Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
initial painting operation and subsequent maintenance work. They have
not been used extensively in the UK because frequent rain inhibits the
formation of the stable oxide layer and rusting continues (athough it is at
a reduced rate). Long dry summer periods are desirable in order to develop
the adherent oxide layer.
3.4.3 Stress, strain and corrosion
Both dry and wet corrosion may be exacerbated in the presence of applied
stress or strain.
Stress-corrosion cracking
Stress-corrosion cracking (SCC) may appear as mechanical failure by cracking
under circumstances where, in the absence of corrosion, no failure would
have been expected. The stress may arise by external application or be present
as a residual internal stress, and the corrosive medium is highly specific to
each alloy. Under fluctuating stress, the phenomenon is known as corrosion
fatigue. Some form of surface pit must initially be formed, but there appears
to be no common mechanism of crack propagation for all the different systems
of alloys and corrodents that produce cracking. The rate of propagation of a
stress-corrosion crack (da/dt) as a function of stress intensity (K) varies in
the manner shown in the idealised curve of Fig. 2.21.
At low K values, the crack velocity is highly K-dependent, becoming
vanishingly small at K I SCC . Considerable experimental patience is required
to establish the latter value, which may correspond to crack growth rates as
low as 10 –11 m s –1 . There is a K-independent velocity plateau at intermediate
values of K, and stage III in Fig. 2.21 shows a rapid acceleration as the value
of K Ic is approached. It is striking that an aggressive environment can commonly
reduce the value of K Ic by 95% down to that of K I SCC . The problem is
normally tackled by metallurgical means, i.e. by modifying the composition
of the alloy to increase its corrosion resistance, rather than by changing the
state of stress or the environment.
Hydrogen embrittlement
Hydrogen embrittlement is a special case of SCC caused by absorbed hydrogen
and it is particularly important for high-strength steels and aluminium alloys.
There are numerous potential sources of hydrogen, including welding,
electroplating and corrosive attack.
In the case of ferritic steels, it has been suggested that dissolved hydrogen
atoms can diffuse into internal cavities and recombine to form hydrogen gas.
The gas can exert an internal pressure to assist the fracture process. An