Hydrocarbon Processing - September 2021 - 79

Maintenance
and Reliability
S. RUPESH, Independent Consultant, Kerala, India
Corrosion under insulation on stainless steel
Corrosion under insulation (CUI) is
a severe form of localized external corrosion
that occurs on stainless steel (SS)
that has been insulated. It is a costly
problem in the process, chemical and
pharmaceutical industries, with millions
of dollars spent on inspection and repair
of equipment over the past several years.
Although CUI is common in carbon
steel, it also affects SS, which is otherwise
considered corrosion resistant.
This corrosion manifests in the form of
severe pitting. The main reasons for CUI
occurrence in SS are the presence of
chlorides and fluoride ions in insulation,
the presence of an electrolyte (mainly
water) and the presence of oxygen.
Sometimes, the presence of acids gives
rise to accelerated acid corrosion, which
is not discussed in this article.
Furthermore, metal temperatures in
the range of 50°C-175°C are prone to external
stress corrosion cracking (ESCC)
failures under these circumstances.
Relevant standards and test methodologies.
A number of standards define
the requirements for thermal insulation
that is to be used in contact with
SS. However, only ASTM C795, the standard
specification for thermal insulation
for use in contact with austenitic SS, is
considered for this article.
ASTM C795 references two other test
methods, ASTM C692 and ASTM C871.
FIG. 1 from ASTM C795 shows the reference
standard for determining the chemical
acceptability of the insulation material.
It indicates the acceptability of insulation
material on the basis of the plot points of
Cl + F and Na +SiO3
analyses. Tests must
be conducted in accordance with ASTM
C692 and ASTM C871.
ASTM C692, the standard test method
for evaluating the influence of therFIG.
1. Acceptability of insulation material on the basis of the plot points of Cl + F and Na + SiO3
analyses. Figure: ASTM C795: Standard specification for thermal insulation for use in contact
with austenitic SS (2018).
Hydrocarbon Processing | SEPTEMBER 2021 79
mal insulations on ESCC tendency of
austenitic SS (otherwise known as the
" 28-d coupon test " ) is a test method that
evaluates the corrosion possibility of insulation
in contact with stressed SS coupons.
In this test, water is wicked through
insulation or dripped onto non-wicking
insulation that is in contact with the SS
coupons. After a period of 28 d, the SS
coupons are examined for any signs of
ESCC. Four coupons must be tested to
comply with the requirements of the
standard, and none of them can show
ESCC. If any of the coupons show cracking,
then the insulation has failed the test.
ASTM C871, the standard test method
for chemical analysis of thermal insulation
materials for leachable chloride,
fluoride, silicate and sodium ions, is the
test method for determining leachable
chloride, fluoride, silicates and sodium
ions in an insulation. The results from this
test are checked with the empirical graph
in FIG. 1 in ASTM C795 for " chemical acceptability "
for use on SS, which is determined
if the plot points of these analyses
fall within the acceptable area of FIG. 1.
For an insulation material to be acceptable
for use in contact with SS, the
material must meet the chemical acceptability
criteria and pass the 28-d coupon
test. Passing only one of the two tests is
not sufficient.
Pre- and post-insulation considerations.
Once the insulation material selection
is finalized, there are several important
considerations for pre-insulation

Hydrocarbon Processing - September 2021

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