Corrosion of Metallic Coatings
261
where i is the current density, D is the diffusion coefficient, C is the concentration of reactive species in the bulk solution, C o is the concentration of
the reactive species at the interface, and δ is the thickness of diffusion layer.
When the concentration of reactive species at the interface is zero (C o = 0),
the current density shows some critical values, i L (called the limiting current
density); that is,
i
nFDC
L = δ
(8.2)
From Equations 8.1 and 8.2,
C
C
i
i
o
L
= −
1
(8.3)
while the concentration overpotential η c is given as:
η
c
=


   


   


   


   
2 3
.
log
RT
nF
C
C
O
(8.4)
From Equations 8.3 and 8.4,
TabLE 8.2
Hydrogen Overpotentials of Various Metals
Metal
Temperature
(°C)
Solution
Hydrogen
Overpotential
η c V/mA/cm
2
(
)
Tafel
Coefficient
β c V
( )
Exchange
Current
Density
i A/cm
oc
2
( )
Pt (smooth)
20
1 N HCl
0.00
0.03
10 −3
Mo
20
1 N HCl
0.12
0.04
10 −6
Au
20
1 N HCl
0.15
0.05
10 −6
Ag
20
0.1 N HCl
0.30
0.09
5 × 10 −7
Ni
20
0.1 N HCl
0.31
0.10
8 × 10 −7
Bi
20
1 N HCl
0.40
0.10
10 −7
Fe
16
1 N HCl
0.45
0.15
10 –6
Cu
20
0.1 N HCl
0.44
0.12
2 × 10 −7
Al
20
2 N H 2 SO 4
0.70
0.10
10 −10
Sn
20
1 N HCl
0.75
0.15
10 –8
Cd
16
1 N HCl
0.80
0.20
10 −7
Zn
20
1 N H 3 SO 4
0.94
0.12
1.6 × 10 −11
Pb
20
0.01–8 N HCl
1.16
0.12
2 × 10 −13