Civil Engineering Reference
In-Depth Information
system. Acid production induced bond loss is relevant for both carbon and
titanium based CP systems.
Anode-copper connections in CP systems may corrode due to anodic
polarisation if their isolation is less than perfect. If a connection fails due
to corrosion, the part of the system fed by it stops receiving current and
its corrosion protection will fail. Anode-copper connections are either
isolated and embedded in the concrete or located in junction boxes. Their
service life depends on the design and execution of the isolation against
water penetration. A strategy of avoiding corrosion of connections seems
appropriate, based on careful design and execution of connections, backed
by experience. It seems possible to set up a test method for embedded
connections. In practice, corrosion of connections occurs, but is felt to be a
minor problem.
Reference electrodes in CP systems have failed, probably due to shrinkage
and drying out, resulting in loss of electrical contact. Improved RE types and
redundancy in their number per zone seem to be successful.
An inventory of CP systems in The Netherlands operating between 2
and 16 years showed that out of 52 well-documented systems, 33 operated
without failure or unforeseen maintenance. The highest rate of failure
reported related to conductive coating failure, ranging from local disbonding
to complete replacement being necessary between 6 and 10 years, out
of 38 conductive coating systems. Most of these failures were related to
leakage, which causes current density to increase locally. Probably at those
spots oxidation of carbon is accelerated, in accordance with our model.
Nine cases were reported of failure of anode-copper connections due to
corrosion of less than perfect isolation. This represents a significant amount
of maintenance that should be avoided by using well-proven connection
details, either embedded in the concrete or in junction boxes. Failure of
reference electrodes was reported in only one case. However, more individual
failures have probably occurred, but have been compensated by replacing
failing electrodes or ignoring them. As reference electrodes are critical for
evaluating system performance, having redundant numbers is preferred.
Overall, the inventory provides a positive picture of CP system
performance over time. The highest failure rate, for conductive coating CP
systems, suggests that avoiding (local) leakage is a critical factor; nevertheless,
some systems will need local touching up or replacement of the anode after
about 10 years.
Acknowledgements
The authors gratefully acknowledge fruitful discussions with the members
of COST 534 working group Electrochemical Maintenance Methods and
CP project information received from Dutch CP companies. Comments
on a previous paper from which this chapter was developed (Polder et al.
2006) by Dr Jürgen Mietz of BAM, Berlin, are greatly appreciated. We thank
