where
n
a
a Z activation overpotential in the anodic reaction
n
a
c Z activation overpotential in the cathodic reaction
b a Z anodicT afel coefficient
b c Z cathodic Tafel coefficient
a Z transfer coefficient
i a Z anodiccurrent density
i c Z cathodic current density
i oa Z exchange current density of anode
i oc Z exchange current density of cathode
The degree of contribution of electrical energy for the activation energy in
the electrode reaction (0! a ! 1) is indicated by the energy transfer factor a ,
that, in mostc ases, is in the range of 0.3–0.7. The exchange current density
i a or i c is the flux of charge that passes through the electrical doublelayer at
the single-equilibrium potential E a or E c .Alinear relationship existsbetween
n
a
and log i a or log i c .The Tafel coefficient b a or b c is the slopedn
a
/d (log i a or
log i c )ofthe polarization curve.Therefore, b is one of the important factors
controlling the corrosion rate.
The electrode reaction at the low reaction rate is controlled by the
activationo verpotential. Oneo ft he processes controlled by the activation
overpotential is the cathodic reaction (in the acid solution):
2H
C C 2 e Z H 2 :
(7.13)
Table 7.2 shows the hydrogen overpotentialso fv arious metals. The
activationo verpotential varies with the kind of metal and the electrolyte
TABLE 7.2
Hydrogen Overpotentials of Various Metals
Metal
Te mperature ( 8 C)
Solutions
Hydrogen Overpotential j n 8 j
(V/mA/cm
L 2
)
Ta tel
Coefficient
j b c j (V)
Exchange
Current Density
j i oc j (A/cm
2
)
Pt smooth
20
1 N HCl
0.00
0.03
10
K 3
Mo
20
1 N HCl
0.12
0.04
10
K 6
Au
20
1 N HCl
0.15
0.05
10
K 6
Ag
20
0.1 N HCl
0.30
0.09
5 ! 10
K 7
Ni
20
0.1 N HCl
0.31
0.10
8 ! 10
K 7
Bi
20
1 N HCl
0.40
0.10
10
K 7
Fe
16
1 N HCl
0.45
0.15
10
K 6
Cu
20
0.1 N HCl
0.44
0.12
2 ! 10
K 7
Al
20
2 N H 2 SO 4
0.70
0.10
10
K 10
Sn
20
1 N HCl
0.75
0.15
10
K 8
Cd
16
1 N HCl
0.80
0.20
10
K 7
Zn
20
1 N H 2 SO 4
0.94
0.12
1.6! 10
K 11
Pb
20
0.01–8 N HCl
1.16
0.12
2 ! 10
13
Coatings
409
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
n
a
a Z activation overpotential in the anodic reaction
n
a
c Z activation overpotential in the cathodic reaction
b a Z anodicT afel coefficient
b c Z cathodic Tafel coefficient
a Z transfer coefficient
i a Z anodiccurrent density
i c Z cathodic current density
i oa Z exchange current density of anode
i oc Z exchange current density of cathode
The degree of contribution of electrical energy for the activation energy in
the electrode reaction (0! a ! 1) is indicated by the energy transfer factor a ,
that, in mostc ases, is in the range of 0.3–0.7. The exchange current density
i a or i c is the flux of charge that passes through the electrical doublelayer at
the single-equilibrium potential E a or E c .Alinear relationship existsbetween
n
a
and log i a or log i c .The Tafel coefficient b a or b c is the slopedn
a
/d (log i a or
log i c )ofthe polarization curve.Therefore, b is one of the important factors
controlling the corrosion rate.
The electrode reaction at the low reaction rate is controlled by the
activationo verpotential. Oneo ft he processes controlled by the activation
overpotential is the cathodic reaction (in the acid solution):
2H
C C 2 e Z H 2 :
(7.13)
Table 7.2 shows the hydrogen overpotentialso fv arious metals. The
activationo verpotential varies with the kind of metal and the electrolyte
TABLE 7.2
Hydrogen Overpotentials of Various Metals
Metal
Te mperature ( 8 C)
Solutions
Hydrogen Overpotential j n 8 j
(V/mA/cm
L 2
)
Ta tel
Coefficient
j b c j (V)
Exchange
Current Density
j i oc j (A/cm
2
)
Pt smooth
20
1 N HCl
0.00
0.03
10
K 3
Mo
20
1 N HCl
0.12
0.04
10
K 6
Au
20
1 N HCl
0.15
0.05
10
K 6
Ag
20
0.1 N HCl
0.30
0.09
5 ! 10
K 7
Ni
20
0.1 N HCl
0.31
0.10
8 ! 10
K 7
Bi
20
1 N HCl
0.40
0.10
10
K 7
Fe
16
1 N HCl
0.45
0.15
10
K 6
Cu
20
0.1 N HCl
0.44
0.12
2 ! 10
K 7
Al
20
2 N H 2 SO 4
0.70
0.10
10
K 10
Sn
20
1 N HCl
0.75
0.15
10
K 8
Cd
16
1 N HCl
0.80
0.20
10
K 7
Zn
20
1 N H 2 SO 4
0.94
0.12
1.6! 10
K 11
Pb
20
0.01–8 N HCl
1.16
0.12
2 ! 10
13
Coatings
409
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485