15
Appendix B
IMBALANCE MEASUREMENT FUNCTION OF
CORRATER
INSTRUMENTS
Metal surfaces, no matter how uniform they may appear, have numerous microscopic
imperfections. Metals such as iron alloys are crystalline in structure and surface
imperfections such as small intergranular cavities will tend to grow, especially in liquids
that have large concentrations of dissolved oxygen.
The corrosion processes (iron oxidation) for iron alloys can be described by the following
anodic reaction:
Fe
→
Fe
+2
+ 2e
-
which, in an oxygen rich environment can be "driven" by the following cathodic reaction:
O
2
+ 2H
2
O + 4e
→
4 OH
When large amounts of oxygen are available at a portion of a metal surface, oxygen not
only maintains this cathodic reaction, but it promotes the reaction. At other locations on
the metal surface where oxygen is less available, the anodic reaction proceeds to
balance the cathodic reaction.
A small intergranular cavity would represent an excellent site for the anodic reaction to
take place because there is less available oxygen. In the case of an iron alloy, the
reaction causes the rapid localized conversion of iron atoms to ferrous ions since a small
anodic area can be supported by the larger cathodic area. As this iron oxidation
proceeds, the small cavity grows which in turn exposes a larger iron surface that is
essentially void of oxygen causing it to be a very active anode. This process which has
the natural tendency to accelerate describes the growth of a corrosion pit.
Since susceptible pitting sites tend to be randomly distributed and generally are not too
numerous on a metal surface, there is a high probability that on two seemingly identical
metal electrodes, one of the electrodes will have a greater number of susceptible pitting
sites than the other electrode. If these two electrodes are the electrodes of a two-
electrode CORRATER
probe and they are submersed in a conductive solution which
tends to promote pitting, one electrode will exhibit a more positive corrosion potential
(E
corr
) than the other. The polarity of the open-circuit potential difference (E
oc
) will
indicate which electrode has the greater pitting tendency. That electrode will be the
more negative of the two.
If these electrodes are electrically connected through a zero-resistance ammeter (ZRA),
the measured short-circuit current is a measure of the pitting tendency of the electrode
Summary of Contents for CORRATER 9020
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