magnesium anodes is approximately -1.55 volts to

a copper-copper sulfate half-cell. The open-circuit

potential of high-manganese magnesium anodes is

required when designing galvanic cathodic pro-

approximately -1.75 volts to a copper-copper sul-

tection systems. Appendix C gives examples of

fate half-cell.

galvanic cathodic protection designs.

(a) The potential of iron in contact with

(1) Review soil resistivity. The site of lowest

soil or water usually ranges around -0.55 volt rela-

resistivity will likely be used for anode location to

tive to copper-copper sulfate. When cathodic pro-

minimize anode-to-electrolyte resistivity. In addi-

tection is applied using magnesium anodes, the iron

tion, if resistivity variations are not significant, the

potential assumes some value between -0.55 and -

average resistivity will be used for design

1.0 volt, depending on the degree of protection

calculations.

provided. In highly corrosive soils or waters, the

(2) Select anode. As indicated in paragraph

natural potential of iron may be as high as -0.82

1-4, galvanic anodes are usually either magnesium

volt relative to copper-copper sulfate. From this, it

or zinc. Zinc anodes are used in extremely corro-

is evident that -0.55 volt should not be used to

sive soil (resistivity below 2000 ohm- centimeters).

calculate the net driving potential available from

Data from commercially available anodes must be

magnesium anodes.

reviewed. Each anode specification will include

(b) A more practical approach is to con-

anode weight, anode dimensions, and package

sider iron polarized to -0.85 volt. On this basis,

dimensions (anode plus backfill), as table 2-3

standard alloy magnesium anodes have a driving

shows for magnesium-alloy anodes. In addition, the

potential of 0.70 volt (1.55-0.85 0.70) and high-

anode's driving potential must be considered (para

potential magnesium anodes have a driving poten-

a[3] below). The choice of anode from those

tial of 0.90 volt (1.75-0.85 0.90). For cathodic

available is arbitrary; design calculations will be

protection design that involves magnesium anodes,

these potentials, 0.70 and 0.90 volt, should be

made for several available anodes, and the most

used, depending on the alloy selected.

economical one will be chosen.

(4) Calculate number of anodes needed to

meet groundbed resistance limitations. The total

resistance (RT) of the galvanic circuit is given by

equation 2-2:

Packaged

Weight

Packaged

size

RT = Ra + Rw +Rc,

(eq 2-2)

(lb)

Size (in.)

wt (lb)

(in.)

where Ra is the anode-to-electrolyte resistance, Rw

3

3.75 x 3.75 x 5

12

6 x 10

5

3.75 x 3.75 x 7.5

17

6 x 12

structure-to-electrolyte resistance. The total resis-

9

2.75 x 2.75 x 26

35

6 x 31

9

3.75 x 3.75 x 13.25

27

6 x 17

tance also can be found by using equation 2-3:

12

3.75 x 3.75 x 18

36

6 x 23

)E

14

2.75 x 2.75 x 41

50

6 x 46

RT '

,

14

3.75 x 3.75 x 21

42

6.5 x 26

(eq 2-3)

I

17

2.75 x 2.75 x 50

60

6 x 55

17

3.75 x 3.75 x 26

45

6.5 x 29

where )E is the anode's driving potential discussed

20

2.5 x 2.5 x 59.25

70

5 x 66

24

4.5 x 4.5 x 23

60

7 x 30

in a(3) above and I is the current density required

32

5.5 x 5.5 x 21

74

8 x 28

to achieve cathodic protection (para 2-1). Rc in

40

3.75 x 3.75 x 59.25

105

6.5 x 66

equation 2-2 can be calculated by using equation

48

5.5 x 5.5 x 30

100

8 x 38

2.4:

48

8 x 16

100

12 x 25

60

4.5 x 4.5 x 60

--

--

R

Rc '

,

(eq 2-4)

Note: Core material is a galvanized 20-gage perforated steel strip.

A

Anodes longer than 24 inches have a 9-gage core. The connecting

wire is a 10-foot length of solid No. 12 AWG TW insulated copper

where R is the average coating resistance, in ohms

wire, silver-soldered to the core with joints sealed against moisture.

per square feet, at the end of the proposed lifetime

Special wires or other lengths are available.

for the system (R is specified by the supplier), and

A is the structure's surface area in square feet.

U.S. Air Force.

Assuming Rw in equation 2-2 is negligible, that

(3) Calculate net driving potential for

anode-to-electrolyte resistance can then be calcu-

anodes. The open-circuit potential of standard alloy

lated from equation 2-5:

Integrated Publishing, Inc. |