The cathodic reaction is controlled by the activation overpotential n
a
c and
the concentration overpotential n
a
c .
The cathodic overpotential is
n c Z n
a
c C n
c
c :
(7.20)
The cathodic overpotential n c can be writtenint he form:
n c Z b c log
i c
i oc
C
2 : 3 RT
nF
log 1 K
i c
i cL
:
(7.21)
See Equation 7.12, Equation 7.19, and Equation 7.20. In most cases, the
corrosion rate can be determined from the anodic and cathodic overpotentials since the rate determiningp rocess is determined by the slopeso ft he
two polarization curves.
As previously mentioned, the roleofthe coating is to isolate the substrate
from the environment. The coating accomplishes this based on two characteristics of the coating material: (1) the corrosion resistance of the coating
material when the coating is formed by the defect free continuous layer and
(2) the electrochemical action of the coating material when the coating layer
has somed efect such as ap ore or crack. The mechanism of the corrosion
cell can explain the actionr equired of the coating layer.F or better understanding, Equation 7.7 is rewritten as follows:
i corr Z
ð E c K E a Þ K j n c j n a
R
:
(7.22)
Ac orrosion resistant coating is achievedb yo ne of the five different
methods to decrease i corr based on Equation 7.22:
1. EMF control protection:D ecrease in electromotive force( E c K E a )
2. Cathodic control protection:Increase in cathodic overpotential j n c j
3. Anodic control protection:I ncrease in anodic overpotential j n a j
4. Mixed controlp rotection:I ncrease in both anodic overpotential j n a j
and cathodic overpotential j n c j
5. Resistance control protection:Increase in resistance of corrosion cell R
7.1.3 EMF Control Protection
The difference in potential between the anodeand the cathode ( E c K E a )isthe
EMF of the corrosioncell. It is also the degree of thermodynamic instability
of the surfacem etal for the environment. In other words, the less the EMF,
the lower the corrosion rate. By covering the surface of the active metal with
acontinuous layer of amore stable metal,the active metal surface becomes
more thermodynamically stable. Dissolved oxygen and hydrogen ions,
Coatings
411
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
a
c and
the concentration overpotential n
a
c .
The cathodic overpotential is
n c Z n
a
c C n
c
c :
(7.20)
The cathodic overpotential n c can be writtenint he form:
n c Z b c log
i c
i oc
C
2 : 3 RT
nF
log 1 K
i c
i cL
:
(7.21)
See Equation 7.12, Equation 7.19, and Equation 7.20. In most cases, the
corrosion rate can be determined from the anodic and cathodic overpotentials since the rate determiningp rocess is determined by the slopeso ft he
two polarization curves.
As previously mentioned, the roleofthe coating is to isolate the substrate
from the environment. The coating accomplishes this based on two characteristics of the coating material: (1) the corrosion resistance of the coating
material when the coating is formed by the defect free continuous layer and
(2) the electrochemical action of the coating material when the coating layer
has somed efect such as ap ore or crack. The mechanism of the corrosion
cell can explain the actionr equired of the coating layer.F or better understanding, Equation 7.7 is rewritten as follows:
i corr Z
ð E c K E a Þ K j n c j n a
R
:
(7.22)
Ac orrosion resistant coating is achievedb yo ne of the five different
methods to decrease i corr based on Equation 7.22:
1. EMF control protection:D ecrease in electromotive force( E c K E a )
2. Cathodic control protection:Increase in cathodic overpotential j n c j
3. Anodic control protection:I ncrease in anodic overpotential j n a j
4. Mixed controlp rotection:I ncrease in both anodic overpotential j n a j
and cathodic overpotential j n c j
5. Resistance control protection:Increase in resistance of corrosion cell R
7.1.3 EMF Control Protection
The difference in potential between the anodeand the cathode ( E c K E a )isthe
EMF of the corrosioncell. It is also the degree of thermodynamic instability
of the surfacem etal for the environment. In other words, the less the EMF,
the lower the corrosion rate. By covering the surface of the active metal with
acontinuous layer of amore stable metal,the active metal surface becomes
more thermodynamically stable. Dissolved oxygen and hydrogen ions,
Coatings
411
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485