Nace CP 1, Course Manual

Page: 336 / 535

RP0176-2003
14
NACE International
ial does not passivate when covered by mud. The
reduction in the current output and efficiency from
a galvanic anode covered by mud should also be
considered if it is at or below the mudline.
5.8.2.6 Whenever practicable, the larger sizes of
galvanic anodes should be attached to the struc-
ture in such a manner that stresses are minimized
at the point of attachment.
5.8.2.7 When the larger offshore-type anodes are
used, cracks may occur while cooling during
casting because of the different coefficients of
expansion of the anode and the core. These
cracks are not detrimental, provided they do not
converge in a manner to allow loss of anode mat-
erial. Warpage is acceptable if it does not ad-
versely affect the anode installation. More specific
guidelines can be found in NACE Standard
RP0387.
11
5.8.2.8 When galvanic anodes are mounted flush
with the steel surface, either the back face of the
anode or the surface of the steel should be coated
or fitted with a dielectric shield to prevent prema-
ture consumption of the anode material from the
back face and possible failure of the mount
because of the pressure developed by the corro-
sion products of the anode.
5.8.3 Impressed current systems are more critical with
respect to mechanical damage because relatively few
anodes, each discharging a substantial amount of pro-
tective current, are involved. The loss of an anode can
seriously reduce system performance.
5.8.3.1 The electrical connection between the
anode lead cable and the anode body must be
made watertight and mechanically sound.
5.8.3.2 Cable and connection insulating materials
should be resistant to chlorine, hydrocarbons, and
other deleterious chemicals.
5.8.3.3 Care must be taken to provide suitable
mechanical protection for both the anode and its
connecting cable. On suspended systems, the in-
dividual anodes or anode strings may be equipped
with winches or other retrieval means as a dam-
age-preventing measure during severe storms or
for routine inspection and maintenance. The loss
of protection during these periods should be con-
sidered.
5.8.3.4 Acceptable methods of installing fixed-
type impressed current anodes include, but are
not limited to, the following:
(a) Anodes can be installed at the lower ends of
protective vertical steel pipe casings or conduits.
Casings should be attached to above-water
structure members and supported at repeating
members below water. The anodes should be
lowered through the casings (which protect the
anode lead wires) and should be allowed to extend
below a termination fitting at the bottom of each
casing. This method provides a means of anode
retrieval or replacement using the anode cable,
without diver assistance. Marine growth or
corrosion scale may make anode retrieval difficult.
(b) Anodes with essentially flat configuration-
mounted, insulating-type holders can be attached
directly to submerged structure members or to
auxiliary structural members, such as vertical
pipes, which can be removed for anode
replacement. Properly designed sys-tems of the
latter type permit anode retrieval without diver
assistance.
(c) Anodes can be installed on submerged
structure members using offset steel structural
supports attached to the structure members. Diver
assistance is required for this type of anode
replacement.
(d) Anodes may be bottom-installed on the ocean
floor in special instances, such as in Cook Inlet,
Alaska. Anodes are typically mounted on specially
designed concrete sleds for stability; this
minimizes the possibility of their becoming covered
with mud or silt. Steel reinforcing in the concrete
has been reported to be subject to interference
current corrosion.
5.8.3.5 Impressed current anodes should be loc-
ated as far as practical from any structure member
(usually a minimum distance of 1.5 m [5.0 ft], but
proportional to current magnitude [see Paragraph
8.5.2]). If a spacing of 1.5 m (5.0 ft) is not feasible
a dielectric shield should be used to minimize was-
tage of protective current by localized overprotect-
tion. Anode holders should be designed to avoid
such wastage and to minimize the possibility of a
short circuit between the anode and the structure.
NOTE: Satisfactory performance of coating mater-
ials used as cathode shields has not been fully
demonstrated over long periods of time. Providing
a suitable anode-to-cathode spacing is the pre-
ferred method of ensuring good anode current dis-
tribution.
5.8.3.6 Simple, rugged, easily maintained im-
pressed current equipment should be used. Be-
cause conditions at an offshore location seldom
vary, except for weather-related factors, manually
adjustable oil-cooled rectifiers with both alternating
current (AC) and direct current (DC) overload pro-
tection are preferred. In certain special cases,
either constant current or automatic potential con-
trol
rectifiers may prove desirable, but the cost and
maintenance of these units should be compared.
5.8.3.7 There may be a delay of several months
to a year or more between the time a structure is