Corrosion, Rusting and How to Fight it. Cairney McAteer
What is Corrosion • Corrosion is the changing of the surface of the metal from an element into a compound. Corrosion is an example of oxidation because it involves a loss of electrons. • Fe ----------> Fe2+ + 2e-
Rusting • Rusting is the special name given to the corrosion of iron. As iron, in the form of steel, is the most commonly used metal in the world, the corrosion of iron is important.
Speeding up Rusting • Rusting can be sped up by the use of sulphur dioxide which causes acid rain. • The quantity of air and water affects the speed.
Iron and Other Metals • If a nail and a carbon rod are connected by a wire and dipped into salt solution containing ferroxyl indicator, a blue colour quickly appears around the nail, and a pink colour around the carbon rod. This means that electrons are flowing from the nail to the carbon. When electrons flow from iron, it rusts. • In Topic 10 it was found that metals higher in the electrochemical series could push electrons onto metals lower in the electrochemical series. Magnesium stops iron rusting, while copper makes iron rust quicker. • Metals that push electrons onto iron stop rusting, but metals that let electrons flow from iron increase the speed of rusting.
Cathodic Protection • Iron has to lose electrons in order to rust. The negative terminal of the battery is pushing electrons onto this nail and this prevents this nail from losing any electrons. This nail cannot rust. Electrons flowing to the nail stop rusting.
Galvanising • Galvanising is when galvanised iron in made by dipping iron into molten zinc which coats the iron with zinc. It is used to protect dustbins, car exhausts and special nails.
Sacrificial Protection • Sacrificial protection is when a more reactive metal sacrifices itself to protect the less reactive metal in the electrochemical series.
Ways to Protect Iron • painting e.g. the Forth Rail Bridge. • greasing or oiling - protects moving parts of machinery. • coating with plastic - dish drainers have a metal core and a plastic coating. • coating with other metals such as tin, zinc, silver, gold.
Rusting • Corrosion is an example of oxidation because it involves a loss of electrons. • Fe ----------> Fe2+ + 2e- • The rusting process continues when iron(II) ions lose another electron to form iron(III) ions. • Fe2+ ----------> Fe3+ + e- • Oxygen and water accept the electrons lost by the iron. • 2H2O + O2 + 4e- ----------> 4OH-