INVESTIGATION OF CHLORIDE STRESS CORROSION CRACKING IN

AUSTENITIC STAINLESS STEEL

MOHAMAD KHAIRUAZLAN BIN KAMARUZAMAN

A report submitted in partial fulfilment of the requirements for award of the degree

of

Bachelor of Mechanical Engineering

Faculty of Mechanical Engineering

UNIVERSITI MALAYSIA PAHANG

JUNE 2013

vii

ABSTRACT

Chloride stress corrosion cracking (CSCC) of AISI 304 austenitic stainless steel are the

most important corrosion processes that affect the performance of this stainless steel. SCC

occurs when susceptible material under the stress is exposed to a specific environment such

as corrosive environment. Since austenitic stainless steel are widely used in power boiler,

nuclear power, chemical plant and marine industry, SCC can cause a major losses to these

industry. The purpose of this project is to study the effect of chloride stress corrosion

cracking in austenitic stainless steel based on the sensitization treatment of the specimens

and different concentration mixtures of sulphuric acid and sodium chloride. Method use in

this study was weight loss method. From the measurement and experiment result, it shows

that mass loss for specimens with sensitization treatments is higher than mass loss for

specimens without sensitization treatment. The experimental result also indicates that SCC

occurs only at certain concentration mixtures of sulphuric acid and sodium chloride. This

was confirmed by using scanning electron microscope and digital weighing balance.

viii

ABSTRAK

Klorida tekanan retak kakisan (CSCC) daripada AISI 304 austenit keluli tahan karat adalah

proses hakisan penting yang memberi kesan kepada prestasi keluli tahan karat ini. SCC

berlaku apabila bahan mudah berkarat di bawah tekanan yang terdedah kepada persekitaran

yang khusus seperti alam sekitar yang menghakis. Disebabkan austenit keluli tahan karat

ini digunakan secara meluas di dalam dandang kuasa, kuasa nuklear, loji kimia dan industri

marin, SCC boleh menyebabkan kerugian besar kepada industri ini. Tujuan projek ini

adalah untuk mengkaji kesan hakisan tegasan klorida keretakan dalam austenit keluli tahan

karat berdasarkan rawatan haba terhadap spesimen dan campuran berbeza kepekatan asid

sulfurik dan natrium klorida. Kaedah yang digunakan dalam kajian ini adalah kaedah

kehilangan jisim spesimen. Dari pengiraan dan keputusan experimen, ia menunjukkan

bahawa kehilangan jisim bagi spesimen dengan rawatan haba adalah lebih tinggi daripada

kehilangan jisim bagi spesimen tanpa rawatan haba. Hasil eksperimen juga menunjukkan

bahawa SCC berlaku hanya pada kepekatan tertentu campuran asid sulfurik dan natrium

klorida. Ini dapat ditentukan dengan menggunakan mikroskop pengimbas elektron dan

penimbang berat digital.

ix

TABLE OF CONTENTS

Page

SECOND EXAMINER’S DECLARATION

SUPERVISOR’S DECLARATION

ii

STUDENT’S DECLARATION iv

DEDICATION v

ACKNOWLEDGEMENTS vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xvii

CHAPTER 1 INTRODUCTION

1.1 Introduction 1

1.2 Background of study 1

1.3 Problem statement 3

1.4 Objectives 3

1.5 Scopes 4

1.6 Thesis outline 4

x

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction 6

2.2 Stress corrosion cracking 6

2.3 Heat treatment 8

2.3.1 Sensitization 9

2.3.2 Annealing 10

2.3.3 Quenching 11

2.4 Austenitic stainless steel 11

2.4.1 AISI 304 stainless steel 12

2.4.2 AISI 316 stainless steel 12

2.5 Types of SCC test specimens 13

2.5.1 U-bend 13

2.5.2 C-ring 13

2.5.3 Bent-beam 14

2.6 Engineered stress corrosion cracking studies 14

2.6.1 Inhibition effect of metal cations to intergranular stress

corrosion cracking of sensitized type 304 stainless steel

14

2.6.2 Influence of local stress on initiation behaviour of stress

corrosion cracking for sensitized 304 stainless steel

15

2.6.3 Effect of sensitizing temperature on stress corrosion

cracking type 316 austenitic stainless steel in hydrochloric acid

17

2.6.4 Stress corrosion susceptibility of 316L stainless steel in H2S 18

xi

and Cl- aqueous solution

CHAPTER 3 METHODOLOGY

3.1 Introduction 20

3.2 Flowchart 21

3.3 Preparation of test specimens 23

3.4 Chemical composition of AISI 304 stainless steel 32

3.5 Preparation of test solution 33

3.5.1 Sodium chloride 33

3.5.2 Sulphuric acid 35

3.5.3 Reaction between sodium chloride and sulphuric acid 36

3.6 Stress corrosion cracking test and measurement 37

3.7 Determination of corrosion rate 38

CHAPTER 4 RESULT AND DISCUSSION

4.1 Introduction 41

4.2 Result analysis 41

4.3 Effect concentration mixtures of sulphuric acid and sodium

chloride on corrosion mass loss of austenic stainless steel 42

xii

4.3.1 Experiment data and corrosion rate (mmpy) 42

4.4 Effect of sensitization on the corrosion mass loss 47

4.4.1 SEM micrograph for specimens without sensitization

treatment 50

4.4.2 SEM micrograph for specimens with sensitization treatment 51

CHAPTER 5 CONCLUSION AND RECOMMENDATION

5.1 Introduction 53

5.2 Conclusion 53

5.3 Recommendation 54

References 55

Appendix A 59

Appendix B 60

Appendix C 61

xiii

LIST OF TABLES

Table No. Title Page

3.1 Chemical composition of AISI 304 stainless steel 33

3.2 Various corrosion test solution 37

4.1 Data for U-bend specimens austenitic stainless steel with

sensitization treatment

43

4.2 Table mixtures of concentrations of NaCl and

Concentration of H2SO4 versus corrosion rate

44

4.3 Data for U-bend specimens austenitic stainless steel

without sensitization heat treatment

45

4.4 Table mixtures of concentrations of NaCl and

Concentration of H2SO4 versus corrosion rate

45

xiv

LIST OF FIGURES

Figure No. Title Page

2.1 SEM fractrograph of SCC showing intergranular fracture

modes

7

2.2 Microstructure of sensitized austenitic stainless steel 10

2.3 Schematic illustration of a constant load testing apparatus

with an in situ observation system

16

2.4 Representative fracture appearances of the sensitized

type 316

18

2.5 Fracture microstructure of the specimen in 210 ppm

H2S and 70 ppm Cl- solution

19

3.1 Project flow chart 22

3.2 Preparation of test specimen flow chart 23

3.3 Corrosion test specimens before shaped into U-bend 24

3.4 Shearing Machine 24

3.5 Bench Drill Machine 25

3.6 Furnace 25

3.7 Grinding Machine 26

3.8 Grinding/Polishing machine 26

3.9 (a) 0.05 micron colloidal silica (b) microid extender 27

3.10 Digital Weighing Balance 27

3.11 Schematic Diagram of U-bend sample preparation stages 29

xv

3.12 Example of typical U-bend specimens dimensions 30

3.13 Stress-strain curve for austenitic stainless steel type 304 31

3.14 U-bend specimen of 304 austenitic stainless steel 32

3.15 Sodium Chloride powder (NaCl) 35

3.16 Concentrated Sulphuric acid (H2SO4) 36

3.17 SCC test 38

3.18 Corrosion rate equation constant 39

3.19 Diagram of the flow of experiment 40

4.1 Graph for data for U-bend specimens with sensitization

treatment

44

4.2 Graph for data for U-bend specimens without

sensitization treatment

46

4.3 Type of corrosion in 304 stainless steel without

sensitization treatment

48

4.4 Type of corrosion in 304 stainless steel with sensitization

treatment

48

4.5 SEM micrograph in AISI 304 stainless steel 49

4.6 SEM micrograph of the cross section of the specimens

with constant concentration of NaCl of 0.5 kmol.m-3.

50

4.7 SEM micrograph of the cross section of the specimens

with constant concentration of NaCl of 1.5 kmol.m-3

51

4.8 SEM micrograph of the cross section of the specimens

with constant concentration of NaCl of 0.5 kmol.m-3

51

4.9 SEM micrograph of the cross section of the specimens

with constant concentration of NaCl of 1.5 kmol.m-3

52

xvi

LIST OF SYMBOLS

δ Delta

ε Strain

Plastic strain

K Constant in corrosion rate equation

T Time of exposure in hours

A Area in cm2

W Mass loss in grams

D Density in g/cm3

xvii

LIST OF ABBREVIATIONS

SCC Stress Corrosion Cracking

Cl- Chloride ions

H2S Hydrogen sulphide

H2SO4 Sulphuric acid

NaCl Sodium chloride

SEM Scanning electron microscope

AISI American Iron and Steel Institute

ASTM American Society for Testing and Materials

NaNO2 Sodium nitrite

Na2SO4 Sodium sulfate

Ca(OH)2 Sodium hydroxide

Cr23C6 Chromium carbide

IGSCC Intergranular stress corrosion cracking

N2 Nitrogen gas

Pt Platinum

AgCl Silver chloride

Ag Silver

CCD Charge-coupled devices

VTR Video tape recording

Cr Chromium

xviii

HCl Hydrochloric acid

Ys Yield strength

UTS Ultimate tensile strength

tss Transition time

tf Time to failure

SSRT Slow strain rate test

PTFE Polytetrafluoroethylene

NaHSO4 Sodium hydrogen sulfate

SO2-

4 Sulphide ion

TGSCC Transgranular stress corrosion cracking

1

CHAPTER 1

INTRODUCTION

1.1 INTRODUCTION

This chapter explains about the background of study, problem statement,

objectives and the scopes of this study. The main purpose for this study can be

identified by referring at the problem statement of this study. Furthermore, the details of

this study and outcome can be achieved on the objectives and its scopes.

1.2 BACKGROUND OF STUDY

One of the major problems in the petroleum refining and the petrochemical

operations is corrosion. Process of corrosion is said to happen when essential properties

within a given material begin to deteriorate, after exposure to elements that recur within

the environment and corrosion of most metals (and many materials for that matter) is

inevitable. All material types are susceptible to degradation. The corrosion of metals

will also occur when the metals exposed to gaseous materials like acid vapors,

formaldehyde gas, ammonia gas, and sulfur containing gases. The best known case of

corrosion is that of the rusting of steel. Corrosion processes are usually electrochemical

in nature, having the essential features of a battery.

Billions of dollars are lost each year because of corrosion and a huge amount of

money is spent on prevention of corrosion and tarnishing of metals. Corrosion causes

2

damage to car bodies, buildings, bridges, iron railings, underground water and sewage

pipes, ships and all objects made of metals. Much of this loss is due to the corrosion of

iron and steel, although many other metals may corrode as well.

When the material under stress is exposed to an environment specific to the

material, stress corrosion cracking (SCC) takes place with the formation of the local

cracking (Satoshi Sunada et al., 2006). The criteria for the stresses are simply that they

have tensile stress and sufficient magnitude. The source of these stresses may be due to

the applied stress, residual stress, and thermal or from the welding. Austenitic stainless

steel type 304 and 316 are the common materials used in a wide range of industries

because of the ability to withstand very many corrosive conditions and high temperature

without the need for protective measures. The presence of the stress element is needed

since stress is one of the requirements in SCC mechanism. As stated by Cottis and

Newman (1993), austenitic stainless steel will not undergoes cracking process or fails

mechanically unless the stress applied is not high enough. Whereas Fathia et al. (2009)

prove that temperature also give an effect to stress corrosion cracking since by

increasing the temperature, the stress corrosion life is decrease as well.

Other than that, other factors that contribute to SCC in stainless steel is

sensitization treatment since sensitization occurs because of the chromium depletion

within or near regions of the grain boundaries in austenitic stainless steels in which

chromium rich carbides precipitate. Sensitization represents both a microchemical and

microstructural condition at an interface. Both heat treatment and thermomechanical

processing appear to have an important influence on sensitization or the degree of

sensitization reached, as well as intergranular stress corrosion cracking (Murr et al.,

1990). The understanding of the factors that contributes to SCC will help in avoiding

losses and damaged in the industry especially in the marine industry.

3

1.3 PROBLEM STATEMENT

Stress Corrosion Cracking (SCC) is one of the major problem that controls and

determines the suitability of materials as they are very expensive modes of failures, of

particular relevance to desalination and power plants. Stress corrosion cracking, SCC is

one of the most significant corrosion issues concerned. Chloride stress corrosion

cracking can be defined as the cracking that caused by the presence of tensile stress, a

susceptible material and a specific corrosive medium such as the presence of chloride

ions in the environment. This type of corrosion usually difficult to be detected since it

occurs at a certain condition especially when the structure undergoes sensitization

treatment. Stainless steel with sufficient carbon content will exposed to the grain

boundaries attack when heated in the range of 415-8500C as the microstructure becomes

susceptible to the precipitation of chromium rich carbide along the grain boundaries.

This will lead to occurrence of stress corrosion cracking. SCC occurs resulting from the

sensitization treatment, a corrosive environment, a susceptible materials and the

presence of stress. Sensitization treatment will accelerate intergranular stress corrosion

cracking (IGSCC) susceptibility by shortening failure time and accelerating crack

initiation and propagation rates. Different types of environment will also result in

different types of corrosion in austenitic stainless steel. The study of the effect of

sensitization treatment and effect of concentration mixtures of sulphuric acid and

sodium chloride on the austenitic stainless steel is important in order to reduce and

prolong the formation of SCC in austenitic stainless steel and predict types of corrosion

that might occur at a certain environment with the information obtained in this study.

4

1.4 OBJECTIVES

The objectives of this study are as follows:

i. To study the sensitization effect to the chloride stress corrosion cracking of

stainless steel.

ii. To investigate the influence of different concentrations mixtures of sulphuric

acid (H2SO4) and sodium chloride (NaCl) on the chloride stress corrosion

cracking of stainless steel.

1.5 SCOPES

Scopes of this study are outline as follows :

i. Material used in this study is AISI 304 stainless steel.

ii. Sensitization treatment of the specimens is done at 8000C for 2 hours in a

furnace.

iii. Specimens used are U-bend specimens according to ASTM G30 standard.

iv. Different concentrations mixtures of H2SO4 (0 kmol.m-3

to 3.0 kmol.m-3

) and

NaCl (0.5 kmol.m-3

to 2.0 kmol.m-3

) are used for immersion test at room

temperature.

v. Corrosion rate measurement is determined by using weight loss method.

vi. Microstructure analysis of the specimens is done by using scanning electron

microscope (SEM).

1.6 THESIS OUTLINE

This thesis consists of 5 chapters which illustrate the flow work of this project

from introduction until conclusion. There are different contents present in each chapter.

5

The reader will understand more on the detail and the outcome of the project after they

read the entire chapters in the thesis.

Chapter 1 discusses about the background of the study, problem statement,

objectives and the scope of this study.

Chapter 2 contains all the literature reviews and some progress of earlier work.

This chapter also discusses some detail of material that used in this study (austenitic

stainless steel 304). The concepts which are related to stress corrosion cracking also

discuss in Chapter 2. Some explanation of microstructure analysis also will be explained

in this chapter.

Chapter 3 discusses about summary of research methodology of this project. The

progress of flow work of this project which includes the steps of this project conducted.

Chapter 4 contains the results that achieved during the experiment in this study.

This chapter also explains the analysis and discussion of the results that obtained.

Chapter 5 discusses about the conclusions of the project. This chapter also

contains some future recommendation of this study.

6

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

This chapter will discuss about the previous related study and researches on

chloride stress corrosion cracking. The sources of the review are extracted from

journals, articles, reference books and internet. The purpose of this section is to

provide additional information and relevant facts based on past researches which related

to this project. This chapter will cover the corresponding terms such as the stress

corrosion cracking, medium for corrosion and type of specimen used which had been

proved experimentally.

2.2 STRESS CORROSION CRACKING

Expensive modes of failures is one of the major problem which is essentially

controls and determines the compatibility of materials from a wide range of

materials, which is particular relevance to desalination and power plants is stress

corrosion cracking (Prakash et al., 1999). Stress corrosion cracking (SCC) is one of the

corrosion failure commonly encountered due to combined action of stress and corrosion

medium. Stress corrosion cracking is only possible in selective environments where a

passive film form at the metal surface. Under tensile stresses if the environment is too

aggressive and the film formation rate is very slow then general corrosion occurs. If the

7

rate of surface film formation is very fast, then the passive film on the metal surface will

immediately repassivate upon the rupture. In SCC, the surface is able to passivate, but

when the film ruptures the surface of the sample temporarily remains exposed to

the environment and locally corrode. Stress corrosion cracks can initiate at

abnormalities on the metal surface, including pits, grain or phase boundaries,

inclusions, secondary phases, or physical defects such as scratches . Crack can

propagate intergranularly or transgranularly through the microstructure once it initiates.

Crack growth occurs when the reactions occur faster at the crack tip than on the

metal surface or crack sides. When the critical crack length reaches, the crack

continues to spread through the metal and causes the remainder surface to fail (Gary wu,

2011). Figure 2.1 shows SEM fractrograph when the sample undergoes intergranular

stress corrosion cracking.

Figure 2.1: SEM fractrograph of SCC showing intergranular fracture modes

Source: Prakash et al. (1999)

8

The fracture surface is indicative of brittle failure and other fractographic

features are dependent on the SCC mechanism. Stress corrosion cracking is an insidious

form of corrosion and produces a drastic loss of mechanical strength without significant

metal loss (Bhattacharya, 2008). Stress corrosion cracking causes a rapid, brittle failure

of the steel without any prior indication and hence is considered catastrophic. Others

major disasters have been attributed to stress corrosion cracking of steel equipment,

including rupture of high-pressure gas transmission pipes, boiler explosions and

severe damage in power stations and oil refineries (Bhattacharya, 2008). The steel is

exposed more to stress corrosion cracking in carbonate, bicarbonate, acetates and

phosphate environments and is identified as the main reason of cracking in natural gas

transmission lines. In low alloy steels, oxygenated water at high temperature,

NaNO2–Na2SO4 solutions, alkaline chloride solutions such as NaCl-Ca(OH)2 under

pitting conditions, and anhydrous ammonia-methanol solution in the presence of

chloride caused stress corrosion cracking to occurred (Prakash et al., 1999).

2.3 HEAT TREATMENT

Heat treatment is defined as the operation or combination of operations which is

involving heating materials at a specific rate, soaking at a certain temperature for a

period of time and cooling at some specified rate. The purpose of this heat treatment is

to obtain a desired microstructure to achieve certain predetermined properties such as

physical, mechanical, corrosion, magnetic or electrical (Baderestani et al., 2004). In

general, heat treatment is the term for any process employed which changes the physical

properties of a metal by either heating or cooling.

The purpose of heat treatment is to cause desired changes in the metallurgical

structure and thus the properties of metal parts. Heat treatment is sometimes done due to

manufacturing processes that either heat or cool the metal such as welding or forming.

In addition, heat treatment also to increasing the strength of material, but it can

also be used to alter certain manufacturability objectives such as improve

9

machining, improve formability and restore ductility after a cold working

operation. Annealing, normalizing, quench hardening, tempering, and austempering are

five of the important heat treatments often used to modify the microstructure and

properties of steels (Sanjib, 2008).

2.3.1 Sensitization

Sensitization is a major problem in stainless steels that affects the alloy’s

durability. Chromium additions in steel were the main contributor to sensitization.

Sensitization is defined when a carbide precipitation induced by the welding process

or heat treatment can cause chromium-depletion near the grain boundaries (Sourmail

et al., 2003). Chromium is extremely reactive with oxygen and will form a very thin

chromium oxide layer on the surface of stainless steel. The film that is created is on the

order of nanometers in size and is what protects the underlying metal alloy from

corrosion and further oxidation. Sensitization treatment significantly modifies the stress

corrosion cracking behavior and the cause of this is the intergranular precipitation and

the grain boundary chromium depletion. The depletion zone of chromium making the

material vulnerable to corrosion particularly intergranular corrosion (Garcia et al.,

2000). Figure 2.2 shows microstructure of austenitic stainless steel that undergoes

sensitization treatment.

10

Figure 2.2: Microstructure of sensitized austenitic stainless steel

Source: Sarvesh Pal et al. (2011)

Sensitization is attributed due to the alloy element, degrees of temperature and

time of heat exposure. The precipitation of Cr23C6 formed when austenitic stainless

steels have been exposed for period of time of high temperature and slowly

cooled in air. As stated by Garcia et al.(2000), chromium Cr rich carbide precipitation

developed at intergranular and the precipitation of δ-ferrite formed at transgranular

in the region of heat affected zone . These precipitations of δ-ferrite and chromium

depleted zones were attacked in a medium that containing chloride anion. Therefore, it

shows that heat input was one of the factors that affecting microstructure of materials.

2.3.2 Annealing

In general, annealing is the opposite of hardening, the metals were annealed to

relieve internal stresses, soften them, make them more ductile, and refine their grain

structures. Annealing consists of heating a metal to a specific temperature, holding it at

11

that temperature for a set length of time, and then cooling the metal to room temperature

(Fargas et al., 2009). The method of the cooling depends on the metal and the properties

desired. Some of the metals are furnace-cooled, and others are cooled by burying in

ashes, lime, or other insulating materials.

2.3.3 Quenching

Quenching of steel in heat treatment is a technological phase which has an

important influence on microstructure changes and consequently on quality of a

machine part (Janez et al., 2001). The cooling rate of an object depends on many things.

The size, composition and initial temperature of the part and final properties are the

deciding factors in selecting the quenching medium. A quenching medium must cool

the metal at a rate rapid enough to produce the desired results.

2.4 AUSTENITIC STAINLESS STEEL

Stainless steels are mainly iron based with 12% to 30% chromium, up to

22% nickel and minor amounts of carbon, copper, molybdenum, selenium and

titanium (Maricica, 2012). The AISI (American Iron and Steel Institute) designation of

these materials is well known with the number series 300 referring to austenitic

stainless steels and the 400 series covering the ferritic and martensitic stainless steels.

Because of their high chromium and nickel content, austenitic stainless steel is one of

the most corrosion resistant of the stainless steel group that provide fine mechanical

properties.