Difference between Corrosion, Rusting and Rancidity

Introduction: 

What is redox reaction? 

The process of losing electrons is oxidation, and the process of gaining electrons is reduction.  The reaction in which both oxidation and reduction occur simultaneously is said to be a redox reaction. We can define oxidation as the loss of electrons, or the gain of oxygen atoms, or the loss of hydrogen atoms. Similarly, we can define reduction process as the gain of electrons, or loss of oxygen atoms or gain of hydrogen atoms. 

Explanation: 

Oxidation reactions in daily life: 

Substances made of silver and copper look shiny when they are new and turn dull after usage. This is because of oxidation of metal with air and moisture in the environment. Such a process is called corrosion. 

Corrosion: 

The reaction of metals with air or oxygen or moisture, acids etc., present around it is called corrosion. The elements react with the moisture in the environment or the oxygen in the air to form different compounds. The process of the formation of compound in this manner is called corrosion. 

Examples: Black coating on silver 

Green coating on copper 

Causes of corrosion: 

• Corrosion causes damage to car bodies, bridges, ships, metal railings etc. 
• Corrosion of Iron metal causes great damage called rusting. 

Corrosion of copper: 

Copper when exposed to air converts to copper(I)oxide which is red in color and further turns to black copper(II)oxide. 

Methods to prevent corrosion: 

• Electroplating 
• Galvanization 
• Anodization 
• Protective coatings 

Rusting of iron: 

The brown layer developed by the iron when treated or come in contact with air or moisture is called rusting of Iron. We can simply call it corrosion of iron. The color of the iron changes to brown and ferrum metal becomes weak. 

Activity: 

• Take three test tubes and label them as A,B,C.  
• In test tube A, add cold water. 
• In B, add hot water with oil.  
• In C, add no water and remove moisture by adding anhydrous Calcium chloride CaCl₂. 
• Put iron nails to all the test tubes and cork all the test tubes and leave it for some time. 

Observations: 

We observe that the Iron nail in test tube labeled A gets rusted as it is exposed to air and moisture and the Iron nails present in test tubes B and C are not rusted. 

Factors responsible for rusting: 

• The iron nails that are exposed to air and moisture get rusted easily like in A. 
• Test tube B has moisture but oil being lighter forms a layer on the top and does not allow air to pass through. 
• Test tube C has no moisture or air as CaCl₂ is hygroscopic and removes moisture from the test tube. 
• So, in B, C iron nails are protected. 

What is rust? 

When Iron is exposed to air or moisture, iron converts to Iron oxide which is reddish brown in color. This is referred to as rust. Rust is hydrated Iron oxide.  

4Fe    +     3O₂    +     2xH₂O     →     Fe₂O₃.xH₂O 

Fe₂O₃.xH₂O is the chemical formula of the rust. 

Principles of rusting: 

Rusting or corrosion of a compound involves redox reaction where Iron loses electrons and Oxygen gains electrons. 

Prevention of rusting: 

Rusting can be prevented by avoiding contact with air or moisture. Painting or applying grease can help in preventing rusting.  

Differences in Rust and Corrosion: 

Rancidity: 

The foods containing fats or oils when exposed to air for a long time their smell and taste change as they become rancid. This property is called rancidity. 

Prevention of rancidity: 

To prevent rancidity of oil or fat containing foods, antioxidants are added which prevent the oxidation of food.  Food is preserved by adding nitrogen gas which is an inert material.   

Summary

1. The reaction of metals with air or oxygen or moisture, acids etc present around it is called corrosion.
2. Corrosion in case of iron metal is called Rusting.
3. Oxidation of oils or fats in food is called Rancidity.
4. These processes are unwanted and can be prevented by avoiding contact with air or by painting.