Engineering Chemistry

Corrosion Science Viva Questions

Corrosion Science Viva Questions

Corrosion Science Viva Questions, Viva Questions on Corrosion Science, Science of Corrosion Viva Questions, Engineering Chemistry Viva Questions, Engineering Viva Questions

Water Treatment Viva Questions

Short Answer Questions

Q.1. Define corrosion.

Ans. Any process of deterioration and consequent loss of solid metallic materials through an unwanted chemical or electrochemical attack by its environment is called corrosion.

Q.2. What is meant by rusting of iron?

Ans. The attack of atmospheric gases on iron or steel, formation of a layer of the reddish scale of hydrated ferric oxide fe2O3 · 3H2O on its surface is known as rusting of iron.

Q.3. What is dry corrosion?

Ans. Dry corrosion takes place due to the direct chemical action of atmospheric gases like CO2, SO2, O2, H2, etc, or anhydrous liquids on the metal surfaces.

Q.4. Formation of which types of metal oxide film cause rapid and continuous corrosion.

Ans. Volatile oxide film and porous oxide film.

Q.5. Formation of which types of metal oxide film prevents corrosion.

Ans. Highly unstable oxide film and finely grained tightly adhering, impervious oxide film.

Q.6. State the two conditions for wet corrosion to take place.

Ans.

  • Immersion or partial dipping of two dissimilar metals or alloy in a solution.
  • A metal in contact with the conducting liquid.

Q.7. Bolt and nut made of the same metal are preferred in practice. Why?

Ans. Because such a combination will not permit galvanic corrosion to take place.

Q.8. What is wet corrosion?

Ans. Wet corrosion is due to the flow of electrons from the metal surface anodic area towards the cathodic area through a conducting solution. It is also known as electrochemical corrosion.

Q.9. What is galvanic corrosion?

Ans. When two dissimilar metals are electrically connected and exposed to an electrolyte, the metal higher in the electrochemical series undergo corrosion.

Q.10. The rate of metallic corrosion increases with an increase in temperature. Give reason.

Ans. With the increase in temperature of the environment, the rate of reaction as well as the rate of diffusion increases, thereby corrosion rate increases.

Q.11. Iron corrodes faster than aluminium, even though iron is placed below aluminium in the electrochemical series, why?

Ans. This can be explained by the fact that aluminium forms a non-porous, very thin, highly adhering protective oxide film (Al2O3) on its surface and this film does not permit corrosion to occur.

Q.12. Wire mesh corrodes faster at the joints, why?

Ans. The joints of wire mesh are stressed due to welding, so that part acts as an anode, Hence oxidation takes place easily at such joints leading to faster corrosion at the joints of wire mesh.

Q.13. Impure metal corrodes faster than pure metal under identical conditions. Why?

Ans. Because the presence of impurities in metal cause heterogeneity and form minute electrochemical cells at the exposed parts, and anodic parts get easily corroded.

Q.14. How is galvanization different from cathodic protection.

Ans. In galvanization, the iron object is protected from corrosion by coating it with a layer of zinc, whereas in cathodic protection, the iron object is made cathodic by connecting it with a more anodic metal like Al, Mg, etc.

Q.15. Where the electrochemical corrosion takes place.

Ans. At the anodic area.

Q.16. Rusting of iron is faster in saline water than in ordinary water. Give reason.

Ans. The presence of sodium chloride in saline water leads to increased conductivity of water, so when saline water comes in contact with the iron surface, corrosion current increases, and rusting is speeded up.

Q.17. Why does part of a nail inside the wood undergo corrosion easily?

Ans.: Corrosion is due to differential aeration Because part of the nail inside the wood is not exposed to atmospheric conditions, whereas the nail outside is exposed to atmospheric air. Thus nail inside the wood becomes anodic while the remaining part acts as cathodic. So due to differential aeration, a differential current starts flowing, and the anodic parts get corroded easily.

Q.18. Why should nickel-plated steel articles be free from pores and pinholes?

Ans. with respect to nickel, steel is anodic and if there are pinholes and pores in nickel-plated steel article, they will expose the anodic steel to the atmosphere. A galvanic cell is set up and intense localized corrosion at these small exposed parts occurs.

Q.19. Corrosion of water-filled steel tanks occurs below the waterline. Why?

Ans. This is because, the area above the waterline is highly oxygenated and acts as cathodic, while the part below the waterline is poorly oxygenated and acts as anodic. So due to differential aeration, an electrochemical cell is set up which results in corrosion of steel tanks below the waterline.

Q.20. What is meant by the term passivity?

Ans. It is the phenomenon by which a metal or alloy shows higher corrosion resistance due to the formation of a highly protective, very thin, and quite an invisible surface film on the metal surface.

Q.21. What is the effect of pH on corrosion?

Ans. The lower the pH (or more acidic), the greater is the corrosion.

Q.22. Can we use aluminium in place of zinc for cathodic protection of rusting of iron, comment?

Ans. Standard electrode potential of

Al3+/Al= -1.66V

Zn2+/Zn= -0.76V

In cathodic protection, the metal (iron) to be protected from corrosion is connected by a wire to a more anodic metal (like Al, Zn, etc.), so that all the corrosion occurs at this more active metal. Thus, the parent metal is protected while the more active metal gets corroded slowly. As the standard potential of aluminium is more than zinc, so Al is more anodic than Zn, so we can better use aluminium in place of zinc for cathodic protection of rusting of iron.

Q.23. Why are galvanized utensils not used?

Ans. Because galvanized articles get dissolved in dilute food acids and form highly toxic compounds. So, galvanized utensils cannot be used for preparation and storing foodstuffs.

Q.24. Why are brass utensils usually tinned?

Ans. Because Tin (Sn) is a noble metal and protects the brass utensils from corrosion, moreover, tin is non-toxic in nature. Hence, it is widely used for coating copper and brass utensils.

Q.25. Galvanization of the iron article is preferred to tinning, why?

Ans. Galvanization (coating iron with zinc) is preferred to tinning (coating iron with tin) due to the following reason: zinc(Zn) is more electropositive than iron, so zinc coating acts as anode; while the exposed iron portions of coating act as a cathode, If by chance, the zinc coating is broken at someplace, the zinc (being more anodic than iron), undergoes corrosion, protecting iron from rusting. So, zinc coating protects iron specifically.

On the other hand, tin is a noble metal (i.e. having higher reduction potential than iron), so it protects the iron due to its higher corrosion resistance than iron, If by chance, the tin coating is broken at someplace, much more corrosion of iron takes place because small exposed part of iron cuts as anode and tin acts as a cathode, a galvanic cell is set up, thereby intense corrosion at the small exposed iron part occurs.

Q.26. What is chromizing?

Ans. The process of heating a mixture of chromium powder, alumina, and iron/steel article in a revolving drum at 1300-1400 c for 3-4 hours, increases the corrosion resistance of the article.

Q.27. What are the main constituents of oil varnish?

Ans. synthetic resin, drying oil, and volatile solvent

Q.28. Give two functions of plasticizers.

Ans.

  • It provides elasticity to the paint film.
  • To minimize the cracking of dried paint film.

Q.29. Give three functions of drier in paints.

Ans.

  • It acts as a carrier of pigments.
  • It helps in forming a thin, homogeneous and protective film.
  • It supplies to paint film adhesion, toughness, durability and water-proofness.

Q.30. Give two functions of extenders or fillers.

Ans.

  • It reduce the cost of paint.
  • It reduce the cracking of the paint film.

Q.31. What is enamel?

Ans. enamel is an intimate dispersion of pigment in a varnish.

Q.32. Give three functions of thinner in a paint.

Ans.

  • To suspend pigment particles
  • To dissolve film-forming materials.
  • To reduce the consistency of paint for getting smooth finish.

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