Corrosion and Its Control Questions and Answers

Answer: D

Correct answer: non-toxic and does not taint the contents

Tin is chosen for food use because its salts are harmless and it does not affect the taste of the contents, and inside a sealed can there is little oxygen for it to fail against. Zinc would protect the steel better at a scratch but its salts are not acceptable in food, so the electrochemically inferior coating is the right engineering choice here.

Answer: A

Correct answer: diffusion into the surface at high temperature

In cementation the article is packed in a powder of the coating metal and heated, so the coating metal diffuses into the surface and forms an alloyed layer rather than sitting on top of it. Sherardising with zinc and calorising with aluminium are the common examples, and the diffused layer follows a complex shape and cannot be chipped off.

Answer: B

Duralumin sheet clad with pure aluminium is the classic example, the strong but corrodible alloy being sandwiched between corrosion-resistant layers. The bond is mechanical and metallurgical rather than electrochemical.

Answer: C

The crystalline phosphate is not protective enough by itself but grips the subsequent paint far better than bare metal and stops corrosion creeping under the film. Almost every painted car body receives such a pretreatment.

Answer: D

Correct answer: giving opacity and, in some cases, inhibiting corrosion

The pigment gives the film its opacity, blocking the ultraviolet light that would otherwise degrade the binder, and an inhibitive pigment such as zinc phosphate does chemical work at the steel surface as well. Red lead and zinc chromate were the classic inhibitive pigments; both have been displaced on toxicity grounds rather than on performance.

Answer: A

Correct answer: form the continuous film that holds the pigment

The vehicle is what actually becomes the paint film: it wets the surface, carries the pigment, and on drying or curing forms the continuous barrier that adheres to the metal. Only the solvent evaporates - the binder stays behind, and the quality of that film is what determines how long the system lasts.

Answer: B

Standing water and oxygen-starved gaps set up concentration cells, so smooth continuous surfaces that drain freely last far longer. Continuous welds, good drainage and a single metal are all desirable rather than to be avoided.

Answer: C

The volume lost is 0.5 over 7.9, that is 0.0633 cubic centimetres, and dividing by the 100 square centimetres of area gives 6.33 \( \times \) 10\( ^{-4} \) centimetres. Uniform penetration is assumed, which pitting would badly violate.

Answer: D

Correct answer: 100

Converting the area to square metres first: \[ 200\ \mathrm{cm}^2 = 0.02\ \mathrm{m}^2, \qquad \frac{2.0}{0.02} = 100\ \mathrm{g\,m^{-2}\,yr^{-1}}. \] The factor here is 10,000 cm² to the square metre, not 100, and treating it as 100 gives the 10 offered as a distractor.

Answer: A

A constant rate makes the loss proportional to time, which is the assumption behind any coupon test. A rate that falls with time indicates a protective film building up, and one that rises indicates breakdown of the surface.