Page 103 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 81

3. Regions close to critical potential for localized corrosion, which is observed in
the case of austenitic stainless steels in near-neutral solutions (80, 114).

Thus it is obvious from the foregoing discussion that complex relationships exist
between corrosion fatigue cracking (CFC), SCC and hydrogen embrittlement crack-
ing (HEC), and the distinction between CFC, SCC, and HEC is difficult as it is likely
that hydrogen–metal interaction near the crack tip is the controlling process of SCC
or CF crack propagation (73).

1.8.10.15 Mechanism of SCC The crack initiation of environmentally assisted
cracking (EAC) is complex and not well understood until now. The majority of SCC
systems exhibit short initiation times ranging from minutes to weeks, and cracking
often occurs because of a change in environment rather than long initiation time. The
stress corrosion growth rates are in the range 10 −11 to 10 −6 m/s. In the case of stain-
less steels in chloride solutions, localized corrosion may create local conditions prone
to crack development, but it is difficult to rationalize initiation of crack development
in the absence of localized corrosion in environmental conditions different from that
of propagation. It is useful to note that dealloyed surface layers such as copper alloys
in ammonia solutions tend to undergo SCC (44).
Surface films appear to play a major role in the initiation of SCC and may also
contribute to HE effects. The main role of the surface film is to localize the dam-
age inflicted on the material by the environment. The damage can be caused by the
mechanical breakdown of the passive film by slip step or electrochemical breakdown
of the passive film (73). SCC may be related to the nature of the surface film. The
∘
SCC of carbon steels is related to the presence of magnetite in environments at 90 C
except when pitting is involved in the crack initiation process, as in nitrate medium
or in high-temperature water (115, 116).

1.8.10.16 Mechanism of HE The hydrogen from the environment is adsorbed at
the crack tip resulting in reduction in the effective bond strength and lowered sur-
face energy leading to the diffusion of hydrogen atoms into the metal (Fig. 1.23a, b)
with decohesion of atoms by hydrogen influx to the dilated lattice (Fig. 1.23c). Some
interactions may occur in advance of the crack tip where the stress and/or strain con-
ditions are favorable for the nucleation of a crack. The nucleation of a crack may be
followed by the formation of a brittle phase such as metal hydride (Fig. 1.23d). It
is also possible that the hydrogen atoms can combine to form molecular hydrogen,
which can cause pressure inside the metallic network and can cause inflation, and this
is the mechanism of formation of blisters.
Hydrogen-induced crack growth as the dominant SCC mechanism has been sug-
gested for ferritic steels, nickel alloys, titanium alloys, and aluminum alloys. The
effects of factors such as yield strength, impurity segregation, and the temperature on
the crack growth behavior of ferritic materials in aqueous solutions follow the trends
of HE. Tin and antimony, known as hydrogen recombinant poisons, segregated to the
grain boundaries of nickel alloy, increase the uptake of atomic hydrogen (117).