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13.1 Corrosion of metals

Chapter 13. Corrosion of metals and their protection. 13.1 Corrosion of metals. 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.

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13.1 Corrosion of metals

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  1. Chapter 13 Corrosion of metals and their protection 13.1 Corrosion of metals 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. 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 Figure 13.2 Golden Bauhinia Square. The surface of bauhinia is covered by a layer of gold which does not corrode at all. 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. Think about Concept check air anhydrous calcium chloride oil layer cotton wool boiled distilled water distilled water iron nail iron nail iron nail tube 2 tube 3 tube 1 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. 13.2 Corrosion of iron — rusting

  9. Rusting refers to the corrosion of iron. For rusting to occur, two things must be present: water and air (oxygen). Key point Table 13.1 Results for the experiments shown in Figure 13.3. 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). O2(aq) + H2O(l) + 2e– 2OH–(aq) Fe(s)  Fe2+(aq) + 2e– • The dissolved oxygen and water accept the electrons to form hydroxide ions, OH– (aq). Learning tip The uneven, scratched, bent or sharp area of an iron piece usually loses electrons more readily. 13.2 Corrosion of iron — rusting

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

  14. How to increase the rate of iron rusting?

  15. 13.3 Factors that speed up rusting Presence of acidic pollutants • Acids speed up the rusting of iron. Factors that speed up rusting Figure 13.5 Rusting occurs at a higher rate in industrial areas. Learning tip Acids speed up the rusting of iron because they promote the formation of Fe2+(aq).

  16. Think about 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. 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. Class practice 13.1 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. 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. Experiment 13.1 iron nail Experiment 13.1 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. 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. Learning tip 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.

  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 13.10 Plastic is coated on (a) coat hangers and (b) paper clips. (a) (b) 13.5 Methods used to protect iron from rusting

  26. 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 zincGalvanized iron • The layer of zinc prevents iron from contacting air and water. Figure 13.12 A bucket made of galvanized iron. 13.5 Methods used to protect iron from rusting

  28. Tin-plating • 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 13.13 ‘Tin cans’ are made from iron coated with a thin layer of tin. 13.5 Methods used to protect iron from rusting

  29. 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) (b) • Figure 13.14 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 electron flow iron-made object (as cathode) graphite or platinum alloy (a conductor) electrolyte Figure 13.15 An experimental set-up illustrating the principle of cathodic protection.

  33. Concept check Sacrificial protection • During rusting, iron loses electrons to form iron(II) ions. Learning tip 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. • 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 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 broken surface zinc coating oxygen and water cannot reach iron, so no rusting occurs iron iron oxygen reacts with zinc instead of iron— no rusting Figure 13.16 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. zinc blocks (a) (b) • Figure 13.17 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. ground damp soil connecting wire electron flow bag containing a magnesium block iron pipeline Example 13.1 Figure 13.18 Protecting underground iron pipelines from rusting by sacrificial protection. It is much easier and cheaper to replace magnesium blocks than iron pipelines. 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 13.19 Stainless steel cookware. 13.5 Methods used to protect iron from rusting

  41. Class practice 13.2 STSE connections 13.1 • 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. 13.5 Methods used to protect iron from rusting

  42. 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. 13.5 Methods used to protect iron from rusting

  44. Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting

  45. Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting

  46. Activity 13.1 Experiment 13.2 Experiment 13.2 Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting

  47. Reading to learn 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 13.20 The Interstate 35W bridge over the Mississippi River in Minnesota in the United States collapsed in 2007. It was found that there was corrosion in the steel parts of the bridge.

  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. surface attacked by oxygen in the air thin protective layer of aluminium oxide aluminium aluminium Figure 13.21 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. Concept check 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. 13.7 Corrosion resistance of aluminium

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