Introduction:

Have you ever noticed an old iron gate covered in a reddish-brown coating, or a silver spoon that has turned dull and dark over time? These are everyday examples of metals undergoing chemical changes. When metals react with substances in their environment — particularly oxygen and moisture — they change in appearance and structure. This process is called corrosion, and its two most common forms are rusting (in iron) and tarnishing (in other metals such as silver, copper, and brass).

1. What is Rusting?

Rusting is a chemical change that affects iron and its alloys, such as steel. When iron is exposed to both oxygen and water (moisture), it reacts to form a new substance called iron oxide, which is the reddish-brown, flaky coating we recognise as rust.

The chemical reaction can be written as:

Iron + Oxygen + Water → Hydrated Iron Oxide (Rust)

Rust is a completely different substance from iron. It is weak, crumbly, and flaky, unlike iron which is strong and shiny. Because a new substance is formed and the process cannot be reversed, rusting is an irreversible chemical change.

Rusting causes serious damage to iron structures such as bridges, railings, vehicles, tools, and machinery. It weakens the metal over time until it crumbles and falls apart.

2. What is Tarnishing?

Tarnishing is a similar process that affects other metals such as silver, copper, brass, and aluminium. Unlike rusting, tarnishing does not always involve water. It mainly occurs when the surface of a metal reacts with oxygen or sulphur compounds in the air, forming a thin layer of a new compound on the surface.

For example:

• Silver reacts with sulphur compounds in the air to form silver sulphide, which appears as a dark, dull coating on the surface of silver objects.
• Copper reacts with oxygen and carbon dioxide in moist air to form copper carbonate, which is the greenish coating (called patina) seen on old copper statues and rooftops.
• Aluminium reacts with oxygen to form aluminium oxide, which forms a very thin, hard layer on its surface.

Tarnishing is generally a surface-level process. The thin layer that forms on the metal surface can sometimes protect the metal underneath from further reaction. This is why tarnished aluminium and copper do not always deteriorate as badly as rusted iron.

3. Oxidation and Corrosion

Oxidation is the chemical process in which a substance reacts with oxygen. When metals oxidise, they combine with oxygen from the air to form metal oxides.

• When iron oxidises, it forms iron oxide (rust).
• When copper oxidises, it forms copper oxide or copper carbonate.
• When aluminium oxidises, it forms aluminium oxide.

Corrosion is the broader term for the gradual destruction of a metal through chemical reactions with its environment. Rusting and tarnishing are both forms of corrosion. Corrosion is always a chemical change because new substances are formed and the process is generally irreversible.

The key difference between oxidation and corrosion is that oxidation refers specifically to the reaction with oxygen, while corrosion includes any chemical reaction — with oxygen, water, acids, or other substances in the environment — that degrades the metal.

4. Environmental Factors That Speed Up Rusting and Tarnishing

Several environmental conditions affect how quickly rusting and tarnishing occur:

• Moisture (water): Water is essential for rusting. Iron rusts much faster in wet or humid conditions than in dry conditions. Water acts as a medium that speeds up the chemical reaction between iron and oxygen. Coastal areas, where there is high humidity and salt in the air, see very rapid rusting.
• Oxygen: Oxygen from the air is the other essential ingredient for both rusting and tarnishing. In environments with a higher concentration of oxygen, corrosion happens more quickly.
• Salt: Salt water speeds up rusting dramatically. This is why ships, coastal structures, and cars in snowy regions (where road salt is used) rust much faster than those in dry, inland areas.
• Acids: Acidic environments, such as acid rain, accelerate corrosion. Acid rain is caused by air pollution from burning fossil fuels. It reacts with metal surfaces and speeds up the breakdown process.
• Temperature: Higher temperatures generally speed up chemical reactions, including rusting and tarnishing. Metals in hot and humid climates corrode faster than those in cool, dry climates.
• Sulphur compounds in the air: These gases, released by burning fossil fuels and industrial processes, react with metals such as silver and copper to cause tarnishing more rapidly.

5. Prevention of Rusting and Tarnishing

Since rusting and tarnishing cause great damage and economic loss, humans have developed several methods to protect metals:

Painting Applying a coat of paint to the surface of iron or steel creates a physical barrier between the metal and the air and moisture. As long as the paint is intact, no rusting can occur. This is why iron bridges, gates, and railings are regularly painted. However, if the paint chips or cracks, rusting begins at that spot.

Galvanisation Galvanisation is the process of coating iron or steel with a thin layer of zinc. Zinc is more reactive than iron, which means it reacts with oxygen and moisture preferentially, protecting the iron underneath. Even if the zinc coating is scratched, it continues to protect the iron around it. Galvanised iron is widely used in roofing sheets, buckets, pipes, and fences.

Electroplating In electroplating, a thin layer of a non-corroding metal — such as chromium, nickel, or tin — is deposited onto the surface of iron using an electric current. This creates a shiny, protective layer. Taps, car bumpers, and bicycle parts are often electroplated.

Oiling and Greasing Applying oil or grease to metal surfaces prevents air and moisture from coming into contact with the metal. This method is commonly used for tools, machinery, and mechanical parts such as bicycle chains and engine components.

Alloying An alloy is a mixture of two or more metals. Adding other metals to iron or steel can make it resistant to corrosion. Stainless steel, for example, is an alloy of iron, carbon, and chromium. The chromium reacts with oxygen to form a very thin, hard, invisible layer of chromium oxide on the surface, which prevents further corrosion. Stainless steel is used in kitchen utensils, surgical instruments, and cutlery.

Using Non-Corrosive Metals In some situations, engineers choose to use metals that do not corrode easily, such as aluminium, titanium, or gold, instead of iron. Although these are more expensive, they last much longer without deteriorating.

Storing Metals in Dry Conditions Since moisture is a key factor in rusting, keeping iron and steel objects in dry, low-humidity environments significantly slows down corrosion. Silica gel packets are sometimes placed in storage containers to absorb moisture.

6. Summary

Rusting and tarnishing are chemical changes in which metals react with oxygen, moisture, or other substances in the environment to form new compounds. These processes are forms of corrosion and are generally irreversible. Environmental factors such as moisture, oxygen, salt, acids, and high temperatures speed up corrosion. However, humans have developed effective methods — including painting, galvanisation, electroplating, oiling, alloying, and careful storage — to slow down or prevent these processes and protect valuable metal structures and objects.

Key Terms to Remember

• Rusting — the corrosion of iron due to reaction with oxygen and water, forming iron oxide
• Tarnishing — the corrosion of metals such as silver and copper due to reaction with oxygen or sulphur compounds
• Oxidation — the chemical reaction of a substance with oxygen
• Corrosion — the gradual destruction of a metal through chemical reactions with its environment
• Galvanisation — coating iron with zinc to protect it from rusting
• Alloy — a mixture of two or more metals
• Stainless steel — an alloy of iron, carbon, and chromium that resists corrosion
• Electroplating — depositing a thin layer of protective metal onto a surface using electricity