Dry or chemical corrosion occurs by the direct attack of a gas on a metal in the
Answer: A
Oxygen, the halogens or hydrogen sulphide react with the metal surface directly, most importantly at high temperature, and no ionic conduction through a solution is involved. Wet corrosion by contrast requires an electrolyte and proceeds through separate anodic and cathodic sites.
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An oxide film protects the metal beneath it if the film is
Answer: B
Aluminium and chromium owe their durability to exactly such a film, which seals the surface and stops further attack almost at once. A porous, cracked or volatile film leaves the metal exposed and the corrosion continues unchecked.
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The Pilling-Bedworth rule states that an oxide film is protective when the volume of oxide formed is
Answer: C
An oxide of larger volume covers the surface completely and remains under compression, whereas one of smaller volume cannot cover what it replaced and leaves a porous layer. Magnesium and the alkali metals fail the rule, which is why their films offer no protection.
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Molybdenum oxidises rapidly at high temperature because its oxide is
Answer: D
Molybdenum trioxide sublimes, so the surface is continually stripped of whatever film forms and fresh metal is always exposed. Tungsten behaves similarly, which is why both need protective coatings for service in air at temperature.
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Iron's oxide scale is only weakly protective because it is
Answer: A
The three oxides formed have different volumes and expansion coefficients, so the scale is stratified and spalls away as the temperature changes. Fresh metal is exposed at every cycle, which is why mill scale is removed rather than relied upon.
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Wet or electrochemical corrosion requires
Answer: B
Anodic and cathodic areas on the surface are connected through the metal and through the solution, so a short-circuited galvanic cell is set up. Rain, condensation, soil moisture or a film of electrolyte a few molecules thick is enough to start it.
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In wet corrosion the metal is attacked at the
Answer: C
Oxidation of the metal to its ions is by definition the anodic reaction, and the electrons released travel through the metal to the cathodic areas. Whatever consumes them there determines which of the two wet mechanisms is operating.
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The hydrogen evolution mechanism of wet corrosion occurs in
Answer: D
The cathodic reaction is the reduction of hydrogen ions to hydrogen gas, which requires a supply of them and no better electron acceptor present. The whole surface tends to be attacked uniformly, so the corrosion is general rather than localised.
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The oxygen absorption mechanism of wet corrosion has as its cathodic reaction the reduction of
Answer: A
Correct answer: oxygen to hydroxyl ions
In neutral or alkaline solution the cathodic half-reaction is \[ \mathrm{O_2 + 2H_2O + 4e^- \rightarrow 4OH^-}, \] which is why this mechanism is called oxygen absorption. Hydrogen evolution is the competing mechanism and takes over only in acid, where hydrogen ions are plentiful enough to be reduced instead.
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Rust is formed when the ferrous ions produced anodically meet the hydroxyl ions produced cathodically and are then
Answer: B
Ferrous hydroxide precipitates first and dissolved oxygen oxidises it to hydrated ferric oxide, the familiar brown rust. Because it forms where the two ion streams meet rather than at the anode itself, the deposit does not protect the pit that produced it.
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