Page 116 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)

Section 3.8 Corrosion Resistance

and type of impurities (dopants), such as phosphorus and boron in silicon, electrical
conductivity can be controlled. This property is utilized in semiconductor (solid-
state) devices, used extensively in miniaturized electronic circuitry (Chapter 28).
Ferromagnetism and Ferrimagnetism. Ferrornagnetism is a phenomenon charac-
terized by high permeability and permanent magnetization that are due to the align-
ment of iron, nickel, and cobalt atoms into domains. It is important in such
applications as electric motors, electric generators, electric transformers, and mi-
crowave devices. Ferrimagnetisrn is a permanent and large magnetization exhibited by
some ceramic materials, such as cubic ferrites.
Piezoelectric Effect. The piezoelectric effect (piezo from Greek, meaning “to
press ”) is exhibited by what are called smart materials. Two basic behaviors are
involved: (a) When subjected to an electric current, these materials undergo a
reversible change in shape, by as much as 4%, and (b) when deformed by an exter-
nal force, the materials emit a small electric current. This unique property is utilized
in making transducers, which are devices that convert the strain from an external
force into electrical energy. Piezoelectric materials include quartz crystals and some
ceramics and polymers, Typical applications are sensors, force or pressure transduc-
ers, inkjet printers, strain gages, sonar detectors, and microphones. For example, an
air bag in an automobile has a sensor that, when subjected to an impact force, sends
an electric charge which deploys the bag.

Magnetostriction. The phenomenon of expansion or contraction of a material
when it is subjected to a magnetic field is called niagnetostriction. Materials such as
pure nickel and some iron-nickel alloys exhibit this behavior. Magnetostriction is
the principle behind ultrasonic machining equipment (Section 26.6).
Magnetorheostatic and Electrorheostatic Effects. When subjected to magnetic or
electric fields, some fluids undergo a major and reversible change in their viscosity
within a fraction of a second, turning from a liquid to an almost solid state. For
example, magnetorheostatic behavior is attained by mixing very fine iron filings with
oil. Called smart fluids, these materials are being developed for applications such as
vibration dampeners, engine mounts, prosthetic devices, clutches, and valves.
Optical Properties. Among various other properties, color and opacity are partic-
ularly relevant to polymers and glasses. (These two properties are described in
Sections 7.2.2 and 8.4.3, respectively.)

3.8 Corrosion Resistance

Metals, ceramics, and plastics are all subject to forms of corrosion. The word
corrosion itself usually refers to the deterioration of metals and ceramics, while sim-
ilar phenomena in plastics are generally called degradation. Corrosion not only
leads to surface deterioration of components and structures (bridges and ships), but
also reduces their strength and structural integrity. The direct cost of corrosion to
the U.S. economy alone has been estimated to be over $400 billion per year, approx-
imately 3% of the gross domestic product; indirect costs of corrosion are estimated
at twice this amount.
Corrosion resistance is an important aspect of material selection for applica-
tions in the chemical, food, and petroleum industries, as well as in manufacturing
operations. In addition to various possible chemical reactions from the elements and
compounds present, environmental oxidation and corrosion of components and