This document discusses ways to prevent coatings from peeling on pipelines. It reports on several pipeline coating failures in recent years caused by extreme operating environments and discrepancies between actual service conditions and coating design conditions. Recent case studies show disbonding of coal tar, FBE, and 3LPE coatings. Investigations aim to enhance adhesion between coatings and steel substrates. New coating materials and longer hot water immersion tests provide better adhesion. A silane-based chemical treatment is presented as an environmentally-friendly alternative to chromate treatments for improving coating-steel bonding and corrosion protection. Test results show the silane treatment provides similar adhesion benefits as chromate after hot water immersion.

1. Preventing
peeling
P. Collet, BS Coatings, France,
discusses ways of tackling
coatings disbondments.
S
everal pipeline coating failures have been
reported by corrosion experts over the
last few years. The major causes of the
various corrosion problems reported may be
summarised in the following two groups:
The operating environment of the pipelines, where
extreme conditions may be encountered due to
high temperature or humidity or a combination of
both.
The discrepancy between the real service
conditions and the conditions the coatings were
actually designed to meet under daily operating
conditions, which consequently may have
adversely impacted the choice of the coating
system and the associated surface preparation of
the steel pipes.
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Facing disbondment issues
Recent case studies have reported large-scale
coating disbondments, which could involve corrosion
problems beneath the coating and lead to potential
damage. Examples have recently been reported for
‘old’ and ‘modern’ coating systems:
Loss of adhesion of coal tar coatings, reported in
Kuwait1
, where the general poor adhesion was the
main source of disbondment.
Blistering problems of FBE coatings appearing
above 90 ˚C, observed in France and Angola.2
Problems of massive loss of adhesion between
epoxy and steel reported in several countries,
such as India3
and South America4
with a 3LPE
system.
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2. Disbonding of an HSS field joint coating, leading to
corrosion caused by the “cathodic protection shielding
effect”, observed in Angola.2
Investigation and required coating properties
In-depth technical evaluations and R&D efforts have been
sponsored in order to enhance our understanding of the
phenomenon and its causes.
As far as the investigation and the possible explanation for
disbonding of 3LPE is concerned, various assumptions are
listed hereafter: water and oxygen diffusion through PE, water
saturation of the FBE layer, superficial corrosion of the steel
surface forming magnetite, all these steps being accelerated
by temperature.2
Therefore, one consequence has been to investigate
the adhesion strength of FBE primers on steel, taking into
account the water diffusion through the topcoat within the
lifetime of the pipeline and the water permeating through
the polyolefin layer in less than 300 days at 60 ˚C.5
A
methodology based on the ageing of bi-layer FBE/adhesive
coatings was then proposed to evaluate the FBE adhesion
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to steel during wet ageing. Other technical experts have also
recommended a change to longer term (120 days) hot water
immersion tests at operating temperature so as to provide a
better prediction of loss of adhesion after ageing.6
A property
known as “wet Tg” has also been newly considered to assess
the mechanical and electrical properties of the FBE.2, 7
These efforts have led to new criteria for selecting
materials, so that mono-layer and multi-layer coatings achieve
optimum bonding to steel and chemical bonding between the
layers.
At this point, it is also important to mention that the
disbondment of three-layer coatings is possibly caused by
internal stresses.8
Although this has to be considered, this
influence is not covered in this article.
Materials selection
Extensive works have been led by the major FBE producers
to develop new grades to match the forthcoming requests
from operators to solve the above-mentioned technical
issues.
These works have focused on the improvement of the
barrier effect against corrosive components such as water
and salts by increasing the Tg and the hydrophobic behaviour
of the epoxy primer. These developments have led to major
improvements in terms of adhesion performance at high
temperature, less sensitivity to hot water absorption, higher
wet Tg, tensile strength improvement even after immersion
in water and better cathodic disbondment resistance at high
temperatures.7
Necessary but not good enough
The optimisation of the anti-corrosion protection power
of the coating is therefore linked. Firstly, to its intrinsic
characteristics that enable the material to withstand the
service constraints and, secondly, by maintaining its
adhesion vis-à-vis the substrate.
The interest of strengthening the adhesion of FBEs used
as mono-layer coatings or as primers in multi-layer systems
is linked to two reasons:
The abrupt variations in temperature that occur during
the coating application process lead to thermal shocks
within each constituent (including the steel), which cause
mechanical stresses at each interface, possibly reducing
inter-layer adhesion.8
The penetration of species such as water, oxygen and
salts into the coating causes a reduction in the affinity
between the metal substrate and its protective coating at
the interface, resulting in blistering and loss of adhesion.
The purpose of the surface preparation consists of two
objectives:
Obtaining a clean substrate.
Optimising the surface roughness.
This is to create a strong link between the coating and
the substrate by improving the ‘wettability’ of the steel to be
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Figure 2. Disbonding HSS. (Photo courtesy of TOTAL, France).
Figure 1. Disbonding 3LPE. (Photo courtesy of TOTAL, France).
Reprinted from April
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3. protected and the (mostly) physical/mechanical anchoring of
the coating to be applied.
However, the force of such linkages is less than that of a
covalent chemical bond.
A technical answer but not an
environmentally-friendly
solution
Chemical treatments have been used
for many years to prepare steel prior
to it being coated in order to obtain
maximum adhesion of the (FBE)
coating on the (steel) substrate, thanks
to this chemical bond.
The main chemical treatments
(chromation, acid washing) efficient in
assuring chemical bonding between
the coating and the steel substrate
have been restricted or banned due to
health, safety and environment issues
(toxic substances, waste disposal).
As far as the required technical performance for the
external pipe coatings are concerned, the current situation
is hence ‘bottlenecked’ by the HSE limitations of the current
solutions for chemical surface treatment, as illustrated by
Figure 3.
A sustainable solution for sustainable
performance
Silpipe®
SCT is a silane-based solution specifically designed
by additivation for the chemical surface treatment of steel
pipes as a technical environmentally friendly alternative to
the chromation technique.9
The associated patented SILPIPE®
process is based on
this Silane-based Chemical Treatment (SCT) of a grit blasted
surface, before heating the steel pipe up to the correct
temperature for epoxy powder coating application, as shown
in Figure 4. This process takes place in several steps, as
described in Figure 4.
As far as the application of the Silpipe®
SCT solution
is concerned, the amount applied to the metal substrate
is around 50 g/m2
. No surface rinsing is necessary before
heating up the steel pipe to the required temperature for
applying the FBE.
SILPIPE®
SCT has to be specially designed for its
compatibility with the epoxy groups of the FBE and modified
to improve the anticorrosion properties. It is an aqueous
solution in the form R-Si—(OH)n, presenting the following
advantages:
No Si-OR alkoxy functions, avoiding the presence of
co-solvent (through hydrolysis).
Very high stability of the aqueous solution.
Fast direct reaction of the Si—(OH) function with the metal.
Furthermore, this solution is a solvent-free and VOC-free
alternative.
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Figure 5. Peeling vs. immersion time of the three-layer coating,
with SILPIPE® SCT chemical metal treatment.
Figure 6. Peeling vs. immersion time of the three-layer coating,
without SILPIPE® SCT chemical metal treatment.
Figure 3. Process for selecting and applying coating at the
required quality level.
Figure 4. Diagram of the SILPIPE® process.
Reprinted from April
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4. Benefits for the coating performance
The performance of this new Silpipe®
SCT solution has
been tested through the evaluation of both FBE coatings
and three-layer coatings and the discussed results are
focused on the adhesion performance after hot water
immersion.
Mono-layer coating
The characteristics of the selected FBE are shown in
Table 1. It was applied by means of an electrostatic spray,
on grit blasted (roughness between 70 and 90 µm/Sa 2.5
surface cleanliness) steel plates and the coating applied
between 350 µm and 450 µm was post cured for
10 minutes at 200 ˚C.
Tests on the FBE monolayer were run by monitoring
the adhesion following hot water immersion, as described
hereafter:
The metal plates were immersed in tap water at 80 ˚C,
for 40, 80 and 120 days.
Pull-off adhesion tests, according to the ISO 4624
Standard, were carried out after 120 days immersion
time. Peeling tests, according to the EN 10289
Standard, were carried out after 40, 80 and 120 days
immersion times.
The compared adhesion performance between
chromate and Silpipe®
SCT chemical treatments,
summarised in Table 2, shows that the treatment efficiency
of the SILPIPE®
SCT solutions used matches the efficiency
of the chromate solutions in coating adhesion after hot
water immersion for 120 days.
Three-layer epoxy-polyolefin coating
The following results are based on a study conducted on a
three-layer coating applied on a pilot scale application line
and further at an industrial scale.
Notes relative to the coatings materials used:
The coating comprised an epoxy powder primer, an
adhesive and a top coat.
The epoxy powder was identical to that previously
described, while the adhesive (ca. 250 µm thickness)
was a polyolefin grafted with maleic anhydride-based
groups, the softening point of which, determined by
DSC, was 135 ˚C and the top coat (ca. 3 mm thickness)
was made of HDPE.
Pilot trial
The coating was applied to the external surface of a 7 mm
thick steel pipe, with an external diameter of 116 mm. Two
series of test pieces were considered:
Series 1: coating applied on clean grit blasted surface.
Series 2: coating applied on clean grit blasted surface
with Silpipe®
SCT surface treatment.
Before immersion in water at 60 °C, on each test piece
(10 cm long sections), the three-layer coating was cut
through its whole thickness. Two incisions, 2.5 cm apart,
were made over the whole circumference of each test
piece to facilitate the ingress of water at the level of the
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Table 2. Adhesion assessment of FBE coatings versus immersion time in tap
water at 80 ˚C
Surface
treatment
Loss of adhesion (mm)
EN10289
Pull off test (N/mm2
) ISO4624
After
40 days
After
80 days
After
120 days
After 120 days
5% chromate
solution
1 mm 2 mm 2 mm 20 +/-2
10% chromate
solution
1 mm 2 mm 2 mm 20 +/-2
5% Silpipe®
SCT
solution
1 mm 2 mm 2 mm 20 +/-2
10% Silpipe®
SCT
solution
1 mm 2 mm 2 mm 20 +/-2
Table 3. Adhesive characteristics
Melt index (190 ˚C/2.16 kg) ISO 1133 5 g/10 mm
Melting point DSC 134 ˚C
Vicat softening point ISO 306 121 ˚C
Table 4. Top coat characteristics
Melt index (190 ˚C/2.16 kg) ISO 1133 5 g/10 mm
Melting point DSC 128 ˚C
Vicat softening point ISO 306 120 ˚C
Table 1. Main characteristics of the FBE
Methods Results
Particle size Laser
spectroanalyser
Average: around
50 µm < 10% above
96 µm
Specific gravity ISO 2811 Around 1.4 g/ml
Moisture content Weight loss at
105 ˚C
< 5%
Gel time at 180 ˚C ISO 8130-06 Around 70 sec
Tg of cured film NFA 49-706 Around 105 ˚C
Minimum time of cure DSC analysis
180 ˚C 160 sec
220 ˚C 70 sec
240 ˚C 50 sec
Performance of 400 µm cured film applied on steel substrate
Methods Results
Impact resistance ISO 6272 > 20 J
Flexibility CAN CSA Z245.20.02
0 ˚C Pass
-30 ˚C Pass
Water absorption after immersion at
100 ˚C for 100 days
ISO 62 Less than 7%
Cathodic disbondment at 23 ˚C for
28 days
CAN CSA Z245.20.02 Less than 6 mm
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5. epoxy-metal substrate interface. This process quickly leads
to a loss of adhesion of the epoxy vis-à-vis the steel. This
performance can be visually observed in Figures 5 and 6.
It demonstrates the improvement in adhesion after
hot water (60 ˚C) immersion for more than 120 days as
suggested test6
to predict loss of adhesion after ageing.
Industrial scale experiments
The purpose of this study was to check if these results
could be duplicated at an industrial scale on an existing
installation without any special investment.
To carry out this assessment, a 3LPE coating was
tested, because it appears to be more sensitive to
delamination in hot and humid environments than a
single layer FBE coating. Adhesive (Table 3) and Top Coat
(Table 4) were applied with the FBE corresponding to the
one previously used and described.
The application process is described according to
Figure 4 with indicative dwelling times for applying a multi-
layer coating on steel pipes with the following dimensions:
Length: 12 m.
External diameter: 114 mm.
Thickness of steel: 3.6 mm.
The 5% Silpipe®
SCT solution was applied at a rate
of around 50 g/m2
on the steel surface after grit blasting
to achieve a Sa. 2.5 (ISO 8501-1) surface cleanliness and
55 - 65 µm Rz roughness.
Two series of test pieces were used for the adhesion
evaluation: the first series being chemically treated with
a Silpipe®
SCT solution, the second series without any
chemical treatment.
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Figure 7. Peeling determination after immersion in hot water for
40 days.
Figure 8. New perspective for selecting and applying coating
with the required quality level.
The immersions were carried out in tap water at
65 ˚C and 80 ˚C for 40 days.
The adhesion evaluations were carried out by an
independent laboratory according to the prEN ISO
21809-1 Standard.
Figure 7 shows the drastic improvement compared
to conventional surface preparation based on
measurements of the peeling force after immersion in
hot water for 40 days at two different temperatures.
New perspectives
Firstly, the new approach initiated through this
patented Silpipe®
process opens new perspectives
regarding the problems linked to the disbondment of
various external coatings, thus meeting the concerns
of operators when designing new coating systems.
The benefit of the chemical surface treatment
has been demonstrated to maximise the adhesion
performance, specifically after hot water immersion
for a long period (120 days) as suggested by experts
for ageing simulation to predict disbondment.
Secondly, this innovative process gives the
opportunity to coating applicators to offer this
technical solution without any HSE restrictions. It
allows the process (Figure 8) to be ‘debottlenecked’
when choosing and applying the right coating system
with a sustainable approach to guarantee durable
lifetimes of pipelines.
References
‘Integrity management of non-piggable pipelines – a KOC Direct
Assessment Case Study’, Allied Engineers & KOC, 9th
Oil and Gas
Pipelines in the Middle East Conference, The Energy Exchange,
Abu Dhabi, 4 - 6 May 2009.
ROCHE, M., MELOT, D., PAUGAM, G., TOTAL SA, ‘Recent
experience with pipeline coating failure’, 16th
International
Conference on Pipeline Protection, BHR Group, Cyprus,
2 - 4 November 2005.
TANDON, KK., SWAMY, G.V., SAHA, G., ‘Performance of three-layer
polyethylene coating on a cross country pipeline – a case study’,
14th
International Conference on Pipeline Protection, BHR Group,
Barcelona, 29 - 31 October 2001.
PORTESAN, G., TAVES, J., GUIDETTI, G., ‘Cases of massive
disbondment with three-layer PE pipeline coatings’, Cathodic
protection and associated coatings, Cefracor, EFC Event nr 254,
Aix-en-Provence, France, 6 - 7 June 2002.
SAUVANT-MOYNOT, V., DUVAL, S., KITTEL, S., LEFÈBVRE, X.,
‘Contribution to a better FBE selection for 3 layer polyolefin
coatings’, IFP, 16th
International Conference on Pipeline Protection,
BHR Group, Cyprus, 2 - 4 November 2005.
JOHN, R., ALAERTS, E., Tyco Adhesives, ‘The importance of
the hot water immersion test for evaluating the long-term
performance of buried pipeline coatings’, 16th
International
Conference on Pipeline Protection, BHR Group, Cyprus,
2 - 4 November 2005.
GAILLARD, G., BOULIEZ J. L., BS Coatings, ‘The interest of new
hydrophobic epoxy primers for three-layer coatings’,
16th
International Conference on Pipeline Protection, BHR Group,
Cyprus, 2 - 4 November 2005.
CHANG, B. T. A., JIANG, H., SUE, H-J., GUO, S., STJEAN, G., PHAM
H., WONG D., KEHR, A., MALOZZI, M., LO ,K. H., ‘Disbondment
mechanism of 3LPE pipeline coatings’, 17th
International
Conference on Pipeline Protection, BHR Group, Edinburgh,
17 - 19 October 2007.
GAILLARD, G., BOULIEZ, J. L., ‘A new application process that
assures good adhesion of fusion bonded epoxy coatings exposed
to very severe conditions’, 18th
International Conference on
Pipeline Protection, BHR Group, Antwerp, 4 - 6 November, 2009.
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