TM 5-811-7
magnesium anodes is approximately -1.55 volts to
a. Sacrificial anode (galvanic) cathodic protec-
a copper-copper sulfate half-cell. The open-circuit
tion system design. The following eight steps are
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
Table 2-3. Weights and dimensions of selected high-potential
resistance (RT) of the galvanic circuit is given by
magnesium-alloy anodes for use in soil or water
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:
2-4
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