Civil Engineering Reference
In-Depth Information
6.4.5 Galvanic Anode Materials Performance
Aluminum and zinc are the main basic materials that are traditionally used in off-
shore structures. Note that the conceptual design in the front end engineering phase
will report the generic type of anode material (i.e., aluminum- or zinc-based) that
has been selected, and this information will be used in the next step of detailed
engineering for the CP system. In general, aluminum-based anodes are tradition-
ally used due to their higher electrochemical capacity. However, zinc-based anodes
have sometimes been considered more reliable with respect to electrochemical per-
formance for applications in marine sediments or internal compartments with high
bacterial activity, which are both environments with anaerobic conditions.
Based on practical experience, ferritic and ferritic-pearlitic structural steels
with specified minimum yield strength (SMYS) up to at least 500 MPa have
proven compatibility with marine CP systems. Laboratory testing has revealed
that hydrogen will induce stress cracking during extreme conditions of yielding.
It is recommended that all welding be carried out according to a qualified pro-
cedure with 350 HV as an absolute upper limit. With a qualified maximum
hardness in the range 300 to 350 HV, design measures should be implemented
to avoid local yielding and to apply a reliable coating system as a barrier to CP-
induced hydrogen absorption.
Welding of material that can change its formation should be followed by
post-weld heat treatment to reduce the heat-affected zone hardness and residual
stresses from welding.
Bolts made of martensitic steel heat-treated to SMYS up to 720 MPa, as per
ASTM A193 grade B7 and ASTM A320 grade L7, have well-documented com-
patibility with CP. However, failures due to inadequate heat treatment have
occurred, and, for critical applications, batch testing is recommended to verify
a maximum hardness of 350 HV.
Design precautions should include measures to avoid local plastic yielding
and use of coating systems qualified for resistance to disbonding by mechanical
and physical or chemical effects.
In most design cases, anodes should have a closed-circuit potential of
−
1.10 V
(or more negative) to copper/copper sulfate electrodes and should have a mini-
mum efficiency of 80% (maximum consumptionrateof3.68kgperampere-
year). Each foundry pour should be tested for chemical composition and
closed-circuit potential. A suitable sample should be retained from each pour
for performing a rate-of-consumption test. Aminimum of three rate-of-consump-
tion tests should be performed during the production run. In the event that any test
indicates a consumption rate of more than the maximum limit mentioned above,
tests should be performed on additional samples to confirm the adequacy of the
anode composition.
The active anode material should be a proven aluminum-zinc-indium alloy
suitable for long-term continuous service in seawater, saline mud or alternating
seawater and saline mud environments.
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