48 Fundamentals of Corrosion
inclusive; newer combinations and newer environments that cause SCC in a
particular alloy are constantly being discovered and are being added to the
list.
An ammonia environment can induce SCC in copper-containing alloys
whereas with low-alloy austenitic stainless steels, a chloride-containing
environment is necessary. It is not necessary to have a high concentration of
corrodent to cause SCC. A solution containing only a few parts per million
of the critical ion is all that is necessary. Temperature and pH are also fac-
tors. There is usually a threshold temperature below which SCC will not take
place and a minimum or maximum pH value before cracking will start.
The alloy content of stainless steel, particularly nickel, determines the sen-
sitivity of the metal to SCC. Ferritic stainless steel, which is nickel-free, and
the high-nickel alloys are not subject to SCC. An alloy with a nickel content
greater than 30% is immune to SCC. The most common grades of stainless
steel (304, 304L, 316, 316L, 321, 347, 303, and 301) have nickel contents in the
range of 7 to 10% and are the most susceptible to SCC.
Examples of SCC include the cracking of austenitic stainless steels in the
presence of chlorides, caustic embrittlement cracking of steel in caustic solu-
tions, cracking of cold-formed brass in ammonia environments, and the
cracking of monel in hydrouorosilicic acid.
In severe combinations such as type 304 stainless steel in a boiling magnesium
chloride solution, extensive cracking can be generated in a matter of hours.
Unalloyed titanium with an oxygen content of less than 2% (ASTM grades
1 and 2) is susceptible to SCC in absolute methanol and higher alcohols, cer-
tain liquid metals such as cadmium and possibly mercury, red-fuming nitric
acid, and nitrogen tetraoxide. The presence of halides in the alcohol acceler-
ates cracking tendencies. The presence of water (<2%) tends to inhibit SCC in
alcohols and red-fuming nitric acid. Titanium is not recommended for use in
these environments under any anhydrous conditions.
Fortunately, in most industrial applications, the progress of SCC is much
slower. However, because of the nature of the cracking, it is difcult to detect
until extensive corrosion has developed, which can lead to unexpected failure.
Normally, SCC will not occur if the part is in compression. Fatigue is trig-
gered by a tensile stress that must approach the yield stress of the material. The
stresses may be induced by faulty installation or they may be residual stresses
from welding, straightening, bending, or accidental denting of the equipment.
Pits, which act as stress concentration sites, will often initiate SCC.
Tensile stresses can lead to other corrosion processes, such as the simple
mechanical fatigue process. Corrosion fatigue is difcult to differentiate from
simple mechanical fatigue but it is recognized as a factor when the environ-
ment is believed to have accelerated the normal fatigue process. Such sys-
tems can also have the effect of lowering the endurance limit so that fatigue
will occur at a stress level below which it would be normally expected.
It is important that any stresses that may have been induced during the
fabrication be removed by an appropriate stress-relief operation. Care should