corrosion behaviour of ductile cast iron mohamed
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CORROSION BEHAVIOUR OF DUCTILE CAST IRON
MOHAMED ASSNOUSI ALI
A Project report submited in partial fulfilment of the requirements for the award of the
degree of Master of Engineering ( Mechanical Material )
Faculty of Mechanical EngineeringUniversiti Teknologi Malaysia
MAY 2009
iii
To my late mother, my late father, my brothers and sisters
for their support and care
iv
ACKNOWLEDGEMENT
First of all, Praise to Allah, the Most Gracious and Most Merciful, Who has
created the mankind with knowledge, wisdom and power.
I would like to express my utmost gratiude to my supervisor, Dr Astuty Amrine
and AssociateProfessor Dr Ali Ourdjini for benig a dedixated mentor as well as for his
valuable and valuable and constructive suggestions that enabled this project to run
smoothly.
Also,not forgetting my friends and classmates, I convey my full appreciation for
his valuable and contributions toward this project , whether directly or indirectly.
Last but not least, I am forever indebted to all my family member for their
constant support throughout the entire duration of this project . their words of
encouragement never failed to keep me going even through the hardest of times and it is
here that I express my sincerest gratitude to them.
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ABSTRACT
In this investigation the corrosion behavior of ductile cast iron as function of the
microstructure and electrolyte solution has been conducted. The change in
microstructure of the ductile cast iron is obtained by austenetising at different
temperatures of 850°C, 900°C,950°C and 1000°C for 90 minutes followed by water
quench. Corrosion tests included both immersion tests and electrochemical test.
Corrosion rates measured from the immersion test using the weight loss method
revealed that the cast iron investigated suffer less corrosion when exposed to sodium
hydroxide compared to sodium chloride and that the corrosion rates are not significantly
affected by the microstructure of the material. Observation of the corrosion attack also
showed that the type of corrosion is that of uniform instead of localized. The low
corrosion rates of the ductile iron are probably the results of the high Si content in the
ductile iron, which provide a thin and protective hydrate layer. This observation is
reconciled with previous research which investigated high Si containing ductile cast
irons.
vi
ABSTRAK
Dalam kajian ini, ciri- ciri kakisan besi tuang mudah tempa sebagai fungsi
terhadap mikrostruktur dan larutan elektrolit telah dijalankan. Perubahan mikrostruktur
besi tuang mudah tempa didapati dengan proses austenising pada suhu yang berbeza
iaitu 850°C, 950°C dan 1000°C untuk 90 minit, diikuti dengan lindap kejut di dalam air.
Ujian kakisan termasuklah ujian rendaman dan elektrokimia. Kadar kakisan diukur
melalui ujian rendaman menggunakan teknik kehilangan jisim. Ini telah menunjukkan,
besi tuang mudah tempa mengalami kakisan yang sedikit apabila didedahkan kepada
Sodium Hidrokside berbanding Sodium Kloride dan kadar kakisan tidak dipengaruhi
secara jelas oleh mikrostruktur bahan. Pemerhatian terhadap serangan kakisan juga
telah menunjukkan bahawa jenis kakisan adalah secara menyeluruh dan bukan secara
tertumpu. Kadar kakisan besi tuang mudah tempa yang rendah, mungkin disebabkan
oleh kandungan Silikon yang tinggi di dalam bahan, yang mana ia menghasilkan
lapisan pelindung hydrate yang nipis. Secara keseluruhannya, kajian ini disokong oleh
kajian sebelum ini berkenaan kandungan Silikon yang tinggi dalam besi tuang mudah
tempa.
vii
TABLE OF CONTENTS
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOWLEDGEMENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLE x
LIST OF FIGURES xii
1 INTRODUCTION 1
1.1 Background of the Research 1
1.2 Problem Satement of Research 2
1.3 Objectives of the Research 3
1.4 Scopes of the Research 3
viii
2 LITERATURE REVIEW 1
2.1 General Review of Cast Iron 4
2.2 Classification of Cats Iron 5
2.2.1 White Cast Irons 6
2.2.2 Gray Cast Irons 8
2.2.3 Malleable Cast Irons 9
2.2.4 Nodular Cast Irons 10
2.2.5 Compacted Graphite Cast Irons 11
2.3 Typically Microstructure of Cats Iron 12
2.3.1 Ferrite (α-Fe) 13
2.3.2 Pearlite 13
2.3.3 Cementite (Fe3C) 14
2.3.4 Phosphide eutectic ( melting point about 930°C ) 15
2.3.5 Martensite 15
2.3.6 Acicular or bainitic 16
2.3.7 Austenite 17
2.3.8 Graphite 17
2.4 Properties of Cast Iron 18
2.5 Ductile Cast iron 19
2.5.1 Mechanical Properties 20
2.5.2 Chemical Composition 21
2.5.3 Grade of Ductile Cast Iron 21
2.5.4 Hardness 23
2.5.5 Tensile Properties 25
2.6 Heat Treatment 25
2.6.1 Austenitisation 26
2.6.2 Cooling rate During Quenching 26
2.7 Corrosion Of Metals 26
2.8 Electrochemical Reactions 27
2.9 Corrosion of Cast Iron 29
ix
2.9.1.1 Effect Structure on Corrosion Resistance 30
2.9.1.2 Effect Composition on Corrosion Resistance 30
2.10 Corrosion of Cast Iron In Nature Environment 31
2.10.1 Atmospheric Corrosion 31
2.10.2 Corrosion by Waters of Cast Iron 31
2.11 Soil Corrosion of Cast Iron 32
2.12 Corrosion in industrial Environment of Aast Iron 32
2.12.1 Corrosion by Acids 32
2.12.2 Mineral Acids 33
2.12.3 Organic Acids 33
2.13 Corrosion by Alkalis
2.13.1 Corrosion by Salt Solution Sf Cast Iron
33
2.14 Corrosion Under Stress 34
2.15 Corrosion f Ttwo Types of Cast Iron 34
2.15.1 high nickel Cast Iron 34
2.15.1.1 Composition and Properties 35
2.15.1.2 Aqueous Corrosion Behaviour 35
2.15.1.3 Nature waters 35
2.15.2 High Chromium Cast Iron 36
2.15.2.1 Corrosion Resistance 36
2.15.2.2 Atmopheric Corrosion 36
2.15.2.3 Nature and Industrial Waters 37
2.16 Corrosion of Ductile Cast Iron 37
2.16.1 Cavitation Erosion of Ductile iron 38
2.16.2 Erosion–Corrosion of Ductile Cast Iron 39
2.16.3 High Temperature Corrosion of Ductile Cast
Irons40
2.16.4 Corrosion fatigue of ductile iron 41
2.16.4.1 Fatigue Behaviour In Various Environment 42
x
3 METHODOLOGY 43
3.1 Introduction 43
3.2 Materials 44
3.3 Samples Preparation 44
3.4 Compositional Analysis 48
3.5 Metallography Analysis 50
3.6 Heat Treatment 51
3.7 Hardness Measurement 52
3.7 Microstructure Analysis 53
3.8 Electrochemical Testing 53
3.8.1 Principle of Measurement 53
3.8.2 Preparation of Working Electrode 55
3.9 Immersion Test 57
4 RESULTS AND DISCUSSION 62
4.1 Compositional Analysis 62
4.2 Microstructural Examination of As-Received Sample 63
4.3 Hardness Test 65
4.4 Immersion Test 66
4.5 Elechtrochemical ( Polraisation Results) 72
4.6 Microstructure Analysis of Samples after Immersion Corrosion Test 79
xi
5 CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions 92
5.2 Recommendations for Future Work 93
REFERENCES 94
xii
LIST OF TABLES
TABLE NO TITLE PAGE
2.1 Grade of Ductile Cast Iron in ASTM A- 536-77 22
2.2 Grade of Ductile Cast Iron in SAE specification No. J434c for
Automotive Castings 23
2.3 Typical Hardness Brinell for Ductile cast Iron 24
3.1 Potentiodynamic Polarization Test Parameters 58
3.2 Parameters for immersion test 61
4.1 Chemical composition of as-received Ductile Cast Iron 63
4.2 Analysis Hardness Rate for ductile Cast Iron 66
4.3 Corrosion rate of specimens expressed in mm/yr after 1 day in
( NaCl ) 67
4.4 Corrosion rate of specimens expressed in mm/yr after 7days in
( NaCl ) 68
4.5 Corrosion rate of specimens expressed in mm/yr after 14 days
in ( NaCl ) 68
4.6 Corrosion rate of specimens expressed in mm/yr after 28days
in ( NaCl) 69
4.7 Corrosion rate of specimens expressed in mm/yr after 1
day in ( NaOH ) 70
4.8 Corrosion rate of specimens expressed in mm/yr after 7 days in
( NaOH ) 70
xiii
4.9 Corrosion rate of specimens expressed in mm/yr after14 days
in ( NaOH ) 71
4.10 Corrosion rate of specimens expressed in mm/yr after 28 days
in ( NaOH ) 73
xiv
LIST OF FIGURES
FIGURE NO TITLE PAGE
2.1 Schematic of iron- iron carbide systems 6
2.2 Microstructure of white cast iron Fe3.6C0.1Si, dentrites of
pearlite 7
2.3 Microstructure of Gray cast iron ( graphite flakes ) 8
2.4 Microstructure of Malleable cast iron 9
2.5 Microstructure of spheroidal graphite cast iron as cast
Fe3.5C-2.5Si-0.5Mn-0.15Mo-0.31Cu-0.042Mg wt% 10
2.6 Microstructure of spheroidal graphite cast iron as cast
Fe3.2C-2.5Si-0.05Mg wt% 11
2.7 Flowchart for Classification of Cast Iron 12
2.8 Microstructure of cast iron under cooled graphite 13
2.9 Microstructure of cast iron consist of alternate lamellae
of ferrite and cimentite 14
2.10 Microstructure of cast iron consist cementite 14
2.11 Grey cast iron with a high phosphorus content 15
2.12 Microstructure of cast iron with some retained austenite 16
2.13 Acicular structure of iron of composition total carbon 2.9%
silicon 1.67%. magnesium 1.6 16
2.14 Microstructure of white iron matensitic 17
xv
2.15 Microstructure consists Nodular graphite is produced in the as
– cast state by the joint addition of magnesium &ceramic 18
2.16 The basic corrosion cell consists of an anode, a cathode , a
electrolyte , and a metallic path for electron flow. 28
3.1 A flow chart showing a summary of research methodology 45
3.2 Cutting Machine (Mecotome T255/300) 46
3.3 Sample Preparation for Heat Treatment Process 47
3.4 Sample Preparation for immerssion Test 47
3.5 Grinding machine 48
3.6 Nikon optical microscope (U-LBD-2 OLYMPUS) 48
3.7 Micro balance (METTER AT 400) 49
3.9 EDX-FESEM (SUPRA 35VP) 50
3.10 Polishing machine 51
3.11 Flow chart illustrating the heat treatment processes for
austempered ductile iron samples 52
3.12 Furnaces for heat treatment process 53
3.13 Vickers Hardness 53
3.15 Cell kit set-up 55
3.16 Photographs of (a) Connection of specimen to copper wire by
brazing technique; (b) Mounting of samples 56
3.17 Photographs of (a) Working Electrode (WE); (b) Showing
typical surface area of sample 57
3.18 Photographs showing (a) Immersion test at room temperature;
(b) In oven at 25C 60
3.19 Photographs showing (a) Ultrasonic cleaning; (b) Drying 61
4.1 Microstructure of ductile cast iron consiste martensite (850C) 64
4.2 Microstructure of ductile cast iron consiste of martensite(900) 65
4.3 Microstructure of ductile cast iron consiste of martensite ( not
fully resolved )(950) 65
4.4 Microstructure of ductile cast iron consiste of plate
martensite(1000) 65
xvi
4.5 Hardness Rate for ductile Cast Iron specimens 66
4.6 Chart showing corrosion rate in NaCl solution 69
4.7 Chart showing corrosion rate in NaOH solution 72
4.8 Bar chart of icorr in 3.5% NaCl at 24±2°C 74
4.9 chart of icorr in 3.5% NaCl at 850°C 74
4.10 chart of icorr in 3.5% NaCl at 900°C 75
4.11 chart of icorr in 3.5% NaCl at 950°C 75
4.12 chart of icorr in 3.5% NaCl at 1000°C 76
4.13 Bar chart of icorr in 10% NaOH at 24±2°C 78
4.14 chart of icorr in 10% NaOH at 850°C 78
4.15 chart of icorr in 10% NaOH at 900°C 79
4.16 chart of icorr in 10% NaOH at 950°C 79
4.17 chart of icorr in 10% NaOH at 1000°C 80
4.18 Optical micrographs of specimens at 850 in 3.5% NaCl +
10% NaOH 850°C 81
4.19 Optical micrographs of specimens at 900°C in3.5% NaCl +
10% NaOH at 24°C 82
4.20 Optical micrographs of specimens at 950 °C in3.5% NaCl +
10% NaOH at 24°C 83
4.21 Optical micrographs of specimens at 1000°C in3.5% NaCl +
10% NaOH at 24°C 84
4.22 Optical micrographs of specimens at 850 °C in3.5% NaCl +
10% NaOH at 24°C 86
4.23 Optical micrographs of specimens at 900°C in3.5% NaCl +
10% NaOH at 24°C 87
4.24 Optical micrographs of specimens at 950°C in3.5% NaCl +
10% NaOH at 24°C 88
4.25 Optical micrographs of specimens at 1000°C in3.5% NaCl +
10% NaOH at 24°C 89
4.26 Optical micrographs of specimens at 850 °C in3.5% NaCl +
10% NaOH at 24°C 90
xvii
4.27 Optical micrographs of specimens at 900°C in3.5% NaCl +
10% NaOH at 24°C 91
4.28 Optical micrographs of specimens at 950 °C in 3.5% NaCl +
10% NaOH at 24°C 92
4.29 Optical micrographs of specimens at 950°C in 3.5% NaCl +
10% NaOH at 24°C 93
CHAPTER 1
Introduction
1.1 General Review of the Research
Ductile iron also known as nodular cast iron or spheroid-graphite (SG) cast
iron contains nodules of graphite, embedded in a matrix of ferrite or pearlite or both,
the graphite separates out as nodules from iron `during solidification because of the
additives like `cerium (Ce) and magnesium (Mg) introduced into the molten iron
before casting. These nodules act as crack arresters, thereby improving the
mechanical properties of ductile iron.
The formation of graphite nodules during solidification causes an internal
expansion of ductile iron as it solidifies, and is responsible for the absence of
shrinkage defects in most ductile iron castings. The major difference in the structure
of ductile and grey iron is the flaky and spheroid graphite in the grey and ductile iron
respectively. However, the spheroid graphite in ductile iron does not weaken the
matrix and hence its mechanical properties are superior to those of grey iron and
comparable to that of steel [1].
2
The corrosion resistance of ductile cast iron is attributed to the formation of a
thin passive barrier film of hydrated oxides of silicon on the metal surface. The film
develops with time due to the dissolution of iron from the metal matrix leaving
behind silicon which hydrates due to the presence of moisture. The passive hydrated
silicon film is thought to bridge over and form an impervious barrier layer on a fine
grained high silicon cast iron with spheroidal graphite areas much more readily than
on a high silicon cast iron with coarse graphite flakes.
While a lot is known on the effect of alloyed elements on the mechanical
properties of ductile cast iron, not much is known of the effect of microstructure, and
the corrosion behavior of these materials, in natural and acidic environments. Hence
the need to investigate the effect of heat treatment on the microstructure and
corrosion resistance of as-cast ductile iron, in Sodium Chloride and Sodium
Hydroxide solutions.[1,2]
1.2 Problem Statement of the Research
While much is known about the effect of alloying elements on the
mechanical properties of cast irons, little is probably known about their corrosion
resistance. The corrosion resistance of (DCI) is related to its microstructure
which is determined by heat treatment parameters (austenitising temperature and
austenitising time)
Thus, the aim of this research is to assess the relationship between the heat
treatment, corrosion behavior and microstructure of ductile cast iron. .
3
1.3 Objective of the Research
To investigate the influence of heat treatment process on the microstructure and
corrosion behavior of Ductile Cast Iron in neutral and acidic environments.
1.4 Scope of project of the Research
The scope of this project is as follows:
(a) Heat treatment of ductile cast iron which includes:
(i) Austenitization
(ii) Quenching
(b) Corrosion test measurement by:
(i) Immersion test (ASTM G67)
(ii) Electrochemical test (ASTM G5)
(c) Corrosion performance and analysis of samples
CHAPTER 2
LITERATURE REVIEW
2.1 General Review
The term cast iron, like the term steel, identifies a large family of ferrous
alloys. [1]. Cast irons are multi-component ferrous alloys. They contain major (iron,
carbon, silicon), minor (<0.01%), and often alloying (>0.01%) elements [1]. Cast
iron has higher carbon and silicon contents than steel. Because of the higher carbon
content, the structure of cast iron, as opposed to that of steel, exhibits a rich carbon
phase. Depending primarily on composition, cooling rate and melt treatment, cast
iron can solidify according to the thermodynamically metastable Fe-Fe3C system or
the stable Fe-Gr system Figure 2.1.
Cast iron is an alloy of Fe, Si and C. Iron (Fe) accounts for more than 95%
of the alloy material, while the main alloying elements are carbon (C) and silicon
(Si).[1]. Cast irons contain appreciable amounts of silicon, normally 1-3%, and
consequently these alloys should be considered as Fe-C-Si alloys. The carbon
concentration is between 1.7 and 4.5 %, most of which is present in insoluble form
(e.g. graphite flakes or nodules). Such material is, however, normally called
UNALLOYED CAST IRON.
5
During solidification, the major proportion of the carbon precipitates in
the form of graphite or cementite. When solidification is just complete the
precipitated phase is embedded in a matrix of austenite which has an equilibrium
carbon concentration of about 2 wt%. [1.2].on further cooling, the carbon
concentration of the austenite decreases as more cementite or graphite precipitates
from solid solution. For conventional cast irons, the austenite then decomposes into
pearlite at the eutectoid temperature. However, in grey cast irons, if the cooling rate
through the eutectoid temperature is sufficiently slow, then a completely ferritic
matrix is obtained with the excess carbon being deposited on the already existing
graphite. White cast irons are hard and brittle; they cannot easily be machined.
2.2 Classification of Ductile Cast iron
Classifications are determined by the eutectic graphite/carbide forms
present in the iron microstructure. Classifications are controlled by alloying,
solidification rates and heat treatment [2].
A) White Irons
B) Malleable Irons
C) Gray Irons
D) Ductile Irons
6
Figure 2.1 Schematic of iron- iron carbide systems
2.2.1 White Cast Irons
Exhibits a white, crystalline fracture surface because fracture occurs
along the iron carbide plates; it is the result of metastable solidification (Fe3C
eutectic) as show in figure (2.2). White cast irons form eutectic cementite during
solidification. The white iron microstructure is due to fast solidification rates and
alloying that promotes eutectic carbide formation. [2]
7
White irons typically have low ductility, high hardness and great wear
resistance. White irons get their name from the shininess of their crystalline
fractures in comparison to the dull gray fractures of graphite irons.[2] The white
cast iron has properties including: Very hard but brittle, High wear & abrasion
resistance, extremely difficult to machine, Is used to produce malleable cast iron.
heat treatment to 800 °C – 900 °C causes decomposition of ( F C ).[2].
Typical application of white cast iron:
- Rollers in rolling mills.
- Brake shoes
- Extrusion nozzles.
Figure 2.2 Microstructure of white cast iron Fe3.6C0.1Si, dentrites of
pearlite
2.2.2 Gray Cast Irons
Exhibits a gray fracture surface because fracture occurs along the
graphite plates (flakes); it is the result of stable solidification (Gr eutectic) as shown
in Figure (2.3). [3] Gray cast irons form graphite flakes during solidification. The
gray iron microstructure is due to slow solidification rates and silicon alloying that
8
promotes graphite formation. [4].Gray irons typically have low ductility and
moderate strength, but they have high thermal conductivity and excellent vibration
damping properties. Gray irons get their name from their dull gray fracture features
[3]. The grey cast iron has properties depends on the shape of graphite ( flakes ,
spheroids or nodules ) & including: least expensive of metals, high fluidity,
complex shapes can be cast, graphite flakes , high damping capacity, and good
machine ability.[2].
Typical application of grey cast iron: Cylinder blokes, Base structure for
machines and heavy equipment.
Figure 2.3 Microstructure of Gray cast iron ( graphite flakes )
2.2.3 Malleable Cast Irons
Malleable cast irons are formed by annealing white irons to transform the
eutectic cementite to graphite. [2] Malleable irons have good ductility and good
strength. Matrix microstructure is dependent upon the cooling rate from the
graphitization annealing. Before the discovery of nodular irons, malleable irons
were the only ductile class of cast irons.[4] The malleable cast iron produce by
9
annealing white cast iron at 900 °C – 1600°C for 50 – 80 hrs ( slow cooling to room
temperature ).
Typical application:
Casting mould.
Railroad
Pipe fittings & bridges.
Connecting rods.
Figure 2.4 Microstructure of Malleable cast iron
2.2.4 Nodular Cast Irons
Nodular cast irons form graphite spheres during solidification. The nodular
iron microstructure is due to slow solidification rates and magnesium or cerium
alloying that promotes spherical graphite formation [3.5]. Removing the graphite
flakes improves the tensile strength, toughness & ductility. Nodular irons typically
have high ductility and strength. Nodular irons were first discovered in the
10
1940’s.Nodular irons are also called “ductile irons” or“spheroidal graphite irons.
[5].
Typical application:
Camshafts.
Gears.
Valves.
Figure 2.5 Microstructure of spheroidal graphite cast iron as cast Fe3.5C-
2.5Si-0.5Mn-0.15Mo-0.31Cu-0.042Mg wt%
2.2.5 Compacted Graphite Cast Irons
Compacted graphite cast irons form graphite particles with a shape between
graphite flakes of gray cast iron and graphite nodules of nodular cast iron.[3]
Compacted graphite cast irons have properties between those of gray cast iron and
nodular cast iron.[5.6.] Compacted graphite irons require very tight control of the
modularizing alloying (magnesium or cerium). [5]
11
Figure 2.6 Microstructure of spheroidal graphite cast iron as
cast Fe3.2C-2.5Si-0.05Mg wt%
Figure 2.7 Flowchart for Classification of Cast Iron
12
2.3 Typically Microstructure of Cats Iron
The matrix structure of cats iron usually contains one or more of the
following constituents.[1]
2.3.1 Ferrite (α-Fe)
In cats iron is essentially a single phase solid solution of silicon in amounts.
[4] Varying with graphite structure, cooling rate and silicon content, tending to
increase in amount as the cooling rate decrease the silicon content increase. [6] .
And the graphite approaches the under- cooling form fully ferritic structure are
normally only obtained by annealing. [7]
Figure 2.8 Microstructure of cast iron under cooled graphite
2.3.2 Pearlite
In cast iron is consisting of alternate lamellae of ferrite and cementite which
can be distinguished in figure (2.8a) and (2.8b). At low magnification it appears as
13
half- tone colour. This structure is formed by the transformation of austenite during
normal cooling in the mould or in air through the critical rang ( 720- 900 )
Figure ( 1.1.7 ) Random graphite matrix and low phosphorus
Figure 2.9 Microstructure of cast iron consist of alternate lamellae
of ferrite and cimentite
2.3.3 Cementite (Fe3C )
In cast iron , in the massive eutectic form is a hard white constituent formed
during solidification as in mottled or white irons . and in the lamellar in pearlite it is
formed by the transformation of austenite through the critical temperature .
Figure 2.10 Microstructure of cast iron consist cementite
14
2.3.4 Phosphide eutectic ( melting point about 930 °C )
Occurs in two distinct forms in cast iron with more than 0.60 per cent
phosphorus. The pseudo – binary form is the normal , consisting of ferrite and iron
phosphide (Fe3P).[6] The true eutectic forms from the liquidus as austenite plus iron
phosphide and on cooling the austenite transforms to ferrite and pearlite and with
iron phosphide gives a bulk hardness of 420- 600 H. [8].
Figure 2.11 Grey cast iron with a high phosphorus content
2.3.5 Martensite
Is fine acicular , slow etching structure , normally produced by very rapid
cooling ( quenching ) of austenite through the critical temperature range , or by
alloying , the structures are martensite .
15
Figure 2.12 Microstructure of cast iron with some retained austenite
2.3.6 Acicular or bainitic
The transformation structures (rapid etching) which can be produced by
isothermal quenching or alloying.[4].These structures are often referred to as
acicular ferrite and are softer and tougher than martensite , but harder and stronger
than pearlite .[6] A range of these acicular structures exists from "the upper
bainites" or acicular ferrites, to martensite , depending upon the transformation time
, composition [7].
Figure 2.13 Acicular structure of iron of composition total carbon
2.9% silicon 1.67%. magnesium 1.6
16
2.3.7 Austenite
Can be made stable at room temperature by the addition of alloys as ( nickel
, manganese ) which depress the critical temperature at the which change
occur.[5] Although, however, the transformation may have been suppressed at room
temperature, it may still take place at a low temperature, depending upon the
amount of alloy present.[8] Figure 2.12 shows the structure of an iron which was
austenite at room temperature 60c.
Figure 2.14 Microstructure of white iron matensitic
2.3.8 Graphite
Can beproduced in forms other than flake and the nodular from is produced
from the melt by the joint addition of magnesium and cerium. The nodular and
aggregate forms shown in Figure 2.13 have been produced by annealing a white
iron.
17
Figure 2.15 Microstructure consists Nodular graphite is produced in the as
– cast state by the joint addition of magnesium &ceramic
2.4 Properties of cast iron
1) High hardness & brittle.
2) Low ductility.
3) Can not be cold worked / deformation at room temperature.
4) Easily melt & can be cast to the desired shapes (can be sand cast to
intricate shapes ).
5) Cheapest alloy.
18
2.5 Ductile Cast Iron
Ductile iron has several engineering and manufacturing advantages when
compared with cast steels.[9] These include an excellent damping capacity, better
wear resistance, 20– 40% lower manufacturing cost and lower volume shrinkage
during solidification.[10].The combination of good mechanical properties and
casting abilities of ductile cast iron makes its usage successful in structural
applications especially in the automotive industry. Gears, camshafts, connecting
roads, crankshafts, front wheel spindle supports and truck axles are some of the
application areas of ductile iron in the automotive industry.[9].
As known these machine parts and many of others are often subjected to
fluctuating loads in service.[9] For example; connecting roads are pushed and pulled
in piston engines. Crankshafts are generally subjected to torsional stress and
bending stress due to self-weight or weight of components or possible misalignment
between journal bearings.[9.12]
Ductile cast iron frequently referred to as nodular or spheroid graphite iron is
a recent member of the family of cast irons.[12] It contains spheroid graphite in the
as cast condition, through the addition of nucleating agents such as cerium or
magnesium to the liquid iron. [11.13].In fact ductile cast iron provides a wide
spectrum of mechanical properties that can be obtained either by altering certain
processing variables or through various heat treatments which present different and
better combination of properties for application with special requirements.
Previous works showed that the main factors affecting the mechanical
properties are the metallurgical structures. Most of published researches for ductile
cast iron were devoted to study the microstructure and other properties. [12].
19
Cast iron is a complex alloy containing mainly a total of up to 10% carbon,
silicon, manganese, sulfur and phosphorous as well as varying amount of nickel,
chromium, molybdenum, vanadium and copper.[3] The metallic matrix of common
boundary cast iron consists of pearlite and ferrite. An increase in pearlite in the
structure with the same form of graphite precipitation improves the mechanical
properties. [7].
2.5.1 Mechanical properties of ductile cast iron
Ductile iron is characterized by having all of its graphite occur in
microscopic spheroids. [10].Although this graphite constitutes about 10% by
volume of ductile iron, its compact spherical shape minimizes the effect on
mechanical properties. The graphite in commercially produced ductile iron is not
always in perfect spheres. [10].
It can occur in a somewhat irregular form, but if it is still chunky as Type II
in ASTM Standard A247, the properties of the iron will be similar to cast iron with
spheroidal graphite. Of course, further degradation can influence mechanical
properties. The shape of the graphite is established when the metal solidifies, and it
cannot be changed in any way except by remelting the metal.
The difference between the various grades of ductile iron is in the
microstructure of the metal around the graphite, which is called the matrix. This
microstructure varies with composition and the cooling rate of the casting. It can be
slowly cooled in the sand mold for a minimum hardness as-cast or, if the casting has
sufficiently uniform sections, it can be freed of molding sand while still at a
temperature above the critical and normalized.[9.13].
20
The matrix structure and hardness also can be changed by heat treatment.
The high ductility grades are usually annealed so that the matrix structure is entirely
carbon-free ferrite.[2].
The intermediate grades are often used in the as-cast condition without heat
treatment and have a matrix structure of ferrite and pearlite. The ferrite occurs as
rings around the graphite spheroids. [14]Because of this, it is called bulls-eye ferrite.
The high strength grades are usually given a normalizing heat treatment to make the
matrix all pearlite, or they are quenched and tempered to form a matrix of tempered
martensite.[10.13] However, ductile iron can be moderately alloyed to have an
entirely pearlitic matrix as-cast.
2.5.2 Chemical Composition
The chemical composition of ductile iron and the cooling rate of the casting
have a direct effect on its tensile properties by influencing the type of matrix
structure that is formed. [15].All of the regular grades of ductile iron can be made
from the same iron provided that the chemical composition is appropriate so that the
desired matrix microstructure can be obtained by controlling the cooling rate of the
casting after it is poured or by subsequent heat treatment. [15].
2.5.3 Grades of Ductile Iron
The common grades of ductile iron differ primarily in the matrix structure
that contains the spherical graphite.[15] These differences are the result of
differences in composition, in the cooling rate of the casting after it is cast, or as the
result of heat treatment. Minor differences in composition or the addition of alloys
may be used to enhance the desired microstructure.Five grades of ductile are
21
classified by their tensile properties in ASTM Specification A-536-80, as shown in
Table 2.1.
The common grades of ductile iron can also be specified by only Brinell
hardness although the appropriate microstructure for the indicated hardness is also a
requirement.[8.15] This method is used in SAE specification J434c for automotive
castings and similar applications, Table 6. Other specifications for special
applications not only specify tensile properties but also have limitations in
composition.[15].
Table 2.1: Grade of Ductile Cast Iron in ASTM A- 536-77
Gradeand Heat
Treatment
TensileStrengthminimum
YieldStrengthminimum
Percent elongation
min. 2'
TypicalBrinell
Hardness
MatrixMicrostructure
60-40-18414 276 18
149-187 ferrite
65-45-12448 319 12
170-207 ferrite &pearlite
80-55-06552 379 6
197-255 pearlite &ferrite
100-70-03690 483 3
217-269 pearlite
120-90-02828 621 2
240-300 temperedmartensite
22
Table 2.2: Grade of Ductile Cast Iron in SAE specification No. J434c for
Automotive Castings
GradeCasting Hardness Range
Description
D4018 179Bhn.max4.6-4.10BIDFerritic
D4512 156-217Bhn 4.80-4.10BIDFerritic-pearlitic
D5506 187-255Bhn 4.4-3.8 BIDFerritic-pearlitic
D7003241-302 Bhn3.90-3.50 BID Pearlitic
DO&TRange as specified Martensitic
2.5.4 Hardness
Because of the minimum influence of the spheroidal graphite on mechanical
properties, the hardness of ductile iron is a very useful test and, with some
reservations, can be directly related to other properties. [15].The relation between
tensile properties and hardness is dependable when the microstructure and chemical
analysis are typical.This relation is not dependable if, for example, the graphite is
very irregular or if the matrix contains primary carbides. The presence of unusual
constituents in the microstructure such as primary carbides or the occurrence of other
forms of graphite can affect some properties quite differently than others[16].
23
The hardness of all graphitic irons is essentially the hardness of the matrix metal
reduced to a somewhat lower value by the presence of the graphite[16]. Graphite
in a spheroidal shape does influence the hardness values obtained with
conventional testers, but not as much as graphite in flake form[11.15]. A
martens tic ductile iron with an actual matrix hardness of Rockwell "C" 63-65
will indicate a hardness of 55-58. This effect presents no problem if it is
recognized. T structures are listed in Table 2.3. Where Brinell Hardness for
Ductile Cast Iron
Table 2.3 Typical Hardness Brinell for Ductile cast Iron
Matrix Structure Brinell Hardness 10/3000
Typical of Grade
Ferrite 149-187 60-40-18
Ferrite + Pearlite 170-207 65-45-12
Pearlite + Ferrite 187-248 80-55-06
Pearlite 217-269 100-70-03
Acicular or Banite 260-350 120-90-02
Tempered Martensite 350-550 120-90-02
Austensite 140-160 High Alloy
24
2.5.5 Tensile Properties
The commonly established tensile properties are tensile strength, yield
strength and percent elongation.[15] The minimums for these properties are typically
established by the specification or implied by specifying the hardness of the casting.
Because of the nominal and consistent influence of spheroidal graphite, the tensile
properties and the Brinell hardness of ductile iron are well related. [15].The relation
between tensile properties and hardness depends on microstructure. Ferritic matrix
irons, often annealed, have a very low combined carbon content.[13].
Hardness and strength are dependent upon hardening of the ferrite by the
elements dissolved in it, silicon being the most important. Manganese and nickel are
also common ferrite hardeners. [11].Pearlitic matrix irons have lamellar carbide as
the principal hardening agent. Pearlitic irons containing free ferrite are in this group.
A uniform matrix of tempered martensite produced by heat treatment has a
somewhat higher strength to hardness relation. [10].The acicular or bainitic matrix
irons have a similar relation, but generally have a lower ductility at a given strength.
The properties of ductile iron are also affected to some extent by processing
considerations including inoculation, post inoculation, and shakeout temperatures.
Reduced cooling times in the mold and a hot shakeout temperature increases strength
because the castings are effectively normalized by this treatment.[10].
2.6 Heat treatment
The heat treatment process consists of two stages .
25
2.6.1 Austenitisation
The cast component is heated to temperatures between 850 and 1000 C for
2h. In contrast to steels, the austenitising temperature determines the matrix carbon
content because the graphite nodules serve as a source or sink for carbon and because
the solubility of graphite in austenite increases with temperature.[18] The
temperature of the isothermal transformation is lower than that associated with
pearlite but greater than the martensite start temperature. The heat treatment
produces different types of bainitic microstructures, depending on the temperature
and time of treatment. [18] A schematic diagram of the austenitising heat treatment
cycle is shown in Fig
2.6.2 Cooling rate during quenching
The rapid reduction of temperature from the austenitising temperature to the
room temperature is achieved when the component is placed in water.[18] The
cooling rate during this stage is of importance since it determines the matrix
microstructure of the ductile iron.
2.7 Corrosion of Metals
The corrosion is the destructive chemical reaction between a metal or metal
alloy and its surrounding (environment). Metal atoms in nature are present in
chemical compounds (minerals).[19]
The same amount of energy needs to extract metals from their minerals are emitted
during the chemical reactions that produce Corrosion. Corrosion is returns the metals
26
to its combined state in chemical compounds that are similar or even identical to the
minerals from which metals were extracted. [19]
Thus, Corrosion has been called extractive metallurgy in reverse. May
nonmetallic materials, such as ceramic, consist of metals that have their chemical
reactivity satisfied by formation of bonds with other reactive ions , such as oxides
and silicates.[19].Thus, materials are chemically unreactive , and they degrade by
physical breakdown at high temperature or by mechanical wear or erosion. Similarly
organic polymers ( plastic ) are relatively unreactive , because they have very stable
covalent bounds , primary between carbon atoms .[19].
2.8 Electrochemical reactions
Corrosion occurs by an electrochemical process. The phenomenon is similar
to that which takes place when a carbon-zinc “dry” cell generates a direct current.
Basically an anode (negative electrode), a cathode (positive electrode), an electrolyte
(environment), and a circuit connecting the anode and the cathode are required for
corrosion to occur (see Figure 2-24). [19].Dissolution of metal occurs at the anode
where the corrosion current enters the electrolyte and flows to the cathode. The
general reaction (reactions, if an alloy is involved) that occurs at the anode is the
dissolution of metal as ions:[19].
M Mn- + en-
Where
M = metal involved
n = valence of the corroding metal species
e = electrons
27
Figure 2.16 The basic corrosion cell consists of an anode, a cathode , a
electrolyte , and a metallic path for electron flow.
Examination of this basic reaction reveals that a loss of electrons, or oxidation,
occurs at the anode. Electrons lost at the anode flow through the metallic circuit to
the cathode and permit a cathodic reaction (or reactions) to occur. In alkaline and
neutral aerated solutions, the predominant Cathodic reaction is
O2 + 2H2O + 4e 4(OH)
In aerated acids, the cathodic reaction could be
O2 + 4H- + 4e- 2H2O
All of these reactions involve a gain of electrons and a reduction process.
2.9 Corrosion of Cast Iron
Cast Iron has, for hundreds of years, been the preferred piping material
throughout the world for drain, waste, and vent plumbing applications and water
28
distribution. Gray iron can be cast in the form of pipe at low cost and has excellent
strength properties. [20].Unique corrosion resistance characteristics make cast iron
soil pipe ideally suited for plumbing applications. Cast iron and steel corrode;
however, because of the free graphite content of cast iron (3% - 4% by weight or
about 10% by volume), an insoluble graphitic layer of corrosion products is left
behind in the process of corrosion.[20] These corrosion products are very dense,
adherent, have considerable strength, and form a barrier against further corrosion.
Because of the absence of free graphite in steel, the corrosion products have little or
no strength or adherence and flake off as they are formed, thus presenting fresh
surfaces for further corrosion. [20].
The corrosion of metals underground is an electrochemical phenomenon of two main
types: galvanic and electrolytic:
Galvanic corrosion is self-generating and occurs on the surface of a metal
exposed to an electrolyte (such as moist, salt-laden soil). The action is similar to that
which occurs in a wet, or dry, cell battery. [20].
Electrolytic corrosion occurs when direct current from outside sources enters
and then leaves an underground metal surface to return to its source through the soil;
metal is removed and in this process and corrosion occurs. [20].
29
2.9.1 Effect of structure and composition on corrosion resistance of cast
iron:
2.9.1.1 Structure
An essential difference may be observed between the behaviors of steel and
cast iron components immersed in an environment in which rust is precipitated at
some distance from the corroding surface, the steel will waste away at steady rate
and its overall dimensions will steadily diminish, whereas cursory examination of the
cast iron may suggest that it has not corroded at all, since its dimensions appear to be
substantially unchanged.[21] This difference arises from the fact that the cast iron
contains in its microstructure several more or less corrosion resistant constituents.
The most important of these corrosion resistance micro- constituents are graphite,
phosphide eutectic and to a lesser extent, carbide, when cast iron corrodes in such a
way that the corrosion product is deposited at some distance from the corroding
surface , a skeleton is left behind comprising graphite flakes stiffened [21]..
2.9.1.2 Composition
Small variations in the composition of cast irons, or even the addition of
small amount of alloying elements, generally have little effect on the corrosion
resistance. [21].For example, Graham , however , working on the corrosive wear of
automobile cylinder and piston rings exposed to high sulfur fuels, showed that irons
exposed to 70% sulfur acid at 130oC are attacked at rates dependent on the silicon
content of the iron, the rate being relatively low at below 1% Si but rises to a peak at
about 2%.[21].
Addition of 0.06%Cu to irons containing 2% Si gave a significant improvement in
corrosion resistance , but the addition to irons containing less than 1.5%Si decreased
30
the copper addition appear to have the particular effect of reducing the corrosion
stimulating effect of the sulfur content of an irons exposed to acid .[21]
2.10 Corrosion of cast iron in neutral environments:
2.10.1 Atmospheric corrosion
Atmospheric corrosion rates are determined by the relative humidity and
pollution. At relative humidity greater than 70% corrosion proceeds even though
there is no visible moisture film on the metal surface because of the cast iron
components are normally very heavy in section, the relatively low rates of attack
associated with atmospheric corrosion do not constitute a problem and little work has
been carried out on the phenomenon. [21].
2.10.2 Corrosion of cast iron by waters
The corrosivity of natural water depends on the concentration and type of the
impurities dissolved in it and especially on its oxygen content. [15.21]. Waters of
similar oxygen content have generally similar corrosivities , e.g. well aerated
quiescent sea water corrodes cast iron at rates of 0.05 – 0.1 mm/y while most well
aerated quiescent fresh water corrode iron at 0.01-0.1 mm/y . these waters in which
the carbon dioxide content is in excess of that required as bicarbonate ion to balance
the bases present are among the most dangerous of the fresh water.[21].
Hard waters usually, though not invariably deposit a carbonate scale and are
generally not appreciably corrosive to cast iron. Water softening do not increase the
31
corrosivity of the water provided that the processes does not result in the
development of excess of dissolved carbon dioxide.[21].
2.11 Soil corrosion of cast iron
Soil corrosion since one of the major uses of cast iron in the manufacture of
pipes, the problem of the corrosion buried cast iron structure is very important, it is
however also very complex and only relatively general observations can be made on
the subject. The corrosion observed on a pipe buried in soil is the result of two
separate effects:[21]
1. Interaction of the metal with the soil electrolyte – the intrinsic
corosivity of the soil.
2. Development of a very large scale galvanic cell , due to for example
to variations in salt concentration or oxygen availability form point to point along
the pipeline or to the present of stray electrical currents.
32
2.12 Corrosion in industrial Environment of cast iron
2.12.1 Corrosion by acids
In general , unalloyed grey cast irons possess no useful resistance to dilute
mineral acids. In very dilute acids the presence of air , or other oxidizing agents such
as ferric salts , appreciably increase the corrosion rate.[22] .
If corrosion rates are be held below 0.25 mm/y in moderately aerated
solutions it's unwise to exceed a total acid concentration of 0.001N of the acid
concerned.
2.12.2 Mineral acids
Unalloyed cast irons possess on useful resistance to hydrochloric acids at any
concentration or temperature. [22].Dilute sulphuric , nitric and phosphoric acids are
also very aggressive, corrosion rates amounting to several centimeters per year in
some cases . owing to the insolubility of surface films of ferrous sulphate in strong
sulphuric acid.
2.12.3 Organic acids
Dilute solution of organic acids , especially if well aerated , attack cast iron a
uneconomical rates .temperature and velocity are also accelerating factors
33
2.13 Corrosion by alkalis
Dilute alkali solution do not corrode cast iron at any temperature , but hot
solution exceeding about 30% concentration will attack it , with an accompanying
evolution of hydrogen , to form a ferrite [22].Broadly speaking , if corrosion rates are
to be held below 0.2mm/y the temperature should not exceed 80c
2.13.1 Corrosion by salt solution of cast iron
The corrosively of a salt solution depends upon the nature of the ions present
in the solution . those salts which give an alkaline reaction will retard the corrosion
of iron as compared with the action of pure water , and those which give a natural
reaction will not normally accelerate the corrosion rate appreciably except in so far
as the increased conductivity of the solution in compression with water permits
galvanic effect to assume greater importance .[23] chloride are dangerous because of
the ability of the anion to penetrate otherwise impervious barriers of corrosion
products.
2.14 Corrosion under stress
As far as is known , cast iron is not subject to stress corrosion cracking ,
although it has been suggested that this can happen with ductile irons exposed to
strong caustic alkali solution , iron exposed under conditions of cyclic stress are ,
however , liable to corrosion fatigue due to water spray could be eliminated or
mitigated by some of the inhibitor systems.[23]
34
2.15 Corrosion of two types of cast iron
2.15.1 High nickel cast irons
2.15.1.1 Composition & properties
The addition of about 20% Ni to cast iron produces materials with a stable
austenitic structure , these materials are sometimes known as austenitic cast irons but
are more often referred to commercially as Ni-resist cast irons. these austenitic
matrix of these irons gives rise to very different mechanical & physical properties to
those obtained with the nickel – free grey cast irons . the austenitic matrix is more
noble than the matrix of unalloyed grey irons.[17.23].
2.15.1.2 Aqueous corrosion behavior
The austenitic cast irons show is better corrosion resistance than the ferritic
irons primarily due to the nickel content of the austenitic matrix. in general the
austenitic cast irons is more favorable corrosion characteristic than ferrite irons in
both the active and passive states .[23
2.15.1.3 Nature waters
Water which is used for cooling purposes in refineries and chemical plant can causes
severe problems of corrosion and erosion .ordinary Cast iron usually fail in this
environment due to graphitic corrosion or corrosion erosion .Ni-resist is better
resistance corrosion due to the nobility of the austenitic matrix and are preferred for
use in the more aggressive environment.[24].
35
2.15.2 High chromium cast iron
Composition there is not clear demarcation between high-chromium steels and high-
chromium cast irons other than the fact that components are fabricated from the steel
. the high chromium irons , those used for components requires high degree of
corrosion resistance normally content 25- 35% chromium.[23].
2.15.2.1 Corrosion resistance
The high chromium irons undoubtedly owe their corrosion resistance
properties to the development on the surface of the alloys of impervious and high
tenacious film , probably consisting of complex mixture of chromium and iron
oxides , since the chromium oxide will be derived from the chromium present in the
matrix and not from that combined with the carbide[23].
2.15.2.2 Atmospheric corrosion
Provided there is suitable excess of chromium over carbon in the alloy the irons will
not rust when exposed to the atmosphere in the as cast state . alloys which have been
found to tarnish in the cast state because of an inadequate excess of chromium may
be found to completely stainless in the machined and polished state , presumably
because a thin film is more likely to be continuous on a smooth surface than rough
one. [23].
36
2.15.2.3 Nature and industrial waters
Because of it's mechanical properties and the difficulties associated with it's
production , high chromium iron mostly used in environments which are particularly
aggressive to other cast alloys [23].It's most useful for handling acid waters
containing oxidizing agents , for example mine waters and industrial effluents ,
because many of these waters tend to contain solid matter in suspension , which can
lead to abrasion of metals exposed to them the very hard high chromium iron is often
the most suitable material for pumps handling these solutions.
2.16 Corrosion of Ductile Cast Iron ( DAI )
Ductile cast iron, which has excellent mechanical properties, has been widely
used as a structural material in the machine,[20].
The automobile industry, the mining industry, This cast iron is expected to be
used as a replacement of traditional cast irons such as nodular cast irons and also for
extensive engineering applications as a structural material, because it has not only
high strength but also excellent wear resistance and the mechanical properties [24].
Ductile iron also known as nodular cast iron or spheroid-graphite (SG) cast
iron contains nodules of graphite embedded in a matrix of ferrite or pearlite or both,
the graphite separates out as nodules from iron during solidification because of the
additives like cerium (Ce) and magnesium (Mg) introduced into the molten iron
before casting. These nodules act as crack arresters thereby improving the
mechanical properties of ductile iron.[24].
37
The formation of graphite nodule during solidification causes an internal
expansion of ductile iron as it solidifies and is responsible for the absence of
shrinkage defects in most ductile iron castings.
The major difference in the structure of ductile and grey iron is the flaky and
spheroid graphite in the grey and ductile iron respectively.[25] However, the
spheroid graphite in ductile iron does not weaken the matrix and hence its
mechanical properties are superior to those of grey iron and comparable to that of
steels.
The corrosion resistance of high silicon cast iron is attributed to the formation
of a thin passive barrier film of hydrated oxides of silicon on the metal surface. The
film develops with time due to the dissolution of iron from the metal matrix leaving
behind silicon which hydrates due to the presence of moisture. The passive hydrated
silicon film is thought to bridge over and form an impervious barrier layer on a fine
grained high silicon cast iron with spheroidal graphite areas much more readily than
on a high silicon cast iron with coarse graphite flakes. [25}
2.16.1 Cavitation Erosion of Ductile iron
Ductile iron has a steel-like matrix with high tensile strength, ductility,
toughness and good casting characteristics. It is a special type of cast iron containing
0.04 0.08% residual Mg by the sandwich or plunging process [26]. It has a spheroidal
form of free graphite interspersed in the matrix and has a wide application in
automobile components such as crank shafts, cam shafts and cam lobes, as well as in
the components used for marine applications, viz. diesel-engine cylinder liners, and
hydraulic machine parts. Laser surface treatment of the critical areas of ductile-iron
components enhances their hardness, fatigue resistance, fracture toughness, corrosion
resistance and wear due to adhesion or abrasion, and hence their service life.[26].
38
The cavitation erosion of ductile iron, a material employed extensively for
components in marine applications, including diesel-engine cylinder liners, valves
and pumps, is addressed here. Pearlitic ductile iron has only modest resistance to
cavitation erosion; the mean depth of penetration rate Ferritic ductile iron had
consistantly better resistance to erosion than pearlitic iron, particularly in corrosive
media.
The resistance to cavitation erosion of cast irons is influenced by the size,
shape and distribution of graphite, apart from the strength of the matrix. In grey
irons, the notch action of the graphite flakes and the notch sensitivity of the matrix
affect the erosion rate [27].Ductile iron with graphite in the form of spheroids has
better erosion resistance than that of grey iron. The cavitation erosion rate of pearlitic
and ferritic ductile iron was respectively 20% and 35% less than that of grey iron of
similar matrix.
2.16.2 Erosion–corrosion of ductile cast iron
Ductile cast iron, which has excellent mechanical properties, has been widely
used as a structural material in the machine, the automobile industry, the mining
industry, and so on. especially as the slurry pumps material, which is one of the most
important equipments in slurry transportation system in ore dressing industry, its
application is little or no. [28].
The parts of slurry pumps endure the serious erosion of solid particles with a high
speed and some corrosions of slurry in ore slurry or fly ash transportation Because
the ductile iron has an appropriate combination of impact fatigue resistance and
abrasive wear resistance through the control of its microstructures, as well as lower
breakage, lower fatigue spalling and lower production cost, it has been used
gradually to make grinding balls and mill liners used under the condition of dry
39
grinding and wet grinding in recent decade, such as that used in cement ball mill and
ore dressing ball mill.
The better application effects of ductile cast iron as grinding balls have been
obtained. [28] However the erosion–corrosion wear characteristics of martensitic
ductile iron with different contents of alloying elements are studied under the
conditions of different slurries in order to provide the proper material for the
producing parts of slurry pumps, as well as to enlarge the application of ductile cast
iron in the modern industry.
2.16.3 High temperature corrosion of ductile cast irons ( oxidation )
The oxidation behavior of two ductile cast irons was investigated in synthetic
diesel and gasoline exhaust gases. The alloys were a SiMo (Fe3.86Si0.6Mo3C). For
this reason, car manufacturers are designing cleaner and more efficient engines
running at higher temperatures.[29] As a consequence, the demands regarding the
high temperature corrosion and mechanical behavior of engine components are
increasingly tougher. This is particularly true for the hot end of the exhaust system,
that is, the exhaust manifold and the turbocharger housing, since these components
are not actively cooled [29].
The materials under investigation are four commercial castalloys intended for
exhaust systems of trucks and cars. and a Ni-Resist (Fe32Ni5.3Si2.1C) Indeed,
nitrides precipitates harden the surface which improves the fatigue and wear
properties.On the other hand, for applications where the component integrity should
be preserved, the effects of nitrogen-based gases may be harmful and require a
special attention to control the activity of the gas and avoid undesirable phases
nitrous oxide, NOx, present in exhaust gases of internal combustion engines, on the
high temperature corrosion behavior of ductile cast irons intended for exhaust
manifolds.
40
Increased engines efficiency together with a decrease in exhaust gas pollutant
emissions push automobile manufacturers to raise the top combustion temperature
and therefore the exhaust gas temperature in engines.As a result, the corrosive effect
of the gas is enhanced and subscale precipitation may occur more easily. [26.29].The
general oxidation of grey and ductile cast irons in air is nowadays well
understoodThe oxide consists of Fe2O3 at the gas/oxide interface, then Fe3O4
and finally FeO. The porous iron oxide allows fast diffusion of oxygen and
thusinternal oxidation of the alloy.
2.16.4 Corrosion fatigue of ductile iron
Ductile irons (ADI) offer excellent combinations of high tensile strength,
ductility, toughness, fatigue strength and wear resistance compared with other grades
of cast irons [30].These desirable mechanical properties are comparable carbide-free
ferrite with carbon-enriched austenite. Low density and low cost in near-net-shape
forming of complex geometries are additional advantages of DCI Therefore, DCI is
an attractive material for many engineering applications in heavy machinery,
transportation equipment and other industries.
As the use of DCI castings in many applications involves long-term,
mechanical variable loads under corrosive environment it is important to understand
the corrosion fatigue (CF) behavior of this advanced cast iron [31].Fatigue properties
of DCI were mainly focused on atmospheric environments including, e.g., high-cycle
fatigue (HCF) , strain controlled low-cycle fatigue (LCF) , and fatigue crack growth
(FCG) .
41
2.16.4.1 Fatigue behavior in various environments at room temperature
The S-N curves for HCF specimens tested in lubrication oil, air and three
aqueous media (distilled water, salt water and sulfuric acid solution) at room
temperature.[31] The straight solid and dash lines in represent the best-fit S-N curves
by a linear regression analysis clearly demonstrates deleterious effects of the three
aqueous environments on the HCF response of DCI, as a remarked reduction of
fatigue life was found in each aqueous solution compared to atmospheric air.The
counterpart results of the FCG tests using pre-cracked CT specimens in the
corresponding environments were plotted as (da/dN) vs. (_K) in Fig. 2 where the
FCGRs in air were not significantly different from those in the aqueous
environments except at the _K region near final fast fracture.
Figure 2.17 Fatigue Corrosion of Ductile Cast iron
CHPTER 3
Experimental Procedures
3.1 Introduction
This chapter introduces the experimental procedures utilized to characterize
ductile irons studies. Figure 3.1 is the general flow chart of experimental
procedures. First, the ductile is subjected to heat treatment in order to obtain
various microstructures and second, corrosion tests are conducted to study the
corrosion behavior as function of the structure. Two methods of corrosion tests are
conducted: immersion test (ASTM G-67) and electrochemical test (ASTM G-34).
The heat treatment consists of austenitising the cast iron at four different
temperatures (850°C, 900°C,950°C and 1000°C ) followed by water quenching. For
the corrosion test samples were exposed in both alkaline and acidic solutions.
44
Figure 3.1 A flow chart showing a summary of research methodology
Literature Review
Microstructure Compositional Analysis
Sample preparation
Cutting
Heat treatment
Hardness Test
Weight before Corrosion
Corrosion Test
Microstructure after Heat Treatment
Immersion Test Electrochemical Test
Weight, Visual &Microstructure after Corrosion Test
Result Dissection & Analysis
3.2 Material
The as-received sample of
cutting machine (Mecotome T255/300)
10mm x 20mm x 40mm
Figure
3.3 Sample Preparation
The samples were cut into pieces with the dimensions of approximately
20mm x10mm x 20mm for the purpose of immersion and electrochemical tests as
shown in Figure 3.3.
For the electrochemical test,
using the SiC paper to 600 grit finishes by using the grinding machine (Figure
received sample of ductile cast iron was cut into the desired size using
chine (Mecotome T255/300) as shown in Figure 3.2. The sample size is
0mm.
Figure 3.2: Cutting Machine (Mecotome T255/300)
Sample Preparation
he samples were cut into pieces with the dimensions of approximately
20mm x10mm x 20mm for the purpose of immersion and electrochemical tests as
For the electrochemical test, surface preparation was carried out by grinding
SiC paper to 600 grit finishes by using the grinding machine (Figure
45
cast iron was cut into the desired size using
sample size is
utting Machine (Mecotome T255/300)
he samples were cut into pieces with the dimensions of approximately
20mm x10mm x 20mm for the purpose of immersion and electrochemical tests as
urface preparation was carried out by grinding
SiC paper to 600 grit finishes by using the grinding machine (Figure 3.5).
46
Figure 3.3 : Sample Preparation for Heat Treatment Process
Figure .3.4 Sample Preparation for immerssion Test
Figure 3.5: Grinding machine
47
Before the tests all specimens were measured by using the digital
caliper in the machine shop. While the microstructure of as received sample
before heat treatment and corrosion test was examined by using the Nikon optical
microscope as shown in Figure 3.6. The etching solution used throughout this
study consists of glycerol: HCl: HNO3 to (2:2:1) reveal the grain boundaries.
Figure 3.6: Nikon optical microscope (U-LBD-2 OLYMPUS)
The initial weight of all the specimens was measured by using the METTER
AT 400 microbalance, Figure 3.7. The weighing process also was performed after
the immersion test to determine the corrosion rate using the weight loss technique.
Figure 3.7: Micro balance (METTER AT 400)
48
3.4 Compositional Analysis
A chemical composition analysis was performed on the ductile cast iron in
order to determine the content of alloying elements of ductile cast iron. It was
conducted by using a combination of Emission Arc Spark Spectrometer
(SPECTROLAB M5) carried out at ANTARA STEEL Mills Sdn. Bhd., Pasir
Gudang (Figure 3.8 (a) and (b)) while elemental analysis using Energy Dispersive X-
ray Spectrometer which is attached to Field Emission Scanning Electron Microscopy
(FESEM) (Figure 3.9 )
(a)
(b)
49
Figure 3.9: EDX-FESEM (SUPRA 35VP)
3.5 Metallographic Analysis
The microstructure analysis was conducted on the cross section and surface
of specimens before and after the corrosion tests. The specimens were ground to
produce a plane surface with minimal scratches and polishing to obtain a mirror-like
surface. The specimen was grounded using the silicon carbide paper with 240, 320,
400, 600, 1000 and 4000 grit whereas polishing of specimens was carried out by
using diamond powder 3 micron paste which is placed on the nylon cloth covered
surface of a rotating polishing wheel (Figure 3.10). The last stage was the etching
process in order to reveal the grain boundaries of the microstructures. 2 parts of
glycerol: 2 parts HCl : 1 part HNO3 used to etch the specimens. Finally, specimens
were observed under Nikon Microscope with magnification of 100µm, 200µm and
500µm and Field Emission Scanning Electron Microscopy
50
Figure 3.10: Polishing machine
3.6 Heat Treatment
The prepared samples were austenitized at different temperatures (850 °C,
900°C, 950 °C, 1000 °C) for period time around 1.5 hours. This process was done to
ensure that the matrix was transformed to austenite. Upon completion of the 1.5
hours austenitization the samples were water quenched. The flow chart of the heat
treatment process is as shown in Figure 3.11. The heat treatment was conducted in
the furnace shown in Figure 3.12.
51
Figure 3.11 Flow chart illustrating the heat treatment processes for austempered
ductile iron samples
Figure .3.12: Furnaces for heat treatment process
Austenitized at varied temperatures for 1.5 hours
850°C
Quenching for 1.5 hours
900°C 950°C 1000°C
Cooling at room temperature
52
3.7 Hardness measurement
A Vickers hardness testing machine as shown in Figure 3.13 was used to
measure the hardness using 10 kg load. The final hardness values are the average o 5
readings.
Figure.3.13 Vickers hardness test machine
3.8 Microstructure Analysis
After heat treatment, the samples were prepared from metallographic
analysis. First they were mounted, ground using SiC paper to 4000 grit and then
polished with 1µm cloth coated with diamond paste. The samples were finally
etched using 2%nital (2%concentrated nitric acid in methanol solution).
Examination was made using Nikon optical microscope.
53
3.9 Electrochemical Testing
An electrochemical corrosion test was carried out by the potentio-dynamic
anodic polarization using Potentiostat Galvanostat instrument according to the
ASTM Standard G-5. Two replicate tests of each measurement were performed. The
test was carried out in two different solutions: Sodium Chloride (NaCl) solution at
concentration (3.5%) and Sodium Hydroxide (NaOH) at concentration (10%). The
temperature of solution was at 24+2°C. All the parameters are tabulated in Table 3.1.
The experimental set up for the instrument is as shown in Figure 3.14.
3.9.1 Principle of Measurement
Figure 3.15 shows the schematic circuitry for potentiostatic electrochemical
polarization measurements apparatus. The potentiostatic measuring equipment
consists of three electrodes procedure.
They are Working Electrode, WE, Reference Electrode, RE and Auxiliary
Electrode, AE. Working electrode represents the specimen to be tested, reference
electrode to provide datum against which the potential of the working electrode is
measured and the auxiliary electrode which carries the current created in the circuit.
A filtered direct current (dc) power supply, PS, supplies current (I) to the working
electrode is measured with respect to a reference electrode, with a series-connected
potentiometer, P.
The experimental arrangement placed the reference electrode which is
Saturated Calomel electrode separately from the electrochemical cell where the
junction test tube was filled with saturated KCl solution. The reference electrode was
then placed into the test tube. The Luggin probe is usually included to minimize
ohmic resistance interferences in the electrolyte. The luggin probe was placed as
near as possible to the surface of the metal being studied, as it allows potential to be
54
detected close to the metal surface. The working electrode becomes the anode while
the auxiliary electrode becomes the cathode.
Figure 3.15: Cell kit set-up
3.9.2 Preparation of Working Electrode
The Ductile cast iron specimens were cut using precision cutter into small
pieces approximately 30mm x 20mm. Brazing technique was applied to connect the
specimen to the steel rod for ease of connection to the electrochemical cell (Figures
3.16 (a) and (b)). Then the specimen was mounted by embedding in epoxy resin for
24 hours as shown in Figures 3.17 (a) and (b). The surface of each ductile iron
sample was smoothened and cleaned to remove any unwanted particles or grease.
Working electrode
Reference electrode
Auxiliary electrode
Test solution
55
(a)
(b)
Figure 3.16: Photographs of (a) Connection of specimen to copper wire
by brazing technique; (b) Mounting of samples
(a)
56
(b)
Figure 3.17: Photographs of (a) Working Electrode (WE); (b) Showing
typical surface area of sample
Table 3.1 Potentiodynamic Polarization Test Parameters
Parameters Unit
Exposure time 10 minutes
Corrosive solution
i) 3.5% Sodium Chloride
(NaCl)
ii) ) 10% Sodium Hydroxide
(NaOH)
Temperature
Room temperature
(24°C)
57
5 Immersion Test
Immersion test was conducted to determine corrosion rate using weight loss
method in which a specimen known initial weight is exposed to the corrosive
environment for a specified period of time. By the end of the test, the specimen is
cleaned and weighed to determine the weight loss and the pits behaviour. The
method for specimen preparation is in accordance to ASTM G1 and the immersion
test is in accordance to ASTM G67.
In this test, the maximum corrosion depth and mass loss was measured for a
set period of different time duration (24,168, 336 and 672) hours at a set temperature
(25°C) as shown in Table 3.2. Temperature change was measured to indicate any
changing in test solution. Two different concentrations and two type of solution were
used as the corrosive medium.3.5%, NaCl and 10% NaOH and.
The concentration of solution varied from 3.5% to 10% to indicate the most
corrosive environment toward specimen was testing. Based on the guidelines in the
ASTM G67 standard, the testing procedure involves the evaluation of related to
weight-loss and location of pits. This technique proved gainful by revealing pits
overlooked by microscopy. Before the immersion test, all the specimens were
cleaned and the weight of each specimen had been taken. Cleaning procedure
include grounding with SiC paper to 800 grit finish by using the grinding machine.
All specimens were then immersed in the test solution (Figure 3.17 (a)).
After the immersion test the specimens were ultrasonically cleaned with ethanol
(Figure 3.19 (a)), dried (Figure 3.19 (b)) and the final weight was taken. Practice for
cleaning corrosion test specimen was carried out in accordance to ASTM G1-90
“Standard Practice for Preparing, Cleaning and Evaluating Corrosion Test
Specimens”. Subsequently visual inspection and microstructure examination were
performed. To reveal the microstructure of specimens, the procedure is described in
Section 4.5. Figure 3.17
Figure 3.18
performed. To reveal the microstructure of specimens, the procedure is described in
7 shows a summary of immersion test.
(a)
(b)
: Photographs showing (a) Immersion test at room
temperature; (b) In oven at 25C
58
performed. To reveal the microstructure of specimens, the procedure is described in
Photographs showing (a) Immersion test at room
59
(a)
(b)
Figure 3.19 : Photographs showing (a) Ultrasonic cleaning; (b) Drying
60
Table 3.2 Parameters for immersion test
Parameters Unit
Exposure time 1.7,14 and 28 days
Corrosive solution
i) 3.5% Sodium Chloride
(NaCl)
ii) ) 10% Sodium Hydroxide
(NaOH)
Temperature Room temperature (24°
The corrosion rate measurement was based on the weight loss method and
calculated using the following equation (3.1)
( mm/yr ) = ×× × (3.1)
Where:
K = a constant (8.76 x 104)
W = mass loss, g
A = exposed surface area, cm2
T = time of exposure, hour
D = density of specimen, g/cm3
61
CHAPTER 4
RESULTS AND DISCUSSION
4.1 Composition analysis
Chemical composition of the as-received sample was performed by using
Emission Arc Spectrometer (EAS-SPECTROLAB M5) (ANTARA STEEL, Pasir
Gudang) and Energy Dispersive X-ray Spectrometer (EDX) detector attached to
Field Emission Scanning Electron Microscopy (FESEM
Fable.4.1 : Chemical composition of as-received Ductile Cast Iron
Element Weight %
C 2.06
Si 3.35
Mn 0.558
P 0.0288
S 0.00529
Cr 0.119
Fe 91.1
cr2 0.0675
4.1 Microstructure
The microstructures of the heat treated samples at different temperatures
shown in Figures 4.1 (a), (b), (c) and
austenitized heat treatment.
austenitization temperatures of 850,
of martensite.
However, there are differences between the different austenitising temperatures as at
the low temperature of 850 C, not all the sample has transformed to austenite as
indicated by the presence of the white phase which is probably is untransformed
austenite. While at higher austenetising temperature of 1000 C the austenite has
transformed to plate martensite.
Fiqure 4.1: Microstructure of ductile cast iron consiste martensite
Microstructure
The microstructures of the heat treated samples at different temperatures
in Figures 4.1 (a), (b), (c) and (d),. All these samples were given identical
heat treatment. From the figures it is quite clear that a all the
austenitization temperatures of 850, 900, 950 and 1000 C the microstructure is made
However, there are differences between the different austenitising temperatures as at
e of 850 C, not all the sample has transformed to austenite as
indicated by the presence of the white phase which is probably is untransformed
austenite. While at higher austenetising temperature of 1000 C the austenite has
transformed to plate martensite.
Microstructure of ductile cast iron consiste martensite
62
The microstructures of the heat treated samples at different temperatures are
(d),. All these samples were given identical
From the figures it is quite clear that a all the
the microstructure is made
However, there are differences between the different austenitising temperatures as at
e of 850 C, not all the sample has transformed to austenite as
indicated by the presence of the white phase which is probably is untransformed
austenite. While at higher austenetising temperature of 1000 C the austenite has
Microstructure of ductile cast iron consiste martensite (850C)
Fiqure .4.2 :Microstructure of ductile cast iron consiste of martensite
Fiqure .4.3 :Microstructure of ductile cast iron consiste of martensite ( not fully
Fiqure 4.4 :Microstructure of ductile cast iron consiste of plate martensite
Microstructure of ductile cast iron consiste of martensite
Microstructure of ductile cast iron consiste of martensite ( not fully
resolved )(950)
Microstructure of ductile cast iron consiste of plate martensite
63
Microstructure of ductile cast iron consiste of martensite(900)
Microstructure of ductile cast iron consiste of martensite ( not fully
Microstructure of ductile cast iron consiste of plate martensite(1000)
4.3 Hardness Test:
The results of the h
Table 4.3. Once the ductile cast iron is
in the hardness. From the data it is clear that as the austenitising temperature is
increased the hardness decrease.
Table .4.2
Figure 4.5
0
100
200
300
400
500
600
700
800
:
The results of the hardness for the ductile cast iron samples
ductile cast iron is austenitized, it is seen that there is an increase
From the data it is clear that as the austenitising temperature is
increased the hardness decrease.
Analysis Hardness Rate for ductile Cast Iron
Hardness Rate for ductile Cast Iron specimens
800 850 900 950 1000 1050
Samples Hardness Rate
850°C 620
900°C 510
950°C 445
1000°C 323
64
are shown in
there is an increase
From the data it is clear that as the austenitising temperature is
Analysis Hardness Rate for ductile Cast Iron
Hardness Rate for ductile Cast Iron specimens
1050
65
4.4 Immersion Test
Immersion test was carried out at different exposure time (24, 168, 336 and
672 hours) with two different types of solutions (NaCl and NaOH), with different
concentrations ( 3.5% NaCl and 10% NaOH), at room temperature (24 ± 2°C).
All the data gathered are indicated in Table 4.4. The main purpose is to
investigate the sodium chloride and sodium hydroxide attack on ductile cast iron.
The exposure time was extended to 672 hours to provide a sufficient time for the
reaction to take place. The time factor on corrosion rate was also investigated.
Corrosion rates were then calculated in mm/yr as shown in Figure (4.6 ) .
Table 43: Corrosion rate of specimens expressed in mm/yr after 1 day in ( NaCl )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaCl 3.5% 24 7.6 240.019
0.14
900 °C Nacl 3.5% 24 7.6 240.012
0.19
950 °C NaCl 3.5% 24 7.6 24 0.015 0.12
1000°C NaCl 3.5% 24 7.6 24 0.017 0.14
66
Table 4.4: Corrosion rate of specimens expressed in mm/yr after 7days in ( NaCl )
Sample
Test
solution Percentage
%
T
°
C
PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaCl 3.5% 24 7.6 168 0.010 0.187
900 °C Nacl 3.5% 24 7.6 168 0.0052 0.167
950 °C NaCl 3.5% 24 7.6 168 0.0072 0.152
1000°C NaCl 3.5% 24 7.6 168 0.0071 0.13
Table4.5: Corrosion rate of specimens expressed in mm/yr after 14 days in ( NaCl )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaCl 3.5% 24 7.6 336 0.05 0.34
900 °C Nacl 3.5% 24 7.6 336 0.034 0.49
950 °C NaCl 3.5% 24 7.6 336 0.05 0.21
1000°C NaCl 3.5% 24 7.6 336 0.048 0.19
67
Table 4.6: Corrosion rate of specimens expressed in mm/yr after 28days in ( NaCl
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaCl 3.5% 24 7.6 672 0.011 0.67
900 °C Nacl 3.5% 24 7.6 672 0.073 0.82
950 °C NaCl 3.5% 24 7.6 672 0.026 0.97
1000°C NaCl 3.5% 24 7.6 672 0.020 0.95
Figure 4.6 Chart showing corrosion rate in NaCl solution.
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
Series1
Series2
Series3
Series4
Cor
rosi
on R
ate
mm
/yr
Time Exposure
68
Table 4.7: Corrosion rate of specimens expressed in mm/yr after 1 day in ( NaOH )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaOH 10% 24 7.6 24 0.026 0.035
900 °C NaOH 10% 24 7.6 24 0.025 0.055
950 °C NaOH 10% 24 7.6 24 0.028 0.056
1000°C NaOH 10% 24 7.6 24 0.020 0.040
Table 4.8 : Corrosion rate of specimens expressed in mm/yr after 7 days in ( NaOH )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaOH 10% 24 7.6 168 0.07 0.11
900 °C NaOH 10% 24 7.6 168 0.04 0.13
950 °C NaOH 10% 24 7.6 168 0.01 0.127
1000°C NaOH 10% 24 7.6 168 0.07 0.123
69
Table 4.9: Corrosion rate of specimens expressed in mm/yr after14 days in ( NaOH )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaOH 10% 24 7.6 336 0.040 0.31
900 °C NaOH 10% 24 7.6 336 0.088 0.61
950 °C NaOH 10% 24 7.6 336 0.040 0.30
1000°C NaOH 10% 24 7.6 336 0.022 0.19
Table 4.10 Corrosion rate of specimens expressed in mm/yr after 28 days in ( NaOH )
Sample
Test
solution Percentage
%
T
°C PH
Exposure
Time
( Hours)
Weight
Loss
( g )
Corrosion
Rate
mm/yr
850 °C NaOH 10% 24 7.6 672 0.113 0.57
900 °C NaOH 10% 24 7.6 672 0.27 0.98
950 °C NaOH 10% 24 7.6 672 0.11 0.66
1000°C NaOH 10% 24 7.6 672 0.12 0.74
70
Figure 4.7 Chart showing corrosion rate in NaOH solution.
From the results it was shown sodium hydroxide has lesser attack on the
ductile cast iron compared to sodium chloride over the entire duration of the
immersion. Figure 4.6 shows that the austenitising temperature has little effect on the
corrosion rate as the corrosion rates measured are more or less the same for all the
temperatures. The corrosion rate however, increases as the exposure duration
increases.
Similar results are also obtained when the samples are exposed to sodium
hydroxide (Figure 4.7) although the corrosion rates are significantly lower to those
observed when the ductile iron was exposed to sodium chloride. The higher
corrosion rates observed in sodium chloride are probably due to the presence of
chloride ions. The extent of corrosion naturally increases with an increase in time. It
is believed that a more sufficient time has provided for the reaction of metal
hydrolyze to form metal hydroxide and consequently enhance the growth of pits.
Increasing the temperature of a corrosive system will normally have the effect of
increasing corrosion rates. The kinetics (rate of motion or reaction) of the action was
increased, thus leadings to high speed of electron transfer and metal dissolution of
iron in an electrolyte.
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15 20 25 30
Series2
Series3
Series4
Time Exposure
Cor
rosi
on R
ate
mm
/yr
71
4.5 Electrochemical ( Polarization results )
Table 4.12 shows the potentiodynamic anodic polarization data obtained
when the test was carried out in alkaline solution (3.5% NaCl) at room temperature.
From the results it is observed that the sample heated to 850℃ corroded the fastest
compared to the other Specimens (7.99 mpy). The values of icorr are shown
graphically in Figure 4.8.
Table 4.11: Tafel polarisation curve parameters in 3.5% NaCl and at 24±2°C
Parameters
850℃A=2
900℃A-2
950℃A=
1000℃A=
i corrosion (µA) 1.222 9.137 7.287 1.109
Icorrosion (A/cm2 1.222 9.137 7.287 1.109
BCa(mV) 82.060 107.325 170.020 160.968
BAn(mV) 318.554 123.290 66.124 70.216
Ecorr(calculate) (mV) -889.31 -762.17 -661.178 -679.692
Initial potential ( V ) -0.2500 -0.2500 -0.2500 -0.2500
Final potential ( V ) -0.2500 0.2500 0.2500 0.2500
Corrosion
penetration rate
(mpy)
7.99 5.977 4.767 7.254
Figure
Figure 4.9
0
2
4
6
8
850
7.99
4.8: Bar chart of icorr in 3.5% NaCl at 24±2°C
Figure 4.9 chart of icorr in 3.5% NaCl at 850°C
Ser900
9501000
5.41
4.22
7.25
Series
72
Ser…
Series1
73
Figure 4.10 chart of icorr in 3.5% NaCl at 900°C
Figure 4.11 chart of icorr in 3.5% NaCl at 950°C
74
Figure 4.12 chart of icorr in 3.5% NaCl at 1000°C
Table 4.13 shows the potentio-dynamic anodic polarization data obtained
when the test was carried out in alkaline solution (10% NaOHl) at room
temperature. In this case the corrosion rates observed follow the same trend as
those exposed to sodium chloride, meaning that the sample heated at 850℃corroded the fastest. However, for all specimens at all four austenetising
temperatures the corrosion rates were significantly lower when the material was
exposed to sodium hydroxide expect for the sample heated at 850 oC.
Table 4.12: Tafel pola
Parameters
i corrosion (µA)
Icorrosion (A/cm2
BCa(mV)
BAn(mV)
Ecorr(calculate) (mV)
Initial potential ( V )
Final potential ( V )
Corrosion
penetration rate
(mpy)
Figure 4.
01
2
3
4
5
6
7
850
6.56
Tafel polarisation curve parameters in 10% NaOH at 24±2°C
850℃A=2
900℃A-2
950℃A=
1000
A=
1.003 2.314 2.253 2.664
1.003 2.314 2.253 2.664
50.608 52.381 111.582 26.691
1616.25 407.045 54.354 1216.60
-918.930 -890.013 -1087.91 -922.83
-0.2500 -0.2500 -0.2500 -0.2500
0.2500 0.2500 -0.2500 -0.2500
6.560 1.513 1.47 1.742
4.13: Bar chart of icorr in 10% NaOH at 24±2°C
900950
1000
56
1.513 1.7421.474
Series
75
at 24±2°C
1000℃A=
2.664
2.664
26.691
1216.60
922.83
0.2500
0.2500
1.742
S…
Series1
76
Figure 4.14 chart of icorr in 10% NaOH at 850°C
Figure 4.15 chart of icorr in 10% NaOH at 900°C
77
Figure 4.16 chart of icorr in 10% NaOH at 950°C
Figure 4.17 chart of icorr in 10% NaOH at 1000°C
4.6 Microstructure Analysis of Samples after Immersion Corrosion Test
The corrosion of all samples was analyzed by examining
before and after the corrosion test. Figure 4.10 shows the microstructure of ductile
iron before (a) and (b, c) after the immersion test in 3.5% NaCl and 10% NaOH at
24±2°C. Visual observation of the specimens exposed for 1 day in both NaCl and
NaOH solutions showed that the color of
clear from microstructures and corrosion rate measurements that very little corrosion
occurred. However, the corrosion appears to be uniform and there is no evidence of
localized corrosion.
Microstructure Analysis of Samples after Immersion Corrosion Test
The corrosion of all samples was analyzed by examining the microstructure
before and after the corrosion test. Figure 4.10 shows the microstructure of ductile
iron before (a) and (b, c) after the immersion test in 3.5% NaCl and 10% NaOH at
observation of the specimens exposed for 1 day in both NaCl and
NaOH solutions showed that the color of the solution remained colorless.
clear from microstructures and corrosion rate measurements that very little corrosion
e corrosion appears to be uniform and there is no evidence of
( a )
(b)
78
Microstructure Analysis of Samples after Immersion Corrosion Test
the microstructures
before and after the corrosion test. Figure 4.10 shows the microstructure of ductile
iron before (a) and (b, c) after the immersion test in 3.5% NaCl and 10% NaOH at
observation of the specimens exposed for 1 day in both NaCl and
the solution remained colorless. It is also
clear from microstructures and corrosion rate measurements that very little corrosion
e corrosion appears to be uniform and there is no evidence of
Figure 4.18 Optical micrographs of specimens at 850 (a) before (b) and (c) after
immersion test in 3.5% NaCl + 10% NaOH at 24°C (24h)
(c)
Optical micrographs of specimens at 850 (a) before (b) and (c) after
3.5% NaCl + 10% NaOH at 24°C (24h)
(a)
(b)
79
Optical micrographs of specimens at 850 (a) before (b) and (c) after
Figure 4.19 Optical micrographs of specimens at 900 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (24h)
( c)
Optical micrographs of specimens at 900 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (24h)
(a)
(b)
80
Optical micrographs of specimens at 900 (a) before (b) and
Figure 4.20 Optical micrographs of specimens at 950 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (24h)
(c)
Optical micrographs of specimens at 950 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (24h)
(a)
(b)
81
Optical micrographs of specimens at 950 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (24h)
Figure 4.21 Optical micrographs of specimens at 1000 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C
(24h)
When the samples were exposed to 7 days, the microstructures of the samples
revealed little corrosion has occurred in
no significant degradation in the specimens as shown in the structures
temperatures. Comparison between electrolyte solutions reveals that sodium chloride
still produce more attack than sodium hydroxide. The micr
exposure are shown in Figure 4.14 to Figure 4.17.
(c)
Optical micrographs of specimens at 1000 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C
When the samples were exposed to 7 days, the microstructures of the samples
corrosion has occurred in terms of rate. It is also evident that there was
no significant degradation in the specimens as shown in the structures
temperatures. Comparison between electrolyte solutions reveals that sodium chloride
still produce more attack than sodium hydroxide. The microstructures after 7 days
exposure are shown in Figure 4.14 to Figure 4.17.
(a)
82
Optical micrographs of specimens at 1000 (a) before
(b) and (c) after immersion test in3.5% NaCl + 10% NaOH at 24°C
When the samples were exposed to 7 days, the microstructures of the samples
. It is also evident that there was
no significant degradation in the specimens as shown in the structures for all
temperatures. Comparison between electrolyte solutions reveals that sodium chloride
ostructures after 7 days
Figure 4.22 Optical micrographs of specimens at 850 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(b)
(c)
Optical micrographs of specimens at 850 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(a)
83
Optical micrographs of specimens at 850 (a) before (b) and
Figure 4.23 Optical micrographs of specimens at 900 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(b)
(c)
Optical micrographs of specimens at 900 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(a)
84
Optical micrographs of specimens at 900 (a) before (b) and
Figure 4.24 Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(b)
(c)
Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
(a
85
Optical micrographs of specimens at 950 (a) before (b) and
Figure 4.25 Optical micrographs of specimens at 1000 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
After 28 days of exposure, however, the corrosion rate increased
significantly, particularly when exposed to the NaCl solution.
revealed that those specimens that
had the better the corrosion behavior.
complete dissolution in the austenite phase and upon quenching a fu
structure is obtained.
(b)
(c)
Optical micrographs of specimens at 1000 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (168h)
After 28 days of exposure, however, the corrosion rate increased
particularly when exposed to the NaCl solution. The results also
specimens that were heated at higher austenitisation temperature,
e better the corrosion behavior. Higher austenitisation temperature results in
complete dissolution in the austenite phase and upon quenching a fully marten
86
Optical micrographs of specimens at 1000 (a) before (b) and
After 28 days of exposure, however, the corrosion rate increased
The results also
higher austenitisation temperature,
Higher austenitisation temperature results in
lly martensitic
Figure 4.26: Optical micrographs of specimens at 850 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (672h)
(a)
(b)
(c)
Optical micrographs of specimens at 850 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (672h)
87
Optical micrographs of specimens at 850 (a) before (b) and
Figure 4.27: Optical micrographs
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (672h)
(a)
(b)
(c)
Optical micrographs of specimens at 900 (a) before (b) and
(c) after immersion test in3.5% NaCl + 10% NaOH at 24°C (672h)
88
of specimens at 900 (a) before (b) and
Figure 4.28: Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in
(a)
(b)
(c)
Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in 3.5% NaCl + 10% NaOH at 24°C (672h)
89
Optical micrographs of specimens at 950 (a) before (b) and
3.5% NaCl + 10% NaOH at 24°C (672h)
Figure 4.29 Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in
(a)
(b)
(c)
Optical micrographs of specimens at 950 (a) before (b) and
(c) after immersion test in 3.5% NaCl + 10% NaOH at 24°C (672h)
90
Optical micrographs of specimens at 950 (a) before (b) and
3.5% NaCl + 10% NaOH at 24°C (672h)
91
CHAPTER5
CONCLUSIONS &RECOMMENDATIONS
5.1 Conclusions
The following conclusion points can be deduced from the present study:
I. Ductile Cast Iron showed four distinct Heat Treatment temperatures {850℃,
900℃, 950℃ and1000℃ ).
i) Electrochemical test results showed that the highest corrosion rate was
obtained in sample tested in NaCl and NaOH solution at 24±2°C and the most
critical temperature attacked was at 850°C. and the better corrosion resistance it’s
was at 950°C then 900°C.
ii) Results for immersion test, at room temperature, indicated that the corrosion
rate of DCI is lower (0.67 mm/yr for 28 days) compared to the high temperature
environment (0.95 mm/yr) for sample in 3.5% NaCl and (0.57mm/yr) to the high
temperature environment (0.74mm/yr) in 10% NaOH.
iii) No significant effect was observed when the specimen was tested using
immersion test when the specimens were exposure at one day and seven days this is
92
due to not more time exposure but at the fourteen and twenty eight days there is
evidence that show the is corrosion happened
iv) The present results showed that there is a relationship between microstructure
of ductile cast iron (produced by varying heat treatment) and corrosion behavior.
The ductile cast iron investigated exhibited better corrosion resistance when
exposed to NaCl and NaOH, particularly when austenitised at 1000 oC, probably
due to the homogenous structure produced
v) High Si content is believed to play an important part in imparting better
corrosion to the ductile cast iron.
vi) The ductile cast iron investigated exhibited better corrosion resistance when
exposed to NaCl and NaOH, particularly when austenitised at 1000 oC, probably
due to the homogenous structure produced.
vii)High Si content is believed to play an important part in imparting better corrosion to
the ductile cast iron.
5.2 Recommendations for future work
Further study can be carried out to enhance the current study and the following
areas are recommended for further investigation:
i. Use micro-electrochemical methods to study the mechanisms of localized
corrosion processes on small areas of metals.
ii. Conduct immersion test for long period to investigate the corrosion behavior
of Ductile Cast Iron.
iii. Study on other type of Ductile Cast Iron and Heat treatment processes .
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