Nace CP 1, Course Manual

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Underground Corrosion Control 3:6
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Cathodic Protection
This approach to control of corrosion using electrical current is limited to
metals exposed to an electrolyte such as soil, water, and concrete. It does
not work in the atmosphere.
Cathodic protection is often used in conjunction with other corrosion
control methods including protective coatings and electrical isolation.
Theory
On a corroding surface, there are hundreds of local, or microscopic, corrosion
cells. Figure 3.1 illustrates such a cell on the surface of a pipeline. There is a
potential, or voltage, difference between the anode and cathodes of these
cells; this potential difference drives the corrosion current.
Anode
Cathode
Microscopic Corrosion Cell on
the Surface of a Pipeline
Figure 3.1 Microscopic Corrosion Cell
The concept of cathodic protection involves reducing the potential difference
between local anodic and cathodic sites on a metal surface to zero, resulting in
zero corrosion current flow. This can be accomplished by causing a current to
flow onto the structure from an external anode which current polarizes the
cathodic sites in an electronegative direction. As the potentials of the cathodic
sites polarize toward the potentials of the anodic sites, corrosion current is
reduced. When the potentials of all cathodic sites reach the open circuit
potential of the most active anodic sites, the voltage difference between local
anodes and cathodes is eliminated and corrosion ceases.
Cathodic protection does not actually eliminate corrosion. Instead, it
transfers it from the structure to be protected to the cathodic protection
anode(s). The structure is now the cathode of an intentional corrosion cell.