UFC 3-570-06
JANUARY 31 2003
Normally, a test station has three wires as shown in Figure 2-23. Some test
stations may not have current shunts, may not have a separate structure potential
measurement lead, may have multiple anode leads, may just have one structure lead
(anodes connected directly to structure), or there may be multiple anodes with a single
lead.
Other types of test stations are described in the sections that describe the
various tests performed using the test stations. It is important that the connections in
the test stations be maintained in their original condition after use or when damaged. If
the identities of the test station leads are unknown, the leads can be disconnected and
the potential of each lead to a copper/copper sulfate reference electrode taken. Using
the galvanic series (Table 2-2), the leads may be easily identified:
High purity magnesium -1.75
Standard magnesium -1.55
Zinc -1.2, aluminum -1.1
Structure leads -0.2 to -0.8 or slightly higher if with other anodes are still
attached
2-7
IMPRESSED CURRENT CATHODIC PROTECTION. As in galvanic anode
systems, impressed current systems supply current for cathodic protection of a metal
surface. However, in the case of an impressed current system, the protective current is
supplied by a rectifier (or other DC power source) instead of by the natural potential
difference of the anode to the structure. As shown in Figure 2-19, the potential
difference between the anode and cathode is forced from a non-reactive anode bed by
the action of additional energy from a rectifier to force the electron flow that would be
normally produced in the corrosion reaction. The energy for the "electron energy pump"
action of the rectifier is provided by ordinary alternating current. The effect of these
electrons at the structure being protected is the same as that derived from the sacrificial
anode type of cathodic protection system. However, the anode material serves only as
a source of electrons and anodic (oxidation) electrochemical reactions.
In practice, materials such as graphite, high silicon cast iron (HSCI), platinum
or mixed metal oxide, are used for impressed current cathodic protection system
anodes because they are slowly consumed (they have a very low kilogram (pound) per
amp year weight loss). To provide a uniform electrolyte, a lower resistance to earth,
and venting of gases and acids, a special backfill is used. This earth contact backfill is
normally coke breeze or calcined fluid petroleum coke. Anodes in impressed current
systems must be periodically inspected and replaced if consumed or otherwise
damaged. As is the case for any electrical equipment, rectifiers used for impressed
current cathodic protection systems require preventive maintenance and recurring
2-41
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