Physical and Chemical Mechanisms of Preparation Techniques - Sample Preparation Topic for Transmission Electron Microscopy - page 91

regulating the dissolution of the material. The electrolytic solutions used generally
contain three components: an acid or a base, in order to ensure oxidation of the
material; an agent that regulates the speed of dissolution (passivator); and a final
element that promotes a viscous state in the solution-metal interfacial zone. This
viscous layer becomes loaded with dissolution products and the result is a layer
whose electrochemical properties, as well as its viscosity, are modified. A concen-
tration gradient forms from the crevices to their projections, i.e., in the different
areas of roughness on the material's surface. There is then a higher current den-
sity on the projections, which dissolve faster than the crevices because the current
flow is facilitated and therefore the current density is higher. The surface gradually
levels out. This preferential dissolution eliminates the sample roughness caused by
mechanical polishing.
Several theories (Jacquet, Elmore, Edwards, Levin, Batashev and Nikitin,
Shigolev, etc.) explain this effect, but none of them are regarded as a universal the-
ory. Figures 5.6 and 5.7 show a model of the mechanism involved in this electrolytic
abrasion.

Fig. 5.6 Current density is
proportional to the
concentration gradient; it is
lower in the crevices and
higher in the projections

Current Lines

Electrolyte Bath

Material to be Polished

Fig. 5.7 Electrolytic
polishing principle (according
to Jacquet). The projections
dissolve faster than the
crevices