•Corrosion is oxidation of metals by materials in their environment.
•Dry corrosion occurs when a metal reacts directly with oxygen. Eg: Burning of magnesium ribbon to produce magnesium oxide.
•Wet corrosion occurs when a metal reacts directly with oxygen in the presence of water. Eg: Corrosion of iron in water to form rust.
•The corrosion of iron is commonly known as rusting.
•Iron metal is oxidised to form iron ions.
Fe (s) → Fe 2+ (aq) + 2e-
•Oxygen in the presence of water is reduced to form hydroxide ions.
O2 (g) + 2H20 (l) + 4e- → 4OH – (aq)
•Overall equation for this reaction:
2Fe (s) + O2 (g) + 2H20 (l) → 2Fe(OH)2 (s)
•Product is insoluble iron(II) hydroxide.
•Iron(II) hydroxide is very unstable in air and quickly reacts to form brown iron(III) hydroxide.
4Fe(OH)2 (s) + O2 (g) + 2H20 (l) → 4Fe(OH)3 (s)
•This brown precipitate iron(III) hydroxide then partially hydrates (loses water) to produce iron(III) oxide, FeO3.xH20, where x is less than three.
•This compound is known as rust.
EFFECTS OF CORROSION
•Corroded metals have less tensile strength and elasticity.
--> affects buildings, bridges, machinery.
•Severly corroded metals cannot conduct electricity.
-->affects car battery.
•Corroded copper pipes and hot water tanks leak as the copper compounds dissolve in the water flowing through them.
Prevents air and water from coming into contact with metal.
•Plastic – household items.
•Paint – large objects – ships, bridges.
•Grease or oil - moving parts machinery.
•Metal coatings – used on steel.
•Metal coatings – using a more reactive metal than steel is called sacrificial coating.
•Zinc is the most common form of sacrifical coating, called galvanising.
•Zinc reacts (corrodes) instead of steel.
•Iron can be alloyed with small quantities of metals, such as chromium, nickel, manganese and molybdenum, to produce stainless steel.
•Stainless steel – kitchen sinks and cutlery.
3. Electrochemical protection.
•Involves placing a more reactive metal in electrical contact with the metal that needs protection.
•It involves ‘sacrificing’ one metal to save another.
•The metal being sacrificed is known as the sacrificial anode.