A zinc coating of 86 \( \mu \)m corroding at 2 \( \mu \)m per year would last about
Answer: C
Dividing the thickness by the annual loss gives 43 years, which is why galvanising is specified by mass for a stated design life. In a marine or industrial atmosphere the rate is several times higher and the life correspondingly shorter.
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A magnesium anode of 10 kg delivering a mean current of 0.1 A, with an equivalent mass of 12.15 g and one faraday as 96 500 C, would last about
Answer: D
Correct answer: 25 years
Faraday’s law gives the charge the anode can deliver, and the current gives the time: \[ Q = \frac{10{,}000}{12.15} \times 96{,}500 = 7.94\times10^7\ \mathrm{C}, \qquad t = \frac{7.94\times10^7}{0.1} = 7.94\times10^8\ \mathrm{s} = 25\ \mathrm{years}. \] In practice a utilisation factor of about 0.85 and some self-corrosion shorten this, so a design life of 20 years would be claimed for such an anode.
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A steel rivet of 1 cm\( ^2 \) in a copper plate of 100 cm\( ^2 \) corrodes, relative to equal areas of the two metals,
Answer: A
The whole galvanic current generated by the large copper cathode is concentrated on the small steel anode, so its current density and hence its penetration rate rise in proportion to the area ratio. Reversing the metals would give slight general attack instead.
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Match the form of corrosion with what causes it.
1. Waterline corrosion 2. Intergranular corrosion of welded stainless steel 3. Erosion corrosion
P. Chromium carbide precipitation Q. Differential aeration at the surface R. Film removal by turbulent flow
Answer: B
Each form has its own mechanism and its own remedy, and treating one as another wastes the effort. Waterline attack needs a coating or cathodic protection, sensitisation a low-carbon grade, and erosion a lower velocity.
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Match the protection method with how it works.
1. Sacrificial anode 2. Anodic inhibitor 3. Phosphating
P. Films over the anodic sites Q. Supplies electrons to hold the metal cathodic R. Keys a subsequent paint film
Answer: C
Correct answer: 1-Q, 2-P, 3-R
Three quite different mechanisms. A sacrificial anode supplies electrons from a more active metal, holding the protected structure cathodic so it cannot dissolve. An anodic inhibitor forms a film over the anodic sites and stops the dissolution there - and is dangerous if underdosed, since a partial film concentrates the attack on what it leaves exposed. Phosphating produces a crystalline layer whose function is to key the paint that follows.
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Put the events of rusting in order.
1. Ferrous hydroxide precipitates where the ion streams meet 2. Iron dissolves anodically to ferrous ions 3. Hydrated ferric oxide forms as rust 4. Oxygen is reduced to hydroxyl ions at the cathodic area
Answer: D
The anodic and cathodic reactions supply the two ions, they meet and precipitate the hydroxide, and dissolved oxygen then oxidises it further to rust. Because the rust forms away from the anode it does not seal the pit that produced it.
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Consider the following statements about corrosion.
1. A scratch in galvanised steel does not rust because zinc is anodic to iron 2. An oxide of smaller volume than the metal consumed is protective 3. Crevice corrosion arises from oxygen depletion in the gap
Answer: A
Correct answer: 1 and 3 only are correct
Statements 1 and 3 are correct: zinc is anodic to iron and protects a scratch sacrificially, and crevice corrosion is a differential aeration effect driven by oxygen depletion in the gap. Statement 2 is inverted - an oxide of smaller volume than the metal it replaced cannot cover the surface and cracks, as magnesium’s does. A protective oxide needs a Pilling–Bedworth ratio slightly greater than one.
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Which of the following would NOT reduce the corrosion of a buried steel pipeline?
Answer: B
Correct answer: connecting it to a copper earthing grid
Copper is strongly cathodic to steel, so bonding the pipeline to a copper grid creates a large galvanic cell with the pipe as the anode - it accelerates the corrosion rather than reducing it, and is a classic cause of premature failure where earthing has been installed without regard to it. Impressed current, coating and magnesium anodes are all genuine protection.
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Three of the following are forms of differential aeration corrosion. The exception is
Answer: C
Correct answer: intergranular corrosion of a sensitised weld
Intergranular attack is a metallurgical effect: chromium carbide precipitates at the grain boundaries in the heat-affected zone and leaves them depleted in chromium, so they lose their passivity. Oxygen has nothing to do with it. Crevices, waterlines and deposits are all differential aeration cells, in which the oxygen-starved region becomes the anode.
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A stainless steel vessel is pitted although the rest of its surface is bright and unattacked. The likely cause is
Answer: D
Correct answer: chloride ions breaking down the passive film locally
Chloride penetrates the passive film at isolated weak points, and once a pit is initiated its interior becomes acidic and depleted in oxygen while the vast passive surface around it acts as the cathode. That area ratio is what makes pitting so dangerous: the surface looks sound, and a very small anode can perforate the wall.
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