Nace CP 1, Course Manual

Page: 335 / 535

RP0176-2003
NACE International
13
the calculated total current requirement, using the
surface areas and selected design current den-
sities.
5.6.4.4 With either galvanic or impressed current
systems, the anodes should be distributed vertic-
ally and/or horizontally in accordance with the cal-
culated current requirements of the structure
members. Computer modeling can be used to ver-
ify proper current distribution.
5.7 Factors Determining Anode Current Output, Operating
Life, and Efficiency
5.7.1 Various anode materials have different rates of
consumption when discharging a given current density
from the anode surface in a specific environment.
Therefore, for a given current output, the anode life
depends on the anode material as well as on its weight.
Established anode performance data should be used
to calculate the probable consumption rate (see
Appendixes B and C).
5.7.2 Galvanic anodes and anode systems should be
designed to provide sufficient exposed anode area to
produce the required protective current for the life of
the anode system. Two alternative protocols for calcu-
lating anode output capability are shown in Appendixes
D and E. The anode system must also provide suffi-
cient weight of anode material to supply current for the
desired number of years of protection, based on estab-
lished anode performance data.
5.7.3 Proper design of a galvanic anode system must
consider structure-to-anode potential and resultant
anode current output and, when applicable, anode lead
wire resistance. The net driving voltage between a typ-
ical aluminum-zinc-mercury or zinc anode of -1.50 V
(Ag/AgCl [sw] reference) is only 0.25 V (1.05 - 0.80),
although the initial net driving voltage is higher because
the structure potential is more positive at this time.
The anode lifetime (L) of galvanic anodes may be
determined using Equation (2):
I E
u W
L
×
×
=
(2)
where:
L = effective lifetime of the anodes in years
W = net mass of the anodes in kg or lb
U = utilization factor determined by the amount of
anode material consumed when the remain-
ing anode material cannot deliver the current
required
E = consumption rate of the anode in kg/A-yr or
lb/A-yr
I = mean current output during the lifetime in A
The shape of the anode affects the utilization factor.
Proper selection of length, diameter, and core diameter
can result in utilization factors ranging from 0.75 to
almost 1.0. For standoff-type anodes, utilization factors
of 0.90 to 0.95 are common. For bracelet anodes, val-
ues of 0.75 to 0.90 may be used.
5.8 Special Mechanical and Electrical Considerations
5.8.1 The performance of a CP system is dependent
on the ability of the various components to withstand
the physical stresses to which they are subjected. The
type of system, the various components thereof, and
the physical installation should all be durable and
dependable.
5.8.2 Galvanic anode systems are relatively simple,
but consideration must be given to the following:
5.8.2.1 Anodes should be located on the structure
to ensure polarization of node welds as early as
possible. Node welds are critical for structural
strength of structures. Polarization of node welds
is of primary importance to prevent development of
unacceptable weld profiles such as undercutting in
the heat-affected zone (HAZ) and pitting in the
weld. Both types of corrosion cause additional
stress concentration that can lead to fatigue crack-
ing. Nodes are also zones of high metal surface
area and complex geometry requiring greater cur-
rent density for protection and to overcome the
effects of shielding. Weld consumables that can
cause welds to be anodic to the steel shall not be
used. Tests shall be carried out as a part of the
welding pre-qualification test (WPQT).
5.8.2.2 Anode cores must be structurally suitable
for the anode weight and the forces to which the
anode is subjected, both during structure setting
and pile-driving operations and during storms and
hurricanes. It is important that the core be able to
withstand anticipated wave forces in the later
stages of anode consumption, when the body of
the anode is no longer adding its strength to that of
the anode core.
5.8.2.3 The core should be designed to maintain
electrical integrity with the anode body for the full
design life of the anode. A large-diameter core
aids in achieving the required electrical output to-
ward the end of the life of the anode.
5.8.2.4 Reinforcement to the structure members
may be required to support the larger sizes of gal-
vanic anodes. If so, it should be made at the point
of anode attachment by gussets, doubler plates, or
another approved method.
5.8.2.5 Because the efficiency of some aluminum
alloys is adversely affected when they are covered
with mud, attaching such anodes to structures loc-
ated at or below the mudline should only be done
after testing the anode in representative mud, or if
experience shows that the intended anode mater-