Evaluation On Embrittlement Phenomena For Welded Pressure Vessel Steel Exposed To Hydrogen.

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UNIVERSITI TEKNIKAL MALAYSIA MELAKA

EVALUATION ON EMBRITTLEMENT PHENOMENA FOR

WELDED PRESSURE VESSEL STEEL EXPOSED TO HYDROGEN

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

Engineering (Engineering Materials) with Honours.

SERI IZZATIE BT ABD. AZIZS

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DECLARATION

I hereby, declared this report entitled “Effect of Hydrogen Adsorption to Ductile to Brittle Transition Temperature (DBTT) of Welded Pressure Vessel Steel” is the results

of my own research except as cited in references.

UNIVERSITI TEKNIKAL M ALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PSM

JUDUL: Evaluation on Embrittlement Phenomena for Welded Pressure Vessel Steel Exposed t o Hydrogen

SESI PENGAJIAN: Semester 2 2008/ 2009

Saya SERI IZZATIE BINTI ABD. AZIZS

m engak u m em benarkan laporan PSM / t esis ( Sarj ana/ Dok tor Falsafah) ini disim pan di Perpust akaan Univer sit i Tek nik al Malaysia Melaka ( UTeM) dengan sy arat- sy arat k egunaan seper t i berik ut :

1 . Laporan PSM / t esis adalah hak m ilik Univ ersit i Tek nikal Malay sia Melak a dan penulis.

2 . Perpustakaan Univ er sit i Tek nik al Malay sia Melak a dibenark an m em buat salinan unt uk t uj uan pengaj ian sahaja dengan izin penulis.

3 . Perpustakaan dibenark an m em buat salinan lapor an PSM / t esis ini sebagai bahan per t uk ar an ant ara inst it usi pengaj ian t inggi.

4 . * Sila t andak an (√) SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarj ah keselamat an at au kepent ingan Malaysia yang t ermakt ub di dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang t elah dit ent ukan ol eh organisasi/ badan di mana penyel idikan dij alankan)

(SERI IZZATIE BT ABD. AZIZS) Alamat Tet ap:

No. 149 (F) Tembangau 02, 28380 Kemayan,

Pahang.

Tarikh: 16 Apr il 2009

Disahkan oleh:

(EN. M. HAIDIR B. MASLAN)

Cop Rasmi:

Tarikh: 16 April 2009

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DECLARATION

I hereby, declared this report entitled “Evaluation on Embrittlement Phenomena for Welded Pressure Vessel Steel Exposed to Hydrogen” is the results of my own research

except as cited in references.

Signature :

Author’s Name : Seri Izzatie Binti Abd. Azizs

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APPROVAL

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

……….. (Mr. Mohamad Haidir bin Maslan)

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ABSTRACT

Pressure vessel especially welded pressure vessel steel often failed catastrophically due to hydrogen embrittlement. Some examples of pressure vessels case histories of failure cases are due to hydrogen embrittlement phenomena. In accordance to that reason, evaluation on embrittlement phenomena of welded pressure vessel exposed to hydrogen must be carried out. ASTM A516 Gr 70 carbon steel which is commonly used for pressure vessel is selected for the study. The specimens are submerged arc welded in X-groove type. Electrolysis is carried out to expose the specimen towards hydrogen at three different exposure times which are 5 hours, 7 hours and 17 hours. As comparison, non-hydrogenated specimens are tested. In weldment, there are three main zones; base metal (BM) zone, heat affected zone (HAZ) and weld metal (WM) zone. However, microstructure and mechanical tests are only done on BM and HAZ as they have greater tendency to cause failures due to hydrogen embrittlement. Optical microscope and Scanning Electron Microscope (SEM) analysis are used to study the microstructure of the specimens. From the experiment, the difference between BM and HAZ overall microstructure, grain size and shape are clearly observed. Dislocations are more severely occur in the HAZ region and the hardness of the specimens is increased by the increasing of cathodic charging time. The complexity of HAZ microstructures in a slim region is expected to cause the dislocation in the specimens’ structure and initiate the cracks. Long hydrogen exposure times could cause the decrement in some important properties of the specimens which lead to hydrogen damages.

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ABSTRAK

Dandang bertekanan terutamanya dandang tekanan besi berkimpal seringkali gagal dengan dahsyat disebabkan oleh perapuhan hidrogen. Beberapa contoh kes sejarah kegagalan dandang bertekanan adalah disebabkan oleh fenomena perapuhan hidrogen. Bertepatan dengan alasan itu, taksiran ke atas fenomena perapuhan dandang bertekanan besi berkimpal terdedah kepada hidrogen perlu dijalankan. Besi karbon ASTM A516 Gr 70 yang mana biasa digunakan untuk dandang bertekanan telah dipilih untuk kajian ini. Spesimen-spesimen telah dikimpal separa cantum dalam alur jenis X. Electrolisis telah dijalankan untuk mendedahkan spesimen kepada hidrogen pada tiga masa pendedahan yang berlainan iaitu 5 jam, 7 jam dan 17 jam. Sebagai perbandingan, spesimen tidak terhidrogenasi telah dikaji. Dalam pengimpalan, terdapat tiga zon utama; logam spesimen (BM), zon terkesan haba (HAZ), dan zon kimpalan(WM). Bagaimanapun, ujian mikrostruktur dan mekanikal hanya dijalankan ke atas BM dan HAZ disebabkan mereka lebih cenderung untuk menyebabkan kegagalan berpunca daripada perapuhan hidrogen. Mikroskop optik dan analisis Mikroskop Imbasan Electron (SEM) digunakan untuk mengkaji mikrostruktur spesimen-spesimen. Daripada eksperimen, perbezaan diantara keseluruhan mikrostruktur, saiz bijian dan bentuk bijian BM dan HAZ dapat diperhatikan dengan jelas. Kehelan adalah lebih ketara terjadi di dalam bahagian HAZ dan kekerasan spesimen-spesimen adalah bertambah berdasarkan penambahan masa pngecasan katodik. Kekompleksan mikrostruktur HAZ di dalam kawasan yang sempit telah disyaki menyebabkan kehelan di dalam struktur spesimen-spesimen dan memulakan keretakan. Masa pendedahan hidrogen yang lama boleh menyebabkan penurunan di dalam beberapa sifat-sifat penting spesimen-spesimen yang membawa kepada kerosakan hidrogen.

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DEDICATION

“Special thanks to my parent,dedicated supervisor; En. Mohamad Haidir Bin Maslan,

Examiner; En Jeefferie Bin Abd. Razak, technicians; En. Azhar, En. Farihan, En Asari, En. Azwan, En. Sarman, and En. Naim, my family, my friends and all involved in

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ACKNOWLEDGEMENT

I would like to express my deepest gratitude to my dedicative supervisor, Mr. Mohamad Haidir Bin Maslan and my supportive examiner, En Jeefferie Bin Abd. Razak for relentless effort and endless patience to acquire this project, guidance in every single step in preparing this report and for their advices and encouragement that lead me to complete this report successfully. Unforgettable, thanks to my parents for their financial support, endless moral support and encouragement throughout the course of my study at Universiti Teknikal Malaysia Melaka. Special thanks to all UTeM’s technicians who had lend their hand in order to complete this research. I also would like to express my special thanks to all my housemates for their opinion and help from the beginning until this report is successfully completed. Finally I would like to express my special thanks to all coursemates of 4BMFB for brilliant comments and ideas in improving my research.

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TABLE OF CONTENT

Abstract i

Abstrak ii

Dedication iii

Acknowledgement iv

Table of Content v

List of Tables vii

List of Figures viii

List of Abbreviations x

1.0 INTRODUCTION 1

1.1 Background 1

1.2 Problem Statement 2

1.3 Research Objective 3

1.4 Research Scope 3

2.0 LITERATURE REVIEW 4

2.1 Hydrogen Damage 4

2.1.1 Classification of Hydrogen Damage 6

2.1.2 Hydrogen Embrittlement 7

2.1.3 Embrittlement Mechanism 8

2.1.3.1 Hydrogen-enhanced localized plasticity – HELP 8

2.1.3.2 Hydrogen-enhanced decohesion – HEDE 9

2.1.3.3 Adsorption-induced dislocation emission – AIDE 10

2.1.4 Experimental of Hydrogen Adsorption 10

2.2 Pressure Vessel 13

2.2.1 ASTM A516 Grade 70 14

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2.3.1 Submerged Arc Welding (SAW) 16

2.3.2 Weld Metal 18

2.3.3 Heat Affected Zone (HAZ) 19

3.0 METHODOLOGY 21

3.1 Introduction 21

3.2 Materials: ASTM A516 Gr. 70 21

3.3 Sample Preparation 23

3.3.1 Submerged Arc Welding 24

3.3.1.1Chemical Composition Determination 25

3.3.2 Post Weld Heat Treatment (PWHT) 26

3.4 Exposure to Hydrogen (Electrolysis) 26

3.5 Mechanical Testing 28

3.5.1 Tensile Test 28

3.5.2 Hardness Test 20

3.6 Microscopic Observation 30

4.0 RESULT & DISCUSSION 32

4.1 Introduction 32

4.2 Glow Discharge Spectrometer 32

4.3 Tensile Test 34

4.4 Electrolysis Observation 36

4.5 Hardness 38

4.6 Microstructure Analysis 41

5.0 CONCLUSION & RECCOMMENDATION 45

5.1 Conclusion 45

5.2 Suggestion 45

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APPENDICES A Gantt Chart

B Photomicrograph through BM > HAZ > WM of 5hours hydrogen charging. C Raw Data of Tensile Test Result.

D Material Safety Data Sheet of Sulfuric Acid.

E ASTM E 3 – 01: Standard Guide for Preparation of Metallographic Specimens. F ASTM E 8M – 04: Standard Test Methods for Tension Testing of Metallic

Materials [Metric]

G ASTM E 92 – 82 (Reapproved 2003)e2: Standard Test Method for Vickers Hardness of Metallic Materials

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LIST OF TABLES

2.1 Some examples of hydrogen damages at two different temperatures. 6 2.2 Hydrogen charging parameters from previous research. 12 2.3 Advantages and disadvantages of elements in alloy steel. 14 2.4 Chemical composition (%) in ASTM A516 with thickness <12.5mm 15

2.5 Tensile requirement for ASTM A516 Gr. 70 15

2.6 Recommended current range and wire stick-out for various

diameter of wire. 17

3.1 Electrolysis parameters with variance in exposure time. 26 3.2 Specification of rectangular tensile test specimen. 28 3.3 Preparation method (general use) as suggested by ASTM E3-01. 31

4.1 Elements and their composition in three distinct zones of welded

pressure vessel ASTM A416. 32

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LIST OF FIGURES

2.1 Electrochemical reactions occurring during corrosion of metal M

in hydrochloric acid. 5

2.2 Hydrogen Damages (a) Hydrogen blistering. (b) SCC in weld metal. 6

2.3 Hydrogen atom penetration through metal. 7

2.4 Optical Micrograph showing 45˚ slip band of 70-30 brass (α-brass)

which had been heat treated at 320˚C for 180s. 9

2.5 Brittle intergranular fracture. 9

2.6 Fracture surface observation by SEM showing mixed mode of

fracture of ZRY-4. 10

2.7 Electrolytic cell for hydrogen adsorption. 11

2.8 Illustration of submerged arc welding. 16

2.9 An example of extended stick-out technique using a 2.5in stick-out. 17 2.10 The effect of polarity and current type on weld beads: (a) DC

current with straight polarity; (b) DC current with reverse polarity;

2.11 Microstructures: (a) base metal, (b) heat affected zone,

2.12 Three distinct zone in typical weld joint. 20

2.13 Microstructure of HAZ of actual weldment (a) coarse-grain

martensite with δ-ferrite, (b) coarse-grain martensite, (c) fine-grain

martensite, and (d) intercritical structure. 20

3.1 Flow process of project methodology. 22

3.2 (a) Dimension of specimen before cutting process. (b) welded specimen with classification of 3 distinct zones; Base Metal (BM),

Weld Metal (WM) and Heat Affected Zone (HAZ). 23

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3.4 Schematic of Glow Discharge Spectrometer (GDS) source. 25 3.5 NFPA Diamond of sulfuric acid. (0) in red box represent that H2SO4

is none flammable, (3) in blue means extremely hazardous to the

health, and (2) in yellow is moderate reactivity hazards. 27

3.6 Electrolytic cell for hydrogen adsorption. 27

3.7 Dog-bone tensile specimen. 28

3.8 Universal Tensile Machine (UTM) 29

3.9 Hardness testing machine 29

3.10 Schematic diagram of optical microscope 30

3.11 Abrasive cutting machine. 32

4.1 Graph of weight distribution of elements in welded A516 pressure

vessel steel. 33

4.2 Stress-strain curve of base metal ASTM A516 35

4.3 A516 base metal steel after 17hours exposed to hydrogen. ‘A’ is the initial size of the specimen and ‘B’ is final sized of specimen immersed in H2SO4. Red circle shows level of specimen immersed

in electrolyte and point of size changes. 36

4.4 Specimen with by-products. (a) powder-like by-products

surrounding specimen with exposure time 5H and 7H. (b)crystal-like

by-products for 17H. 37

4.5 SEM micrograph of electrolysis by-products at 5H, 7H and 17H

reaction time, (1000x Magnification). 38

4.6 Hardness of base metal A516 steel exposed to different time of

hydrogen charging. 39

4.7 Mean hardness of A516 steel at different hydrogen-charging time. 39

4.8 Microhardness profile across PWHT 9Cr-1Mo 40

4.9 Micrograph of neutral BM with bands of preutectoid ferrite (white)

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4.10 Micrograph of neutral HAZ microstructure with acicular ferrite surrounded by prior austenite grain boundary and cementite network,

(200x Magnification). 42

4.11 Micrograph of neutral WM microstructure showing grain boundary of ferrite, side-plate ferrite, Widmanstätten ferrite, and bainite, (200x

Magnification). 42

4.12 Photomicrograph of A516 exposed to hydrogen, (200x Magnification). 43 4.13 SEM images of polished and etched HAZ17 (left) and BM17 (right) ,

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LIST OF ABBREVIATIONS

AC - Alternating Current

AIDE - Adsorption-induced dislocation emission AHW - Atomic Hydrogen Arc Welding

ASME - American Society of Mechanical Engineers ASTM - American Society of Testing and Materials AWS - American Welding Society

BCC - Body Cubic Centered BIF - Brittle intergranular fracture

BM - Base Metal

CTTW - Contact Tip to Work CVN - Charpy V-Notch

DBTT - Ductile to Brittle Transition Temperature

DC - Direct Current

DCEN - Direct Current Electrode Negative DCEP - Direct Current Electrode Positive ESO - Electrode Stick-Out

GDS - Glow Discharge Spectrometer GMAW - Gas Metal Arc Welding GTAW - Gas Tungsten Arc Welding HAZ - Heat Affected Zone

HB - Hydrogen Blistering

HE - Hydrogen Embrittlement

HEDE - Hydrogen-enhanced decohesion

HELP - Hydrogen-enhanced localized plasticity LPG - Liquefied Petroleum Gas

MEA - Monoethandamine

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PV - Pressure Vessel

PWHT - Post weld heat treatment SAW - Submerged Arc Welding SCC - Stress Corrosion Cracking SEM - Scanning Electron Microscope SMAW - Shielded Metal Arc Welding TIG - Tungsten Inert Gas Welding

UTeM - Universiti Teknikal Malaysia Melaka UTM - Universal Testing Machine

UTS - Ultimate Tensile Strength VHN - Vickers Hardness Number

WM - Weld Metal

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CHAPTER 1 INTRODUCTION

1.1Background

Hydrogen is one of the mostly known environment which surrounds in human daily lives. It contained in water (H2O), use as fuel including rocket fuel, filling balloon, preparing others useful chemical such as hydrochloric and methanol, and others. However, it also has a wide range of harmful effects on metals; known as hydrogen damages. Occurrences of hydrogen had caused several damages on pressure vessels, since a few decades ago.

Hayes (1996) stated six case histories of pressure vessels failures within 1963 to 1984 in her research study. Three of the failures were clearly caused by hydrogen; John Thompson ammonia converter pressure vessel for ICI Immingham (1965), Robert Jenkins pressure vessel (1970) and Union Oil Co. Amine absorber tower (1984). Both failures at John Thompson and Robert Jenkins pressure vessels were initiated by crack at the heat affected zone (HAZ) of weldment. The probable cause of the original crack was hydrogen cracking. Meanwhile, Union Oil Co. Amine absorber pressure vessel failure was caused by the hydrogen blistering which later formed four cracks at the HAZ. According to Hayes study, it can be concluded that hydrogen penetration is more sensitive towards the HAZ rather than the base metal and the fusion zone. Another case of failure is on December 13, 1984, at the Las Piedras Refinery, Venezuela in which a crack was found in the heat affected zone about 1-1.5 inches from weld and fatigue is dominantly occurred due to hydrogen embrittlement (Abduh, 2008).

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Hydrogen damages can be classified into several types and one of them is Hydrogen Embrittlement (HE). HE occurred when hydrogen is absorbed into the material and results in reduction of its mechanical performance. Propagation of hydrogen atoms into metals will cause severe failures. It can be initiator to others problems and corrosion of the metals as well. Hydrogen could be developed from water vapor in atmosphere, melting metal process, electrolysis in electroplating, welding and others.

Reduction of the mechanical properties such as decreasing in ductility and loss in tensile strength are the main problem occurred from HE. Hence, microscopic examination is carried out in order to study in detail the embrittlement process that causes reduction of materials properties. Microstructure of ASTM A516 metal specimens exposed to hydrogen at three different exposure times will be further observed and explained throughout this research.

Pressure vessel is a closed container used for liquid or gases storage at high pressure. ASTM A516 is one of the steel commonly used for pressure vessel at low and moderate temperature. In this study, welded pressure vessel specimen is used for the determination of hydrogen adsorption effects. The pressure vessel has broad application such as liquid and gases storage in oil refinery, diving cylinder, LPG gas storage and others. Welded pressure vessel has different properties than non-welded pressure vessel and it will be explain in detail throughout this thesis.

1.2Problem Statement

Pressure Vessel steel had been designed so that it capable to operate in the elevated temperature and high pressure environmental service application. However, due to long time exposure to hydrogen environment, hydrogen embrittlement occurred and significantly embrittled the steel. As the pressure vessel is welded, the tendency of hydrogen embrittlement occurs in the weld part could be slightly different from the vessel steel part, because of the differences in chemical composition and some other

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factors. Hence, an investigation is carried out to check the durability of pressure vessel starting with the crack inspection of two types of samples; hydrogen exposed steel and non-hydrogen exposed steel, and then each type of samples are divided into two distinct parts; base metal and heat affected zone. Weld metal will not included in detail for this study as there is very little and almost none failure cases regarding to the weld metal. The microstructure of each samples are evaluated to analyze the differences of hydrogen embrittlement by means of different exposure times. Elements in the samples are determined by using Energy Dispersive X-ray Microanalysis (EDX) which also known as Energy dispersive Spectroscopy (EDS).

1.3Research Objective

a) To evaluate the hydrogen embrittlement mechanism.

b) To analyze the microstructure of two distinct zones in the welded pressure vessel steel and to differentiate their properties.

1.4Research Scope

This study is mainly focused into three major scopes. First is to analyze the welded pressure vessel steel characteristics and its behavior towards the mechanical properties. Microstructure of two out of three distinct zones of the welded pressure vessel steel; base metal (PV steel) and heat affected zone, are further investigated to differentiate their properties. Then, electrolysis is carried out in order to expose the sample towards hydrogen. Finally, the hardness test is done to differentiate the hardness of the zones for the hydrogen exposed and hydrogen exposed specimens. Besides three non-destructive tests; optical microscopy test, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Microanalysis (EDX), were also carried out.

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CHAPTER 2 LITERATURE REVIEW

Hydrogen Embrittlement is a failure phenomenon where the ductile metal becomes brittle due to hydrogen exposure. National Board Bulletin (January 1986) describes the cause of a monoethandamine (MEA) absorber vessel that ruptured in the state of Illinois in 1984, resulting in 17 fatalities and property damage in excess of $100 million (Schmeilski, 1986). This article is describing the affect of welding factors and corrosion factors that contributed to the rupture. Besides that, hydrogen adsorption is investigated over MEA adsorption as another possible factor. Several type of hydrogen damage will be further investigated especially damages which occurred at room temperature. Metallurgical of two different zones in welded pressure vessel of ASTM A516 Gr. 70 will be observed. The following literature review attempt to explain in detail each component included in this research.

2.1Hydrogen Damage

Hydrogen damage is a metal degradation processes cause by the interaction with hydrogen gases (Fontana, 1986). The hydrogen atom is the smallest of all atoms and hydrogen attack is similarly insidious and hidden from the simple inspection techniques. Hydrogen is present practically everywhere, in the atmosphere, several kilometers above the earth and inside the earth. Engineering materials are exposed to hydrogen and they may interact and resulting various kinds of the structural damage. Damaging effects of hydrogen in metallic materials have been known since 1875 when W. H. Johnson reported that some remarkable changes are produced in iron by the action of hydrogen

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and acids (Hayes, 1996). During the intervening years many similar effects have been observed in the different structural materials like steels, aluminum, titanium, zirconium, etc. Because of the technological importance of hydrogen damage, many people explored the nature, causes and the control measures of hydrogen related degradation of metals. Hardening, embrittlement and internal damage are the main hydrogen damage processes in metals. Figure 2.1 schematically shows the mechanism of the degradation begins with the formation of free hydrogen atom at the steel surface. Technically, the hydrogen atoms are developed from the sulphide attack of the process medium at the steel surface. As a result of electrochemical reaction, hydrogen atoms are generated and the steel surfaces are corroded.

Figure 2.1: Electrochemical reactions occurring during corrosion of metal M in hydrochloric acid

(Fontana, 1986)

The process variables for hydrogen occurrence are; (a) Acidic environment with acidity of pH 4.0 and below has greater tendency for hydrogen damage with the presence of H2S. However, if both H2S and cyanide are present, hydrogen damage can occur at acidity level above the pH 7.0. (b) Normally, H2S level above 50ppm is required for the attack. Meanwhile, in the presence of cyanide, H2S threshold level for the attack is reduces remarkably. (c) Basically, hydrogen damage takes place at two temperature regions which are temperatures at the ambient and slightly above 26oC-150oC and at the elevated temperatures. At elevated temperature, moisture does not required to develop the damage.

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2.1.1 Classification of Hydrogen Damage

As stated earlier, hydrogen damage will takes place at the two temperature region; elevated temperatures and at the ambient and slightly above. Ambient hydrogen damage, also known as wet Hydrogen Sulfide (H2S) damage can be classified in terms of a number of distinct classes as shown in Table 2.1.

Table 2.1: Some examples of hydrogen damages at two different temperatures ranges.

Low / Moderate Elevated

i) Hydrogen Embrittlement i) Decarburization ii) Hydrogen Blistering ii) Hydrogen Attack iii) Stress Corrosion Cracking

Stress corrosion cracking is a class of corrosion which is typically develops in hard spots of weld; cracking occur in weld metal itself (Mostert & Sharp, 2005). In SCC, the ductile metal is said to be failed in the brittle manner or known as ductile to brittle transition. Two main conditions must be correlated in SCC are stress is being applied towards the metal together with the presence of hydrogen. Meanwhile, hydrogen blistering (HB) results from the penetration of hydrogen same as HE, instead the effect of the penetration of hydrogen towards the material physical. In HB, hydrogen gas nucleates at the internal defects and inclusions, forming voids which eventually generate enough pressure to locally rupture the metal (Fontana, 1986).

Figure 2.2: (a) Hydrogen blistering. (b) SCC in weld metal. (Mostert and Sharp, 2005)

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Both of the corrosion types shown in Figure 2.2 slightly have potential to be developed in this research. However, more focus are given on the hydrogen embrittlement as the main hydrogen damage assumed to develop rather than both stated earlier. Furthermore, hydrogen embritlement is more prevalent at room temperature and decreases with the increasing temperature (Kim, 2002).

2.1.2 Hydrogen Embrittlement

Hydrogen embrittlement is a form of environmentally assisted failure which is caused by the action of hydrogen often in combination with residual or applied stress resulting in the reduction of the load bearing capacity of a component. Many metallic alloys such as those of Fe, Ni, Al, Ti, Zr, Ta, Hf, Nb, V, W, Mo and U are susceptible to hydrogen embrittlement and in fact no structural alloy is totally immune to this type of embrittlement (Dayal & Parvathavarthini, 2003). According to Kim (2002), the presence of hydrogen in steel reduces the tensile ductility and causes premature failure under the static loads that depends on the stress and time. The mechanism starts with the single hydrogen atoms diffusing through the metal as shown in Figure 2.3.

Figure 2.3: Hydrogen atom penetration through metal (Toyer, 2004).

Typical conditions of hydrogen embrittlement are at the pressure of 10-6 to 10 8 N/m2 and usually observed at -100˚C to 700˚C; most severe near 20˚C as stated by ASTM. Besides that, embrittlement is more severe at low strain rate and in notched or pre-cracked

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