Environmental Engineering Reference
In-Depth Information
the formation of ferrous ferricyanide by reacting with ferrous ions available from
corrosion of steel, if the protection is inadequate.
4.4.3 Stray Current Corrosion
Stray currents are those that travel away from the intended circuit. A part of the
current discharged by the ground bed in an impressed-current cathodic protection
system may enter a metallic structure in the vicinity of the protected structure.
To complete the circuit it leaves the structure, enters the electrolyte (soil), and
then goes to the cathode (i.e., the protected structure). Figure 4.12 illustrates such
a situation.
The steel pipe is the neighboring structure to the cathodically protected steel
tank. It is clear from Section 4.4.1 that the portion of the steel pipe where the
current is entering from the soil gets the benefit of cathodic protection, whereas
the portion where the current leaves the pipeline to enter the soil acts as anode
and undergoes corrosion. This type of corrosion is called
stray current corrosion
.
Eventually, a leak may appear in the pipeline. Application of a paint coating to
the pipeline simply aggravates the situation as the current now gets discharged
from a few defect sites in the paint coat where anodic current density increases
enormously. The structure can be saved from stray current corrosion by simply
short-circuiting it to the protected tank. In that case the current will follow the
metallic path to complete the circuit and the pipeline will also be a part of the
protected structure. Such a situation will not be uncommon in congested industrial
areas. A common cathodic protection system should be organized by the owners
of the installations for their mutual benefit.
Although stray current corrosion has been discussed in the context of cathodic
protection, the stray current may emerge from other sources as well. In a metropo-
lis stray currents from tram car lines often bring hazards to the metallic structures
Figure 4.12
Strong current corrosion resulting from an impressed current cathodic
protection system.
