Page 57 - Corrosion Engineering Principles and Practice

38 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 39

sites on the metal surface where hydrogen ions are reduced to
hydrogen gas according to Eq. (3.5) (Fig. 2.7 in Chap. 2).
−
Anodic reaction: Zn(s) → Zn 2+ + 2e (3.4)
−
+
Cathodic reaction: 2H + 2e → H (g) (3.5)
2
Equations (3.4) and (3.5) illustrate the nature of an electrochemical
reaction typically illustrated for zinc in Fig. 3.3. During such a
reaction, electrons are transferred, or, viewing it another way, an
oxidation process occurs together with a reduction process. The
overall corrosion processes are summarized in Eq. (3.6):

Overall corrosion reaction: Zn 2H → Zn 2+ + H (g) (3.6)
+
+
H
2
Briefly then, for corrosion to occur there must be a formation of
ions and release of electrons at an anodic surface where oxidation or
deterioration of the metal occurs. There must be a simultaneous
reaction at the cathodic surface to consume the electrons generated at
the anode. These electrons can serve to neutralize positive ions such
+
as the hydrogen ions (H ), or create negative ions. The anodic and
cathodic reactions must go on at the same time and at equivalent
rates. However, what is usually recognized as the corrosion process
occurs only at the areas that serve as anodes.

3.2 Anodic Processes

Let us consider in greater detail what takes place at the anode
when corrosion occurs. For instance, reconsider Eq. (3.6). This re-
action involves the reduction of hydrogen ions to hydrogen gas,
according to Eq. (3.5). This hydrogen evolution reaction occurs
with a wide variety of metals and acids, including hydrochloric,
sulfuric, perchloric, hydrofluoric, formic, and other strong acids.
The individual anodic reactions for iron, nickel, and aluminum are
listed as follows:
Iron anodic reaction: Fe(s) → Fe 2+ + 2e (3.7)
−
Nickel anodic reaction: Ni(s) → Ni 2+ + 2e (3.8)
−
Aluminum anodic reaction: Al(s) → Al 3+ + 3e (3.9)
−
Examining the Eqs. (3.7) to (3.9) shows that the anodic reaction
occurring during corrosion can be written in the general form:
General anodic reaction: M(s) → M n+ + ne (3.10)
−
That is, the corrosion of metal M results in the oxidation of metal
M to an ion with a valence charge of n+ and the release of n electrons.
The value of n, of course, depends primarily on the nature of the metal.