3.RUSTING AS A REDOX REACTION -

 State the condition for rusting of iron

 State what corrosion of metal is

 Describe the process of rusting in terms of oxidation
and reduction

 Generate ideas on the use of other metals to control
rusting

 Explain with examples on the use of a more
electropositive metal to control metal corrosion

 Explain with examples on the use of a less
electropositive metal to control metal corrosion

 Describe efforts to prevent corrosion of metals

3.2 : RUSTING AS A
REDOX REACTION

 Rusting is the corrosion of iron.

 The conditions for the rusting of iron are the
presence of oxygen (air) and water.

 Corrosion of metal is a redox reaction in which a
metal is oxidized naturally to its ions.

Example: Zn  Zn2+ + 2e-

 Zn loses electrons to form zinc ions, Zn2+.

 Zn is said to have corrosion. Zn undergoes
oxidation.

 The more electropositive a metal is, the easier for it
to corrode.

 This is because a more electropositive metal easily
loses its electrons to form positive ion

The edges of the water droplet, where the
concentration of oxygen is higher is the cathode

( positive terminal) , where reduction occur

The surface of iron in the middle of the water droplet is
the anode ( negative terminal), where oxidation occur.

 The iron (II) ions, Fe2+ formed combine with the hydroxide ions,OH-
to form iron (II) hydroxide, Fe(OH)2
2Fe + O2 + 2H2O  2Fe(OH)2

 The iron (II) hydroxide, Fe(OH)2 is then rapidly oxidized by oxygen
to brown hydrated iron (III) oxide , represented by the formula

 Fe2O3.xH2O, whereby x is varies.
 This hydrated iron (III) oxide is called rust.

 Rusting occurs faster in the presence of acids and salt solution.

 Iron objects can be protected against rusting in
a variety of ways.

1 •Alloying

2 •Sacrificial Protection

3 •Protective Coating

1. In stainless steel, iron is alloyed with carbon,
chromium and nickel.

2. It is expensive and mainly used in cutlery or
decorative items.

3. Chromium and nickel form corrosion-resistant
oxide film on the surface. Furthermore, this oxide
film is shiny, thus giving it an attractive, mirror-like
finish.

 Iron is connected to a more electropositive
metal such as Magnesium or zinc.

 It is used in bridge pillar, hulls of ship and
underground pipelines.

 More electropositive metal acts as the sacrificial
metal. It corrodes first, thus inhibit rusting.

Example : using zinc, Zn to protect iron, Fe from K
corrosion Na
Ca
1. Oxidation Mg
Al
~ Zinc, Zn is more electropositive than iron, Fe Zn
~ Zn loses electron easily than Fe Fe
~ Zn (s)  Zn2+(aq) + 2e Sn
~ The electrons flow through the iron to the metal surface Pb
where there is oxygen, O2 and water H
Cu
Ag

2. Reduction To protect
~ The electrons gained by oxygen: metal from
O2 (g) + 2H2O(l) + 4e  4OH-(aq) corrosion:

Use a metal
that is more
electropositive
than the metal
we want to
protect

I. Paints , plastic, enamel and grease.

II. Galvanizing
 Iron or steel is coated with a thin layer of zinc.
 Zinc protects iron in two ways.

a) It produces a protective oxide coating.

b) When galvanized iron is scratched, zinc corrodes instead of iron because it
is more electropositive than iron.

 It is used in iron roofings, water tanks and rubbish bins.

III. Tin plating
 Sheets of steel are coated with a very thin coating of tin, Sn.
 The tin produces strong protective oxide coating.
 As soon as the can is dented or scratched, rusting occurs

because tin less electropositive than iron.
 Tin coated steel is used to make tin cans for food.

What will happen if iron, Fe is in contact with tin, Sn? K

1. Oxidation Na

~ Iron, Fe is more electropositive than tin, Sn Ca
Mg

~ Fe loses electrons more easily than Sn Al

Fe  Fe2+(aq) + 2e Zn
Fe

Sn

Pb

2. Reduction H

~ The electrons that are lost are gained by oxygen: Cu
O2 (g) + 2H2O(l) + 4e  4OH-(aq) Ag

A less

electropositive

3. Formation of rust metal will
~ Iron (II) hydroxide, Fe(OH)2 is formed speed up the
corrosion of

~ It is oxidized to iron (III) hydroxide, Fe(OH)3 that metals
eventually becomes rust , Fe2O3.3H2O