Page 213 - Materials Chemistry, Second Edition

200 3 Metals

but is not easily drawn into a wire without the use of die-extrusion techniques. It should
be noted that excessive cold working of metals may cause brittleness, where the metal
willfracture rather than exhibitingplastic flow under stress. As previously mentioned, a
high concentration of carbon in the metal lattice (e.g., cast iron) will also cause
brittleness, which explains the lack of structural applications for these metals.

3.2.3. Stainless Steels
Technological advancements in iron ore processing and metal doping have resulted
in the fabrication of many types of high-strength steels for diverse applications. By
contrast, earlier generations worked exclusively with wrought iron, an inferior
material containing >20 wt.% C, formed through simple annealing of the ore with
coal. We have seen that the concentration and form (e.g.,Fe 3 C, graphite, etc.) of
dopant species will alter the physical properties of the material. Hence, the bulk and/
or surface thermal and physical processing of steels is the most important consider-
ation for ultimate material performance, as these treatments greatly affect its
microstructure.
Thus far, we have focused primarily on Fe–C alloys, with carbon atoms posi-
tioned within vacant interstitial sites within the iron lattice. As you may expect, a
variety of other elements may also be present in steel that will alter its overall
physical properties. For example, all steels contain manganese that assists in hard-
ening mechanisms, as well as facilitating the removal of sulfur and oxygen atoms in
the matrix. This prevents FeS formation and removes bubbles in the molten state of
steels, both of which would greatly contribute to brittleness of the final product.
Typically, large transition metal dopants will exist as substitutional alloys, ran-
domly replacing iron sites throughout the lattice. Steel containing <0.30 wt.% C
and chromium concentrations >10.5 wt.% are referred to as stainless steels. The
addition of Cr results in the formation of a native layer of Cr 2 O 3 , providing corrosion
resistance. As the concentration of Cr is increased, the material is concomitantly less
predisposed to rust. Such protection occurs from the comparative oxidation poten-
tials between Cr and Fe (Eq. 20). For redox processes, the spontaneity (Gibbs free
energy) is governed by Eq. 21, where a negative DG indicates a spontaneous
reaction at equilibrium. If both chromium and iron are present together, chromium
atoms are considered as a sacrificial anode, being preferentially oxidized leaving
the iron untouched. Other metals with large positive oxidation potentials such as
Zn (þ 0.763 V), Al (þ 1.68 V), Ni (þ 0.257 V), and Ti (þ 2.00 V) are also useful
additives that serve as corrosion barriers.

Fe ! Fe 3þ þ 3e E ¼ 0.331 V
ð20Þ

Cr ! Cr 3þ þ 3e E ¼þ1:32 V;

ð21Þ DG ¼ nFE
where n is the number of electrons involved in the redox process, F is Faraday’s
1
4

constant (9.64853 10 C mol ) and E is the reaction potential, measured at STP.