Page 62 - Corrosion Engineering Principles and Practice

42 C h a p t e r 3 C o r r o s i o n E l e c t r o c h e m i s t r y 43

Note that all of the above reactions are similar in one respect—they
consume electrons. All corrosion reactions are simply combinations of
one or more of the above cathodic reactions, together with an anodic
reaction similar to Eq. (3.10). Thus, almost every case of aqueous
corrosion can be reduced to these equations, either singly or
in combination.
Consider the corrosion of zinc by water or moist air. By multiplying
the zinc oxidation reaction (3.4) by 2 and summing this with the
oxygen reduction reaction, one obtains Eq. (3.21).
2Zn(s) → 2Zn 2+ + 4e (oxidation) (3.20)
−
+ O 2 + 2H O + 4e → 4OH (reduction) (3.17)
−
−
2
2Zn 2H O O → 2Zn 2+ + 4OH → 2 Zn(OH) (p) (3.21)
+
+
−
2
2
2
2+
−
The products of this reaction are Zn and OH , which immediately
react to form insoluble Zn(OH) . Likewise, the corrosion of zinc by a
2
solution containing copper ions is merely the summation of the
oxidation reaction for zinc and the metal deposition reaction involving
cupric ions in Eq. (3.19).
−
Zn(s) → Zn 2+ + 2e (oxidation) (3.4)
−
+ Cu 2 + + 2e → Cu(s) (reduction) (3.19)
Zn + Cu 2+ → Zn 2+ + Cu(p) (3.22)
During corrosion, more than one oxidation and one reduction
reaction may occur. For example, during the corrosion of an alloy, its
component metal atoms go into solution as their respective ions.
Thus, during the corrosion of a chromium-iron alloy, both chromium
and iron are oxidized. Also, more than one cathodic reaction can
occur on the surface of a metal.
Consider the corrosion of zinc in a hydrochloric acid solution
containing dissolved oxygen. Two cathodic reactions are possible:
the evolution of hydrogen and the reduction of oxygen (Fig. 3.6).
Since there are two cathodic reactions or processes which consume
electrons, the overall corrosion rate of zinc is increased. Thus, acid
solutions which either contain dissolved oxygen or are exposed to
air are generally more corrosive than air-free acids. Therefore,
removing oxygen from acid solutions will often make these solutions
less corrosive.
If a piece of mild steel is placed in a solution of hydrochloric
acid, a vigorous formation of hydrogen bubbles is observed. Under
such conditions, the metal corrodes very quickly. The dissolution
of the metal occurs only at anodic surfaces. The hydrogen bubbles
form only at the cathodic surfaces, even though it may appear they