Civil Engineering Reference
In-Depth Information
Anode Resistance
The anode resistance, R (ohm), is calculated using the following formulas,
which are applicable to the actual anode shape. Calculations are performed
for the initial anode dimensions and for the estimated dimensions when the
anode has been consumed to its utilization factor.
For long, slender stand-off anodes, L
≥
4r:
ρ
ln
4L
=
r
−
R
1
(6.14)
2
π
L
where L is the anode length,
ρ
is the seawater resistivity (ohm/m) and r is the
effective anode radius.
For short, slender stand-off anodes, L
<
4r:
"
(
!
)
#
r
1
+
r
1
+
2
2
ρ
2L
r
r
2L
r
2L
−
r
2L
R
=
ln
1
+
+
(6.15)
2
π
L
The equation is valid for anodes a minimum distance of 0.30 m from the pro-
tection object. For an anode-to-object distance less than 0.30 m but a minimum of
0.15 m, the same equation may be applied with a correction factor of 1.3.
For long, flush-mounted anodes with L greater than or equal to 4 times the
width and thickness:
ρ
R
=
⋅
(6.16)
2
S
), where c (in m) is the anode cross-
sectional periphery. S is the arithmetic mean of anode length and width.
For short, flush-mounted, bracelet and other anode types:
For noncylindrical anodes, r = c/(2
π
0
:
315
ρ
R
=
p
A
(6.17)
where A is the anode surface area.
Resistivity of Seawater and Sediment
Seawater resistivity,
⋅
m), influences the design of the CP system and
this resistivity is affected by both the seawater salinity and temperature. Note
that, in the open sea, salinity does not vary significantly and temperature is
the main factor. On the other hand in the case of shore areas, particularly at
river outlets and in enclosed bays, the salinity will vary significantly. It is
recommended that the design of CP systems in such locations be based on resis-
tivity measurements reflecting the annual mean value and the variation of resis-
tivity with depth.
In comparison to seawater, the resistivity of marine sediments is higher by a
factor ranging from about 2 for very soft clays to approximately 5 for sand. If
ρ
(ohm
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