Unit 6: Rusting & Prevention

Understanding the electrochemical corrosion of iron and the methods used to prevent it.

6.25 The Rusting of Iron (Conditions & Process)

Rusting is the common term for the corrosion of iron and its alloys, such as steel. It is an electrochemical process that is a major cause of economic damage. Rust itself is hydrated iron(III) oxide, $Fe_2O_3 \cdot xH_2O$.

Necessary Conditions for Rusting:

Experiments show that two substances are essential for iron to rust:

  1. Oxygen (usually from the air)
  2. Water
Rusting is accelerated by the presence of dissolved salts (like NaCl) or acids (like acid rain), as these electrolytes increase the conductivity of the water and speed up the electrochemical process.

The Electrochemical Process:

A water droplet on the surface of iron acts like a tiny galvanic cell.

  • Anodic Region (Oxidation): At the center of the droplet where oxygen concentration is lower, iron is oxidised. This is the anode.
    $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$
  • Cathodic Region (Reduction): At the edge of the droplet where oxygen concentration is higher, oxygen from the air is reduced. This is the cathode.
    $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$
  • Formation of Rust: The $Fe^{2+}$ ions and $OH^-$ ions migrate and combine to form iron(II) hydroxide, which is then further oxidised by oxygen to form reddish-brown hydrated iron(III) oxide (rust).
    $Fe^{2+}(aq) + 2OH^-(aq) \rightarrow Fe(OH)_2(s)$
    $4Fe(OH)_2(s) + O_2(g) \rightarrow 2Fe_2O_3 \cdot H_2O(s) + 2H_2O(l)$ (Simplified)
Solved Examples:
  1. What are the two essential conditions for iron to rust?
    Solution: The presence of both oxygen and water.
  2. What is the chemical name for rust?
    Solution: Hydrated iron(III) oxide.
  3. Why does a car rust faster in areas where roads are salted in winter?
    Solution: The salt dissolves in water to form an electrolyte solution, which speeds up the electrochemical process of rusting.
  4. In the rusting process, what happens to the iron atoms?
    Solution: They are oxidised to iron(II) ions ($Fe^{2+}$).
  5. What is the role of oxygen in rusting?
    Solution: It acts as the oxidising agent; it is reduced at the cathodic region.
  6. Is rusting a redox reaction? Explain.
    Solution: Yes, because iron is oxidised (loses electrons) and oxygen is reduced (gains electrons).
  7. An iron nail is placed in a test tube with boiled water (to remove dissolved air) and a layer of oil on top. Will it rust?
    Solution: No, because oxygen is absent.
  8. An iron nail is placed in a test tube with a drying agent like anhydrous calcium chloride. Will it rust?
    Solution: No, because water is absent.
  9. Where does reduction occur on a water droplet on an iron surface?
    Solution: At the edge of the droplet, where the concentration of dissolved oxygen is highest.
  10. Write the half-equation for the oxidation of iron during rusting.
    Solution: $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$.

6.26 Methods of Rust Prevention

Preventing rust involves stopping either oxygen or water from reaching the iron surface, or by using electrochemical principles.

1. Barrier Methods:

These methods create a physical barrier between the iron and its environment.

  • Painting, Oiling, Greasing: Provide a waterproof and airproof layer. This is effective as long as the barrier is intact. If scratched, rusting will occur underneath.
  • Coating with Plastic: Similar to painting, provides a durable protective layer.
  • Plating with a Less Reactive Metal (e.g., Tin): Tin-plating is used for food cans. Tin is unreactive and protects the steel. However, if the tin layer is scratched, the iron underneath will rust very rapidly because it is more reactive than tin and becomes the anode in an electrochemical cell.

2. Sacrificial Protection:

This method involves connecting the iron to a more reactive metal. The more reactive metal corrodes (is oxidised) in preference to the iron, thus "sacrificing" itself.

  • Galvanising: This is a specific type of sacrificial protection where iron or steel is coated with a layer of zinc. Zinc is more reactive than iron, so it will be oxidised first ($Zn \rightarrow Zn^{2+} + 2e^-$), even if the surface is scratched. This is because zinc has a more negative electrode potential than iron.
  • Sacrificial Anodes: Large blocks of a more reactive metal (like zinc or magnesium) are attached to the hulls of ships or underground pipelines. The blocks corrode away over time and must be replaced, but the steel structure remains protected.

Solved Examples:
  1. How does painting a gate prevent it from rusting?
    Solution: The paint acts as a physical barrier, preventing oxygen and water from coming into contact with the iron.
  2. What is galvanising?
    Solution: It is the process of coating iron or steel with a layer of zinc for rust protection.
  3. Why is galvanising a particularly effective method of rust prevention?
    Solution: It provides both a physical barrier and sacrificial protection. If the zinc is scratched, it will still corrode in preference to the iron.
  4. A steel food can is coated with tin. What happens if the can gets a deep scratch?
    Solution: The exposed steel will rust very quickly. Iron is more reactive than tin, so the iron becomes the anode and is oxidised preferentially.
  5. What is meant by "sacrificial protection"?
    Solution: It is a method where a more reactive metal is deliberately allowed to corrode to protect a less reactive metal (like iron) to which it is connected.
  6. Which metal could be used as a sacrificial anode for an iron pipe: copper or magnesium? ($E^\circ_{Mg} = -2.37V$, $E^\circ_{Fe} = -0.44V$, $E^\circ_{Cu} = +0.34V$)
    Solution: Magnesium. It is more reactive (has a more negative electrode potential) than iron and will be oxidised in its place.
  7. Why are the hulls of ships often fitted with blocks of zinc?
    Solution: To act as sacrificial anodes, protecting the steel hull from rusting in the saltwater environment.
  8. Name two barrier methods of rust prevention.
    Solution: Painting and oiling/greasing.
  9. In a galvanised bucket, which metal acts as the anode if the surface is scratched?
    Solution: Zinc.
  10. Why doesn't aluminium corrode away like iron, even though it is more reactive?
    Solution: Aluminium reacts quickly with oxygen in the air to form a very thin, tough, and unreactive layer of aluminium oxide ($Al_2O_3$), which protects the metal underneath from further corrosion.

Knowledge Check (20 Questions)

Answer: Rusting.

Answer: Oxygen and water.

Answer: Galvanising.

Answer: Oxidation ($Fe \rightarrow Fe^{2+} + 2e^-$).

Answer: A method that physically separates the iron from air and water, such as painting.

Answer: Anode.

Answer: Hydrated iron(III) oxide ($Fe_2O_3 \cdot xH_2O$).

Answer: It creates an electrolyte solution, which speeds up the movement of ions in the electrochemical cell.

Answer: Zinc.

Answer: At the center, where oxygen concentration is lowest.

Answer: It is reduced.

Answer: Iron is more reactive than tin, so the iron rusts preferentially. Zinc is more reactive than iron, so the zinc corrodes preferentially (sacrificial protection).

Answer: Sacrificial protection (or galvanising).

Answer: It becomes the cathode.

Answer: $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$.

Answer: Temporary, as the oil can be washed or worn away.

Answer: Zinc.

Answer: Iron(II) hydroxide, $Fe(OH)_2$.

Answer: No, because copper is less reactive than iron.

Answer: Rust prevention.
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