MIL-HDBK-1004/10
Composition. The compositions of most aluminum alloy anodes are
7.8.3.1
proprietary. Typical compositions of three proprietary alloys are given
below:
Type I
Type II
Type III
Element
Zinc
0.35% - 0.50%
3.5% - 5.0%
3.0%
Silicon
0.10% max
-
0.1%
Mercury
0.035% - 0.048%
0.035% - 0.048%
-
Indium
-
-
0.015%
Aluminum
Remainder
Remainder
Remainder
The Type I alloy is formulated for submersion in full strength seawater. The
Type II alloy is formulated for use when the anode may become immersed in
bottom sediments. The Type III alloy is formulated for use in bottom
sediments, full strength seawater, or in brackish water.
Anode Efficiency. Type I aluminum anodes have a consumption rate
7.8.3.2
of approximately 1,250 A hrs/lb or 6.8 lbs/A yr. Type II aluminum anodes in
bottom sediments have a reduced efficiency and a consumption rate of
approximately 770 A hrs/lb or 11.4 lbs/A yr. Type III aluminum anodes have a
consumption rate of approximately 1,150 A hrs/lb or 7.6 lbs/A yr.
Potentials. The potential of Type I and Type II aluminum anodes is
7.8.3.3
-1.10 V versus copper/copper sulfate. Type III anodes have a slightly higher
driving potential of -1.15 V versus copper/copper sulfate.
Sizes. Aluminum alloy anodes have been developed primarily for the
7.8.3.4
protection of marine structures. They are available in a wide variety of
sizes and shapes as shown in Tables 22 through 26. The bracelet anodes
described in Table 24 are shown in Figure 74. These bracelet anodes are used
for the protection of submerged pipelines and may also be used on pipe
pilings.
Current Output. The current output of aluminum anodes can be
7.8.3.5
determined either by the calculations outlined in para. 7.4.2 or by field
measurements as described in para. 7.4.3. The current output for some sizes
of aluminum anodes is provided by anode manufacturers and is calculated using
an assumed structure potential of -850 mV versus copper/copper sulfate and an
environmental resistivity of 20 ohm-cm. These values should be considered as
estimates only and should be verified by calculations of tests.
7.9
Other System Components
7.9.1
Connecting Wires. Proper selection of cable size, type of
insulation, and routing is necessary for proper and reliable system operation.
Only copper cables should be used in any cathodic protection installation.
High connection resistances and difficulty in making welded connections
associated with the use of aluminum wires precludes their use in cathodic
protection.
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