Environmental Engineering Reference
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Ita
nF
W
(2.36)
where F is the Faraday constant (96,500 coulombs/equivalent)
n is the number of equivalents exchanged
a is the atomic weight of the metal, and
t is the time in seconds
The rate of metal dissolution, or corrosion, r , is obtained by dividing Eq. 2.36
by the surface area A and time t . Thus,
W
tA
ia
nF
r
(2.37)
where i , defined as current density, equals I / A . The total current I or the current
density i , which is equivalent to corrosion rate, will appear in all discussion re-
lated to the kinetics of corrosion.
2.3.1 Exchange Current Density
For any electrode reaction at equilibrium current does not flow through the circuit,
but there is always a finite exchange of ions and atoms at the interface. For
example, for the reaction:
Zn i Zn 2
2e
(2.20)
some moles of zinc atoms are leaving the surface and entering the electrolyte as
zinc ions; at the same time, an equal number of zinc ions from the electrolyte
are getting reduced on the electrode surface. Since electron transfer is involved,
the rate of exchange can be expressed in terms of current density using Faraday's
law:
i 0
nF
r oxid
r red
(2.38)
where r oxid and r red are the equilibrium oxidation and reduction rates, and i 0 is the
exchange current density. Thus, exchange current density can be defined as the
rate of oxidation or reduction at an equilibrium electrode expressed in terms of
current density.
The magnitude of exchange current density is a measure of how easily a reac-
tion attains equilibrium. It varies for different reactions and, for a particular reac-
tion, the magnitude of exchange current density is a function of the electrode
composition, surface roughness, surface impurities, and temperature.
The equilibrium of hydrogen evolution reaction on different metal surfaces
presents interesting data in terms of exchange current density, which is shown
 
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