UFC 3-570-06
JANUARY 31 2003
Table 2-11. Electrochemical Equivalents Of Common Structural Metals
WEIGHT LOSS
METAL
KILOGRAMS (POUNDS)
PER AMPERE YEAR
Carbon (Graphite)
1 (2.2)
Iron
9.1
(20.1)
Aluminum
2.9
(6.5)
Lead
33.9
(74.5)
Copper
10.3
(22.8)
Zinc
10.7
(23.6)
Magnesium
4 (8.8)
Nickel
9.6
(21.1)
Silver
35.2
(77.6)
Tin
19 (42)
2-7.2.1
High Silicon Cast Iron. Cast iron anodes are produced in two alloy types
with the principal addition of chromium, developed in 1959, to improve the life in
chloride soils and water. Table 2-12 shows the metallurgical composition of the two
alloys. The principal reason for superior cast iron performance is the formation of a
silicon oxide (Si02) film that forms on the anode surface, reducing the rate of oxidation,
and retarding the consumption rate. They have good electrical properties, and the
resistance of the alloy is 72 micro-ohms per cubic centimeter at 20 C. To be
successful in soils, the anodes are backfilled with metallurgical or petroleum coke
breeze to reduce the anode resistance and increase the effective anode surface area.
Even when the anodes are pre-packaged in coke breeze filled canisters, filling the
annular space between the canister and the soil with additional coke breeze is a
recommended practice. In seawater, the anode will discharge current at very low
voltages and, therefore, does not require backfill. Manufacturers of cast iron anodes
developed cable-to-anode connection procedures which distributors are required to
follow. Therefore, there has been a lower incidence of connection failures, compared to
2-45
Previous Page
