pressure Vessel Inspectors.
3. Patel, R., 2005, “Digital Applications of Radiography,” Proc. of
3rd MENDT, Manama, Barain, pp. 210-215.
4. Black, J.T., Kohser, R, 2011, Materials and Processes in
Manufacturing, 11th Edition, John Wiley & Sons.
5. Robers, M., Scottini, R., 2002, "Pulsed Eddy Current in
Corrosion Detection," NDT.net Issue 2002-10.
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A Primer on Corrosion Under Insulation
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