Protective Coatings - Questions and Answers

1. What is a paint?

Ans. Paints is a viscous, opaque, mechanical dispersion mixture of one or more colouring matter called pigment in a dry oil called vehicle.

2. What are the characteristics of a good paint?

Ans.

It has high covering power.
It should spread easily over the metal surface.
The film formed should be tough and uniform on the metal surface.
It should not crack on drying.
It should possess high adherent capacity.

3. Explain the functions of drying oil in paint.

Ans. (a) It is a main film-forming constituents.
(b) It gives better adhesion to the surface.
(c) It imparts high durability, water repellency and toughness to the film.
(d) It forms the protective film by oxidation and to act as a barrier to atmosphere.
(e) It is used to bind the pigments.

4. Explain the functions of dries in the paint.

Ans. (i) They improve the drying quality of the oil film.
(ii) They act as oxygen-carriers (or) catalysts.
(iii) They provide oxygen, which is essential for oxidation and polymerisation of drying oil.

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

Ans. (i) Cracking (ii) Flaking (iii) chalking (iv) Erosion (v) Blistering

6. What are metallic coatings ?

Ans. The surface coating made up of noble metal are called metallic coating. It is mostly applied on the steel and iron surfaces.

7. What are the objectives of electroplating on non-metals?

Ans. (i) To preserve and decorate the surfaces of non-metals like plastic, wood, glass, etc.
(ii) To increase strength.
(iii) For making the surface conductive by utilization of light weight, non-metallic materials.

8. Write down the factors affecting the quality of an electro deposit.

Ans. (i) Current density
(ii) PH of the bath
(iii) Throwing power
(iv) Temperature

9. What is electroplating?

Ans. Electroplating is electro deposition on metals, non-metals and alloy. It is the process by which the coating metal is deposited on the base metal or alloy by passing a direct current through an electrolytic solution containing the soluble salt of the coating metal.

10. What is electroless plating?

Ans. Electroless plating is a technique of deposition of noble metal (noble metal salt solution) on a catalytically active metal surface by using a suitable reducing agent without using electrical current.

11. Mention any two differences between electroplating and electroless plating.

S. No	Electro plating	Electroless plating
1.	It is carried out by passing current.	It is carried out by auto catalytic redox reaction.
2.	Separate anode is employed.	Catalytic surface of the substrate acts as an anode.
3.	Anodic reaction is M → Mⁿ⁺+ ne^–	Anodic reaction is R → O + ne^–
4.	Objected to be coated is cathode.	Object to be coated, after making its surface catalytically active.

12. What is the use of applying protective coating on the metal surface ?

Ans. Protective coating is one of the methods used to prevent theme tall from corrosion. These protective coatings form a continuous physical-barrier between the coated-surface and its environment. Protective coating also provides mechanical and physical properties such as wear-resistance, hardness, electrical properties, oxidation resistance and thermal-insulating properties.

13. What do you know about electroless plating ?

Ans. Electroless plating is defined as the controlled auto catalytic deposition of a continuous film by the interaction of a solution of a metal salt and a chemical reducing agent.

The reducing agent reduces the metallic salt to metal which gets deposited over the catalytically activated surface giving a uniform thin coating.

Metal salt + Reducing agent → Metal + Oxidised products

14 What are the applications of electroless Ni-plating ?

Ans. Electroless Ni-plating is used
(i) In electronic appliances
(ii) In domestic as well as automotive fields like jewellery, tops of perfume bottles etc.
(iii) In decorative and functional works.
(iv) In digital as well as electronic instruments.

15. What is electroplating or electrodeposition ?

Ans. Electroplating is the process by which the coating metal is deposited on the base metal by passing a direct current through an electrolytic solution, containing the soluble salt of the coating metal.

16. What are the objectives of electroplating on metal?

Ans. (i) To increase the resistance to corrosion and chemical attack of the plated metal.
(ii) To obtain a polished surface.
(iii) To improve hardness and wear resistance.

17. What are the various steps of electroless plating?

Ans. (i) Pretreatment and activation of the surface
(ii) Preparation of plating bath
(iii) The pretreated object is immersed in the plating bath

18. How is Ni plating done by electroless plating?

Ans. The pretreated object is immersed in the plating bath containing NiCl₂ and sodium hypo-phosphite. During which Ni²⁺ ion gets reduced to Ni and deposited on the object.

19. What are the applications of electroless Ni plating?

Ans. (i) Plastic cabinets coated with copper or nickel by electroless plating is used in digital and electronic instruments.
(ii) Electroless Ni-P Coatings are used in electronic applications
(iii) Electroless Ni plating specifically used in tops of perfume bottles, costume jewellery and car trim.

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