V. Mitchell Liss
Engineering consultant, La Grange, TX
Summary: The following article is a part of the
National Board Technical Series. This article was originally published in the
January 1988 National Board BULLETIN. (6 printed
Galvanic corrosion, alkaline acidic corrosion, and stress-corrosion cracking
have all been known to happen in certain conditions. Here are ways to reduce
Although the extent and resultant cost of corrosion under insulation (CUI) are
not known exactly, J. F. Delahunt in "Corrosion Control Under Thermal
Insulation and Fireproofing"1 shows the seriousness of the problem
by presenting case histories. These cover deep pitting, as well as general
corrosion, that have occurred on galvanized steel tanks under 12-year-old
Delahunt advises that metal loss can be the least of the problem. For example,
an eight-inch carbon steel pipeline carrying heavy fuel oil was insulated with
calcium silicate block and protected with a metal weather jacket. The pipe
corroded, resulting in a leak. The oil was ignited and a large fire ensued,
causing hundreds of thousands of dollars in damage to process equipment.
Types of Corrosion Under Insulation
By understanding the types of corrosion that can occur under insulation, the
proper materials and construction can be employed to prevent them. Intruding
water is the key problem in CUI. Special care must be taken during design not
to promote corrosion by permitting water to enter a system either directly or
indirectly by capillary action. Moisture may be external or may be present in
Corrosion may attack the jacketing, the insulation hardware, or the underlying
piping or equipment. Depending on other factors, chloride, and galvanic, acidic
or alkaline corrosion may occur.
Galvanic corrosion generally results from wet insulation with an electrolyte or
salt present that allows a current flow between dissimilar metals (i.e., the
insulated metal surface and the outer jacket or accessories). The extent and
severity of the attack on the less noble metal depends not only on the
difference in potential of the two metals, but also on their relative areas.
The complete galvanic series and the voltage potential for each metal or alloy
appear in handbooks and other standard references. The mechanism of galvanic
corrosion is detailed by G. Butler and H. C. Ison in "Corrosion and Its
Prevention in Waters."2
Alkaline or acidic corrosion results when an alkali or acid and moisture, are
present in certain fibrous or granular insulations. For hot service above 250o
F, most of the water is driven off. This water vapor may condense at the edge
of the insulation, and dissolve the alkaline or acidic chemicals there,
resulting in corrosion of the aluminum or steel jacketing.
Some alkaline waters with aluminum produce etching and pitting. Pitting can be
severe, especially when chloride ions are present. Insulating cement may also
contain alkaline chemicals and water (while the cement is still drying). Below
250° F, an alkaline water may cause corrosion if the substrate or insulated
surface is stainless steel, copper, brass or aluminum. Steel would normally not
be affected in the time needed for the cement to dry. Fresh, potable water is
recommended when mixing insulating cement.
Delahunt reported on leaching tests performed on polyurethane foam insulation
containing fire retardant chemicals (i.e., brominated or chlorinated
compounds). Distilled water was used, and aggressive acidic solutions were
formed. The same was found true for phenolic foams. The pHs of the solutions
were often two to three. Laboratory corrosion rates have been shown to be 15-20
mils/yr. Of the two foams, the phenolics are by far the more corrosive.
Chloride corrosion 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 140o
F. Concentration of the chloride ion usually results from the evaporation of
rain water,or of water used to fight fires, or of process water.
Stress-corrosion cracking of insulating jackets often results from airborne
salts in coastal regions.
The probability of failure and the speed of crack propagation are governed by
the temperature of the stainless steel and the chloride concentration at the
metal surface. Solutions containing less than 1 ppm are normally considered
safe. Below 176°> F, levels of 100 ppm are not particularly dangerous if
continuous surface-wetting occurs; but at higher temperatures, lower levels can
result in failure.
In practice, it should be assumed that evaporation of the solution will
inevitably occur. Because local concentration of chlorides take place at the
metal surface, the bulk concentration may be of little importance. Above 390°
F, external stress-corrosion cracking is normally not experienced. The stress
required to cause cracking of stainless steel may result either from
fabrication or operation (or shutdown).
Water entering the insulation and diffusing inward will eventually reach a
region of dryout at the hot pipe or equipment wall. Next to this dryout region
is a zone in which the pores of the insulation are filled with a saturated salt
solution-this includes any chlorides present. When a shutdown or process change
occurs and the metal-wall temperature falls, the zone of saturated salt
solution moves into the metal wall. Upon reheating, the wall will temporarily
be in contact with the saturated solution (e.g., chlorides), and
stress-corrosion cracking may begin.
The major factor in preventing CUI is to keep liquid from intruding into the
insulation. Water decreases the effectiveness of the insulation and leads to
corrosion of pipe or equipment. Poor conditions caused by wet insulation can be
aggravated by weathering, vibration or abuse from people.
There are five factors in preventing CUI: (1) insulation selection; (2)
equipment design; (3) protective paints and coatings; (4) weather barriers; and
(5) maintenance practices. If an area is subject to spills or high humidity,
special consideration must be given to selecting the insulation. Some
insulations leave the system less sensitive to defects in weatherproofing or
paint films because the insulations are nonabsorbent and chemically
Unfortunately, the insulation picked is normally based on installed costs
versus energy saved, and maintenance or corrosion costs are not considered. The
following should be considered:
The cost of repairing the insulation if corrosion is detected. Insulation
should be removed in limited sections for inspection. If insulation is subject
to damage by abuse, the cost of periodic replacement must be considered.
The cost of the protective paint.
For nonabsorbent insulation, a "credit" should be given for the energy saved by
eliminating periodic water invasion to absorbent insulation during washdowns
Insulations such as calcium silicate, glass fiber and, to some extent, cellular
plastic foams absorb and retain liquids and vapors. Additional flashing is
required where spills, leaks or drippings may occur, or where washing and
hosing are carried out. The only fully nonabsorbent insulation is cellular
glass. Cellular glass should be used where corrosive or flammable liquids are
The proper design of insulation for pressure vessels, tanks and piping includes
consideration of the support and connection of the material. Details can be
found in a handbook from Midwest Insulation Contractor's Association.3
According to plant operators, weather barriers for insulation are frequently
broken either because inappropriate details were originally given for equipment
or not enough space was allotted around the insulation. Improvement in design
can be accomplished by handling the insulation specifications early during the
vessel design and by "simplifying" the surface to be insulated.
The coating system must protect for long periods against water or corrosives.
Highly permeable coatings allow corrosion to start behind the coating even in
the absence of breaks or pinholes.
An epoxy or epoxy-phenolic should be applied in two coats over an abrasive
blast cleaned surface. Inspection of the surface preparation is critical at
In selecting coatings, consider temperature and abrasion resistance, and a
service rating for water (or corrosive-chemical) immersion. It is difficult to
visually inspect coatings under insulation to find points that need touching
up. Unless corrosion or insulation failure causes reinsulating an entire
insulation setup, recoating is done every 10-15 years.
The weather/vapor jacket of the insulation provides the primary barrier to
water. This covering is the only part of the system that can be inspected
quickly and repaired economically. It must not only keep liquids out but also
allow for evaporation of any liquid that manages to get into the insulation
system. For weatherproofing, a rating of two perms, measured according to ASTM
Standard E 398, is acceptable. Also, it should be durable, offer flame-spread
resistance, and be economical. The material must be maintained periodically
(usually, two to five years) to remain effective.
Further, routine maintenance is needed to catch defects due to deterioration or
abuse. If the system is opened in any way for maintenance or inspection, it
should be closed promptly after work is completed. One plant reported that
openings made in a metal vessel's insulation for acoustic emission tests were
never closed. Severe corrosion occurred.
Extensive use of a nonbreathing 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
Since this form of corrosion results from water invasion in a wet or humid
atmosphere, selecting cellular insulation may be the only answer. Also, it
might be best to use a plastic or synthetic-rubber jacket. Such jackets are
factory applied and are fire and weather resistant. They offer protection from
normal abuse and from easy penetration of water. Commercial plastic jacketings
normally will not corrode. For example, Hypalon resists sodium chloride
solutions to 260 degrees F.
Metal surfaces can, of course, be painted. Paints can inhibit cathodic and
anodic reactions, and provide a highly resistive path to current flow. However,
some pigments can promote corrosion, especially in the first coat of primers.
Common ones are red oxides, gypsum, ochre, graphite and lamp black. Even in the
normal corrosion process, alkali is formed at the cathode; this alkaline area
grows, even under paint films pigmented with zinc or aluminum.
In selecting coatings to prevent galvanic corrosion, consider the following in
regard to sacrificial materials: (a) as temperature increases, there is a
chance of reversal in the polarity of galvanic couples; (b) salts carried into
insulation and deposited onto surfaces interfere with, or destroy, the
effectiveness of corrosion inhibitors; and (c) the insulation system is not
freely ventilated and may have inadequate oxygen for sacrificial reactions to
Preventing Alkaline or Acidic Corrosion
Metal jackets should contain moisture barriers on the inside. When corrosion of
the jacket is the problem, plastic weather types are a good answer. Precautions
with water and the insulation cement are needed if the piping or equipment is
stainless steel. If certain combinations of metals cannot be avoided, paint the
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 a coal-tar epoxy or an extruded polyethylene jacket. A polyethylene jacket
should also be placed over the insulation.
The most critical part of this system is protection of the field-made joints.
Rigorous inspection methods are needed to ensure that joints are done
correctly. An internally mounted anode beneath the primary weathering barrier
and above the secondary coating has been found effective as an additional
Preventing Chloride Corrosion
With stainless steels, problems have resulted because of leachable chlorides
contained in some insulation. For the 300 series stainless, use insulation that
meets MIL-I-24244 or 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:
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.
For specifications that require less than 10 ppm of chloride in the insulation,
attention must be paid to the mortars and cements used, since these may contain
To prevent water and chlorides from reaching the stainless steel, the external
barrier must be designed properly and maintained periodically.
When steam tracing is used within insulation, extra precautions are needed to
resist stress-cracking corrosion.
Be sure that stainless steel is not coated. In case of a fire, catastrophic
embrittlement could result if zinc, titanium or other metallics were used in
previously applied paints.
The metal jacketing should be securely fastened and banded to prevent water
entry at joints, or where the insulation is supported with attachment angles.
Cement-coated insulation can be finished with a suitable waterproof mastic to
prevent water ingress. The type of insulation and method of application chosen
should assure the absence of shrinkage cracks.
Wrapping equipment with aluminum foil before applying insulation will reduce
the risk of corrosion, since the foil provides a physical barrier that prevents
the saturated chloride solution from reaching the metal surface. Due to its
high thermal conductivity, the aluminum will be at substantially the same
temperature as the equipment, and the chloride solution will shift to the foil,
rather than the stainless.
When steam tracing is used within insulation, extra precautions are needed to
resist stress-cracking corrosion. Extra protection via insulation selection or
use of foil wrap is necessary to protect the costly stainless steel surface.
Knowing where corrosion is likely to occur, such as in low spots and at certain
temperatures, helps establish an inspection regime. The National Board
Inspection Code 4
and the American Petroleum Institute's API 510 require removal of some
insulation at least every five years on all vessels where external corrosion is
possible. Records of the frequency of trouble spots can assist in setting up an
adequate inspection program.
Delahunt, J. F. "Corrosion Control Under Thermal Insulation and Fireproofing."
Proceedings: Exxon Research & Engineering Co. Internal Conference on
Corrosion Under Insulation (1984): p 554.
Butler, G., and H. C. Ison. "Corrosion and Its Prevention in Waters."
Melbourne, FL: Robert E. Krieger. (1976): Ch. VI, p lO2.
Midwest Insulation Contractors Association. "Commercial and Industrial
Insulation Standards." Omaha, NE. (1983): Plate No.1-50.
National Board Inspection Code, NB-23, Rev.6. Columbus, OH: The National Board
of Boiler and Pressure Vessel Inspectors. (1987).
Reprinted by special permission from Chemical Engineering ©
1987. New York, NY: McGraw Hill, Inc. (March 1987): pp 97-100. Reprinted text
has been edited for clarity and space requirements.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.