Corrosion
DOE-HDBK-1015/1-93
GENERAL CORROSION
Rev. 0
CH-02
Page 15
Figure 7 Effect of pH on the Corrosion Rate
of Iron in Water
Figure 8 Effect of pH on the Relative Attack Rate of Iron in Water
First, consider the exposure of iron to aerated
water at room temperature (aerated water will
contain dissolved oxygen). The corrosion
rate for iron as a function of pH is illustrated
in Figure 7. In the range of pH 4 to pH 10,
the corrosion rate of iron is relatively
independent of the pH of the solution. In this
pH range, the corrosion rate is governed
largely by the rate at which oxygen reacts
with absorbed atomic hydrogen, thereby
depolarizing the surface and allowing the
reduction reaction to continue. For pH values
below 4.0, ferrous oxide (FeO) is soluble.
Thus, the oxide dissolves as it is formed
rather than depositing on the metal surface to
form a film. In the absence of the protective
oxide film, the metal surface is in direct contact with the acid solution, and the corrosion reaction
proceeds at a greater rate than it does at higher pH values. It is also observed that hydrogen is
produced in acid solutions below a pH of 4, indicating that the corrosion rate no longer depends
entirely on depolarization by oxygen, but on a combination of the two factors (hydrogen
evolution and depolarization). For pH values above about pH 10, the corrosion rate is observed
to fall as pH is increased. This is believed to be due to an increase in the rate of the reaction of
oxygen with Fe(OH) (hydrated FeO) in the oxide layer to form the more protective Fe O (note
2
2
3
that this effect is not observed in deaerated water at high temperatures).
A plot of the relative corrosion
rate for iron at various pH
values in 590?F, oxygen-free
water is presented as Figure 8.
The curve illustrates that the
corrosion rate of iron in high
temperature water is lower in
the pH range of 7 to 12 than it is
at either lower or higher pH
values (at very high pH values,
greater than pH 13.0, the oxide
film becomes increasingly more
soluble because of increased
formation of soluble FeO at
2
high
temperatures,
and
corrosion rates increase). As a
result of the data plotted in
Figure 8 and other similar measurements, it is general practice to maintain high temperature
water in the alkaline condition (but below very high pH values) to minimize the corrosion of iron
and the steels exposed to the high temperature water.
