Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
the lowest content of dissolved oxygen and acts as an anode; Fe 2+ ions thus
pass into solution (i.e. the metal will corrode) in this region, equation [3.13]
and electrons pass up the specimen and are discharged near the surface,
where OH – ions form in the electrolyte by equation [3.14].
Diffusion occurs in the liquid over a period of time and the Fe 2+ ions and
the OH – ions interact to form an iron hydroxide (rust). Two points are significant
here: firstly, there is the geometrical fact that the metal goes into solution in
one region, electrons are discharged in another region (that where oxygen is
most readily available) and the corrosion product is formed in a third place.
Since the corrosion product does not form at the site where the metal is
dissolving, there can be no stifling of the attack (as in the case of dry corrosion,
when a protective film progressively reduces the rate of oxidation). The
second point to emphasize is the importance of the gradient in oxygen
concentration in the electrolyte: this phenomenon of differential aeration is
the origin of the EMF of the corrosion ‘cell’ and is, in practice, a very
common source of corrosion in iron and steel. This is the reason why corrosion
is often concentrated in crevices in structures, the regions of minimum oxygen
availability forming anodes. Conversely, if oxygen is totally excluded from
the system, electrons cannot be discharged by the process of equation [3.14]
and so corrosion ceases. This is why steel wrecks immersed in very deep
seawater (e.g. the Titanic) survive for long periods, whereas those on or
near the seashore quickly disintegrate by corrosion since they are so well
aerated.
If two different metals are in electrical contact in an electrolyte, a galvanic
EMF may exist and corrosion of the more reactive metal can proceed
preferentially, even in the absence of oxygen. An example of this would be
a galvanic couple between Al and Fe in an electrolyte,where the Al would
form the anode and be preferentially dissolved. The relative areas of anode
and cathode in contact with the electrolyte are important here, most dangerous
being a situation where the area of the anode is much smaller than that of the
cathode leading to a high local intensity of anodic attack. For example, it
would be unwise to fasten a steel plate with aluminium bolts or rivets since,
under corrosive conditions rapid attack of the bolts will occur.
Galvanic corrosion can be employed deliberately to reduce the corrosion
of metals such as iron by coupling them to more reactive ones (such as Zn
or Al) which corrode ‘sacrificially’. In terms of their susceptibility to galvanic
attack in seawater, metals and alloys have been empirically grouped as follows:
Galvanic series for alloys immersed in seawater
Titanium alloys
Nickel alloys
Stainless steels
Silver alloys