Civil Engineering Reference
In-Depth Information
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There are good trade associations and learned societies with members
skilled in the design, installation and operation of systems.
•
There are certification schemes for engineers who design, install and
operate the systems.
•
It has a proven track record of over 20 years standing.
•
Systems should last 20 to 50 years and can be designed for longer
durability of the major components.
The key limitations of impressed current cathodic protection are:
•
They require specialist knowledge.
•
Extreme caution and specialist advice is required to apply impressed
current cathodic protection to:
•
Structures containing prestressing steel
•
Structures with coated steel (epoxy or galvanised)
•
Structures that have been epoxy injected or have any other
impediment to current reaching the steel.
•
It requires a permanent power supply (and telephone line if it is remotely
monitored).
•
It requires ongoing monitoring and maintenance.
•
Initial cost is high and requires 10 to 20 years remaining life to justify
the life-cycle cost.
4.2 Galvanic (sacrificial anode) cathodic protection
Galvanic cathodic protection has been in existence far longer than impressed
current cathodic protection but has only recently been applied to steel in
concrete. This is because it relies on a small current generated by a corroding
anode to protect the steel and the driving voltage is very small so that high-
resistance concrete can reduce the current to inadequate levels to achieve
protection.
4.2.1 Theory
The method used is to connect the steel to a sacrificial or galvanic anode
such as zinc. This anode corrodes preferentially, liberating electrons with the
same effect as the impressed current system, e.g.:
Zn
→
Zn
2+
+ 2e
-
This system is illustrated schematically in
Figure 4.6.
The same phenomenon is used in galvanising where a coating of zinc is
applied over steel to corrode preferentially, protecting the steel. However,
the main restriction on this system is that the zinc has a small driving voltage
when coupled to steel. This is only a few hundred millivolts, and gets smaller
