ABSTRACT
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.
