Page 573 - Corrosion Engineering Principles and Practice

536 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 537

When this occurs, there may be relatively low potentials between
the pipe and adjacent earth. This is indicated by the −0.7 V reading in
Fig. 13.11, which is less than the full protection criterion of −0.85 V
versus a copper sulfate reference electrode. The shielding effect will
tend to be greatest near the center of the congested area.
Where congested area shielding is a problem, it may not be
practical to rely solely on a remote CP system. However, CP can still
be attained by the use of additional anodes (either galvanic or
impressed current) distributed strategically throughout the congested
area in such a way that the areas of influence surrounding the anodes
overlap sufficiently to permit development of protective potentials
on the innermost structures.

13.3.1 Sacrificial Cathodic Protection
Figure 13.12 illustrates the essential differences between sacrificial CP
and ICCP systems for soil applications. As shown in Fig. 13.12, a
voltage can be measured between the corroding structure and the
material to be used as an anode. The structure must be positive (+)
with respect to the anode before the anode can discharge current.
There are certain requirements for a metal to be a practical galvanic
anode material:

• The potential between the anode and corroding structure
must be large enough to overcome the anode-cathode cells on
the corroding structure.
• The anode material must have sufficient electrical energy
current to permit reasonably long life (10 to 15 years plus)
with a practical amount of anode material.
• Anodes must have good efficiency, meaning that a high
percentage of the electrical energy content of the anode
should be available for useful CP current output. The balance
of the energy that is consumed in self-corrosion of the anode
itself should be ideally small.

13.3.2 Impressed Current Cathodic Protection
With the ICCP system shown in Fig. 13.10, the groundbed anodes are
not really providing the driving power. Instead, some external source
of direct electric current is connected (or impressed) between the
structure to be protected and the groundbed anodes. The positive
terminal of the power source is connected to the groundbed, which is
then forced to discharge as much CP current as is desirable. If a
mistake is made and the positive terminal is connected erroneously
to the structure to be protected, the structure will become an anode
instead of a cathode and will corrode actively.
Groundbed anodes forced to discharge current will corrode. It is
important to provide anode materials which are consumed at