Handbook of Electrical Installation Practice - BeKnowledge

194 Handbook of Electrical Installation Practice
as of prime importance by most owners and operators as an integral part of their
corrosion control programme.
INTERACTION
Interaction is the presence of stray earth or other external d.c. or a.c. currents in
the electrolyte which may affect the performance of installed cathodic protection
systems. The installation of a cathodic protection system itself, particularly of the
impressed current type, may cause corrosion interaction on neighbouring or adjacent
structures. The effect of such interaction may be to increase the corrosion rate
thus leading to shorter service life. All newly installed cathodic protection systems
should therefore be subject to interaction notification and testing. Where tests show
unacceptable levels of interaction, safeguard measures should be taken to correct
the situation. Where cathodic protection systems are installed close to low-voltage
d.c. circuits (e.g. railway track signalling systems), serious disruption of circuit
balance and operation can occur.
Stray current leading to accelerated corrosion effects may also be detected as a
result of natural geomagnetic effects. In all cases a test programme is required
so that appropriate mitigating actions can be taken. Any subsequent changes in
operating circumstances of the cathodic protection system will necessitate repeat
interaction testing.
Reference should be made to British Standard Code of Practice 7361: Part 1 for
interaction testing and other requirements regarding installation.
PROTECTION OF STEEL IN CONCRETE
The application of cathodic protection systems as a corrosion prevention measure
is being extended to structures hitherto not considered appropriate. Significant areas
of development have included the protection of steel in concrete and metallic structures
which are not continuously in contact with an electrolyte but operate in corrosive
conditions. Protection of steel in concrete is now widely used to ensure
structural integrity whereby cathodic protection impressed currents are applied to
reinforcing steel within concrete where chlorides are likely to be present. Anode
systems generally comprise titanium or mixed metal oxide mesh or electrically conductive
paint/coating systems laid on the external concrete surface so that current
may flow uniformly to the reinforcing bars within the structure. Testing and monitoring
are carried out via fixed reference electrodes installed at representative sites
throughout the protected structure. These may be connected permanently to a centralised
data logger or alternatively used with portable measurement equipment.
The application of cathodic protection to steel within concrete is of a specialist
nature and does not accord with the criteria set out in this chapter. Further information
is available from the Society for the Cathodic Protection of Reinforced
Concrete.