1. Chapter 13 Corrosion of metals and their protection
13.2 Corrosion of iron — rusting
13.3 Factors that speed up rusting
13.4 To observe the rusting of iron using rust indicator
13.5 Methods used to protect iron from rusting
13.6 Socio-economic implications of rusting
13.7 Corrosion resistance of aluminium

2. Key terms
Progress check
Summary
Concept map

3. 13.1 Corrosion of metals
Corrosion: gradual deterioration of a metal, resulting from the reaction of the metal with air, water or other substances in the environment
(a)
(b)
(c)
Figure 13.1 Some corroded metal objects.
(a) Corroded iron water pipes. Some reddish brown solid is found on their surface.
(b) Corroded copper roofs. Some green solid is found on their surface.
(c) Corroded silver spoons. Due to corrosion, they have become tarnished.

4. The more reactive a metal, the more rapidly it corrodes.
13.1 Corrosion of metals

5. Learning tip
When a metal corrodes, metal atoms at the surface lose electrons and change to metal ions. As a result, a solid (which is usually a metal oxide), forms on the metal surface.

6. Figure 13. 2 Golden Bauhinia Square
Figure 13.2 Golden Bauhinia Square. The surface of bauhinia is covered by a layer of gold which does not corrode at all.
The reaction of a metal with air, water or other substances in the environment, leading to gradual deterioration of the metal, is called corrosion.
Key point
13.1 Corrosion of metals

7. 13.2 Corrosion of iron — rusting
Conditions for rusting
Rusting: corrosion of iron
To find out whether both water and air are necessary for rusting to occur, prepare the tubes as shown in Figure 13.3.
The test tubes are allowed to stand for several days.

8. tube 1
anhydrous calcium
chloride
cotton wool
iron nail
tube 2
iron nail
oil layer
boiled
distilled water
tube 3
distilled
water
air
iron nail
Figure 13.3 Experimental set-up used for investigating the conditions for rusting.
SBA note
Anhydrous calcium chloride is a drying agent. It removes water (moisture) from the air.
Concept check
Think about
13.2 Corrosion of iron — rusting

9. Tube
Has rusting occurred?
Explanation 1 No
Anhydrous calcium chloride removes moisture from the air. Hence, tube 1 has air but no water. 2
Boiling the water removes dissolved air. The oil layer on top prevents air from dissolving in water again. Hence, tube 2 has water but no air. 3
Yes
Distilled water contains dissolved air. Thus, both water and air are present.
Table 13.1 Results for the experiments shown in Figure 13.3.
Rusting refers to the corrosion of iron. For rusting to occur, two things must be present: water and air (oxygen).
Key point
13.2 Corrosion of iron — rusting

10. Chemistry of rusting Rusting is a slow and complex chemical process.
In the initial stage of rusting, some iron atoms lose electrons to form iron(II) ions, Fe2+(aq).
Fe(s)  Fe2+(aq) + 2e–
The dissolved oxygen and water accept the electrons to form hydroxide ions, OH– (aq).
O2(aq) + H2O(l) + 2e–  2OH–(aq)
The uneven, scratched, bent or sharp area of an iron piece usually loses electrons more readily.
Learning tip
13.2 Corrosion of iron — rusting

11. Fe2+(aq) + 2OH–(aq)  Fe(OH)2(s)
air
water film
iron
electron flow
Figure 13.4 A diagram illustrating the initial stage of rusting.
The Fe2+(aq) and OH–(aq) ions formed then combine in the water film, forming iron(II) hydroxide precipitate, Fe(OH)2(s).
Fe2+(aq) + 2OH–(aq)  Fe(OH)2(s)
13.2 Corrosion of iron — rusting

12. hydrated iron(III) oxide (rust)
The precipitate further reacts with dissolved oxygen and water, forming iron(III) hydroxide, Fe(OH)3(s).
4Fe(OH)2(s) + O2(aq) + 2H2O(l)  4Fe(OH)3(s)
Iron(III) hydroxide changes to hydrated iron(III) oxide rust
The overall reaction can be represented by the following equation:
4Fe(s) + 3O2(g) + 2nH2O(l)  2Fe2O3nH2O(s)
hydrated iron(III) oxide (rust)
(reddish brown solid)
13.2 Corrosion of iron — rusting

13. Rusting continues until the iron piece corrodes completely.
Rust is in fact hydrated iron(III) oxide (Fe2O3nH2O), where n is a variable number. It is a reddish brown solid.
Key point
13.2 Corrosion of iron — rusting

14. How to increase the
rate of iron rusting?

15. Factors that speed up rusting
Presence of acidic pollutants
Acids speed up the rusting of iron.
Figure 13.5 Rusting occurs at a higher rate in industrial areas.
Acids speed up the rusting of iron because they promote the formation of Fe2+(aq).
Learning tip
Factors that speed up rusting

16. Presence of soluble ionic compounds
Soluble ionic compounds, such as sodium chloride, also speed up the rusting of iron.
Figure 13.6 Iron-made objects near the seashore rust more quickly.
Think about
13.3 Factors that speed up rusting

17. High temperature
An increase in temperature always increases the rate of chemical reactions, including rusting.
car exhaust pipe
Figure 13.7 The car exhaust pipe corrodes easily.
13.3 Factors that speed up rusting

18. Attachment of less reactive metals to iron
When iron is attached to a metal lower in the reactivity series (such as tin or copper), rusting becomes faster.
Learning tip
Tin is less reactive than iron but more reactive than lead.
13.3 Factors that speed up rusting

19. Scratched, bent or sharp area of an iron-made object
Rusting also becomes faster where the iron surface is scratched or bent.
Rusting occurs faster at the sharp area of an iron-made object.
Class practice 13.1
13.3 Factors that speed up rusting

20. 13.4 To observe the rusting of iron using rust indicator
Place an iron nail in a warm gel containing a rust indicator.
Rust indicator contains
potassium hexacyanoferrate(III) K3[Fe(CN)6],
phenolphthalein, and

21. When iron rusts, iron(II) ions and hydroxide ions form.
Iron(II) ions turn potassium hexacyanoferrate(III) blue.
Hydroxide ions turn phenolphthalein pink.
Appearance of blue and pink colours near the iron nail indicates that rusting has occurred.
13.4 To observe the rusting of iron using rust indicator

22. iron nail Petri dish warm gel containing rust indicator (a) (b)
Figure 13.8 Investigating rusting of an iron nail using a rust indicator.
(a) An iron nail (before rusting) is placed in a Petri dish with a rust indicator added.
(b) The iron nail rusts. The appearance of blue colour near the iron nail indicates the presence of iron(II) ions, while the appearance of pink colour indicates the presence of hydroxide ions.
Experiment 13.1
Experiment 13.1
13.4 To observe the rusting of iron using rust indicator

23. 13.5 Methods used to protect iron from rusting
Several methods can be used to protect iron from rusting or to slow down the rusting process.
Steel is an alloy of iron. It is produced by mixing 0.15% to 1.5% carbon with iron. It can also undergo rusting as it contains iron.
Learning tip

24. Applying a protective layer
Coating with paint, plastic, oil or grease
A layer of paint prevents the iron from contacting air and water cheap way
This method can be applied to bridges, ships, car bodies, fences and other large iron-made objects.
Figure 13.9
(a) The fence and (b) the car body are painted to prevent rusting.
(a)
(b)
13.5 Methods used to protect iron from rusting

25. Small iron-made objects like coat hangers and paper clips are often protected by coating them with a layer of plastic.
Objects coated with plastics can look better and last longer but is more expensive than painting.
Figure Plastic is coated on (a) coat hangers and (b) paper clips.
(a)
(b)
13.5 Methods used to protect iron from rusting

26. ∵ the paint or plastic would be scratched off easily.
Moving parts of machines and woodworking tools are not painted or coated with plastic.
∵ the paint or plastic would be scratched off easily.
They are protected from rusting by oiling or greasing.
Oil and grease can serve as a lubricant.
Figure 13.11
(a) Bicycle gear and chains and (b) woodworking tools are oiled or greased to prevent rusting.
(a)
(b)
13.5 Methods used to protect iron from rusting

27. Coating with another metal
Galvanizing (zinc-plating)
Galvanizing: to coat the surface of iron with a thin layer of zinc Galvanized iron
The layer of zinc prevents iron from contacting air and water.
Figure A bucket made of galvanized iron.
13.5 Methods used to protect iron from rusting

28. Tin-plating: to coat the surface of iron with a thin layer of tin
It protects iron from rusting by preventing it from contacting air and water.
Tin-plating is commonly used in making food cans since tin and tin ions are not poisonous.
Figure ‘Tin cans’ are made from iron coated with a thin layer of tin.
13.5 Methods used to protect iron from rusting

29. Common metal to be electroplated: Chromium
Electroplating
Electroplating: an electrical process in which a thin layer of metal is plated on an object
Common metal to be electroplated: Chromium
Iron plated with chromium has a beautiful shiny appearance but this method is quite expensive.
13.5 Methods used to protect iron from rusting

30. A chromium-plated water tap. A motor cycle with chromium plated parts.
(b)
Figure The corrosion resistant chromium protects the iron underneath from rusting.
A chromium-plated water tap.
A motor cycle with chromium plated parts.
13.5 Methods used to protect iron from rusting

31. Cathodic protection
Iron can be protected from rusting by cathodic protection.
An iron-made object is connected to the negative terminal of a d.c. power supply.
A conductor (such as graphite or platinum alloy) is connected to the positive terminal.
The battery supplies electrons to the iron-made object prevents iron from losing electrons
13.5 Methods used to protect iron from rusting

32. Cathodic protection has many uses:
To protect car body, underground water pipelines and storage tanks, and the steel pier legs
d.c. power supply
graphite or platinum alloy (a conductor)
iron-made object (as cathode)
electron flow
electrolyte
Figure An experimental set-up illustrating the principle of cathodic protection.

33. Sacrificial protection
The electrode connected to the negative terminal of the d.c. power supply is called the cathode, which will be further discussed in Book 3B, Chapter 31.
Learning tip
Sacrificial protection
During rusting, iron loses electrons to form iron(II) ions.
If iron is attached or connected to a more reactive metal, that metal will lose electrons more readily than iron prevent iron from forming iron(II) ions
Sacrificial protection
Concept check
13.5 Methods used to protect iron from rusting

34. 1. Galvanized iron
When the zinc coating of galvanized iron is undamaged, the iron is protected from rusting.
In case the coating is partly scratched, the exposed iron is still protected
∵ zinc is more reactive than iron
Zinc will corrode instead of iron zinc is ‘sacrificed’ to ‘save’ iron
13.5 Methods used to protect iron from rusting

35. zinc coating
iron
oxygen and water cannot reach iron, so no rusting occurs
broken surface
oxygen reacts with zinc instead of iron — no rusting
Figure Sacrificial protection of iron by zinc.
Galvanized iron is not used in making food cans because zinc ions are poisonous.
13.5 Methods used to protect iron from rusting

36. 2. Attaching zinc blocks to ship hulls
Most ships are made of steel.
To protect steel from corrosion, zinc blocks are attached to the ship hull.
Zinc will corrode instead of iron.
Zinc blocks need to be replaced before they have completely corroded.
13.5 Methods used to protect iron from rusting

37. The ship is protected from corrosion by zinc blocks.
(a)
(b)
Figure The zinc blocks need to be replaced regularly, but this is certainly cheaper than replacing the ship.
The ship is protected from corrosion by zinc blocks.
Zinc blocks for attachment to the ship hull.
13.5 Methods used to protect iron from rusting

38. 3. Connecting magnesium blocks to underground pipelines
Sacrificial protection is also used to protect underground iron pipelines from rusting.
Magnesium blocks are connected to the underground pipelines.
Magnesium corrodes instead of iron.
Magnesium blocks should therefore be replaced from time to time.
13.5 Methods used to protect iron from rusting

39. bag containing a magnesium block damp soil connecting wire
Figure Protecting underground iron pipelines from rusting by sacrificial protection. It is much easier and cheaper to replace magnesium blocks than iron pipelines.
ground
bag containing a magnesium block
damp soil
connecting wire
electron flow
iron pipeline
Example 13.1
13.5 Methods used to protect iron from rusting

40. Using alloys of iron Stainless steel is an alloy of iron.
It is produced by mixing the right amounts of carbon (0.15–1.5%) and other metals (such as chromium, nickel and manganese) with iron.
Figure Stainless steel cookware.
13.5 Methods used to protect iron from rusting

41. Stainless steel is corrosion resistant.
∵ A layer of chromium(III) oxide is formed on the surface.
The oxide layer is very tough and can protect the iron underneath from contacting air and water.
Alloying is the most expensive rust prevention method.
STSE connections 13.1
Class practice 13.2
13.5 Methods used to protect iron from rusting

42. Method of rust prevention Simple chemistry Advantage(s)
Disadvantage(s)
Example(s)
(a) Painting
the added layer prevents the iron object from contact with air and water
• cheap
• scratched off easily
bridges, ships, fences, car bodies
(b) Coating with plastic
• lasts long
• looks good
• more expensive than painting
coat hangers, paper clips
(c) Oiling or greasing
• does not fall off like paint or plastic
• has lubricating effect
• not ‘once and for all’
• dirt would stick to oil or grease
moving parts of machines, wood-working tools
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting

43. Table 13.2 Different methods of rust prevention.
Method of rust prevention
Simple chemistry
Advantage(s)
Disadvantage(s)
Example(s)
(d) Galvanizing
the added layer prevents the iron object from contact with air and water
• in case the zinc coating is damaged, the iron is still protected
• zinc ions are poisonous
galvanized iron plate used in construction, buckets
(e) Tin-plating
• tin is corrosion resistant
• tin and tin ions are not poisonous
• when the tin coating is damaged, rusting will occur more quickly than iron alone
‘tin cans’ for storing food
(f) Electroplating (e.g. chromium- plating)
• has a beautiful shiny appearance
• quite expensive
water taps, car bumpers
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting

44. Method of rust prevention Simple chemistry Advantage(s)
Disadvantage(s)
Example(s)
(g) Cathodic protection
the negative terminal of an electric source is connected to the iron object, supplying electrons to prevent it from rusting
• convenient
• not applicable to many objects
car bodies, under-ground water pipelines, storage tanks, steel pier legs
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting

45. Method of rust prevention Simple chemistry Advantage(s)
Disadvantage(s)
Example(s)
(h) Sacrificial protection
a more reactive metal (e.g. magnesium, zinc) in contact is ‘sacrificed’ to form ions; this would prevent iron from forming iron(II) ions
• an effective way of protection
• the ‘sacrificed’ metal needs replacement from time to time
galvanized iron, zinc blocks attached to the ship hull, magnesium blocks connected to the under-ground pipelines
Table 13.2 Different methods of rust prevention.
13.5 Methods used to protect iron from rusting

46. Method of rust prevention Simple chemistry Advantage(s)
Disadvantage(s)
Example(s)
(i) Using alloys of iron
iron is alloyed with carbon, chromium, nickel and manganese which are corrosion resistant
• has a beautiful appearance
• a very effective way of protection
• most expensive
cookware, cutlery
Table 13.2 Different methods of rust prevention.
Experiment 13.2
Activity 13.1
Experiment 13.2
13.5 Methods used to protect iron from rusting

47. 13.6 Socio-economic implications of rusting
Huge sums of money are spent every year to prevent rusting and replace rusted objects.
Rusting also causes damages to buildings and even loss of human lives.
Figure The Interstate 35W bridge over the Mississippi River in Minnesota in the United States collapsed in It was found that there was corrosion in the steel parts of the bridge.
Reading to learn

48. 13.7 Corrosion resistance of aluminium
Protective oxide layer on aluminium
When aluminium is exposed to air, a thin but tough layer of aluminium oxide forms on its surface.
This layer is impermeable to air and water and hence can protect the aluminium underneath from further corrosion.
Aluminium appears to be less reactive than it really is.

49. The protective oxide layer on aluminium is very thin.
surface attacked by oxygen in the air
thin protective layer of aluminium oxide
aluminium
aluminium
Figure Explaining the corrosion resistance of aluminium.
The protective oxide layer on aluminium is very thin.
It can be thickened by a process called anodization.
13.7 Corrosion resistance of aluminium

50. Thickening the protective oxide layer on aluminium
An aluminium sheet is rolled into cylindrical shape and made the negative electrode (cathode).
The aluminium object to be anodized is made the positive electrode (anode).
Dilute sulphuric acid is the electrolyte.
During anodization, a layer of aluminium oxide forms on the surface of the aluminium object.
the oxide layer on the object is thickened.
Concept check
13.7 Corrosion resistance of aluminium

51. Overall chemical equation of aluminium anodization:
2Al(s) + 3H2O(l)  Al2O3(s) + 3H2(g)
aluminium sheet (as cathode)
aluminium object to be anodized (as anode)
dilute sulphuric acid (as electrolyte)
Figure Experimental set-up for anodization.
13.7 Corrosion resistance of aluminium

52. Anodized aluminium can be dyed easily to give attractive colours.
Beside an aluminium sheet, a graphite rod can be used as the negative electrode of the set-up for anodization.
Learning tip
After anodization, the aluminium object becomes more corrosion resistant.
Anodized aluminium can be dyed easily to give attractive colours.
13.7 Corrosion resistance of aluminium

53. Figure 13.24 (right) Window frames made of anodized aluminium.
Figure (left) The milk caps and the packaging of chocolate are made of anodized aluminium.
Figure (right) Window frames made of anodized aluminium.
Class practice 13.3
13.7 Corrosion resistance of aluminium

54. Key terms anodization 陽極電鍍 cathodic protection 陰極保護 corrosion 腐蝕作用
electroplating 電鍍
galvanized iron 鍍鋅鐵
galvanizing 鍍鋅
rust 鐵銹
8. rust indicator 鐵銹指示劑
9. rusting 銹蝕

55. sacrificial protection 犧牲性保護 stainless steel 不銹鋼 tin-plating 鍍錫
Key terms

56. Progress check What is the meaning of corrosion?
What is the meaning of rusting?
What are the essential conditions for rusting?
What are the factors that speed up rusting?
What can we observe when rusting is shown by a rust indicator?
What methods can we employ to prevent rusting? How does each of these methods work?
What are the socio-economic impacts of rusting?

57. Why is aluminium more corrosion resistant than expected?
How can we enhance the corrosion resistance of aluminium?
Progress check

58. Summary 13.1 Corrosion of metals 13.2 Corrosion of iron — rusting
Corrosion is the gradual deterioration of a metal due to reaction with air, water or other substances in the surroundings.
In general, a metal higher in the metal reactivity series corrodes faster.
13.2 Corrosion of iron — rusting
Rusting is the corrosion of iron. Rusting requires the exposure of iron to both water and air. Rust is in fact hydrated iron(III) oxide, Fe2O3nH2O.

59. 13.3 Factors that speed up rusting
Factors that speed up rusting include:
Presence of acidic pollutants or soluble ionic compounds
High temperature
Attachment of less reactive metals to iron
Scratched, bent or sharp area of an iron-made object
Summary

60. 13.4 To observe the rusting of iron using rust indicator
We can observe rusting conveniently using a rust indicator. It shows blue and pink colours where rusting occurs.
13.5 Methods used to protect iron from rusting
To prevent rusting, we can make use of a suitable method. Refer to Table 13.2 on p.17 for different methods of rust prevention.
Summary

61. 13.6 Socio-economic implications of rusting
Rusting causes socio-economic problems and rust prevention methods have to be developed.
13.7 Corrosion resistance of aluminium
Aluminium is resistant to corrosion because it has a protective oxide layer on its surface.
Anodization is a process used to thicken the aluminium oxide layer on aluminium. Anodized aluminium becomes more corrosion resistant, and can be easily dyed to give attractive colours.
Summary

62. Gradual deterioration
Concept map
Gradual deterioration
of metals
Acidic
pollutants
presence of
is called
CORROSION
Soluble ionic
compounds
of iron is called
speeded up by
High
temperature
Rusting
Scratched, bent
or sharp area
contact with a
Less reactive
metal

63. Gradual deterioration
of metals
is called
can be protected by
CORROSION
Aluminium
Anodization
of iron is called
Air (oxygen)
Rusting
conditions required
Water
Concept map

64. Protective layer Plastics Tin
Rusting
can be prevented/ slowed down by
coating with
Protective
layer
Paint
Plastics
Oil/ grease
Other metals
example
Tin
Concept map

65. Cathodic protection Stainless steel Galvanizing
Rusting
can be prevented/ slowed down by
Using alloys of iron
Sacrificial
protection
Cathodic protection
example
example
Stainless steel
Galvanizing
Concept map