Environmental Engineering Reference
In-Depth Information
Figure 3.52
Atomic surface mobility mechanism. Atoms migrate out of the crack tip
from A to B, then subsequently to C and D. The vacancy created at A amounts to the
advancement of crack by one atomic distance.
Remedial Measures
Control of Stress
. Lowering of applied stress below threshold stress or stress
intensity by suitable changes in design and lowering or removal of residual stress
by annealing treatment are effective means to reduce the incidence of SCC in
service. The season cracking of brass cartridge cases, mentioned earlier was pre-
vented by proper stress relief annealing treatment. It is a recommended practice
to give such a treatment to brass tubes after drawing or to steel tubes after welding
for use in sour oil wells. However, annealing may be impractical for some stain-
less steels that sensitize and become susceptible to intergranular attack.
Since a tensile component of stress is required in SCC, it can be prevented
by putting the surface of a component into compression, e.g., by shot peening.
The treatment needs to be applied uniformly. It will not be effective if pitting
occurs on the compressive layer.
Control of Corrosion.
The elimination or reduction of the damaging species is
desirable, but often it is difficult to achieve in practice. The initial nondamaging
concentration of the species may become high in the crevices or under high-
temperature conditions bringing about failures. However, the prevention of inter-
granular SCC of sensitized stainless steel, a recurring and expensive problem in
cooling-water piping of boiling water nuclear power plants, has been achieved
by minimizing both dissolved oxygen and chloride [44].
Use of inhibitors reduces or even eliminates SCC, possibly by moving the
