Page 52 - Corrosion Engineering Principles and Practice

32 C h a p t e r 2 C o r r o s i o n B a s i c s 33

is also present. Water solutions rapidly dissolve oxygen from the air,
and this is the source of the oxygen required in the corrosion process.
The most familiar corrosion of this type is the rusting of iron when
exposed to a moist atmosphere.

+
4Fe 6H O + 3O → 4 Fe(OH) ↓ (2.17)
2 2 3
In Eq. (2.17), iron combines with water and oxygen to produce an
insoluble reddish-brown corrosion product that falls out of the
solution, as shown by the downward pointing arrow.
During rusting in the atmosphere, there is an opportunity for
drying, and this ferric hydroxide dehydrates and forms the familiar
red-brown ferric oxide (rust) or Fe O , as shown in Eq. (2.18):
2 3
2Fe(OH) → Fe O + 3 H O (2.18)
2
3 2 3
Similar reactions occur when zinc is exposed to water or moist air
followed by natural drying:

(
2Zn + 2H O + O → Zn(OH) s) (2.19)
2 2 2
Zn(OH) → ZnO + H O (2.20)
2
2
The resulting zinc oxide is the whitish deposit seen on galvanized
pails, rain gutters, and imperfectly chrome-plated bathroom faucets.
As discussed previously, the iron that took part in the reaction with
hydrochloric acid in Eq. (2.15) had a valence of 2, whereas the iron that
takes part in the reaction shown in Eq. (2.17) has a valence of 3. The clue
to this lies in the examination of the equation for the corrosion product
−
Fe(OH) . Note that water ionized into H and OH . It is further known
+
3
that a hydrogen ion has a valence of 1 (it has only one electron to lose). It
would require three hydrogen ions with the corresponding three positive
−
charges to combine with the three OH ions held by the iron. It can thus
be concluded that the iron ion must have been Fe or a ferric ion.
3+
Also note that there is no oxidation or reduction (electron transfer)
during the reaction in either Eq. (2.18) or (2.20). In both cases the
valences of the elements on the left of each reaction remain what it is
on the right. The valences of iron, zinc, hydrogen, and oxygen
elements remain unchanged throughout the course of these reactions,
and it is consequently not possible to divide these reactions into
individual oxidation and reduction reactions.

Reference
1. Mapes RS, Berkey WW. X-ray diffraction methods for the analysis of
corrosion products. In: Ailor WH, ed. Handbook on Corrosion Testing and
Evaluation. New York: John Wiley & Sons, 1971; 697–730.