To investigate the ability of metals to act as a sacrificial protector of iron, preventing or reducing rusting. Theory Iron oxide is more stable than iron metal, so when given the opportunity iron will be readily oxidised. Iron reacts with oxygen and water to form a hydrated form of iron(III) oxide (Fe2O3i?? xH2O), known as rust, which is permeable to air and water so the metal underneath the rust layer continues to corrode1. This diagram shows the reactions which occur in a droplet of water on the surface of a piece of iron or steel:
Source 1 The iron atoms readily give away two electrons each, which react with oxygen and water molecules to form hydroxyl ions. These ions reacts with the iron ions to form iron hydroxide, which is then further oxidised and hydrated to give the iron(III) oxide. The higher concentrations of dissolved oxygen at the edges of the drop mean that the reduction reactions happen around the edge, leaving a pit in the metal under the middle of the drop. The rust forms away from the surface of the iron, as the iron and hydroxyl ions diffuse away.
Iron has a more negative electrode potential than oxygen + water (-0. 44V compared to +0. 40V), which explains why it gives away electrons (oxidation) instead of accepting them (reduction). By coating the iron in a substance with a more negative electrode potential, the coating will give up electrons more readily than the iron. Therefore the coating will be oxidised and the iron will be protected. Ferroxyl indicator is a mixture of Phenolphthalein in sodium chloride solution, and potassium hexacyanoferrate(III).
It turns from orangey-yellow to pink in the presence of hydroxyl ions, and to a blue colour in the presence of Fe3+ ions (the charge of the iron ions in the iron(III) oxide)2. Therefore if a piece of iron placed in the indicator rusts, a blue colour will appear. If nothing happens, there will be no colour change. If the coating is oxidised, the mixture will turn pink (as the coating’s electrons will react with water and oxygen to produce hydroxyl ions, but no iron(III) ions will be produced). The metals that will be used as a coating are magnesium, copper, zinc and lead.