CORROSION or RUSTING :
Corrosion is defined as the oxidation of metal by air. Although presence of moisture makes corrosion faster but it is not necessary condition.
In corrosion, the metal is slowly oxidized by the air.
Corrosion of Iron is also called Rusting of iron. Iron articles are shiny when new, but get coated with a reddish brown powder called rust (Fe2O3.n H2O) when left for some time. This process is commonly known as rusting of iron.
The black coating on silver and the green coating on copper are other examples of corrosion.
Silver is a very noble metal and is often found in a native state combined with gold, tin, copper, and platinum. It is completely stable in aqueous solutions. In addition, silver is not affected by dry or moist air that is free from ozone, halogens, ammonia, and sulfur compounds .
In a marine environment, with its abundance of soluble sulfates and oxygen-consuming, decaying organic matter, sulfate-reducing bacteria utilizes available sulfates under anaerobicconditions to form hydrogen sulfides as a metabolic product. The hydrogen sulfide reacts with the silver to form silver sulfide. The overall reaction proceeds in the same process as described earlier for iron:
2Ag + H2S → Ag2S + H2
The green coating on copper metal called a Patina. The green coating is mainly Copper (II) Carbonate CuCO3 caused by slow chemical reactions involving the copper metal and water and carbon dioxide.
2 Cu + H2O + CO2 + O2 → Cu (OH)2 + CuCO3
The chemistry of the reaction resulting in the formation of rust can be summarized as follows:
The chemical equations for rust formation
2Fe(s) + 2H2O(l) + O2(g) → 2Fe2+(aq) + 4OH–(aq)
Fe2+(aq) + 2OH–(aq) → Fe(OH)2(s)
4Fe(OH)2 (s) + O2(g) + 2H2O(l) → 4 Fe(OH)3 (s)
Fe(OH)3 (s) dehydrates into Fe2O3. nH2O (s) or rust.
The chemical formula for rust is Fe2O3. nH2O
Methods of preventing Corrosion or Rusting :
Corrosion of metals is prevented by not allowing them to come in contact with moisture and O2.
This is achieved by the following methods:
- By coating with paints: Paint coated metal surfaces keep out air and moisture.
- By coating with oil and grease: Application of oil and grease on the surface of iron tools prevents them from moisture and air.
- By alloying with other metals: Alloys (mixture of metals) is more resistant to corrosion. Example: stainless steel.
- By the process of Galvanization: This is a process of coating zinc on iron sheets by using electric current. In this Zinc forms a protective layer of zinc carbonate on the surface of iron.This prevents corrosion.
- Electroplating: It is a method of coating one metal over the surface of Iron (or other metal) by passing electric current. Example: silver plating, nickel plating. This method not only lends protection but also enhances the metallic appearance.
- Sacrificial Protection: Sometime more electropositive metal like Magnesium is used to protect Iron. The more electropositive metal is connected with iron surface or articles . The electropositive reduces Iron if it is oxidized and saves Iron. Thus finally Electropositive metal like Magnesium is sacrificed to prevent Iron from oxidation.
When fats and oils are oxidised, they become rancid and their smell and taste is changed. This is called Rancidity. Usually substances which prevent oxidation (antioxidants like Nitrogen, B.H.T.) are added to foods containing fats and oil. Keeping food in air tight containers helps to slow down oxidation or Rancidity.