Coatings
are an incredibly important part of maintaining safe pipeline operations and
asset integrity, including being a first line of defense against corrosion and
corrosion-induced failures.
In
2022, corrosion accounted for 22.4% of all incidents in hazardous liquid lines
and 11.7% of all incidents in gas transmission lines (PHSMA, 2024). These statistics
show that corrosion is a major threat that pipeline operators face, and
problems with the protective coating are typically the root cause of corrosion.
Since
the 2000’s, fusion-bonded epoxy (FBE) coatings have been and continue to be the
standard for most new assets. These shop-applied coatings provide excellent
longevity and corrosion protection; however, they are typically welded together
in the field, requiring a field-applied coating to be applied. This article
will focus on common limitations and challenges having to do with these field-applied
pipeline coatings.
Liquid Epoxy
Liquid
epoxy coatings are often used for corrosion prevention on large-diameter
pipelines because they can be applied in the field, generally do not require
extra heat to fully cure, adhere well to steel and FBE Coatings and have less
stringent temperature and humidity requirements. These types of coatings are typically
2-part, made up of a base resin and curing agent, and can be applied via brush,
roller or airless spray equipment.
Many
of the challenges with field-applied liquid epoxy coatings derive from in-field
applications. Unlike fusion-bonded epoxy (FBE) coatings, which are typically
applied in the lab under very controlled environments, there is much less
control over the application environment for field-applied coatings.
In
addition, liquid epoxy coatings are typically applied in maintenance and repair
situations or over weld seams in new pipeline construction projects. These
types of scenarios are associated with complex local geometries, like irregular
weld seams, dents, flanges or corrosion pitting. They must also be compatible
with the FBE coating and bond effectively with the substrate.
External Factors
As
mentioned before, the challenges associated with the lack of control over
environmental factors are significant hindrances to effective, field-applied
liquid epoxy coatings.
The
coating formulation, as well as the material temperature, surface temperature
and atmospheric temperature all affect how well the coating will bond with the
pipe substrate and any existing coating. Temperature has a significant effect
on the viscosity and cure rate of an epoxy coating.
When
the coating isn’t applied with the correct material and surface temperature, it
will not properly wet out the profile created during the grit blasting stage
during surface preparation. Coatings rely on both physical bonding — like an
interlocking zipper — and chemical bonding to provide the optimal adhesion
level, so proper wet-out is essential.
Repairs
on operating natural gas pipelines pose a particular challenge because the steel
is usually colder than the air around it, which can lead to condensation forming
on the substrate. It is critical that the surface temperature of the pipe is at
least 5 degrees F above the dewpoint to prevent
condensation.
Further, epoxy coatings that cure in low-temperature, high-humidity environments (or in environments with high levels of carbon dioxide present) can form an amine blush. An amine blush is a surface phenomenon unique to epoxy-type coatings that will completely prevent subsequent layers from adhering properly.
Applying
and inspecting coatings in a trench differs vastly from in the shop. It is
easier in the shop to inspect the full 360 degrees of the pipe and to pull
aside a spool that does not pass the QA/QC check. In the field, finding and
repairing defects is much more difficult.
Coating
inspectors may have only a few inches of clearance around a pipe, requiring the
use of mirrors and other devices to see the bottom. Lighting is always a
challenge in the field, and many specifications do not require the use of
high-voltage holiday testing to find voids in the coating.
Liquid-applied
coatings generally require an SSPC SP-10 Near White metal blast cleanliness
with a 2-3 mil angular profile for proper adhesion. Foreign materials and weld
spatter should be removed, and sharp edges must be ground to a radius of 3 mm
prior to grit blasting.
Coating
over weld seams is inherently more difficult than coating a smooth
surface. The topography of a weld seam
makes blasting more challenging and significantly increases the risk of air
entrapment under the coating.
Solvent-borne
coatings are typically cheaper and easier to apply than solvent-free epoxy
coatings. They have volatile organic materials like xylenes or
methylethylketone (MEK) incorporated in the coating to make them lower in
viscosity. When applied too thickly or over pitting and irregular surfaces,
these coatings have a tendency to entrap solvent.
Entrapped
solvent will often create a premature coating failure. Zero VOC, 100% solid
coatings do not have solvents incorporated, eliminating the risk of solvent
entrapment and preventing environmental compliance issues with VOC emissions.
When
repairs are performed over weld seams, the surface area is fairly small, so the
liquid epoxy coatings are most commonly applied via brush or paint roller. Hand
application of high-viscosity materials increases the risk of air entrapment. These
coatings typically require two or three layers to help reduce the likelihood of
any entrapped air pockets extending through the entire coating. When air
pockets extend through the coating, water can permeate through the film
quickly, allowing subsurface corrosion to occur.
Spray
application techniques reduce the risk of entrapped air bubbles but can
increase the cost of small projects. If a high-voltage holiday test is
possible, it will help eliminate the risk of air entrapment. The formulation of
the coating itself significantly affects adhesion, temperature requirements and
air entrapment.
Saving
money on a low-cost coating can create major problems and actually make the
installation cost more because of tighter temperature and application
requirements. Consulting a coating expert can help ensure the best material
selection, eliminate potential failures and save money on the installation.
When
applying a repair to an existing pipeline, it is critical that the repair
material adheres well to any existing coatings. Coating materials interact best
with like materials. Any epoxy-based system should provide the best levels of
adhesion to existing epoxy-type coatings, including fusion-bonded epoxies. Polyureas,
polyurethanes and vinyl esters do not adhere as well to existing epoxy base
coatings.
Polarity
For
wet and semi-wet applications, the polarity of the coating will play a
significant role in the longevity of the system. Water permeates through
coating systems at different rates, depending on the polarity and cross-link
density of the system.
Water
will permeate most slowly through low polarity, or hydrophobic coatings. This
allows the overall film thickness to be lower without sacrificing service life.
Conversely, if water condensation on the surface during application is a major concern, higher polarity and more hydrophilic coatings can displace the water on the surface and provide proper adhesion even with condensation on the surface. Some epoxy-type coating systems can even be applied underwater. Matching the coating to the application and service conditions is critical to maximizing a coating system's service life.
During
a routine inspection, a New York gas pipeline operator found corrosion to a key
distribution pipeline. The pipe was located inside a tunnel and had been coated
with a wax tape system. Installed in the 1940’s, this vintage pipeline was not
compatible with smart pigging. The wax tape stopped visual inspection from
being able to identify the corrosion without removing the tape coat.
The
operator decided to remove the wax tape in sections, to determine the extent of
the corrosion. A plan was formulated to protect the pipe from additional
corrosion by installing a field applied, solvent free epoxy coating system. The
system was designed to be installed in three coats, at 15-20 mils per coat, to
provide a 50-year life extension.
Unfortunately,
many areas of the pipe had more than 20% wall loss and required a structural
reinforcement to continue to operate at the design pressure. Instead of welding
external metal sleeves, the operator found it more cost effective to apply a
composite repair system around the entire section of pipe. The composite was
coated with a solvent free epoxy, and the pipeline was able to return to full
operating pressure.
Pipe
support areas were reinforced with an aramid composite to provide additional
abrasion resistance. Working with a coating expert, this utility was able to
adjust to the conditions found and provide the best possible solution for their
system.
To
prevent premature coating failures, operators should perform due diligence
research and educate stakeholders to raise awareness of these challenges. A
high-quality application specification will help address expected challenges
and prevent costly change orders or unqualified material trade-outs.
Consulting
with a coating expert can help mitigate any project-specific challenges, ensure
the quality of an application specification and create a customized QC plan. Onsite
quality control should always be carried out by third-party, NACE-certified coating
inspectors to ensure the success of any field application. P&GJ
Russell “Rusty” Giudici is the president of Advanced FRP
Systems. Having spent his entire career in corrosion prevention for industrial
processes, he supports process engineers, maintenance managers, operators and
asset managers in protecting vital infrastructure.