Civil Engineering Reference
In-Depth Information
Reaction (4.5) is the reverse of reaction (4.2), i.e. alkalinity is formed at
the steel cathode (enhancing the passivity of the steel) and consumed at the
anode. These and related reactions can carbonate the area around the anode
(especially where carbon-based anodes are used, where the carbon also turns
into carbon dioxide) and can lead to etching of the concrete surface and
attack on the cement paste and even some aggregates once the alkalinity is
consumed.
We can therefore see that three factors must be taken into account when
controlling our cathodic protection system:
1
There must be sufficient current to overwhelm the anodic reactions and
stop or severely reduce the corrosion rate.
2
The current must stay as low as possible to minimise the acidification
around the anode and the attack on the anode for those that are
consumed by the anodic reactions.
3
The steel should not exceed the hydrogen evolution potential, especially
for prestressed steel to avoid hydrogen embrittlement.
The balancing of these requirements will be discussed below, under
criteria for control of impressed current cathodic protection systems.
One of the more confusing facts of cathodic protection is that when we
carry out a reference electrode potential survey of a reinforced concrete
structure (
Chapter 1)
the most negative areas are those that are at highest
risk of corrosion while the areas with a positive potential are at lowest risk
of corrosion, i.e. cathodic. However, to achieve cathodic protection, we
must depress the potential of the steel. This is because there must be excess
electrons on the steel to force the production of hydroxyl ions (OH-) and
suppress the formation of iron ions (Fe
2
+
) which want to release electrons
(equation (4.1)).
One of the earliest-identified criteria for achieving effective cathodic
protection is to depress the potential of the cathodes to that of the most
anodic areas (Mears and Brown, 1938). This stops the current flow from
anode to cathode. It works because cathodes are more easily polarised than
anodes. For a fixed current, an actively corroding area shifts its potential less
than a non-corroding area. Therefore, once we can depress all the cathodes
below the potential of the anodes, the corrosion will stop.
4.1.2 Impressed current cathodic protection system components
An impressed current cathodic protection system consists of an anode system
in permanent contact with the concrete, an adjustable direct current power
supply, reference electrodes embedded in the concrete near the steel and
possibly other monitoring probes. There will also be a monitoring and control
system to measure voltages and currents and to adjust the power supply.
These components are all wired together. A structure will be broken down
