Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 413

In case of a steel structure with some rust or mill scale, it is necessary to
effect polarization of an initially bare metal surface. This is done by the cathodic
current density that is called the initial design current.
In addition to that, the initial design cathodic current density is necessarily
higher than the final design current density because the calcareous scale and
possibly marine fouling layer developed during the initial phase reduce the sub-
sequent current demand as the polarization resistance is reduced. A sufficient
initial design current density enables rapid formation of protective calcareous
scale and hence efficient polarization.
During the offshore steel structure s lifetime, it usually accumulates calcar-

eous scale and marine growth, so the final design current density must be suffi-
cient to repolarize a structure if such layers are partly damaged.
An appropriate final design current density and hence CP polarizing capacity
will further ensure that the protection of the structure

'

' s jacket remains polarized to

apotentialof 1.05 V throughout the design life. In this potential range,

the current density demand for maintenance of CP is lowest.
The initial and final current densities are the main parameters used to calcu-
late the required number of anodes of a specific type to achieve a sufficient
polarizing capacity. The CP system designer uses Ohm

− 0.95 to −

s law and assumes
that the anode potential is in accordance with the design closed-circuit potential
and the potential of the protection object is at the design protective potential for
C-steel and low-alloy steel, i.e.,

'

0.80 V.
The anode current decreases linearly when the cathode is polarized toward
the closed-circuit anode potential, reducing the driving voltage for the galva-
nic cell. The total CP current for a CP unit, I t , can be calculated from Equa-
tion (6.9) :

−

E c − E a

I t = (6.9)

R

where R (ohm) is the total anode resistance, E c (V) is the global protection
potential and E a (V) is the actual anode (closed-circuit) potential.
The mean design current density, I cm (A/m 2 ), is a measure of the anticipated
cathodic current density once the CP system has attained its steady-state protec-
tion potential; this is typically 0.15 to 0.20 V more negative than the design pro-
tective potential.
The initial polarization period preceding the steady-state condition is nor-
mally short compared to the design life, and the time-weighted cathodic current
density becomes very close to the steady-state cathodic current density.
The water depth and the geographical location are the main parameters
affecting CP. Recommendations for initial/final and average (mean) design cur-
rent densities, based on climatic regions and depth, are given in Table 6.11 .
These design current densities have been selected conservatively to account