Corrosion Mechanisms
17
2.4.2 Concentration Polarization
A buildup or depletion of ions at the electrode surface as a result of reaction will change the value of the electrode potential according to the Nernst
equation. For example, for a corroding zinc electrode, the concentration of
zinc will increase with dissolution in the vicinity of the electrode. The value
of a oxid in the equation increases, causing the electrode potential to shift in a
positive direction.
For the hydrogen evolution reaction, the higher rate of discharge of hydrogen ions at the electrode surface brings down the value of a oxid and the
electrode potential, according to the Nernst equation, shifts in a negative
direction. However, the rate of discharge of hydrogen ions at the electrode
surface depends on the diffusion of hydrogen ions from the bulk of the solution to the surface:
i =
DnFC
x
l
(2.16)
100
10
1
0.1
0.01
–0.3
–0.2
–0.1
0
0.1
2 H +
H
2
H
2
2 H +
+
2 e –
2 e –
+
Noble
Active
Overvoltage η
a Volts
0.2
Current Density
i o
H 2 /H
+
FigurE 2.6
Activation polarization curve of a hydrogen electrode.