Page 117 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Chapter 3 Physical Properties of Materials
structures is a major concern, particularly at elevated temperatures and in automo-
biles and other transportation vehicles.
Resistance to corrosion depends on the composition of the material and on the
particular environment. Corrosive media may be chemicals (acids, alkalis, and salts),
the environment (oxygen, moisture, pollution, and acid rain), and water (fresh or salt
water). Nonferrous metals, stainless steels, and nonmetallic materials generally have
high corrosion resistance. Steels and cast irons generally have poor resistance and
must be protected by various coatings and surface treatments (Chapter 34).
Corrosion can occur over an entire surface, or it can be localized, called
pitting. (Pitting is a term that is also used for fatigue wear or failure of gears and in
forging; see Section 33.2.) Corrosion can also occur along grain boundaries of met-
als as intergranular corrosion, and at the interface of bolted or riveted joints as
crevice corrosion.
Two dissimilar metals may form a galvanic cell (after L. Galvani, 1737-1798)-
that is, two electrodes in an electrolyte in a corrosive environment that includes
moisture-and cause galvanic corrosion. Two-phase alloys (Chapter 4) are more sus-
ceptible to galvanic corrosion, because of the physical separation of the two different
metals involved, than are single-phase alloys or pure metals; as a result, heat treat-
ment can have a significant influence on corrosion resistance.
Stress-corrosion cracking (Section 2.l0.2) is an example of the effect of a cor-
rosive environment on the integrity of a product that, as manufactured, had residual
stresses. Likewise, cold-worked metals are likely to have residual stresses; hence,
they are more susceptible to corrosion than are hot-worked or annealed metals.
Tool and die materials also can be susceptible to chemical attack by lubricants
and by coolants; the chemical reaction alters their surface finish and adversely influ-
ences the metalworking operation. One example is that of carbide tools and dies
having cobalt as a binder (Section 22.5). The cobalt is attacked by elements in the
metalworking fluid (selective leaching). Thus, compatibility of the tool, die, and
workpiece materials with the metalworking fluid under actual operating conditions
is an important consideration in the selection of materials.
Chemical reactions should not be regarded as having only adverse effects.
Advanced machining processes such as chemical and electrochemical machining are
indeed based on controlled chemical reactions (Chapter 27). These processes re-
move material by chemical action, in a manner similar to the etching of metallurgi-
cal specimens. The usefulness of some level of oxidation is exhibited in the corrosion
resistance of aluminum, titanium, and stainless steel. Aluminum develops a thin (a
few atomic layers), strong, and adherent hard-oxide film (Al2O3) that better protects
the surface from further environmental corrosion. Titanium develops a film of tita-
nium oxide (TiO2). A similar phenomenon occurs in stainless steels, which, because
of the chromium present in the alloy, develop a protective film on their surfaces.
These processes are known as passivation. When the protective film is scratched and
exposes the metal underneath, a new oxide film begins to form.

EXAMPLE 3.1 Selection of Materials for Coins

There are five general criteria in the selection of mate- this criterion is the feel of the coin; this term is
rials for coins. difficult to describe, because it combines many
I. The subjective factors, such as the appearance human factors. It is similar in effect to the feel
of the coin, its color, weight, and its ring (the of a fine piece of wood, polished stone, or fine
sound made when striking). Also included in leather.