Question 1
Passivation plays an important role in corrosion of metals and alloys.
(a) Describe the behaviour of an "active-passive" metal when a fresh surface is exposed to a corrosive
environment. Draw a schematic Evans diagram to illustrate your discussion, labelling the various regions
on the diagram. Indicate important potentials and current densities.
[10]
(b) Give an example of such a metal or alloy.
[1]
Question 2
The formation of mill scale during certain processing of e.g. steel profiles is considered a form of
corrosion.
(a) Compare this type of corrosion to the other broad class of corrosion mechanisms, and discuss
how this type may be modeled.
[7]
(b) Discuss the effect of metal oxide volume on the protective ability of an oxide layer on a metal.
[8]
Question 3
lntergranular corrosion {IGC) is a localised corrosion that occurs at or near the grain boundaries in a metal or
alloy.
(a) State at least three general factors that can cause this type of corrosion.
[3]
(b) State what the main preventative measures against IGC are that you can take during the metal alloying
stage.
[2]
Question 4
Consider an unpainted mild steel item exposed to the atmosphere. After some time, examination of the
piece shows general corrosion. Hint: keep in mind the practical experiment you did.
Explain, in terms of the basic aqueous corrosion cell, how this type of corrosion has occurred.
[9]
Question 5
A brass (copper-based alloy) fitting used in a marine application is joined by soldering with lead-tin solder. This
application is used even in fresh water plumbing systems.
(a) Do you expect corrosion to occur? Explain your answer.
[3]
{b) Explain the advantages and disadvantages of this type of joining.
[2]
Question 6
Cathodic protection may be achieved by the use of an impressed current from an electrical source, or by the
use of sacrificial anodes. Discuss the merits and demerits of sacrificial anodes approach.
[6)
Question 7
Consider a pipeline made of high strength steel, carrying a flow of seawater to a desalination plant. One part
is encased in concrete, running into the sea. The rest is buried in moist soil, running to the plant. At the plant
it is connected via bolted flanges to an electrical pump. The pump housing is a casting in naval brass.
(a) Is stress corrosion cracking likely, in your opinion? Explain your answer.
[3]
(b) What (other) type(s) of corrosion may be expected in this situation? What could be done to manage the
problem(s)?
[9]
2