Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
crack from time to time under this compressive stress, leading to ‘quasilinear’
behaviour as illustrated in Fig. 3.34. Rectilinear oxidation occurs
when the film is non-protective, and logarithmic behaviour occurs when the
films develops flaws parallel to the interface, which limit the effective
area available for diffusion. Logarithmic laws are also observed at low
temperatures, possibly due to the reduced electrical conductivity under these
conditions.
Improvement in oxidation resistance may therefore be expected if the
driving force, E = –∆G/NF, is reduced, by making the alloy more ‘noble’.
The addition of 40% Cu to Ni to make the alloy Monel increases the oxidation
resistance, as does the addition of Ni to Fe (as in ‘Ni-resist’ cast iron).
The second approach is to reduce the electronic and ionic conductivities
of the film itself, as well as its resistance to cracking and spalling. The
inherently protective film Ti, Zr and Ta arises for this reason, but it must be
borne in mind that these metals will not exhibit inertness under reducing
conditions which may remove the protective film. Al-based alloys usually
have an inherently protective film, although the addition of Cu to the alloy
usually leads to a fall in its oxidation resistance.
c
b
Film thickness
a
d
Time
3.34 Growth laws for oxide films on metals: (a) parabolic,
(b) rectilinear, (c) quasi-rectilinear and (d) logarithmic.