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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.