Page 587 - Corrosion Engineering Principles and Practice

550 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 551

The main advantage of sacrificial CP over ICCP is that it does not
require a power supply or control system and therefore has low
monitoring requirements. Its disadvantages are that it cannot be
controlled to ensure complete corrosion control and that the anode
types and anode lives are more limited.

13.5 Cathodic Protection Components

13.5.1 Reference Electrodes
The main criterion for monitoring and controlling the cathodic
protection of most metals is based on a measure of the potential of the
metal relative to a half cell or reference electrode. As described in
Chap. 4, Corrosion Thermodynamics, reference electrodes consist of
a metal electrode immersed in a specific solution of its own ions and
this solution is connected to the corroding electrolyte through a salt
bridge. This bridge may consist of some electrolyte-holding medium
such as a gel, a sintered or porous ceramic, or a small wood plug
saturated with the reference electrode solution, the corroding
electrolyte, or a liquid compatible with both. An ideal reference
electrode would possess the following properties [12]:

• Its potential should be constant irrespective of the electrolyte
or environment in which it is used and it should not vary
greatly with changes in electrolyte temperature or other
parameters.
• Any changes should be predictable and should have no
hysteresis effect.
• The device should not polarize at small currents either when
these are cathodic or anodic.
• It should have a low internal resistance.
• These properties should be constant with time.
• A robust construction.
• It should be easy to replenish in the field if necessary.

The reference electrode generally used for monitoring cathodic
protection in soils is the copper sulfate electrode. This consists of an
electrode of electrolytic copper in a solution saturated with copper
sulfate crystals. However, the most universal reference electrodes are
of the silver chloride type. These consist of a silver wire, plate, or
mesh coated with silver chloride so that both silver and silver chloride
are in contact with a chloride ion rich electrolyte. The potential of the
Ag/AgCl reference electrode will depend upon this chloride ion
concentration.