MIL-HDBK-1004/10

d) Minimum weight of anode material:

W = YSI/E

where

Y

=

15 years

S

=

8.8 lbs/A yr

E

=

0.50 efficiency

I

=

0.73 A

W

=

15 x 8.8 x 0.73/0.50

W

=

193 pounds

It should be noted that the 193 pounds arrived at above is based on

an output current of 0.73 A for the full designed life of the cathodic

protection system, 15 years. Strictly speaking, this is not the case because

the current output following a new installation is much less due to the high

coating efficiency. The average current requirement at first may be as low as

0.015 mA/ft of pipe.

e) Current output of a single 9-pound packaged, "high-potential"

magnesium anode:

i = Cfy/P

where

C = 120,000, constant for magnesium on well-coated

structures

f = 0.71, anode size factor (refer to Table 3)

y = 1.00, structure potential factor (refer to Table 4)

P = 5,000 ohm-cm, soil resistivity

i = 120,000 x 0.71 x 1.00/5,000

i = 17.0 mA

Because the structure is well coated, the anode spacing is

relatively great. Therefore, the multiplying factor for magnesium anode

groups is not used.

f)

Number of anodes (n):

n = I/i

where

I

=

726 mA, total current required

i

=

17.0 mA, current output of one anode

n

=

726/17

n

=

42.7 (use 43 anodes)

198

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