Corrosion

Introduction:
Corrosion is a natural process that occurs when certain metals react with their environment, resulting in the deterioration of the metal. It is a common phenomenon that can have detrimental effects on the appearance, strength, and functionality of various metallic objects. In this case, the examples provided are all related to the corrosion of different metals.

Examples of Corrosion:
The correct answer is option 'D', which includes all the given examples of corrosion. Let's discuss each example in detail:

1. Black coating on Silver:
When silver comes into contact with sulfur-containing compounds present in the air, it undergoes a chemical reaction known as tarnishing. This reaction forms a black coating on the surface of the silver, which is mainly composed of silver sulfide (Ag2S). Tarnishing is a form of corrosion specific to silver and silver-plated objects.

2. Green coating on Copper:
Copper is highly susceptible to corrosion, especially in the presence of moisture and oxygen. When exposed to these elements, copper undergoes a process known as oxidation, resulting in the formation of copper oxide (CuO) or copper carbonate (CuCO3). The green coating, often seen on copper roofs or statues, is called patina and is formed by the reaction of copper with carbon dioxide and moisture in the air.

3. Brown coating on Iron:
Iron is prone to a specific type of corrosion known as rusting. When iron reacts with oxygen and water, it forms hydrated iron(III) oxide, commonly known as rust (Fe2O3·nH2O). Rust appears as a reddish-brown coating on the surface of iron objects and is a common problem in outdoor structures, vehicles, and machinery made of iron or steel.

Conclusion:
Corrosion is a natural and inevitable process that affects various metals, leading to the deterioration of their properties. The examples mentioned in the given options illustrate different forms of corrosion, such as tarnishing of silver, patina formation on copper, and rusting of iron. Understanding the causes and effects of corrosion is essential for implementing preventive measures and maintaining the longevity of metallic objects.