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50.10 CORROSION OF METALS

The process of slow conversion of metals into their undesirable compounds (usually oxides) by reaction with moisture and other gases present in the atmosphere is called
corrosion of metals.

For example,
(i) Iron when exposed to moist air, gets corroded and a layer of reddish brown flaky substance, known as rust, is formed on its surface. Rust is chemically hydrated iron (ill) oxide, Fe2O3.xH2O.
(ii) Copper metal on exposure to moist air gets coated with greenish white powdery substance which is basic copper carbonate, CuCO3.Cu(OH)2

(iii) Silver on exposure to air, loses it shine and its surface becomes black due to formation of silver sulphide, Ag2S.

The metals occupying higher position in the activity series undergo corrosion rapidly. Only a few metals such as gold, platinum, palladium, etc., which are at the bottom of the activity series, are resistant to corrosion.

Sometimes corrosion of metals is an advantage because it prevents the metal underneath from further corrosion. For example, aluminum when exposed to air becomes coated With a layer of aluminium oxide which protects the metal underneath from further corrosion.

Now let us study corrosion of iron (rusting) in somewhat detail:

RUSTING OF IRON

The corrosion of iron with moist air is known as rusting.

When an iron object is exposed to air in the presence of moisture, it gets corroded and reddish brown flaky substance, which is known as rust, is formed. Rust is hydrated iron (III) oxide FE2O3. xH2O. Unlike the oxide layers of other metals such as aluminium and chromium, rust does not stick to the surface and does not protect the metal from further corrosion. Rust once formed, causes more and more rusting until the whole of the metal is eaten up. Thus, rusting weakens the
structures that are made up of iron.

Moisture
4Fe + 3O2 → 2Fe2O3 xH2O
Hydrated iron (III) oxide
(Rust)

The two conditions necessary for the rusting of iron to
1. Presence of moisture, and
2. Presence of oxygen.

Conditions for rusting can be investigated by the following simple experiment.

Activity 50.1 Investigation of Conditions for Rusting

Take three test tubes and label them as A, B and C. Add a few clean iron nails to each of them. To the tube A add .tap water. To the tube B add anhydrous calcium chloride and cork it. To the tube C add boiled distilled water and then add about 1 cm3 oil and cork it. Leave the test tubes as such for a few days.
Fig.-50.6. Investigation of conditions for rusting.

Fig. 50.6. Investigation of conditions for rusting.

What do you abserve?

After a few days it is observed that the nails in the tube A rust but the nails in the tubes B and C do not
rust. In tube A both air and water are present. In tube B nails are exposed only to air while in the tube C
nails are exposed to water but the layer of oil prevents the air from dissolving in water. Thus, presence of water and air both is essential for rusting.

 

The following factors further catalyse the process of rusting.

 

1. Presence of carbon dioxide,

2. Presence of acids,

3. Presence of impurities in the iron.

Due to rusting, iron object loses its strength. Rusting can be prevented by alloying iron with other metals and non-metals. For example, stainless steel is an alloy of iron which contains about 0.05% carbon, 8% nickel and

18% chromium. It is quite hard and is resistant to corrosion.

 

Add To Your Knowledge

Anodising is a process of forming a thick layer of aluminium oxide on the surface of aluminium so as to protect it from corrosion.

Aluminium which has been exposed to atmosphere is covered with a thin layer of aluminium oxide, Al2O3 which protects the metal from further corrosion. In order to protect the aluminium even more, it is possible to increase the thickness of the oxide layer
to about 10-5 m by anodising. The aluminium is anodised by making it the anode during electrolysis of dilute sulphuric acid. Oxygen, released at the anode combines with the aluminium and thickens the oxide layer.

The oxide layer at this stage readily absorbs dyes. By carrying out the electrolytic anodising process in the presence of dyes, the anodised material can be coloured attractively.

 

  • Native state. An element is said to exist in native state, if it is found in nature in its elementary form. Less reactive elements such as gold, platinum, etc., are found in native state.

 

  • Combined state. An element is said to exist in combined state if it exists in nature in the form of its compounds. Reactive elements occur in nature in combined state.

 

  • Minerals. A mineral is a naturally occurring material obtained by mining from the earth’s crust that contains metal in its native state or combined state.

 

  • Ore. A mineral from which metal can be extracted conveniently and economically called an ore.

 

  • Gangue or Matrix. The earthly and silicious impurities associated with the ore are known as gangue or matrix.

 

  • Metallurgy. The process of extraction of pure metal from one of its ores is known as metallurgy.

 

  • Concentration. The process of removal of earthly impurities from the pulverized ore is called
    concentration or benefication of ore.
    This is also sometimes called dressing of ore.

 

  • Calcination. It is the process of heating the ore in a limited supply of air below its melting point. This process is employed for oxide ores and carbonate ores.

 

  • Roasting. It is the process of heating the ore in the excess supply of air below its melting point. This process is employed for sulphide ores.

 

  • Smelting. In this process, the roasted or calcined ore is mixed with suitable quantity of coke or charcoal (which act as reducing agent) and is heated to a high temperature
    above its melting point. During reduction, an additional reagent is also added to the ore to remove the impurities still present in the ore. This additional reagent is called flux. Flux combines with the impurities to form a fusible product called slag.

 

  • Reduction with aluminium. The process of reduction of oxides with aluminium is called aluminothermy.

 

  • Electrolytic reduction. The highly electropositive. metals such as alkali metals, alkaline earth metals and aluminium are extracted by electrolysis of their fused salts.

 

  • Liquation. This process is applied for purification of metals having low melting points, such as tin and lead.

 

  • Distillation. Metals having low boiling points, such as mercury and zinc, can be purified by distillation.

 

  • Electro-refining. Metals such as copper, silver, aluminium and gold are refined by process of
    electrolysis. In this method impure metal is made anode, while a thin strip of pure metal is made cathode and solution of some salt of the metal is used as electrolyte.

 

  • Gold occurs in native form in alluvial sand or gravel. Gold is extracted by hydrometallurgy.

 

  • Aluminium is the most abundant metal in the earth’s crust.

Its most important ore is bauxite (Al2O3 . 2H2O)

 

  • Bauxite is concentrated by leaching process. Aluminium is obtained by electrolysis of pure alumina dissolved in molten cryolite. The process is known as Hall-Heroult process.

 

  • Iron is the second most abundant metal in the earth’s crust

 

  • The important ores of iron are:

(i) Haematite, Fe2O3 (red oxide of iron)

(ii) Magnetite, Fe3O4 (Magnetic oxide of iron)

 

EVALUATE YOURSELF

I. Objective Type Questions _

 

Select the’ most appropriate choice from the options given as
(a), (b), (c) and (d) after each question:

 

1. The impurities associated with’ the ore after mining are collectively called

  (a) flux                            (b) slag

(c) minerals                  (d) gangue.

 

2. An ore after levigation is found to have acidic impurities. Which of the following can be used as flux during smelting operation?

  (a) H2SO4                     (b) CaCO3

(c) SiO2                        (d) Both CaCO3 and SiO2.

 

3. In which of the following minerals, aluminium is not present?

   (a) Fluorspar                (b) Cryolite

(c) Feldspar                  (d) Mica

 

4. The process in which metal oxide is reduced to metal by aluminium is called

(a) smelting                  (b) aluminothermy

(c) hydrothermy           (d) no specific name.

 

5. Which of the following metals can be extracted by smelting?

(a) Aluminium             (b) Magnesium

(c) Iron                        (d) None of these.

 

6. In the froth-floatation process for benefication of the ores, the ore particles float because

(a)       they are light

(b)      their surface is not easily wetted by water

(c)       they bear electrostatic charge

(d)      they are insoluble.

 

7. Which of the following benefication processes is used for the mineral bauxite ?

(a) Froth floatation                   (b) Leaching

(c) Liquation                            (d) Magnetic separation.

 

  1. Heating an ore in the absence of air below its melting point is called

       (a) leaching                              (b) roasting

(c) smelting                              (d) calcination.

 

II. Fill in the Blanks

Complete the following sentences by supplying appropriate words:

(i) Froth floatation process is generally employed for….ores.

(ii) The most abundant metal in earth’s crust is…..

(iii) The purification of crude metal is referred to as…..

(iv) The process of reduction of oxides by aluminium is known         as…..

(v) The process of removal of gaugue from ore is known       as…..

(vi) Aluminium is obtained from Al2O3 by…..reduction.

(vii) In the metallurgical process for electrorefining of the metal,        the anode is made of…. metal.

(viii) Slag is a product formed during smelting by combination           of…..and impurities.

 

III. Discussion Questions

10.       (a) Name the three most abundant metals in the decreasing order of abundance.

(b) Name any two metals which exist in native state.
(c) Define the following:

(i) mineral                (ii) are

(iii) gangue                (iv) metallurgy.

11. What is the significance of leaching in the extraction of aluminium?

12. What type of ores are concentrated by hydraulic washing?

13. Which process is generally used for the benefication of sulphide ores?

14. An ore has impurities which are attracted by magnet. Suggest process for its benefication.

15. What is the difference between calcination and roasting?

16. What name is given to carbon reduction process for extracting the metal?

17. When is electrolytic reduction applied for getting metal?

18. What is flux and in which operation is it used?

19. Explain the terms: mineral and ore. Suggest the formula of the following minerals:

(i) Bauxite                    (ii) Haematite

(iii) Magnetite

 20. Describe the froth floatation process.

21. Describe magnetic separation method for the concentration of ores.

22. What are ores? How do they differ from minerals? Justify the statement that ‘all the ores are minerals but all the minerals are not ores’.

23. What is meant by leaching? Give reactions involved during leaching of alumina from bauxite ore.

24. Describe the process of extraction of gold. What are the important uses of gold?

25. Describe how pure aluminium oxide is obtained from its ore.

26. Explain why aluminium, though an electropositive metal, finds extensive use as a structural material.

27. Give important uses of aluminium.

28. What is magnalium?

29. Give name and formula of the flux used during extraction of iron.

30. What is difference between pig iron and cast iron? How is pig iron converted into cast iron? -

31. Name the most pure form of iron. What is the percentage of carbon in it?

32. Give equations for the reactions taking place in different regions of the blast furnace during extraction of iron from haematite.

33. Name any two ores of iron.

  1. What are the components of stainless steel?
  2. What is rust? List two ways by which rusting of iron can be prevented.
  3. Explain why the surface of sonie metals acquires a dull appearance when exposed to air for a long time.
  4. What are the factors which catalyse the process of rusting? Explain, why rusting is considered harmful?
  5. Which metals do not corrode easily?
  6. Define corrosion. Name three metals which when exposed to air get corroded. Identify the substance formed after corrosion of these metals.

 

Answers

  1. I.                   Objective Type Questions

 

                        1. (d)                            2. (b)                            3. (a)

                        4. (b)                            5. (c)                            6. (b)

                        7. (b)                            8. (d)

 

  1. II.                Objective Type Questions

 

9.   (i) sulphide                  (ii)aluminium
     (iii) refining                 (iv) aluminothermy

     (v) concentration          (vi) electrolytic

     (vii) impure                  (viii) flux