Cathodic Shielding of Pipeline Coatings

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Cathodic Shielding of Pipeline Coatings

• Presented by Jim Utley
• Denso North America, Inc.

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QUALITIES OF A PIPELINE COATING

• Coating must possess excellent adhesion and
  cohesion to steel substrate
• Coating must have good dielectric strength
• Coating must be impervious to water absorption
• Coating must have good hardness characteristics
• Coating must be compatible with other coatings

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Field Coating Systems

• Bitumen/Butyl Tape and Liquid Coatings
• Petrolatum/Wax Type Coatings w/ appropriate outer
  wraps
• Liquid Epoxy (100 Solids)- Plural Component
  Spray and Brush-Applied
• Coal-Tar Epoxies
• Powder Type Fusion-Bond Epoxy (Flocking in the
  Field)
• Heat-Shrink Sleeves (Tubular Split-Sleeve)

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DEFINITION OF CATHODIC SHIELDINGThe
inability to drive current in the form of
cathodic protection under a disbonded coating or
structure. On larger diameter pipe, certain
types of coatings possessing high dielectric
polyethylene backings are most vulnerable

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What types of field coatings are most susceptible
to shielding and whyTypically, the field
coatings that are most susceptible to shielding
issues are cold-applied tapes and shrink-sleeves
that are composed of a high dielectric polymer
backing and a bitumen/butyl adhesive.Coating
disbondment of tape or shrink-sleeves must occur
for cathodic shielding to take place. This can
occur through the separation of layers or from
total coating disbondment. If a polyethylene
backed coating is applied improperly or subjected
to severe soil-stress , conditions are ripe for
cathodic shielding to take place. The use of
select backfill and/or the use of rockshield are
preventative measures to combat coating
disbondment. On large diameter cross-country
pipelines, it is impractical to assume from a
cost standpoint that those measures will be
followed 100 of the time.
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Qualities of liquid epoxy coatings and
fusion-bonded coatings that make them fail-safe
from shielding issuesWhen liquid epoxy or
fusion-bonded coatings disbond, they are
considered fail safe from a cathodic shielding
standpoint. That is determined by the way they
disbond from the steel substrate. If cracking
or disbondment occurs from impact during
backfill, then cathodic protection can still be
administered to the disbonded areas. In these
instances, there is little possibility that
cathodic shielding will occur.
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COATING DISBONDMENT
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• Lower pipeline until full weight is
  re-established on support areas

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Hot-Applied BitumenTape80 mil hot applied tape
monolithic in composition and where extra
hardness is required.
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100 Solids Liquid Spray-Applied Epoxy Coating -
Applied with a plural component unit with 31
spray equipment.
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High Build 100 Solids Liquid Epoxy Coating
Applied to girth weld at 25 - 30 mils.
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SUMMARYIt is now possible with the use of
smart pigs and direct current voltage gradient
(DCVG) inspection techniques to locate cathodic
shielding found under certain pipeline coatings.
All pipeline coatings will weaken due to age and
eventually possibly fail. Some coatings will
fail prematurely due to poor application and
stress.For public safety reasons, it is
extremely important that no disbonded coating
areas go cathodically unprotected.