kwame nkrumah university of science and technology … · weight loss measurements as well as the...
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KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY
KUMASI
COLLEGE OF ENGINEERING
DEPARTMENT OF MATERIALS ENGINEERING
EFFECT OF CASSAVA EXTRACT (CYANIDE) ON THE CORROSION
BEHAVIOUR OF MILD STEEL
BY
IBRAHIM, MUBARAK BABA
AYAIM, JOOJO ENU
BODZAH, ALFRED
May, 2018
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KWAME NKRUMAH UNIVERSITY OF SCIENCE AND TECHNOLOGY
KUMASI
COLLEGE OF ENGINEERING
DEPARTMENT OF MATERIALS ENGINEERING
EFFECT OF CASSAVA EXTRACT (CYANIDE) ON THE CORROSION
BEHAVIOR OF MILD STEEL
A PROJECT REPORT SUBMITTED TO THE MATERIALS ENGINEERING
DEPARTMENT IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR
THE BACHELOR OF SCIENCE DEGREE IN METALLURGICAL ENGINEERING
BY
IBRAHIM, MUBARAK BABA
AYAIM, JOOJO ENU
BODZAH, ALFRED
SUPERVISORS
DR. EMMANUEL KWESI ARTHUR
DR. EMMANUEL GIKUNOO
May, 2018
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DECLARATION
We hereby declare that we the students mentioned below under supervision personally
undertook this project and research work. It is being submitted as a final year project in
partial fulfilment of the requirement in awarding B.Sc. Degree in Metallurgical
Engineering in Kwame Nkrumah University of Science and Technology, Kumasi.
IBRAHIM, MUBARAK BABA …………………… …....……………
2204014 Signature Date
AYAIM, JOOJO ENU …………………… ……….…………….
2202914 Signature Date
BODZAH, ALFRED .….……………… …….………………
9777789 Signature Date
CERTIFIED BY
DR. EMMANUEL KWESI ARTHUR ……………. …..….………
(Supervisor) Signature Date
DR. EMMANUEL GIKUNOO ….…….…… …..…………
(Supervisor) Signature Date
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ABSTRACT
The most available and less expensive metal for the fabrication agro-processing
equipment (particularly milling machines) in Ghana is mild steel. However, cassava has
been found to contain aggressive elements that have detrimental effects on the
composition of mild steel resulting in corrosion of these machinery, leading to their
premature failure in service. This work was done to study the effect of cassava extract
(squeezed manually from cassava tubers) on the corrosion behaviour of mild steel. The
corrosion rates of mild steel samples immersed in sweet and bitter cassava juice were
determined under two different immersion times ─ continuous and intermittent. Periodic
weight loss measurements as well as the relationship between weight loss and immersion
times were determined. The results show the highest corrosion rate in the bitter cassava
juice, recording a peak value of 1.3 mm/yr. The bitter cassava recorded 0.972mg of HCN
(48.6 mg HCN/kg) while the sweet cassava recorded 0.864 mg of HCN (42.3 mg
HCN/kg), implying a higher cyanide content in the bitter cassava. The sweet cassava
recorded a moisture content of 28.33g while the bitter cassava recorded 25.48g. The
functional group analysis revealed alkenes, aliphatic amines, alkynes etc. present in the
cassava extracts. In food processing industries and more specifically cassava processing
industries, product quality and health concerns are paramount. These industries cannot
and must not tolerate corrosion deposits in their products. Therefore, material selection
for machine fabrication is very important. The results of this study show that mild steel
is not suitable for use in cassava processing without some form of surface treatment or
protection.
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ACKNOWLEDGEMENT
Our first debt of gratitude is to the Almighty God for granting us life, good health and
protection throughout the work. The team expresses their profound appreciation to our
supervisors Dr. Emmanuel Kwesi Arthur and Dr. Emmanuel Gikunoo for their advice,
corrections and patience.
In addition, the team is equally grateful to Mr. Gyasi, the sports master of St. Louis Senior
High School for his support in acquiring cassava tubers from his farm and also to Hajia
Humu-Kursum, Head of Chemistry Laboratory of KNUST for her assistance in the
preparation of our various reagents for the work.
We are also grateful to Mr. Desmond Appiah of Tema Steels Company Limited for his
assistance in carrying out the chemical analysis of our mild steel samples.
Finally, we are most grateful to the laboratory technicians at the Chemical Engineering
Laboratory and Materials Engineering Laboratory of KNUST for their assistance and
direction in performing our experiments.
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TABLE OF CONTENTS
DECLARATION ..........................................................................................................ii
ABSTRACT ................................................................................................................ iii
ACKNOWLEDGEMENT ........................................................................................... iv
TABLE OF CONTENTS .............................................................................................. v
LIST OF TABLES .................................................................................................... viii
LIST OF FIGURES ...................................................................................................... ix
CHAPTER ONE ........................................................................................................... 1
1.0 INTRODUCTION ................................................................................................... 1
1.1 Background ............................................................................................................. 1
1.2 Problem Statement .................................................................................................. 1
1.3 Aim and Objectives ................................................................................................. 2
1.5 Scope of Study ........................................................................................................ 3
CHAPTER TWO .......................................................................................................... 4
2.0 LITERATURE REVIEW ........................................................................................ 4
2.1 Introduction ............................................................................................................. 4
2.2 Metal ....................................................................................................................... 7
2.2.1 Non-Ferrous Metal ............................................................................................. 10
2.2.2 Ferrous Metal ..................................................................................................... 11
2.3 Mild Steel .............................................................................................................. 16
2.3.1 Classification of Mild Steel ................................................................................ 17
2.3.2 Application of Mild Steel ................................................................................... 18
2.3.3 Advantages of Mild Steel ................................................................................... 18
2.3.4 Disadvantages of Mild Steel .............................................................................. 19
2.4 Corrosion of Materials .......................................................................................... 19
2.4.1 Types and Classification of Corrosion ............................................................... 19
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2.4.2 Corrosion Behaviour of Mild Steel .................................................................... 21
2.4.3 Effect of Cyanide on Corrosion Behaviour of Mild Steel .................................. 21
2.5 Prevention of Corrosion ........................................................................................ 21
2.5.1 Inhibitors ............................................................................................................ 22
2.6 Cassava .................................................................................................................. 26
2.6.1 Types of cassava tuber ....................................................................................... 27
2.6.2 Processing of Cassava ........................................................................................ 27
2.7 Cyanide ................................................................................................................. 28
2.7.1 Sources of Cyanide ............................................................................................ 28
2.7.2 Cyanide Content in Cassava Tuber .................................................................... 29
2.7.3 Health Effect of Cyanide from Cassava ............................................................. 30
2.7.4 Obtaining cassava extract ................................................................................... 31
2.8 Characterization .................................................................................................... 31
2.8.1 Cyanide Content Determination ......................................................................... 31
2.8.2 Functional Group Analysis ................................................................................ 32
2.8.3 Microscopy Analysis .......................................................................................... 32
2.8.4 Corrosion Experiment ........................................................................................ 33
3.0 METHODOLOGY ................................................................................................ 39
3.1 Materials and Equipment ...................................................................................... 39
3.2 Material Acquisition .............................................................................................. 39
3.3 Specimen Preparation ............................................................................................ 40
3.3.1 Preparation of cassava extract ............................................................................ 40
3.3.3 Processing of corrosion inhibitor ....................................................................... 42
3.3.4 Reagents Preparation .......................................................................................... 43
3.4 Cyanide Content Determination by Alkali Titration Method ............................... 44
3.5 Moisture Content Determination ........................................................................... 46
3.6 Corrosion Experiment ........................................................................................... 46
3.6.1 Weight loss Measurement without inhibitor ...................................................... 46
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3.6.2 Weight loss Measurement with inhibitor ........................................................... 47
3.6.3 Electrochemical Measurement ........................................................................... 47
3.7 Characterization Process ....................................................................................... 47
3.7.2 Microstructural Analysis .................................................................................... 48
CHAPTER FOUR ....................................................................................................... 49
4.0 RESULTS AND DISCUSSION ........................................................................... 49
4.1 Cyanide Content Determination ............................................................................ 49
4.2 Moisture Content ................................................................................................... 50
4.3 Weight loss Method .............................................................................................. 50
4.4 Electrochemical Analysis ...................................................................................... 57
4.6 FTIR ...................................................................................................................... 58
4.7 Microstructure Analysis and surface morphology of corroded samples ............... 60
CHAPTER FIVE ......................................................................................................... 62
5.0 CONCLUSIONS AND RECOMMENDATIONS ............................................... 62
5.1 CONCLUSIONS ................................................................................................... 62
5.2 RECOMMENDATIONS ...................................................................................... 62
REFERENCES ............................................................................................................ 63
APPENDICES ............................................................................................................. 68
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LIST OF TABLES
Table 2.1: Analysis of cassava liquid ................................................................................ 6
Table 4.1: Cyanide content of the cassava extracts ......................................................... 49
Table 4.2: Moisture content and % moisture content ...................................................... 50
Table 4.3: Weight loss determination without inhibitor ................................................. 51
Table 4.4: Weight loss determination with inhibitor ....................................................... 53
Table 4.5: Weight loss determination of mild steel in cassava dough ............................ 55
Table 4.8: Functional groups present in the sweet extract .............................................. 59
Table 4.9: Functional groups present in the bitter extract ............................................... 59
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LIST OF FIGURES
Figure 2.1: (a) A round mild steel pipe and (b) A square mild steel pipe (Cereda, 1994)
......................................................................................................................................... 17
Figure 2.2: (a) Cassava plant and (b) Cassava tubers ..................................................... 27
Figure 2.3: an experimental arrangement for making corrosion potential measurement
(Zaki, 2006). .................................................................................................................... 34
Figure 2.4: Circuitry associated with controlled potential measurements. (Zaki, 2006) 35
Fig 2.5: Actual and measured polarization curves for active metals .............................. 36
Figure 2.6: A hypothetical Tafel plot (Zaki, 2006). ........................................................ 38
Fig 2.7: Hypothetical linear polarization plot (Zaki, 2006). ........................................... 38
Figure 3.1: (a) Acquisition of the bitter cassava tubers and (b) Acquisition of sweet
cassava tubers .................................................................................................................. 39
Figure 3.2: (a) Peeling, cutting and dicing of cassava and (b) Blending of cassava tuber
with an electric blender ................................................................................................... 40
Figure 3.3: (a) Extracted cassava juice and (b) Cassava dough ..................................... 41
Figure 3.4: Preparation of mild steel samples ................................................................. 41
Figure 3.5: (a) Washing of orange fruits, (b) Sun drying of orange peels, (c) Pounding of
dried orange peels and (d) Powdered peels used as inhibitor ......................................... 42
Figure 3.6: The distillation process ................................................................................. 45
Figure 3.7: (a) The Titration process and (b) Final titrated solution showing the colour
change ............................................................................................................................. 45
Figure 4.1: Graph of Corrosion rate against Immersion time (Without inhibitor) .......... 52
Figure 4.2: Graph of Corrosion rate against Immersion time (With inhibitor) ............... 54
Figure 4.3: Graph of Corrosion rate against Immersion time (With and without inhibitor)
......................................................................................................................................... 54
Figure 4.4: A graph for weight loss in sweet and bitter cassava dough .......................... 56
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Figure 4.5: A graph of current vs potential for the bitter cassava extract ....................... 57
Figure 4.6: A graph of current vs potential for the sweet cassava extract ...................... 57
Figure 4.7: A graph of Intensity against Wave length for Sweet species ....................... 58
Figure 4.8: A graph of Intensity against Wave length for Bitter species ........................ 59
Figure 4.9 Microstructure of mild steel sample .............................................................. 60
Figure 4.10: Surface morphology of corroded mild steel ............................................... 61
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CHAPTER ONE
1.0 INTRODUCTION
1.1 Background
Cassava (also known as tapioca or manioc) is an important root crop in many African and
Pacific Islands countries both for food and as a cash crop. Its tubers are the main edible
parts and its related products are of high demand in these countries. Cassava has a flexible
harvest period and has been stored to be used as a reserve in case of natural disasters such
as droughts (Burns, 1991). The culinary uses of cassava include cassareep/ tucupay, fufu,
mingao, ampesi, macaroni, cassava pudding etc. (Balagolan, 1996). Milling is one of the
methods of processing cassava. Milling is a common practice in Africa, where the cassava
tubers are ground to produce cassava flour or cassava dough. The milling machines are
usually made of mild steel which is susceptible to corrosion. Cyanogenic glucosides are
found throughout the cassava plant and in all varieties of cassava. Varieties referred to as
sweet, have low levels of cyanogenic glucoside in the flesh of the root and the bitter types
have high levels of cyanogenic glucosides in the flesh of the root. (Balagolan, 1996).
Corrosion is a wearing process which causes the flaking off of the surface of the mild
steel. These flakes have a high tendency of entering the milled product, which would
cause negative health implications upon consumption including headache, weakness, loss
of consciousness, cardiac arrest, seizures etc. Most Ghanaians are ignorant of this, hence
the milling process goes on unchecked, with people being oblivious of the harmful
outcomes.
The reasons stated above makes this undertaken very essential as it helps in determining
the effect the cyanide content of the cassava processed by mild steel milling machines has
on the corrosion behaviour of the mild steel and methods of controlling the corrosion
process to prevent contamination of consumable cassava products.
1.2 Problem Statement
Mild steel is a very useful manufacturing material however; it tends to corrode in acidic
media. This phenomenon is not desirable especially in the food processing industry. Food
products produced by machines and tools made of mild steel may be contaminated by
coming into contact the wearing products as a result of corrosion. This may lead to food
poisoning upon consumption.
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Cassava is a very popular food in Ghana and other tropical countries. It is processed using
milling machines. Milling machines are made of mild steel which tends to corrode upon
reaction with the cyanide present in the cassava at the high temperatures of the milling
operation. However, many are ignorant of the negative effects the rusting of the milling
machine has on the quality of the cassava flour/ dough produced and its health
implications. This has led to less work being done to control the rate of corrosion of mild
steel in the milling.
1.3 Aim and Objectives
The aim of this project is to observe the effect of cassava extract (cyanide) on the
corrosion behaviour of mild steel.
The specific objectives of study are:
• To obtain extracts from the cassava tubers.
• To determine the cyanide content of the cassava extracts.
• To investigate the effects of the extract on mild steel samples under varying conditions.
• To propose how to prevent corrosion behaviour on the various mild steel samples.
1.4 Justification
Milling is one of the most important processes of processing cassava and other food
products in Ghana. However, one of the limitations of the process is the corrosion of the
milling machine which is made of mild steel. Works have been done in this field, however
little has been done to find a lasting solution. Corrosion reduces the integrity of the milling
machine. It may also result in the contamination of the food product which may not proper
for human consumption. This scenario would in turn result in lack of food security and
financial losses for agricultural and agro-processing industry.
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1.5 Scope of Study
This report has been assembled into five chapters. Chapter one is the introduction, which
consists of the background, problem statement, the aim and objectives, justification and
the scope of study. Chapter two presents a review of available literature. Chapter three
describes the research methodology. Chapter four presents the results and discussions and
the Chapter five outlines the conclusions and recommendations.
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CHAPTER TWO
2.0 LITERATURE REVIEW
2.1 Introduction
Works (Olawale et al., 2006, May, 2016, Bodude et al., 2012, Osarolube et al., 2008,
Durowoju et al.,2014) have been done on the corrosion behaviour of mild steel in various
acidic media and it was observed that nitric acid environment was the most corrosive
medium for both mild steel and high carbon steels because of its oxidizing nature,
followed by perchloric acid, and then hydrochloric acid (Osarolubeet et al., 2008). The
factors that affect corrosion of metals are the corrodent concentration and exposure time
(Osarolubeet et al., 2008).
Studies (Bodude et al., 2012) have also been done to determine the influence of
austempering (heat-treatment method) on the corrosion-wear resistance of ST60Mn steel
in cassava extract. The heat-treatment was done by varying the austenitizing temperature,
austempering temperature and time. The corrosion- wear resistance was investigated
under an instrumented pin-on-disc wear testing machine with the steel samples dipped in
the cassava juice. The results obtained showed that the austempered ST60Mn steel had a
wear rate of 3.0µg/cycle (Bodude et al., 2012). While, the non-heat-treated sample
possessed 70.1µg/cycle. This study showed a significant improvement in the corrosion -
wear rate through the austempering (Bodude et al., 2012).
Research (Bodude et al., 2012, Durowoju et al.,2014) has been done to study the corrosion
rate of mild steel (with and without surface protection) immersed in sweet cassava juice
subjected to two different exposure durations, i.e. continuous and intermittent. Periodic
weight loss measurements and the relationship between weight loss and exposure time
were determined. The results obtained showed that galvanized steel had the highest
corrosion rate during the time of exposure as compared to painted mild steel. Normal mild
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steel with no surface protection however, had the least corrosion rate throughout the
immersion period (Durowoju et al.,2014).
Cassava is the staple food of more than 500 million people in the tropics, many of whom
are very poor. The leaves and roots of the plant contain the cyanogenic glucosides
(Linamarin and a small amount of lotaustralin). The Linamarin is readily hydrolysed to
glucose and acetone cyanohydrin in the presence of the enzyme Linamarase, which is also
produced by the plant. The acetone cyanohydrin decomposes rapidly in neutral or alkaline
conditions liberating hydrogen cyanide and acetone. The sum of the amounts (HCN
equivalent) of Linamarin, acetone cyanohydrin, hydrogen cyanides and cyanide ion equal
the cyanogenic potential of the cassava sample. The cyanogenic potential of cassava roots
and leaves range from 2 to > 1000 ppm HCN, fresh weight. It is generally considered that
cassava roots that contains >100 ppm HCN equivalent should be processed to reduce the
cyanogenic potential before use for human consumption. The traditional methods of
peeling and grating, dewatering and fermentation for 72h reduce the cyanogens in cassava
roots to a considerably safe level. There are several health disorders which have been
associated with regular intake of sub lethal quantities of cyanogens, some of which have
resulted into outright poisoning and death due to cyanide intake from consumption of
poorly processed cassava products. These include the exacerbation of goitre, cretinism
and cardiovascular diseases such as encelopathy and neuropathy. Severe cyanide
poisoning can lead to heart, brain and optic nerve degeneration (Soto-Blanko et al., 2002).
Corrosion problems of steels in cassava fluid become more pronounced as the cassava
tubers are broken into pulps and thereafter left to ferment naturally by yeasts and other
microorganisms in which the sugars in the mash are hydrolysed (Cereda, 1994; Cereda et
al., 1996). Water used in cassava processing carry high concentrations of these
glucosides, which explains the high amounts of toxic compounds in the residual liquid
waste (Cereda, 1994; Cereda et al., 1996). Linamarin and lotoaustralin are hydrolysed in
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the presence of acids and enzymes to produce CN-2 and subsequently, hydrocyanic acid
(HCN) (Williams, 1979; Cereda et al., 1996). The average analysis of cassava liquid as
reported by Cereda is given in Table 2.1
Table 2.1: Analysis of cassava liquid
Composition Percentage % (average)
Humidity 93.71
Protein 0.49
Starch 5.23
Carbohydrate -
Lipid -
Ash 1.06
Fibres -
pH 4.10
Acidity 2.70
HCN (mg/c) 444.00
source: Cereda, (1994)
Such liquids are characterized by high humidity and cyanide composition.
The estimated annual global production of cassava between 1998 and 2000 was 168
million tonnes fresh weight out of which about 70 percent was produced in Nigeria,
Brazil, Thailand, Indonesia and Democratic Republic of Congo (FAO, 2001). Literature
is replete with information on the effect of corrosion on metals in different environments.
Water pollution is harmful not only to mankind but also to metallic equipment such as
pipelines, storage tanks, pumps, heat exchangers, etc. ((Nemerow et al., 1987). Corrosion
by industrial waste water could lead to eventual failure of metallic equipment as well as
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causing water pollution due to the dissolved metal ions such as Cu2+, Cd2+ and Pb2+ which
themselves might be toxic (Salvato et al.,1982; Stuckey et al., 1981),
Corrosion is a complex process characterized by a chemical reaction which proceeds in a
relatively uniform manner over the entire area of a material or structure (Durowoju et al.,
2009; Odidi, 2002). Steel is used in petroleum industry, chemical and nuclear
engineering, power generation, food processing industries and other engineering fields
(Zhang et al., 2005). Under the conditions of highly polluted environment of chloride
ions, stress corrosion cracking and serious pitting corrosion often take place in 304
stainless steel. However, corrosion is said to be a destructive phenomenon of which its
economic effects is detrimental to the appearance of metals and eventually causes
equipment/ machinery failure (Fontana et al., 1987; Daramola et al., 2011).
2.2 Metal
A metal is a hard, opaque and usually lustrous material that has good electrical and
thermal conductivity. Metals are generally malleable, that is they can be hammered or
pressed permanently out of shape without breaking or cracking, they are also generally
fusible and ductile. About 91 of the 118 elements in the periodic table are metals.
Astrophysicists use the term ―metal‖ to collectively describe all elements other than
hydrogen and helium.
From left to right on the periodic table, there are the highly reactive alkali metals, the less
reactive alkaline earth metals, lanthanides and radioactive actinides, the archetypal
transition metals and the physically and chemically weak post-transition metals.
Specialized subcategories are the refractory and noble metals.
All metals may be classified as ferrous or nonferrous. A ferrous metal has iron as its main
element. A metal is still considered ferrous even if it contains less than 50 percent iron,
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as long as it contains more iron than any other compound. A metal is nonferrous if it
contains less iron than any other compound.
General properties of metals include:
I. Tensile strength: the ability of a metal to resist being pulled apart by opposing forces
acting in a straight line. It is expressed as the number of pounds of force required to pull
apart a bar of the material 1-inch-wide and 1 inch thick.
II. Compressive strength: the ability of a metal to withstand pressures acting on a given plane
III. Elasticity: the ability of metal to return to its original size and shape after being stretched
or pulled out of shape.
IV. Ductility: the ability of a metal to be drawn or stretched permanently without rupture or
fracture. Metals that lack ductility will crack or break before bending.
V. Malleability: the ability of a metal to be hammered, rolled, or pressed into various shapes
without rupture or fracture.
VI. Toughness: the ability of a metal to resist fracture plus the ability to resist failure after the
damage has begun. A tough metal can withstand considerable stress, slowly or suddenly
applied, and will deform before failure.
VII. Hardness: the ability of a metal to resist penetration and wear by another metal or material.
It takes a combination of hardness and toughness to withstand heavy pounding. The
hardness of a metal limits the ease with which it can be machined, since toughness
decreases as hardness increases. The hardness of a metal can usually be controlled by heat
treatment.
VIII. Machinability and weld ability: are the ease or difficulty with which a material can be
machined or welded.
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IX. Corrosion resistance: the resistance to eating or wearing away by air, moisture, or other
agents.
X. Heat and electrical conductivity: the ease with which a metal conducts or transfers heat
or electricity.
XI. Brittleness: the tendency of a material to fracture or break with little or no deformation,
bending, or twisting. Brittleness is usually not a desirable mechanical property. Normally,
the harder the metal, the more brittle it is.
XII. The structures are closely packed with a low ionization energies and electro negativities,
Below is a list of certain metals and their peculiar properties:
• Aluminium and bronze: Serve as good thermal and electrical conductors and are fairly
corrosion resistant. Aluminium Light weight
• Bronze: Best metal fatigue unit. More expensive than steel
• Titanium: Highest-strength-to-density ratio of any metallic element
• Carbon steel: Low tensile strength
Application of metals include:
• Suitable for construction purposes.
• Transportation and packaging industries
• For heat sinks to protect sensitive equipment from overheating
• Used heavily in the military
• Automobile transmission for pilot bearings (Terence bell, 2010)
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2.2.1 Non-Ferrous Metal
In metallurgy, a non-ferrous metal is a metal including alloys that does not contain Iron
(ferrite) in appreciable amounts. Generally non-ferrous metals are used because of desired
properties such as low weights (e.g. aluminium), higher conductivities (e.g. copper),
nonmagnetic properties and resistance to corrosion (e.g. zinc). Examples of non-ferrous
metals include aluminium, copper, lead, nickel, tin, titanium etc.
They were the first metals to be used in the art of metallurgy. Silver, copper and gold have
been collected and used since ancient times as currency, jewellery, crucible, holy objects,
weapons and architectural components. These metals are either transformed into finished
products or intermediary metals.
The advantages of non-ferrous include:
Resistance to corrosion
High electrical conductivity
Non-ferrous metals also have certain disadvantages including:
Not used in structural applications More expensive
Non-ferrous metals are mostly applied in;
Valves and plumbing fixtures for brass castings
Structural applications requiring reduced weight, higher strength, nonmagnetic
properties, higher melting points and atmospheric corrosion
Also used in electrical and electronic applications.
(Callister, 2010)
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2.2.2 Ferrous Metal
Outside chemistry, ferrous indicate the presence of iron. The word is derived from the
Latin word ferrum (―iron‖). Ferrous metals include steel, pig iron and alloys of iron with
other metals (such as stainless steel). Manipulation of atom-to-atom relationships
between iron, carbon and various alloying elements establishes the specific properties of
ferrous metals. Generally, ferrous metals are used because of their desirable properties
such as; excellent hardness, durability, and high tensile strength. They are widely used in
almost all industries including the manufacture of shipping containers, industrial piping,
automobiles, and many commercial and domestic tools.
Ferrous metals are highly susceptible to corrosion when exposed to moisture or acidic
environments. Most ferrous metals have good magnetic properties and are considered to
be good conductors of electricity. This property makes them suitable for electronics.
Ferrous metals include cast iron, steel, and the various steel alloys, the only difference
between iron and steel is the carbon content. Cast iron contains more than 2-percent
carbon, while steel contains less than 2 percent. An alloy is a substance composed of two
or more elements. Therefore, all steels are an alloy of iron and carbon, but the term
―alloy steel‖ normally refers to a steel that also contains one or more other elements. For
example, if the main alloying element is tungsten, the steel is a ―tungsten steel‖ or
―tungsten alloy. If there is no alloying material, it is termed ―carbon steel.
Ferrous metals consist of:
Stainless steel: a poor conductor, does not corrode or rust like ordinary steel. Used for
cutlery, knives, milled alloys, surgical tools, etc.
Wrought iron: very malleable and used in home décor, jail and prison fences and high-
end carpentry tools.
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Cast iron: usually comes from Pig Iron. It is liquefied at high temperatures and moulded
into shape for solidification. Cast iron is found in buildings, bridges and life-lasting
cookware.
Tool steel: it is repellent against corrosion, wear and high temperature. Used for things
like injection moulding and machine parts.
High Carbon Steel: this steel usually has a low threshold for temperature. It is fairly well
at resisting rust. It is very rich in carbon with concentrations over 1%.
Advantages of ferrous metals include:
Good speed for constructional works
High quality material
Durability
Aesthetics
Disadvantages include:
High cost of final finishing and polishing products
Environmental issue
Costly waste
Ferrous metals are applied in:
Marine industries for manufacturing hulls, structures and engines of a ship
Construction of building structures and concrete reinforcement
Automotive industries for producing body parts, engine part chassis and drive train
Military for making weapons
Producing utensils, tools, recreational vehicles, etc.
(Callister, 2010)
2.2.2.1 Steels
Steels have been produced in bloomer furnaces for thousands of years but its large-scale
and industrial use began only after more efficient production methods were derived in the
17th century, with the production of blister steel and then crucible steel. With the invention
13
of the Bessemer process in the mid-19th century, a new era of mass-produced steel began.
This was followed by the Siemens-Martin process and then the Gilchrist-Thomas process
that refined the quality of steel. With their introductions, mild steel replaced wrought
steel. Further refinements in the process, such as basic oxygen steelmaking (BOS), largely
replaced earlier methods by further lowering the cost of production and increasing the
quality of the product. Today, steel is one of the most common man-made materials in
the world with more than 1.6 billion tons produced annually. Modern steel is generally
identified by various grades defined by assorted standards organizations. (Terence Bell,
2010).
Steel can be produced by smelting an iron from its ore which contains more desirable
carbon content. The carbon content is reproduced to contain the appropriate amount,
whiles other elements are added to produce an ingot. The ingots are then heated in a
soaking pit and hot rolled into slabs, billet or blooms to strengthen it by heat treatment
process.
Steel can be defined as an alloy of iron and carbon. Because of its high tensile strength
and low cost, it is a major component in buildings, infrastructure, tools, ship, automobiles,
machines appliances and weapons.
Steel’s base metal is iron, which is able to take on two crystalline forms, namely body
cantered cubic (BCC) and face cantered cubic (FCC) depending on its temperature. It is
the interaction of these allotropes with the alloying elements and carbon that gives steel.
(Callister, 2010).
Varying the amount of carbon and other elements in steel, as well as controlling their
chemical and physical makeup, slows the movement of the dislocations that make pure
iron ductile and thus control and enhance its qualities. These qualities/properties include:
Hardness
Quenching behaviour
Need for annealing
Tempering behaviour
Yield strength
Tensile strength
14
The increase in steel’s strength compared to pure iron is only possible by reducing its iron
ductility. There are four main classification of steel which contribute to the development
of the economy including:
1. Alloy Steels are made up of the addition of small percentages of one or more metals to
iron. The addition of alloys changes the properties of steels. For example, steel made from
iron, chromium and nickel produces stainless steel. The addition of aluminium can make
steel more uniform in appearance.
2. Stainless Steels also contain between 10%-20% chromium, making the steel extremely
resistant to corrosion (rusting). When a steel contains over 11% chromium, it about 200
times more resistant to corrosion as steel that does not contain chromium. There are three
groups of stainless steels namely;
A. Austenitic steels: which are very high in chromium and also contain small amounts of
nickel and carbon. These are commonly used for food processing and piping. They are
non-magnetic.
B. Ferritic steels: contain about 15% chromium but only trace amounts of carbon and metal
alloys such as molybdenum, aluminium or titanium. These steels are magnetic, very hard
and strong and can be strengthened by cold working.
C. Martensitic steels contain moderate amounts of chromium, nickel and carbon. They are
magnetic and heat treatable. They are often used for cutting tools such as knives and
surgical equipment.
3. Tool Steels are durable, heat resistant metals containing tungsten, molybdenum, cobalt
and vanadium. They are used not to make tools such as drills. There are a variety of
different types of tool steels containing varying amounts of different alloying metals.
15
4. Carbon Steels are also alloys made from a combination of iron and carbon with several
other elements of low percentages. By varying the percentage of carbon, it is possible to
produce steel with a variety of different qualities. These elements are manganese (1.65%
maximum), silicon (0.60% maximum) and (0.60% maximum). Other elements may be
present in quantities too small to affect its properties. There are three classifications of
carbon steel which includes; high carbon steel, medium carbon steel and low carbon steel.
High Carbon Steel
It is composed of 0.55%-0.95% carbon with 0.30%-0.90% manganese. It is very strong
and holes shape memory well making it ideal for springs and wire. The properties of high
carbon steel are based on the purpose of heat treatment of carbon steel to change the
mechanical properties of steel, usually ductility, hardness, yield strength or impact
resistance.
Properties of High Carbon Steel: increase in carbon content increases its hardness,
strength, improves hardenability and increases brittleness.
Medium Carbon Steel
It is composed of 0.29%-0.54% carbon with 0.60%-1.65% manganese. It is ductile and
strong with long wearing properties. Usually used for crankshafts, couplings and headed
parts.
Properties of Medium Carbon Steel: high tensile strength, impact strength, hardness and
ductility of the material.
Low Carbon Steel
It also composed of 0.05%-0.25% carbon and up to 0.4% manganese. Also known as mild
steel, it is low-cost material that is easy to shape. While not as hard as high carbon steels,
carburizing can increase its surface hardness.
Properties of Low Carbon Steel: relatively low tensile strength but it is cheap and easy to
form. Surface hardness can be increased by carburizing.
16
Applications of Steels
• The construction of roads
• Most large modern structures such as stadiums and skyscrapers
• The construction of bridges
• The construction of railways and other infrastructure
• Manufacturing of appliances and building equipment
Advantages of Steels
• Good thermal and electrical conductor
• Theoretically 100% recyclable without any loss of its natural qualities
Disadvantages of Steels
• Susceptible to fatigue
• Susceptible to corrosion
2.3 Mild Steel
In the recent quest for improved performance of materials, they are specified by numerous
criteria comprising of low carbon content, low tensile strength, high malleability and
ductile. Thus, material researchers, engineers and scientists are always determined to
produce either improved materials or completely newer materials. Mild Steels are
examples of the improved materials category. Mild steel has been in existence for many
decades, however there are no records of its earliest usage (Callister, 2010)
Mild Steel can be referred as steel with a low carbon content, typically the AISI grades
1005 through 1025 which are usually used for structural applications while it can also be
referred as a metal with high strengthening through the addition of carbon. The difference
is that it has a small carbon content and is alloyed with chromium, nickel, molybdenum
and other elements to improve its mechanical and chemical properties.
Mild Steel is a steel containing a small percentage of carbon, it contains approximately
0.05-
0.25% carbon making it malleable and ductile. It is strong and tough but readily tempered.
Properties of mild steel include:
17
• Has a relatively low tensile strength, meaning it will break more easily under tension than
other steels. Luckily, there is a solution of enhancing the strength, wear properties and
improving its fatigue strength
• Relatively inexpensive
• It is ductile
• Has a good machinability
• It is easily to form due to its low carbon content
• Its surface hardness can be increased through carburizing
• It is Weldable
• It is very durable (although it corrodes)
• It is relatively hard and easily annealed
Generally, the density of Mild Steel is approximately 7850 kg/m3 or 0.284 Ib/in3 and the
Young’s modulus (elasticity) is 200 GPa (29,000,000 psi). It suffers from yield point run
out where the material has two yield points. The first yield (upper yield) point is higher
than the second while the yield drops dramatically after the upper yield point. (Callister,
2010).
Mild steel objects are illustrated in the figures below:
(a) (b)
Figure 2.1: (a) A round mild steel pipe and (b) A square mild steel pipe (Cereda,
1994)
2.3.1 Classification of Mild Steel
Mild steel is the alloy of iron and cementite (not carbon) with latter being in the range of
0.05% to 0.3% by weight. Other metallic and non-metallic constituents are also present
18
in very less percentages. As per the Indian Standard 2062, there are nine mild steel grades
specified and they include;
1. Fe250 or E250
2. Fe275 or E275
3. Fe300 or E300
4. Fe350 or E350
5. Fe410 or E410
6. Fe450 or E450
7. Fe550 or E550
8. Fe600 or E600
9. Fe650 or E650
(Quora, 2012)
2.3.2 Application of Mild Steel
Mild Steel materials are widely used and are applicable to almost all areas in the world.
Engineers have been able to produce and formulate mild steel materials to fit and suit a
lot of applications and specifications. Mild Steel applications can be found in the areas of
construction, aerospace, sports, domestic, food processing, etc. In the aerospace sector,
mild steel materials are used for making of aircraft body and also for car parts. In the
construction sector, products such as bolts and nuts, nails and screws and various outdoor
uses (Wilson, 2010).
2.3.3 Advantages of Mild Steel
• Cost effective
• Weldable
• Ductile
• Can be carburized
• Recyclable
19
2.3.4 Disadvantages of Mild Steel
Poor resistance to corrosion
2.4 Corrosion of Materials
Corrosion is the degradation and deterioration of a material due to reactions with its
environment. Some school of thought argue that the definition should be restricted to
metals, however corrosion engineers must consider both metallic and non- metallic
components in solving corrosion problems as well as in manufacturing. The process of
metal corrosion can be considered as extractive metallurgy in reverse- metals return to
their more stable forms (Fontana, 1987).
Most corrosion processes are electrochemical in nature and consist of at least two
reactions on the surface of the corroding metal. There is the oxidation reaction (e.g.
dissolution of iron) also referred to as the anodic partial reaction. The other is the
reduction reaction (e.g. reduction of oxygen), and it is also referred to as the Cathodic
partial reaction. The products of the electrochemical reactions can react with each other
in a chemical reaction to form the final corrosion product (May, 2016).
Some of the effects of corrosion includes:
• Plant shutdowns.
• Loss of products, container leakages, storage tank leaks- at least 25% of water is lost due
leakage
• Loss of efficiency: Corroded machinery reduce heat transfer, piping capacity etc.
• Contamination: Corrosion products may contaminate chemicals, pharmaceutical products
etc.
(Zaki, 2006).
2.4.1 Types and Classification of Corrosion
Varying degrees and large amounts of corrosion problems are observed in industry as a
result of the combination of different materials in varying environments as well as the
20
various service conditions. Corrosion may not have an adverse effect on a material
immediately but it would gradually affect its strength, mechanical properties and physical
appearance. It may lead to serious operational problems. Corrosion may present itself as
a cosmetic/ aesthetic problem only, but it can be very serious if the degradation involves
critical components (Zaki, 2006).
The occurrence of corrosion has therefore been classified as follows:
• Uniform corrosion: It is the uniform thinning of a metal with no localized attack.
Corrosion occurs at the surface does not penetrate very deep inside. A common example
is the rusting of steel in air.
• Galvanic corrosion: Occurs when two metals with different electrochemical potentials are
in contact with each other in a corrosive electrolyte.
• Dezincification: It is a type of corrosion in which zinc is selectively attacked in an alloy
containing zinc.
• Crevice corrosion: This is a localized form of corrosion that is caused by the deposition
of dirt, dust, mud and other deposits on a metal‘s surface or by the existence of voids,
gaps and cavities between adjoining surfaces.
• Pitting corrosion: It is a form of localized corrosion of a metal’s surface where small areas
are preferentially corroded, which leads to the formation of cavities, however the bulk of
the metal’s surface remains not attacked. Metals that form passive films, such as
aluminium and steels, are highly susceptible to this type of corrosion.
• Intergranular corrosion: It is as a form of localized attack on the grain boundaries of a
metal/ alloy in corrosive media, that causes the loss of strength and ductility of the
metal/alloy.
• Stress corrosion cracking: Stress corrosion is the failure of a metal as a result of the
conjoint action of stress and chemical attack.
21
(Zaki, 2006).
2.4.2 Corrosion Behaviour of Mild Steel
The corrosion behaviour of mild steel has been investigated in various concentrations of
HNO3, HCL, HCLO4, etc. Results show that the corrosion rates of mild steel in all the
acidic media studied were higher than that of high carbon steel. This may imply that the
carbon content may have little or no effect on general corrosion resistance of steel.
(Osarolube et al, 2008).
Mild steel and high carbon steel have been found to corrode in different concentrations
of HNO3, HCl, and HClO4 solutions. This was made evident by the decrease in the initial
weight of the metal coupons. HNO3 has been found to be more corrosive, followed by
HClO4 and then HCl. The corrosion of mild and high carbon steels in HCl, HNO3 and
HClO4 solutions have been attributed to the presence of water, air, and H+ which speed
up the corrosion process. The weight loss of the steel samples increased with time and
concentration. This observation proves that the rate of the corrosion reaction increases
with increasing concentration (Osarolube et al, 2008).
The rate of corrosion of mild steel in NaCl media has also been investigated by the weight
loss method. The weight loss of mild steel in NaCl at room temperature i.e. 25◦C, has
been found to be very significant, indicating poor corrosion resistance (May, 2016).
2.4.3 Effect of Cyanide on Corrosion Behaviour of Mild Steel
Cyanide occurs in cassava as HCN which is acidic. This reduces the pH thereby providing
suitable conditions for the increase in corrosion rate of iron and hence mild steel.
2.5 Prevention of Corrosion
In virtually all situations, metal corrosion can be managed, slowed or even stopped by
using the appropriate techniques. Corrosion prevention can take different number of
forms depending on the circumstances of the metal being corroded. Corrosion prevention
can take a number of forms depending on the environmental conditions. The corrosion
prevention techniques can be classified into six (6) major groups and these are;
22
Environmental Modifications: Corrosion is caused by chemical interactions between
metal and gases in the surrounding environment. Methods to reduce the sulphur, chloride
or oxygen content in the surrounding environment can limit the speed of metal corrosion.
Metal Selection and surface conditions: Proper material selection can also lead to
significant reductions in corrosion.
Cathodic Protection: It works by converting unwanted anodic(active) sites on a metal’s
surface to Cathodic(passive) sites through the application of an opposing current. This
opposing current supplies free electrons and forces local anodes to be polarized to the
potential of the local cathodes.
Corrosion Inhibitors: Slowing down corrosion depends on changing the anodic or
Cathodic polarization behaviour by decreasing the diffusion of ions to the metal’s surface
and increasing the electrical resistance of the metal’s surface.
Coating: Paints and other organic coatings are used to protect metals from the degradative
effect of environmental gasses.
(Terence, 2017)
2.5.1 Inhibitors
Corrosion rate reduction and corrosion prevention can be achieved by various processes.
One such process is the use of inhibitors. Formerly chromates were used as corrosion
inhibitors.
However environmental scientists discourage this due to the toxic nature of chromates.
Extracts of plant materials are environmentally friendly and contain many active
principles. They contain polar atoms such as S, N, O, P etc. Due to this, the lone pair of
electrons present on these atoms is pumped onto the metal surface, hence loss of electrons
from the metal surface can be avoided thus corrosion inhibition takes place. Because of
the adsorption of inhibitor molecules on the metal surface, a protective film is formed and
corrosion is controlled. (Nano, 2015)
23
Various works have been done (Amuda et al., 2005 and 2006) on the effects of certain
inhibitors on the corrosion behaviour of mild steel in cassava extract. The results of these
works revealed that a higher inhibitor concentration provides better corrosion rate
reduction than a lower inhibitor concentration, irrespective of the type of the inhibitor/
inhibitors. Further works by (Olawale et al., 2006) looked at the contribution of volumes
of inhibitor in controlling the corrosion rate of mild steel in Hydrogen Cyanide. This study
showed that the corrosion rates of mild steel in hydrogen cyanide inhibited with a mixture
of zinc oxide/triethylene amine progressively reduced. The best result, was obtained when
the zinc oxide was of a higher volume (15/5) in the zinc oxide/triethylene amine mixes.
The corrosion rate in this medium was very low
The use of chemical inhibitors to decrease the rate of corrosion processes is quite varied.
In the oil extraction and processing industries, inhibitors have always been considered to
be the first line of defence against corrosion. A great number of scientific studies have
been devoted to the subject of corrosion inhibitors. However, most of what is known has
grown from trial and error experiments, both in the laboratories and in the field. Rules,
equations, and theories to guide inhibitor development or use are very limited. By
definition, a corrosion inhibitor is a chemical substance that, when added in small
concentration to an environment, effectively decreases the corrosion rate. The efficiency
of an inhibitor can be expressed by a measure of this improvement.
Inhibitor efficiency (%) = 𝐶𝑅1−𝐶𝑅2
𝐶𝑅1 × 100 (Equation 2.1)
where CR1 = corrosion rate of the uninhibited system
CR2 =corrosion rate of the inhibited system
In general, the efficiency of an inhibitor increases with an increase in inhibitor
concentration (e.g., a typically good inhibitor would give 95% inhibition at a
concentration of 0.008% and 90% at a concentration of 0.004%). A synergism, or
cooperation, is often present between different inhibitors and the environment being
24
controlled, and mixtures are the usual choice in commercial formulations. The scientific
and technical corrosion literature has descriptions and lists of numerous chemical
compounds that exhibit inhibitive properties. Of these, only very few are actually used in
practice. This is partly because the desirable properties of an inhibitor usually extend
beyond those simply related to metal protection. Considerations of cost, toxicity,
availability, and environmental friendliness are of considerable importance
Classification of Inhibitors
Inhibitors are chemicals that react with a metallic surface, or the environment this surface
is exposed to, giving the surface a certain level of protection. Inhibitors often work by
adsorbing themselves on the metallic surface, protecting the metallic surface by forming
a film. Inhibitors are normally distributed from a solution or dispersion. Some are
included in a protective coating formulation. Inhibitors slow corrosion processes by;
Increasing the anodic or Cathodic polarization behaviour (Tafel slopes)
Reducing the movement or diffusion of ions to the metallic surface
Increasing the electrical resistance of the metallic surface
(Jonas, 1988)
Selection of an Inhibitor System
In choosing between possible inhibitors, the simplest corrosion tests should be done first
to screen out unsuitable candidates. The philosophy of initial screening tests should be
that poorly performing candidates are not carried forward. An inhibitor that does poorly
in early screening tests might actually do well in the actual system, but the user seldom
has the resources to test all possible inhibitors. The inhibitor user must employ test
procedures that rigorously exclude inferior inhibitors even though some good inhibitors
may also be excluded. The challenge in inhibitor evaluation is to design experiments that
simulate the conditions of the real-world system. The variables that must be considered
include temperature, pressure, and velocity as well as metal properties and corrosive
25
environment chemistry. System corrosion failures are usually localized and attributed to
micro conditions at the failure site. Examples of microenvironments are hot spots in heat
exchangers and highly turbulent flow at weld beads. The practice of corrosion inhibition
requires that the inhibitive species should have easy access to the metal surface. Ideally,
surfaces should therefore be clean and not contaminated by oil, grease, corrosion
products, water hardness scales, and so forth. Furthermore, care should be taken to avoid
the presence of deposited solid particles. This conditioning is often difficult to achieve,
and there are many cases where less than adequate consideration has been given to the
preparation of systems to receive inhibitive treatment. It is also necessary to ensure that
the inhibitor reaches all parts of the metal surfaces. Care should be taken, particularly
when first filling a system, that all dead ends, pockets, and crevice regions are contacted
by the inhibited fluid. This will be encouraged in many systems by movement of the fluid
in service, but in nominally static systems it will be desirable to establish a flow regime
at intervals to provide renewed supply of inhibitor. Inhibitors must be chosen after taking
into account the nature and combinations of metals present, the nature of the corrosive
environment, and the operating conditions in terms of flow, temperature, and heat
transfer. Inhibitor concentrations should be checked on a regular basis and losses restored
either by appropriate additions of inhibitor or by complete replacement of the whole fluid
as recommended, for example, with engine coolants. Where possible, some form of
continuous monitoring should be employed, although it must be remembered that the
results from monitoring devices, probes, coupons, and so forth, refer to the behaviour of
that particular component at that particular part of the system. Nevertheless, despite this
caution, it must be recognized that corrosion monitoring in an inhibited system is well
established and widely used. (Brasunas, 1984).
26
2.6 Cassava
Cassava is a perennial crop native to the tropical regions including Africa, South and
Central America, the Caribbean and some parts of Asia (namely India, Indonesia etc.). It
is a woody shrub that belongs to the Manihot genus of the Euphorbiaceous family. The
Manihot genus is reported to have approximately 100 species, among which only Manihot
esculenta Crantz (cassava) is cultivated commercially (Ubwa et al., 2015). This shrub
grows to about 1 to 4 meters in height with large, spirally arranged, lobed leaves which
is grown mainly for its tubers. It produces several tuberous roots that serve as reserves of
up to 35% starch. These tuberous roots may grow up to 1 m in length and may weigh up
to 40 kg all together. The stems are however used as planting material (Kouakou et al.,
2016).
Cassava (also known as tapioca or manioc) is an important root crop as it provides food
security and can be cultivated as a cash crop. It can produce appreciable yields on
relatively infertile soil as it has a flexible harvest period (William, 2011). Cassava root
can be consumed raw as a snack or just after being boiled (Fakir et al., 2012) – this is
after removing the skin and rind. The culinary uses of cassava include cassareep/ tucupay,
fufu, mingao, ampesi, macaroni, cassava pudding etc. (Balagolan, 1996).
In general, cassava is used as raw material in the production of processed food, animal
feed and industrial products (Balagolan, 1996). In the Pharmaceutical, garment (textile),
bakery and food processing industries, cassava tubers serve as raw materials (Fakir et al.,
2012). This shrub has over twenty products derived from it and is also used in the
manufacture and production of paper, glues, alcohol and starch. The cultivation and
marketing of cassava and its products provides both small and large producers with
significant income (Kouakou et al., 2016).
Cassava contains naturally occurring, but potentially toxic compounds called cyanogenic
glycosides, which upon maceration of the plant tissue, releases hydrogen cyanide (HCN)
27
as a result of enzymatic hydrolysis (William, 2011). An image of a cassava plant and
cassava tubers are shown below,
(a) (b)
Figure 2.2: (a) Cassava plant and (b) Cassava tubers
2.6.1 Types of cassava tuber
Cassava has many varieties that have been classified into three main types based on the
cyanogenic glucoside (cyanide) content of the tuber as well as its bitterness. The types
are sweet, average toxic and bitter with concentrations <50, 50-100 and >100 ppm of
Linamarin respectively, calculated as mgCN-/kg of fresh weight cassava that is
consumable. The bitterness of the tubers has often been used as a means of determining/
approximating their relative cyanide contents and toxicity (Jansz, 1997).
2.6.2 Processing of Cassava
Primarily cassava is processed into gari, flour, etc. by peeling, washing, slicing, grating,
dewatering/fermentation, sifting, drying/frying, post-grinding, packaging and storage. All
these operations involve the use of different levels of machine sophistication through
which corrosion problems are likely to be encountered.
Cassava products are obtained using artisanal and semi-industrial processes. Some of
these products are:
Chips: The roots are peeled, washed and cut into small pieces and then dried in the sun.
28
Flour: The roots are peeled, washed and crushed. The crushed and pressed extracted pulp
is dried in the sun. The dried pulp is ground and sieved through a 710 μm sieve.
Gari: The roots are peeled, washed and crushed then the slurry is fermented for 48 hours
before pressed. The obtained pulp is sieved and dried using stainless cooking devices.
Attieke: The roots are peeled, washed, crushed and fermented for 48 hours before
pressed.
The obtained pulp is rolled, sifted then steamed.
(Diallo et al., 2014).
2.7 Cyanide
Cyanides are a wide range of chemically complex compounds, all of which possess a CN
moiety. Many chemical forms of cyanide are used in industry or are present in the
environment. However, the cyanide anion, CN- is the primary toxic agent. Hydrogen
cyanide is either a colourless or pale blue liquid/gas with a slight bitter almond-like odour.
Hydrogen cyanide is used mainly in the production of adiponitrile, methyl methacrylate,
chelating agents, cyanuric chloride, methionine and its hydroxylated analogues etc.
Cyanides, such as sodium and potassium cyanide, are solid/crystalline hygroscopic salts
that are mostly used in ore extracting processes for gold recovery (Medical Ecology,
2004)
2.7.1 Sources of Cyanide
Cyanide has long been considered a toxic and deadly substance. It is ubiquitous and
present in some plants as well as in the products of the combustion of synthetic materials
(Poisoning, C). Leaves and stems of all sorghum species as well as cassava contain
varying amounts of cyan glycosides (Carlson, 2013).
29
There are at least 2,650 species of plants - including fruits and vegetables- that produce
cyanogenic glycosides and usually an accompanying hydrolytic enzyme (beta-
glycosidase), which are brought together when the cell structure of the plant is disrupted
by a stimulus/predator, with subsequent breakdown to sugar and a cyanohydrin, that
rapidly decomposes to hydrogen cyanide (HCN) plus an aldehyde or a ketone. The
glycosides, cyanohydrins and hydrogen cyanide are collectively known as cyanogens
(Standards, 2005).
In air, cyanide is present as hydrogen cyanide gas, while small amounts are present in fine
dust particles. Cyanides have the tendency to be transported over very long distances from
their points of emission (Medical Ecology, 2004).
2.7.2 Cyanide Content in Cassava Tuber
Cassava roots contain appreciable quantities of cyanide which occurs in the form of
cyanogenic glycosides, mainly Linamarin and smalls amount of lotaustralin. The amount
of cyanide present in the tubers is dependent on their ages (William, 2011).
As stated above, the main cyanogenic glycoside in cassava is Linamarin, with small
amounts of lotaustralin (methyl Linamarin) also present, as well as the enzyme
linamarinase. Upon enzymatic catalysis by Linamarinase, Linamarin is readily
hydrolysed to glucose and acetone cyanohydrin while lotaustralin is hydrolysed to a
related cyanohydrin and glucose. Under neutral conditions, the acetone cyanohydrin
produced decomposes to acetone and hydrogen cyanide (Standards, 2005).
There are many varieties of cassava and the cyanide content differs. The types are the
sweet type hat has a concentration of <50 ppm of Linamarin and the bitter type that has
concentrations of >100 ppm of Linamarin (Jansz, 1997).
30
2.7.3 Health Effect of Cyanide from Cassava
Cyanides are well absorbed via the gastrointestinal tract as well as the skin and rapidly
absorbed through the respiratory tract. Upon absorption, cyanide is rapidly distributed
throughout the body, the highest levels are usually found in the liver, lungs, blood, and
brain. Prolonged consumption of cassava containing high levels of cyanogenic glycosides
has been linked with the occurrence of tropical ataxic neuropathy, spastic Para paresis,
and in areas where there is low consumption of iodine, there is the high tendency of the
development of hypothyroidism, goitre, and cretinism (Medical Ecology, 2004).
Konzo is an adverse health condition that results from the excessive ingestion of cyanide
compounds from inadequately prepared cassava and cassava products and is
characterized by permanent paralysis of the legs and other developmental disorders. It
occurs mainly in children and women of child bearing age (William, 2011).
Despite the presence of these naturally occurring toxic compounds, millions of people all
over the world have been safely consuming cassava for many years. Usually, cassava is
well processed before consumption. Inadequate processing may however result in
appreciable amounts of cyanogenic glycosides remaining thereby posing health risks
(William, 2011).
Symptoms of acute cyanide intoxication include: rapid respiration, drop in blood pressure,
rapid pulse, dizziness, headache, stomach pains, vomiting, diarrhoea, mental confusion,
twitching and convulsions. If the hydrogen cyanide exceeds the limit an individual is able
to detoxify/tolerate, death may occur due to cyanide poisoning. The acute oral lethal dose
of hydrogen cyanide for human beings is reported to be 0.5-3.5 mg/kg bodyweight.
Approximately 50-60 mg of free cyanide from cassava and its processed products
constitutes a lethal dose for an adult man (Standards, 2005).
31
2.7.4 Obtaining cassava extract
The cassava extract which is used as a corrosive medium is obtained by grinding freshly
harvested cassava tubers followed by pressing which involves using a screw press or
hydraulic press with a bucket underneath to collect the cassava extract which is further
preserved in a refrigerator (Bodude et al.,2012).
2.8 Characterization
The corrosion testing procedure involves various steps that have to be undertaken
meticulously to obtain accurate and reliable information. The surface of the samples
should be well cleaned as surface dirt could affect the integrity of the experiment. This
experiment would involve extraction of cyanide from cassava tubers, sample preparation,
weight loss determination, electrochemical testing, corrosion rate determination,
morphology as well as functional group determination.
2.8.1 Cyanide Content Determination
There are various methods of determining the cyanide content of cassava. For example,
the cyanide concentration in cassava parenchyma was determined using ninhydrin-based
spectrometer of trace cyanide at 485 nm maximum wavelength. A calibration graph was
first constructed using standard solutions of CN- at concentrations of 0.02, 0.04, 0.08, 0.1
and 0.2 μg/mL (which is within the linear range) and was prepared by adding appropriate
volumes of cyanide solutions at concentration of 20 μg CN-/mL to 1 mL of 2% Na2CO3.
Ninhydrin solution (0.5 mL) containing 5 mg/mL in 2% NaOH was added to each
standard cyanide solution. The mixture was homogenized and incubated for 15 minutes
for colour development. Similarly, the blank was prepared in the same way as above,
except that instead of 1 mL 2% Na2CO3 containing CN-, 1 mL of 2% Na2CO3 without
CN- was added. UV Visible absorption of the reaction product (Cyanide-ninhydrin
adduct) of the different concentrations of cyanide was measured using UV
Spectrophotometer at 485 nm. Total cyanide in the samples was determined by adding
0.1 g of the ground sample in a standard volumetric flask (5 mL) and made up to mark
with 0.1% NaHCO3. The samples were sonicated for 20 minutes in a water bath and the
mixture centrifuged at 10,000 rpm for 10 minutes. The supernatant was pipetted with
automatic pipette, two aliquots (2 mL each) and added to 0.5 mL ninhydrin in NaOH,
32
allowed for fifteen minutes for colour development and absorbance measured at 485 nm
(Ubwa et al., 2015)
2.8.2 Functional Group Analysis
Fourier transform infrared spectrometry (FTIR) has been used to evaluate the possible
functional groups present in various compounds such as biodiesel. It is a less complex
way to determine the presence of functional groups in a sample and its structure, based
on the energies associated with the molecular vibration when irradiated. When samples
are run through FTIR, the bonds as well as functional groups present, respond differently
to the incoming radiation, due to the differences in their molecular vibration of stretching
and bending. The response of these functional groups is characterized by observing the
transmission of infrared radiations and comparing it with known standards in order to
identify the type and the nature of functional groups present in the samples. The presence
and the nature of functional groups among other factors provide information on the
stability of the compound.
The major components of the FTIR system are; the radiation source, the interferometer,
the slit, beam splitter and the detector. The radiation source generates a radiation in form
of light which is directed to the sample through the interferometer. The interferometer
separates the source radiation into its different wavelengths, and the slit selects the
collection of wavelengths that passes through the sample at any given time. The beam
splitter separates the incident beam into two; half of the incident beam goes to the sample
and the other half to the reference standard. The sample absorbs light according to its
chemical properties. A detector collects the radiation that passes through the sample and
compares its energy to that going through the reference, and then put the electric signer
which is normally sent directly to a recorder linked back to the interferometer so as to
allow the interpretation of energy as a function of frequency or wavelength translated into
a finger print which appears in a computer monitor attached directly to the detector and
printed out as hard copy. The functional groups are identified by comparing the peaks
generated by the sample to that of the reference standard.
(Grace, 2013).
2.8.3 Microscopy Analysis
Scanning microscopy analyses are beneficial tools in characterizing the corrosion
behaviour of metallic samples immersed in corrosion media. An optical microscope with
high digital camera is used to identify the effects of exposing mild steel samples to
33
cassava extract. Images are obtained for all samples immersed in the corrosive media to
characterize the corrosion product that is formed on the surface of the samples.
The surface morphology of the samples is carried out by using an electronic microscope
after ̀ removing the corrosion product. In cases where NaCl has been used as the corrosive
media, it has been observed that the surface morphology indicated a uniform attack.
(May, 2016).
2.8.4 Corrosion Experiment
This stage consists of a corrosion testing process and a corrosion rate determination
process. Both the corrosion test process and the corrosion rate determination process are
made up of a weight loss and an electrochemical method.
Corrosion testing process
Corrosion tests include the weight loss method and the electrochemical method.
Weight loss method
Mild steel samples of size 50 x 40 x 2 mm are cut from a sheet. Samples are machined
and abraded sequentially with silicon-carbide papers of grades 180. The samples are then
washed, cleaned with ethyl alcohol and dried up. The initial weights of the samples are
taken, (to an accuracy of three decimals) before immersion in the cassava extract in order
to conduct the weight loss experiments. The samples are then immersed in the cassava
extract.
At the end of each interval testing period the samples are taken out of the extract, washed
with tap water, distilled water and then with ethanol. Afterwards, the samples are weighed
for the weight loss calculations. The process of washing, drying, weighing, determination
of weight loss and recording are repeated consistently (May, 2016).
Electrochemical method
This involves the measurement of the electrode (mild steel samples) potential (Ecorr) and
the corrosion current (icorr). These are the potential and the current at which corrosion
would occur for a particular metal, in this case mild steel.
Measurement of Ecorr (Corrosion Potential)
The potential of the electrode also known as the working electrode is measured with
respect to a standard, non-polarizable Calomel electrode. The Calomel electrode which is
34
the reference electrode is kept in a separate container and it is electrically connected to
the working electrode which is placed in a container in contact with the electrolyte via a
salt bridge. A voltmeter of high impedance is connected between the working electrode
and the reference electrode- the negative terminal of the voltmeter is connected to the
working electrode and the positive terminal is connected to the reference electrode. The
corrosion potential (open circuit potential) in the freely corroding state is displayed by the
voltmeter. The corrosion potential is also known as the mixed corrosion potential as it
represents the compromise/ combined potential of the anodes and the cathodes (Zaki,
2006). A set up of this experiment is shown below
Figure 2.3: an experimental arrangement for making corrosion potential
measurement (Zaki, 2006).
Measurement of Corrosion Current (icorr)
The corrosion current is calculated from the anodic and Cathodic polarizations. For the
anodic polarization, an over-potential (E-Ee) is impressed in the noble direction/
decreasing direction starting from the corrosion potential. The over-potential (η) is a
measure of how far the reaction is from the equilibrium i.e. where η, is zero. If the
potential becomes more positive than the equilibrium potential, then the rate of the anodic
reaction (forward reaction) is greater than the rate of Cathodic reaction (reverse/
backward reaction) and metal dissolution/ oxidation continues. Impress potentials E1, E2
and E3 in the noble direction/ decreasing direction. The values of i (current density) are
measured experimentally from the equation 2.2 where I, is the current and A is the surface
area of the samples. i is plotted in the graph E against log i. The three values i are plotted
35
in the graph. The three points are connected to obtain the measured anodic polarization
curve and the potential lines E1, E2 and E3 intersect the anodic curves at i1, i2 and i3
i =I
A (Equation 2.2)
In a similar but opposite direction, the measured Cathodic polarization curve is obtained
by increasing the potential from Ecorr to E1, E2 and E3 in the negative direction also known
as the active potential direction.
(Zaki, 2006). The set up for this experiment is shown in Figure 2.4, a feedback circuit
(not shown) automatically adjusts the voltage held between test electrode and reference
electrode to any pre-set value (Zaki, 2006)
Figure 2.4: Circuitry associated with controlled potential measurements. (Zaki,
2006)
36
Fig 2.5: Actual and measured polarization curves for active metals
(Zaki, 2006).
Corrosion rate determination
Weight loss method
After surface preparation, the dimensions of the specimens would be carefully measured
to enable the calculation of the surface area. The surface area would be used in the formula
below to calculate the corrosion rate.
CR = kW
A×D×T (Equation 2.3)
Where k = 8.76 × 10-4, W = weight loss (mg), A = area in square cm, D = mild steel
density (g/cm3), T = immersion time (hr.). The initial area is used to calculate the rate of
corrosion throughout the test (May, 2016).
Electrochemical method
The electrochemical method of corrosion determination can be done by the Tafel
extrapolation method or the Polarization Resistance (Linear Polarization)
Tafel extrapolation method:
In this method, the polarization curves for the anodic and Cathodic reactions are obtained
by applying potentials of approximately 300 mVSCE far above the corrosion potential and
recording current. Plotting the graph of potential vs log i and extrapolating the currents in
the two Tafel regions gives the corrosion potential E and the corrosion current icorr.
Knowing icorr, the corrosion rate can be calculated in preferred units by using Faraday’s
law.
37
R= ia
nF (Equation 2.4)
where i is icorr, a is the atomic weight of the metal, n is the number of electrons exchanged
in the reaction, F is the Faraday constant which is 96500C. The corrosion rate expression
is also multiplied by 1
D, where D is the density of the metal and this is done to determine
the penetration rate of the corrosion process.
For alloys such as mild steel the corrosion rate is expressed as equation 2.5.
𝑟 =𝑖(𝐸𝑊)
𝐹𝐷 (Equation 2.5)
Where EW is the equivalent weight of the alloy.
EW = 1
Neq (Equation 2.6)
Neq is the total number of equivalents.
Neq = ∑fiNi
a (Equation 2.7)
fi is the mass fraction, Ni is the electrons exchanged and ai is the atomic weight.
(Zaki, 2006).
38
Figure 2.6: A hypothetical Tafel plot (Zaki, 2006).
Polarization Resistance (Linear Polarization):
Under this method, the Polarization resistance (Rp) of a metal corroding is defined by
Ohm‘s Law as the slope of a potential (E) vs log of current density (log i) plot at the
corrosion potential (Ecorr). Here 𝑅𝑝 =∆𝐸
∆𝐼at E= 0. By measuring this slope, the rate of
corrosion can be determined. The correlation between icorr and slope is given by equation
2.8 where ba and
bc are Tafel slopes (Zaki, 2006).
dE
dI=
babc
2.3icorr(babc) (Equation 2.8)
Fig 2.7: Hypothetical linear polarization plot (Zaki, 2006).
39
CHAPTER THREE
3.0 METHODOLOGY
In this chapter, the methods that were employed in the determination of the effect of
cassava extract (cyanide) on the corrosion behaviour of mild steel are outlined. The
experimental procedures and techniques used are also discussed.
3.1 Materials and Equipment
The materials used for the project were mild steel samples, sweet and bitter cassava
tubers, ethyl alcohol, silicon carbide papers and green inhibitor. Equipment for the
experimental process were an electronic balance, oven, optical microscope, potentiostat,
a household electric blender, an FTIR machine and a mass spectrometer.
3.2 Material Acquisition
Cassava tubers (bitter and sweet type) were purchased from a farmland situated behind
the Cassley Hayford block on KNUST campus (Figure 3.1). Mild steel samples of size
50 x 40 x 2 mm3 (Quora, 2012) were purchased and cut from a sheet at Suame magazine.
The elemental composition of the mild steel samples was determined with the help of a
mass spectrometer at Tema Steel Company. For the inhibitors, peels of orange fruits were
obtained and pounded to obtain the powder.
(a) (b)
Figure 3.1: (a) Acquisition of the bitter cassava tubers and (b) Acquisition of sweet
cassava tubers
40
3.3 Specimen Preparation
This section details processes undertaken such as cassava juice extraction, mild steel
sample preparation, preparation of inhibitors and reagent preparation.
3.3.1 Preparation of cassava extract
The extract in the two types of cassava i.e. sweet and bitter were obtained separately but
with the same procedure. The cassava tubers were washed thoroughly, peeled and diced
(Figure 3.2a). 500g of the diced tubers was weighed and put into a household electric
blender, 50mL of distilled water was added to it and the mixture blended for 60 seconds
(Figure 3.2b). The blended sample was placed in a clean white cloth and squeezed
manually to obtain the cassava extract (Figure 3.3a).
(a) (b)
Figure 3.2: (a) Peeling, cutting and dicing of cassava and (b) Blending of cassava
tuber with an electric blender
41
(a) (b)
Figure 3.3: (a) Extracted cassava juice and (b) Cassava dough
3.3.2 Preparation of Mild Steel Samples
The surfaces of fifteen (15) mild steel samples of size 50 x 40 x 2 mm3 were polished
with grinding papers of progressively fine grades (Figure 3.4). The samples were then
rinsed in distilled water, cleaned with ethanol and dried.
Figure 3.4: Preparation of mild steel samples
42
3.3.3 Processing of corrosion inhibitor
Ten (10) orange fruits were purchased, washed (Figure 3.5a) and peeled. The peels were
dried for 24 hours, partly in the sun and partly in the oven at 105 oC. They were then
pounded (Figure 3.5c) to powder form (Figure 3.5d).
(a) (b)
(c) (d)
Figure 3.5: (a) Washing of orange fruits, (b) Sun drying of orange peels, (c)
Pounding of dried orange peels and (d) Powdered peels used as inhibitor
43
3.3.4 Reagents Preparation
Four (4) reagents used in the Alkali Titration Method (AOAC, 1990) were prepared at the
Chemistry laboratory of KNUST in order to determine the cyanide content of the cassava
extract. They were prepared as follows:
0.02M Sodium hydroxide (NaOH);
2.02 grams of NaOH pellets were weighed and recorded. The pellets were transferred to
a 500ml volumetric flask, distilled water was added and shaken to obtain a properly mixed
solution. The solution was topped up to the 500ml mark with distilled water.
5% Potassium iodide (KI):
5 grams of KI pellets were weighed and recorded. The pellets were transferred to a 100ml
volumetric flask, distilled water was added and shaken to obtain a properly mixed
solution.
The solution was topped up to the 100ml mark with distilled water.
0.02M Silver nitrate (AgNO3):
1.69 grams of AgNO3 powder was weighed and recorded. The powder was transferred to
a 500ml volumetric flask, distilled water was added and shaken to obtain a properly mixed
solution. The solution was topped up to the 500ml mark with distilled water.
6M Ammonium solution:
225.6ml of concentrated ammonia solution was added to 150ml of distilled water in a
500ml volumetric flask and shaken to obtain a properly mixed solution. The solution was
topped up to the 500ml mark with distilled water.
44
3.4 Cyanide Content Determination by Alkali Titration Method
The cyanide contents of the various cassava extracts were determined at the Chemical
Engineering laboratory of KNUST using the Alkali Titration method below:
• Cassava tubers were peeled, washed and diced (Figure 3.2a).
• 20g of the diced tubers were weighed and ground using a household electric blender
(Figure 3.2b).
• The ground product was placed in a clean white cloth and squeezed to obtain the cassava
juice.
• The extract was transferred into a distilled flask and left to stand for 3 hours. It was then
distilled until 150 cm3 of the distillate was obtained (Figure 3.6).
• 20 cm3 of 0.02M Sodium Hydroxide (NaOH) was added to the distillate using a pipette
and the volume completed to 250 cm3 in a volumetric flask using distilled water.
• Three aliquots of the diluted distillate, two of 100 ml each and one of 50 ml were obtained.
• 8 cm3 of 6 M Ammonium Solution (NH4OH) and 2 cm3 of 5% Potassium Iodide (KI)
were added to the 100 mL aliquots.
• 4 cm3 of 6M Ammonium Solution (NH4OH) and 1 cm3 of 5% Potassium Iodide (KI) were
added to the 50 mL aliquots.
• These were then titrated against 0.02 M Silver Nitrate (AgNO3), the 50 ml aliquot was
used as the trial run (Figure 3.7a).
• A change from clear solution to a turbid one indicated the end point (Figure 3.7b).
• Equation 3.1 was used to calculate the amount of HCN present in the extract.
1 mL of 0.02 M Silver Nitrate = 1.08 mg HCN. (Equation3.1)
AOAC (1990)
45
Figure 3.6: The distillation process
(a) (b)
Figure 3.7: (a) The Titration process and (b) Final titrated solution showing the
colour change
46
3.5 Moisture Content Determination
The moisture contents of the dough obtained from the sweet and bitter cassava were
determined according to the ASTM D4442 at the KNUST Materials Engineering
laboratory. 50g each of bitter and sweet cassava dough were weighed and recorded. They
were placed in an electric oven at 105 oC for 24 hours. The samples were removed from
the oven, placed in a desiccator and re-weighed. The moisture content and percentage
moisture content were calculated using Equations 3.2 and 3.3.
Moisture content M = Mi – Mf. (Equation 3.2)
% moisture content = 𝑀𝑖−𝑀𝑓
𝑀𝑖 × 100 (Equation 3.3)
Mi is the initial weight of the cassava dough and Mf is the weight after drying.
3.6 Corrosion Experiment
The corrosion test was done in two forms, i.e. the weight loss method and the
electrochemical method. In both methods, the experiment was carried out with and
without the application of inhibitor to the set up.
3.6.1 Weight loss Measurement without inhibitor
Weight loss measurements were performed on rectangular mild steel samples of sizes 50
x 40 x 2 mm3 (Quora, 2012). Every sample was weighed and recorded and then placed in
the cassava extract (300mL) or dough. 8 set ups were made ─ 2 set ups each for sweet
cassava extract, bitter cassava extract, sweet cassava dough and bitter cassava dough.
From this, 4 set ups were never touched for 8 weeks. The other 4 were inspected at interval
of 3 days for the 8 weeks. At every inspection, the surfaces of the specimen were cleaned
with distilled water followed by rinsing with ethanol and dried. They were reweighed and
recorded in order to calculate the corrosion rate (CR).
Weight loss W = Wi – Wf. (Equation 3.4)
Wi is the initial weight of sample and Wf is the weight after immersion.
The corrosion rates were calculated using Equation 2.3
The experiment was conducted according to ASTM G1-88 standard. (Singh, 2010)
47
3.6.2 Weight loss Measurement with inhibitor
The process detailed above was repeated with the addition of inhibitor to the set up. At
every inspection, the surfaces of the specimen were cleaned with distilled water followed
by rinsing with ethanol and then reweighed in order to calculate the corrosion rate (CR),
inhibitor efficiency (η %), the corrosion rate (CR) and the surface coverage (θ).
The weight loss was calculated using Equation 3.4, the inhibitor efficiency
was calculated using Equation 2.1 and the corrosion rate was calculated using
Equation 2.3
θ = W−Wih
W (Equation 3.5)
W is weight loss without inhibitor and Wih is weight loss with inhibitor
(Singh et al, 2010)
3.6.3 Electrochemical Measurement
The electrochemical experiment was performed using a typical three electrode cell. A
platinum rod was used as a counter electrode, silver nitrate as the reference electrode and
the mild steel sample as the working electrode in 150 ml of the cassava extract acting as
the electrolyte. An attempt was made to generate polarization curves at a constant sweep
rate of 5 mV/s at the interval from -500 to +500 mV using a Potentiostat. The potentiostat
used had low sensitivity hence the exercise was a failure.
3.7 Characterization Process
The characterization processes done were the determination of the chemical composition
of the mild steel, determination of the functional groups present in the cassava extract
(FTIR) and microstructural analysis of the mild steel surface.
48
3.7.1 Chemical Composition and Functional Group Determination
Smooth grinding was done on a portion of the mild steel sample, followed by polishing
of the ground portion. The mild steel sample was then placed under a mass spectrometer
while sparking was applied to determine the elemental composition of the sample. The
sparking was repeated in order to determine the average elemental composition of the
sample. The process was done at Tema Steels.
FTIR spectroscopy was done at the Central Lab of KNUST to determine the functional
groups present in the cassava extract using PerkinElmer, UATR Two.
3.7.2 Microstructural Analysis
A small piece of the mild steel sample was mounted in epoxy, ground using a grinding
machine, polished and etched. It was observed under the AmScope MT130 optical
microscope at the Materials Engineering lab of KNUST.
49
CHAPTER FOUR
4.0 RESULTS AND DISCUSSION
4.1 Cyanide Content Determination
Table 4.1 shows the cyanide content of the cassava types (i.e. sweet and bitter) that were
extracted for the corrosion experiment. It was observed that the bitter cassava had a higher
cyanide content than the sweet type. The bitter cassava recorded 0.972mg of HCN (48.6
mg HCN/kg) while the sweet cassava recorded 0.864 mg of HCN (42.3 mg HCN/kg),
implying a higher cyanide content in the bitter cassava and supporting the findings of
previous works.
Table 4.1: Cyanide content of the cassava extracts
Titrations Sweet Cassava Bitter Cassava
Control
(50 mL)
Initial level in the
burette
9.6 mL 1.8 mL
Final level in the
burette
10.3 mL 3.0 mL
Amount of AgNO3
reacted at the end point
0.7 mL 1.2 mL
Cyanide 0.756 mg HCN
(37.8 mg HCN/kg)
1.296 mg HCN
(64.8 mg HCN/kg)
First
(100 mL)
Initial level in the
burette
11.7 mL 3.7 mL
Final level in the
burette
12.5 mL 4.3 mL
Amount of AgNO3
reacted at the end point
0.8 mL 0.6 mL
Cyanide 0.864 mg HCN
(43.2 mg HCN/kg)
0.648 mg HCN
(32.4 mg HCN/kg)
Second
(100 mL)
Initial level in the
burette
13.5 mL 4.8 mL
Final level in the
burette
14.4 mL 5.7 mL
Amount of AgNO3
reacted at the end point
0.9 mL 0.9 mL
Cyanide 0.972 mg HCN
(48.6 mg HCN/kg)
0.972 mg HCN
(48.6 mg HCN/kg)
Total
cyanide
content
0.864 mg HCN
(43.2 mg HCN/kg)
0.972 mg HCN
(48.6 mg HCN/kg)
50
4.2 Moisture Content
Moisture is a major element in the corrosion and corrosion rate of metals. Table 4.2 shows
the moisture content and the percentage moisture content of both the sweet and bitter
types.
Table 4.2: Moisture content and % moisture content
Type of dough Initial weight
(g)
Final weight
(g)
Moisture
content(g)
% Moisture
content
Sweet cassava 50.00 21.67 28.33 56.66
Bitter cassava 50.00 24.52 25.48 50.96
The table above shows that the sweet cassava dough had a higher moisture content than
the bitter cassava dough.
Corrosion Measurement and analysis
4.3 Weight loss Method
There were similarities in the trends of corrosion behavior of the mild steel samples
observed with respect to time for the various extracts. Table 4.3 shows the corrosion rates
of the mild steel samples in the extract without inhibitors, Table 4.4 shows the corrosion
rates of the mild steel samples in the extract with inhibitor and Table 4.5 shows the
corrosion rates of mild steel samples in both the sweet and bitter cassava dough. These
are data for the samples placed in the extracts intermittently.
51
Table 4.3: Weight loss determination without inhibitor
Sweet type Bitter type
Immersion
time (days)
Weight of
sample (g)
(Initial)
Weight
loss (g)
Corrosion
rate
(mm/yr)
Weight of
sample (g)
(Initial)
Weight
loss (g)
Corrosion
rate
(mm/yr)
0 30.971 0 0 33.09 0 0
3 30.869 0.102 0.79 32.994 0.096 0.74
6 30.854 0.015 0.058 32.935 0.059 0.23
9 30.851 0.003 0.0078 32.827 0.108 0.28
12 30.849 0.002 0.0039 32.825 0.002 0.004
15 30.845 0.004 0.006 32.801 0.024 0.04
18 30.843 0.002 0.003 32.775 0.026 0.034
21 30.841 0.002 0.002 32.534 0.241 0.27
24 30.839 0.002 0.0019 32.342 0.192 0.19
27 30.83 0.009 0.008 32.152 0.19 0.16
30 30.826 0.004 0.003 32.149 0.003 0.002
33 30.819 0.007 0.005 32.09 0.059 0.04
36 30.81 0.009 0.006 32.079 0.011 0.007
39 30.808 0.002 0.001 32.067 0.012 0.007
42 30.8 0.008 0.004 32.056 0.011 0.006
45 30.798 0.002 0.001 32.051 0.005 0.003
48 30.796 0.002 0.001 32.044 0.007 0.003
52
Figure 4.1: Graph of Corrosion rate against Immersion time (Without inhibitor)
Figure 4.1 shows the change in corrosion rates per immersion time of the samples placed
in the extracts without the addition of inhibitors. The curves show that the mild steel
sample in the bitter cassava extract had higher corrosion rates than the mild steel sample
in the sweet cassava extract. This can be attributed to the higher cyanide content of the
bitter cassava.
The weight loss due to the corrosion of the samples can be attributed to the more intense
chemical reactivity’s and corrosion processes of the anodic and Cathodic reactions that
occurred at the specimen/solution interface─ the solution being the cassava extract. These
corrosion reactions might have been enhanced by the mild steel’s heterogeneous
microstructure, which basically consists of ferrite, pearlite and some inclusions. This led
to the establishment of anodic and cathodic areas. With the immersion of the samples in
the extracts, the anodic sites corroded and the inclusions served as stress raisers leading
to more chemical (corrosion) reactions.
This is also evident in the samples that were immersed in the extract with inhibitors as
well as those in the dough.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48
Co
rro
sio
n r
ate
(mm
/yr
Immersion time (days)
Corrosion rate in sweetcassava juice (mm/yr)
Corrosion rate in bittercassava juice (mm/yr)
53
Table 4.4: Weight loss determination with inhibitor
Sweet type Bitter type
Immersion
time (days)
Weight of
sample
(g)
(Initial)
Weight
loss
(g)
Corrosion
rate
(mm/yr)
Weight of
sample (g)
(Initial)
Weight
loss (g)
Corrosion
rate
(mm/yr)
0 28.29 0 0 33.155 0 0
3 28.153 0.137 1.06 32.978 0.177 1.37
6 28.063 0.09 0.35 32.751 0.227 0.88
9 28.053 0.01 0.03 32.658 0.093 0.24
12 28.018 0.035 0.07 32.649 0.009 0.02
15 28.013 0.005 0.01 32.523 0.126 0.2
18 27.984 0.029 0.04 32.414 0.109 0.14
21 27.966 0.018 0.02 32.305 0.109 0.12
24 27.954 0.012 0.01 32.297 0.008 0.01
27 27.941 0.013 0.01 32.254 0.043 0.04
30 27.91 0.031 0.02 32.244 0.01 0.01
33 27.861 0.049 0.04 32.195 0.049 0.04
36 27.856 0.005 0.003 32.125 0.07 0.05
39 27.551 0.305 0.18 31.997 0.128 0.08
42 27.449 0.102 0.06 31.885 0.112 0.06
45 27.419 0.03 0.02 31.875 0.01 0.01
48 27.369 0.05 0.02 31.846 0.029 0.01
54
Figure 4.2: Graph of Corrosion rate against Immersion time (With inhibitor)
Figure 4.3: Graph of Corrosion rate against Immersion time (With and without
inhibitor)
From Figure 4.3 it was observed that the samples in the extract with inhibitor had higher
corrosion rates in the initial stages of the experiment than those in the extracts without
inhibitors. This is an unexpected result and may be due to usage of oranges (processed
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48
Co
rro
sio
n r
ate
(mm
/yr)
Immersion time (days)
Corrosion rate in sweetcassava juice (mm/yr)
Corrosion rate in bittercassava juice (mm/yr)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 10 20 30 40 50 60
Co
rro
sio
n r
ate
(mm
/yr)
Immersion time (days)
Corrosion rate in sweetcassava juice withoutinhibitor (mm/yr)Corrosion rate in bittercassava juice withoutinhibitor (mm/yr)Corrosion rate in sweetcassava juice with inhibitor(mm/yr)Corrosion rate in bittercassava juice with inhibitor(mm/yr)
55
for inhibitors) that were not cut directly from the plant. Hence they might have been more
acidic and contained compounds that might have caused undesirable reactions with the
extract.
For the mild steel samples immersed in the extracts continuously for 8 weeks, the sample
in the sweet cassava extract without inhibitor recorded a corrosion rate of recorded
0.051mm/yr. The sample in the sweet cassava extract with inhibitor recorded 0.016
mm/yr.
Table 4.5: Weight loss determination of mild steel in cassava dough
Sweet type Bitter type
Immersion
time (days)
Weight of
sample (g)
(Initial)
Weight
loss (g)
Corrosion
rate
(mm/yr)
Weight of
sample (g)
(Initial)
Weight
loss (g)
Corrosion
rate
(mm/yr)
0 30.816 0 0 29.964 0 0
3 30.808 0.008 0.616 29.913 0.051 0.3925
6 30.701 0.107 0.4118 29.723 0.190 0.7312
9 30.954 0.107 0.2745 29.401 0.322 0.8261
12 30.954 0 0 29.346 0.055 0.1058
15 30.586 0.008 0.0123 29.343 0.003 0.0046
18 30.583 0.003 0.0038 29.324 0.019 0.0244
21 30.578 0.005 0.0055 29.293 0.031 0.0341
24 30.571 0.007 0.0067 29.293 0 0
27 30.562 0.009 0.0077 29.292 0.001 0.0009
30 30.555 0.007 0.0054 29.289 0.003 0.0023
33 30.531 0.024 0.0168 29.280 0.009 0.0063
36 30.512 0.019 0.0122 29.275 0.005 0.0032
39 30.502 0.010 0.0059 29.271 0.004 0.0024
42 30.499 0.003 0.0016 29.269 0.003 0.0016
45 30.491 0.021 0.0108 29.264 0.004 0.0021
48 30.478 0.013 0.0063 29.253 0.011 0.0053
56
Table 4.5 shows the behaviour of mild steel samples in both the sweet and bitter cassava
dough.
Figure 4.4: A graph for weight loss in sweet and bitter cassava dough
Figure 4.4 also shows a higher corrosion rate of mild steel in the bitter cassava dough
further buttressing the point that the higher cyanide content of the bitter cassava results
in the accelerated corrosion of mild steel.
The graphs describe the similarities in the trends of corrosion behaviour observed in the
various media under study. It is observed that there was a sharp increase in the corrosion
rate of the samples in the initial stages of the experiment. This is because the samples
were immersed in the extracts immediately after their surfaces had been cleaned using
emery papers. This exposed an almost bare fresh surface for corrosion. This phenomenon
is known as active corrosion where there were immediate anodic dissolution reactions at
the specimen/ solution interface.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48
Co
rro
sio
n r
ate
(mm
/yr)
Immersion time (days)
Corrosion rate in sweetdough(mm/yr)
Corrosion rate in bitterdough (mm/yr)
57
The initial rise in corrosion rates can be attributed to the reduction in pH of the extracts
due to the increasing acidity of the fermenting solution. As the experiment progressed
there was a continuous increase in the pH of the extracts and hence a decrease in the
corrosion rates of the mild steel samples. This could be attributed to the production of
some acetic and lactic acids which are weaker acids compared to the HCN in the fresh
extract.
4.4 Electrochemical Analysis
Figure 4.5 and Figure 4.6 show a straight line for the electrochemical measurements
which is attributed to the lack of sensitivity of the potentiostat used for the experiment.
Figure 4.5: A graph of current vs potential for the bitter cassava extract
Figure 4.6: A graph of current vs potential for the sweet cassava extract
1
10
100
1000
10000
-600 -400 -200 0 200 400 600
Cu
rren
t (u
A)
mV
current(BT1)
1
10
100
1000
10000
-600 -400 -200 0 200 400 600
Cu
rren
t (u
A)
mV
current(ST1)
58
4.5 Chemical Composition
The elemental composition of the mild steel samples is detailed in Table 4.7
Table 4.7: Elemental composition of mild steel plate
Run Fe C Mn Cu Ni Cr Mo Co
1 99.476 0.054 0.286 0.013 0.047 0.032 0.068 0.016
2 99.479 0.061 0.28 0.013 0.052 0.033 0.063 0.015
Average 99.373 0.058 0.28 0.013 0.049 0.032 0.065 0.016
Mild steels are characterized by their carbon content which ranges from 0.05 to 0.25% by
mass. Table 4.7 shows the mild steel samples used for this experiment as falling in this
range and hence suitable for the tests. Carbon is an important element in steel, since it
hardens and strengthens it.
Mn helps in grain structure, wear resistance and hardening, Mo prevents brittleness and
maintains steel strength at high temperatures while Cr offers corrosion resistance.
4.6 FTIR
Figure 4.7 and 4.8 show the FTIR spectra for the sweet and bitter cassava extracts
respectively. The spectra revealed a number of peaks, indicating the various functional
groups present in the extracts and their complex nature.
Figure 4.7: A graph of Intensity against Wave length for Sweet species
0
20
40
60
80
100
120
0 1000 2000 3000 4000 5000
Inte
nsi
ty (
% T
)
Wavelength (cm - 1)
%T
N-H
-C≡C-
=C-HC-N=C-H
C-N N-H
59
Figure 4.8: A graph of Intensity against Wave length for Bitter species
Table 4.8: Functional groups present in the sweet extract
Sweet cassava
Peaks Wave length,
X (cm – 1)
Intensity, Y (%
T)
Bonds Functional Groups
1 1000 71.63 =C-H bend Alkenes
2 976 80.76 -C-H bend Alkenes
3 1156 85.29 C-N stretch Aliphatic amines
4 1344 88.43 C-N stretch Aromatic, Amines
5 1644 70.74 N-H bend 1 Amines
6 2139 95.68 -C≡C- stretch Alkynes
7 3353 50.58 N-H stretch 1, 2 Amines, Amides
Table 4.9: Functional groups present in the bitter extract
Bitter cassava
Peaks Wave length,
X (cm – 1)
Intensity, Y (%
T)
Bonds Functional Groups
1 1000 76.70 =C-H bend Alkenes
2 967 84.21 =C-H bend Alkenes
3 1160 87.97 C-N stretch Aliphatic amines
4 1349 89.72 C-N stretch Aromatic, Amines
5 1644 71.29 N-H bend 1 Amines
6 2172 96.81 -C≡C- stretch Alkynes
7 3332 51.73 N-H stretch 1, 2 Amines, Amides
The FTIR spectra confirmed the existence of C-N, amines, amide and aliphatic amines in
the extract. It was observed that both the sweet and bitter types had the same functional
groups.
0
20
40
60
80
100
120
0 1000 2000 3000 4000 5000
Inte
nsi
ty (
% T
)
Wavelength (cm - 1)
%T
N-H
-C≡C-
=C-HC-N=C-H
C-N N-H
60
4.7 Microstructure Analysis and surface morphology of corroded samples
The microstructural analysis showed two phases present (ferrite and pearlite). Images are
shown below:
(a)
(b) (c)
Figure 4.9 Microstructure of mild steel sample
61
Metallographic Preparation: Figure 4.91 shows the micrographs of the corroded parts
obtained using optical microscope AmScope MT130 model at the Materials Engineering
lab of KNUST.
(a) (b)
(c) (d)
Figure 4.10: Surface morphology of corroded mild steel
62
CHAPTER FIVE
5.0 CONCLUSIONS AND RECOMMENDATIONS
5.1 CONCLUSIONS
From this study, the mild steel samples showed a destructive rate of corrosion in the
cassava extracts in both the continuous (permanent immersion) and intermittent
conditions. However, the rate of corrosion in the bitter extracts were the highest.
The general increase in the corrosion rates of the samples in the initial stages of the
experiment, indicate the susceptibility of mild steel to active corrosion.
The orange peel powder used as inhibitors were unable to reduce the corrosion rate of the
mild steels in the cassava extract.
The electrochemical measurements were unsuccessful due to lack of sensitivity of the
potentiostat used.
Furthermore, the cassava extract does not only corrode the mild steel but the corrosion
products formed have detrimental effect on the final food product.
5.2 RECOMMENDATIONS
Results from this study will enable further works to be done on this subject. Works such
as the determination of the factors that result in the reduction in corrosion rate of the mild
steel samples after 30 days of being in the extract. For the works to be done after this, we
recommend the use of cassava tubers of varying ages and species. This would help
determine the effect of the ages of the cassava have on the corrosion behaviour of mild
steel. It would also help determine the aggressive/ corrosive nature of the different
cassava species.
We also recommend the use of different inhibitors in order to determine the most effective
one for the protection of the surface mild steel samples from corrosion. Also if orange
peels are to be used as corrosion inhibitor, then fresh oranges (of the right species) cut
directly from the plant should be used.
For the electrochemical measurement, potentiostat of higher sensitivity and range should
be used.
63
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LIST OF EQUATIONS
1 mL of 0.02 M Silver Nitrate = 1.08 mg HCN (AOAC (1990))
Weight loss W = Wi – Wf
Moisture content M = Mi – Mf.
% moisture content = 𝑀𝑖−𝑀𝑓
𝑀𝑖 × 100
Corrosion rate CR = kW
A×D×T
Inhibition weight loss, Wih = Wi - Wh
Inhibition efficiency, η (%) = 𝐶𝑅1−𝐶𝑅2
𝐶𝑅1 × 100
Surface coverage θ = W−Wih
W
68
APPENDICES
APPENDIX A
A1: Surface coverage and Inhibition efficiency for sweet type
Weight of
sample (g)
Weight loss
(g)
Surface Coverage Inhibition efficiency (%)
28.29 0 28.29 28.29
28.153 0.137 28.14813373 27.66637335
28.063 0.09 28.05979293 27.74229302
28.053 0.01 28.05264353 28.01735319
28.018 0.035 28.0167508 27.89308031
28.013 0.005 28.01282151 27.99515114
27.984 0.029 27.98296369 27.88036935
27.966 0.018 27.96535636 27.90163613
27.954 0.012 27.95357072 27.91107233
27.941 0.013 27.94053473 27.89447339
27.91 0.031 27.90888929 27.7989287
27.861 0.049 27.85924127 27.68512692
27.856 0.005 27.85582051 27.83805055
27.551 0.305 27.53992962 26.44396214
27.449 0.102 27.44528402 27.07740176
27.419 0.03 27.41790587 27.30958682
27.369 0.05 27.36717312 27.18631156
69
A2: Surface coverage and Inhibition efficiency for bitter type
Weight of
sample (g)
Weight loss
(g)
Surface coverage Inhibition efficiency (%)
33.155 0 33.155 33.155
32.978 0.177 32.97263279 32.44127855
32.751 0.227 32.74406891 32.05789139
32.658 0.093 32.65515231 32.37323057
32.649 0.009 32.64872434 32.62143407
32.523 0.126 32.51912582 32.13558187
32.414 0.109 32.41063726 32.07772555
32.305 0.109 32.30162591 31.96759093
32.297 0.008 32.2967523 32.2722299
32.254 0.043 32.25266683 32.1206832
32.244 0.01 32.24368986 32.21298648
32.195 0.049 32.19347802 32.04280245
32.125 0.07 32.12282101 31.90710117
31.997 0.128 31.99299962 31.5969625
31.885 0.112 31.88148738 31.53373765
31.875 0.01 31.87468627 31.84362745
31.846 0.029 31.84508937 31.75493676