Effect Of Chromium Carbide (Cr23C6) Formation On Microstructure And Corrosion Passivity Behavior Of AISI 304 Stainless Steel.

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

EFFECT OF CHROMIUM CARBIDE (Cr23C6) FORMATION ON

MICROSTRUCTURE AND CORROSION PASSIVITY BEHAVIOR OF AISI 304 STAINLESS STEEL

This report submitted in accordance with requirement of the Universiti Teknikal Malaysia Melaka (UTeM) for the Bachelor Degree of Manufacturing Engineering

(Engineering Materials) with Honours.

By

SITI FAIZAH BINTI MAD ASASAARI

FACULTY OF MANUFACTURING ENGINEERING 2010

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS TESIS*

TAJUK: EFFECT OF CHROMIUM CARBIDE (Cr23C6) FORMATION ON MICROSTRUCTURE AND CORROSION PASSIVITY BEHAVIOR OF AISI 304 STAINLESS STEEL

SESI PENGAJIAN: 2009/2010

Saya SITI F AIZAH BINTI MAD ASASAARI

mengaku membenarkan tesis (PSM/Sarjana/Doktor Falsafah) ini disimpan di Perpustakaan Universiti Teknikal Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

1. Tesis adalah hak milik Universiti Teknikal Malaysia Melaka .

2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan untuk tujuan pengajian sahaja.

3. Perpustakaan dibenarkan membuat salinan tesis ini sebagai bahan pertukaran antara institusi pengajian tinggi.

4. **Sila tandakan (-1)

D

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan atau kepentingan Malaysia yang termaktub di dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan oleh organisasi /badan di mana penyelidikan dijalankan)

Disa;Jleh:

(SIT I FAIZAH MAD ASASAARI) (DR. MOHD WARIKH B. ABD RASHID)

Alamat Tetap:

B3, Kompleks Niaga Parit Raja, 86400 Parit Raja, Batu Pahat, Johor.

Tarikh: 20 May 2010

OR. MOHO WARIKH BIN ABO. JlltASHI Cop Rasmi 7imNlan De/can (Penvelidik:fn & Pengajlan Siswa ah)

Fakultf KeJuruteraan Pembuatan Universiti Teknikal Malaysia Melaka

Tarikh:

* Tesis dimaksudkan sebagai tesis bagi ljazah Doktor Falsafah dan Sarjana secara penyelidikan, atau disertasi bagi pengajian secara kerja kursus dan penyelidikan, atau Laporan Projek Sarjana Muda (PSM) ** Jika tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi berkenaan dengan menyatakan sekali sebab dan tempoh tesis ini perlu dikelaskan sebagai SULIT atau TERHAD.

DECLARATION

I hereby, declared this report entitled “Effect of Chromium Carbide (Cr23C6)

Formation on Microstructure and Corrosion Passivity Behavior of AISI 304 Stainless Steel” is the results of my own research except as cited in references.

Signature :

Author’s Name : SITI FAIZAH BINTI MAD ASASAARI Date : 16th April 2010

APPROVAL

This report is submitted to the Faculty of Manufacturing Engineering of UTeM as a partial fulfillment of the requirements for the degree of Bachelor of Manufacturing Engineering (Engineering Materials) with Honours. The member of the supervisory committee is as follow:

………..……… (DR. MOHD WARIKH BIN ABD. RASHID)

i

ABSTRACT

This project aimed to study the effect of chromium carbide, Cr23C6 formation on the

microstructure and corrosion resistance behavior of AISI 304 stainless steels in 3.5 % NaCl solution. Five specimens of AISI 304, 15 mm x 15 mm x 2 mm each size with various heating time, 5 seconds, 10 seconds, 20 seconds, 40 seconds, and 60 seconds respectively are used. All specimens are immersed in 3.5 % NaCl solution, for cyclic potentiodynamic polarization technique with a scan rate of 0.5 mV/s, then, the corrosion behavior of AISI 304 was analyzed. The Cr23C6 formation on the

microstructure of AISI 304 to resist pitting corrosion is studied by exposed the specimens to the heating time and five different pH solutions which is pH 3, pH 5, pH 7, pH 9, and pH 12. The number of Cr23C6 formation is high at 60 s of heating

time by observing on the optical microscope. X-ray diffraction (XRD) analysis is used to study the Cr23C6 formation that presence on the structure of AISI 304. The

increase in number and size of Cr23C6 performed in the structure of AISI 304, the

corrosion resistance of the specimens decrease. At high heating time and more acidic solution, the pitting corrosion attack is increased. Based on the cyclic polarization scan, it is clear that at 60 s, the value of high potential stressis 0.325 V, decreases from 0.420 V as performed by 5 s heating time. While for the effect of pH solution, specimen immersed in pH 3 shows the lowest potential, 0.32 V as performed by specimen number one.

ii

ABSTRAK

Projek ini bertujuan untuk mempelajari pengaruh pembentukan kromium karbida, Cr23C6 terhadap mikrostruktur dan perilaku rintangan pengaratan bagi keluli tahan

karat AISI 304 dalam larutan 3.5 % NaCl. Lima spesimen AISI 304 bersaiz 15 mm x 15 mm x 2 mm dipanaskan pada pelbagai masa pemanasan iaitu 5 saat, 10 saat, 20 saat, 40 saat, dan 60 saat. Kesemua spesimen telah direndam dalam larutan 3.5 % NaCl dengan kadar imbasan sebanyak 0.5 mV/s bagi teknik kitaran potentiodinamik polarisasi. Dengan itu, perilaku pengaratan bagi AISI 304 dianalisis. Pembentukan Cr23C6 pada mikrostruktur AISI 304 dipelajari untuk menangani pengaratan bopeng

yang terdedah pada spesimen apabila didedahkan dengan masa pemanasan yang berbeza dan lima larutan pH yang berbeza iaitu pH 3, pH 5, pH 7, pH 9 dan pH 12. Jumlah pembentukan Cr23C6 adalah tinggi pada masa 60 saat dengan mengamati

pembentukannya pada mikroskop optik. Analisis pembelauan X-ray (XRD) digunakan untuk mengkaji pembentukan Cr23C6 yang hadir pada struktur AISI 304.

Peningkatan jumlah dan saiz Cr23C6 pada struktur AISI 304 menunjukkan rintangan

pengaratan semakin menurun. Pada masa pemanasan yang tinggi dan keadaan berasid, serangan pengaratan bopeng meningkat. Berdasarkan imbasan kitaran polarisasi, jelas menunjukkan bahawa pada masa pemanasan 60 saat, nilai potensi stres menurun sebanyak 0.325 V dari 0.420 V iaitu ketika 5 saat masa pemanasan. Manakala, untuk pengaruh larutan pH, spesimen yang direndam pada pH 3 menunjukkan potensi terendah iaitu 0.32 V untuk spesimen nombor satu.

iii

iv

ACKNOWLEDGEMENT

Alhamdulillah, thank to Allah for giving me a chance to complete this report with successfully.

First and foremost I offer my sincerest gratitude to my supervisor, Dr. Mohd Warikh Bin Abd Rashid, who has supported me throughout my final year project and report writing with his patience and knowledge that he give to me in this research. Thank you to the other lecturers, technicians, and staff that help me in fulfilling this final year project and conducted me during the experimental process in the laboratory.

My colleagues from Bachelor Degree of Manufacturing in Engineering Materials, Mohd Ashaari Omar, Nur Farizan Ayoob, Nikmah Abd. Muin, Nurhashima Shafiee, Nur Rifhan Rosli, and Nurul Aina Abd Talib to help me in giving a lot of idea and information about this research.

I would also like to say thanks to my beloved parent for supporting me and providing a space for me in which to complete my writing up.

v

TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables ix

List of Figures x

List of Abbreviations xii

List of Symbol xiii

1.0 INTRODUCTION 1

1.1 Background of Study 1

1.2 Problem Statement 2

1.3 Objectives 2

1.4 Scope of Study 3

2.0 LITERATURE REVIEW 4

2.1 Stainless Steel 4

2.1.1 AISI 304 Stainless Steel 5

2.1.1.1 Mechanical, Thermal and Electrical Properties of AISI 304

5

vi

2.2 Passivity 8

2.2.1 Corrosion Phenomena on Passivated Metal Surfaces 9

2.2.2 Passivity Breakdown 9

2.3 Corrosion Behavior 9

2.4 Optical Microscope 10

2.4.1 Chemical and Microstructure Analysis 10

2.5 Previous Study on Corrosion Behavior of AISI 304 11

2.6 Corrosion of Stainless Steel 14

2.6.1 Types of Corrosion 15

2.6.1.1 Pitting Corrosion 15

2.6.1.2 Intergranular Corrosion 15

2.6.1.3 Galvanic Corrosion 16

2.6.1.4 Crevice Corrosion 17

2.6.1.5 Stress Corrosion Cracking 17

2.6.1.6 Erosion Corrosion 17

2.6.1.7 Selective Leaching Corrosion 18

2.6.1.8 Uniform Corrosion 19

2.6.2 Factors Affecting Corrosion 19

2.6.2.1 Effect of Temperature 19

2.6.2.2 Effect of pH 19

2.6.3 Effect of Chloride Environments 20

2.7 Corrosion Tests 21

2.7.1 Electrochemical Methods 21

2.7.2 Corrosion Rates 24

vii

3.0 METHODOLOGY 28

3.1 Introduction 28

3.2 Specimens Size Preparation 28

3.3 Sample Preparation 30

3.3.1 Grinding and Polishing Method 34

3.4 Corrosion Test 35

3.4.1 Electrochemical Test 36

3.4.1.1 Conducting the Electrochemical Test Experiment 37

3.4.1.2 Sample and Solution Preparation 37

3.4.1.3 Experimental Analysis of Electrochemical Test 38

4.0 RESULTS AND DISCUSSION 39

4.1 Introduction 39

4.2 Electrochemical Results 39

4.2.1 Effect of Temperature on Corrosion Behavior of AISI 304 40

4.2.2 Effect of pH Solution on Corrosion Behavior of AISI 304 44

4.3 Characterization of Corrosion Attack 46

4.4 Microstructural Analysis 49

4.5 X-ray Diffraction Analysis 50

4.6 Hardness of Tested Specimen 53

5.0 CONCLUSION AND RECOMMENDATION 55

5.1 Conclusion and Finding 55

5.2

viii

REFERENCES 57

APPENDICES

A Gantt Chart PSM 1

B Gantt Chart PSM 2

C Cyclic Polarization Scan for Temperature Effect

ix

LIST OF TABLES

Table 2.1 Chemical composition of stainless steels 5

Table 2.2 Mechanical properties of AISI 304 6

Table 2.3 Chemical composition (in weight percent) of AISI 304 SS 6

Table 3.1 Summary of sample preparation for temperature and pH effect 31

Table 3.2 Preparation Method (ASTM E3) 33

Table 3.3 Sample Preparation of pH Effect at specific aqueous solution and condition

38

Table 3.4 Temperature Effect on Aqueous Solution 38

Table 4.1 The electrochemical parameters where Ecorr, icorr, and Epit collected in the cyclic potentiodynamic polarization curves

43

x

LIST OF FIGURES

Figure 2.1 Anodic polarization and overvoltage curve in H2SO4 8

Figure 2.2 (A) and (B) shows the backscattered SEM images of AISI 304

stainless steel before electrochemical test

(C) and (D) shows the backscattered SEM images of AISI 304 stainless steel after electrochemical test

11

Figure 2.3 SEM analysis of pit formed around MnS inclusions of AISI 304 specimen after electrochemical polarization test

12

Figure 2.4 SEM analysis of a pit formed in stainless steel after polarized close to Epit

13

Figure 2.5 Typical corrosion morphology of as-received samples 14

Figure 2.6 Effect of pH on the corrosion rate 20

Figure 2.7 Potentiodynamic polarization behavior of the three alloys in deaerated 0.5 M NaCl solution at 80°C using scan rate 0.5 mV/s

21

Figure 2.8 Wiring diagram for potentiostatic experiments 22

Figure 2.9 Overvoltage curves and corresponding polarization curves 23

Figure 2.10 Polarization curves in non carbonated saturated Ca(OH)2 solutions with 0.5 % NaCl of AISI 304 stainless steel before and after welding as respect to heating exposure

26

Figure 2.11 Cyclic Polarization Curves for AISI 304 after expose in 3.5 % NaCl solution

27

Figure 3.1 Flowchart of research study on AISI 304 29

Figure 3.2 Illustration of AISI 304 specimens’ size in 3D view 30 Figure 3.3 (a) Side view of AISI 304 specimen

(b) and (c) Actual length and width of AISI 304 specimen which indicate (15 mm x 15 mm) respectively

xi

Figure 3.4 (a) The grinding machine used in the laboratory testing

(b) The sand paper grit used in this practice expressed in the ANSI system units

33

Figure 3.5 Grinding process performed on the rotating wheels 34

Figure 3.6 Polishing process with diamond powder 35

Figure 3.7 Process Flow of Corrosion Test 36

Figure 3.8 Electrochemical Polarization Test Setup 36

Figure 3.9 Instrumentation setup 37

Figure 4.1 Corrosion attack of AISI 304 stainless steel at various heating temperatures

41

Figure 4.2 Analysis of high potential stress for AISI 304 specimens 42

Figure 4.3 Polarization scan of AISI 304 in various pH solutions 46

Figure 4.4 Effect of pH on the potential value of AISI 304 47

Figure 4.5 Microstructure of AISI 304 appears on optical microscope

(a) As-received specimen after exposed to oxalic acid

(b) 20 seconds after exposed to heat of oxy acetylene

48

Figure 4.6 Microstructure of AISI 304 after exposed to heat for 60 seconds 48

Figure 4.7 Microstructure of welded AISI 304 stainless steel

(a) Heat affected zone

(b) Weld Metal as refer to exposed area

50

Figure 4.8 Identification of Cr23C6 presence in the structure of AISI 52

xii

LIST OF ABBREVIATIONS

AISI - American Iron and Steel Institute

ANSI - American National and Standard Institute ASTM - American Society for Testing and Materials SAE - Society for Automotive Engineers

UNS - Unified Numbering System SEM - Scanning Electron Microscopy OM - Optical Microscope

EDS - Energy Dispersive X-Ray Spectroscopy XRD - X-ray Diffraction

EBDS - Electron Backscatter Diffraction DC - Direct Current

xiii

LIST OF SYMBOLS

Ecorr - corrosion potential

Epit - pitting potential

Erep - repassivation potential

iapp - applied current density

icorr - current density

βa - anodic Tafel slope

βc - cathodic Tafel slope

iox - oxidation current density

Iox - oxidation current

F - Faraday’s constant (96,487 C/gram)

n - number of electrons

A - surface area

I - circuit current

t - time in Coulombs (C)

d - metal or alloy density

ni - number of electron of component ith element of alloy

fi - mass fraction of component ith of alloy

Mi - atomic weight of element ith of alloy

EW - equivalent weight

pH - - log a (H+) HCl - hydrochloric acid H2SO4 - sulfuric acid NaCl - sodium chloride SiC - silicon carbide S1 - specimen one

C - capacitance V - voltage

T - temperature (oC)

M - molar

1

CHAPTER 1

INTRODUCTION

1.1 Background of Study

Stainless steels are an important class of alloys. Their importance is revealed in a wide range of applications that rely on their use. From low end applications, like cooking utensils and furniture, to very sophisticated ones, such as space vehicles (Lo, K.H., et al., 2009), the use of stainless steels is crucible. Stainless steels are used for consumer products, machinery, architectural and military applications, and for equipment in the petroleum, chemical, aerospace, power, and process industries. In fact, the omnipresence of stainless steels in our daily life makes it impossible to specify their applications. In order to impart stainlessness to steels, chromium (Cr) must be added to at least about 11 wt% (Lo, K.H., et al_{., 2009). At this Cr level, an} adherent, self-healing chromium oxide can form on the steel surface in relatively benign environments. Stainless steel type 304 containing 18 % chromium, 8 % nickel and also known by the UNS Number S30400 (Schweitzer, P.H., 2007), they do in fact suffer from certain types of corrosion. Corrosion can cause a variety of problems, depending on the applications (Steinberg, A.M., 2008). Corrosion is an expensive process which leads to enormous damage for modern industrial societies. The serious consequences of the corrosion process have become a problem of worldwide significance where due to this problem, more cost had been use for research and development of corrosion engineering as to control corrosion on the material. Hence, this study will introduce the fundamentals and the electrochemical basis of the corrosion of metals in an aqueous environment and the corrosion passivity behavior on AISI 304.

2

1.2 Problem Statement

A lot of stainless steel components are used at elevated temperatures. Upon high-temperature exposure, anumerous of phases may precipitate in the various classes of stainless steels especially AISI 304. Some of these phases, like the Cr23C6, may be

common to all classes, but some of them are not. High corrosion resistance of AISI 304 is primarily attributed to the passive oxide film formed on its surface that exposed to an aqueous solution and chromium oxide enrichment at the metal film interface (Marcus, P., 2002). Formation of grain boundary carbides, with an attendant decrease in Cr content in their regions, degrades corrosion resistance and isknown to be the main culprit for causing sensitization. Susceptibility of the heat applied on stainless steels to intergranular corrosion has been known for a long time (Kokawa, H. and Kuwana, T., 1992), and is principally caused by the diffusion of carbon in the steel to the grain boundaries in the sensitized region. The interrelationship between Cr23C6 and grain boundary irregularity has been studied in detail in several recent

works. Hong and Nam (2002) have discovered that grain boundary irregularity occurs before Cr23C6 precipitation and exerts a dramatic effect on its formation in

austenitic stainless steels. Based on the previous research, the presents of Cr23C6 in

the microstructure of AISI 304 have been affecting the corrosion behavior of this material, which by means, the number of Cr23C6 present will tend to localize

corrosion. This occurs at a short distance from the base metal. Chromium carbide precipitates form at the boundaries with the result that regions adjacent to the grain boundaries are depleted of the chromium necessary for passivity. However, the resistance of this passive film is determined by the environmental effects which exposed to the stainless steel. In this study, AISI 304 will be exposed to heat of oxy acetylene at various times and the corrosion resistance of AISI 304 is analyzed as to study the effect of temperature by using electrochemical test. It becomes clear that the best methods to study susceptibilities to this form of corrosion are the electrochemical techniques.

3

1.3 Objectives

(1) To study the effect of chromium carbide (Cr23C6) formation at the grain boundaries of AISI 304 stainless steel.

(2) To study the corrosion behavior of AISI 304 from electrochemical technique due to the temperature effect and pH effect.

1.4 Scope of Study

The scope of this project is to study the corrosion passivity behavior of AISI 304 towards electrochemical technique in an aqueous environment. The purpose of electrochemical technique is to investigate corrosion properties of AISI 304 due to the temperature effect and pH effect. Five samples of various pH values are exposed to 3.5 % NaCl solution to study the corrosion behavior of AISI 304 in an acidic, neutral, and alkali solution. The as-received specimens of AISI 304 is exposed to heat of oxyacetylene gas welding and another five specimens ranging from 5 seconds, 10 seconds, 20 seconds, 40 seconds, and 60 seconds also exposed to the same heat of oxyacetylene gas welding to investigate the effect of temperature on microstructure and corrosion properties of this materials. The study will focused on the microstructure of AISI 304 such the present of Cr23C6 formation by using optical

microscope (OM) and X-ray diffraction analysis (XRD) is used to determine the peak corresponding to the presence of Cr23C6.

4

CHAPTER 2

LITERATURE REVIEW

This section will reviewed the study of AISI 304 stainless steel properties and the application of electrochemical technique to corrosion research. This literature survey provides information related to corrosion phenomena on passivated metal surfaces and its finding on AISI 304 stainless steel corrosion behavior.

2.1 Stainless Steels

The main reason for existence of the stainless steels is their resistance to corrosion. Chromium (Cr) is the main alloying element, and the steel should contain at least 11 percent (Fontana, M.G., 1987). Cr is a reactive element, but it passivates and exhibit excellent resistance to many environments. Stainless steels are characterized by their corrosion resistance, high strength and ductility referred to mechanical properties, and high Cr content. They are called stainless because in the presence of oxygen, they develop a thin, hard, adherent film of chromium oxide that protects the metal from passivation corrosion. Stainless steels generally are divided into five types decomposed of austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, precipitation-hardening, and duplex structure (Kalpakjian, S., 2006). Table 2.1 lists the compositions of stainless steels. The American Iron and Steel Institute (AISI) type numbers shown designate wrought compositions. A recent study by the Society for Automotive Engineers (SAE) and the American Society for Testing Materials (ASTM) resulted in the Unified Numbering System (UNS). UNS numbers are also listed in the Table 2.1 where the letter S identifies stainless steels.

5

Table 2.1: Chemical compositions of stainless steels (Fontana, M.G., 1987)

AISI Type

UNS Number

Carbon Content (%)

Chromium Content (%) 304 S 30400 0.08 maximum 18 - 20 316 S 31600 0.10 maximum 16 - 18

2.1.1 AISI 304 Stainless Steel

The most common stainless steel used in industries is AISI 304 stainless steel. Type 304 stainless steel is a T 300 series stainless steel austenitic. AISI 304 stainless steel is the most common austenitic grades, containing approximately 18 percent chromium and eight percent nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the milder chemicals. It has high corrosion or oxidation resistance due to the chromium content, it has a deep drawing quality, excellent toughness, low temperature properties responding well to hardening by cold working and it is easy to clean, fabrication and has a beauty appearance.

2.1.1.1Mechanical, Thermal and Electrical Properties of AISI 304

Table 2.2 lists mechanical properties of AISI 304. The high strength materials exhibit good strength. High hardness is desirable for wear and some applications where resistance to corrosion is required. The austenitic steels retain good ductility and impact resistance at very low temperatures. Stainless steel, an alloy of Cr, nickel (Ni) and iron (Fe), requires at least 12 % Cr for passivity. If stainless steel is heated to a high temperature such as 425 degree celcius (oC), chromium carbide precipitates will start to form along grain boundaries, leaving a zone depleted of Cr. The precipitates will dissolve back into the grain structure when heated above 850 oC and fast cooled to room temperature.

6

Table 2.2: Mechanical properties of AISI 304 (Fontana, M.G., 1987)

2.1.1.2Effect of Alloying Compositions

Table 2.3 shows the chemical composition of AISI 304 which decomposed of chromium, manganese, silicon, carbon, and nickel. The element that can influence the corrosion behavior and properties of AISI 304 is discussed.

Table 2.3: Chemical composition of AISI 304 stainless steels (Schweitzer, P.A., 2006)

Element Carbon Manganese Silicon Chromium Nickel

Weight % 0.08 maximum 2.00 maximum 1.00

maximum 18 - 20 8 - 10

Chromium

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance or to improve high temperature strength. It is added for wear resistance and most importantly for corrosion resistance of AISI 304. Chromium has a tendency to increase hardness penetration. When five percent chromium or more is used in conjunction with manganese, the critical quenching speed is reduced to the point that the steel becomes air hardening. Chromium can also increase the toughness of steel, as well as the wear resistance.

Material Condition

Tensile strength,

lb/in.2

Yield point, lb/in.2 0.2 % offset

Elongation, % in 2 in.

Hardness

Rockwell Brinell

7

Nickel

Nickel is used as alloying compositions in AISI 304 to improve low-temperature toughness and to increase hardenability. It appears to reduce the sensitivity of a stainless steel to variations in heat treatment and to distortion and cracking during quenching. Nickel is particularly effective when used in combination with chromium and molybdenum in producing AISI 304 that has high strength, toughness and hardenability.

Manganese

Manganese always present in steels to reduce the negative effects of impurities carried out forward from the production process for example sulfur embrittlement (Schweitzer, P.A., 2006). Manganese combines with sulfur to form manganese sulfides stringers. It considered having a detrimental effect on the general corrosion resistance of stainless steels. Manganese also used in stainless steels in order to improve hot ductility and increase the solubility of nitrogen and is used to obtain high nitrogen contents in austenitic stainless steels.

Carbon

Carbon is the most important single alloying element in steel. Toughness and ductility are reduced by increases in carbon content. The hardness of AISI 304 stainless steel is increased by raising the carbon content of stainless steel.

2 1.2 Problem Statement

A lot of stainless steel components are used at elevated temperatures. Upon high-temperature exposure, anumerous of phases may precipitate in the various classes of stainless steels especially AISI 304. Some of these phases, like the Cr23C6, may be common to all classes, but some of them are not. High corrosion resistance of AISI 304 is primarily attributed to the passive oxide film formed on its surface that exposed to an aqueous solution and chromium oxide enrichment at the metal film interface (Marcus, P., 2002). Formation of grain boundary carbides, with an attendant decrease in Cr content in their regions, degrades corrosion resistance and isknown to be the main culprit for causing sensitization. Susceptibility of the heat applied on stainless steels to intergranular corrosion has been known for a long time (Kokawa, H. and Kuwana, T., 1992), and is principally caused by the diffusion of carbon in the steel to the grain boundaries in the sensitized region. The interrelationship between Cr23C6 and grain boundary irregularity has been studied in detail in several recent works. Hong and Nam (2002) have discovered that grain boundary irregularity occurs before Cr23C6 precipitation and exerts a dramatic effect on its formation in austenitic stainless steels. Based on the previous research, the presents of Cr23C6 in the microstructure of AISI 304 have been affecting the corrosion behavior of this material, which by means, the number of Cr23C6 present will tend to localize corrosion. This occurs at a short distance from the base metal. Chromium carbide precipitates form at the boundaries with the result that regions adjacent to the grain boundaries are depleted of the chromium necessary for passivity. However, the resistance of this passive film is determined by the environmental effects which exposed to the stainless steel. In this study, AISI 304 will be exposed to heat of oxy acetylene at various times and the corrosion resistance of AISI 304 is analyzed as to study the effect of temperature by using electrochemical test. It becomes clear that the best methods to study susceptibilities to this form of corrosion are the electrochemical techniques.

3 1.3 Objectives

(1) To study the effect of chromium carbide (Cr23C6) formation at the grain boundaries of AISI 304 stainless steel.

(2) To study the corrosion behavior of AISI 304 from electrochemical technique due to the temperature effect and pH effect.

1.4 Scope of Study

The scope of this project is to study the corrosion passivity behavior of AISI 304 towards electrochemical technique in an aqueous environment. The purpose of electrochemical technique is to investigate corrosion properties of AISI 304 due to the temperature effect and pH effect. Five samples of various pH values are exposed to 3.5 % NaCl solution to study the corrosion behavior of AISI 304 in an acidic, neutral, and alkali solution. The as-received specimens of AISI 304 is exposed to heat of oxyacetylene gas welding and another five specimens ranging from 5 seconds, 10 seconds, 20 seconds, 40 seconds, and 60 seconds also exposed to the same heat of oxyacetylene gas welding to investigate the effect of temperature on microstructure and corrosion properties of this materials. The study will focused on the microstructure of AISI 304 such the present of Cr23C6 formation by using optical microscope (OM) and X-ray diffraction analysis (XRD) is used to determine the peak corresponding to the presence of Cr23C6.

4

CHAPTER 2

LITERATURE REVIEW

This section will reviewed the study of AISI 304 stainless steel properties and the application of electrochemical technique to corrosion research. This literature survey provides information related to corrosion phenomena on passivated metal surfaces and its finding on AISI 304 stainless steel corrosion behavior.

2.1 Stainless Steels

The main reason for existence of the stainless steels is their resistance to corrosion. Chromium (Cr) is the main alloying element, and the steel should contain at least 11 percent (Fontana, M.G., 1987). Cr is a reactive element, but it passivates and exhibit excellent resistance to many environments. Stainless steels are characterized by their corrosion resistance, high strength and ductility referred to mechanical properties, and high Cr content. They are called stainless because in the presence of oxygen, they develop a thin, hard, adherent film of chromium oxide that protects the metal from passivation corrosion. Stainless steels generally are divided into five types decomposed of austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, precipitation-hardening, and duplex structure (Kalpakjian, S., 2006). Table 2.1 lists the compositions of stainless steels. The American Iron and Steel Institute (AISI) type numbers shown designate wrought compositions. A recent study by the Society for Automotive Engineers (SAE) and the American Society for Testing Materials (ASTM) resulted in the Unified Numbering System (UNS). UNS numbers are also listed in the Table 2.1 where the letter S identifies stainless steels.

5

Table 2.1: Chemical compositions of stainless steels (Fontana, M.G., 1987) AISI

Type

UNS Number

Carbon Content (%)

Chromium Content (%)

304 S 30400 0.08 maximum 18 - 20

316 S 31600 0.10 maximum 16 - 18

2.1.1 AISI 304 Stainless Steel

The most common stainless steel used in industries is AISI 304 stainless steel. Type 304 stainless steel is a T 300 series stainless steel austenitic. AISI 304 stainless steel is the most common austenitic grades, containing approximately 18 percent chromium and eight percent nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the milder chemicals. It has high corrosion or oxidation resistance due to the chromium content, it has a deep drawing quality, excellent toughness, low temperature properties responding well to hardening by cold working and it is easy to clean, fabrication and has a beauty appearance.

2.1.1.1Mechanical, Thermal and Electrical Properties of AISI 304

Table 2.2 lists mechanical properties of AISI 304. The high strength materials exhibit good strength. High hardness is desirable for wear and some applications where resistance to corrosion is required. The austenitic steels retain good ductility and impact resistance at very low temperatures. Stainless steel, an alloy of Cr, nickel (Ni) and iron (Fe), requires at least 12 % Cr for passivity. If stainless steel is heated to a high temperature such as 425 degree celcius (oC), chromium carbide precipitates will start to form along grain boundaries, leaving a zone depleted of Cr. The precipitates will dissolve back into the grain structure when heated above 850 oC and fast cooled to room temperature.

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Table 2.2: Mechanical properties of AISI 304 (Fontana, M.G., 1987)

2.1.1.2Effect of Alloying Compositions

Table 2.3 shows the chemical composition of AISI 304 which decomposed of chromium, manganese, silicon, carbon, and nickel. The element that can influence the corrosion behavior and properties of AISI 304 is discussed.

Table 2.3: Chemical composition of AISI 304 stainless steels (Schweitzer, P.A., 2006)

Element Carbon Manganese Silicon Chromium Nickel

Weight %

0.08 maximum

2.00 maximum

1.00

maximum 18 - 20 8 - 10

Chromium

Chromium is commonly added to steel to increase corrosion resistance and oxidation resistance or to improve high temperature strength. It is added for wear resistance and most importantly for corrosion resistance of AISI 304. Chromium has a tendency to increase hardness penetration. When five percent chromium or more is used in conjunction with manganese, the critical quenching speed is reduced to the point that the steel becomes air hardening. Chromium can also increase the toughness of steel, as well as the wear resistance.

Material Condition

Tensile strength,

lb/in.2

Yield point, lb/in.2 0.2 % offset

Elongation, % in 2 in.

Hardness

Rockwell Brinell

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Nickel

Nickel is used as alloying compositions in AISI 304 to improve low-temperature toughness and to increase hardenability. It appears to reduce the sensitivity of a stainless steel to variations in heat treatment and to distortion and cracking during quenching. Nickel is particularly effective when used in combination with chromium and molybdenum in producing AISI 304 that has high strength, toughness and hardenability.

Manganese

Manganese always present in steels to reduce the negative effects of impurities carried out forward from the production process for example sulfur embrittlement (Schweitzer, P.A., 2006). Manganese combines with sulfur to form manganese sulfides stringers. It considered having a detrimental effect on the general corrosion resistance of stainless steels. Manganese also used in stainless steels in order to improve hot ductility and increase the solubility of nitrogen and is used to obtain high nitrogen contents in austenitic stainless steels.

Carbon

Carbon is the most important single alloying element in steel. Toughness and ductility are reduced by increases in carbon content. The hardness of AISI 304 stainless steel is increased by raising the carbon content of stainless steel.