Metals and alloys undergo rusting and corrosion. The process by which some metals when exposed to atmospheric condition i.e., moist air, carbon dioxide form undesirable compounds on the surface is known as corrosion. The compounds formed are usually oxides. Rusting is also a type of corrosion but the term is restricted to iron or products made from it. Iron is easily prone to rusting making its surface rough. Chemically, rust is a hydrated ferric oxide.
Rusting, an electrochemical mechanism:
Rusting may be explained by an electrochemical mechanism. In the presence of moist air containing dissolved oxygen or carbon dioxide, the commercial iron behaves as if composed of small electrical cells. At anode of cell, iron passes into solution as ferrous ions. The electron moves towards the cathode and form hydroxyl ions. Under the influence of dissolved oxygen the ferrous ions and hydroxyl ions interact to form rust, i.e., hydrated ferric oxide.
Methods for the prevention of corrosion and rusting: Some of the methods used to prevent corrosion and rusting are discussed here: 1) Barrier Protection – In this method, a barrier film is introduced between iron surface and atmospheric air. The film is obtained by painting, varnishing etc. 2) Galvanization – The metallic iron is covered by a layer of more reactive metal such as zinc. The active metal loses electrons in preference of iron, thus protecting from rusting and corrosion.
The aim of this project is to investigate the effect of metal coupling on the rusting of iron. Metal coupling affects the rusting of iron. If the nail is coupled with a more electro-positive metal like zinc, magnesium or aluminium, rusting is prevented but on the other hand, if it is coupled with less electro-positive metals like copper, the rusting is facilitated.
Aim: To investigate the effect of metal coupling on the rusting of iron.
Apparatus: Two petri-dishes, four test-tubes, four iron nails, a beaker, sand paper, wire guage, gelatin (agar agar), copper, zinc & magnesium strips, potassium ferricyanide solution and phenolphthalein solution.
1. At first, we have to clean the surface of the iron nails with the help of sand paper.
2. After that, wind the zinc strip around one nail, a clean copper wire around the second and a clean magnesium strip around the third. Then, put all these three and a fourth nail in petri-dishes so that they are not in contact with each other.
3. Then, fill the petri-dishes with hot agar agar solution in such a way that only the lower halves of the nails are covered with the solution. Cover the petri-dishes for a day or so.
4. The liquids set to a gel on cooling. Two types of patches are observed around the rusted nail. One is blue while the other is pink. The blue patch is due to the formation of potassium ferro-ferricyanide while the pink patch is due to the formation of hydroxyl ions which turns the colorless phenolphthalein solution pink.