Environmental Engineering Reference
In-Depth Information
uniformly with the passage of time. This model of corrosion being analogous
to a short-circuited electrochemical cell is often termed the
local cell theory of
corrosion
. It will be useful to remember that in the corrosion cell the electrode
from which current enters the electrolyte is the anode and this undergoes corro-
sion.
2.1.3 Types of Local Cell Formation
Three main types of local cell formation leading to corrosion are encountered in
practice:
1.
Dissimilar electrode cells
2.
Concentration cells
a. Salt concentration cell
b. Differential aeration cell
3.
Differential temperature cells
Dissimilar Electrode Cells
Dissimilar electrode cells may be formed when two dissimilar metals are in con-
tact or due to the heterogeneity of the same metal surface. The Daniell cell is an
example of the former. In practice, a copper pipe connected to a steel pipe or a
bronze propeller in contact with the steel hull of a ship provides example for this
type of corrosion cell. This is often referred to as
galvanic coupling
, in which
the less noble metal becomes anode (discussed later in Section 3.3). A cold-
worked metal in contact with the same metal annealed leads to a similar situation,
i.e., the cold-worked metal remaining anodic. On the same metal surface such
type of cell formation may result from dissimilar phases and impurities, grain
boundaries, differentially strained areas, and scratches or abrasions. In a single
crystal, the different crystal faces differ in their electrochemical characteristics
because of the difference in atomic orientation and, as a result, one crystal face
tends to become anodic compared to others.
Concentration Cells
These are formed when the electrodes are identical but are in contact with solu-
tions of differing composition. A salt concentration cell forms when one electrode
is in contact with a concentrated solution and the other electrode with a dilute
solution. On short circuiting, the electrode in contact with the dilute solution will
be anodic (Fig. 2.3).
The local variation of composition of the process stream inside the pipeline
in a chemical plant may lead to such a situation in practice.
A differential aeration cell forms when the identical electrodes are exposed to
solutions of identical chemical composition that differ in oxygen content, which is
