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Chapter VII Reactivity at Dislocation….
from the surface during etching. This is known as preferential etching. This
technique is very simple and gives plenty of information, if it is used properly.
(b) Electrolytic Etching
Current flow is employed to cause dissolution of the material in a suitable
electrolytic. When a metal dissolves it forms positive ions, thereby each atom
should lose one or more electrons, i.e., it must be oxidized. This oxidation
process can be achieved either by an oxidizing substance in the solution or by
selecting the metal as an anode in the electrolytic cell; in that case the electrons
are removed by the battery or generator in the circuit. The rate of reaction in
electrolytic method is directly measured by the current flowing in the circuit.
(c) Thermal Etching
The material is removed from the surface by vaporization in a vacuum or in an
inert atmosphere. Thermal etching depends upon the conditions of atmosphere,
pressure and temperature of the media in which it is carried out. The temperature
is required sufficiently high for the surface atoms to become mobile so that the
thermal etching takes place. The conditions of temperature and pressure for
thermal etching, are such that either the reaction product is removed by
evaporation or it becomes chemically unstable.
(d) Ionic Etching
The material is removed by sputtering of ions. If the metal is placed as cathode
in a gas discharge tube, the current in the tube is carried out by positive ions of
the gas, which bombard the cathode and remove atoms from its surface. This
process is known as “sputtering”. Depending upon the tube conditions, such as
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