Corrosion

Corrosion reactions

Corrosion is the loss of metallic properties of a metal as the metal reacts with the atmosphere or water. e.g. strength, lustre or shine and electrical conductivity.

Rust is brownish red in color and is formed from the corrosion of iron. Other metals like copper and aluminium also corrode or weaken.

For corrosion to occur both water and oxygen need to be present.

Examining the corrosion of iron using nails:

Oxygenated water (shaken)

Humid air

Dry air (anhydrous CaCl2)

Deoxygenated water

(Boiled water/ oil layer)

Salt water

Rust

Rust

No rust

No rust

Extensive rust

If a nail is placed in deoxygenated water it will not rust. If a nail is placed in dry air it will not rust. For rust to occur both oxygen and water need to be present. The presence of salt accelerates or speeds up the rate of corrosion.

The chemical formula of rust is Fe2O3.nH2O

The process of rust formation is due to a series of chemical reactions. Points of stress acts in the nail allow the iron to be easily oxidised. Carbon impurities in the nail allow water to be reduced. An internal battery or galvanic cell is set up.

Oxidation of iron at points of stress in the crystal lattice: 2Fe(s) 2Fe2+(aq) + 4e-
Reduction of water at the site of carbon impurities: O2(g) + H2O(l) + 4e- 4OH-(aq)

Overall equation: 2Fe(s) + O2(g) + H2O(l) Fe(OH)2

The iron(II) hydroxide is converted to rust through a serious of reactions.

The iron(II) hydroxide firstly oxides to iron(III) oxide.

1. Fe(OH)2(s) Fe(OH)3

oxidation

The iron(III) oxide then changes to rust through a dehydration reaction.

2. Fe(OH)3(s) Fe2O3.nH2O(s) or rust

dehydration

Rust adheres loosely to the surface of the metal. This exposes the metal to more and more water and oxygen allowing the process of rusting to continue.