MIL-HDBK-1110
a) The desired properties of coatings for buried,
cathodically protected pipelines are:
(1)
Good electrical insulation
(2)
Good moisture resistance
(3)
Good adhesion
(4)
Resistance to cathodic disbonding
(5)
Resistance to damage during handling
(6)
Ease of repair
b) Coatings for piping to be buried should
be applied
in a shop under controlled conditions. Blasting with
automatic
equipment that recycles the abrasive should provide a
high level
of cleanliness (SSPC SP 5 or 10). Coatings that have
been
commonly used on buried piping include:
(1) Coal Tar Enamels - Coal tar enamels
use
different combinations of fiberglass and felt wraps to
provide
mechanical strength and thickness (1/8 inch or more).
Their use
has greatly declined because of environmental concerns
about coal
tar.
(2) Asphalt Mastics - Asphalt mastics are
combinations of asphalt, sand, and other materials that are
extruded over pipes at thicknesses up to 1/2 inch. They are
quite moisture-resistant but lack resistance to both hydrocarbons
and sunlight and, like coal tar enamels, are a health concern.
(3) Extruded Coatings - Extruded coatings
typically have a 10 to 15 mil mastic base covered with a
polyethylene or polypropylene outer jacket. They are sensitive
to sunlight and must be covered if they are to be exposed to it
for long periods of time.
(4) Fusion-Bonded Powder Coatings - Fusion-bonded
powder coatings are usually applied by electrostatic spray to
pre-heated pipe (400 to 500 degrees F) cleaned to a near-white
surface (SSPC SP 10). The 10 to 30 mil coating is water-cooled
before storage and subsequent use.
(5) Plastic Tapes - Polyethylene, vinyl, and coal
tar tapes are available with different adhesives and thicknesses.
Because they are relatively easily damaged, they are often
installed with a secondary rock shield. Portable wrapping
machines are also available for over-the-ditch application.
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