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Chapter VII Reactivity at Dislocation….
to be important; the dissolution takes place at moderate under-saturations.
However, both the edge and the screw dislocations are etched in nearly the
same manner and nearly the same rate in many crystals. Different theories of
dissolution, thermodynamic and topo-chemical adsorption are of interest since
they deal with the formation of dislocation etch pits on a crystal surface [15, 16].
The topo-chemical theories suggest the dissolution rate in terms of chemical
reactions on the crystal surface [16, 17]. It is considered that etch pits at the sites
of dislocations are formed as a result of enhanced dissolution caused by the
preferred adsorption of a reactant at that site because of the strain associated
with the dislocation. If the reactants are absorbed more strongly at the site of
dislocation it produces good contrast etch pits.
Topo-chemical theories are attractive for crystalline substances. The
dissolution of crystals involves the formation of reaction products, however, they
poses certain limitations. One limitation is that each etching system has to be
treated independently. Whereas, the thermodynamic theories are based on the
postulate that the energy localized in the vicinity of a dislocation lowers the free
energy needed for the nucleation of unit depth in the surface at the site of the
dislocation.
This decrease in the free energy is the cause of preferred dissolution of the
surface at the emergence points of dislocations. The free energy change
associated with the formation of a mono-molecular pit at dislocation site, ∆G is
given by the following expression,
∆G = 2ra -
r a
2
- aEd (7.1)
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