Aqueous Corrosion: Fundamentals - Environmental Degradation of Metals - page 6

The attack on the surface is almost immediately visible. As the reaction pro-
ceeds, the piece of zinc thins down and loses its weight. The process continues
until the entire piece disappears in the solution as zinc chloride.
Equation 2.1 does not, however, reveal the entire picture of the reaction. The
basis of this reaction is charge transfer, which is represented by two partial reac-
tions:

Zn → Zn 2 2e (2.2)

2H 2e → H 2 (2.3)

The former represents the ionization of zinc in the acid solution with the liberation
of valence electrons and the latter represents the discharge of the available disso-
ciated hydrogen ions in the acid solution by the liberated electrons, which takes
place on the metal surface itself. In terms of electrochemistry, the electron libera-
tion reaction or the deelectronation reaction, as represented by Eq. 2.2, is called
oxidation or anodic reaction , and the electron consumption or electronation reac-
tion, as represented by Eq. 2.3, is called reduction or cathodic reaction. Figure 2.1
shows a schematic representation of these two reactions taking place at spatially
separated sites on the metal surface. The site for oxidation or anodic reaction is
called the anode and the site for reduction or cathodic reaction is called the cath-
ode . The corrosion of zinc in hydrochloric acid is therefore an electrochemical
reaction involving anodic and cathodic reactions of charge transfer.

Figure 2.1 Electrochemical reactions occurring during corrosion of zinc in air-free
hydrochloric acid.