1. Paper No.
00000 (Official use)
NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Corrosion under Insulation – Facts and Prevent
KUMAR KOLUR VADIVELU
ABSTRACT
Corrosion under insulation (CUI) has become one of the utmost important mechanical integrity
issues with the following sectors chemical plants, petroleum refineries, power generating
facilities and other plants in all over the world.
Corrosion under insulation is very difficult to detect and find-out because of the insulation
material hide the beneath problem.
Old-style methods of addressing this issue include removal of insulation for visual inspection
operating temperatures ranges from -3°C (25 ºF) to 150 °C (300 ºF.) This method will not
address about cold service and cyclic service operation.
Oxygen with moisture is the largest promoting factor for corrosion. Under the insulation the
evaporated media transferred from one place to another place and not allowing the evaporated
media in to the environment so this is leads to the corrosion under insulation.
Carbon and Stainless Steel with 300 Series are more susceptible for Corrosion under insulation
there are various reason to cause corrosion. But the process upsets or cyclic process conditions
will be the primary factor.
This paper focus about the different type of corrosion under insulation it is include the substrate
being protected and cladding . material. The primary reasons to promote the corrosion under
insulation. Discussion about the selection of insulation material, impact on coating material, CUI
detection logics and modern industry practice to detect the CUI.
This paper discuss about the possibility of CUI. The best way to minimize or detect if any failure
occur always refer to relevant standards.
Key words:
Corrosion under Insulation (CUI), Galvanic corrosion, Acid, alkaline corrosion, weather barriers. Insulation
material, Coating

2. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Corrosion under Insulation
Is a severe form of localized external corrosion that occurs in carbon and low alloy steel
equipment that has been insulated. This form of corrosion occurs when water is absorbed by or
collected in the insulation. The equipment begins to corrode as it is exposed to water and
oxygen. CUI is common in refineries and process plants that typically operate equipment at high
temperatures (Source_ Wiki).
Type of Corrosion under Insulation
The following type of corrosion is mostly occurs under the Insulation
 Galvanic Corrosion
 Alkaline or Acidic Corrosion
 Chlorine stress corrosion
Galvanic couple occurs when two dissimilar material contact together based on Potential
difference of both metals. The most active material corrode first compare to least active metal,
in this concept the insulation material when wet condition the contaminated (Salt) electrolyte
that allows a current between insulated material and outer jacket or its accessories, The degree
and rigorousness of the attack on the less noble metal depends not only on the difference in
potential of the two metals, but also on their comparative areas.
Alkaline or Acidic corrosion results when contaminants in the moisture or vapor are present in
certain type of insulation material. For hot service above 300o
F, (150°C) most of the water is
driven off and the contaminants with water vapor may condense at the edge of the insulation
jacket, based on the contaminate type whether acidic or alkali in the condensed moisture react
with the insulating jacket resulting the corrosion particularly aluminum jackets affects well.
Alkaline contaminants react with aluminum jackets produce sever etching and pitting effect.
When chlorine present in the contaminants the pitting corrosion in the aluminum jacket is more
severe.
Cement type insulating material has alkaline in condition or it has alkaline chemical and water if
the steel immersed in the cement and still drying off particularly below 150°C(300°F), the
alkaline water may cause corrosion in case the insulating material stainless steel , aluminum or
Brass. Potable water is recommended to mix the insulating cement.
Corrosion due to Chloride is the combination of insulation material contain leachable chlorides
and austenitic-stainless-steel surfaces (300 Series), Chloride corrosion is more prone when the
moisture is present with the temperatures are higher than 60°C (140o F). Chloride
concentration is normally results from the evaporation of rain water, or in the industry water
utilized for fight fire or water used for process. In general the insulation jackets corroded due to
airborne salts blown thru air in the coastal regions.

3. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
The Temperature of the surface particularly stainless steel and the chloride ion concentration
will directly proportional to the failure and the speed of crack propagation. Based on several
study shows below 60°C (176 °F) the moisture containing less than 510pmm Chloride ion and
pH is less than 10 considered safe. But at higher the temperatures with the lower levels of
Chloride ion can result in failure. (Referred API 571 Table 13.3M)
In practice, it should be expected that vaporization of the solution will certainly occur. Because
local concentration of chlorides takes place at the metal surface, the bulk concentration may be
of little importance. Above 200°C (390° F) the external stress-corrosion cracking is normally not
evidenced, the stress required to cause cracking of stainless steel may result from either the
fabrication, operation or shutdown activity.
Carbon steel, LTCS and low allow steels are prone to affect CUI with the following normal
operating temperature between -12°C (10°F) to 176. If the Carbon steel equipment’s spends
more than 10% of the total time below 175°C (350°F) should be considered more susceptible to
CUI.
Insulation surface normally water entering and diffusing inward will ultimately reach a region of
dry out at the hot pipe or equipment wall. Next to this dry out area is a region in which the pores
of the insulation are filled with a soaked contaminated solution-this includes any chlorides or
other ions. When a shutdown or process upset occurs and the metal-wall temperature fluctuates
(particularly falls).The zone of soaked contaminated solution passages into the metal wall. Upon
resuming the normal process condition (by reheating) the wall will momentarily be in contact
with the saturated solution (e.g., chlorides), and stress-corrosion cracking may activate.
How to Prevent CUI – General Practice
Foremost factor to minimize or avoiding the Corrosion under insulation is to keep liquid from
engrossing in to the insulation. Engrossing the water will limit the insulation property and
promote the corrosion under the insulation of equipment, piping and other insulated items.
Various factors may contribute to invade the water in to the insulated items in general those are
poor weathering, vibration and or abuse from the people.
Good practice to preventing CUI are
 Selection of Insulation Type
 Design of item being insulated
 Protective paints and coatings
 Weather barriers being used
 Good Maintenance practices.
If an area is subject to spills or high humidity, special attention must be given to selecting the
insulation. Some insulation leave the system less sensitive to defects in weatherproofing or
paint films because the insulations are nonabsorbent and chemically nonreactive.

4. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Unluckily, the insulation are normally picked based on installed costs versus energy saved, and
not considered maintenance or corrosion cost. The selecting the insulation type the following
should be considered:
 The cost of revamping the insulation if corrosion is spotted. Insulation should
be removed in limited sectors for inspection. If insulation is subject to damage
by abuse, the cost of periodic removal and replacement must be considered.
Insulations such as calcium silicate, glass fiber and to some degree cellular plastic foams will
absorb and hold liquids and vapors. Additional flashing is required where spills, leaks, drippings
or where washing and hosing are carried out. The only fully nonabsorbent insulation is cellular
glass. Cellular glass must be used where corrosive or flammable liquids are present.
Design consideration of vessels, piping including consideration of supports and connects can be
found NACE SPO198 and handbook from Midwest Insulation Contractor's Association. Based
on practical experience stand pond the reason for frequently broken of weather barriers in the
insulation either because inappropriate details were originally given for equipment during
construction or not enough space was allotted around the insulation.
To minimize the corrosion under insulation, development of design can be accomplished by
handling the insulation specifications early during the vessel design and by "simplifying" the
surface to be insulated.
Protective coating system shall protect the item for long periods against the corrosive
contaminants. Normally under insulation highly permeable coating is not advisable, because this
type of coating will lead to permeate the moisture in to the coating to start corrosion behind the
coating.
Selection of coating is a vital role to prevent the corrosion under insulation, modern days several
coatings are developed under insulation to prevent the corrosion under insulation for long
lasting like Phenolic Epoxy, Silicone Aluminum and as stated in the NACE SPO 198.
In general cyclic temperature Zinc Silicate base coating not advisable.
The coating being used under insulation has the following basic parameter , good withstanding
of temperature, the selected protective coating must with stand the design temperature, good
abrasion resistance and also the coating will with stand the good immersion property
particularly hot water immersion and corrosive- chemical immersion.
The main purpose of weather/vapor jacket of the insulation provides barrier to water. This
weather / vapor jacket is the only part of the insulation system that can be verified and inspected
quickly and repaired easy an economically.
Routine preventive maintenance is required to identify and catch defects due to deterioration or
mishandling. If the insulation system is opened in any way for maintenance or inspection activity
it should be closed promptly after work is completed. If the insulation opening was not closed
properly then heavy corrosion problem will be encounter with in a short period that to more
possible where the cyclic process service and climatic condition.

5. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Extensive use of a non-breathing metallic jacket is believed to contribute greatly to corrosion of
warm equipment. Without a permeable jacket, water is trapped. Water in the insulation reaches
a point where it is vaporized. Vapor travels to the jacket and condenses; the cycle repeats itself
Preventing galvanic corrosion under CUI
GC main contribution factor is results from contaminated water invasion in a wet or humid
atmosphere., Selecting cellular insulation may be the only answer. Also, Plastic or synthetic-
rubber jacket may be used to prevent engrossing of water to prevent galvanic couple. Such
special jackets are factory applied and are fire and weather resistant. This type of jackets
greatly offer protection from normal abuse and from easy penetration of water. Advantage of
plastic jacketing’s normally will not corrode. For example, chlorosulfonated polyethylene
(CSPE) synthetic rubber (CSM) resists sodium chloride solutions to 125°C (260°F)
The alternate method to minimize or inhibit the cathodic and anodic reactions by apply suitable
painting over the substrate, these protective coating are provide a highly resistive path to
current flow. However, some type pigments can promote corrosion, especially in the base coats.
Such a pigments are red oxides, gypsum, ochre, graphite and lamp black. In a corrosion
reaction, alkali is formed at the cathode; this alkaline area grows, even under paint films
pigmented with zinc or aluminum.
Selecting the coatings to stop galvanic corrosion, the following parameter to be considered.
Consideration about reversal in the polarity of galvanic couple when temperature
increases or Cyclic.
Contaminants particularly salts are carried into the insulation and placed onto surfaces
inhibit with, or abolish, the effectiveness of corrosion inhibitors
Preventing alkaline or acidic Corrosion under CUI
To prevent Chemical corrosion under the metal jacket, the metal jackets should contain
moisture barriers on the inside. As stated in the preventing galvanic corrosion the same as
plastic jacket is another option to preventing the Alkaline and Acidic corrosion problem, or
plastic weather types are a good answer.
If certain type or combination of metals cannot be avoided then painting over the substrate is a
good option. Precautions with water and the insulation cement are needed if the stainless steel
piping or equipment is being protected by insulating cement then more care to be taken with
respect to water used for insulating cement and type of insulating cement.
For underground pipes, the pipe should be painted before it is insulated, and then a coating
should be applied over the insulation. The pipe should be coated with suitable paint system or an
extruded polyethylene jacket. A polyethylene jacket should also be placed over the insulation.

6. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Field-made joints. Rigorous inspection methods are needed to ensure that joints are done
correctly. Internally mounted anode beneath the primary weathering barrier and above the
secondary coating has been found effective as an additional measure.
Preventing chloride Corrosion under CUI
With stainless steels, problems have been resulted due to Leachable chlorides contained in
some insulation result in severe corrosion problem. To avoid the corrosion problem
recommended to use insulation that meets ASTM C-795 specifications. This provides sufficient
control, unless there is an external invasion of chlorides. A further way to reduce the chance of
chloride attack is to use an inhibited insulation.
Other points should also be considered to prevent chloride induced corrosion:
 With some insulation, such as polyurethane, it is not possible to add
inhibitors. Thus, fire-retarded polyurethanes are not recommended for use
over austenitic stainless steels.
 Some specification more stringent that require less than 10 ppm of leachable
chloride in the insulation, more attention must be paid to the mortars and
cements used, since these may contain chlorides.
 To prevent water and chlorides from reaching the stainless steel, the external
barrier must be designed properly and maintained periodically.
Stress – Cracking corrosion is more probable when steam tracing is used within insulation, extra
precautions are needed.
Certain type of pigment like zinc, titanium or other metallic used in the coating and applied over
stainless steel surface, lead to catastrophic embrittlement in case of any fire accident.
The metal jacketing must be securely fastened to prevent water entry at joints, or where the
insulation is supported with attachment angles.
Suitable water proof mastic is recommended over Cement-coated insulation to prevent water
ingress. The type of insulation and method of application chosen should assure the absence of
shrinkage cracks.
Prior to insulation any stainless steel equipment it is highly advisable to wrap the equipment by
aluminum foil, this foil act as a physical barrier to prevent to leach out any contaminants over
the surface particularly chloride containing solution. The aluminum will be at considerably the
same temperature as the equipment, and the chloride solution will shift to the foil, rather than
the stainless.

7. NIGIS * CORCON 2016 * Sept. 18 – 21, 2016 * New Delhi
Copyright 2016 by NIGIS. The material presented and the views expressed in this paper are solely those of the author(s) and do not necessarily by NIGIS.
Detection
Several method is currently practiced to detect the corrosion under insulation, in general the
following logics are followed
By the previous inspection history the corrosion like to occur.
API 510 recommends the process vessel to be remove at least every five year on all
vessel where there is a possibility of external corrosion.
Without removal of insulation there a several tools and techniques are used in modern current
industry practices those are
 Real time Radiography
 Guided wave Ultrasonic
 Temperature survey
Acknowledgments
I would like to thank Sadara Management and my dear wife Unsasi Kumar, Friends and
Colleagues helped me a lot in finalizing this technical literature within the limited time frame.
References
 Delahunt, J. F. "Corrosion Control under Thermal Insulation and
Fireproofing." Proceedings: Exxon Research & Engineering Co. Internal
Conference on Corrosion Under Insulation (1984): p 554.
 NACE -SPO 198-2010