Page 43 - Corrosion Engineering Principles and Practice

24 C h a p t e r 2 C o r r o s i o n B a s i c s 25

FIGURE 2.4
Brass door knob
with evidence of
crystal structure
etched by corrosive
perspiration.
(Courtesy of
Kingston Technical
Software)

patterns that are, in fact, large grains visible without any etching. Brass
door knobs are normally bright and shiny, however, after some time
the corrosive perspiration from hands etches the crystalline features of
the alloy on the surface as shown in this second example (Fig. 2.4).
The procedure to determine the grain size or microstructure of a
metal or alloy is to first prepare a sample for microscopic study by
grinding and polishing the surface of a specimen mounted in a plastic
material. The polished surface is then corroded with a suitable etching
reagent, such as those briefly described in Table 2.3, that attacks more
readily the grain boundaries of a metallic microstructure to reveal its
characteristic features.
The grains shown in Figs. 2.3 and 2.4 are extremely large for most
metal crystals. Normally, metal grains are so small they can only be
satisfactorily observed with a microscope. The general range of grain
size usually runs from 25 to 250 µm in diameter. The American Society
for Testing and Materials (ASTM) grain number (G), defined in
Eq. (2.1) and illustrated in Fig. 2.5, is a convenient way to describe the
size of grains in a material:

N = 2 G−1 (2.1)
where N is the number of grains per square inch at a magnification of
100 times.