Standard Grade Chemistry Topic 12 - Corrosion

Corrosion Corrosion is the changing of the surface of the metal from an element into a compound. This natural change of metals into compounds is very costly.

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.

Corrosion is an example of oxidation because it involves a loss of electrons. Fe  Fe2+ + 2e- The corrosion process continues when iron(II) ions lose another electron to form iron(III) ions. Fe2+  Fe3+ + e-

The Cause of Rusting This shows that oxygen and water are both needed for rusting to occur.

The Role of Oxygen and Water in Rusting Oxygen and water accept the electrons lost by the iron. 2H2O + O2 + 4e-  4OH-

Detecting Rusting The typical brown colour of rust is the end result of rusting. The first stage of the rusting process can be detected by ferroxyl indicator. As a blue colour is only made with iron(II) chloride solution, and with a rusting nail, it proves that iron(II) ions (Fe2+) are made during rusting.

Test for iron(II) ions Iron(II) ions (Fe2+) give a blue colour with ferroxyl indicator. Fe2+ are produced when iron atoms lose 2 electrons. Test for Hydroxide ions Ferroxyl indicator turns pink when hydroxide ions are present (in greater numbers than in water).

Speed of Corrosion Most metals corrode, but the speed at which they corrode is related to the chemical activity series e.g. potassium corrodes very quickly, copper corrodes very slowly, while gold does not seem to corrode.

Corrosion requires an electrolyte such as dissolved salt or acid rain. Acid can speed up corrosion in two ways: (a) by acting as an electrolyte (b) by reacting with the metal.

Electrolytes increase the speed of rusting, and cars rust faster in winter when salt is spread on the roads. An electrolyte is needed for rusting to occur. This can even be dissolved carbon dioxide. The electrolyte helps to carry ions away from the rusting iron and this speeds up the oxidation (corrosion).

Rusting and Redox As iron atoms lose electrons in rusting and oxygen/water molecules gain these electrons, rusting is described as a Redox reaction.

PREVENTING CORROSION Physical Protection This is where a metal is given a coating to stop it coming in contact with air and water which prevents corrosion.

Methods of Physical Protection painting e.g. the Forth Rail Bridge. greasing or oiling e.g. bike chains coating with plastic coating with other metals such as tin, zinc, silver, gold.

Tin-plating - metals can be coated with other metals which are less likely to corrode. Food cans are steel cans dipped into molten tin giving a layer of tin. Electroplating - e.g. chromium-plating of car bumpers and the silver-plating of cutlery are done using this process to give an attractive appearance which provides protection against corrosion. Galvanising - galvanised iron in made by coating iron in zinc. It is used to protect dustbins, car exhausts and special nails.

PAINT GALVANISING GALVANISING GALVANISING

PREVENTING CORROSION Using Electricity to Prevent Corrosion The battery causes the nail connected to the positive terminal to rust rapidly, but the nail connected to the negative terminal does not rust.

Why does the nail connected to the negative terminal not rust? 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.

Why does the nail connected to the positive terminal rust rapidly? The positive terminal of the battery is removing electrons from the nail connected to it. This nail rusts rapidly, changing into iron(II) ions which turn ferroxyl indicator blue. Electrons flowing from the nail increase rusting.

PREVENTING CORROSION (c) Sacrificial Protection Metals higher in the electrochemical series can push electrons onto metals lower in the electrochemical series. Metals that push electrons onto iron stop rusting, but metals that let electrons flow from iron increase the speed of rusting.

Magnesium stops iron rusting, while copper makes iron rust quicker. This is an example of sacrificial protection - where a more reactive metal is allowed to corrode in order to protect a less reactive metal.

Comparison of Plating of Metals Tin-plating works well provided the layer of tin remains unbroken. If the tin layer to scratched, the iron corrodes quickly because electrons travel to tin from iron. (Only physical protection). Zinc-plating works well if the layer of zinc remains unbroken and also when scratched because then the zinc corrodes quickly and electrons are pushed onto the iron. (Physical protection AND sacrificial protection).