The Foundations of Chemistry

A point of strain in a steel object acts as an anode where the iron is oxidized to iron(II)

ions, and pits are formed (Figure 21-12).

Fe88nFe^2  2 e (oxidation, anode)

The electrons produced then flow through the nail to areas exposed to O 2. These act as

cathodes where oxygen is reduced to hydroxide ions, OH.

O 2 2H 2 O 4 e88n4OH (reduction, cathode)

At the same time, the Fe^2 ions migrate through the moisture on the surface. The overall

reaction is obtained by balancing the electron transfer and adding the two half-reactions.

2(Fe88nFe^2  2 e) (oxidation, anode)

O 2 2H 2 O 4 e88n4OH (reduction, cathode)

2FeO 2 2H 2 O88n2Fe^2 4OH (net reaction)

The Fe^2 ions can migrate from the anode through the solution toward the cathode

region, where they combine with OHions to form iron(II) hydroxide. Iron is further

oxidized by O 2 to the 3 oxidation state. The material we call rust is a complex hydrated

form of iron(III) oxides and hydroxides with variable water composition; it can be repre-

sented as Fe 2 O 3 xH 2 O. The overall reaction for the rusting of iron is

2Fe(s)^32 O 2 (aq)xH 2 O(
)88nFe 2 O 3 xH 2 O(s)

CORROSION PROTECTION

There are several methods for protecting metals against corrosion. The most widely used

are

1.Plating the metal with a thin layer of a less easily oxidized metal

2.Connecting the metal directly to a “sacrificial anode,” a piece of another metal that

is more active and therefore preferentially oxidized

3.Allowing a protective film, such as a metal oxide, to form naturally on the surface

of the metal

4.Galvanizing, or coating steel with zinc, a more active metal

5.Applying a protective coating, such as paint

21-18

874 CHAPTER 21: Electrochemistry

Figure 21-12 The corrosion of
iron. Pitting appears at the anodic
region, where iron metal is oxidized
to Fe^2 . Rust appears at the cathodic
region.

Compare the potentials for the
reduction half-reactions to see which
metal is more easily oxidized. The
more positive the reduction potential
for a metal, the more stable the metal
is as the free element and the harder it
is to oxidize.

E^0 reduction

Mg^2  2 e88nMg 2.37 V

Zn^2  2 e88nZn 0.763 V

Fe^2  2 e88nFe 0.44 V

Sn^2  2 e88nSn 0.14 V

Cu^2  2 e88nCu 0.337 V

H 2 O layer

Iron Anodic

region

Fe2+

O 2

Rust OH– H 2 O

O 2

Cathodic

region

O 2 + 2H 2 O + 4e– → 4OH–

Reduction

Fe → Fe2+ + 2e–

Oxidation

e–

Overall process: 2Fe(s) + –O^322 (aq) + xH 2 O(
) → Fe 2 O 3 • xH 2 O(s)