Civil Engineering Reference
In-Depth Information
Figure 4.6
Schematic of a galvanic cathodic protection system. The steel is made
generating the electrons needed to sustain the cathodic reaction on the steel
surface.
with actively corroding steel. While a galvanising system puts the two
metals in direct contact, with galvanic anode cathodic protection there is an
electrolyte to carry the current. The resistance of the electrolyte is crucial to
the performance of the system.
Galvanic cathodic protection systems have been used extensively since the
early 1990s in Florida on prestressed concrete bridge support piles in the
sea. One of the reasons the galvanic system is used there is because concrete
resistivity is low due to the marine exposure conditions. The Florida
systems frequently incorporate a distributed anode of zinc fixed on the
atmospherically exposed concrete and bulk zinc anodes in the water which
pass current through the low-resistance sea water to protect the submerged
area as shown in
Figure 4.7.
The distributed anodes used in Florida are principally electric arc sprayed
zinc, a few tenths of a millimetre thick
(
Figure 4.8),
or zinc metal mesh
mounted in a permanent form and grouted in place. The sprayed zinc anodes
are also widely used in the USA for impressed current cathodic protection.
The mesh anodes were originally mechanically clamped to the pile. In later
versions a glass reinforced plastic (GRP) jacket containing the zinc mesh is
