Corrosion Mechanisms
15
be preferable to either cathodic or anodic reactions. Understanding the
influence of environmental changes on polarization can offer insight into
controlling corrosion. For example, in the iron–hydrochloric acid example, hydrogen gas formation at the cathode can actually slow the reaction
(increased current resistance) by blocking access of hydrogen ions to the
cathode site. This results in cathodic polarization and lowers the current
flow and corrosion rate. If oxygen is bubbled through the solution, the
hydrogen is removed more rapidly by combining to form water and the
corrosion rate increases significantly. Although this is an oversimplified
view of the effects of oxygen, it does indicate that the degree of polarization can be affected by changes in the environment, either natural or
induced.
There are three basic causes of polarization, termed activation, concentration, and potential drop. Potential drop is the change in voltage associated
with effects of the environment and the circuit between the anode and cathode sites. It includes the effects of the resistivity of the media, surface films,
corrosion products, etc.
2.4.1 activation Polarization
Activation polarization arises out of a slow step in the electrode reaction
for which an activation energy in the form of an increment in potential is
required for the reaction to proceed. This will best be illustrated by the
hydrogen evolution reaction.
The hydrogen evolution reaction consists of several steps as shown
in Figure 2.5. Either the electron transfer step (step 2) or the formation of
Current, i
i corr
2H
+ + 2e
H 2
Fe
Fe
2+ + 2e
–0.5
–0.3
–0.1
Potential, E
E (H 2 /H
+ ) Cathodic polarization curve
0.1
E corr
E (Fe/Fe
2+ )
Anodic polarization curve
FigurE 2.4
Polarization of iron in acid.
15
be preferable to either cathodic or anodic reactions. Understanding the
influence of environmental changes on polarization can offer insight into
controlling corrosion. For example, in the iron–hydrochloric acid example, hydrogen gas formation at the cathode can actually slow the reaction
(increased current resistance) by blocking access of hydrogen ions to the
cathode site. This results in cathodic polarization and lowers the current
flow and corrosion rate. If oxygen is bubbled through the solution, the
hydrogen is removed more rapidly by combining to form water and the
corrosion rate increases significantly. Although this is an oversimplified
view of the effects of oxygen, it does indicate that the degree of polarization can be affected by changes in the environment, either natural or
induced.
There are three basic causes of polarization, termed activation, concentration, and potential drop. Potential drop is the change in voltage associated
with effects of the environment and the circuit between the anode and cathode sites. It includes the effects of the resistivity of the media, surface films,
corrosion products, etc.
2.4.1 activation Polarization
Activation polarization arises out of a slow step in the electrode reaction
for which an activation energy in the form of an increment in potential is
required for the reaction to proceed. This will best be illustrated by the
hydrogen evolution reaction.
The hydrogen evolution reaction consists of several steps as shown
in Figure 2.5. Either the electron transfer step (step 2) or the formation of
Current, i
i corr
2H
+ + 2e
H 2
Fe
Fe
2+ + 2e
–0.5
–0.3
–0.1
Potential, E
E (H 2 /H
+ ) Cathodic polarization curve
0.1
E corr
E (Fe/Fe
2+ )
Anodic polarization curve
FigurE 2.4
Polarization of iron in acid.