TM 5-811-7
(5) Find the current output to ground for a single 17-pound standard packaged magnesium anode using
equation C-2:
Cfy
i'
,
(eq C-2)
P
where C = 120,000, a constant for well coated structures using magnesium, f = 1.00 (table C-4), y = 1.00
(table C-5), P = 4500 ohm-centimeters. Thus,
120,000 1.00 1.00
i'
4500 ohm&cm
i = 26.7 mA.
Because the structure is well coated, anode spacing will be relatively large.
Table C-4. Galvanic anode size factor (f)
Size
Anode
factor
weight
Standard anodes
(f)
3
(packaged)
0.53
5
(packaged)
0.60
9
(packaged)
0.71
17
(packaged)
1.00
32
(packaged)
1.06
(packaged-anode dimension 8" dia x 16")a
50
1.09
50
(packaged-anode dimension 5" x 5" x 31"
1.29
Long anodes
9
(2.75" x 2.75" x 26" backfill 6" x 31")
1.01
10
(1.5" x 1.5" x 72" backfill 4" x 78")
1.71
18
(2" x 2" x 72" backfill 5" x 78")
1.81
20
(2.5" x 2.5"x "60" backfill 5" x 66")
1.60
40
(3.75" x 3.75" x 60" backfill 6.5" x 66")
1.72
42
(3" x 3" x 72" backfill 6" x 78")
1.90
Extra-long anodes
(1.6" dia x 10* backfilled to 6" din)
15
2.61
(1.3" x 20* backfilled to 6" din)
20
4.28
(2" dia x 10* backfilled to 8" din)
25
2.81
a
In this table, "denotes inches, ` denotes feet.
Reprinted from DA. Tefankjian, "Application of Cathodic Protection," Materials Protection and Performance. Vol 11 (No. 11),
November 1972. Used with permission.
Table C-5. Structure potential factor (y)
Structure-to-electrolyte
Magnesium
Zinc
potential (volts, relative
structure
structure
to copper-copper sulfate)
factor (y)
factor (y)
-0.70
1.14
1.60
-0.80
1.07
1.20
-0.85
1.00
1.00
-0.90
0.93
0.80
-1.00
0.79
0.40
-1.10
0.64
0.00
-1.20
0.50
0.00
Reprinted from l).A. Tefankjian, "Application of Cathodic Protection.** Materials Protection and Performance. Vol 11 (No. 11),
November 1972. Used with permission.
C-7
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