Environmental Engineering Reference
In-Depth Information
Figure 2.15
Polarization of constituent electrode reactions representing corrosion of
zinc in air-free acid solution giving mixed potential,
E
corr
, and corrosion current,
i
corr
.
oxidation and reduction reactions at the same rate. This theory, propounded by
Wagner and Traud in 1938 [1], is known as mixed-potential theory of corrosion.
Mixed-potential theory paves the way for understanding the homogeneous
theory of corrosion (Section 2.1.4). For corrosion to occur, spatially separated
anodic and cathodic sites are not required if the system provides a potential where
there is simultaneous occurrence of oxidation and reduction reactions. This poten-
tial obviously will be more positive than the equilibrium potential of the anodic
reaction and more negative than the equilibrium potential of the cathodic reaction.
2.3.4 Importance of Kinetic Considerations
The electrode kinetic parameters
i
o
,
i
L
,
β
c
determine the rate of a corrosion
reaction. The rate of corrosion in terms of current density for a single-electron
transfer reaction is expressed by the Bulter-Volmer equation [2]:
β
a
, and
i
i
o
(e
(1
β)
F
η/
RT
e
β
F
η/
RT
)
(2.42)
The difference between the redox potentials of the cathodic and anodic reac-
tions is the driving force for a corrosion reaction. However, as Eq. 2.42 shows,
