Environmental Engineering Reference
In-Depth Information
advisable to test the compatibility of a given alloy to a new environment where
it may be expected to perform.
Sources of Stress
The existence of a tensile stress, either applied or present as a residual stress, is
essential for the cracking process. In order for SCC to occur a threshold stress
or stress intensity (see section ''Testing Methods'') must be exceeded.
Residual stresses of high magnitude, 70% of the yield strength of the material
or even higher, may be produced by cold-forming operations like bending, roll-
ing, deep drawing, etc., or by thermal processing like welding, solidification of
casting with large differences in section thickness, severe quenching processes,
etc. Often design stress in service is sufficiently high to precipitate the occurrence
of SCC. The presence of stress raisers may aggravate the situation. These stress
raisers may be the geometrical stress raisers or notches related to design, inclu-
sions, rough machining marks, welding strikes, and so on, and also may result
from localized corrosion attack such as pitting, selective leaching, or intergranular
corrosion.
Exposure of metal parts to high and low temperatures results in nonuniform
heating rates and sharp thermal gradients which, in turn, may give rise to thermal
stresses of very high magnitude. This is quite common in heat exchangers. If the
environment on the cold side of the heat exchanger is conducive to stress corro-
sion, cracking is produced by these thermal stresses.
Corrosion products accumulating in fissures or tightly jointed parts may pro-
duce wedging action providing sufficiently high stress for SCC.
General Features of Stress Corrosion Cracks
Stress corrosion environments being usually mildly corrosive from the viewpoint
of general attack, the surface of the stress-corroded part exhibits only faint signs
of corrosion while fine cracks penetrate deeply into the part. The mode of crack-
ing can be intergranular or transgranular (Figs. 3.35 and 3.36). The cracks proceed
in a direction perpendicular to the stresses and often show branching.
One type of cracking usually occurs more readily in a given alloy. For exam-
ple, carbon steels exhibit intergranular mode of cracking in nitrate solutions,
whereas austenitic stainless steels crack transgranularly in boiling MgCl
2
solu-
tions. However, mixed mode of cracking may also be observed; depending on
environment, alloy composition, cold work, or metallurgical conditions, there can
be a transition from one type of cracking to the other. For example,
α
brass
cracks intergranularly in ammoniacal solutions, but
β
brass (
40% Zn) cracks
transgranularly in the same environment. Again,
brass shows transgranular
cracking if heavily cold-worked or in highly alkaline ammoniacal solutions. Aus-
tenitic stainless steels in sensitized condition crack intergranularly.
Stress corrosion cracks have the appearance of a brittle mechanical fracture.
α
