Background
of the Shielding Problem:
When underground
pipelines are constructed today for transmission of oil, products,
or natural gas, and even water or sewage, the pipeline design almost
always includes corrosion coating with a cathodic protection system.
The cathodic protection system is intended to act as backup corrosion
protection in the event that the corrosion coating system were to
fail.
The surprising
thing is the frequency that coating systems are used which can
partially or completely block the effectiveness of the cathodic
protection system.
In recent years,
knowledgeable engineers and scientists have begun to try to highlight
this incompatibility. Beavers and Thompson summarized the shielding
incompatibility in the newly released Volume 13 of the ASM Handbook:
“Ultimately,
the effectiveness of a coating system in preventing corrosion
is related to two primary factors: (a) the resistance of a coating
to degradation over time and (b) the ability of the coating to
conduct CP current should the coating fail (minimize shielding).
For SCC resistance, these factors as well as the type of surface
preparation used with the coating are important.”
Beavers,
J.A. and Thompson, N.G., External Corrosion of Oil and Natural
Gas Pipelines,
ASM Handbook, Volume 13C, Corrosion: Environments and Industries (#05145),
ASM International, Materials Park Ohio 44073-0002, p1021.
www.asminternational.org
The
above definition of an effective coating system is what some people
have started to call "non-shielding coating systems".
Others call it a “fail/safe
coating system”.
Plenty
of research and papers are available on the topic of shielding and
non-shielding coating systems. Click
here for Technical Reference. We have organized
the frequently asked questions about shielding and
non-shielding
coatings
below, drawing
on
the conclusions found in the research and papers. Where possible
a citation with a link is included.
FREQUENTLY
ASKED QUESTIONS ABOUT SHIELDING AND FAIL/SAFE COATINGS
1. What
are some other things NACE says about shielding?
a. NACE SP0169,
Standard Recommended Practice for underground or submerged piping,
says:
"Materials
and construction practices that create electrical shielding
should not be used on the pipeline."
Nace
Standard SP0169-2002, Control of External Corrosion on Underground
or submerged Metallic Piping Systems,
NACE International, Houston, Texas 77084-4906, Section 4.2.3, p4.
www.nace.org/nacestore/dept.asp?Cat%5FID=2905
b.
NACE has initiated a course entitled; “Coatings in
Conjunction with Cathodic Protection”. This 6 day
course “was developed for corrosion control personnel who
must deal with the selection and application of protective coatings
that will also be exposed to cathodic protection.”
For
more information on the CCCP course, go to www.nace.org/nace/content/Education/courses/crm/crsdesc_cccpp.asp#about
2. What
is the shielding behavior of various pipeline coatings in use
throughout the world?
a. Fusion
Bond Epoxy (FBE)
In the early 1990’s PRCI (Pipeline Research Committee International)
funded a research program to investigate three factors which prevent
SCC failure. One of the factors was “the ability to pass
CP current should the coating fail”. In this research,
single layer FBE coatings were found to conduct CP current in the
absence of holidays.
Therefore
FBE may be considered a non-shielding coating. If disbondment
of FBE occurs (a failure), cathodic protection currents probably
will be able to reach the disbonded area.
Beavers,
J.A. 1992. Assessment of the Effects of Surface Preparation
and Coating on the Susceptibility of Line Pipe to Stress Corrosion
Cracking.
PRCI, Arlington, VA, Report L51666
www.prci.com/publications/L51666.htm
This disbonded
FBE had no corrosion underneath. The pH of the water under the
blister
was elevated (made more alkaline) as the result of the CP currents
being able to reach
underneath the disbondment.
b. Shrink
Sleeves
Shrink sleeves were included in research by CC Technologies on shielding
behavior of pipeline coatings which was published in 2006. But they
were included as the extreme “complete shielding”. In
the words of the study; “Shrink sleeves and polyethylene
tape wrap coatings for girth welds are known to cause problems with
cathodic protection shielding. Polyethylene is an excellent water
barrier and absorbs virtually no moisture, making it an excellent
packaging material for food (Glad Wrap, Ziploc bags, garbage bags,
etc.) but promoting CP shielding by not allowing ionic conductivity.
The small pores and cracks which may be present in heterogeneous
coatings are not present in polyethylene based coatings. When combined
with polyethylene’s naturally hydrophobic chemistry, the result
is a water impermeable coating without defect pathways for ionic
migration”.
As expected, “No
evidence of CP was seen in the shrink sleeve coating…..although
destructive testing found that it was not well bonded in all
locations on the panel.”
Ruschau,
G.R. and Chen, Y. 2006 Determining the CP Shielding Behavior
of PipelineCoatings in the Laboratory.
NACE International, Houston, TX 77084, Paper No.
06043
www.nace.org/nacestore/default_0.asp
Therefore
shrink sleeves should be considered to shield cathodic protection
currents. If disbondment of shrink sleeves occurs, cathodic protection
currents probably will not be able to reach the disbonded area.
c. Polyguard
RD-6™
Polyguard
Products RD-6™ is designed to be non-shielding in two ways:
1. The geotextile
backing is invisible to CP currents because of its open weave.
Therefore if the coating compound underneath the backing is
damaged, and a holiday is created in the corrosion coating,
the backing will not shield the passage of protective currents. Click
here for information.
2. If the
corrosion coating becomes disbonded, and water penetrates between
the coating and the substrate underneath the overlap, CP current
can penetrate underneath the overlap to the disbonded area.
This
has been proven in three ways:
a. Lab
testing has shown that current can pass from the outside
of the overlap to the steel substrate under a disbonded area.
b. Lab
testing has shown that pH of water in disbonded areas (voids)
at overlaps is raised to a level of 9 or higher. A high (alkaline)
pH has been shown to minimize corrosion activity.
c. Over
the 19 year field history of RD-6™, Polyguard Products personnel
have been present to observe numerous of digups where RD-6™
was used as the corrosion coating. Only six areas
have been found where any coating was disbonded. Disbondment
was attributed to failure to strip weld seams, insufficient
tension being applied during application of the RD-6™, and/or
application to wet pipe.
In the
above instances where disbondment was found, there has been
no significant surface corrosion. On occasions where the
pH of the water underneath the disbondment was measured,
pH was 9 or above.
The above
testing is described in Richard Norsworthy's 2004 article,
referenced below.
Norsworthy, R., June 2004, “’Fail/safe’ System
Used in Conjunction with Cathodic Protection”, Materials
Performance, p. 34-38
www.polyguardproducts.com/match/publishedpapers/FailSafe.pdf
Water
underneath a disbonded area of this improperly applied
RD-6™ had a pH of 9-10,
as compared to water under nearby disbonded coal tar coating, which has a pH
of 5-6.
The CP current had increased the alkalinity of the water under the RD-6™ to
a level
which does not support corrosion. |
Therefore
RD-6™ may be considered a non-shielding coating.
If disbondment of RD-6™ occurs cathodic protection
currents probably will be able to reach the disbonded area.
d. 3
Layer Systems:
Three layer
systems are covered in detail by a 2005 NACE paper by Argent
and Norman;
“The
universal strategy for external corrosion protection on buried
or sub-sea pipelines accepts that coating damage will occur
and CP is built into the pipeline design to prevent metal
loss at these sites of coating damage. If the failed coating
does not impede the flow of CP current onto the pipe steel
then normal CP monitoring will ensure continued pipeline
integrity.
When
a PE based coating loses adhesion from the metal substrate
then a condition of CP shielding can be created. The corrosion
risk created by CP shielding can only be eliminated by excavation
and recoating. Coatings that can fail to create conditions
of CP shielding include cold applied tapes, heat shrink materials
(particularly mastic backed heat shrink), 2LPE, 3LPE, and
3LPP.”
Argent,
C.A. and Norman, D., 2005, “Fitness for Purpose Issues
Relating to FBE and Three Layer PE Coatings”’,
David Norman Corrosion Control, Cornwall, TR8 5SA, England
www.davidnormancorrosioncontrol.com/html/papers/PDF/DavidNormanNACE2005.pdf
Therefore
three layer systems should be considered shielding
coatings.
If disbondment of three layer systems occurs (a failure), cathodic
protection currents may not be able to reach
the disbonded area.
e. Liquid
Coatings
Four types of liquid coatings (epoxy, epoxy-polyurethane, polyurethane,
and wax) were investigated for their ability to allow CP current
to be transmitted. Results showed that; “the liquid coatings,
when applied extra thin to accelerate the kinetics of absorption
and current transmission…..all allowed CP current to be transmitted.”
Ruschau,
G.R. and Chen, Y. 2006 Determining the CP Shielding Behavior
of PipelineCoatings in the Laboratory.
NACE International, Houston, TX 77084, Paper No.
06043
www.nace.org/nacestore/default_0.asp
From a practical
viewpoint this result is inconclusive at best regarding non-shielding
properties of liquid coatings. “Extra thin” coatings
are achievable in the lab, but in field application can be difficult
to achieve. When liquid coatings were applied thick in the lab,
no evidence of pH change or CP current was noted.
Thus, it appears
that liquid coatings will need some work to obtain and prove
non-shielding and fail/safe properties.
f. Polyethylene
Backed Tapes
Polyethylene backed tapes were the
first shielding culprit to be identified. In 1988,
the Pipeline Research Committee concluded that
shielding of cathodic protection currents was a
problem for over half of the 27 gas industry members
surveyed.
Moreover,
the report stated; “PRC research indicates that SCC
is enhanced by cathodic protection shielding at the disbonded
areas...”
As
stated above in the section on Shrink Sleeves, researchers believe
that: “Shrink sleeves and polyethylene tape wrap coatings
for girth welds are known to cause problems with cathodic protection
shielding. Polyethylene is an excellent water barrier and absorbs
virtually no moisture, making it an excellent packaging material
for food (Glad Wrap, Ziploc bags, garbage bags, etc.) but promoting
CP shielding by not allowing ionic conductivity. The small pores
and cracks which may be present in heterogeneous coatings are
not present in polyethylene based coatings. When combined with
polyethylene’s naturally hydrophobic chemistry, the result
is a water impermeable coating without defect pathways for ionic
migration”.
Pipeline
Research Committee, “A Review of Gas Industry Pipeline
Coating Practices”,
Pipeline Research Council International, 1988,
p.3, published by Technical Toolboxes, Inc., 3801 Kirby
Drive, Houston, Texas 77098
www.prci.com/publications/index_titles.htm
3. Tapes
are widely condemned above. Isn’t Polyguard RD-6™ a tape?
RD-6™ is not
a tape because it was designed to overcome the fatal flaws that
have almost destroyed the North American tape market. These flaws
are shielding
of cathodic protection currents and vulnerability to soil
stress. For
more on "RD-6™ Is Not A Tape, or is it?" click here.
4. What
are the cost implications of using non-shielding coatings?
The use of non-shielding
coatings will reduce the possibility of catastrophic failure. That
is a benefit difficult to quantify.
The cost benefit
of non-shielding coatings was estimated by one 3rd party however.
In a 2006 Pipeline & Gas Journal article entitled A
Strategy to Target Aging Pipeline Decisions, the
author says that aging pipeline systems with deteriorated coating
systems can be addressed with
$15-25/foot linear anodes, vs. $125-150/foot for recoating. However,
as shown in the decision chart below, the lower cost CP treatment
will not work if the corrosion coating is one which shields cathodic
protection surrents.
A Strategy To Target Aging Pipeline Decisions
Huck,
T. June 2006 "Linear
Anodes Target Aging Pipeline Coating Threats",
Pipeline & Gas Journal, pgs. 34-36.
http://www.oildompublishing.com/PGJ/pgj_archive/June06/linear.pdf
5. If
users knew 20 years ago that solid film backed corrosion coatings
created shielding problems, how come there are so many specifications
in place today for them?
It is true that
many specs are in place today, although the number in North America
has dropped significantly. But the other part of the answer is
that engineering specifications have long tails. Which reminds
us of a story.....click
here to view the story.
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