Civil Engineering Reference
In-Depth Information
A brief description is given in
Table 4.1.
For further information on anode
systems see Broomfield (2007).
Monitoring probes
The reference electrodes are generally silver/silver chloride/potassium
chloride electrodes or proprietary manganese/manganese dioxide electrodes
designed and constructed for permanent embedment in concrete. They
should be installed with minimal disruption to the concrete around the steel
for representative measurement.
For systems on structures with very long lives, simpler, pseudo-reference
electrodes may be used along with 'true' reference electrodes. This is because
true reference electrodes have a life of about 20 years while a pseudo-
reference electrode of graphite or mixed metal oxide coated titanium will
last indefinitely but is less accurate.
Power supplies
The transformer/rectifier (T/R or rectifier) is the DC power supply that
transforms mains AC to a lower voltage and rectifies it to DC. The positive
terminal is connected to the anode and the negative to the cathode. The level
of the output is controlled as described below. T/Rs can be run at constant
voltage, constant current or constant potential (against a half cell). They
can be adjusted manually, automatically by circuitry or computer control,
or remotely using a telephone line and modem link or similar remote
connection as described later.
Transformer/rectifiers for conventional cathodic protection systems of
steel piles in docks or on pipelines can be very large and powerful, capable
of delivering hundreds of amps, with oil-cooled transformers. However,
for steel in concrete, the requirements are far more modest. Most systems
are designed for a current density of about 10 to 20 mA per square metre
of steel surface for actively corroding structures and for 0.2 to 2.0 mA/m
2
for new structures where there is no pitting and so no need to passivate
pits.
Most steel in concrete needs less than 10 mA/m
2
to provide protection,
usually at less than 10 V. The power for a 100 watt light bulb will typically
protect 10,000 m
2
. This means that a single phase, air cooled T/R will
usually protect even the largest structure and power consumption is rarely
an economic concern.
When calculating the current demand in a system, there may be a
requirement for allowances to be made for the current flow to lower layers
of steel as well as the outer, corroding layer. Calculations must also make
allowances for the voltage drops down the connecting cables and anode
strings.
The T/R must be rugged and reliable with minimal maintenance
requirements. It should be easy to maintain with good instruction manuals,
