Download - CORROSION BEHAVIOUR OF DUCTILE CAST IRON MOHAMED

i

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.

v

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




4.13 Bar chart of icorr in 10% NaOH at 24±2°C 78

4.14 chart of icorr in 10% NaOH at 850°C 78




4.18 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


10% NaOH at 24°C 84


10% NaOH at 24°C 86


10% NaOH at 24°C 87


10% NaOH at 24°C 88


10% NaOH at 24°C 89


10% NaOH at 24°C 90

xvii


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


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


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


7 shows a summary of immersion test.

(a)

(b)

: Photographs showing (a) Immersion test at room

temperature; (b) In oven at 25C

58


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


e of 850 C, not all the sample has transformed to austenite as



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


e of 850 C, not all the sample has transformed to austenite as



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



74


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


77



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



observation of the specimens exposed for 1 day in both NaCl and

NaOH solutions showed that the color of the solution remained colorless.


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



observation of the specimens exposed for 1 day in both NaCl and

the solution remained colorless. It is also


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


(a)

(b)

80


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


(a)

(b)

81



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)




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


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




. It is also evident that there was

no significant degradation in the specimens as shown in the structures for all


ostructures after 7 days



(b)

(c)



(a)

83




(b)

(c)



(a)

84




(b)

(c)



(a

85




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)




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



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


(a)

(b)

(c)



87


Figure 4.27: Optical micrographs


(a)

(b)

(c)



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)


(c) after immersion test in 3.5% NaCl + 10% NaOH at 24°C (672h)

89


3.5% NaCl + 10% NaOH at 24°C (672h)


(c) after immersion test in

(a)

(b)

(c)


(c) after immersion test in 3.5% NaCl + 10% NaOH at 24°C (672h)

90


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 .

REFERENCES

[1] http://engineeronline.ws/Ductile Cast Iron%20Selection.htm

Retrieved on: Mei 29, 2008

[2] R. A Lula, Consultant, Stainless Steel, American Society for Metals, Metals

Park, Ohio, 1986.

[3] J. Gordon Parr & Albert Hanson, An Introduction to Cast Iron, American Society

for Metals, Metals Park, Ohio, 1966.

[4] en. Wikipedia.org/wiki/stainless_steel

Retrieved on 29/5/2008

[5] http://www.bssa.org.uk Retrieved on 29/5/2008

[6] Ashby, Michael F.; & David R. H. Jones [1986] (1992). “Chapter 10”

Engineering, Materials 2, with corrections (in English), Oxford: Pergamon Press,

[7] Peckner, D., and Bernstein, I.M. 1977. Handbook of Ductile Cast Iron, McGraw-

Hill, New York.

[8] Lacombe, P., Baroux, B., and Beranger, G. 1993. Stainless steel, Les Editions de

Physique, Les Ulis, France.

[9] Sedriks, A.J. 1996. Corrosion of Cast Iron, Wiley-Interscience, New York.

[10] Jones, D.A. 1995. Principles and Prevention of Corrosion, 2nd ed., Prentice Hall,

Upper Saddle River, NJ.

[11] http://www.sandmeyersteel.com/3Ductile Iron.htm Retrieved on 22/5/2008

[12] .http://en.wikipedia.org/wiki/Wrought_iron.

[13] Lippold, J. C., Juhas, M. C and Dalder, E. N. C. 1985. The relationship between

microstructure and fracture behavior of fully austenitic Ductile Cast Iron

materials at 4.2K, Metallurgical Transactions.

[14] H. S. Khatak & Baldev Raj, Corrosion of Ductile Cast Iron,

Mechanism, Mitigation and Monitoring, Narosa Publishing House, 2002

94

[15] George E. Linnert., Welding metallurgy:Carbon and alloy steels, 3rd edition,

American Welding Society, New York, 1967.

[16] ASM. 1987. ASM Metals Handbook, 10th ed., Vol. 13., Corrosion, ASM

International, Materials Park OH.

[17] Gooch, T. G. 1996. Corrosion behavior of Ductile Iron journal.

[18] Copson, H.R. 1959. Effect of composition on stress corrosion cracking of some

alloys containing Ni, in Physical Metallurgy of stress corrosion fracture,

Interscience, New York, pp 247-272.

[19] Garner, A. 1983. Pitting corrosion of high Cast Iron in Soil environments,

Journal, 62(1):27-34

[20] Baker, Dr. M, “AFM Study of pitting corrosion initiation in Ductile Cast Iron,”

http://www.surrey.ac.uk/MME/Research/SPM/8-1.html

[21] Corrosion of Cast Iron

http://www.rj-sanitary.com. Retrived on 26/5/2008

[22] Calister, William D., Jr. Materials science and engineering, An introduction,

Second, edition. John Wiley & Sons, Inc. 1991.

[23] Takano, M., Teramoto, K., and Nakayama, T., Corrosion Science, 1981.

[24] Gooch, T.G., Proceedings of Conference “The influence of Microstructure and

Heat Treatment On, Corrosion Behaviour of Ductile Cast Iron”, IIW Annual

Assembly, Tel Aviv, (1975)

[25] Cole, C. L and Jones, J.D., Proceedings of conference on “Ductile Cast Iron”,ISI

Publication 117, (1969) 71.

[26] R. A. Farrar, Ductile Cast Iron 1984, The Institute of Metals, London

[27] Jones. D.A (1996). Principles and Prevention of Corrosion, (2nd ed.).Pipelines of

Ductile Iron

[28] National Association of Corrosion Engineers, Corrosion Basics, An Introduction

Download - CORROSION BEHAVIOUR OF DUCTILE CAST IRON MOHAMED

Top Related