A High Performance, Damage Tolerant Fusion Bonded Epoxy Coating
D.G. Enos, J.A. Kehr, C.R. Guilbert
3M Company
Corrosion Protection Products Dept.
3M Austin Technical Center
6801 River Place Blvd.
Austin, TX 78726
USA
ABSTRACT
A novel technique has been developed which significantly improves the damage tolerance of epoxy coated
components in a variety of corrosive environments. Characterization of these coatings via an array of AC and DC
electrochemical techniques has revealed considerable improvements in the overall corrosion performance as a result
of this technology. The quality of the protective coating is thereby maintained, despite minor damage, which may
occur on the job site. Furthermore, this improved corrosion performance enhances cathodic disbondment (CD)
performance while the impact resistance and flexibility of the fusion-bonded epoxy (FBE) coating are unaffected.
INTRODUCTION
Corrosion is a global problem, consuming three to four percent of gross national product in the developed
countries of the world.[1] Selecting economical and effective techniques for minimizing the effects of corrosion is a
critical design decision for pipeline systems. Corrosion protection is essential to prevent leaks, environmental
disasters, fire and explosion, personal injury, service disruption and costly maintenance. Thus, in addition to
effectively dictating the lifetime of the pipeline, the corrosion prevention system also significantly influences the
pipelines operational costs such as general maintenance, pumping energy, and capacity upgrades. Although these
protective measures are critical, they represent only a small fraction of the overall long-term cost of a pipeline
system.
Today, higher operating temperatures and a host of hostile environmental conditions that pipeline materials
encounter during installation and use require a new generation of coatings [2] to protect both the interior and
exterior of the pipe. Hydrogen sulfide (H2S), increased levels of carbon dioxide (CO2) and other factors contribute to
a pipeline environment that increases levels of internal corrosion. There are many possible solutions to these
problems including use of corrosion resistant alloys, inhibitors, and corrosion protective linings and coatings. FBE
coatings have offered an effective solution to these problems for nearly 40 years.
As the times and exposure conditions have changed, FBE coatings have evolved as well. More demanding
transportation, storage, installation, and pipeline operating conditions require the use of advanced FBE-based worldclass coatings to protect the pipe exterior. In addition, improved field application technology provides comparablequality same-system coatings for the girth weld. FBEs have been formulated to operate in these very harsh service
environments.
Background, History, and Advantages of FBE Linings and Coatings
An FBE is a one part, heat curable, thermosetting epoxy resin powder. It is a material that utilizes heat to
melt and adhere to a metal substrate. It provides a coating with no trapped solvents, excellent adhesion, and a tough,
smooth finish resistant to abrasion and chemicals. FBE coatings have been in use since 1960 to protect pipelines
from corrosion. It is estimated that over sixty thousand miles (ninety-five thousand kilometers) of FBE coated
pipelines are installed around the world.
FBE is currently specified in the oil, gas, and water pipeline industries. It has been used as an internal
lining in desalination plants in Australia and the Middle East and on gas [5] and oil transmission lines It has been in
use protecting downhole tubing for over twenty years. More recently, it has been used in sour crude pipelines [4]. In
addition to its use as a stand-alone exterior coating, FBE is the favored primary coating for three-layer polyolefin
corrosion coatings, where it has been in use since 1979.
In the water industry, FBE provides a thinner lining than competing materials such as concrete, enabling
smaller pipe sizes and reduced bulk and weight during handling and installation of pipe. The smooth, hard FBE
coating provides reduced friction compared to uncoated or concrete-lined pipe. This results in more efficient flow,
reduced energy costs, and lower-installed pump or compressor investment. It has been used on valves, pumps, and
fittings in water districts for both water and sewer systems in California for nearly thirty years. It is in use protecting
brine-pit piping systems, solving both erosion-corrosion problems as well as general corrosion [5]. FBE has been
used in high-sand-content seawater cooling pipework for ten years and is still in excellent condition. It has been
applied to valves and pipework handling seawater for the US Trident Submarine program and has a twenty-year
history in the pump manufacturing industry effectively protecting against cavitation and slurry damage. In the UK, it
has protected drinking water pipework since 1978, with coating on over two hundred thousand square meters [6].
Specific formulations meet the drinking-water requirements in many countries.
There are reports of six to eighteen percent flow efficiency improvements in gas transportation when using
FBE internally lined pipe as opposed to bare steel pipe [3]. Using the six percent figure on an eight-hundred mile
(thirteen-hundred kilometer), DN 750 (NPS 30) pipeline with a discharge pressure of 140 kPa (960 psig) and a
compressor station every eighty miles (one-hundred-thirty kilometers), the potential savings are over four million
dollars in compressor equipment cost and an annual energy savings of about a million dollars.
Damage Tolerant FBE Coatings
Although FBE coatings are extremely effective corrosion protection materials in a wide variety of
environments, their overall effectiveness is still closely tied to the quality of the applied coating. As with nearly all
barrier coatings, defects caused by mishandling or misuse of the coated pipe significantly reduce it’s ability to
provide the superior corrosion protection required for many applications. Coating damage frequently occurs during
the handling and installation of a pipeline, or is the result of rock damage occurring during backfill [7]. The solution
to this problem is seemingly quite simple: if the FBE coated pipe is treated properly, and the coating remains
structurally sound, the pipeline should readily meet or exceed its design life. Unfortunately, the type of damage
which is causing the problems discussed above is nearly impossible to avoid. Policing jobsites and preventing the
use of questionable construction practices is problematic at best. Similarly, preventing rock damage during backfill
is nearly impossible, and even more difficult to verify. Nevertheless, irrespective of the pipe coating condition, or
the abuse level, it still needs to get the job done—preventing the onset of corrosion and subsequent structural
problems. It is better to assume that damage to the coating is inevitable, and, instead of trying to prevent it, design
the coating to survive it with much of its corrosion-preventive properties intact. In this study, such a coating is
pursued.
Through the addition of microencapsulated materials, a self-healing FBE coating has been designed. As
illustrated below, this coating is considerably more damage tolerant than traditional FBE coatings. The mechanical
damage which weakens conventional coatings instead ruptures the microcapsules which in turn “heal” the FBE
coating, preserving its barrier properties. What follows is a brief description of the coating technology under
evaluation, along with promising preliminary results.
Microencapsulation is a technique through which liquid materials, such as oils, are encapsulated within a
seamless, solid shell. An example of microencapsulated oil is presented in Figure 1. A wide variety of shell wall
materials are available – the appropriate choice is determined by a combination of the application, the material to be
encapsulated, and the desired stimulus to rupture the capsule (e.g., impact, pH, or solution chemistry). In general,
microcapsules are between 5 m to 200 m (0.2 mils to 8 mils) in size with wall thicknesses on the order of 1 m to
2 m (0.04 mils to 0.08 mils). Fill materials may be either aqueous or non-aqueous in nature. What is important to
note is that, once encapsulated, liquid-based fill materials behave as a solid. As a result, considerably larger
quantities of the fill material may be added to a coating without adversely affecting its properties.
As an example, suppose a liquid inhibitor has been demonstrated to effectively halt corrosion of the metal
to be coated. The coating formulator then makes the decision that the inhibitor should be added to the current
coating formulation. However, in the case of a powder coating such as an FBE, only very limited concentrations of
a liquid additive may be added before the powder begins to clump and hinder application. If that same liquid is
encapsulated prior to addition to the powder, large concentrations of the inhibitor may be added with little or no
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detrimental effect on the handling or application of the coating material. A similar scenario holds for liquid-based
coatings, where large additions of an inhibitor may hinder cure of the coating or degrade physical properties.
In this study, a series of modified coatings were formulated and produced, as illustrated in Figure 2. As can
be seen in the figure, the coating consists of two layers. The first layer is a primer layer containing the
microcapsules. The microcapsules contain approximately 80% active fill, and are approximately 50 m (2 mils) in
size. The epoxy matrix of the primer layer is an unmodified, flexible FBE. The second layer of the coating, or the
topcoat, is composed entirely of an unmodified FBE. More details on the actual formulation may be found below in
the Experimental Methods section.
Also illustrated in Figure 2 is the manner in which the coating is designed to function. The coating will
behave the same as a traditional FBE until it has been subjected to mechanical damage. This damage may be the
result of an impact to the coating, microcracking of the coating during bending on the job site, etc. Once damaged,
the microcapsules release their protective fill. By placing the microcapsules close to the steel surface in the primer
layer, they are able to deliver their protective fill directly to the regions near the metal surface where they are
needed. A variety of fill materials have been investigated, consisting essentially of combinations of corrosion
inhibiting materials and sealants. Once released, the fill material reseals the coating and prevents the onset of
corrosion, preserving the protective benefits of the FBE.
Although placing the microcapsules in the primer layer is the simplest embodiment of this coating concept,
it is not the only one. Figure 3 illustrates a coating that has microencapsulated additions in both the topcoat and the
primer layer. In this example, a series of different microcapsules could be envisioned, each serving a different
purpose. Microcapsules containing corrosion inhibiting materials could be placed in the primer layer, ensuring their
delivery directly to the metal surface. In the topcoat, capsules containing sealants could be used, their purpose being
to reseal the damage site and hold in place the inhibitive materials released from the primer layer. In addition,
microcapsules containing a dye could also be incorporated into the topcoat. Upon damage, the dye capsules would
rupture, releasing their fill and locally discoloring the coating (e.g., turn a green coating red), easing the
identification of damage sites along the pipe for later repair with appropriate patch materials.
EXPERIMENTAL METHODS
Coating Composition and Preparation
Prior to coating, mild steel bars were degreased in 2-butanone (MEK) and isopropanol, then grit blasted to
a near-white metal finish in accordance with NACE No.2/SSPC-SP 10. Each bar was preheated for 45 minutes to a
temperature of 400F (205C), after which they were coated via a fluidized bed system. A two-step coating was
accomplished by first dipping the sample into the primer bed, after which the sample was immediately transferred
to, and coated in the topcoat bed. Samples were left in each fluidized bed for sufficient time to allow the coating to
build to the desired thickness. Coating thickness was verified using a magnetic thickness gauge.
The FBE coatings investigated in this study were based on a flexible fusion bonded epoxy resin. Control
samples were coated with 400 m (16 mils) in a single coating operation. Microcapsule containing coatings were
produced by first depositing a primer layer 150 m (6 mils) in thickness which consisted of 85% unmodified FBE +
15% microcapsules, followed by a 250 m (10 mils) top coat of unmodified FBE. The two layers were applied
sequentially (i.e., no additional preheating).
Electrochemical Testing
Solution Preparation. All solutions were prepared using distilled water and reagent grade chemicals. The cathodic
delamination testing solution was made in accordance with ASTM G8 [8] and contained 1% each of sodium
chloride, sodium carbonate, and sodium sulfate.
Corrosion initiation tests were conducted in mildly acidic, 3.5 wt% sodium chloride solutions. The sodium
chloride solutions were first made to the desired concentration after which the pH was adjusted (via HCl) to 5.
3
Cathodic Delamination (CD). Samples were prepared by machining a 6 mm (0.25 inch) defect in the center of a
11.4 cm x 11.4 cm (4.5 inch x 4.5 inch) coated panel. The samples were then placed in the cell pictured in Figure 4.
Next, the solution was added and the samples polarized for 30 days at –1.44 V vs. a saturated calomel reference
electrode. Periodic electrochemical impedance spectroscopy (EIS) spectra were taken from each sample throughout
the 30 days. Upon completion of the test, the coating around the intentional holiday was removed with a knife and
the delamination radius measured in accordance with ASTM G8. Four replicates of both the control and the capsule
containing samples were investigated.
Impact Corrosion Testing. Following coating, samples were subjected to an impact of 80 in-lbs (9 Nm) with a
0.625-inch (15.9-mm) diameter tup. A holiday detector was utilized to verify that the coating had been disrupted
and that bare metal was exposed. Next, the samples were placed into a pH 5, 3.5% NaCl solution for 30 days.
Throughout the testing, periodic electrochemical impedance spectra were taken from each sample. Four replicates
of both the control and microcapsule containing coatings were investigated.
Electrochemical Impedance Spectroscopy. EIS testing was performed utilizing a PAR Model 273A Potentiostat in
combination with a Solartron 1255 FRA and two PAR Model 314 Multiplexers. All experiments were conducted
under software control via ZPlot (Scribner Associates, Inc.). Experiments were performed about a DC bias of –1.44
VSCE for the CD experiments, and about the open circuit potential for the impact-damaged samples. The
perturbation frequency was scanned between 106 Hz and 1 mHz. A waveform of 25 mVRMS was used in all cases.
RESULTS
Cathodic Delamination Testing
Cathodic delamination testing was performed for traditional FBE coated samples as well as for the
microcapsule loaded coating. Four replicates of each coating were evaluated. Figure 5 illustrates the typical results
for the two coatings. As can be seen in the figure, the delamination radius was reduced by 60% from 9.8mm (0.38
inches) for the traditional FBE to 4.2mm (0.17 inches) for the capsule loaded coating. There was very little
variation among the four replicates of each coating. On an area basis, the delaminated area was reduced by 68%.
Neither coating was discolored, blistered, or observably swelled after the 30-day experiment.
Corrosion Initiation from a Damage Site
To determine if the microencapsulated additions actually prevent the onset of corrosion, coated samples
were damaged as discussed above and placed into a mildly acidic, high chloride solution. While in the bath,
electrochemical impedance spectroscopy was used to qualitatively evaluate changes occurring at the damage site.
Figures 6 and 7 present a comparison of impedance data for the conventional and modified FBE coatings. As is
clearly evident in the figures, a second time constant is present in the case of the modified coating. This second time
constant represents the newly sealed layer resulting from the ruptured microcapsules. Throughout the time of the
test it can be seen that the polarization resistance of the modified FBE coating remained significantly higher than
that of the conventional FBE coating. Based on this information, it may be inferred that the modified coating is
providing increased protection compared to the standard FBE coating.
To further demonstrate the ability of the modified coating to provide increased protection, samples were
prepared as described above, then placed into a pH 5, aerated, 3.5 wt% NaCl solution at 60C (140F). Within a
matter of hours, rust blooms were observed within the damage site for all conventional FBE coated samples. In the
case of the modified FBE coated samples, no corrosion initiation was observed within the defect for two weeks.
This is illustrated in Figures 8a and 8b. Note the minor attack visible on the modified FBE coating as compared to
the severe attack visible on the conventional coating. After 6 weeks, more significant corrosion was visible on the
modified FBE coated sample. However, the corrosion of the conventionally coated sample was again more severe
then the microcapsule containing, modified FBE coating.
DISCUSSION/SUMMARY
ability.
To summarize, a modified FBE coating has been presented which possesses an extraordinary self-healing
This self-healing characteristic is achieved through the use of microencapsulated additions that are
4
positioned close to the metal/coating interface. In the particular example presented in this paper, the microcapsules
are designed to rupture and release their protective fill when the FBE coating is subjected to mechanical damage.
These protective fill materials may include corrosion inhibiting materials as well as sealants for the coating. As a
result, a protective material is delivered to and held in place at the steel surface within a damage site. In addition,
the damaged coating is sealed, and the protective nature of the coating is preserved.
Experiments were conducted on coatings containing a wide variety of microcapsule types and fill materials.
Preliminary results for one of the more promising combinations were presented above. As was discussed
previously, electrochemical testing to determine if the microcapsules increase the corrosion resistance of a damaged
coating revealed that the onset of corrosion was delayed considerably. In addition, in an industry standard cathodic
disbondment test, a significant improvement in performance was observed.
Other advantages of this technology revolve about the self-healing capability of these coatings in
combination with other features that may be attained through the use of microencapsulated additions. For example,
through the use of dye capsules, a coating could be produced that, in addition to possessing the self-healing
capabilities described above, would positively indicate where damage occurred. This indicator would enable more
efficient repair of damage areas via an appropriate patch material. In addition to aiding the patch process, the newly
sealed layer formed by the ruptured microcapsules further enhances the performance of the patch, providing a more
effective barrier to the external environment. Perhaps most important, though, is that the modified FBE coating will
be able to effectively handle the abuse which current FBE experiences en route to and at the job site.
The microcapsule containing coating also provides additional benefits when used in conjunction with a
cathodic protection system. When such a system is utilized, one of the primary factors dictating operational cost and
overall effectiveness is the total defect area present in the coating. As the total defect area increases, the amount of
cathodic protection current required to protect those areas increases – thus increasing the cost of operation. In
addition, as the overall cathodic protection current increases, the effective throwing power of the CP system
decreases, due to IR drop. In other words, a larger portion of the applied potential will be comprised of IR drop –
unless compensated, insufficient cathodic protection will be applied. Since the overall effect of the coating
described above is to minimize the amount of damage experienced by the pipe, such a coating will also reduce the
cost and help maintain the effectiveness of a cathodic protection system as well.
In addition to the benefits described above, the modified FBE coating offers a number of advantages over
similar corrosion prevention technologies. Although this approach appears to be simply the addition of corrosion
inhibitors to a coating, it is much more than that. Corrosion inhibitors are added in such a way that very large
concentrations (considerably more than could be added to a coating via conventional means) are delivered to the
damage site, while very little of the material is wasted. This is in contrast to the typical procedures followed when
using inhibitors. When protecting the internals of a pipe system, the use of chemical corrosion inhibitors requires
that large quantities of inhibitor be added to the process stream to protect a relatively small amount of material (i.e.,
the entire stream must be treated, even though only that portion which is in contact with the steel needs to have
inhibitor present – the remainder is waste). In the case of external protection of the pipe, the use of inhibitors is
typically not possible due to environmental concerns. The system above alleviates many of the problems of using
inhibitors – so, protection of the external surface of a pipe is possible, since the inhibitor is confined to the coating.
By sealing the inhibitive materials in place at the damage site, there is no need to continuously augment the system
with additional inhibitor.
Another advantage of a modified FBE coating as presented above is its similarity in performance and
handling to traditional FBE coatings. By maintaining a form that is handled in the same manner as conventional
FBE coatings, no new technologies must be invested in and learned by applicators. The modified FBE coating may
be applied via electrostatic spray or through a fluidized bed. The only change required is the addition of another
series of spray guns in the existing equipment (this assumes, of course, that a multi-layer coating is applied). Since
the coating behaves identically to a conventional FBE coating, no new pipeline construction practices or procedures
need be adopted.
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CONCLUSIONS
In conclusion, in this study a robust FBE coating has been designed through the application of
microencapsulated additions. Upon being damaged in a manner that would compromise a conventional FBE, the
microcapsules are ruptured, releasing their protective chemistry. As a result, the damaged FBE coating is healed,
retaining much of the protective properties that it possessed prior to damage. The list below summarizes the benefits
of this technology and its advantages:

The modified FBE coating presented in this study possesses superior resistance to corrosion initiation in the
damaged state compared to conventional FBE coatings.

The modified FBE coating presented in this study possesses superior resistance to cathodic delamination
compared to conventional FBE coatings.

Unlike cathodic protection, increased maintenance requirements or expensive equipment that is subject to
failure in the field does not accompany this increased resistance to corrosion.

The overall effect of this coating is the reduction of defect area on a coated pipe. Since the cost to operate a CP
system is a direct function of the defect area, this coating reduces operation costs associated with the use of a
CP system.

Implementation of this technology will be facilitated by the fact that the same technologies are used in its
application. No new technologies must be mastered by coating applicators – the modified FBE can be readily
applied with existing equipment and methods.
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ACKNOWLEDGEMENTS
The authors gratefully acknowledge the technical support provided by Susan Weiland and Stephen Daniell
in the 3M Corrosion Protection Products Department.
REFERENCES
1.
Islam, Moavin, “Condition Evaluation of Reinforced Concrete Structures: A Case Study,” Paper No. 521,
NACE National Corrosion Conference, Corrosion ‘95.
2.
Strobel, Rupert F., “Fusion-Bonded Epoxy Coatings for Pipeline Corrosion Protection,” 1981, 3M Company
publication.
3.
“Corrosion Control Report: Internal Pipe Coatings are a Wise Investment,” Pipeline and Gas Journal, March
1993, pp. 67-69.
4.
Read, Thomas, “Yates Field Crude Line Coated Internally, Externally,” Pipeline and Gas Journal, February
1982.
5.
Carlson, Ron E., Jr., “Internal Pipeline Corrosion Coatings Case Studies and Solutions Implemented,” Paper
No. 27, NACE 1992 Annual Corrosion Conference.
6.
Langford, Paul, Dr., “New Developments in Coatings for the Internal Protection of Water Industry Line Pipe,”
unpublished, 1993.
7.
Norman, D., Gray, D., “Fusion-Bonded Epoxy Pipe Coatings – 10 Years’ Experience,” Materials Performance,
Vol. 32, No. 3 (1993), p.36.
8.
“Standard test Methods for Cathodic Disbonding of Pipeline Coatings,” ASTM Standard G8-96.
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Figure 1: Micrograph of typical microcapsules containing a protective fill.
FBE
Damage
Steel
Self Healing
Newly Sealed
Layer
Figure 2: Schematic illustrating the function of a single primer layer containing corrosion-preventative
microencapsulated additions.
8
Damage
Steel
Damage Indicator
Self Healing, Indicating
Newly Sealed
Layers
Figure 3: Schematic illustrating the function of a coating in which both the primer and top layer contain
microencapsulated additions. By careful selection of the fill materials, the function of the top and bottom layers
may be tailored (e.g., primer provides corrosion inhibiting materials, while the top layer provides materials which
seal the damage site and possibly provide a visual indication of the damaged area.
CE
Clamping System
RE
Acrylic Cell Body
WE
Defect
O-Ring
Seal
Coated Sample
Figure 4: Schematic of electrochemical cell used for the testing and evaluation of the FBE coated plates (i.e., CD
testing, impact testing, etc.).
9
A
B
Figure 5: Typical cathodic disbondment results after 30 days in 1% sodium chloride + 1% sodium sulfate +1%
sodium carbonate at room temperature with an applied potential of –1.44 VSCE for (a) unmodified FBE (9.2 mm) and
(b) unmodified FBE with a 150 m (6 mil) primer layer containing 15 wt% microencapsulated additions (4.7 mm).
Coating thickness was 350 m (14 mils) in both cases.
10
105
-70000
-55000
104
103
102
10-3
-40000
10-2
10-1
100
101
102
103
104
105
Frequency (Hz)
-90
-25000
Phase Angle
Imaginary Impedance (Capacitive)
Magnitude
Scotchkote 413SG
Modified FBE
24 Hours
pH 5, 3.5% NaCl
-10000
0
15000
30000
45000
-40
-15
10-3
60000
Additional Time
Constant
-65
10-2
10-1
Real Impedance (Resistive )
100
101
102
103
104
105
Frequency (Hz)
Scotchkote 413SG
Modified FBE
Figure 6: Typical EIS spectra for an unmodified FBE and the microcapsule containing material after 24 hours.
Note the presence of an additional time constant indicative of the inhibitor film in the case of the microcapsule
containing material.
105
-17500
-15000
-12500
104
103
102
101
10-3
-10000
10-2
10-1
100
101
102
103
104
105
Frequency (Hz)
-7500
-90
Phase Angle
Imaginary Impedance (Capacitive)
Magnitude
Scotchkote 413SG
Modified FBE
360 Hours
pH 5, 3.5% NaCl
-5000
-2500
0
0
2500
5000
7500
10000
12500
15000
-75
-60
-30
-15
0
10-3
17500
Real Impedance (Resistive )
Additional Time
Constant
-45
10-2
10-1
100
101
102
103
104
105
Frequency (Hz)
Scotchkote
413SG
Figure 7: Typical EIS spectra for an unmodified FBE and the microcapsule
containing
material after 360 hours.
Modified FBE
Note that the additional time constant is still present for the microcapsule containing material, illustrating its
persistence.
11
A
Severe corrosion
initiation
B
Minor
corrosion
initiation
Figure 8: Typical results for impact damaged coatings exposed in 60C (140F), pH 5, 3.5% NaCl solution for
(a) an unmodified FBE and (b) a coating containing the microencapsulated materials. The microcapsules
dramatically increased the time to corrosion initiation in this qualitative test (from 4 hours to nearly two weeks).
12