MIL-HDBK-1004/10
For some anode materials, the anode efficiency is dependent upon anode current
density as shown in Figure 73. For these materials, the anode consumption can
be calculated using the formula:
EQUATION:
W
=
YSI/E
(23)
where
W
=
anode consumption in pounds
Y
=
number of years
S
=
theoretical anode consumption rate in lbs/A yr
I
=
current output in A
E
=
anode efficiency
For the standard alloy magnesium material, the anode efficiency is essentially
constant above 250 mA/ft of anode area. If the anode efficiency is low at
the anode current density at which it is operated, anode material is wasted
due to self corrosion. At an anode efficiency of 50 percent, one-half of the
anode material is consumed by self corrosion and one-half is consumed in
providing protective current. If the desired anode life is not obtained using
an initially selected anode material and size, a different sized anode or one
of a different material is substituted and the process repeated in an
iterative manner until a system with the desired characteristics is obtained.
Seasonal Variation in Anode Output. Anode output will vary as the
7.7
resistivity of the environment changes. Seasonal variations associated with
soil moisture in buried systems or seawater dilution in estuaries may result
in changes in anode output. Fortunately, in most cases the current required
for protection is also reduced when the resistivity of the environment
increases so that this effect is partially self compensating. In some cases,
however, anode output will fall below or above the limits for protection and
the system will require seasonal adjustment or augmentation in order to
provide adequate protection.
7.8
Sacrificial Anode Materials
7.8.1
Magnesium. Magnesium is the most commonly used sacrificial anode
material for the protection of buried structures. Magnesium anodes are also
used for the protection of the interiors of water tanks and heaters, heat
exchangers and condensers, and waterfront structures. Magnesium anodes are
available as castings and extrusions weighing from 1 to 200 pounds, and in a
wide variety of shapes (refer to para. 7.8.1.4). Two anode compositions are
commonly used. They are the standard alloy and a "high potential" alloy.
The composition of each alloy is given in para. 7.8.1.1.
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