Environmental Engineering Reference
In-Depth Information
general corrosion as well as from localized corrosion, such as pitting, dezincifica-
tion, intergranular corrosion, and SCC. However, the necessary condition is that
the surface of the protected structure be in intimate contact with the corrosive
medium. That is why, it cannot be applied to the portions above the water line
in an immersed structure. Neither can it protect structures that are electrically
shielded, such as the inner members in a bundle of pipes or rods.
Cathodic protection was introduced as early as in 1924 by Sir Humphrey Davy
to protect the copper seathing of the ship hulls of that time, only to be discon-
tinued as the stoppage of corrosion prevented the release of toxic copper ions
and the growth of marine foulings adversely affected the speed of ships under
sail. It took nearly 100 years to be reintroduced to the concept of the protection
of buried oil pipelines in the United States. Application of cathodic protection
to steel ship hulls started in 1950.
4.4.1 Principle
Cathodic protection is based on an understanding of the simple electrochemical
principle that in the process of corrosion the cathode member in the cell is saved
from corrosion. The corrosion reaction in an acid medium is represented by:
M
→
M
n
ne (anodic)
(2.6)
2H
2e
→
H
2
(cathodic)
(2.3)
Supply of electron to the metal structure to be protected makes the first equation
(2.6) proceed from left to right and the protection is achieved. Since the electrons
flow through the metallic path, the corresponding positive current can be imag-
ined to enter the structure from the electrolyte. Cathodic protection, therefore, is
achieved though the flow of current from an external source through the electro-
lyte to the structure concerned. This will be made clearer from the consideration
of corrosion in a pipeline buried in soil, as discussed below.
Figure 4.8 represents corrosion of a buried pipeline in soil by the action of
local cells, A being the anodic area and C the cathodic area. Electrons flow from
A to B through the pipe. So, the positive current (or, simply, the current) flows
from A to B through the electrolyte (soil). From the point of view of the direction
of current flow, therefore, the area where the current is entering is protected. If
now an external current of a bigger magnitude is allowed to enter the pipeline
at all points countering the action of local cells, the entire pipeline will be pro-
tected, and this forms the basis of cathodic protection.
With regard to the magnitude of the current required for protection, the effect
of external cathodic current on the Evans diagram of the corroding system may
be considered (Fig. 4.9). Impressing a cathodic current is essentially the polariza-
tion of the cathodic line (dc) further toward the active (anodic) direction, as repre-
sented by the dotted line. When the polarization line reaches the level of
E
A
, the