Civil Engineering Reference
In-Depth Information
This is the cathodic reaction. Two interesting observations can be made
here: more hydroxyl ions are generated in the cathodic reaction. These ions
will strengthen the passive layer, warding off the effects of carbonation and
chloride ions. It should also be noted that water and oxygen are needed at the
cathode to allow corrosion to occur, so oxygen is not necessarily required at
the corrosion site, provided the steel at the anodic site is electrically connected
to a suitable cathode with access to oxygen and moisture.
So, if it is possible to make the system favour the cathodic reaction, corrosion
will stop. In electrochemical techniques an external anode is applied to the
concrete surface. This generates the electrons instead of the anodic reaction
(9.3), and the steel has only the cathodic reaction (9.4) occurring on its surface.
For both chloride removal (
Chapter 8)
and realkalisation, the external anode
is temporary and the reactions are driven by a DC power supply. The systems
re-establish a 'passive' environment around the steel that will last many years.
For all electrochemical treatment, good electrical continuity in the steel
reinforcement is needed to ensure that current flows from the anode to all areas
of steel. Electrical continuity must be checked and, if necessary, established
by adding wired connections in all applications of these techniques. Equally,
there must be no short circuits between the steel and the surface. If there are,
current will short circuit the concrete pore structure and the chloride ions in
chloride removal and the hydroxyl ions in realkalisation will not flow.
Figure
9.1
shows a typical transformer rectifier for a temporary system such as is used
for realkalisation.
chloride removal (courtesy of Vector Corrosion Technologies).
