Corrosion and Its Prevention - Metals and Non-metals
1. What is Corrosion?
Corrosion is the gradual destruction of metals through a chemical reaction with elements in their environment like air, moisture, acids, or other substances.
When metals are exposed to these elements, they slowly get eaten away, lose their shine, and become weak.
Important Points:
- Corrosion needs both a reactant (the metal) and environmental factors (like oxygen, water, gases).
- The damaged product is often softer and more fragile than the original metal.
Examples:
- Iron objects: These slowly develop brownish-red rust in damp air.
- Silver jewelry: Turns black after some time due to exposure to H₂S in the air.
- Copper roofs or statues: Become greenish due to formation of copper carbonate.
2. Examples of Corrosion
Below are few examples of Corrosion that we will discuss in detail:
- a) Rusting of Iron
- b) Tarnishing of Silver
- c) Green Coating on Copper
- d) Corrosion of Aluminium
a) Rusting of Iron
- Iron reacts with oxygen (O₂) and water (H₂O) from air to form rust (hydrated iron(III) oxide), a brown flaky substance.
- Rusting can make iron weak and breakable.
Equation:
4Fe(s) + 3O₂(g) + xH₂O(l) → 2Fe₂O₃·xH₂O(s) (rust)
Examples:
- Iron nails left in wet air develop brown rust.
- Iron bridges or gates exposed to weather get rust patches.
- Old iron tools, especially if not used or stored in a damp place, show rusting.
b) Tarnishing of Silver
- Silver reacts with hydrogen sulphide (H₂S) in the air to form a black layer of silver sulphide (Ag₂S).
Equation:
2Ag(s) + H₂S(g) → Ag₂S(s) + H₂(g)
Examples:
- Silver spoons or jewellery turning black when left open.
- Silver idols changing color with time.
- Trophies made of silver losing their shine.
c) Green Coating on Copper
- Copper reacts with water (moisture), carbon dioxide, and oxygen, forming a green coating of basic copper carbonate.
Equation:
2Cu(s) + H₂O(l) + CO₂(g) + O₂(g) → Cu(OH)₂(s) + CuCO₃(s)
Examples:
- Old copper coins appearing green.
- Copper wire left outdoors showing a greenish surface.
- Copper bottoms of old vessels or statues, especially in moist environments.
d) Corrosion of Aluminium
- Aluminium forms a thin, hard layer of aluminium oxide (Al₂O₃) that protects the metal from further corrosion.
- Unlike rust on iron, this oxide layer is protective.
Examples:
- Aluminium window frames stay shiny for years.
- Aircraft bodies made of aluminium rarely corrode.
- Kitchen foil does not rust despite moisture.
3. Why is Corrosion Harmful?
Key Points:
- Weakens Metals: Makes bridges, vehicles, pipes, and machines unsafe and fragile.
- Economic Loss: Requires regular repair and maintenance; leads to wastage of metal.
- Safety Dangers: Broken pipes can cause leaks and pollution.
- Affects Appearance: Makes objects look dull or dirty.
Examples:
- Rusted bridge beams may fail, risking accidents.
- Corroded cooking vessels may contaminate food.
- Leaking petrol pipes due to corrosion can cause fires.
4. Prevention of Corrosion
Below are methods to prevent Corrosion:
- a) Painting
- b) Oiling and Greasing
- c) Galvanization
- d) Alloying
- e) Other Methods
Let's discuss them in detail.
a) Painting
Painting covers the metal to keep out air and moisture.
Examples:
- Iron gates and railings are often painted.
- Cars and bicycles have colorful paintwork.
- Steel lamp posts are painted regularly.
b) Oiling and Greasing
Oil or grease forms a water-resistant barrier. This keeps air and water away from metal.
Examples:
- Greasing the chain of a bicycle.
- Oiling tools before storing them.
- Greasy coatings in machinery in factories.
c) Galvanization
Galvanization: Coating iron or steel with a layer of zinc.
Zinc is more reactive and forms zinc oxide, which protects iron from further attack.
Examples:
- Water supply pipes in homes (galvanized iron pipes).
- Iron buckets used in gardening.
- Fences or wire mesh have a silvery look due to zinc coating.
d) Alloying
Making alloys makes metals stronger and more corrosion-resistant.
Examples:
- Stainless Steel: Iron mixed with chromium and nickel is stainless and rust-free. Used for utensils, surgical tools.
- Brass: Copper and zinc; used for musical instruments and water taps.
- Bronze: Copper and tin; used for statues and medals.
e) Other Methods
i) Anodization
- Especially for aluminium.
- Electric current passed through aluminium in acid forms a thick, protective oxide coat.
Examples:
- Aluminium window frames.
- Fancy coloured aluminium water bottles.
ii) Electroplating
Depositing a thin layer of another, less reactive metal.
Examples:
- Chrome-plated bike handles.
- Nickel-plated kitchen tongs.
- Silver-plated cutlery.
iii) Use of Desiccants
Chemicals like silica gel absorb moisture and protect packed metals.
Examples:
- Silica gel packs in electronic gadgets.
- Small packets in new shoe boxes to prevent corrosion of eyelets.
iv) Sacrificial Protection (Cathodic Protection)
- A more reactive metal is attached to the object to be protected. The sacrificial metal corrodes instead.
Examples:
- Magnesium blocks attached to ship hulls.
- Zinc bars with underground pipelines.
Activity: Observing the Rusting of Iron
Step-by-step Instructions:
-
Take three test tubes: Label them A, B, and C.
-
Put a clean iron nail in each tube.
-
Set up the tubes as follows:
- Test Tube A: Add water to half (nail partially dipped).
- Test Tube B: Add boiled distilled water, then a layer of oil to prevent air from entering.
- Test Tube C: Add some anhydrous calcium chloride (to absorb moisture), keep it dry.
-
Leave the tubes for a few days.
Observations:
- Test Tube A: Nail rusts (needs water + air).
- Test Tube B: No rust (air prevented).
- Test Tube C: No rust (water/moisture prevented).
Conclusion:
Both air (oxygen) and water are necessary for rusting.
Scenario-Based Questions and Answers
1: Scenario: Your bicycle chain often gets wet in the rain.
Question: What should you do to protect your bicycle chain from rusting, and why?
Answer: Apply oil or grease to the chain. This prevents water and air from reacting with the metal, so rusting does not occur.
2: Scenario: Workers use iron pipes for a water supply in a city.
Question: How can they increase the pipes’ life and prevent corrosion?
Answer: They can use galvanized iron pipes (zinc-coated iron) or paint the pipes, forming a barrier against water and oxygen.
3 Scenario: You see a shiny aluminium window frame even after many years.
Question: Why does aluminium not corrode away like iron?
Answer: Aluminium forms a thin, hard, oxide layer that protects the metal from further attack. This layer is stable and stops further corrosion.
4 Scenario: Silver jewellery is turning black after some time.
Question: What chemical reaction is happening? How can you restore the shine?
Answer: Silver reacts with hydrogen sulphide (H₂S) from the air to form black silver sulphide (Ag₂S).
Clean the jewellery with a soft cloth and mild cleaning paste to remove the coating.
5 Scenario: Engineers want to prevent corrosion in an underground iron pipeline.
Question: Which method should they choose, and why?
Answer: They should use sacrificial protection. By attaching blocks of magnesium or zinc, which are more reactive, these metals will corrode first, protecting the iron pipe.
Remember:
Understanding corrosion and its prevention not only saves money but also helps keep people safe!
