302
Fundamentals of Corrosion
= 1200 mA. Magnesium anodes used in this particular soil have a voltage of
–1.65 V, or a galvanic cell voltage of
E =E
E
V
cell
C
A
− =−
− −
=+
0 85
165
0 8
.
( . )
.
The resistance is therefore
R=
V
1
=
=
0 8
1 2
0 67
.
.
. Ω
As the number of anodes is increased, the total resistance of the system
decreases. Each anode that is added provides a new path for current flow,
parallel to the existing system. The relationship between the resistance of the
system and the anodes is shown in the Sunde equation:
R=
0.00521P
NL
L
d–1
L
S
N)
=
+
( . log
.log .
2 3
8
2 2 3 0 656
where R = resistance (ohms), P = soil resistivity (ohm-cm), N = number of
anodes, L = anode length (ft), d = diameter of anode (ft), and S = distance
between anodes (ft).
Figure 9.3 shows the typical plotting of the results of this equation. Different
anodic shapes will have different curves.
Number of Anodes
Resistance
FigurE 9.3
Plot of Sunde equation.
Fundamentals of Corrosion
= 1200 mA. Magnesium anodes used in this particular soil have a voltage of
–1.65 V, or a galvanic cell voltage of
E =E
E
V
cell
C
A
− =−
− −
=+
0 85
165
0 8
.
( . )
.
The resistance is therefore
R=
V
1
=
=
0 8
1 2
0 67
.
.
. Ω
As the number of anodes is increased, the total resistance of the system
decreases. Each anode that is added provides a new path for current flow,
parallel to the existing system. The relationship between the resistance of the
system and the anodes is shown in the Sunde equation:
R=
0.00521P
NL
L
d–1
L
S
N)
=
+
( . log
.log .
2 3
8
2 2 3 0 656
where R = resistance (ohms), P = soil resistivity (ohm-cm), N = number of
anodes, L = anode length (ft), d = diameter of anode (ft), and S = distance
between anodes (ft).
Figure 9.3 shows the typical plotting of the results of this equation. Different
anodic shapes will have different curves.
Number of Anodes
Resistance
FigurE 9.3
Plot of Sunde equation.