Corrosion and Control: Important Two Marks Questions and Answers

UNIT 4: Corrosion and Control

Two Marks Questions and Answers

 

 

1. What is corrosion? What are its types.

Corrosion is defined as the gradual destruction or deterioration of metals or alloys by the chemical or electrochemical reaction with its environment.

Based on the environment, corrosion is classified into

(i) Dry (or) chemical corrosion, and

(ii) Wet (or) electrochemical corrosion.

 

2. What is dry corrosion? Give one example.

Dry corrosion is the corrosion occurs due to the attack of metal surfaces by the atmospheric gases such as oxygen, hydrogen sulphide, sulphur dioxide, nitrogen, etc.

Example: Oxidation corrosion

Anode: M → M²⁺ + 2e^‒

Cathode: ½ O₂ + 2e⁻ → O^2‒

Overall reaction: M + ½O₂ → M²⁺ + O^2‒

 

3. Explain the consequences of corrosion.

1. Due to formation of corrosion product over the machinery, the efficiency of the machine gets lost.

2. The products gets contaminated due to corrosion.

3. The corroded equipment must be replaced frequently.

4. Plant gets failure due to corrosion.

5. It is necessary for over design to compensate for the corrosion.

6. Corrosion releases toxic products, health hazard, etc.

 

4. State Pilling‒Bedworth rule. What is its importance.

(a) According to Pilling‒Bedworth rule, if the volume of the oxide layer formed is less than the volume of the metal, the oxide layer is porous and non‒protective.

(b) On the other hand, if the volume of the oxide layer formed is greater than the volume of the metal, the oxide layer is non‒porous and protective.

Importance:

Protective and non‒protective nature of the oxide film can be found out.

 

5. How does the liquid metal cause corrosion on metals?

(i) Either dissolution of a solid metal by a liquid metal. (or)

(ii) Liquid metal may penetrate into the solid metal.

 

6. What is meant by hydrogen embrittlement?

Formation of cracks and blisters by the hydrogen gases, when it contacts to H₂S, is called hydrogen embrittlement. Hydrogen embrittlement causes loss in ductility of a material.

 

7. What is decarburisation?

The process of decrease in carbon content in steel is termed as "decarburisation" of steel.

 

8. What is electrochemcial corrosion.

Electrochemical corrosion is the one which occur,

(i) When two dissimilar metals or alloys are in contact with each other in presence of an aqueous solution or moisture.

(ii) When a metal is exposed to varying concentration of oxygen or any electrolyte.

 

9. In electrochemical corrosions what is the general anodic reaction of a metal M? What is the cathodic reaction in basic aqueous solution containing dissolved oxygen?

Anodic reaction: M → M²⁺ + 2e ̄

Cathodic reaction: ½O₂ + H₂O + 2e ̄ → 2OH^‒

 

10. What is galvanic cell corrosion.

 (or)

Explain galvanic corrosion with suitable examples.

When two different metals are in contact with each other in presence of an aqueous solution (or) moisture, galvanic corrosion occurs.

Here the more active metal acts as anode and the less active metal acts as cathode.

Example:

In Zn‒Fe couple, Zn acts as anode and undergoes corrosion.

But in Fe‒Cu couple, Fe acts as anode and undergoes corrosion.

 

11. How is galvanic corrosion prevented?

Galvanic corrosion is prevented by

(a) selecting the metals as close as possible in the electrochemical series.

(b) providing smaller area for cathode and larger area for anode.

(c) inserting an insulating material between the two metals.

 

12. Steel screw in a brass marine hardware corrodes ‒ Explain.

This is due to galvanic corrosion. Iron (higher position in electrochemical series) becomes anodic and is attacked and corroded, while brass (lower in electrochemical series) acts as cathodic and is not attacked.

 

13. Bolt and nut made of the same metal is preferred in practice. Why?

It is preferred in practice, because galvanic corrosion is avoided due to homogeneous metals (no sigma anodic and cathodic part).

 

14. What is differential aeration corrosion

Differential aeration corrosion is a concentration cell corrosion, occurs when a metal is exposed to varying concentration of oxygen (or) air.

 

15. Corrosion of a metal is the highest at the metal junction in a galvanic couple. Account the reason.

Metal junction, where air cannot diffuse easily, is less aerated and becomes anodic, the remaining parts are more aerated and becomes cathodic. So, the corrosion concentrates at the metal junction in a galvanic couple.

 

16. What is meant by differential metal corrosion? Explain with an example.

Differential metal corrosion is nothing but galvanic corrosion, which occurs when two different metals are joint (or) if the pure metal contains impurity.

The corrosion on metal depends on their relative position in the emf series. If the impurity (one metal) is less noble than the bulk metal (second metal), the impurity will act as anode and thus corrodes and vice versa.

 

17. Comment the type of corrosion occurring on lead pipeline passing through clay to cinders.

Lead pipeline passing through clay to cinders undergo corrosion. Since the pipeline under cinders is more aerated, it gets corroded easily.

 

18. What is pitting corrosion?

Pitting is a localised attack, resulting in the formation of a hole around which the metal is relatively unattacked.

Example: Metal area covered by a drop of water, sand, dust.

 

19. How does a drop of water, oil, dust, resting on an iron surface lead to corrosion of the metal.

The area covered by the drop of water acts as an anode due to less oxygen concentration and suffers corrosion. The uncovered area (freely exposed to air) acts as a cathode due to high oxygen concentration.

 

20. Why does corrosion of water on filled steel tanks occur below water line?

What type of corrosion occurs in the following cases (i) Wire fence (ii) Riveted joints (iii) Iron tanks, used for storing water.

Differential aeration or concentration cell type corrosion.

 

21. Irons is corroded faster than aluminium even though Fe is placed below Al in electrochemical series. Why?

Al forms a thin, non‒porous, tightly adhering protective film of Al₂O₃ on its surface and this film does not allow Al to undergo further corrosion.

 

22. What is water line corrosion? Explain with an example.

 (or)

What is concentration cell corrosion? Give an example.

Justify the formation of water line corrosion.

This type of corrosion occurs when a metal is exposed to varying concentration of oxygen or any electrolyte or water on the surface of the base metal. Here the metal above the water is more aerated and become cathodic, the part below the water is less aerated and become anodic.

Example: Metals partially immersed in a water (or) conducting solution.

 

23. What is differential aeration?

Differential aeration is nothing but two different concentration of air on a metal surface.

 

24. List out the the differences between chemical corrosion and electrochemical corrosion.

Dry or Chemical corrosion

1. It occurs in dry state.

2. It follows adsorption mechanism.

3. Corrosion product accumulate on the same spot, where corrosion occurs.

Wet or Electrochemical corrosion

1. It occurs in presence of moisture or electrolyte.

2. It follows the mechanism of electro chemical reaction.

3. Corrosion occurs at anode while products gather at cathode.

 

25. What is deactivation? Give examples.

Deactivation is a process of removing dissolved oxygen by adding some chemicals in aqueous solution.

Examples

1. Sodium sulphite: 2Na₂SO₃ + O₂ → 2Na₂SO₄

2. Hydrazine: N₂H₄ + O₂ → N₂ + 2H₂O

 

26. Relate the rate of corrosion with temperature.

Rate of corrosion ∝ Temperature.

 

27. List the environmental factors that influence corrosion.

1. Temperature

2. Humidity

3. Presence of corrosive gases

4. Presence of suspended particles

5. Effect of pH

 

28. What are factors which affects corrosion?

1. Position of the metal in emf series

2. Relative areas of the anode and cathode

3. Purity of the metal

4. Over voltage

5. Nature of the surface film

6. Nature of the corrosion product

7. Temperature & Humidity

 

29. State Pilling Bedworth rule. Name two metals in which the specific volumes of their oxides are greater than that of the metals.

(a) According to Pilling‒Bedworth rule, if the volume of the oxide layer formed is less than the volume of metal, the oxide layer is porous and non‒protective.

(b) On the other hand, if the volume of the oxide layer formed is greater than the volume of metal, the oxide layer is non‒porous and protective.

Ex: Volume of Oxides of Pb & Sn are greater than that of metals.

 

30. Using chemical equations, state the mechanism of corrosion of iron in weakly alkaline solution.

When an iron metal contacts with a neutral (or) weakly alkaline solution of an electrolyte in presence of oxygen, OH^‒ ions are formed.

At anode

Iron dissolves as Fe²⁺ with the liberation of electrons.

Fe → Fe²⁺ + 2e^‒ (oxidation)

At cathode

The liberated electrons flow from anodic to cathodic part through metal, where the electrons are taken up by the dissolved oxygen to form OH^‒ ions.

 ½ O₂ + H₂O + 2e^‒ → 2OH^‒

Thus, the net corrosion reaction is

 Fe²⁺ + 2OH^‒ → Fe(OH)₂↓

If enough O₂ is present Fe(OH)₂ is easily oxidized to Fe(OH)₃, a rust (Fe₂O₃. H₂O).

4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃

 

31. Explain why magnesium corrodes faster when it is in contact with copper than when it is in contact with iron.

The Magnesium corrodes faster when it is in contact with copper because larger difference in their standard reduction potential values between Mg and Cu. Mg have (‒) ve reduction potential value and Cu have (+) ve reduction potential value.

 (Mg = ‒2.3 V; Cu = + 0.34 V)

But, lesser difference in the standard reduction potential values between Mg and Fe and both have (‒) ve reduction potential values.

 (Mg = ‒ 2.37 V; Fe ‒ 0.44 V)

 

32. Why does Mg corrode faster than iron?

Mg possess higher (‒)ve reduction potential (E° = ‒ 2.37 V) than the iron (E° = ‒0.44 V). So Mg corrodes faster.

 

33. Zinc is more readily corroded when coupled with copper than with lead why?

Zinc is anodic to Cu and Pb. When zinc is (E^o = ‒0.76 V) is coupled with copper (E° = +0.34 V) zinc is more readily corroded, because the difference in their possition in emf series is more. Also Cu has (+)ve reducetion potential. But, if Zn is coupled, with lead (E° = ‒0.13 V) rate corrosion is less because the difference in their possition in emf series is less. Also Pb has (‒)ve reduction potential.

 

34. Zinc reacts with dilute sulphuric acid give hydrogen but silver doesn't liberate hydrogen. Why?

Metals with negative reduction potential (like Zn) (i.e., the metals placed above H₂ in the emf series) liberates the hydrogen from an acid solution.

 Zn + H₂SO₄ → ZnSO₄ + H₂↑

 E°_Zn = ‒0.76 volt

But, the silver with positive reduction potential (i.e., the metals placed below H₂ in the emf series) does not liberate the hydrogen from an acid solution.

Ag + H₂SO₄ → No reaction

E°_Ag = +0.80 volt

 

35. Small anodic area results in intense corrosion. Why?

Rate of corrosion is more rapid and intense, if the anodic area is smaller and the cathodic area is larger. When the cathodic area is larger, the demand for electrons will be more and this results in an increased rate of corrosion of metals at anodic area.

 

36. Why corrosion rate increases with temperature?

The rate of chemical reaction and rate of diffusion of the ions increases with rise of temperature.

 

37. What is a sacrificial anode? How does it protect a submerged pipeline?

A sacrificial anode is more active metal than the anodic part of the corrosion cell.

The sacrificial anode is connected to the submerged pipeline. So the submerged pipeline will be converted to cathode and corrosion concentrates over the sacrificial anode.

 

38. Which of the following metals could provide cathodic protection to iron: Al, Zn, Cu, Ni

Al and Zn (Both are placed above the Fe in emf series)

 

39. How does impressed current cathodic method carried out?

 (or)

What is the principle involved in impressed cathodic current method of prevention of corrosion?

In impressed current cathodic protection, current is applied in the opposite direction of the corrosion current to nullify it.

 

40. Compare sacrificial anode method and impressed current method.

Sacrificial anode method

1. No external power supply is necessary.

2. This method requires periodical replacement of sacrificial anode.

3. Investment is low.

4. Soil and microbiological corrosion effects are not taken into account.

5. This is most economical method especially when short‒term protection is operations.

6. This method is suitable when the current requirement and the resistivity of the electrolytes are relatively low.

Impressed current method

1. External power supply must be present.

2. Here anodes are stable and do not disintegrate.

3. Investment is more.

4. Soil and microbiological corrosion, effects are taken into account.

5. This method is well suited for large structures and long term operations.

6. But this method can be practiced even if the current requirement and the resistivity of the electrolytes are high.

 

41. Write the effetct of pH of the conducting medium on corrosion of metals.

When pH of the conducting medium increases, rate of corrosion increases.

 pH ∝ corrosion rate

 

42. Give the importance of pilling‒bedworth rule.

1. Rate of dry corrosion can be predicted.

2. Protective or non‒protective nature of the oxide film can be found out.

 

43. Define glavanic series.

A galvanic series is a chart (or) list that orders metals and alloys based on their oxidation potential (activity) in a particular environment (seawater (or) soil)

 

44. Mention any two applications of galvanic series.

1. It is used to prevent galvanic corrosion.

2. Engineers use the series to select materials that are close to each other to minimize corrosion.

3. It is used to prevent corrosion of ship hulls and other metallic components.

 

45 How is ceramic coating produced?

Ceramic coatings are made of mixing high refractory oxides such as chromium trioxide with silica materials. The paste of silica material and refractory oxide is sprayed and then fired.

 

46. What are enamels?

Enamels are pigmented varnish, ie., a colloidal dispersion of pigments and varnish. Enamels dry slowly but form hard, lustrous and glossy film.

 

47. What are the uses of enamels?

1. Enamels are used to protect the materials against corrosion.

2. Japans are used for painting bicycles, automobiles, etc.,

 

48. What are the requisites of a good paint?

(i) It should spread easily on the metal surface.

(ii) It should have high hiding (covering) power.

(iii) It should not crack on drying.

(iv) It should adhere well to the surface.

(v) The colour of the paint should be stable.

(vi) It should be a corrosion and water resistant.

(vii) It should give a glossy film.

 

49. What is the role of pigment in paint? Give two examples.

Pigments are solid and colour producing substances in the paint.

White pigments ‒ White lead, lithophone.

Black pigments ‒ Lamp black, carbon black.

 

50. Explain the functions of driers in the paint.

(i) They act as oxygen carriers (or) catalysts.

(ii) They provide oxygen, which is essential for oxidation, polymerisation of drying oil.

Examples: Metallic soaps, linoleats and resinates of Co, Mn and Pb.

 

51. What is the function of extender in the paint?

(i) It reduces the cost of the paint.

(ii) It retards the setling of the pigment in all paints.

(iii) It modifies the shades of the pigments.

(iv) It prevents shrinkage and cracking.

Examples: Talc, gypsum, chinaclay, etc.

 

52. What is mechanism of drying of oil paint?

The mechanism involves oxidation, polymerisation and condensation reactions. The oil containing conjugated double bonds dry very faster than the non‒conjugated oils.

 

53. What are the structural arrangement required for drying of a drying oil?

Conjugated oil dry very faster than non‒conjugated oil.

(i) Conjugated oil structure

‒CH=CH‒CH=CH‒CH=CH‒

(ii) Non‒conjugated oil structure:

‒CH=CH‒CH₂‒CH=CH ‒

 

54. How to calculate P.V.C of paint? Mention its significance.

It is an important property of a paint. The following equation is used to calculate the P.V.C.

P.V.C = Volume of pigment in the paint / [ Volume of pigment in the paint + Volume of non‒volatile vehicle in the paint ]

Higher the volume of P.V.C, lower will be the durability, adhesion, consistency of the paint.

 

55. What are the reasons for a failure of a paint?

1. Chalking

2. Cracking

3. Erosion

4. Blistering.

 

56. What is a varnish?

Varnish is a homogeneous colloidal solution of natural or synthetic resins in spirit or oil. Varnish is used for both protection and decoration of metal surfaces.

 

57. How is an oil varnish prepared?

Oil varnish is prepared by dissolving natural or synthetic resins in a drying oil and volatile solvent. This type of varnish dries by the evaporation of the solvent followed by oxidation and polymerisation of the drying oil.

 

58. Explain the characteristics of a good varnish.

(i) It should be soft.

(ii) It should produce a shining and glossy film on drying.

(iii) It should dry quickly.

(iv) It should not shrink or crack after drying.

 

59. What are the applications of varnishes?

1. It is used to protect the materials against corrosion.

2. It is a coating prier to paint coating.

3. It is also used for improving the appearance of wooden surfaces.

 

60. How do paints differ from varnishes?

(i) Paint: Contains pigments- Vamish: Does not contain pigments.

(ii) Paint: Opaque, colloidal solution - Vamish: Transparent, homogenous solution.

 

61. What is a lacquer? How does it differ from varnish. Mention its uses.

Lacquer is a colloidal dispersion of cellulose derivatives, resins and plasticiers in solvents and diluents. It is used for interior decoration. It does not contain drying oil like varnish.

 

62. What are Japans?

When coloured pigments are used, such enamels are called japans. If black pigments are used, the enamels are called black japans.

 

63. What is anodising?

Anodising (or) anodic oxidation is an electrolytic process, in which a thick oxide coating is produced on the base metal.

 

64. What are the advantages of anodization?

(i) an insulating coat for the electrically conducting base metal (aluminium).

(ii) very good resistance to corrosion,

(iii) thicker oxide coating and hence it is more protective,

(iv) the film may be coloured with organic (or) inorganic dyes,

(v) anodized metal (like anodised aluminium) is used in automobile engine piston.

 

65. How is anodising of aluminium carried out?

Anodised coating on aluminium is done by making aluminium as an anode in an electrolytic bath of H₂SO₄ (or) oxalic acid and the cathode is a plate of lead or stainless steel.

 

66. Why is anodising is best suited to aluminium?

Since aluminium has high negative reduction potential it can undergo oxidation readily in presence of electrolyte like chromic acid. By making aluminium as anode in the electrolytic cell, a thick oxide coating can be produced on its surface.

 

67. What is the difference between chromising and chromate coating?

Chromising is cementation, obtained by heating the base metal with chromium powder.

Chromate coating is produced by dipping the metal object in a bath of acidic potassium chromate, followed by dipping in neutral chromate solution.