MIL-HDBK-1004/10
structure. When suspended in water, the core is extended by welding on a
steel extension. For suspended systems, the use of a cable continuity bond is
recommended to insure that the resistance between the anode and the structure
is minimized. Sizes and shapes of commercially available zinc alloys for
cathodic protection are given in Tables 16 through 19. In addition, zinc
ribbon anodes 5/8 by 7/8 inch weighing 1.2 pounds per foot (lb/ft) for
seawater use are available. Two sizes of zinc ribbon anodes are available in
the standard alloy: 1/2 by 9/16 inch weighing 0.6 lb/ft, and 11/32 by 15/32
inch weighing 0.25 lb/ft. All three of these commercially available zinc
ribbon anodes have a 1/10-inch steel core.
Current Output. The current output of zinc anodes may be
7.8.2.5
determined either by the calculations outlined in para. 7.4.2 or by field
measurements as described in para. 7.4.3. When used without backfill, zinc
anodes can become covered with nonconductive corrosion products which can
reduce their current output. Seawater alloy anodes are specially formulated
to reduce this tendency in seawater. When used in soil containing high levels
of oxygen, carbonates, or phosphates, backfill should be used with zinc anodes
in order to reduce the possibility of the buildup of these corrosion products.
Backfill. Two typical compositions of backfill used with zinc
7.8.2.6
anodes in soils are given below:
Type 1
Type 2
Material
Hydrated gypsum
75%
50%
Bentonite
20%
50%
Sodium sulfate
5%
Aluminum. Aluminum sacrificial anodes are a more recent
7.8.3
development than either zinc or magnesium alloys. Their primary use is in the
protection of structures in seawater. However, there is a potential for their
use in fresh water or in soil. When the original anodes used are aluminum
alloy and their performance has been satisfactory they should be replaced with
anodes of the same type. Early formulations of aluminum alloys for use as a
sacrificial anode contained mercury. While the amount of mercury contained in
the alloy is small, the mercury tends to concentrate in the anode stubs which
remain after the bulk of the anode has been consumed. Precautions should be
taken during removal of the stubs, especially by methods which generate heat,
to prevent mercury poisoning. Mercury containing aluminum alloy anode stubs
must be disposed of properly.
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