Corrosion and Its Control Questions and Answers

Answer: C

Zinc's more negative potential makes it dissolve while the steel is held cathodic, and the blocks are renewed at each dry docking. Sea water's high conductivity makes the arrangement effective over a considerable area.

Answer: D

A small anodic current can push a passivating alloy such as stainless steel into the range where its film is stable and the corrosion rate collapses. Applied to a non-passivating metal the same current would accelerate the attack instead.

Answer: A

Chromium, aluminium and titanium passivate spontaneously in air, which is what makes them useful in aggressive service. Chloride ions can break the film locally and start a pit even while the rest of the surface stays passive.

Answer: B

Chromates, phosphates and nitrites blanket the anodic sites and stifle the metal dissolution. An insufficient dose is worse than none at all, because it leaves small unfilmed anodes surrounded by a large cathode and provokes intense pitting.

Answer: C

Sodium sulphite or hydrazine deprives the cathodic reaction of its electron acceptor and the whole process stops for want of it. In acid solutions organic amines and thiourea instead raise the hydrogen overvoltage at the cathode.

Answer: D

The unfilmed patches carry the entire corrosion current from the filmed remainder, so penetration at those points is far faster than with no inhibitor present. Cathodic inhibitors are safer in this respect, since underdosing merely reduces the benefit.

Answer: A

Hot dipping or electrodeposition gives a layer that both excludes the environment and protects sacrificially where it is scratched. It is the commonest protection for structural steel and sheet in ordinary atmospheric exposure.

Answer: B

Correct answer: anodic to steel and protects it sacrificially

Zinc is anodic to iron, so at a scratch it is the zinc that dissolves and the exposed steel that becomes the cathode. The protection is therefore active rather than merely a barrier, and it continues as long as any zinc remains nearby - which is why a galvanised sheet can be cut and drilled on site without special treatment of the edges.

Answer: C

Correct answer: cathodic to steel and accelerates its attack

Tin is cathodic to iron, so at a break in the coating the small area of exposed steel becomes the anode against a very large tin cathode. The adverse area ratio concentrates the whole corrosion current into the scratch, and the steel is perforated far faster than it would have been bare. Tin protects only as long as the coating is unbroken - the exact opposite of zinc.

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.