Figure 3 shows that the matrix structure of the material being examined is however only discernible after contrasting in many cases. After a thorough cleaning, the specimens are immersion etched for this purpose with a suitable etchant in the simplest case. This causes a minor and desirable corrosive attack, which amplifies the differences in reflecting capacity between grains with different orientation, different microconstituents and between crystallites and grain boundaries.
With immersion etching, the etchants can react very differently with the specimen surface. Some examples are shown in Figure 4 for materials with a single-phase structure.
Multi-phase materials contain at least two microconstituents, which are attacked by the etchant at different speeds due to their dissolution behaviour.
For immersion etching,the etching conditions (composition of the etchant, temperature, exposure time, pretreatment of the specimen, bath agitation) have to be matched to the material being examined and the preparation objective. Please refer to the numerous etchant formulations found in the literature [1, 5, 7–10].
Table 1 summarises some examples for ferrous materials. Here and in tables 2 through 4, the etchants are listed with the applicable designations of the chemicals.