MIL-HDBK-1004/10
Potentials. The open circuit potential of the standard alloy is
7.8.1.3
approximately -1.55 V versus copper/copper sulfate. The open circuit
potential of the high potential alloy is approximately -1.75 V versus
copper/copper sulfate.
Sizes. Magnesium anodes are available in a wide variety of sizes
7.8.1.4
and shapes as shown in Tables 12 through 17. In addition to the sizes shown,
magnesium alloy anode material is available as a "ribbon" anode which consists
of a 10-gauge steel wire surrounded by standard alloy magnesium 3/8 by 3/4
inch. Magnesium ribbon anodes are used in situations such as inside casings
where the space available is limited, or to protect small diameter utility
cables.
Current Output. Current output from magnesium anodes should be
7.8.1.5
determined from the formulae in para. 7.4.2 or by field measurement as
described in para. 7.4.3.
Backfill. For soil installations, the use of backfill is highly
7.8.1.6
desirable and is required in all cases. Composition of typical backfill
material for use with magnesium anodes is given below:
Composition of Backfill for Magnesium Anodes
Gypsum
75%
Bentonite
20%
Sodium Sulfate
5%
Anodes are available in prepackaged permeable cloth bags filled with prepared
backfill. Prepackaged anodes are commonly supplied with an outer impermeable
wrapping such as plastic. The impermeable wrapping must be removed from these
anodes prior to installation.
Zinc. Zinc anodes are commonly available in weights from 5 pounds
7.8.2
to 250 pounds in the form of plates, bars, and rods as described in Tables 18
through 21. Zinc is also available as ribbon anodes in 5/8- by 7/8-inch, 1/2-
by 9/16-inch, and 11/32- by 15/32-inch sizes, each with a 1/10-inch-diameter
galvanized steel wire core. Zinc anodes are most commonly used in immersion
service either in fresh or salt water. They are, however, occasionally used
in the protection of buried structures when special circumstances are
encountered. Two zinc anode compositions are commonly available. They are a
standard alloy formulated for use in fresh water and soil and an alloy
specially formulated for use in seawater. The composition of these alloys is
given in para. 7.8.2.1.
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