Rusting as a Redox Reaction

• Rust / hydrated iron(III) oxide, Fe₂O₃•xH₂O – formed slowly at the surface of iron when it exposed to the damp air.

• Rusting – a redox reaction that take places between iron and oxygen to form hydrated iron(III) oxide and this is a slow reaction.
  4Fe(s) + 3O₂(g) + 2xH₂O(l) –> Fe₂O₃•xH₂O(s)

Corrosion – a redox reaction that take places between a metal and the gases in air. Metal is oxidised to form an oxide layer on the surface. Metal atoms lose electrons to form positive ions.

1. Group 1 metals are very reactive.
2. Metals are exposed to air will corrode rapidly and become tarnished.
3. Aluminium, lead and zinc corrode rapidly in the air and forms an oxide layer. The oxide layer is hard, non-porous, impermeable and difficult to crack. This protects the aluminium, lead and zinc below it from further corrosion.

Example: Corrosion of metal.
Zn(s) –> Zn²⁺(aq) + 2e
Cu(s) –> Cu²⁺(aq) + 2e

K, Na, Ca, Mg, Al, Zn, Fe, Sn, Pb, Cu, Hg, Ag, Au
<————Tendency of metal to corrode increases.

Process of Rusting of Iron

1. Anode (negative terminal) – concentration of oxygen is lower and iron rust (oxidation process) to form iron(II) ions:
   Fe(s) –> Fe²⁺(aq) + 2e
2. Cathode (positive terminal) – concentration of oxygen is higher and oxygen gains electrons that reduced to hydroxide ions:
   O₂(g) + 2H₂O(l) + 4e –> 4OH^-(aq)
3. Fe²⁺ ions and OH^- ions combine to form iron(II) hydroxide, Fe(OH)₂
4. Oxygen further oxidises iron(II) hydroxide, Fe(OH)₂ to hydrated iron(III) oxide, Fe₂O₃•xH₂O.
   2Fe(OH)₂(s) –> Fe₂O₃•xH₂O(s)

The Effect of Other Metals on the Rusting of Iron
Potassium hexacyanoferrate(III), K₃Fe(NO)₆ is used to detect Fe²⁺ ions
(produces dark blue colour in the presence of Fe²⁺).
Phenolphthalein is used to detect OH^- ions
(produces pink colour in the presence of OH^-).

Test tube	Observation	Reaction
Fe only	Intensity of blue colour is low.	Oxidation:Fe(s) –> Fe2+(aq) + 2e
Control	Pink colour is not present.	Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
	Inference	Fe nail rusts a little.

Test tube	Observation	Reaction
Fe & Mg	Blue colour is not present.	Oxidation:Mg(s) –> Mg2+(aq) + 2e
	Intensity of pink colour is very high.	Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)
	Inference	Mg is corroded and Fe nail does not rust.

i) Fe act as the (+) terminal (cathode)
ii) Mg act as the (-) terminal (anode)

Test tube	Observation	Reaction
Fe & Zn	Blue colour is not present.	Oxidation:Zn(s) –> Zn2+(aq) + 2e
	Intensity of pink colour is high.	Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)
	Inference	Zn is corroded and Fe nail does not rust.

i) Fe act as the (+) terminal (cathode)
ii) Zn act as the (-) terminal (anode)

Test tube	Observation	Reaction
Fe & Sn	Intensity of blue colour is high.	Oxidation:Fe (s) –> Fe2+(aq) + 2e
	Pink colour is not present.	Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
	Inference	Fe nail rusts quickly (high rate).

i) Sn act as the (+) terminal (cathode)
ii) Fe act as the (-) terminal (anode)

Test tube	Observation	Reaction
Fe & Cu	Intensity of blue colour is very high.	Oxidation:Fe (s) –> Fe2+(aq) + 2e
	Pink colour is not present.	Reduction:O2(g) + 2H2O(l) + e –> 4OH-(aq)OH- ions react with Fe2+ ions to form Fe(OH)2.
	Inference	Fe nail rusts very quickly (the highest rate).

i) Cu act as the (+) terminal (cathode)
ii) Fe act as the (-) terminal (anode)

• Iron nail does not rust if it has contacted with more electropositive metals (Mg and Zn).
• Iron nail rusts quickly if it has contacted with less electropositive metals (Sn and Cu).

Prevention of Rusting of Iron
The rate of rusting of iron decreases if the iron (Fe) in contact with any of these metals: K, Na, Ca, Mg, Al and Zn.
The rate of rusting of iron increases if the iron (Fe) in contact

• with any of these metals: Sn, Pb, Cu, Hg, Ag and Au.
• a strong electrolyte (salt and acid) is present.

Ways Used for Prevention of Rusting

1. Painting – Protect iron surface (prevent from contacting with air and water)
2. Coat with plastic – Used in metal netting
3. Apply oil and grease – Protective coating for machine part
4. Alloying the iron – Alloying the iron with 18% chromium and 8% nickel that provide a protective oxide coating.
5. a) Tin plating (less electropositive metal) – Cans of food (iron) is covered with a thin layer of tin to provide a protective oxide coating to the cans.
   b) Chrome plating
6. Cathodic protection / Electrical protection (more electropositive metal)
   a) Galvanising (coat with zinc metal) – Zinc layer provides a protective oxide coating and zinc is oxidized instead of iron. Iron cannot form ions, so it will not rust.
   b) Sacrificial protection – Blocks of magnesium are attached at the intervals of the water piping system & zinc bars are attached to the part of the ship submerged in sea water.