Nace CP 1, Course Manual

Page: 360 / 535

RP0176-2003
38
NACE International
________________________________________________________________________
Appendix C
Consumption Rates in Seawater of Various Commercial Types of Impressed
Current Anodes Available for Cathodic Protection of Offshore Structures
Impressed current anodes, unlike galvanic anodes, can be
driven over a wide range of anode current densities,
depending on the designer’s preferences and the demands
of a particular application. Because anode current loadings
have a definite bearing on consumption rates, any meaning-
ful listing of consumption rates for various anode materials
must necessarily include the approximate anode current
density at which such rates have been established.
Table C1 lists consumption rates of various impressed cur-
rent anode materials used for CP systems on offshore
structures. If titanium is used as the substrate for a plati-
num-type anode, anode/electrolyte voltage must be limited
to about 8 V. For niobium substrates, laboratory data indi-
cate that voltages as high as 100 V are satisfactory; how-
ever, field experience shows that 50 V is a more reliable
and practical limit.
Table C1: Consumption Rates of Impressed Current Anode Materials
Impressed Current Anode Material Typical Anode Current Density
in Saltwater Service A/m
2
(A/ft
2
)
Nominal Consumption Rate
g/A-yr (lb/A-yr)
Pb-6%Sb-1%Ag 160 to 220 (15 to 20) 14 to 90 (0.03 to 0.2
(A)
)
Pb-6%Sb-2%Ag 160 to 220 (15 to 20) 14 to 27 (0.03 to 0.06
(B)
)
Platinum (on Titanium, Niobium, or Tantalum
substrate or mixed metal oxide)
540 to 3,200 (50 to 300) 3.6 to 7.3 (0.008 to 0.016
(B)
)
Graphite 10 to 40 (1 to 4) 230 to 450 (0.5 to 1.0)
Fe-14.5%Si-4.5%Cr 10 to 40 (1 to 4) 230 to 450 (0.5 to 1.0)
___________________________
(A)
Very high consumption rates of Pb-Ag anodes have been experienced at depths below 30 m (100 ft).
(B)
This figure can increase when current density is extremely high and/or in low-salinity waters.
________________________________________________________________________
Appendix D: Typical Method for Calculation of Galvanic Anode Current Output Using Initial, Maintenance,
and Final Current Densities
This method has been commonly employed in the past for
CP design, so practitioners tend to be familiar with it. The
disadvantage is that it is based on an algorithm rather than
being first principles-based and can lead to unnecessary
over-design compared to the slope parameter method (see
Appendix E). The over-design can be minimized if a range
of anode sizes is considered, and the size selected on the
one that gives the lowest installed cost.
36
From a
modification of Dwight’s equation,
39
the resistance of a
cylindrically shaped anode to the electrolyte in which it is
placed is equal to the product of the specific resistivity of the
electrolyte and certain factors relating to the shape of the
anode, as given in Equation (D1):
1
r
L 4
ln
L
K
R
−












ρ =
(D1)
where:
R = anode-to-electrolyte resistance in ohms
ρ
= resistivity of the electrolyte in ohm-cm. (See Table
A1)
K = 0.500/
π
or 0.159 if L and r units are in cm or
0.0627 if L and r units are in inches
L = length of anode in centimeters
r = radius of anode in centimeters (for other than
cylindrical shapes, r = C/2
π
, where C = cross-
section perimeter).
Thus, for a 25 x 25 cm (10 x 10 in.) cross-section, C = 100
cm (40 in.), and r = 15.9 cm (6.27 in.).
To determine the current output from an anode, use Ohm’s
Law: I = E/R.
For a typical calculation for an anode in the Gulf of Mexico,
the number of anodes required for protection must satisfy
three different calculations. There must be enough anodes
to polarize the structure initially (initial current density from
Table A1), to produce the appropriate number of amps of
current over the design life of the structure (mean current
requirement), and to produce enough current to maintain
protection at the end of the design life (final current require-