MIL-HDBK-1004/10

Determination of Current Required for Protection. The first step

7.3

in the design of sacrificial anode type cathodic protection systems (refer to

para. 4.2.6) is the determination of the total current required for the

system. This fixes the current to be supplied by the sacrificial anodes.

Determination of Anode Output. The output of a single anode in the

7.4

environment is determined. This may be determined by a simplified method

which uses standard factors for the type and size of anode to be used and for

the structure-to-electrolyte potential desired. Single anode output can also

be determined by using the driving potential between the anode and the

structure and the total circuit resistance. The anode-to-electrolyte

resistance is a major factor in most cases. This method is essentially

identical to the design procedure for impressed current systems.

Simplified Method for Common Situations. The formula given in

7.4.1

para. 4.2.5 can be used to estimate the output of zinc or magnesium anodes in

environments where the resistivity is above 500 ohm-cm. The following formula

gives a good approximation of current output in many cases and can be used to

check the results of the more detailed procedure outlined in para. 7.4.2.

EQUATION:

i

=

Cfy/P

(21)

where

i

=

current output (mA)

C

=

material constant

f

=

size factor

y

=

potential factor

P

=

environmental resistivity

Determination of Output Using Anode-to-Electrolyte Resistance. As

7.4.2

in the case of impressed current systems, this method determines the total

resistance of the cathodic protection circuit including anode-to-electrolyte

resistance, structure-to-electrolyte resistance, and the resistance of all

electrical connections and splices. Then, using the difference between the

anode potential and the protected structure potential, the current output is

determined using Ohm's Law.

Calculation of Anode-to-Electrolyte Resistance. As in the case of

7.4.2.1

impressed current systems, the resistance between the anode and the

environment is commonly the highest resistance in the cathodic protection

circuit. This is particularly true when the anodes are located a small

distance (10 feet or less) from the structure to be protected. The

anode-to-electrolyte resistance can be calculated using simplified equations

which are adapted to the most common situations, or the more complex but more

general basic equations.

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