Page 54 - Corrosion Engineering Principles and Practice

CHAPTER 3

Corrosion

Electrochemistry

3.1 Electrochemical Reactions
An electrochemical reaction is defined as a chemical reaction involving
the transfer of electrons. It is also a chemical reaction which involves
oxidation and reduction. Since metallic corrosion is almost always an
electrochemical process, it is important to understand the basic nature
of electrochemical reactions. The discoveries that gradually evolved
in modern corrosion science have, in fact, played an important role in
the development of a multitude of technologies we are enjoying
today. Appendix A provides a list of some of these discoveries.
An important achievement early in the history of electrochemistry
was the production of power sources, following the production of the
first batteries by Alessandro Volta. Figure 3.1 illustrates the principle
of a Daniell cell in which copper and zinc metals are immersed in
solutions of their respective sulfates. The Daniell cell was the first
truly practical and reliable electric battery that supported many
nineteenth-century electrical innovations such as the telegraph. In
the process of the reaction, electrons can be transferred from the
corroding zinc to the copper through an electrically conducting path
as a useful electric current. Zinc more readily loses electrons than
copper, so placing zinc and copper metal in solutions of their salts can
cause electrons to flow through an external wire which leads from the
zinc to the copper.
Zinc anode: Zn(s) → Zn 2+ + 2e (3.1)
−
−
Copper cathode: Cu 2+ + 2e → Cu(s) (3.2)
The difference in the susceptibility of two metals to corrode can
often cause a situation that is called galvanic corrosion named after
Luigi Galvani, the discoverer of the effect. The purpose of the
separator shown in Fig. 3.1 is to keep each metal in contact with its

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