Page 50 - Corrosion Engineering Principles and Practice

30 C h a p t e r 2 C o r r o s i o n B a s i c s 31

2.4 Corrosion as a Chemical Reaction

2.4.1 Corrosion in Acids
One of the common ways of generating hydrogen in a laboratory is to
place zinc into a dilute acid, such as hydrochloric or sulfuric. When
this is done, there is a rapid reaction in which the zinc is attacked or
“dissolved” and hydrogen is evolved as a gas (Fig. 2.8). This is shown
in Eqs. (2.10) to (2.14):

Zn + 2HCl → ZnCl + H g) (2.10)
0
(
2
2

Zn + 2H + 2Cl → Zn 2+ + 2Cl + H g) (2.11)
−
0
(
−
+
2

Equations (2.10) and (2.11) are the chemical shorthand for the
following statement: One zinc atom plus two hydrochloric acid
−
molecules dissociate as ions H and Cl and become one molecule of
+
2+
zinc chloride in Eq. (2.10) [written as a soluble salt in the form of Zn
−
and Cl ions in Eq. (2.11)] plus one molecule of hydrogen gas which is
given off as indicated by the vertical arrow. It should be noted that
the chloride ions do not participate directly in this reaction, although
they could play an important role in real corrosion situations.
Similarly, zinc combines with sulfuric acid to form zinc sulfate
(a salt) and hydrogen gas as shown in Eqs. (2.12) and (2.13):
Zn + H SO → ZnSO + H g) (2.12)
0
(
4
2
4
2

Zn + 2H + SO 2− → Zn 2+ + SO 2− + H g) (2.13)
0
+
(
4
2
4

FIGURE 2.8
Bubbling, or
“plating out
of hydrogen” on
zinc immersed in a
0.1 M sulfuric acid
solution. (Courtesy
Kingston Technical
Software)