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Corrosion Tutorial

Key Concepts

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Example of Corrosion: Rusting of Iron

If you look very closely at a piece of iron as it begins to rust you might notice two things:

This is because the site where you see the red rust is not the site where the iron is initially being oxidised.

In order for the rust to form on the iron, the iron needs to contain an impurity, that is, a place where the electrons released from the oxidation of iron can travel to.

In order to complete the electrical circuit, ions need to be able to travel through an electrolyte, and this is provided by moisture, water in the air surrounding the iron, in which ions can dissolve and travel to the site of rust formation.

The process by which rust is formed on iron is shown in the diagram below:

Fe(s) is oxidised to Fe2+ and a pit develops on the surface of the iron.

Fe(s) → Fe2+(aq) + 2e-

At the site of an impurity oxygen is reduced to hydroxide ions.

O2(g) + 2H2O(l) + 4e- → 4OH-(aq)

Electrons flow through the iron bar from the pit to the impurity.

Ions flow through the water electrolyte to the rusting site.

At the rusting site, iron(II) hydroxide, Fe(OH)2(s), is formed which readily undergoes oxidation to form hydrated iron(III) oxide, Fe2O3.H2O(s), (rust).

The equations representing the reactions at the anode, cathode, and at the site where the rust forms are shown below:

Anode
(oxidation)
pitting
develops
Fe(s) Fe2+(aq) + 2e-
Cathode
(reduction)
site of
impurity
O2(g) + 2H2O(l) + 4e- 4OH-(aq)

Cell
Reactions
site of
rust
Fe2+(aq) + 2OH-(aq) Fe(OH)2(s)
REDOX
Reaction
4Fe(OH)2(s) + O2(g) 2Fe2O3.H2O(s) + 2H2O(l)

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Factors Influencing the Rate of Corrosion of Iron

The rate of corrosion of iron, or the rate of rusting of iron, refers to how quickly the iron corrodes (or rusts).
A number of factors can influence how fast the iron will rust:

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Rust Prevention Methods

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1. Biological activity can also corrode metals.