Corrosion of metals and their protection

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 P. 1 / 64

Key terms Progress check Summary Concept map P. 2 / 64

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 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. P. 3 / 64

(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. The more reactive a metal, the more rapidly it corrodes. 13.1 Corrosion of metals P. 4 / 64

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. The reaction of a metal with air, water or other substances in the environment, leading to gradual deterioration of the metal, is called corrosion. 13.1 Corrosion of metals P. 5 / 64

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. P. 6 / 64

anhydrous calcium chloride cotton wool oil layer air iron nail tube 1 iron nail tube 2 boiled iron nail distilled water tube 3 distilled water SBA note Figure 13.3 Experimental set-up used for investigating the conditions for rusting. Anhydrous calcium chloride is a drying agent. It removes water (moisture) from the air. Concept check Think about 13.2 Corrosion of iron rusting P. 7 / 64

Tube Has rusting occurred? 1 No 2 No 3 Yes Key point Explanation Anhydrous calcium chloride removes moisture from the air. Hence, tube 1 has air but no water. 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. 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). 13.2 Corrosion of iron rusting P. 8 / 64

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, Fe 2+ (aq). Fe(s) Fe 2+ (aq) + 2e The dissolved oxygen and water accept the electrons to form hydroxide ions, OH (aq). O 2 (aq) + H 2 O(l) + 2e 2OH (aq) 13.2 Corrosion of iron rusting P. 9 / 64

Learning tip The process of rusting involves redox reactions, which will be further discussed in Book 3B, Chapter 30. Learning tip The uneven, scratched, bent or sharp area of an iron piece usually loses electrons more readily. 13.2 Corrosion of iron rusting P. 10 / 64

air water film iron electron flow Figure 13.4 A diagram illustrating the initial stage of rusting. The Fe 2+ (aq) and OH (aq) ions formed then combine in the water film, forming iron(ii) hydroxide precipitate, Fe(OH) 2 (s). Fe 2+ (aq) + 2OH (aq) Fe(OH) 2 (s) 13.2 Corrosion of iron rusting P. 11 / 64

The precipitate further reacts with dissolved oxygen and water, forming iron(iii) hydroxide, Fe(OH) 3 (s). 4Fe(OH) 2 (s) + O 2 (aq) + 2H 2 O(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) + 3O 2 (g) + 2nH 2 O(l) 2Fe 2 O 3 nh 2 O(s) 13.2 Corrosion of iron rusting hydrated iron(iii) oxide (rust) (reddish brown solid) P. 12 / 64

Learning tip Hydrated means combined with water. In hydrated iron(iii) oxide, the number of water molecules combined with the iron(iii) oxide varies. n is a variable number. Rusting continues until the iron piece corrodes completely. Key point Rust is in fact hydrated iron(iii) oxide (Fe 2 O 3 nh 2 O), where n is a variable number. It is a reddish brown solid. 13.2 Corrosion of iron rusting P. 13 / 64

13.3 Factors that speed up rusting Presence of acidic pollutants Acids speed up the rusting of iron. Learning tip 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 Fe 2+ (aq). Factors that speed up rusting P. 14 / 64

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 Think about P. 15 / 64

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 P. 16 / 64

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 P. 17 / 64

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 ironmade object. 13.3 Factors that speed up rusting Class practice 13.1 P. 18 / 64

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) K 3 [Fe(CN) 6 ], phenolphthalein, and sodium chloride. P. 19 / 64

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 P. 20 / 64

iron nail warm gel containing rust indicator (a) Petri dish 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. (b) Experiment 13.1 Experiment 13.1 13.4 To observe the rusting of iron using rust indicator P. 21 / 64

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. P. 22 / 64

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. (a) (b) Figure 13.9 (a) The fence and (b) the car body are painted to prevent rusting. 13.5 Methods used to protect iron from rusting P. 23 / 64

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. (a) (b) Figure 13.10 Plastic is coated on (a) coat hangers and (b) paper clips. 13.5 Methods used to protect iron from rusting P. 24 / 64

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. (a) (b) 13.5 Methods used to protect iron from rusting P. 25 / 64 Figure 13.11 (a) Bicycle gear and chains and (b) woodworking tools are oiled or greased to prevent rusting.

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 13.12 A bucket made of galvanized iron. 13.5 Methods used to protect iron from rusting P. 26 / 64

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 P. 27 / 64

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 P. 28 / 64

(a) (b) Figure 13.14 The corrosion resistant chromium protects the iron underneath from rusting. (a) A chromium-plated water tap. (b) A motor cycle with chromium plated parts. 13.5 Methods used to protect iron from rusting P. 29 / 64

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 ironmade object prevents iron from losing electrons 13.5 Methods used to protect iron from rusting P. 30 / 64

Cathodic protection has many uses: To protect car body, underground water pipelines and storage tanks, and the steel pier legs d.c. power supply iron-made object (as cathode) electron flow graphite or platinum alloy (a conductor) electrolyte Figure 13.15 An experimental set-up illustrating the principle of cathodic protection. P. 31 / 64

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. 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 13.5 Methods used to protect iron from rusting P. 32 / 64 Concept check

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 sacrificed to save iron zinc is 13.5 Methods used to protect iron from rusting P. 33 / 64

zinc coating zinc coating broken surface iron oxygen and water cannot reach iron, so no rusting occurs iron Figure 13.16 Sacrificial protection of iron by zinc. oxygen reacts with zinc instead of iron no rusting Galvanized iron is not used in making food cans because zinc ions are poisonous. 13.5 Methods used to protect iron from rusting P. 34 / 64

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 P. 35 / 64

zinc blocks (a) (b) Figure 13.17 The zinc blocks need to be replaced regularly, but this is certainly cheaper than replacing the ship. (a)the ship is protected from corrosion by zinc blocks. (b)zinc blocks for attachment to the ship hull. 13.5 Methods used to protect iron from rusting P. 36 / 64

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 P. 37 / 64

damp soil connecting wire ground iron pipeline electron flow bag containing a magnesium block 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 Example 13.1 P. 38 / 64

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 P. 39 / 64

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 P. 40 / 64

Method of rust prevention (a) Painting (b) Coating with plastic (c) Oiling or greasing Simple chemistry the added layer prevents the iron object from contact with air and water Advantage(s) Disadvantage(s) Example(s) cheap lasts long looks good does not fall off like paint or plastic has lubricating effect Table 13.2 Different methods of rust prevention. scratched off easily more expensive than painting not once and for all dirt would stick to oil or grease bridges, ships, fences, car bodies coat hangers, paper clips moving parts of machines, woodworking tools 13.5 Methods used to protect iron from rusting P. 41 / 64

Method of rust prevention (d) Galvanizing (e) Tin-plating (f) Electroplating (e.g. chromiumplating) Simple chemistry the added layer prevents the iron object from contact with air and water Advantage(s) in case the zinc coating is damaged, the iron is still protected tin is corrosion resistant tin and tin ions are not poisonous has a beautiful shiny appearance Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting Disadvantage(s) Example(s) zinc ions are poisonous when the tin coating is damaged, rusting will occur more quickly than iron alone quite expensive P. 42 / 64 galvanized iron plate used in construction, buckets tin cans for storing food water taps, car bumpers

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, underground water pipelines, storage tanks, steel pier legs Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting P. 43 / 64

Method of rust prevention Simple chemistry Advantage(s) Disadvantage(s) Example(s) (h) Sacrificial protection a more reactive metal (e.g. magnesiu m, zinc) in contact is sacrificed to form ions; this would prevent iron from forming iron(ii) ions an effective way of protection Table 13.2 Different methods of rust prevention. 13.5 Methods used to protect iron from rusting the sacrificed metal needs replacement from time to time galvanized iron, zinc blocks attached to the ship hull, magnesium blocks connected to the underground pipelines P. 44 / 64

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 Table 13.2 Different methods of rust prevention. most expensive cookware, cutlery Experiment 13.2 Activity 13.1 Experiment 13.2 13.5 Methods used to protect iron from rusting P. 45 / 64

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. Reading to learn P. 46 / 64

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. P. 47 / 64

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 P. 48 / 64

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 Concept check P. 49 / 64

Overall chemical equation of aluminium anodization: 2Al(s) + 3H 2 O(l) Al 2 O 3 (s) + 3H 2 (g) aluminium sheet (as cathode) dilute sulphuric acid (as electrolyte) aluminium object to be anodized (as anode) Figure 13.22 Experimental set-up for anodization. 13.7 Corrosion resistance of aluminium P. 50 / 64

Learning tip Beside an aluminium sheet, a graphite rod can be used as the negative electrode of the set-up for anodization. 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 P. 51 / 64

Figure 13.23 (left) The milk caps and the packaging of chocolate are made of anodized aluminium. Figure 13.24 (right) Window frames made of anodized aluminium. 13.7 Corrosion resistance of aluminium Class practice 13.3 P. 52 / 64

Key terms 1. anodization 陽極電鍍 2. cathodic protection 陰極保護 3. corrosion 腐蝕作用 4. electroplating 電鍍 5. galvanized iron 鍍鋅鐵 6. galvanizing 鍍鋅 7. rust 鐵銹 8. rust indicator 鐵銹指示劑 9. rusting 銹蝕 P. 53 / 64

10. sacrificial protection 犧牲性保護 11. stainless steel 不銹鋼 12. tin-plating 鍍錫 Key terms P. 54 / 64

Progress check 1. What is the meaning of corrosion? 2. What is the meaning of rusting? 3. What are the essential conditions for rusting? 4. What are the factors that speed up rusting? 5. What can we observe when rusting is shown by a rust indicator? 6. What methods can we employ to prevent rusting? How does each of these methods work? 7. What are the socio-economic impacts of rusting? P. 55 / 64

8. Why is aluminium more corrosion resistant than expected? 9. How can we enhance the corrosion resistance of aluminium? Progress check P. 56 / 64

Summary 13.1 Corrosion of metals 1. Corrosion is the gradual deterioration of a metal due to reaction with air, water or other substances in the surroundings. 2. In general, a metal higher in the metal reactivity series corrodes faster. 13.2 Corrosion of iron rusting 3. 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, Fe 2 O 3 nh 2 O. P. 57 / 64

13.3 Factors that speed up rusting 4. 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 ironmade object Summary P. 58 / 64

13.4 To observe the rusting of iron using rust indicator 5. 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 6. 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 P. 59 / 64

13.6 Socio-economic implications of rusting 7. Rusting causes socio-economic problems and rust prevention methods have to be developed. 13.7 Corrosion resistance of aluminium 8. Aluminium is resistant to corrosion because it has a protective oxide layer on its surface. 9. 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 P. 60 / 64

Concept map Acidic pollutants Soluble ionic compounds High temperature Scratched, bent or sharp area Less reactive metal contact with a presence of speeded up by Gradual deterioration of metals Rusting is called CORROSION of iron is called P. 61 / 64

Gradual deterioration of metals is called CORROSION Aluminium can be protected by Anodization Rusting of iron is called conditions required Air (oxygen) Water Concept map P. 62 / 64

can be prevented/ slowed down by Rusting Protective layer coating with Paint Plastics Oil/ grease Other metals example Tin Concept map P. 63 / 64

Rusting can be prevented/ slowed down by Using alloys of iron example Stainless steel Sacrificial protection example Galvanizing Cathodic protection Concept map P. 64 / 64