Page 84 - Reliability and Maintainability of In service Pipelines

Pipeline Inspection and Maintenance 73

2.4.2 CATHODIC PROTECTION

Cathodic protection is another common method for protecting steel pipelines
from corrosion. In older and simpler galvanic systems this involved coating the
pipeline in an anode, such as zinc, to prevent the steel from reacting to corrosive
environment. The anode corrodes in place of the steel. Because the zinc eventu-
ally corrodes and leaves the steel bare, galvanic systems have a limited lifetime.
Newer pipelines are required to make use of an impressed current protection sys-
tem (Fig. 2.9). In this system, the steel pipeline is connected to an anode made of
a metal that is more reactive than steel (e.g., magnesium, aluminum, zinc).
Because the anode is more reactive, it needs to lose all its ions before the steel
begins to corrode. The anode is then connected to a power source called a recti-
fier. As the anode loses electrons to corrosion in place of the steel, the electrons
are replenished by the power source. This system is limited only by the rectifier
and therefore has a much longer lifespan.
Steel pipeline maintenance can be undergone by cathodic protection. Water
and fuel pipelines which are made of steel are the most common to undergo this
type of protection.

Figure 2.9 Cathodic protection of pipes.