Basic Science Week3
Topic: Rusting
Introduction
When iron is exposed to air and moisture, its surface gradually corrodes and converts into a brown mass. This brown mass is called rust and this phenomenon is known as rusting.
The corrosion of metals results from the combined action of atmospheric oxygen and water. The most important example of metallic corrosion is that of iron. The corrosion of iron is commonly known as rusting. When iron is exposed to damp air, it reacts with oxygen and water to form a reddish-brown coating of an iron oxide called hydrated iron (III) oxide. This formation of iron (III) oxide is called rusting. This process occurs naturally. Iron is of high use and very important to man. Man cannot afford to allow rust to damage and decay away our iron.
Condition for Rusting
Rusting is a natural process, but we must try to prevent moist air and water from getting on to the surface of the metals.
Conditions for Rusting of Iron
- The presence of water and oxygen is essential for the rusting of iron.
- Impurities in the iron, the presence of water vapor, acids, salts and carbon dioxide hastens rusting.
- Pure iron does not rust in dry and carbon dioxide free air. It also does not rust in pure water, free from dissolved salts.
- Metals like chromium, zinc and magnesium prevent rusting to a great extent and alkalis also help to prevent rusting.
Take some clean iron nails and place them in the three test tubes prepared as under:
Test tube A: Contains boiled water and thin layer of oil.
Test tube B: Contains a small cloth bag containing soda lime.
Tent tune C: Contains ordinary water. Some nails are kept under water and some more are suspended above the water surface.
All the test tubes are tightly closed after placing the nails. Wait for a few days and then observe what has happened
Test tube A
The nails do not rust. They have water, but no oxygen. Boiled water does not contain any dissolved oxygen. The thin layer of oil prevents oxygen in air from reaching the water in the test tube.
Test tube B
The nails do not rust. They have air, but no water and no carbon dioxide, for soda lime absorbs all the carbon dioxide and water vapor.
Test tube C
The nails placed in the water, as well as the ones suspended in the air rust. The water contains dissolved air, and the air contains water vapor.
Preventing Rusting
There are several ways of preventing the rusting of iron:
- Painting: Iron parts installed in vehicles, machinery and buildings are often protected with paint. Tar which is cheaper and lasts longer is frequently used instead of paint on those parts which we do not see, such as the bottom of ships
- Greasing: Iron articles are often greased to stop them from rusting.
- Electroplating: Iron is coated by other metals that will not rust. Examples of metals that will not rust are copper, tin, chromium, gold, silver. Chromium is used for plating bicycle parts.
- Galvanizing: Here iron is protected by a coating of zinc. The sheet of iron is dipped into melted zinc, which sticks to the iron, so that when the iron is taken out and allowed to cool, it is coated with a thin layer of zinc. Galvanized iron is used for making roofing metal, baths and buckets.
- Enamel: Used in making cooking utensils, is often coated with enamel. Enamel is hard, impervious to air and water and does not react with food
