Page 220 - Corrosion Engineering Principles and Practice

194 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 195

microscopic in their early stages of development. In many cases, they
are not evident on the exposed surface by normal visual examination,
and can be detected only by microscopic examination with an optical
or a scanning electron microscope. As the cracking penetrates farther
into the material, it eventually reduces the effective supporting cross
section to the point where the structure fails by overload or, in the
case of vessels and piping, escape (seepage) of the contained liquid or
gas occurs.
Cracking is usually either intergranular (intercrystalline) as
shown in Fig. 6.44 or transgranular (transcrystalline) as illustrated in
Fig. 6.45. Occasionally, both types of cracking are observed in a failure.
Intergranular cracks follow the grain boundaries in the metal.
Transgranular cracks cross the grains without regard for the grain
boundaries. The morphology of the cracks may change when the
same material is exposed to different environments.
Failures are not necessarily the result of ordinarily applied
engineering loads or stresses. However, these loads have to be added
to invisible residual stresses already present in a structure due to
various sources such as fabricating processes (e.g., deep drawing,
punching, rolling of tubes into tubesheets, mismatch in riveting,
spinning, welding, and so forth).
Residual stresses will remain in a structure unless it is annealed
or otherwise thermally stress relieved following fabrication, a
practice that becomes increasingly impractical as a system gets larger

FIGURE 6.44 Typical intergranular stress corrosion cracks in cartridge brass
(70 Cu, 30 Zn). Etched 30 percent H O , 30 percent NH OH, 40 percent
2
2
4
H O (×75) (Corrosion Basics: An Introduction, 2nd edn., NACE International,
2
by permission)