Corrosion Protection - Offshore Structures: Design, Construction and Maintenance - page 411

TABLE 6.10 Performance Properties of the Alloy

Minimum
Alloy
Capacity
(A/hr/kg)

Nominal
Resistivity
(Ohm/cm)

Working Potential
(mV w.r.t.)
Ag/AgCl/Seawater

Anode
Temperature ( Environment ° C)

Saline mud 100 40 1500 − 1050

Saline mud 200 25 1865 − 1050

Seawater 25 25 2000 − 1070

Whoever will deliver the anode should propose an alloy offering the mini-
mum electrochemical performance characteristics shown in Table 6.10 ,and
the characteristics should be documented by long-term performance test
reports.
Before starting the manufacturing process, the supplier should prove the
ability of the proposed alloy to satisfy the above requirements, and minimum
anode capacities and working potentials should be determined at operating
anode current density, in the range of 0.25 A/m

² to 1.0 A/m ² for saline mud

and 0.25 A/m for free seawater.

The supplier should submit the full chemical analysis details and the specific
gravity value of the proposed alloy to the client for approval and should demon-
strate that the specification requirements can be satisfied at the compositional
limits.

² to 4.0 A/m ²

6.4.6 CP Design Parameters
Most of the design of the CP system depends simply on providing an electri-
cal current to compensate for the loss of electrons through corrosion. So
all the design parameters depend on providing a quantity of anode that will
be distributed through out the structure to provide the required current
under the environmental conditions that the structure is exposed to during
its lifetime.
So the target is to select the quantity of anode that will be depleted after the
structure s lifetime and that will provide the required current. The design concept

and equations are available in guideline DNV B401, which is traditionally used
by most designers, but it needs to be adapted according to the designer

'

' sexperi-

ence with the geographic location of the structure.
All the parameters are usually defined by the designer of the CP system.
However, sometimes certain or all CP design parameters have already been
defined by the purchaser in a project document.