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Chapter VII Reactivity at Dislocation….
under saturated at a particular kink under consideration. Any one or combination
of these factors controls the dissolution. This has been shown in figure (7.1).
The edges of a crystal are considered to be good source of steps and kinks
during dissolution; therefore, in practical cases the rate of dissolution is not
limited by the requirement of these defects. The first step in dissolution at the
centre of a crystalline face is the generation of a mono-molecular pit, as shown in
figure (7.1). If the bond energy in simple cubic structure is , then 5 energy is
required to remove first atom from a perfect surface. However, this energy is 3
for an atom at a kink, and 4 for an atom on a step edge. One can consider an
atom at center of an edge dislocation is also having energy nearly 4, which is
less than the energy for the perfect surface.
Single lattice vacancies situated at the surface also form small
monomolecular pits and may influence dissolution under certain conditions.
However, the steps produced by them are difficult to observe. On the other hand,
the clusters of lattice vacancies or impurity atoms can some times be detected
[13]. It is important to note that the dislocation lines are quite effective in acting
as nucleation centers for dissolution steps. The effect of dislocation lines on
dissolution nucleation is due to their elastic energies or the chemical potential of
the material near by [14].
The surface layer around a screw dislocation that lies normal to a free surface
is stress free except at the core of the dislocation. However, for an edge
dislocation that intersects a free surface, the most of the strain energy remains
with the edge. The surface steps associated with screw dislocations are thought
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