Problem Sheet 11

The numbers in square brackets refer to the number of the problem in your textbook.

  1. [E17.2] The oxidation kinetics of titanium to TiO2 is limited by oxygen diffusion, with an activation energy Qd of 275 kJ/mol. If the oxide film grows to a thickness of 0.08 $\mu$m after 1 hour at 800oC, how thick a film would you expect if it had been grown at 1000oC for 30 minutes?
  2. [E17.3] A copper and a platinum electrode are immersed in a bath of dilute copper sulphate. What potential difference would you expect to measure between them? If they are connected so that a current can flow, which one will corrode?
  3. For cathodic protection, a ship hull requires a current density of 15 mA m-2. Magnesium is used as the sacrificial anode. What is the quantity of magnesium required per square metre of the hull surface for design life of ten years? [Hint: Since magnesium is divalent, one mole of magnesium will be used up as anode for every two Faraday of charge).
  4. The activation energy for cuprous ion diffusion in Cu2O is 160 kJ mol-1. Cuproous ion diffusion controls the oxidation of copper above 500oC. Find the ratio of oxidation rates of copper at 900 and 600oC.
  5. For good oxidation resistance, the oxide layer should be adherent to the surface. The adherence of an oxide film is dependent on the ratio (known as Pilling-Bedworth ratio) of the volume of oxide formed to that of metal consumed during oxidation. What should this ratio be for a protective film to be formed on the surface? Why? Given the density of CaO and Ca are 4030 and 1550 kg m-3, calculate the Pilling-Bedworth ratio for CaO. [Hint: In metals with Pilling-Bedworth ratio less than unity, the oxidation rate is linear while for metals with ratio substantially larger than unity, the initial rate of oxidation is parabolic).
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