Environmental Engineering Reference
In-Depth Information
Figure 3.33
Cavitation degree of a pump impeller showing a honeycomb type of at-
tack.
Stress corrosion cracking (SCC) involves static tensile stress, whereas corro-
sion fatigue takes place under the conditions of cyclic or alternating stresses with
a tensile component. Hydrogen-induced cracking (HIC) also involves tensile
stresses, but the cracking process is distinctly related to the entry of atomic hydro-
gen into the metal. Cathodic reduction of hydrogen ions in a corrosion process
is one of the sources of atomic hydrogen. Since hydrogen-induced cracking falls
under the broad category of hydrogen damage, this will be discussed in a subse-
quent chapter (Chapter 8).
3.9.1 Stress Corrosion Cracking
Stress corrosion cracking (SCC) is defined as the delayed failure of alloys by
cracking when exposed to certain environments in the presence of static tensile
stress. The importance of a conjoint action of corrosion and stress is reflected in
the definition; an alternate application of stress and corrosive environment will
not produce SCC. The stress level at which the failure occurs is well below the
stress required for a mechanical failure in the absence of corrosion. The minimum
stress below which SCC is not encountered is called threshold stress (Fig. 3.34),
but this may be as low as 10% of the yield strength in some systems. Again,
corrosion alone in the absence of stress does not cause SCC.
The earliest report on SCC is probably the occurrence of ''season cracking''
in brass cartridge cases in ammonia-bearing environments in the beginning of
the present century.
Caustic embrittlement
of riveted steel boiler plates is another
classical example of SCC encountered in the early steam-driven locomotives.
