Nace CP 1, Course Manual

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Underground Corrosion Control 3:13
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Specifications of Galvanic Anode Systems
Size and Shapes
Galvanic anodes come in a variety of sizes and shapes. For seawater
application aluminum anodes are often sized to fit the size of the structure.
The ends of these anodes are equipped with welding tabs for electrical
connection to the structure.
In soil environments, the shape and size of the anode depends more on
resistivity of the soil, current requirement, and other conditions than on the
size of the structure. These anodes are equipped with an insulated wire for
the electrical connection. Spacing between the anode and structure is
operator controlled. Another component required for a soil application
anode is a chemical backfill.
Backfill
The chemical backfill used for galvanic anodes consists of 75% gypsum
(CaSO
4
), 20% bentonite clay, and 5% sodium sulfate. The chemical backfill
surrounding magnesium and zinc anodes provides a uniform environment
that reduces self-consumption of the anode. The chemical composition of
the backfill is such that the anode functions more efficiently because the
backfill reduces polarization of the anode. Also, the backfill swells upon
getting wet, providing a tight fit with the surrounding soil and thus reducing
the resistance of the anode to earth.
Galvanic anodes can be ordered bare or prepackaged with the chemical
backfill. If anodes are not prepackaged with a special chemical backfill,
field installation of the anode will require mixing the backfill on site and
placing it around the anode.
Attachment to Structures
Galvanic anodes must be directly attached to the structure through a
metallic conductor. This is achieved by one of the following methods:
•
using insulated copper wire provided by the manufacturer and thermite
welded or otherwise attached to the structure.