7. Procedure
7.1 The metallographer may be required to electropolish
some metal or alloy which has not previously been encountered.
There are several general principles which can be applied
advantageously in these cases. The problem should be viewed
in comparison with known procedures and information gained
through previous experience. It is generally helpful to compare
the relationship of the major component with elements of the
same general group in the periodic arrangement of the elements,
and to study the phase diagram, if available, to predict
the number of phases and their characteristics. Single-phase
alloys are generally easier to electropolish than multi-phase
alloys. Minor alloying elements may affect the material’s
response to polishing in a given electrolyte.
7.2 The conditions required for optimum polishing of a
metal in a given electrolyte can be ascertained by plotting
current density versus voltage curves. The resulting curves will
approximate one of the two forms shown in Fig. 2. Curve I is
typical of electrolytes that either polish over a very wide range
or will not polish at all. Curve II is characteristic of electrolytes
that form an ionic film. The dotted portion of the curve is added
in recognition of certain published data and the observation
that the formation of a polishing film requires finite time.
Polishing will occur between B and C and is usually best
slightly before C.
7.3 After the polishing range is determined, other constants
such as preparation, electrolyte flow and time can be determined
experimentally. In the majority of cases, a mechanically
prepared surface, as produced by grinding up to a 600-grit
silicon carbide finish, is sufficient; however, with some alloys,
amechanically polished surface is desirable. Metal removal
rates by electropolishing are low, typically around 1 μm per
minute. To remove all prior damage from grinding, either some
rough polishing should be performed after the 600-grit grinding
step, or the electropolishing time may be extended, which
may not be desirable. The surface to be electropolished should
be thoroughly cleaned to obtain uniform attack by the electrolyte.
7.4 Small specimens may be mounted in compression
mounting media or in castable mounting media for ease in
handling for mechanical preparation. Both types of mounting
media are generally impervious to attack by the electrolyte (see
5.7). When mounted samples are to be electropolished, electrical
contact can be made through a small hole drilled through
the back of the mount into the metal.
7.5 To aid in selecting or developing an electrolyte for a new metal or alloy the following characteristics should be
considered: