Connection to Structure. Sacrificial anodes are normally supplied
with either lead wires or cast-in straps to facilitate their connection to the
structure being protected. The lead wires may be attached to the structure by
welding or mechanical connections. These should have a low resistance and
should be insulated to prevent increased resistance or damage due to
corrosion. Where anodes with cast-in straps are used, the straps should be
welded directly to the structure if possible, or, if welding is not possible,
used as locations for attachments using mechanical fasteners. A low
resistance mechanically adequate attachment is required for good protection
and resistance to mechanical damage. Welded connections are preferred to
avoid the increase in resistance that can occur with mechanical connections.
Other Requirements. As for all systems to be protected, the
structure being protected by sacrificial anodes must be electrically
continuous. The system should also include test stations that are used to
monitor the performance and to adjust the system for proper operation. As in
all mechanical and electrical systems, cathodic protection systems require
periodic inspection, maintenance, and adjustment for satisfactory operation.
Impressed Current Systems. From the standpoint of the structure
being protected, cathodic protection using the impressed current method is
essentially the same as in the sacrificial anode system. As shown in Figure
5, the cathodic protection system supplies high energy electrons to the
structure being protected and the circuit of the electrochemical cell is
completed through the soil. However, in the impressed current system, a
supply of direct electrical current is used to develop the potential
difference between the anode and the structure being protected. Consumption
of the anode is not the driving force for the flow-protective current. A
properly designed, installed, and maintained impressed current cathodic
protection system is as effective as the galvanic anode type of system in
preventing corrosion of the structure being protected.
Anode Materials. The materials commonly used for impressed current
cathodic protection have the capability of passing a current into the
environment without being consumed at a high rate. Graphite and high silicon
cast iron are the most commonly used impressed current cathodic protection
anode materials; however, other materials such as magnetite, platinum, and
newly developed oxide coated ceramic materials have been successfully used.
For buried anodes, a backfill consisting of carbonaceous material is normally
used: to decrease the electrical resistance of the anode; to provide a
uniform, low resistivity environment surrounding the anode; and to allow for
the venting of gasses produced at the anode surface.
Direct Current Power Source. The supply of direct electrical
current used to develop the potential difference between the anode and the
structure being protected is normally a rectifier which changes alternating
current to direct current of the appropriate voltage and current output.
However, in special applications, other direct current power sources such as
solar cells, thermoelectric cells, motor-generator sets, and wind-driven