Dissemination of IT for the Promotion of Materials Science (DoITPoMS)



Quick questions

You should be able to answer these questions without too much difficulty after studying this TLP. If not, then you should go through it again!

  1. Which of the following half-reactions represent a general corrosion process?

    a M + 2e → M2+ and H2 → 2H+ + 2e
    b M → M2+ + 2e and H2 → 2H+ + 2e
    c M2+ + 2e → M and H2 → 2H+ + 2e
    d M → M2+ + 2e and 2H+ + 2e → H2

  2. Which of the following are possible cathodic reactions that accompany corrosion

    a 2H+ + 2e → H2
    b O2 + 4H+ + 4e → 2H2O
    c 4OH- → O2 + 2H2O + 4e
    d Mnz+ + ze  → M

  3. Which of the following is not a form of Tafel's equation?


  4. Using the electrochemical series, which of the following metals can be used as a sacrificial anode for steel under standard conditions? Click on this link to launch the electrochemical series in a new window.

    a Nickel
    b Zinc
    c Magnesium
    d Tin

  5. Look at the following Tafel plot. What is the critical current density?

    a 15 μA m-2
    b 1.5 A m-2
    c 32 A m-2
    d 0.3 μA m-2

Deeper questions

The following questions require some thought and reaching the answer may require you to think beyond the contents of this TLP.

  1. Draw the Tafel plot of the following information on graph paper and find:
    a) The corrosion potential
    b) The corrosion current density
    c) How long would a 3 mm thick component survive in this scenario (use Faraday's law)
    Both reactions have ba = -bc = 0.12 V / decade.
    One half-reaction has an equilibrium potential -0.25 V (SHE) and an exchange current density of 10 μA m-2. This reaction has a passivation potential 0.15 V (SHE) and passive current density 10 mA m-2.
    The other half-reaction has equilibrium potential 0.8 V (SHE) and exchange current density 0.1 mA m-2.
    The metal corroding forms 2+ ions, has a molar mass of 35 g mol-1 and has a density of 6400 kg m-3.

  2. a) Write balanced half-reactions and the overall reaction for an iron water pipe corroding in fully aerated water under standard conditions, and with the water flowing at such a rate to maintain a diffusion layer 1 μm thick.
    b) Derive the anodic and cathodic Tafel slopes for the two halves of the reaction if α = 0.5
    Using the fact that the iron does not passivate in the potential range being considered, some of the data in this window and the other information and answers above, draw the Tafel plot on graph paper, and then calculate
    c) The corrosion potential
    d) The corrosion current density
    e) Whether or not the pipe will survive 1 year if it has walls 5 mm thick
    f) What happens if the pipe is part of a sealed system such as central heating?
    g) Why else might your answer to c) be flawed?