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Paper No.
613
LABORATORY COATING EVALUATION AND ITS RELATIONSHIP WITH THE
SELECTION TO PROTECT PIPELINE AGAINST EXTERNAL
CORROSION
Valmore Rodriguez, Elias Perozo, Lino Castafieda, Elena Alvarez
Material Technology Department
Intevep, S.A. P.O.Box. 76343 Caracas 1070 A.
Venezuela
There are several pipeline external coatings classified into
different types, among them are extruded and tape polyethylene,
fusion bonded epoxy (FBE), coal tar enamel, coal tar epoxy,
polyurethane, wax, cementitious epoxy and multilayer systems. These
coatings are assumed to protect the pipeline for a certain period
of time. In order to select the most appropriate coating, several
accelerated laboratory tests need to be carried out to obtain their
properties and correlate them with the performance in the
field.
Cathodic disbonding tests at different temperatures and
voltages, adhesion at different temperatures, impact resistance,
loss of adhesion against time under water immersion conditions,
differential scanning calorimetry analysis and water uptake are the
most important tests to carry out in this study. The results
indicate that coatings perform differently in each test. For this
reason the selection must be done according to the soil
characteristics, considering the results obtained in
laboratory.
This paper will provide a guideline to select a coating for a
specific environment and the results that should be expected in the
field. For example, the multilayer system presented the best
performance like low cathodic disbonding, excellent adhesion, high
impact resistance, excellent behavior under hot water immersion and
high temperature resistance. This system can be used in all types
of soil with high life time expectancy in protecting the pipeline.
On the other hand, the polyurethane coating presented high impact
resistance but very low adhesion and high cathodic disbonding that
limits its use in corrosive soil.
Keywords: Coatings, Pipeline, Evaluation,
Copyright 01998 by NACE International. Requests for permlssion
to publish thismanuscript in any form, in part or in whole must be
made in writmg to NACE International, Conferences Division, P.O.
Box 218340, Houston, Texas 77218-8340. The material presented and
the wews expressed in this paper are solely those of the author(s)
and are not necessarily endorsed by the Association. Printed in the
U.S.A.
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INTRODUCTION
Many types of anti-corrosion coatings are available in the
market to be used on pipelines. The coatings act as an isolating
material between the corrosive environment and the pipeline. It is
important to select the best coating system that will have the
longest useful life for the application, taking into account the
economical factor. There are many variables that need to be
considered when a coating decision has to be made. This implies
that the adhesion, cohesion, chemical stability, water absorption,
mechanical properties, soil stress resistance, application,
microbiological attack resistance and cathodic disbonding should be
necessary to maintain the integrity of the pipe.
Some coatings lose their integrity and ultimately fail under
certain conditions to provide the service for which they were
intended. A failed coating may be more harmful and costly than bare
pipeline. It can be argued that a bare pipeline is more expensive
to maintain than coated pipeline because of the increase expenses
for a larger CP system and larger current demand [l]. However, if a
coating fails, the cost can be much higher.
Each coating has a set of properties that describes its behavior
under certain conditions. These properties are usually part of the
manufacturer material specifications or laboratory coating
evaluation.
This paper intends to provide a laboratory evaluation of the
most frequently used organic coatings applied on pipelines and how
this data should be used to select the most appropriate coating for
a specific environment.
COATING SYSTEMS
Fusion bonded epoxv (FBE): This coating has been commercially
available since the early 1960s. Since that time improvements have
been made. Several brands are available in the market for one layer
of FBE coating. Lately, coatings with two layers of FBE have been
introduced in the market. FBE is considered a thin hard and
relatively brittle coating. Most companies that use FBE coatings
have in-house specifications to control its applications [2].
The FBE samples were obtained either from the manufacturer or
application plants.
Coal tar epoxv: Coal tar epoxy is a relatively hard coating that
can be applied in one or two layers. This coating has been used
extensively to recoat existing linepipe and to repair coatings. In
order to obtain good performance of this coating the surface needs
excellent surface preparation. Several products were evaluated and
the samples were supplied by coal tar epoxy manufacturers.
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Cathodic disbonding test.
The cathodic disbonding test is one of the most important tests
for coating evaluation and selection. For this test the ASTM G-8
and ASTM G-42 standards were used. Two different temperatures were
used to evaluate the coatings (25 and 95C). All the tests were run
at -1,5 volts (vs SCE), 3% NaCl aqueous solution for 30 days.
Peeling Resistance test
This test was done only for the polyolefin tape, extruded
polyethylene, wax and multilayer using tape as external layer due
to the intrinsic characteristics of the coatings. Effect of the
temperature on the adhesion was investigated in this study. All
tests were performed using the DIN 30 670 standard.
Coating adhesion
Adhesion is another important property of coating. The adhesion
test was done according to ASTM D-3359, before and after hot water
immersion. For one layer FBE coating, adhesion to steel substrate
was investigated using pull-off strength (ASTM D-4541).
Impact Resistance
Mechanical damage is reported to be one of the major causes of
coating breakdown and resistance to impact will therefore be an
important coating parameter to be analyzed. The impact resistance
was obtained according to ASTM G-14 standard.
Water absorption
The amount of water that a coating system can absorbe has a
negative effect on the coating life time. A high water absorption
will cause low performance. The test was done at 25,60 and 95C for
48 hours.
Hot water immersion
This test was done at 50 and 95C for 48 hours depending at the
condition where the coating will be exposed. The samples were
analyzed before and after the test in order to find blistering,
color change, softening, hardness, etc.
The adhesion after hot water immersion (90C) was determined as a
function of time. The adhesion test after the immersion period was
carried out under ASTM D-4541 standard specification.
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Soil stress
The soil stress effect on tape coating was study in the field.
For this test, coated pipe was exposed to high soil stress clay for
3 years. After this period, the coatings were visual inspected and
the results were analyzed.
RESULTS AND DISCUSSION
FBE coatings
Table 1 shows the properties of FBE coatings evaluated. It can
be observed in this table that the FBE coatings have excellent
adhesion, relatively low impact resistance, moderate water
absorption and low cathodic disbonding. For one layer FBE, the
coatings are susceptible to loss of adhesion and the cathodic
disbondment increases significantly when temperature increases. On
the other hand, for two layers FBE coatings showed good performance
in the high temperature test.
In order to investigate the adhesion under hot water immersion
for one layer FBE coating, an experiment was carried at 90C and the
adhesion was obtained at different times. Table 2 shows the
adhesion values obtained for one of FBE coatings. It is observed in
this Table there is a loss of adhesion with exposure time at high
temperature (9OC) and this loss of adhesion appeared after 10 days
of testing time. This indicates that most of the adhesion test time
recommended in the literature for hot water immersion may not be
appropriate to evaluate this property. It was shown in previous
work that one layer FBE coating had low performance on a hot
pipeline [3].
Water absorption is a very important parameter to study to
assure the performance of the coating. Figure 1 shows the water
absorption study of a two layered FBE coating at 25,60 and 95C as a
function of time. As it is expected, the water absorption increases
with temperature, nevertheless, the values obtained are considered
in the acceptable range. It is important to mention that at a
higher value of water absorption the performance of the FBE
decreases. An intensive study of the performance of 2 FBE layers
was published by Rodriguez et.al. [3,4]. In this study it was shown
that the properties of the two layered FBE coating will perform
well under a high temperature regime.
Tape coatings
The properties for tape coatings are shown in Table 3. This
coating presents high impact resistance (the values shown in table
3 seem to be low but 2 inch diameter pipes were used), low water
absorption, medium-high cathodic disbondment and a moderate value
of adhesion. All tape products presented
61314
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low properties at high temperature, the cathodic disbondment
increases and the adhesion decreases significantly (see Figures 2).
Results on cathodic disbonding test at 65 C in this study has shown
that the disbonded diameter passes acceptance range (> 40 mm).
From these results the tape coatings are not considered for high
temperature (greater than 60C).
One of the properties that need to be investigated is the
resistance to soil stress. In the laboratory, this test is not easy
to do. For this reason, it was decided to carry out a test in the
field under high soil stress clay. This was done to obtain the
effect of the soil stress on the performance of the tape coating
samples. In Figure 3 can be observed the low resistance of tape to
soil stress, this behavior has been consistent to all type of tape
coating that have been tested. From these results, the tape samples
have been found to be susceptible to soil stress. Several authors
have reported this phenomenon previously [5,6]. The selection of
type of tape coating should be not used in those environments with
soil stress.
Multilayer system
The multilayer coatings performed well under the laboratory test
(see Table 4). The coatings have high adhesion to steel, high
impact resistance, excellent performance under hot water immersion,
low cathodic disbonding, low water absorption and kept the adhesion
after being immersed in hot water. The cathodic disbondment values
for high temperature test were in the acceptance range. This
indicates that the coating will perform well in the field under
high temperature operation.
Coal tar epoxy
The characteristic of the coal tar epoxy evaluated are the
following (see Table 5): good adhesion, very low impact resistance,
medium values of cathodic disbondment, low water absorption and
fairly good behavior in hot water immersion. Due to the properties
described above this coating is not recommended for new
construction, it is recommended for repair or recoat existing
pipe.
Urethane
The Urethane coating evaluated presented high impact resistance,
low adhesion, bad performance under hot water immersion, high
cathodic disbondment and low water absorption (see Table 5). The
product evaluated does not satisfy or guarantee the protection of
the pipeline against external corrosion.
61315
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Extruded polyethylene
The extruded polyethylene coating evaluated has the
characteristic of good adhesion, good impact resistance, low water
absorption, low performance under hot water immersion, acceptable
cathodic disbondment value (see table 5). This coating has
performed quite well in the field for more than 20 years.
Nevertheless, problems of microbiological attack have been found
under the conditions of Maracaibo Lake [7].
Cementitious coating
This coating is a cement based with polymer additive. The
properties of this coating is presented in Table 6. The most
important aspects of this coating are: high impact resistance, good
adhesion, moderate value of cathodic disbondment, loss of adhesion
after hot water immersion and relatively high water absorption. It
cracked during the cathodic disbondment test as shown in Figure 4.
This coating needs some improvement to be acceptable as a reliable
one to protect pipeline against external corrosion.
Concrete epoxy on FBE
The combination of concrete epoxy on a layer of FBE gives
excellent results as is shown in Table 6. The most important
properties are: high impact resistance, excellent adhesion,
excellent behavior under hot water immersion, very low cathodic
disbonding value and relatively high water absorption. Furthermore,
this coating has the advantage of high abrasion resistance. This
product was successfully applied on a directional drilling pipeline
where a high resistance to abrasion was needed. The concrete epoxy
was also evaluated as primary coating and results obtained were
under specification. For this reason the concrete epoxy coating is
not considered to be used alone due to low performance in the
laboratory, but it is highly recommended on top of FBE in those
projects that require high mechanical properties, especially
abrasion resistance for the coating.
Wax
This is the most difficult coating to evaluate due to intrinsic
properties of the wax. Nevertheless, effort was applied to
characterize it since this coating is used to do maintenance and
repair in some pipelines. This coating has very low adhesion to
steel (almost none), very low water absorption, very low impact
resistance and the cathodic disbonding value seemed to be
acceptable (see Table 6). It is very important to mention that it
is very difficult to determine the cathodic disbondment value since
this coating does not have adhesion. This coating is used for
maintenance and minor repair of pipeline.
6136
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CONCLUSIONS
According to the results obtained in this study, the following
conclusions are made:
Multilayer coatings have the best properties and it can be used
in all environments, especially for those with high temperature.
Nevertheless, its selection has to be made with economical studies,
since it is considered a high initial cost coating.
The one layer FBE coating can be selected in new construction
basically in all environments, except for those pipelines that
transport high temperature. fluids (it should be selected below
SOOC). Especial attention needs to be paid in those environments
where the coating will be in continuous contact with water since an
inadequate degree of curing will cause premature failure.
The two layers of FBE coatings have overall good properties and
can be selected in the majority of the environments including high
temperature pipelines.
Tape coatings can be used in those environments where soil
stress is not present and the temperature of the pipeline has to be
below 60-C. Special attention should de paid in those humid
environments since this coating has shielding effect.
Extruded polyethylene coating can be selected in those new
projects where the temperature is below 60C and there is low soil
stress effect.
Coal tar epoxy coating can be used to repair or recoat existing
pipelines, taking into consideration the surface preparation and
the low impact resistance of this coating.
Urethane and cementitious coatings need more development to
increase the performance to be considered as a reliable
coating.
The use of concrete epoxy on FBE coating improves the
performance of it. It is recommended for those projects where high
impact resistance and high abrasion resistance are needed. There is
still a limitation with the temperature. The selection should be
limited to those projects under 70-C.
Wax coating should be used for maintenance and minor repair of
pipelines.
613l7
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1. Werner D.P, Lukezich SJ., Hanckock J.R. and Yen B.C. Survey
results on pipeline coatings selection and use, Material
Performance, 11 (1992):~ 16.
2. Griffiths B. and Edwards H.S., The quality control of factory
applied epoxy coating to linepipe Third International Conference on
the internal and External ptotection of pipes, London-England,
1979.
3. Rodriguez V., Perozo E., Gil R., Paiva A., External coating
evaluation for high temperature pipeline, NACE Corrosion95, Paper
No. 370.
4. Rodriguez V., Perozo E., Alvarez E., Coating application and
evaluation for heavy wall thickness, temperature and pressure
pipeline, NACE Corrosion97, Paper No.577.
5. Rodriguez V., Paiva A., Castaneda L., External coating
failure in Venezuelan pipeline, NACE Corrosion94, Paper No.
571.
6. Coulson K.E.W., Nunez S., Choate L.C., Banach J.L., A review
of gas industry pipeline coating practice NACE Corrosion90, Paper
No. 242.
7. Rodriguez V., Paiva A., Astudillo M., New approach to control
microbiological attack to high density polyethylene external
coating, NACE Corrosion 94, Paper no. 572.
613/6
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Table 1. Properties of the FBE coatings evaluated
A: One layer FBE coating company A AT: One layers FBE coating
company A for high temperature AZL: Two layers FBE coating company
A B: One layer FBE company B BZL: Two layers FBE company B
Table 2. Adhesion for one layer FBE coating at 90C at different
exposure time.
Time (days) 0 1 3 7 10 21 28 Adhesion > 1000 > 1000 >
1000 > 1000 > 1000 720 432 fvsij
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Table 3. Properties for Polyolefin tape coatings evaluated
Coating 1 TA-1 1 TA-2 1 TA-3 [ TA-4 1 TB-1 ( TB-2 1 TC-1
immersion
TA-1,2: Tape coating from company A, normal temperature coating
TA-3: Tape coating company A, high temperature coating. TB-1,2:
Tape coating company B, normal temperature coating. TC-1: Tape
coating company C, normal temperature coating
Table 4. Properties of the multilayer coatings evaluated
Coating MA
Thickness (mild 1 49.1
Peeling resistance 62.4 (N/cm) @ 25C
613iiO
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Table 5. Properties of coal tar epoxy, urethane and extruded
polyethylene.
Coating
Thickness (mils) Adhesion (rating: Impact resist. (lbf.inch) Hot
water immersion Adhesion after hot water Cathodic disbonding (mm),
diameter Water absorption (%) Peeling resistance
/ (Nkm)
CTA
22.3 good
15
Visual Changes
good
20
1.54
61 B and C. CTA, 8, C: coal tar epoxy company 1
UA: Urethane coating company D EPE: Extruded polyethylene
company
CTB CTC UD EPE
19 16 37.1 41 good good 4 good
26 18 56 36
Visual Visual No visual No visual Changes Changes changes
changes
good good bad fair
33 14 > 47 22
1.1 1.1 1,61 very low
613lll
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Table 6. Properties of cementitious, FBE-concrete epoxy and
wax.
(Ldf.inch) Hot water No visual No visual immersion changes
changes
Adhesion after 2,3* 1 hot water (N/mm2) Cathodic 22 6 disbonding
(mm), diameter Water 2,75 2.3 63 absorption (%) 70C
Peeling resistance (N/cm) CA: Cementititous coating company A
CEA: Concrete epoxy coating company A WA: Wax coating
I WA 90 ---
17
23
0.23
0.8
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Figure 1. Water absorption for two FBE layers coating at 25,60
and 90C.
5.0
4.0
3.0
2.0
1.0
0.0 ,...m...n...,... 0 20 40 60 80 100 120 1
Time (hours)
14
12
10
8
6
4
2
0
-t- 25C
--m- 94C
0 5 10 15
Time (days)
Figure 2. Peeling resistance of a tape coating at 25 and 94C for
different exposure time.
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Figure 3. Effect of the soil stress on the performance of tape
coating.
613/14
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Figure 4. Crack found in the cathodic disbonded test on
Cementitious coating.
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