1. M
etal structures subjected to
service constraints such as high
or low temperature, UV exposure,
humidity or soil salinity can be
weakened very quickly, given that they have a
natural tendency to corrode.
This is particularly the case with steel
pipelines that, whether onshore or offshore,
are sited in aggressive environments that
favour corrosion.
It is for this reason that such metallic
structures have to be protected by organic
coatings, the barrier effect of which procures
anticorrosion protection.
The efficiency of the anticorrosion
protection provided by the coating is linked
to its physical and chemical characteristics.
In fact, the thermosetting‑based coating
has to have a glass transition temperature
above the service temperature, be chemically
resistant and be able to slow down the influx
of corrosive elements such as water and salts.
Polyurethane coatings, which have now been
in use for several decades, for instance either
as external corrosion protection of buried
pipes or internal linings of water storage
tanks, are able to offer this anticorrosion
protection.
Polyurethane coatings are technically
adapted to positively answer the
requirements of the pipeline industry,
i.e., good adhesion on the steel substrate,
corrosion barrier effect with good electrical
insulation resistance and chemical resistance
Corrosion
contained
P. Collet, R. Turcas,
BS Coatings, France and
A. Anwar, SIPCO, Saudi Arabia, present
polyurethane coating as an efficient
solution for the corrosion protection
of onshore pipelines.
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2. against soil and water. They also offer good mechanical
resistance, such as impact resistance, indentation resistance,
hardness, etc., in order to withstand stresses during storage,
transport and installation, and to avoid damage from hard and
sharp stones during the pipeline laying phase. The coatings
must also be compatible with cathodic protection. Exterior
pipe PU coating are usually a one‑coat system. This is applied
directly to the metal substrate without (or with) a primer and
has the necessary resistance properties thanks to the high
film thickness of 800 – 1500 µm that is required to ensure
long‑term corrosion protection of the pipelines after laying.
The coating systems used for pipes can be roughly classified
according to the application depending on whether single
pipe sections are coated in the factory (cast iron pipes,
steel pipes) or complete pipelines are coated onsite (for
rehabilitation purposes), and also for onshore joints.
Figure 1. Principle of two‑component hot airless spray equipment
– metering by adjustable ratio pumps.
Table 2. Typical curing time of PU coatings
Curing stage Curing time
At 20 ˚C At 50 ˚C
Touch-dry 15 mins 7 mins
Stackable 8 hrs 30 mins
Figure 2. Ring application of PU coating – pipe rehabilitation.
Polyurethane coatings are also valued as internal linings,
especially for water pipes in compliance with standards such
as AWWA C222 or WRAS drinking water approval. The film
thicknesses vary compared to those of exterior coatings and
depending on the specification and application.
Polyurethane coatings
The polyurethane coating material itself is formed from the
crosslinking of polyol and isocyanate chemical groups. This
reaction between an isocyanate and a polyol takes place at
room temperature and the reaction kinetics are extremely
rapid.
Polyurethane anti‑corrosion coatings can be formulated
as one‑ or two‑component systems. In the case of
two‑component systems, the co‑reactants for both aliphatic
and aromatic polyisocyanate components are H acid
compounds, such as polyalcohols (containing hydroxyl groups)
or polyamines.
Isocyanate group + hydroxyl group:
Urethane group:
Beyond this classical chemical reaction between
two ingredients, in‑depth knowledge of the variety of H acid
compounds and isocyanates is vital in order to consider and
select the right chemicals in the formulation. Indeed, the film
properties of these systems are adjusted to suit, at one and
the same time, the object being coated, the conditions of
application and the requirements of the final client. This is
possible due to the wide variety of available polyisocyanate
and polyol components available. Polymeric MDI products
of NCO prepolymers with different NCO content can be
reacted with, for example, short‑chain diols, polyethers and/or
polyester polyols with varying hydroxyl contents and branching
factors.1
For instance, the selection of polyols can directly influence
the chemical resistance of the polyurethane coating. The
lower the absorption level, the better the barrier effect against
corrosive agents.
This know‑how of the coating formulator is also defined
by the different kinds of PU coating profiles proposed on the
market. Three examples of commercially available PU coating
materials corresponding to different profiles in terms of
mechanical and insulation properties linked to different end use
requirements are discussed in Table 1.
This knowledge about the key chemical components must
be associated with know‑how about the formulation by the
coating manufacturer to define the correct product profiles,
while also taking into account the application conditions. As
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3. far as the application conditions are concerned, the coating
formulation must ideally respond to the preferred component
ratio, the adjustment of the reactivity taking account of the
specificities of the two‑component spray equipment, the
environment of the coating application in a factory or onsite
(relative humidity level, wind probability and associated
dust, etc.). Since PU coating materials are applied in one single
wet‑on‑wet coat, the formulation has to be adapted in terms
of sagging limit of the wet coating, as the required dry film
thickness may vary from 500 up to 1500 – 2000 µm.
As far as the application equipment is concerned, the
PU coating materials are applied by two‑component hot airless
spray equipment with the mixing ratio correlated to the ratio
between the two components, usually called part A (resin) and
part B (hardener) (Figure 1).
As far as the application conditions are concerned, the
following conditions are usually recommended to process and
apply the coating materials correctly:
ÂÂ Temperature of the substrate must be between +10 ˚C and
+60 ˚C and maintained at 3 ˚C above the dew point during
application.
ÂÂ Ambient temperature between +10 ˚C and +40 ˚C.
ÂÂ Relative humidity must not exceed 80%.
In addition to these application conditions, the
temperatures of application of the two components are
monitored in order to apply them in the best conditions,
i.e., part A: 50 ˚C – 80 ˚C; part B: 40 ˚C – 60 ˚C.
Typical curing times of PU coating materials are shown
Table 2.
These fast curing times are linked to the rapid chemical
reaction between the two parts, involving a short pot life
and which therefore requires precise monitoring during the
application.
The importance of well trained, disciplined and specialised
staff cannot be underestimated, or compromised, since this
could be the cause of immense problems in both the selection
and application processes, avoiding huge economic costs
for future repairs. In other words, qualified and approved
applicators/contractors must be employed with experience and
proven track records in the application of multi‑component
liquid coating systems.
PU coatings as a corrosion protection for
pipelines
PU chemistry is capable of providing a corrosion barrier
with good mechanical properties associated with fast curing
without compromising adhesion on metal substrates (steel,
cast iron).
Adhesion on the metal substrate, especially steel, is
ensured by adapting the mechanical surface preparation based
on grit or sand blasting to achieve the requirements in terms
of cleanliness (Sa 2.5 according to the ISO 8501‑1 standard) and
surface profile (roughness [Rz] around 70 µm according to the
ISO 8503‑4 standard).
Since this new PU chemistry offers an environmentally
friendly solvent‑ and VOC‑free solution for replacing
traditional coal tar products, which have a low mechanical
resistance and present health risks (containing products
classified as carcinogenic by the World Health Organisation
such as benz(a)pyrene), the first pipeline projects were
completed in the late 1970s with the following partners:
Gaz de France (as the end user), Vallourec (as the steel pipe
manufacturer), and BS Coatings (as the coating manufacturer).
Furthermore, the coating formulation and associated
application conditions were adapted to the application
process in the factory and onsite and the PU coating
materials have the possibility of being easily applied in one
Figure 3. Shore D hardness as a function of temperature – case of
a selected PU coating.
Table 3. Technical requirements of the EN 10290 standard
Property Requirements Method/remark
Porosity under 8 V/µm No porosity
Hardness – shore D at 23 ˚C
Hardness – shore D at 80 ˚C
For info
For info
ISO 868
ISO 868
Adhesion – peel-off at 20 ˚C
Adhesion – peel-off at 80 ˚C
Adhesion – pull-off at 23 ˚C
Adhesion – pull-off at 80 ˚C
Rating ≤ 3
Rating ≤ 4
≥ 7 MPa
For info
ISO 4624
ISO 4624
Cathod disbondment 2 d at 60 ˚C
Cathod disbondment 28 d at 23 ˚C
Cathod disbondment 2 d at 80 ˚C
≤ 8 mm
≤ 8 mm
For info No cooling
Specific insul. res. 100 d at 23 ˚C
Specific insul. res. 30 d at 80 ˚C
≥ 107
Ω/m2
≥ 104
Ω/m2
Impact resistance at 23 ˚C
Impact resistance at -5 ˚C
≥ 5 J/mm
≥ 3 J/mm
Bending at 23 ˚C No cracking (Mandrel 194 mm in dia.)
Wet adhesion after 100 hrs at 60 ˚C
Wet adhesion after 100 hrs at 80 ˚C
For info
For info
Peel-off test
Peel-off test
Indentation res. after 24 hrs at 23 ˚C
Indentation res. after 24 hrs at 80 ˚C
≤ 0.2 mm
≤ 30%
0.15 mm
15 %
Porosity after ageing 100 d at 100 ˚C
Pull-off adhesion after ageing 100 d
at 100 ˚C
For info
For info
Table 4. Comparison between liquid epoxy/liquid PU2
Test Standard Solvent-free epoxy Solvent-free PU
Sag resistance Typically 1 mm Typically 2 mm
Hard-dry time 4 hrs 1 hr
Flexibility ASTM G42 Typically < 5% Typically < 40%
Impact resistance ASTM G14 Typically 3 – 4 J > 8 J
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4. pass (up to 2 mm). The cure of the polyurethane coating
can progress very fast, thus ensuring sufficient strength for
handling the pipes, making this technology attractive for
onshore field joints and pipeline rehabilitation.
As far as pipeline rehabilitation is concerned, PU coatings
have been selected for many projects due to their high
productivity and their ability to face ongoing challenges,
such as soil conditions at the pipeline location and the
operating conditions of the pipeline. For some rehabilitation
pipeline projects, automatic spray application has been
developed together with equipment manufacturers, making
this coating technology even more attractive in terms of
consistency and productivity (Figure 2).
International standards and norms
The successful development of PU coatings for pipeline
applications (field joints, rehabilitation, factory‑applied
jobs) has been made possible by the introduction of
different norms, standards and end user specifications,
requiring minimum performance. Some of the most
representative include:
ÂÂ EN 10329: Steel tubes and fittings for onshore and
offshore pipelines – external field joint coatings.
ÂÂ DIN 30671: Thermoset plastic coatings for buried steel
pipes.
ÂÂ EN 10290: Steel tubes and fittings for onshore and
offshore pipelines. External liquid applied polyurethane
and polyurethane modified coatings.
Today, DIN 30671 and EN 10290 are benchmark
norms for PU systems used as external pipe corrosion
protection coatings, to be considered in the -20 ˚C/+80 ˚C
temperature window thanks to the thermal resistance of
the PU coating (Figure 3).
To summarise, the requirements of the pipeline industry
are mostly based on the following current technical
requirements, based on the EN 10290 – steel tubes and
fittings for onshore and offshore pipelines. External liquid
applied polyurethane and polyurethane modified coatings
(Table 3).
These requirements will continue to be revised and
updated, driven by the potential influence of new operators
adopting PU coatings or by international organisations, such
as NACE, currently establishing a standard for this kind of
material.
Positioning versus other widely‑used liquid
coatings
Beside the use of polyurethane coatings, liquid epoxy
coatings are also used for onsite application, together with
chemically cured composites or UV cured systems, cold
Table 5. Adhesion values (pull‑off test) before and after seven months of UV ageing
Before ageing – measured at 29 ˚C
Location Adhesion Type of failure
Field joint 13.1 MPa Glue failure 100%
Field joint 25.3 MPa Glue failure 100%
3LPE 18.8 MPa Glue failure 100%
After seven months ageing – measured at 64 ˚C
Location Adhesion Type of failure
Field joint 8.6 MPa Glue failure 100%
Field joint 6.7 MPa Glue failure 100%
3LPE 9.1 MPa Glue failure 100%
3LPE 8.1 MPa Glue failure 100%
NB: These values were measured by a local coating contractor using a
Defelsko – AT-A Automatic type measuring device.
Figure 4. Wet adhesion (peel‑off and pull‑off tests) after 1000 hrs
immersion in saline solution (50 g/l) at 60 ˚C.
Figure 5. Flexibility versus temperature according to the
DIN 30671 method.
Figure 6. Impact resistance of a specially formulated PU coating as
a function of the temperature.
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5. applied tapes and heat shrinkable sleeves for field joints or
rehabilitation jobs.
Compared to tapes, sleeves or similar coating systems,
common liquid coatings, such as epoxy and PU offer
advantages in terms of productivity, coating consistency,
costs, etc. without compromising coating performance. The
application of liquid coatings is specially adapted to large
diameter pipes (from 30 in. in dia.).
Within the family of liquid coatings, the differences
between epoxy and polyurethane coatings can be summarised
as follows in Table 4.
These typical testing values illustrate why solvent-free
polyurethanes are selected as the preferred coating system
for pipeline rehabilitation. Furthermore, compared to
PU tar coatings, which are still being specified by end users,
tar‑free PU are more heat resistant and more environmentally
friendly solutions.
Harsh conditions
Three examples are described below to show how adapted PU
coating formulations and/or coating systems can positively
meet specific demands or special environment conditions.
UV ageing
UV ageing tests were conducted on natural exposure
in a sunny desert environment. The selected location
was Fahaheel, Kuwait and the UV ageing tests started
in early November, 2010. The tests were conducted on
applied PU Endoprene®
880 materials, a kit repair version of
Endoprene®
870 EN.
The materials were applied on pipe sections simulating
the behaviour on field joints with the PU coating applied on
a grit‑blasted steel surface, and the PU coating applied on a
flame treated three‑layer polyethylene (3LPE) coated surface.
A first evaluation was made after seven months on
7th
June, 2011 by checking the appearance of the coatings
and the mechanical properties (adhesion by the pull‑off test
method).
Appearance of the coated material
When applied, the coated PU material has a uniform glossy
grey appearance. After ageing, the appearance becomes less
glossy with slight superficial chalking. This observation has
been made after seven months of UV exposure.
From these observations, the possible appearance
modifications on the mechanical properties of the coated PU
materials were assessed.
Influence on the mechanical performance of the PU coating
The mechanical performance was assessed by measuring
the adhesion, a key property to guarantee good corrosion
protection.
The adhesion was assessed by a pull‑off test carried
out where the PU coating material was applied on the steel
substrate (field joint area), and where the PU coating material
was applied on the 3LPE surface (overlapping zone).
Test dollies were stuck on the surface and the pull‑off
test was carried out after 24 hrs. The original pull‑off tests
were carried out in early November 2010, at a temperature
of 29 ˚C. Due to the Summer season, a significantly higher
temperature – 64 ˚C – was measured for the surface of the
pipe when the tests were carried out after seven months of
UV exposure. The results are summarised in Table 5.
Performance in Subkha conditions
Subkha is an Arabic expression to describe recent coastal
sediments with a high salt content (up to 18%) and are
characterised by very low bearing capacities. Subkha soils
are common in the Arabian Peninsula and are also widely
distributed over the world, such as in India, Australia, the US
and Southern Africa, where subkha soils have different
expressions. The soil resistivity can be as low as 20 Ω/cm.
These soil conditions are very extreme for any organic
coating due to the combination of high temperature, soil
humidity and high salinity, since it has been established that
hydrophobic organic materials are not totally impermeable,
which means that molecules of water and corrosive elements
pass through the coating in the more or less longer‑term.
This phenomenon can concern three‑layer coatings, the
polyolefin part of which is traversed by water in less than
300 days at 60 ˚C.3
By extrapolation,
this phenomenon applies for any other organic coating, such
as PU coatings.
The other major factor, which assures the long lasting
efficiency of anticorrosion protection, is maintaining the
adhesion of the organic coating on the substrate.
The optimisation of the anticorrosion protection power of
the coating is therefore partly linked to the adhesion vis‑à‑vis
the substrate.
A specific highly engineered silane chemical solution, used
as an adhesion promoter by modifying the steel substrate,
has been considered to optimise the anticorrosion protection
power of the coating thanks to the good adhesion on the
substrate.
The test programme with polyurethane coatings was based
on:
ÂÂ Wet adhesion after immersion in saline solution (50 g/l)
for 1000 hrs at 60 ˚C and 80 ˚C.
ÂÂ Specific electrical insulation properties after 1000 hrs
immersion in saline solution (50 g/l) at 60 ˚C and 80 ˚C.
The polyurethane coating used for this test programme
was Endoprene®
870 EN, designed for application onsite
(onshore field joints, pipe rehabilitation) in compliance with
the EN 10290 standard and approved and used by various
clients in Europe, Africa and the Middle East.
The pipe surface was prepared by conventional mechanical
surface treatment with grit blasting to achieve the correct
surface profile (Rz: around 70 µm) and cleanliness (Sa 2.5 as per
the ISO 8501‑1 standard).
The PU coating material was applied by twin‑feed hot
airless spray equipment, targeting 1500 µm dry film thickness.
Two series of test panels were prepared: one as per
the above mentioned surface preparation, another by the
application of the highly engineered silane chemical solution
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6. (Silpipe®
SCT) between the mechanical surface preparation
and the application of the PU coating by spray. The
waterborne chemical solution was applied by brush (50 g/m2
,
corresponding to the ‘surface demand’) and the PU coating was
spray applied once the surface was touch dry (water evaporation).
Similar results to those shown in Figure 4 were observed
after 1000 hrs immersion in saline solution (50 g/l) at 80 ˚C,
showing the benefit of the chemical surface preparation for hot
and humid soil conditions, in combination with the PU coating
for such severe conditions to upgrade the performance of the
corrosion protection thanks to the improved adhesion.
Mechanical coating properties at very low temperature
In this example, the challenge regarding the polyurethane
formulation was to design a coating offering very high mechanical
performance and good balance between hardness and flexibility
with high impact resistance at low temperature (down to ‑40 ˚C)
to cope with the demand of cold countries such as Russia.
The performance of a specially formulated polyurethane
coating (ENDOPRENE®
8500) compared to a conventional
PU coating is demonstrated in Figure 5.
This same PU coating, designed for good flexibility, offers high
impact resistance in a broad temperature range, with ‑40 ˚C as the
lowest temperature to illustrate its performance.
The example in Figure 6 shows that the know‑how developed
by a coating manufacturer makes it possible to design PU coating
materials with very specific compromises linked to operators’
requirements.
Conclusion
Polyurethanes offer wide possibilities for developing products
adapted to specific requirements. With this technology, the
mission of the coating manufacturer is to choose the best
association from the arsenal offered by isocyanate and polyol
chemistries, to offer the right coating solution or system in
partnership with the industrial applicator and end user. It is
possible to reach the best compromise between the different
constraints in a given specification to take maximum advantage of
PU chemistry for serving the various players involved in the buried
steel pipeline market.
References
1. MEIER‑WESTHUES, U., ‘Polyurethanes’, European Coatings Tech files, Vincentz
Edition.
2. YOUNG, G., ‘Middle East Pipelines‑Rehabilitation Procedures and Methodology
Workshop’, MEA Pipes, Abu Dhabi, UAE (May, 2010).
3. SAUVANT‑MOYNOT, V., DUVAL, S., KITTEL, J. and LEFÈBVRE, X., ‘Contribution
to a better FBE selection for 3 layer polyolefin coatings’, 16th
International
Conference on Pipeline Protection, BHR Group (November 2005).
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