Cathodic reaction in acidic solution:
2H
C C 2 e / H 2
Cathodic reaction in neutral and alkaline solutions:
O 2 C 2H 2 O C 4 e / 4OH
K
The Evans diagramshown in Figure 7.3 represents the mechanism of the
corrosion cell. The cathodic current is expressedinthe same direction as the
anodic current.
In Figure 7.3, E a shows the single potential for H 2 /H
C
or for O 2 /OH
K
at
the cathode and E a shows the single potential for metal/metal in equilibria
at the anode. The single potential is given by the Nernst equation:
E Z E 8 C
RT
nF
ln a
(7.4)
where
E Z single potential
E 8 Z standard single potential
R Z absolute temperature
n Z charge on an ion
F Z Faraday constant
a Z activity of the ion
Internal polarization
curve
A n o d ic p o la r iz a t io n c u r v e
C a t h o d ic p o la r iz a t io n c u r v e
External polarization
curve
E corr
E a
E c
Electrode
potential
i oc
i oa
i corr
n c
n a
O
2 + 2 H
2 O + 4 e
4 O H −
M
M
n + − n e
FIGURE 7.3
Mechanism of corrosion cell.
(7.2)
(7.3)
Corrosion of Linings and Coatings
406
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485