This set of Engineering Materials & Metallurgy Multiple Choice Questions & Answers (MCQs) focuses on “Protective Coatings”.
1. Which of the following coatings has a glass composition?
Explanation: Enamel (vitreous) is a protective coating composed of glass containing a metal substrate. Paint is composed of organic material, whereas galvanized coating has anodic metal, both of which contain metal substrate. The anodized coating has an Al composition with an aluminum substrate.
2. Which of the following is not a type of protective coating?
Explanation: A protective coating is generally defined as a layer of an inert substance which is applied to a material to prevent the chemical and electrochemical attack. These are classified into metallic, organic, and inorganic coatings.
3. An example of an anodic coating is _____
Explanation: The anodic coating is a classification of a metallic coating of metals which are anodic to the base metal. Zinc, aluminum, and cadmium are examples of anodic coatings. Cathodic coatings include Cu, Ni, Ag, etc.
4. The method of immersing a material into a molten bath for coating is known as _____
b) Hot dipping
Explanation: Hot dipping is a method of metallic coating in which the product to be coated is dipped into a molten bath of the coating metal. Water pipe fittings coated with by the method of hot dipping.
5. Which of these methods uses a filler wire at a high-temperature flame?
a) Hot dipping
b) Metal spraying
c) Vapor plating
Explanation: In metal spraying, the surface to be coated is sprayed with the coating metal from a filler wire or powder at a high-temperature flame using a spray gun. A few materials like Al, Cu, Pb, Sn, and Zn can be coated by a spraying method.
6. The veneering of metals for coating is known as _______
b) Vapor plating
Explanation: Veneering of two or metals under a pressure is described as the cladding method of metallic coatings. The metal which needs to be applied the protective coating on is kept between two layers of the coating metal. This is then rolled into the required thickness, producing a protective coating.
7. Alclad is the cladding method where _____ is coated with pure aluminum.
Explanation: When two materials are veneered by pressure, it is defined as the cladding process. The cladding of duralumin with pure aluminum is called Alclad.
8. Which method uses the powdered form of a coating to form the protective layer?
b) Hot dipping
c) Vapor plating
Explanation: Cementation is the process of alloying powdered coating metal with the base metal below melting point temperatures. Carburizing and sherardising are types of cementation processes. Al, Zn, Cr, and W are only a few metals used for cementation.
9. Phosphate coating and Chromate coating are classifications of ______ coatings.
Explanation: Chemical or electrochemical conversion is a form of an inorganic coating. They are used to improve corrosion resistance and for decoration. Phosphate, chromate, anodized, and chemical oxide coating are the various classifications of inorganic coatings.
10. The mixture of oil and a pigment is known as _____
Explanation: Paint is a form of an organic coating which is applied to protect against corrosion and to beautify surfaces. Oil is the wet component, whereas pigment is the dry material which adds color. The oil oxidizes to form a protective layer of the dry pigment.
11. A varnish is a mixture of _____ and oil.
Explanation: A mixture of natural or thermosetting resin and drying oil is used to form varnishes. These coatings do not contain pigments. However, reduced viscosity is obtained by adding turpentine to the mixture.
12. A mixture of oil and pigment in water is known as ______
Explanation: An emulsion is a suspension of particles of drying oil and pigment in water. It is an organic type of protective coating. Here, the water evaporates and the mixture of oil and pigment forms the required film. The emulsions are applied for decoration in household appliances.
13. Which organic coating is made from Lac dissolved in alcohol?
Explanation: Shellac is an organic protective coating which is made from the dissolved Lac in alcohol. It usually dries by evaporation of the solvent and leaves an organic finish. Lacquers contain nitrocellulose dissolved in the solvent.
14. Which common application do anodizing and galvanizing serve?
a) Corrosion resistance
b) Improved surface
c) Zinc coating
d) Increased strength
Explanation: Both anodizing and galvanizing processes are carried out to improve the corrosion resistance of materials. Additionally, anodizing improves the surface which helps in painting, whereas a layer of zinc is coating in the galvanizing process.
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