Lectures on

Materials Science
Structure
MEC 3202

Properties Processing

Najib Ahmad Muhammad, Ph.D.

 Introduction to Materials Science

1.1 Overview of Materials Science and its Importance

Material, as defined by the dictionary, is any physical substance used in the composition of an
object. Materials science, in general, on the other hand, can be divided into subdisciplines. The
study of materials science involves exploring the connection between materials’ structures and
properties. Materials engineering designs materials to have specific properties based on their
structure. Materials Scientists create new materials, while Materials Engineers use existing ones
to make new things and develop ways to process materials. Most graduates in materials programs
are trained to be materials scientists and engineers.
The structure is a cloudy term that deserves some explanation. In brief, the structure of a
material usually relates to the arrangement of its internal components. The subatomic structure
involves electrons within the individual atoms and interactions with their nuclei. On an atomic
level, structure encompasses the organization of atoms or molecules relative to one another. The
next larger structural realm is called the microscopic. Large groups of atoms, typically clustered
together, can be directly observed using a microscope and belong to the next larger structural realm.
The structural elements that are visible to the naked eye are referred to as macroscopic. The notion
of property deserves elaboration. While in service use, all materials are exposed to external stimuli
that evoke some response. For example, a specimen subjected to forces will experience
deformation, or a polished metal surface will reflect light. A property is a material trait regarding
the kind and magnitude of response to a specific imposed stimulus. One can define properties
regardless of material shape and size.
There are six categories that cover all important properties of solid materials: mechanical,
electrical, thermal, magnetic, optical, and deteriorative. For each, there is a characteristic type of
stimulus capable of provoking different responses. Mechanical properties relate deformation to an
applied load or force; examples include elastic modulus (stiffness), strength, and toughness.
Electrical properties like conductivity and dielectric constant respond to an electric field. The
thermal behavior of solids can be represented in terms of heat capacity and thermal conductivity.
Magnetic properties show the response of a material to the application of a magnetic field. For
optical properties, the stimulus is electromagnetic or light radiation; index of refraction and

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reflectivity are representative optical properties. Finally, deteriorative characteristics relate to the
chemical reactivity of materials.
In materials science and engineering, processing and performance are essential, as are
structure and properties. The interplay of these four components determines the structure of a
material based on how it is processed. A material’s performance will be a function of its properties.
Fig. 1 shows how processing, structure, properties, and performance are interrelated. We focus on
how these four components are related to material design, production, and use.

Processing Structure Properties Performance

Figure 1 The four components of materials science and engineering discipline and their interrelationship.

1.2 Why is it Important to Study Materials Science/Engineering?

Engineers and scientists will face material design problems in their careers. Some examples
are a gear, farm machine, building frame, oil refinery part, or computer chip. Materials scientists
and/or engineers investigate and design materials. Often, material issues lie in the material’s
selection from a plethora of thousands of options. The final decision is normally based on several
criteria. First, the in-service conditions must be characterized, for these will dictate the properties
required of the material. Only on rare occasions does a material possess the maximum or ideal
combination of properties. Thus, it may be necessary to trade one characteristic for another. The
classic example involves strength and ductility; normally, a material having a high strength will
have only limited ductility. A reasonable compromise between two or more properties may be
necessary in such cases. Second, we consider any material damage during operation. High
temperatures or corrosive environments can weaken mechanical strength. Finally, the overriding
consideration is economics: What will the finished product cost? A material with perfect properties
might be discovered, but its cost may be exorbitant. Here again, some compromise is inevitable.
The cost of a finished piece also includes any expense incurred during fabrication to produce the
desired shape. Knowing the characteristics and properties of materials helps engineers and
scientists make better choices.

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1.3 Classification of Materials
Classifying materials into three classes: metals, ceramics, and polymers, is easy. This system
relies mainly on chemical composition and atomic arrangement. Materials can generally be
categorized into specific groups. There are engineered composites made by combining two or more
different materials. Next, a brief explanation may be provided about the material classifications
and their representative characteristics. Another category is advanced materials used in high-
technology applications, such as semiconductors, biomaterials, smart materials, and
nanoengineered materials.

1. Metals and Alloys

Various types of metals and alloys include steel, aluminum, magnesium, zinc, cast iron,
titanium, copper, and nickel. Metals are a versatile and vital material in many industries. They can
be used for structural support, electrical components (in electrical conductor wire, for instance),
and thermal conductivity; aluminum, magnesium, zinc, cast iron, titanium, copper, and nickel are
commonly used due to their high stiffness, which is measured by the elastic modulus E. They also
have a low yield strength σy, which can be improved through alloying, mechanical, and heat
treatment. This allows them to be formed by deformation processes like rolling or forging while
remaining ductile. Metals are also tough, with a high fracture toughness, K1c. However, they are
susceptible to corrosion if not properly protected. An alloy is a metal that contains additions of
one or more metals or non-metals. It combines two or more mixed metals to create a new substance
with unique properties. Many applications use alloys, from construction to electronics, because of
their strength, durability, and resistance to corrosion. Fig. 2 shows various products made of
different types of metal and alloy. Some common examples of alloys include steel, brass, and
bronze. Steel is an alloy of iron and carbon, brass is an alloy of copper and zinc, and bronze is an
alloy of copper and tin. Alloys are essential in modern manufacturing and vital in many industries.
Metals have good electrical and thermal conductivity. Metals and alloys have relatively high
strength, stiffness, ductility or formability, and shock resistance. They are useful for structural or
load-bearing applications. Although pure metals are occasionally used, alloys improve a desirable
property or permit a better combination of properties.

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Figure 2 A range of metallic and metal alloys objects, comprising silverware (fork and knife), scissors, coins,
a gear, a wedding ring, and a nut and bolt (in a left-to-right configuration).

2. Ceramics
Ceramics, such as porcelain or alumina, are non-metallic inorganic solids—and are often used
as the material for the insulator casing of the spark plug in a petrol engine. They have many
attractive features, including stiffness, hardness, and abrasion resistance. Additionally, they are
able to retain their strength at high temperatures while resisting corrosion well. Most ceramics are
also good electrical insulators. However, they do have their weaknesses. Unlike metals, ceramics
are brittle and have low K1c, which means they have a low tolerance for stress concentrations (e.g.,
holes or cracks) and high contact stresses (e.g., at clamping points).

Figure 3 A range of ceramic materials made objects: scissors, a china teacup, a building brick, a floor tile, and
a glass vase.

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3. Polymers
Polymers are interesting materials made up of long-chain carbon or silicon molecules. They
are light and flexible, yet can also be strong. Therefore, they are suitable for food packaging. The
densities 𝜌 of polymer materials are lower than even the lightest metals. Their properties change
with temperature, so it’s important to consider this when designing with them. For example, a
flexible polymer at room temperature might become brittle in a freezer (e.g., −6⁰ C) or rubbery in
boiling water (~100⁰ C, for instance). However, if you consider these factors when designing with
polymers, they can be incredibly useful. One advantage of polymers is that they are easy to shape,
so they are often called “plastics.” Because of this, you can create complex parts with multiple
functions in a single molding operation, which can be very cost-effective. Polymers are well-suited
for components that snap together, eliminating the need for finishing operations. By taking
advantage of these properties, you can create high-quality products that are both functional and
cost-effective.

Figure 4 A range of polymeric materials made objects: plastic tableware (spoon, fork, and knife), billiard
balls, a bicycle helmet, two dice, a lawn mower wheel (plastic hub and rubber tire), and a plastic milk carton.

4. Glasses, elastomers, and hybrids (Home Exercise)

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Advanced materials
Advanced materials are used in high-tech applications, such as electronic equipment, fiber-
optic systems, and spacecraft. These materials are typically traditional materials with enhanced
properties or newly developed high-performance materials, including semiconductors,
biomaterials, and “materials of the future,” like smart and nano-engineered materials. They are
expensive and can be made of different material types, like metals, ceramics, and polymers. Some
examples of their applications are lasers, integrated circuits, magnetic information storage, liquid
crystal displays (LCDs), and fiber optics.

(1) Semiconductors
Semiconductors are materials with electrical properties that are intermediate between
conductors (metals and metal alloys) and insulators (ceramics and polymers). Their electrical
characteristics are susceptible to the presence of impurity atoms, which can be precisely controlled
over small areas. Semiconductors have enabled integrated circuitry that has transformed the
electronics and computer industries (and our lives) over the past three decades.

(2) Biomaterials
Biomaterials can replace damaged or diseased body parts and must be non-toxic and
compatible with body tissues. Examples of biomaterials include metals, ceramics, polymers,
composites, and semiconductors, some of which are used in artificial hip replacements.

(3) Smart Materials

Smart materials are advanced materials capable of sensing changes in their environment and
responding to them in predetermined ways, similar to living organisms. Sophisticated systems
consist of smart and traditional materials, with components including sensors and actuators that
can change shape, position, natural frequency, or mechanical characteristics in response to changes
in temperature, electric fields, and/or magnetic fields.
(4) Nanomaterials
Nanomaterials are a new material class with unique properties and technological potential.
Unlike other materials, nanomaterials are classified based on size rather than chemistry and can be
made of metals, ceramics, polymers, or composites. In other words, their dimensions in
nanometers (10−9 m), less than 100 nanometers (corresponding to around 500 atom diameters).
With the development of scanning probe microscopes, it has become possible to design and build

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new structures from their atomic-level constituents, one atom or molecule at a time, providing
opportunities to develop mechanical, electrical, magnetic, and other properties that are not
otherwise possible.

1.4 Historical Perspective on Materials Development

With the current technological advancement, take a few moments to ponder what your
existence would be like without the presence of all the materials you are surrounded by. These
materials are necessary for daily things, like cars, phones, computers, and furniture. Materials
influence every fragment of our routine life. Without these, our way of life would resemble our
Stone Age forefathers/ancestors. Throughout history, the progress and evolution of civilizations
have been closely tied to their capacity to control and use materials. Ancient civilizations have
been classified based on the extent of their material advancement. The earliest humans had access
to a scant quantity of materials, only those found in the environment, e.g., stone, clay, wood, and
skin. Over time, they developed strategies for creating materials that outperformed their natural
equivalents, with pottery and an array of metals among the new substances. It was revealed that
the attributes of a material could be changed through heat treatments and incorporating additional
substances. We used material utilization to choose the best material from a limited set for a specific
use. Scientists only recently discovered the link between a material’s structure and properties. This
knowledge, assimilated over the past hundred years, has strengthened them.
Modern society demands unique qualities from materials like metals, plastics, glasses, and
fibers—the evolution of multiple technologies that improve our quality of life. An improvement
in comprehending a type of material often precedes a gradual progression. For instance, creating
automobiles was only possible because there had been readily available and affordable steel or a
similarly adequate alternative. In the modern era, complex electronic devices depend on produced
components.

1.5 Atomic Structure and Bonding

This section encompasses fundamental knowledge of atomic composition and bonding across
diverse materials. Materials Science I was the venue for its introduction. Turn to your lecture notes
for revision.

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 Mechanical Behavior and Properties of Materials
Materials respond to forces based on their stiffness, elasticity, resistance to deformation, and
breaking, among other properties. Understanding the properties of materials is crucial to selecting
the appropriate one for a specific application and using it effectively. For example, let us say you
have a steel ruler. It can bend easily and then spring back into shape—called elastic behavior. The
ruler’s stiffness (or resistance to bending) depends on both its shape and the type of steel it is made
from. Thin steel strips are easier to bend than thick ones, and materials with high elastic modulus
E (like steel) are inherently stiffer than those with low elastic modulus E (like polyethylene). Fig.
5(b) shows what can happen if something is not stiff enough.

Figure 5 Illustration of various conditions for mechanical properties.

A steel ruler has some flexibility or bends elastically, but a good quality one is difficult to bend
permanently. The ruler’s ability to be permanently bent is related to its strength, not stiffness. The
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ease of permanent deformation depends on the ruler’s shape and the property of the steel called
yield strength (represented by σy). Materials with high yield strength, such as titanium alloys, are
resistant to permanent deformation, even if their stiffness (coming from E) is not particularly high.
Materials with low σy, such as lead, can be easily deformed. When metals are deformed, they tend
to become stronger, a process known as “work hardening.” However, there is an ultimate limit
to this strength called the tensile strength (𝜎𝑡𝑠 ). If this limit is exceeded, the material will fail, and
the amount of stretching it undergoes before breaking is called ductility. Hardness (H) is related
to strength (σy), and higher hardness implies greater scratch resistance and wear resistance. Fig.
5(c) illustrates the consequences of insufficient strength.

Stress, strain, and elasticity

Before we dive into the topic, let us start by understanding stress and strain. While you may
think of force and deformation, it is important to note that the response of a material to force is
dependent on its size. For instance, it takes a lot more force to bend a copper pipe than a thin
copper wire. This is why materials engineers use the terms stress and strain instead of force and
deformation. We will focus on tensile stress and tensile strain for now, which result from a “pulling”
force, as shown in Fig. 6. Tensile stress is denoted by the symbol σ (sigma) and can be calculated
using Eq. (1):

𝐹
𝜎=𝐴 (1)
0

The stress calculated based on the initial area is referred to as engineering stress. However,
true stress is calculated based on the actual area, A. The symbol ɛ (epsilon) represents tensile strain
and can be computed using Eq. (2):

∆𝐿 𝐿−𝐿0
𝜀= = (2)
𝐿0 𝐿0

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 𝐴
𝐹

𝐴0

𝐿0

Figure 6 Illustration of deformation behavior of a bar for a metallic material owing to the application of a
tensile force, F. The bar has an initial length L0 and an initial cross-sectional area A0. The force causes the bar
to become longer and thinner, with a new length L, and a new cross-sectional area A.

The term we are referring to is engineering strain. Similar to stress, we can also define true
strain, but it is not commonly used, so we will not be discussing it further. You may recall Hooke’s
law from your physics studies, which pertains to springs in Eq (3):
𝐹 = −𝑘𝑥 (3)
The formula states that the length of a spring increases proportionally with the amount of force
applied to it. The constant k represents the spring constant, which varies based on the type of spring.
Similarly, we can create an equation for a material that experiences stress in Eq (4):
𝜎 = 𝐸𝜀 (4)
The constant E in this equation is known as the elastic modulus, Young’s modulus, or simply
the modulus. Similar to the spring constant, the modulus determines the rigidity of the material—
in other words, how much force is required to stretch it by a specific amount. A material with a
higher modulus is stiffer and experiences less deformation for a given stress. When conducting a
tensile test by pulling on the material at a consistent rate, we obtain a stress-strain curve, which
shows the relationship between stress and strain. Fig. 7 displays a typical stress-strain curve that
we will use to comprehend the information that these curves provide. It is important to note that
no material exhibits stress–strain curve identical to Fig. 7.
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 Figure 7 Illustration of important characteristics of general stress–strain curve.

It is important to understand the distinction between the elastic and plastic regions when
examining stress-strain curves. During the elastic region, the curve is linear, and Hooke’s law
applies, meaning that the slope is the elastic modulus. In this stage, any deformation to the material
is recoverable, and it returns to its original shape once the stress is removed. The proportional limit
marks where the curve is no longer linear, and the resulting deformation is permanent and
irrecoverable, known as the plastic region. When attempting to bend a coat hanger, you can feel
the proportional limit as the point where it starts to give. Fig. 7 depicts the mechanical properties
attainable from a stress-strain curve, including the following:

Below are some important terms related to stress and strain in material science:
❖ Yield stress (𝜎𝑦 ): This is the stress at which a material starts to yield or permanently deform.
It occurs at the proportional limit.
❖ Yield strain (𝜀𝑦 ): This is the strain at which a material starts to yield or permanently deform.
It occurs at the proportional limit.
❖ Ultimate tensile strength ((𝜎UTS ): This is the maximum stress that a material can handle before
breaking. The highest point on the stress-strain curve represents it.

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❖ Fracture strain or ductility (𝜀𝐹 ):): This is the maximum strain that a material can handle before
breaking. It is often expressed as a percentage, i.e., (𝜀𝐹 × 100) of the original length of the
material.
❖ Toughness: This is the energy required to break material, represented by the area under the
stress–strain curve. It is important to note that this is just one of three different definitions of
toughness in material science.
Fig. 8 displays stress-strain curves for various materials, revealing their unique properties.
Ceramics are known for their brittleness, which is demonstrated by their low ductility and lack of
a plastic region on the curve. Notably, the real curves for metals and plastics in Fig. 8 differ from
the generic curve in Fig. 7, needing a clear proportional limit. As a result, alternative methods are
needed to determine the yield point, which varies between metals and plastics based on tradition.
Metals use the 0.2% offset yield, as shown in Fig. 8.

Figure 8 Illustration of typical stress–strain curves for metals, plastics, and ceramics materials. Metals and
plastics have a yield point, while ceramics do not. Other types of curves are also possible.

To determine the yield point, follow these steps:

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❖ Draw a line that is parallel to the initial linear portion of the curve, but make sure it starts at a
strain of 0.2% (0.002 strain).
❖ Locate where this line intersects the stress-strain curve. This point of intersection is the yield
point.
❖ The yield stress and yield strain are the stress and strain at the yield point.

When dealing with polymers, identifying the yield point is a simple matter; it is the first
maximum on the stress-strain curve. Suppose you need to obtain a stress-strain curve for a material.
In that case, the American Society for Testing and Materials (ASTM) has set standards for
conducting tests and specifications for materials. You can find almost any test you need for
measuring density, strength, refractive index, resistance to chemicals, and many others. These tests
are created by experts and checked by different labs to ensure clarity and accuracy. There are
various standards for stress-strain measurements for different materials, but they all require the
same basic steps. The sample is machined or molded into a specified shape and then placed in a
machine that pulls it at a constant rate while measuring the force. By knowing the initial length
and cross-sectional area of the specimen, the force and length are converted to stress and strain.

Problem 1
If a pure electrolytic tough pitch (ETP) copper piece that was originally 475 mm long and under a
stress of 187 MPa is pulled in tension, what will be the resulting elongation? Assume the modulus
of elasticity of pure ETP copper to be 110 GPa.
Problem 2
In the process of designing a metallic material, a cylindrical rod with a diameter of 8 cm was
fabricated. This rod has a yield stress of 207 MPa, and a safety factor of 1.2 is required for the
design. What is the maximum force that can be applied to the rod while still meeting the safety
requirements?
Problem 3
To transport a 2000kVA soundproof diesel Mikano generator from their office in Kano to the
nearby Flour mills manufacturing plant in Bompai, a stainless-steel eyebolt/eyelet (an example of
an eyebolt is given in Fig. 9) was used for lifting. The lifting eyebolt’s material has a yield stress
of 325 MPa and is designed with a factor of safety of 1.6 against yielding. What is the minimum

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required diameter of the shank for the lifting eyebolt, as illustrated in the lifting eyebolt below (Fig.
10)?

Eye

Shoulder

Shank

Figure 9 Illustration of an eyebolt.

𝟓𝟎 𝒌𝑵

Figure 10 Lifting eyebolt.

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 Concept of Metal Processing and Manufacturing
Engineers not only design products but also develop processes to create them.
Manufacturing is the term used to describe the making of products. In broader terms,
manufacturing is the conversion of either raw or semi-finished materials into finished parts. The
product design and manufacturing disciplines are closely related because consideration of how a
component is to be manufactured is often a defining criterion for successful design. Metals are
often selected for engineered parts because of a combination of properties and cost factors. Indeed,
many engineers may not appreciate the fortuitous circumstances that led to the widespread use of
steel. Not only is steel a low-cost choice for many applications, it also has a desirable combination
of the mechanical properties that are often critical. Additionally, many metals have important
engineering applications. Some commonly used engineering materials are broadly classified as
shown in Fig. 11. Nevertheless, some applications cannot be effectively served by metals. Often
these applications require a specific alloy that offers superior mechanical properties. Pure metals
possess low strength and do not have the required properties. So, alloys are produced by melting
or sintering two or more metals or metals and non-metals together. Alloys may
consist of two more components. Metals and alloys are further classified into two major kinds
namely ferrous metals and non-ferrous metals:
a) Ferrous metals are those which have the iron as their main constituent, such as
pig iron, cast iron, wrought iron, and steels.
b) Non-ferrous metals are those which have a metal other than iron as their main
constituent, such as copper, aluminum, brass, bronze, tin, silver zinc, invar etc.

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 Figure 11 Classification of engineering materials.

A variety of metal alloys, such as plain-carbon steels, alloy steel, stainless steels, cast iron, and
copper, are used in manufacturing various gears. For example, chromium steels are used for
automobile transmission gears. Chromium-molybdenum steels are used for aircraft gas turbine
gears, nickel-molybdenum is used for earth-moving equipment, and some copper alloys are used
to manufacture gears for low load levels. The choice of the gear metal and its manufacturing
depends on size, stresses involved, power requirements, and the environment in which they will
operate. Metals and alloys have many useful engineering properties and have widespread
application in engineering designs. Iron and its alloys (principally steel) account for about 90 %
of the world’s production of metals mainly because of their combination of good strength,
toughness, and ductility at a relatively low cost. Each metal has special properties for engineering
designs and is used after a comparative cost analysis with other metals and materials.
Manufacturing operations can be generally classified into primary and secondary processes.
For metals, primary manufacturing usually refers to the conversion of ores into metallic
materials. Secondary manufacturing is generally understood to mean the conversion of products
from the primary operation into semi-finished or finished parts. For example, the fabrication of
automobile engine blocks from a primary melt of iron or aluminum is said to be secondary
manufacturing. It is often difficult to classify a particular metal shaping operation as either a

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primary or secondary process in an absolute sense, as it can be difficult to delineate between the
various steps within an integrated manufacturing process.

Process of Manufacture of Iron and Steel

Pure iron is not readily available since it easily oxidizes in the presence of air and moisture.
The iron industry reduces iron oxides to obtain pure iron, i.e. metallic iron. Steel is an alloy based
on iron and carbon, with carbon concentrations ranging from 0.2 % to 2.14 % in weight. High
carbon content results in higher hardness, tensile strength, and lower ductility. The resulting steel
is also more brittle. Steel alloys can be enriched with other materials to tune the final material
properties that also depend on production techniques and on the quality of the basic materials. The
basic material for iron and steel production is iron ore or ferrous scrap. Iron ores are classified
based on shape and volume. Iron fines have a majority of particles with a diameter of < 4.75 mm;
iron lump ore has a majority of particles with a diameter of > 4.75 mm; iron pellets are a fine-
grained concentrate rolled into balls (with a binder) and indurated in a furnace. Their diameter
ranges from 9.5 to 16.0 mm.
The iron and steel production process can be subdivided into 3 sub-processes: iron-making,
steel-making, and steel manufacturing. In the basic process, the input materials—a combination
of sinter, iron pellets, limestone, and cokes, enter a blast furnace to be converted into molten pig
iron. The pig iron is then loaded into an oxygen furnace to produce steel slabs. Alternative
processes are direct reduction iron and smelting reduction iron. Ferrous scrap can also be processed
in an electric arc furnace to obtain steel. Today, most used steel-making processes consist of a
combination of a blast furnace and a basic oxygen furnace. Some smelting reduction iron processes
can produce steel directly. The process of iron and steel manufacturing is further given in detail:

A. Production of Pig Iron in a Blast Furnace

The vast majority of pig iron produced from iron ores is processed by blast furnaces. Blast
furnace is a process for producing liquid raw iron by smelting pellets or sinter in a reducing
environment. The end products are usually molten metal, slag, and blast furnace gas. In the
reduction process, oxygen (O2) is taken out of the pellets or sinter. Coke is often used as a reducing
agent, as well as fuel. Fuel (coke) and pellets or sinter are supplied continuously through the top
of the furnace, and O2-enriched air is blown out the bottom by electrical air ventilators. The

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chemical reactions take place while the materials move downward. Coke also serves as a carrier
to move the bulk material column downward in the blast furnace. Various alternative reducing
agents are available, such as hydrocarbons, coke, coal, oil, and natural gas (nowadays, in some
cases, also plastics). In the past, a widely used reducing agent was charcoal, in particular charcoal
from eucalyptus trees. Whatever the fuel and reducing agent, the content of the furnace needs to
have optimum permeability to the flow of gaseous and molten products. Blast furnace gas contains
CO (20-28 %), H2 (1– 5 %), inert compounds such as N2 (50-55 %) and CO2 (17-25 %), some
Sulphur and cyanide compounds, and large amounts of dust from impurities of coal and iron ore.
The lower heating value of blast furnace gas ranges from approximately 2.7 to 4.0 MJ/Nm3. The
production of blast furnace gas is approximately 1200 to 2000 Nm3/t pig iron. Much effort is
devoted to increasing efficiency and reducing emissions of the blast furnaces. It is so imperative
to understand that most iron is extracted from iron ores in large blast furnaces. In the blast furnace,
coke (carbon) acts as a reducing agent to reduce iron oxides (mainly Fe2O3) to produce raw pig
iron, which contains about 4 % carbon along with some other impurities according to the typical
reaction.

Fe2 O3 + 3C0 → 2Fe + 3C02

The pig iron from the blast furnace is usually transferred in the liquid state to a steel making
furnace.

Figure 12 Blast furnace.

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The explanation of terms used in the production of Pig Iron in a blast furnace is as follows:
• Coking: Coking or coal pyrolysis is the way coke is produced by heating coal in an oxidation-
free atmosphere. Flue gases at temperatures between 1150°C and 1350°C heat up coal
indirectly to 1000-1100°C for 14-24 hours. At the end of the grate, coke is fully carbonized,
and it is quenched mostly by water or by inert gas. Air cannot be used for this purpose, as the
oxygen would cause the hot cokes to ignite spontaneously. Some 1000 kg of coal usually yields
750-800 kg of coke and approximately 325 m³ COG (Coke Oven Gas).
• Sintering: Sinter and pellets are produced by mixing together raw or recycled materials, which
undergo a physical and metallurgic agglomeration process. The high permeability and the
reducibility of sinter and pellets enhance the BF performance. In the sintering process, ores,
additives, recycled sinter, and coke breeze are blended in a mixing drum. This mixture is then
loaded onto a moving grate and ignited. As the mixture proceeds along with the grate, air is
drawn downwards through the sintering bed by powerful fans, causing the combustion front to
move downwards through the mixture. The sinter is cooled in a separate cooler, after which it
is crushed.
• Palletization: Is a process to convert iron ore into small balls (9–16 mm) while upgrading its
iron content. While sintering is mostly used in integrated steelworks, palletization is mostly
used at mining sites. The process of forming pellets can be divided into four steps: Grinding
and Drying, Green ball preparation, Induration, and Screening and Handling. In the first step,
wet or dry ores are ground (grated), and the resulting slurry is mixed with additives to prepare
the green balls. Induration involves green balls drying, heating and final cooling. During this
process, almost all magnetite is transformed into hematite. This explains the large amount of
heat needed for the process (magnetite ore has low iron content and must be upgraded to make
it suitable for steelmaking). In the last screening/handling step, undersized or broken pellets
are recycled.

B. Steelmaking and Processing of Major Steel Product Forms

Plain-carbon steels are essentially alloys of iron and carbon with up to about 1.2 % carbon.
However, the majority of steels contain less than 0.5% carbon. Most steel is made by oxidizing
the carbon and other impurities in the pig iron until the carbon content of the iron is reduced to the
required level. The most commonly used process for converting pig iron into steel is the basic

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oxygen process (BOF). Steel can also be made in an electric arc furnace (EAF) from scrap steel
and, in some cases, from direct reduced iron. BOF is typically used for the high-tonnage production
of carbon steels, while the EAF is used to produce carbon steels and low-tonnage specialty steels.
In this process (referring to BOF), pig iron and up to about 30 % steel scrap are charged into a
barrel-shaped refractory-lined converter into which an oxygen lance is inserted. Pure oxygen from
the lance reacts with the liquid bath to form iron oxide. Carbon in the steel then reacts with the
iron oxide to form carbon monoxide.
FeO + C → Fe + CO

Immediately before the oxygen reaction starts, slag-forming fluxes (chiefly lime) are added in
controlled amounts. The molten steel from the converter is either cast in stationary molds or
continuously cast into long slabs from which long sections are periodically cut off. Today
approximately 96% of the steel is cast continuously, with about 4000 ingots still being cast
individually. However, about one-half of the raw steel is produced by recycling old steel, such as
junk cars and old appliances.
After being cast, the ingots are heated in a soaking pit and hot-rolled into slabs, billets, or
blooms. The slabs are subsequently hot and cold-rolled into steel sheets and plates. The billets are
hot and cold rolled into bars, rods, and wire, while blooms are hot and cold-rolled into shapes such
as 1beams and rails.
Iron-Carbon Equilibrium Diagrams
The structure and state of aggregation in which a material exists may depend upon the
temperature, pressure, and proximity of other materials. Examples of this bound in our common,
everyday experience. For example, we all know oil and water do not mix. We can add water to oil
and shake vigorously in an attempt to mix them, but the best we can do is to produce a dispersion
of water in oil, which soon separates out into the two components. In this case, the oil doesn’t
affect the water and vice versa. Each exists independently of the other. However, the situation is
entirely different for sugar and water. If we attempt to add a teaspoon of sugar to iced water (e.g.,
in iced tea), we find that only a small amount of sugar dissolves in the iced tea, and the rest can be
seen at the bottom of the glass. However, if we attempt to add the same amount of sugar in a hot
tea, the sugar goes into solution readily. In this case, the solubility of the sugar depends on the
temperature and both the water and sugar are affected by the other. Another example of the
relationship between pressure and temperature, pressure and state of aggregation is seen in ice
20
skating. Careful measurements have shown that ice is not really slippery and actually has a rather
high coefficient of friction. Yet an ice skater is able to glide across the ice gracefully. How can
this be? The answer lies in the pressure/temperature relationship between ice and water. At the
normal temperature of ice, only relatively modest pressure is required to convert ice to water. More
than enough pressure is applied through the blade of a skate to form a thin film of water, which
‘lubricates’ the ice/blade combination and allows the skater to glide. Two solids in close proximity
can react for temperature can react to form a liquid.
In all these examples, a material or combination of materials exists in equilibrium for a given
combination of temperature, pressure, and composition. In this part, we will study the factors that
govern the equilibrium state of materials (called phase equilibria) and develop maps that
incorporate temperature, pressure, and composition (called phase diagrams), which are used to
present such information.

1. The Iron- Iron-Carbide Phase Diagram

The phases present in very slowly cooled iron-carbon alloys at various temperatures and
compositions of iron with up to 6.67% carbon are shown in the Fe-Fe3C phase diagram of Fig. 13.
Under certain conditions, Fe3C, which is called cementite, can decompose into the more stable
phases of iron and carbon (graphite). However, for most practical conditions, Fe3C is very stable
and will, therefore, be treated as an equilibrium phase.

2. Solid phases in the Fe-Fe3C Phase Diagram

The Fe-Fe3C diagram contains the following solid phases: α ferrite, austenite (γ), cementite
(Fe3C), and δ ferrite or delta iron.

a) α ferrite: This phase is an interstitial solid solution of carbon in the BCC iron crystal
lattice. As indicated by the Fe-Fe3C phase diagram, carbon is only slightly soluble in α
ferrite, reaching a maximum solid solubility of 0.025 % at 723 0C. The solubility of carbon
in α decreases to 0.005 % at 0 0C.
b) Austenite (γ): The interstitial solid solution of carbon in γ iron is called austenite. Austenite
has an FCC crystal structure and a much higher solid solubility for carbon than α ferrite.
The solid solubility of carbon in austenite is a maximum of 2.06 % at 1147 0C and decreases
to 0.83 % at 723 0C.

21
 c) Cementite (Fe3C): The intermetallic compound Fe3C is called cementite. Cementite has
negligible solubility limits and a composition of 6.67 % C and 93.3 % Fe. Cementite is a
hard and brittle compound.
d) δ ferrite: The interstitial solid solution of carbon in δ iron is called δ ferrite. It has a BCC
crystal structure like α ferrite but with a greater lattice constant. The maximum solid
solubility of carbon in δ ferrite is 0.09 % at 1465 0C.

Figure 13 The iron-iron-carbide phase diagram.

3. Invariant Reactions in the Fe-Fe3C Phase Diagram

a) Peritectic Reaction: At the peritectic reaction point, liquid of 0.05 % C combines with δ
ferrite of 0.09 % C to form γ austenite of 0.17 % C. This reaction, which occurs at 1495
0
C, can be written as
Liquid (0.53% C) + δ (0.09 % C) → γ (0.17 % C)

22
 δ ferrite is a high – temperature phase and so is not encountered in plain – carbon steels at
lower temperatures.

b) Eutectic Reaction: At the eutectic reaction point, liquid of 4.3 % forms γ austenite of 2.08 %
C and the intermetallic compound Fe3C which contains 6.67 % C. This reaction, which
occurs at 1147 0C, written as

Liquid (4.3 % C) → γ austenite (2.08 % C) + Fe3C (6.67 % C)

This reaction is not encountered in plain- carbon steels because their carbon contents are too low.

c) Eutectoid Reaction: At the eutectoid reaction point, solid austenite of 0.83 % C produce
𝛼 ferrite with 0.025 % C and Fe3C (cementite) that contains 6.67 % C. This reaction which
occurs at 723 0C, can be written as

γ austenite (0.83 % C) → α ferrite (0.025 % C) + Fe3C (6.67 % C)

This eutectoid reaction, which takes place completely in the solid state, is important for some
of the heat treatments of plain-carbon steels. Plain-carbon steel that contains 0.83% C is called
eutectoid steel since all eutectoid structure of 𝛼 ferrite, and Fe3C is formed when austenite of this
composition is slowly cooled below the eutectoid temperature. If plain carbon steel contains less
than 0.8 % C, it is termed a hypoeutectoid steel, and if the steel contains more than 0.8 % C, it is
designated a hypereutectoid steel.
As in the eutectic reaction, the two phases that form have different compositions, so atoms
must diffuse during the reaction. Most of the carbon in the austenite diffuses to the Fe3C, and most
of the iron atoms diffuse to α. This redistribution of atoms is easiest if the diffusion distances are
short, which is the case when the α and Fe3C grow as thin lamellae or plates.

d) Pearlite: The lamellar structure of α and Fe3C that develops in the iron-carbon system is
called pearlite, which is a microconstituent in steel. This was so named because polished
and etched pearlite shows the colorfulness of mother-of-pearl. The lamellae in pearlite are
much finer than the lamellae in the lead-tin eutectic because the iron and carbon atoms
must diffuse through solid austenite rather than through liquid. One way to think about
pearlite is to consider it as a metal-ceramic nano-composite.

23
Problem 4
A plain-carbon steel with 0.80 % C is slowly cooled from 750°C to slightly below 723°C,
assuming complete transformation of austenite to 𝛼 ferrite and cementite.

a) Determine the weight percent eutectoid ferrite formed.

b) Determine the weight percent eutectoid cementite formed.

Problem 5
A hypoeutectoid plain carbon steel with 0.4 % carbon is slowly cooled from 940°C to just above
723°C.

a) Find the weight percent of austenite present in steel.

b) Determine the weight percent proeutectoid plain-carbon steels slowly cooled from 9400C
in the steel.

Problem 6
A hypoeutectoid plain-carbon steel with 0.4 % C is slowly cooled from 940°C to just below 723°C.

a) Calculate the weight percent proeutectoid ferrite present in the steels

b) Calculate the weight percent of eutectoid ferrite and weight percent of eutectoid cementite
present in the steel.

Remark
Below are some selected hints on the steps required to analyze the phase diagram:
1. Determine the phase/phases present at the point (composition vs. temperature)
2. The mass percentage composition of each phase at the point can be determined by the drawing
a horizontal through the point for the length of the entire region.
3. The intersection of the horizontal line and a line on the phase diagram defines the composition
of the solution.

24
 Heat Treatment Processes
Heat treatment is an important factor that can influence the properties and performance of
engineering materials. Which can be described as the controlled heating and cooling of metals to
change their properties to improve their performance or to facilitate processing. An example of
heat treatment is the hardening of a piece of high-carbon steel rod. If it is heated to dull red heat
and plunged into cold water to cool it rapidly (quenching), it will become hard and brittle. If it is
again heated to dull red heat but allowed to cold very slowly, it will become softer and less brittle
(more tough). In this condition, it is said to be annealed. After the heat treatment happens on the
material, it will be in its best condition for flow forming. During flow forming (working), the
grains will be distorted, and this will result in most metals becoming work-hardened if the flow is
formed at room temperature. To remove any locked-in stresses resulting from the forming
operations and to prepare the material for machining, the material has to be normalized.

Heat Treatment of Carbon Steel

Plain carbon steels and alloy steels are among the relatively few engineering materials that can
be usefully heat treated in order to vary their mechanical properties. The other main group is the
heat-treatable aluminum alloys. Steels can be heat treated because of the structural changes that
can take place within solid iron-carbon alloys. The various heat-treatment processes appropriate
to plain carbon steels are:
1. Annealing
2. Normalizing
3. Hardening
4. Tempering
In all the above processes, the steel is heated slowly to the appropriate temperature for its
carbon content and then cooled. It is the rate of cooling that determines the ultimate structure and
properties that the steel will have, providing that the initial heating has been slow enough for the
steel to have reached phase equilibrium at its process temperature.
1. Annealing: All annealing processes are concerned with rendering steel soft and ductile so that
it can be cold worked and/or machined. There are three basic annealing processes, and these
are:
a) Stress-relief annealing at subcritical temperatures.

25
 b) Spheroidized annealing at subcritical temperatures.
c) Full annealing for forgings and castings.

The process chosen depends upon the carbon content of the steel, its pretreatment processing, and
its subsequent processing and use.
a) Stress-relief annealing
It is also called ‘process annealing’, 'interstage annealing', and sub-critical annealing. It is often
used for softening cold worked low carbon (0.4 % carbon content) steel or mild steel. To fully
anneal such a steel would involve heating to a temperature of more than 900 ⁰C, with consequent
high cost. In mild steel, ferrite makes up about 90 % of the structure, and the recrystallization
temperature of cold worked ferrite is only about 500 ⁰C. Annealing a cold-worked mild steel in the
temperature range 550 0C–600 0C will result in complete recrystallization of ferrite, although the
cold worked pearlite will be largely unaffected.
Frequently, however, we must apply a considerable amount of cold work to mild steels, as, for
example, in the drawing of wire. Stress-relief annealing then becomes necessary to often the metal
so that further drawing operations can be carried out. Such annealing is carried out at about 650 °C.
Since this temperature is well above the recrystallization temperature of 500 °C, recrystallization
will be accelerated so that it will be complete in a matter of minutes on attaining the maximum
temperature.
It should be noted that process annealing is a sub-critical operation; that is, it takes place below
the lower critical temperature. For this reason, although recrystallization is promoted, there is no
phase change and the constituents ferrite and cementite remain present in the structure throughout
the process. Process annealing is generally carried out in either batch-type or continuous furnaces,
usually with an inert atmosphere of burnt coal gas, though cast-iron annealing "pots" are still used,
their lids being muted on with clay.
b) Spheroidized annealing
The Spheroidized condition is produced by annealing the steel at a temperature between 650
0
C and 700 0C, just below the lower critical temperature. During this treatment, cementite forms
as spheroidal particles in a ferrite matrix, putting the steel into a soft, but very tough condition.
Since the temperature involved is subcritical, no phase changes take place, and spheroidization of

26
the cementite is purely a surface tension effect. This is referred to as the spheroidization of pearlitic
cementite,
c) Full annealing
It is the treatment given to produce the softest possible condition in hypoeutectoid steel. It
involves heating the steel to a temperature within the range of 30 ⁰C – 50 ⁰C above the upper
critical temperatures and then allowing the steel to cool slowly within the furnace. This produces
a structure containing coarse pearlite. This results in the formation of fine grains of austenite that
transform into relatively fine grains of ferrite and pearlite or pearlite and cementite (depending
upon the carbon content) as the steel is slowly cooled to room temperature, usually in the furnace.
Full annealing is an expensive treatment, and when it is not absolutely essential for the steel to be
in a very soft condition, but a reasonably soft and ductile material is required, the process known
as normalizing is used instead.
Ferrite, which then begins to precipitate in accordance with the equilibrium diagram, deposits
first at the grain boundaries of the austenite, thus revealing, in the final structure, the size of the
original austenite grains. The remainder of the ferrite is then precipitated along certain
crystallographic planes within the lattice of the austenite. This gives rise to a directional
precipitation of the ferrite and plate, representing typically what is known as a Widmanstatten
structure. This type of structure was first encountered by Widmanstatten in meteorites, which may
be expected to exhibit a coarse structure in view of the extent to which they are overheated during
their passage through the upper atmosphere. The mesh-like arrangement of ferrite in the
Widmanstatten structure tends to isolate the stronger pearlite into separate patches so that strength,
and more particularly toughness, are impaired. The main characteristics of such a structure are,
therefore, weakness and brittleness, and steps must be taken to remove it either by heat treatment
or by mechanical work. Hot-working will effectively break up this coarse as-cast structure and
replace it with a fine-grained material, but in this instance, we are concerned with retaining the
actual shape of the casting. Heat treatment must, therefore, be used to affect the necessary
refinement of grain.
2. Normalizing: The process resembles full annealing except that, whilst in annealing, the
cooling rate is deliberately retarded. In normalizing, the cooling rate is accelerated by taking
the work from the furnace and allowing it to cool in free air. Provision must be made for the
free circulation of cool air, but draughts must be avoided. In the normalizing process, as applied

27
 to hyper-eutectoid steels, it can be seen that the steel is heated to approximately 50ْ ⁰C above
the upper critical temperature line. This ensures that the transformation to fine grain austenite
corrects any grain growth or grain distortion that may have occurred previously. Again, the
steel is cooled in free air, and the austenite transforms into fine grain pearlite and cementite.
The fine grain structure resulting from the more rapid cooling associated with normalizing
gives improved strength and toughness to the steel but reduces its ductility and malleability.
The increased hardness and reduced ductility allow a better surface finish to be achieved when
machining. (The excessive softness and ductility of full annealing leads to local tearing of the
machined surface). The type of structure obtained by normalizing will depend largely upon the
thickness of the cross-section, as this will affect the rate of cooling. Thin sections will give a
much finer grain than thick sections, the latter often differing little in structure from an
annealed section. Although highly successful, this procedure tied up an excessive amount of
working capital and space, and nowadays, heat treatment is preferred as the work in progress
is turned around more quickly.
3. Hardening: When a piece of steel containing sufficient carbon is cooled rapidly from above
its upper critical temperature, it becomes considerably harder than it would be if allowed to
cool slowly. This involves rapidly quenching the steel from a high temperature into oil or water.
Hypereutectoid steels are heated to (30 0C- 50 0C) above the upper critical temperature prior
to quenching. It is possible that some cementite grain boundaries. Consequently,
hypereutectoid steels are hardened by quenching from (30 0C- 50 0C) above the lower critical
temperature. At this temperature, the structure is because of one of the spheroidal cementite
particles in an austenite matrix.
The degree of hardness produced can vary and is dependent upon such factors as the initial
quenching temperature, the size of the work, the constitution, properties, and temperature of the
quenching medium, and the degree of agitation and final temperature of the quenching medium.
To harden a piece of steel, it must be heated and then quenched in some media, which will
produce the desired rate of cooling. The medium used will depend upon the composition of the
steel and the ultimate properties required. The quenching medium is chosen according to the rate
at which it is desired to cool the steel. The following list of media is arranged in order of quenching
speeds:

28
 • 5 % Caustic soda
• 5 – 20 % Brine
• Cold water
• Warm water
• Mineral oil.

4. Tempering: A quench-hardened plain carbon steel is hard, and brittle, and hardening stresses
are present. In such a condition, it is of little practical use, and it has to be reheated or tempered
to relieve the stresses and reduce the brittleness. This temperature will remove internal stress
setup during quenching, remove some, or all, of the hardness, and increase the toughness of
the material.
Tempering causes the transformation of martensite into less brittle structures.
Unfortunately, any increase in toughness is accompanied by some decrease in hardness.
Tempering always tends to transform the unstable martensite back into the stable pearlite of
the equilibrium transformations. Tempering temperatures below 200 0C only relieve the
hardening stresses, but above 220 0C the hard, brittle martensite starts to transform into a fine
pearlitic structure called secondary troostite (or just 'troostite'). Troostite is much tougher,
although somewhat less hard than martensite, and is the structure to be found in most carbon-
steel cutting tools.
Tempering above 400 0C causes any cementite particles present to "ball up" giving a
structure called sorbite. This is tougher and more ductile than troostite and is the structure used
in components subjected to shock loads and where a lower order of hardness can be tolerated,
for example springs. It is normal to quench the steel once the tempering. For most steels,
cooling forms the tempering temperature may be either cooling in air, or quenching in oil or
water. Some alloy steels, however, may be become embrittled if slowly cooled temperature
has been reached. From the tempering temperature, and these steels have to be quenched.

29
 Corrosion of Metals
Corrosion (Corrodere (Latin)–To eat away) is the destructive attack of a metal by a chemical
or electrochemical reaction with its environment. Deterioration by physical causes is not called
corrosion but is described as erosion, galling, or wear. In some instances, chemical attack
accompanies physical deterioration, as described by the following terms: corrosion – erosion,
corrosive wear, or fretting corrosion. Non-metals are not included in this definition of corrosion.
Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may
leach away, but the term corrosion, in this section, is restricted to chemical attack of metals.
“Rusting” applies to the corrosion of iron or iron-base alloys with the formation of corrosion
products consisting largely of hydrous ferric oxides. Nonferrous metals, therefore, corrode but do
not rust. During corrosion, the metals are converted to their metallic compounds at the surface.
The loss of materials due to corrosion has become a great problem. The most common example of
corrosion is the rusting of iron when it is exposed to atmospheric conditions. The rusting is due to
the formation of hydrated ferric oxide on the surface. Another example is the formation of a green
film of basic copper carbonate on the surface of Cu when exposed to moist air containing CO2.
A spectacular example of corrosion-induced failure occurred in 1988 a 19-year old Boeing 737
operated by Aloha Airlines suddenly ripped off, and lost a major portion of the upper fuselage in
full flight at 24000 ft, as shown in Fig. 14. The cause of this was related to extensive corrosion of
Aloha Incident
the aluminum alloy canopy materials in a salt air environment.

Figure 14 Corrosion induced failure of an airplane.

6

30
Three factors govern corrosion;
a) The metal from which the component is made.
b) The protective treatment the component surface receives.
c) The environment in which the component is kept.
All metals corrode to a greater or lesser degree; even precious metals like gold and silver
tarnish in time, and this is a form of corrosion. Prevention processes are unable to prevent the
inevitable failure of the component by corrosion; they only slow down the process to a point where
the component will have worn out or been discarded for other reasons before failing due to
corrosion. Let's now look at the three ways in which metals corrode.
1. Direct Chemical Corrosion or Dry Corrosion
This is the direct oxidation of metals, which occurs when a freshly cut surface reacts with the
oxygen of the atmosphere. Most of corrosion-resistant metals, such as lead, zinc, and aluminum,
form a dry oxide film, which protects the metal from further atmospheric attack. It occurs mainly
through the direct chemical action of atmospheric gases such as O2, halogens, H2S, CO2, SO2, N2,
H2, or liquid metals on metal surfaces in the absence of moisture.
2. Electrochemical Corrosion or Wet Corrosion
This type of corrosion occurs in two ways when:
a) The oxidation of metals in the presence of air and moisture, as in the rusting of ferrous
metals.
b) The corrosion of metals by reaction with the dilute acids in rain due to the burning of fossil
fuels (acid rain) - for example, the formation of the carbonate 'patina' on copper. This is
the characteristic green film seen on the copper clad roofs of some public buildings.
3. Galvanic corrosion
This occurs when two dissimilar metals, such as iron and tin or iron and zinc, are in intimate
contact. They form a simple electrical cell in which rain, polluted with dilute atmospheric acids,
acts as an electrolyte as generated and circulates within the system. Corrosion occurs with
(depending upon its position in the electrochemical series) being eaten away. Other metals, in
addition to iron and steel, corrode when exposed to the atmosphere. The green corrosion product
which covers a copper roof, or the white, powdery film formed on some unprotected aluminum
alloys is clear evidence of this. Fortunately, the reactivity of a metal and the rate at which it
corrodes is not related. For example, although aluminium is chemically more reactive than iron,

31
as soon as it is exposed to the atmosphere, it forms an oxide film that seals the surface and prevents
further corrosion from taking place. On the other hand, iron is less reactive and forms its oxide
film more slowly. Unfortunately, the iron hydroxide film (rust) is porous, and the process continues
unabated until the metal is destroyed.

Types of corrosion damage

Corrosion is often thought of only in terms of rusting and tarnishing. However, corrosion
damage occurs in other ways as well, resulting, for example, in failure by cracking or in loss of
strength or ductility. In general, most types of corrosion, with some exceptions, occur by
electrochemical mechanisms, but corrosion products are not necessarily observable, and metal
weight loss need not be appreciable to result in major damage. The five main types of corrosion
classified with respect to outward appearance or altered physical properties are as follows:

1. General Corrosion or Uniform Attack: This type of corrosion includes the commonly
recognized rusting of iron or tarnishing of silver. “Fogging” of nickel and high-temperature
oxidation of metals are also examples of this type. Rates of uniform attack are reported in
various units, with accepted terminologies being millimeters penetration per year (mm/y) and
grams per square meter per day (gmd). Other units that are frequently used include inches
penetration per year (ipy), mils (1mil = 0.001 inch) per year (mpy), and milligrams per square
decimeter per day (mdd). These units refer to metal penetration or to weight loss of metal,
excluding any adherent or non-adherent corrosion products on the surface. Steel, for example,
corrodes at a relatively uniform rate in sea water of about 0.13 mm/y, 2.5gmd, 25mdd, or
0.05ipy. These represent time-averaged values. Generally, for uniform attack, the initial
corrosion rate is greater than subsequent rates. Duration of exposure should always be given
when corrosion rates are reported because it is often not reliable to extrapolate a reported rate
to times of exposure far exceeding the test period.
Conversion of mm/y to gmd or vice versa requires knowledge of the metal density. A given
weight loss per unit area for a light metal (e.g., aluminum) represents a greater actual loss of
metal thickness than the same weight loss for a heavy metal (e.g., lead). For handling chemical
media whenever an attack is uniform, metals are classified into three groups according to their
corrosion rates and intended application. These classifications are as follows:

32
 a) < 0.15mm/y (< 0.005ipy) — Metals in this category have good corrosion resistance to the
extent that they are suitable for critical parts, for example, valve seats, pump shafts and
impellors, springs.
b) 0.15 to 1.5mm/y (0.005 to 0.05ipy) — Metals in this group are satisfactory if a higher rate
of corrosion can be tolerated, for example, for tanks, piping, valve bodies, and bolt heads.
c) 1.5mm/y (> 0.05 ipy) — Usually not satisfactory.

2. Pitting: This is a localized type of attack, with the rate of corrosion being greater at some areas
than in others. Various corrosion pits are shown in Fig. 15. If the appreciable attack is confined
to a relatively small, fixed area of metal, acting as anode, the resultant pits are described as
deep. If the area of attack is relatively larger and not so deep, the pits are called shallow. Depth
of pitting is sometimes expressed by the pitting factor, the ratio of deepest metal penetration
to average metal penetration as determined by the weight loss of the specimen. A pitting factor
of unity represents uniform attack (Fig. 16).

Figure 15 Typical cross-sectional shapes of corrosion pits.

33
 Figure 16 Sketch of deepest pit in relation to average metal penetration and the pitting factor.

Iron buried in the soil corrodes with the formation of shallow pits, whereas stainless steels
immersed in seawater characteristically corrodes with the formation of deep pits. Many metals,
when subjected to high-velocity liquids, undergo a pitting type of corrosion called impingement
attack, or sometimes corrosion-erosion. Copper and brass condenser tubes, for example, are
subject to this type of attack.
a) Fretting corrosion, which results from slight relative motion (as in vibration) of two
substances in contact, one or both being metals, usually leads to a series of pits at the metal
interface. Metal-oxide debris usually fills the pits so that only after the corrosion products
are removed do the pits become visible.
b) Cavitation–erosion is the loss of material caused by exposure to cavitation, which is the
formation and collapse of vapor bubbles at a dynamic metal–liquid interface— for example,
in rotors of pumps or on trailing faces of propellers. This type of corrosion causes a
sequence of pits, sometimes appearing as a honeycomb of small, relatively deep fissures.
3. Dealloying, Dezincification, and Parting: Dealloying is the selective removal of an element
from an alloy by corrosion. One form of dealloying, dezincification, is a type of attack
occurring with zinc alloys (e.g., yellow brass) in which zinc corrodes preferentially, leaving a
porous residue of copper and corrosion products. The alloy so corroded often retains its original
shape and may appear undamaged except for surface tarnish, but its tensile strength and
ductility are seriously reduced. Dezincified brass pipe may retain sufficient strength to resist
internal water pressures until an attempt is made to uncouple the pipe or a water hammer occurs,
causing the pipe to split open.
34
 Parting is similar to dezincification in that one or more reactive components of the alloy
corrode preferentially, leaving a porous residue that may retain the original shape of the alloy.
Parting is usually restricted to such noble metal alloys as gold–copper or gold–silver and is
used in gold refining. For example, an alloy of Au–Ag containing more than 65% gold resists
concentrated nitric acid as well as gold itself. However, on the addition of silver to form an
alloy of approximately 25% Au–75% Ag, a reaction with concentrated HNO3 forms silver
nitrate and a porous residue or powder of pure gold. Copper-base alloys that contain aluminum
are subject to a form of corrosion resembling dezincification, with aluminum corroding
preferentially.

4. Intergranular Corrosion: This is a localized type of attack at the grain boundaries of a metal,
resulting in loss of strength and ductility. Grain –boundary material of limited area, acting as
an anode, is in contact with large areas of grain acting as a cathode. The attack is often rapid,
penetrating deeply into the metal and sometimes causing catastrophic failures. Improperly
heat-treated 18- 8 stainless steels or Duralumin-type alloys (4% Cu–Al) are among the alloys
subject to intergranular corrosion. At elevated temperatures, intergranular corrosion can occur
because, under some conditions, phases of low melting point form and penetrate along grain
boundaries; for example, when nickel-base alloys are exposed to sulfur-bearing gaseous
environments, nickel sulfide can form and cause catastrophic failures. This type of attack is
usually called sulfidation.

5. Cracking: If a metal cracks when subjected to repeated or alternate tensile stresses in a

corrosive environment, it is said to fail by corrosion fatigue. In the absence of a corrosive
environment, the metal stressed similarly, but at values below a critical stress, called the fatigue
limit or endurance limit, will not fail by fatigue even after a very large, or infinite, number of
cycles. A true endurance limit does not commonly exist in a corrosive environment: The metal
fails after a prescribed number of stress cycles, no matter how low the stress. The types of
environment causing corrosion fatigue are many and are not specific. If a metal, subject to a
constant tensile stress and exposed simultaneously to a specific corrosive environment, cracks
immediately or after a given time, the failure is called stress-corrosion cracking. Both stress-
corrosion cracking and cracking caused by the absorption of hydrogen generated by a corrosion

35
 reaction follow this definition. The stress may be residual in the metal, as from cold working
or heat treatment, or it may be externally applied. The observed cracks are intergranular or
transgranular, depending on the metal and the damaging environment. Failures of this kind
differ basically from intergranular corrosion, which proceeds without regard to whether the
metal is stressed.
Almost all structural metals (e.g., carbon and low-alloy steels, brass, stainless steels,
Duralumin, magnesium alloys, titanium alloys, nickel alloys, and many others) are subject to
stress-corrosion cracking in some environments. Fortunately, either the damaging
environments are often restricted to a few chemical species, or the necessary stresses are
sufficiently high to limit failures of this kind in engineering practice. As knowledge
accumulates regarding the specific media that cause cracking and regarding the limiting
stresses necessary to avoid failure within a given time period, it will be possible to design metal
structures without the incidence of stress-corrosion cracking. Highly stressed metal structures
must be designed with adequate assurance that stress-corrosion cracking will not occur.

Factors Affecting Corrosion

1. Structural design
The following factors should be observed during the design stage of a component or assembly
to reduce corrosion to a minimum.
a) The design should avoid crevices and corners where moisture may become trapped;
adequate ventilation and drainage should be provided.
b) The design should allow for easy washing down and cleaning.
c) Joints that are not continuously welded should be sealed, for example, by the use of mastic
compounds or impregnated tapes.
d) Where dissimilar metals have to be joined, high-strength epoxy adhesives should be
considered since they insulate the metals from each other and prevent galvanic corrosion.
e) Materials that are inherently corrosion-resistant should be chosen, or if this is not possible,
an anti-corrosive treatment should be specified.
2. Environment
The environment in which the component or assembly is to spend its service life must be
carefully studied so that the materials chosen, or the anti-corrosion treatment specified, will

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provide an adequate service life at a reasonable cost. It would be unnecessary and uneconomical
to provide a piece of office equipment which will be used indoors with a protective finish suitable
for heavy-duty contractor’s plant which is going to work on construction sites in all kinds of
weather conditions.
3. Applied or internal stresses
Chemical and electrochemical corrosion is intensified when a metal is under stress. This
applies equally to externally applied and internal stresses, although more common in the latter case.
Internal stresses are usually caused by cold working and, if not removed by stress-relief heat
treatment, result in corrosive attack along the crystal boundaries. This weakens the metal
considerably more than simple surface corrosion. An example of intercrystalline corrosion is the
'season cracking' of brass after severe cold working.
Intercrystalline corrosion occurs at the grain boundaries of crystals not only when impurities
are present but also when stress concentrations are present. Grain boundaries are regions of high
energy levels, even in very pure metals, so corrosion tends to occur more quickly at the grain
boundaries. Severely cold-worked α brasses are prone to ‘season cracking'. Here, intercrystalline
corrosion follows the grain boundaries until the component is no longer able to sustain the internal
stresses due to cold working. The component then cracks. This can be prevented by a low-
temperature stress relief annealing process. This low temperature does not cause recrystallisation
but is sufficient to remove the locked up stresses by allowing the atoms to move small distances
nearer to their equilibrium positions.
4. Composition and structure
The presence of impurities in non-ferrous metals reduces their corrosion resistance. Hence the
high level of corrosion resistance exhibited by high- purity copper, aluminum and zinc. The
importance of grain structure has also been mentioned above, and a fine-grain structure is generally
less susceptible to corrosion than a coarse-grain structure. The inclusion of certain alloying
elements such as nickel and chromium can also improve corrosion resistance-for example, the
stainless steels and cupro-nickel alloys.

5. Temperature
For all chemical reactions, there is a critical temperature below which they will not take place.
Since corrosion is the result of chemical or electrochemical reactions, corrosion is retarded or

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stopped altogether at low temperatures. On the other hand, corrosion is at its worst in the hot. The
humid atmosphere of the tropical rain forests and equipment for use in such environments has to
be 'tropicalised' if it is to have a reasonable service life. High temperatures alone do not increase
the rate of corrosion, and corrosion is virtually non-existent in arid desert areas of the world.
Failure of mechanical devices in desert environments is due generally to the abrasive effect of the
all-pervasive sand.

Metals that resist corrosion

It has already been made clear that metals combine with atmospheric oxygen and or
atmospheric pollutants to a greater or lesser extent. The following metals, which resist corrosion,
react to form impervious, homogeneous coatings on their surfaces, which prevent further corrosion
from taking place, providing these coatings remain undisturbed. Such as copper, zinc, aluminum,
lead, stainless steel, nickel and chromium.

Protection of Corrosion
There are two principal methods by which corrosion may be prevented or minimized. First,
the metallic surface can be insulated from the corrosive medium by some form of protective
coating. Such coatings include various types of paints and varnishes, metallic films having good
corrosion resistance, and artificially thickened oxide films. All of these are generally effective in
protecting surfaces from atmospheric corrosion, though zinc coatings are used to protect iron from
the rusting action of water, whilst tin coatings offer protection against most animal and vegetable
juices encountered in the canning industry. In circumstances where corrosive action is severe or
where mechanical abrasion is likely to damage a surface coating, it may be necessary to use a
metal or alloy that has an inherent resistance to corrosion. Such corrosion-resistant alloys are
relatively expensive, so their use is generally limited to chemical-engineering plants, marine-
engineering equipment, and other special applications. There are many types of protection from
corrosion, which are:

1. The Use of a Metal or Alloy Which Is Inherently Corrosion-resistant

The corrosion resistance of a pure metal or a homogeneous solid solution is generally superior
to that of an alloy in which two or more phases are present in the microstructure. As mentioned

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above, the existence of two phases leads to electrolytic action when the surface of the alloy comes
into contact with an electrolyte. Most of the alloys which are used because of their high corrosion-
resistance exhibit solid-solution structures. Aluminum-magnesium alloys containing up to 7-0%
magnesium fulfil these conditions and are particularly resistant to marine atmospheres. Stainless
steel of the "18-8" type is completely austenitic in structure when correctly heat-treated, but faulty
heat-treatment may lead to the precipitation of carbides and, hence, to corrosion. Although such
corrosion is partly due to the impoverishment of the austenitic matrix in chromium (since it is
mainly chromium carbide which is precipitated), this corrosion is accelerated by electrolytic action
between the carbide particles and the matrix. “Weld-decay” in steels of this type occurs for similar
reasons.

2. Protection by Metallic Coatings

The protection afforded by metallic coatings can be either "direct" or "sacrificial". Direct
protection depends on an unbroken film of metal covering the article, and if the film becomes
broken, corrosion may be accelerated by electrolytic action between the film and the metal beneath.
In the case of sacrificial protection, however, the metallic film becomes the anode in the event of
a break in the film and thus dissolves in preference to the surface beneath. It follows that when
protection is limited to the direct type, as in the case of tin coatings on steel, the quality of the
coating is most important since acceleration, and not inhibition, of corrosion, would follow a break
in the film. In both cases, of course, protection of the direct type is the fundamental aim of the
metal-coating process, and it is only in the possibility of the coating becoming broken that the
effects of electrolytic action must be considered. A number of methods are available for the
production of metallic coatings. The most widely used are either electro-plating or dipping the
articles to be coated into a bath of the molten metal. In some cases, a successful coating can be
produced by heating the articles to be coated in the finely powdered metal, whilst specialized use
is made of the process known as "cladding". The rest of the methods available for the production
of metallic coatings are hot-dip metal coating, coatings by means of a spray of molten metal, and
electro-plating.

3. Protection by oxide coating

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 In some instances, the film of oxide which forms on the surface of a metal is very dense and
closely adherent. It will then protect the metal surface beneath from oxidation. Stainless steels
owe their resistance to corrosion to the presence of a high proportion of chromium, which is one
of these elements that form oxide films impervious to oxygen. The "blueing" of ordinary carbon
steel by heating it in air produces an oxide film of such a nature that it affords partial protection
from corrosion.

4. Protection by other non-metallic coatings

Coatings of this type usually offer only a limited protection against corrosion and are more often
than not used only as a base for painting.
a) Phosphating
A number of commercial processes fall under this heading, but in all of them, a coating of
phosphate is produced on the surface of steel or zinc-base alloys by treating them in or with a
solution of acid phosphates. In order for the metal to be made rust-proof, a finishing treatment
with varnish, paint, oil, or lacquer is required.
b) Chromating
Chromate coatings are produced on magnesium-base alloys and on zinc and its alloys by
immersing the articles in a bath containing potassium bichromate along with various other
additions. The color of the films varies with the bath and alloy, from yellow to grey and black.

5. Cathodic protection
This method of protection against corrosion can be used for buried or submerged pipelines and
other structures. The pipeline is made to act as a cathode by burying pieces of metal, which is
much more electropositive than the iron of the pipeline. These pieces of metal will, therefore, be
anodic towards the iron of the pipeline and will corrode sacrificially. Alternatively, a current from
D.C. mains can be passed through the soil or water onto the metallic surface concerned so as to
keep it at a slightly negative potential with respect to its surroundings. When electric power is
available, this will be the cheaper method, since electricity can be obtained more cheaply from the
mains than from any electro-chemical source. To protect the whole surface of a pipe-line by this
means, however, would be expensive, but if the pipe has already been coated with paint or some
other non-metallic substance, so that it is only necessary to protect any defective areas, the power

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cost is small, since very small currents only are necessary. In some parts of America, the current
is generated by dynamos driven by windmills.

Further reading
William D. Callister, and David G. Rethwisch, Materials Science and Engineering: An
Introduction, 8th edition, Wiley, 2009.

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