328 Steels: Metallurgy and ApplicationsStress corrosion crackingStress corrosion cracking (SCC) is a form of failure induced by the conjoint
action of tensile stresses and particular types of corrosive environments. The
stresses can be either applied or residual and cracking takes place in a direction
normal to the tensile stresses, often at stress levels below the yield strength of the
material. A micrograph illustrating SCC in an austenitic stainless steel is shown
in Figure 4.18. Cracking can take place in either a transgranular or intergranular
manner and can proceed to the point where the remaining material can no longer
support the applied stress and fracture then takes place.
SCC occurs in chloride, caustic or oxygen-rich solutions but the majority of
failures in austenitic stainless steel take place in chloride-bearing environments.
Chloride ions may be present in the process stream but can also be introduced
accidentally through the incomplete removal of sterilizing agents, such as
hypochlorite solutions.
It is now generally acknowledged that SCC is initiated by the formation
of a pit or crevice at anodic sites on the surface, e.g. outcropping inclusions
which disrupt the passive film. The reactions controlling pitting corrosion were
described in the previous section. A number of mechanisms has been proposed
for crack propagation but the surviving theories are based on anodic dissolution
and hydrogen embrittlement mechanisms. In the former, cracking propagates by
local anodic dissolution at the crack tip, passivation at that point being prevented
by plastic deformation. Alternatively, the principal role of plastic deformation
Figure 4.18 Stress corrosion cracking in ~pe 304 stainless steel