Low Alloy Steel Susceptibility to Stress Corrosion Cracking in Hydraulic

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LOW ALLOY STEEL SUSCEPTIBILITY TO STRESS CORROSION

CRACKING IN HYDRAULIC FRACTURING ENVIRONMENT

Thesis

Submitted to

The School of Engineering of the

UNIVERSITY OF DAYTON

In Partial Fulfillment of the Requirements for

The Degree of

Master of Science in Chemical Engineering

Ezechukwu J. Anyanwu

Dayton, OH

May, 2014

LOW ALLOY STEEL SUSCEPTIBILITY TO STRESS CORROSION

CRACKING IN HYDRAULIC FRACTURING ENVIRONMENT

Name: Anyanwu, Ezechukwu John

APPROVED BY:

______Douglas C. Hansen, Ph.D. Sean C. Brossia, Ph.D. Advisory Committee Chairman Research Advisor Research Advisor and Professor Senior Vice President and Chemical and Materials Engineering Senior Principal Engineer DYCE USA

______Robert J. Wilkens, Ph.D., P.E. Committee Member Professor Chemical and Materials Engineering

______John G. Weber, Ph.D. Tony E. Saliba, Ph.D. Associate Dean Dean, School of Engineering School of Engineering and Wilke Distinguished Professor

ABSTRACT

LOW ALLOY STEEL SUSCEPTIBILITY TO STRESS CORROSION

CRACKING IN HYDRAULIC FRACTURING ENVIRONMENT

Name: Anyanwu, Ezechukwu John University of Dayton

Research Advisors: Dr. Douglas C. Hansen Dr. Sean C. Brossia

The pipelines used for hydraulic fracturing (aka. “fracking”) are often operating at a pressure above 10000psi and thus are highly susceptible to Stress Corrosion Cracking

(SCC). This is primarily due to the process of carrying out fracturing at a shale gas site, where the hydraulic fracturing fluid is pumped through these pipes at very high pressure in order to initiate fracture in the shale formation. While the fracturing fluid is typically more than 99% water, other components are used to perform various functions during the fracturing process. Research into the occurrence of SCC reveals that SCC is engendered by a number of factors, of which two main contributors are stress in

iii the pipe steel and a particular type of corrosive environment in contact with the pipeline in the service setting. The variety of fracturing fluid formulas which could be used and the insufficient reported information about the fracturing fluid chemistry makes it very important to carry out analysis to ensure the integrity of the pipelines used for this process. The current research described here is focused on the evaluation of the susceptibility of low alloy steel (AISI 4340) to stress corrosion cracking in different environments as it relates to hydraulic fracturing fluid chemistry and operating conditions. These different environments are achieved by varying the solution pH, the pH adjusting agent and the applied stress. Electrochemistry and stress measurements showed that at near neutral pH solution, AISI 4340 showed a higher SCC susceptibility in solutions where Na2CO3 was used as the pH adjusting than in solutions where NaOH was used as the pH adjusting agent. Scanning electron microscopy and Auger electron spectroscopy was used to analyze the oxide film in solution with the two pH adjusting agents at a pH of 7. Depth profiles of the passive film formed in a solution with pH adjusted to 7 using NaOH pH adjusting agent suggests the presence of a complex, FeOCl, which dissolves actively and thus reduces the SCC susceptibility of AISI 4340 in this environment. It is inferred from the SEM image of AISI 4340 material after testing and stress measurements showed that low alloy steel is more susceptible to SCC in solutions with Na2CO3 as the pH adjusting agent than solutions with NaOH pH adjusting agent especially at near neutral pH. Whereas, at high pH environment AISI 4340 showed a higher SCC susceptibility in solution with NaOH as the pH adjusting agent.

ACKNOWLEDGEMENTS

This thesis would not have been accomplished without the support of so many people. In order to show them how grateful I am for their kindness, I wish to acknowledge them.

First I would like to thank my thesis advisors, Dr. D. C. Hansen and Dr. S. C. Brossia for their financial generosity, extraordinary patience and their time which definitely contributed a whole lot to this work. I also want to thank Dr. Robert Wilkens for his role in making this work come to conclusion.

I also want to thank the company that sponsored the major part of this work, DET

NORSKE VERITAS (DNV) Columbus, for their generosity and support in terms finances and other equipment to carry out my experimental work.

I would also like to extend my thanks to Joe Gerst of DNV Columbus and Steven

Goodrich of University of Dayton Research Institute (UDRI) for their invaluable assistance with the slow strain rate tests, and also Kenny Evans for his inputs in my electrochemistry measurements.

My warmest thanks goes to my past and present lab group, William Nelson, Lu Han,

Phil, Rachel, Yuxin, Yaqiu, Dr. Yuhchae Yoon, Dr. Leanne Petry, for creating a very friendly and happy working environment. Also my thanks go to the people of DNV,

Ashiwini, Noi, Barry, Kris, Beth, Nicky, Feng and others for also making my experience at DNV a very memorable one.

I would like to extend my special thanks to my family who have been strongly behind me and for their love and belief I am able to push further in my academic pursuit, namely

Chief Sir and Dr. Lady H. E. Anyanwu, Mr. and Mrs. Ben Anunne, Arch. and Mrs.

Ikenna Anyanwu, Arch and Mrs. Julius Egbeogu, Mr. and Mrs. Chino Ilechukwu, Engr. and Mrs. Ceejay Anyanwu, Rev. Sr. Dr. C. Osuagwu, Rev. Fr. Dr. Dennis Osuagwu and

Rev. Fr. Dr. Reginald Ejikeme.

Finally, I would like to thank Chinenye for her love and belief in me.

I dedicate this work to the memories of Dr. N. I. Onuoha.

TABLE OF CONTENTS

ABSTRACT…………………………………………………………………………iii

ACKNOWLEDGEMENTS…………………………………………………………v

TABLE OF CONTENTS…………………………………………………………... vii

LIST OF FIGURES……………………………………………………………….. x

LIST OF TABLES…………………………………………………………………. xv

NOMENCLATURE……………………………………………………………….. xvi

CHAPTER 1……………………………………………………………………….. 1

INTRODUCTION…………………………………………………………………. 1

1.1 Background…………………………………………………………………….. 1

1.2 Fracturing Fluid Chemistry…………………………………………………….. 2

1.3 Literature Review ………………………………………………………………4

1.4 Hypotheses………………………………………………………………………9

vii

CHAPTER 2……………………………………………………………….. ……....11

MATERIALS AND METHODS………………………………………………….. 11

2.1 Test Materials and Sample Preparation………………………………………...11

2.2 Electrochemical Testing in Environment with Varying Chloride Ion

Concentration………………………………………………………………………. 13

2.3 Electrochemical Testing in Simulated Fracturing Fluid Solution………………16

2.4 Post Test Analysis ……………………………………………………………... 18

2.5 Stress Tests in Simulated Fracturing Fluid Solution…………………………... 19

2.6 Metallographic Analysis ……………………………………………………... 24

CHAPTER 3……………………………………………………………….. ……... 25

RESULTS………………………………………………………………….. ……... 25

3.1 Electrochemistry in Environment with Varying Chloride Ion Concentration….25

3.2 Electrochemistry in Simulated Fracturing Fluid Environment………………… 32

3.3 Slow Strain Rate Test Measurement (SSRT) in Fracturing Fluid Environment..36

3.4 Crack Microstructure………………………………………………………….. 44

3.5 Slow Strain Rate Tests at Potentials Close to the Ecorr………………………… 45

3.6 Long Open Circuit Potential (OCP) Measurement…………………………….. 47

viii

3.7 Static Load Test……………………………………………………………….. 47

3.8 Post Test Analysis……………………………………………………………… 52

CHAPTER 4……………………………………………………………….. ……....62

DISCUSSION………………………………………………………………...... 62

4.1 Effect of Chloride Ion Concentration on the Passivation Behavior of

AISI 4340………………………………………………………………………….. 62

4.2 Electrochemical Behavior of AISI 4340 in Simulated Fracturing Fluid

Solution……………………………………………………………………………. 67

4.3 Relating Test Environment to Field Condition………………………………… 71

4.4 Static Load Test Below and Above Yield……………………………………… 72

4.5 Surface Film Analysis………………………………………………………….. 73

4.6 Conclusion……………………………………………………………………... 74

REFERENCES………………………………………………………………..…… 77

LIST OF FIGURES

Figure 1: Tensile specimen with dimensional measurement………………………..12

Figure 2: Cylindrical sample for electrochemical measurement…………………....13

Figure 3: Slow Strain Rate Test (SSRT) cell assembly…….....…………………….21

Figure 4: Cyclic potentiodynamic polarization curve for AISI 4340 test in nitrate/chloride environment………………....………………….….....…………… 27

Figure 5: Sample after testing in 3.7M NaNO3 solution.….....…………………….. 28

Figure 6: Sample after testing in 3.7M NaCl & 3.7M solution NaNO3…………… 28

Figure 7: Sample after testing in 1M NaCl & 3.7M solution NaNO3………..……. 29

Figure 8: Sample after testing in 3.7M NaCl………………….…………………… 29

Figure 9: Cyclic potentiodynamic polarization curve for AISI 4340 test in varying chloride ion concentration.…...... …………………………………………….…….30

Figure 10: Difference between the breakdown and free corrosion potential in the different chloride ion concentration environment….. ……………………………... 31

Figure 11: Sample after testing in 1M NaCl solution..….....………………………. 31

Figure 12: Sample after testing in 0.1M NaCl solution...…...………………………31

Figure 13: Sample after testing in 0.01M NaCl solution.....……………………….. 32

Figure 14: Sample after testing in 0.001M NaCl solution...... …………………..…. 32

Figure 15: Comparison of CPP curves in solution 2 and solution 3 at pH 7-10 and temperature 50°C...……………..……………………………………………... 33

Figure 16: CPP curves in solution 3 at pH 7-10 and temperature 25°C...... 34

Figure 17: Ecorr summary in solution 3 with varying pH and at temperature 25°C and 50°C……………………………………………………………...……..………35

Figure 18: Polarization resistance summary in solution 3 with varying pH and at temperature 25°C and 50°C………….……………...... …………………. ………35

Figure 19: Slow strain rate chart in air, solution 1 and solution 2...... ………………37

Figure 20: Slow strain rate chart in air, solution 1 and solution 3..………………... 38

Figure 21: Chart showing the % reduction of AISI 4340 in various test environments……………………………………………………………………….. 40

Figure 22: Chart showing the time to failure of AISI 4340 in various test environments……………………………………………………………………….. 40

Figure 23: Chart showing the % plastic elongation of AISI 4340 in various test environments……………...... 41

Figure 24: SEM image of AISI 4340 sample after testing in solution 2 pH 7 environment……………………………………………………………………….. 42

Figure 25: SEM image of AISI 4340 sample after testing in solution 3 pH 7 environment………………………………………..…………………...…………. 43

Figure 26: Microstructure image of cracks on AISI 4340 sample after SSRT in solution 3 pH 7 environment…………………………...... …………………..…… 44

Figure 27: Slow strain rate test results in solution 3 at pH 7at varying potentiostatic hold....……………………………………………………………………………… 45

Figure 28: Sample after SSRT in solution 3 pH 7 at -400mV vs. SCE potentiostatic hold………………………………………………………………………………… 46

Figure 29: Sample after SSRT in solution 3 pH 7 at -500mV vs. SCE potentiostatic hold……………………………………………………...... …………………….. 46

Figure 30: Sample after SSRT in solution 3 pH 7 at -600mV vs. SCE potentiostatic hold……………………………………………………...... …………………..….. 46

Figure 31: Overlay of OCP measurement of AISI 4340 in solution 3 pH 7

environment for 20 hrs and 108 hrs………………...... …………………...... 47

Figure 32: Displacement measurement at stress held below yield strength of material in solution 3 at pH 7…………………………………………………….... 48

xii

Figure 33: Displacement measurement at stress held above yield strength of material in solution 3 at pH 7……………………………………………………… 49

Figure 34: Overlay of current reading at stress held above and below the yield strength of AISI 4340 in solution 3 at pH 7..…….………………………………… 50

Figure 35: Image of sample after test at stress hold below yield strength in solution 3 at pH 7…..…...... ……... 51

Figure 36: Image of sample after test at stress hold above yield strength in solution 3 at pH 7…..…..…………………………………………………………. 51

Figure 37: Potentiostatic test of AISI 4340 material in solution 3 pH 7 and solution 2 pH 7 environment at -400mV vs SCE………………………...……….. 52

Figure 38: SEM image of oxide film formed on AISI 4340 after potentiostatic hold of -400mV vs. SCE for 16hrs in solution 3 pH 7 environment….……..……. 53

Figure 39: SEM image of oxide film formed on AISI 4340 after potentiostatic hold of -400mV vs. SCE for 16hrs in solution 2 pH 7 environment.. 53

Figure 40: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at pH 7 (Test 1)…………………………...... …………………..………. 54

xiii

Figure 41: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at pH 7 (Repeat Test)……………………...... …………………..………55

Figure 42: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 3 at pH 7 (Test 1)…………………………...... …………………..………..56

Figure 43: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 3 at pH 7 (Repeat Test)…………………...... …………………..…………57

Figure 44: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 2 at pH 7 (Test 1)……………………….....…………………..………….. 58

Figure 45: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 2 at pH 7 (Repeat Test)………………………………………..…………..59

Figure 46: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 3 at pH 7 (Test 1)……………………….....…………………..………….. 60

Figure 47: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 3 at pH 7 (Repeat Test)………………………………………..………….. 61

Figure 48: Dependence of pitting potential (Ep) on the activity of Cl- (aCl-) in solution...... 65

xiv

LIST OF TABLES

Table I: The composition and mechanical property of AISI 4340………………… 13

Table II: Solution chemistry of simulated fracturing fluid………………….……... 17

Table III: Data summary from CPP curve for AISI 4340 test in nitrate/chloride environment……………………………………………….....……………………. 28

NOMENCLATURE

Ecorr Corrosion Potential (mV)

Ep Pitting Potential (mV)

ERep Repassivation Potential (mV)

SSRT Slow Strain Rate Test

AES Auger Electron Spectroscopy

XPS X-ray photoelectron spectroscopy

SIMS Secondary ion mass spectrometry (SIMS) and

EP Plastic strain to failure (%)

EF Elongation at failure (in./in.)

EPL Elongation at proportional limit (in./in.)

LI Initial gauge length (in.) (usually 1 in.)

2 σF Stress at failure (lbs/in )

2 σPL Stress at proportional limit (lbs/in )

xvi

Di Initial diameter (in.)

Df Final diameter (in.)

- - aCl Activity of the Cl in solution.

ɣ Activity coefficient

[C] Concentration of Cl-(M)

Activation energy (kJ/mol)

T Temperature (K)

HSS High Speed Steel

ISS Ion Scattering Spectroscopy

SEM Scanning Electron Microscopy

AES Auger Electron Spectroscopy

SCE Saturated Calomel Electrode

OCP Open Circuit Potential (mV)

CPP Cyclic Potentiodynamic Polarization

CE Counter Electrode

RE Reference Electrode

WE Working Electrode

xvii

IR Ohmic Resistance

AISI American Iron and Steel Institute

βa Anodic Tafel Slope (ΔmV/ΔIog i)

βc Cathodic Tafel Slope (ΔmV/ΔIog i)

Rp Polarization Resistance (Ohms)

Icorr Corrosion Potential (amps)

xviii

CHAPTER 1

INTRODUCTION

1.1 Background

Hydraulic fracturing is the process of injecting a fracturing fluid through a wellbore into a shale formation at pressure at such a high pressure (mostly above 10000 psi) that the geologic structure “cracks” or fractures1. The pipelines used for this process are often operating at very high pressure and thus may be highly susceptible to Stress Corrosion

Cracking (SCC). This susceptibility is a result of the high pressure which this process has to perform at in order to initiate fracture in the shale formation. The term fracking is used to describe the process of opening up fractures already present in the formation and to create new fractures1. While the hydraulic fracturing fluid is typically more than 99% water, other components are used to perform different specialized functions during the fracturing process2. These components are generally considered proprietary, so drilling companies are not required to disclose the specific content or formula of their fracturing fluid.

SCC in buried pipelines is a serious problem that may cause significant economic, environmental and human losses3. The variety of fracturing solution which could be used and the insufficient reported information about the fracturing fluid chemistry make it

1 even more important for more investigation to be done to ensure the integrity of the pipeline used for this process.

1.2 Fracturing Fluid Chemistry

Fracturing fluids are typically “slickwater” (water with drag reducer) designed fracture treatments, which are water-based fracture fluids4. Desirable properties of a hydraulic fracturing fluid may include

 high viscosity

 low fluid loss

 low friction during pumping in the well

 stability under the conditions of use such as high temperature deep wells and

 ease of removal from the fracture and well after the operation is completed5.

Depending on the particlar fracturing operation, it may be necessary that the fluid be made viscous to help create the fracture in the reservior and to carry the proppant into this fracture1.

Chemicals used during the fracturing process are a vital component to a successful well completion6.The exact composition of fracturing fluids depends upon the geologic layer to be fractured; however some additives used in fracturing may not be needed for every application as each additive has a specific purpose during the fracturing process7.

However, the chemicals used help reduce the amount of pressure required to fracture the shale formation (known as surface treating pressures), aid in placement of the propping agent within the deep, downhole formation, and help maintain fluid properties that meet design specifications6. Chemicals are mixed in very low concentration with water and

2 make up less than 1% of the total job volume6,7. Temperature has to be taken into account when selecting additives and concentrations for hydraulic fracturing applications 4. The additives used in hydraulic fracturing activities are

 friction reducers

 biocides

 scale inhibitors

 potassium chloride (clay stabilizer)

 surfactants

 hydrochloric acid

 acid inhibitors

 iron control agents,

 gel and crosslinkers4. pH adjusting agents such as sodium carbonate, potassium carbonate, sodium hydroxide or potassium hydroxide are often used to maintain the effectiveness of other components such as the crosslinkers. Some fracturing activities may use fluids with fewer additives2.

The main aim of this research is to study the effect of pH, pH adjusting agents as well as stress on low alloy steel susceptibility to SCC as it relates to hydraulic fracturing activities.

1.3 Literature Review

Role of chloride ion in the breaking down of passive film:

Many engineering alloys are useful because of their ability to passivate by forming thin

(nanometer scale) oxide layers on the metal surface and thus, greatly reducing the rate of corrosion of the alloy8. Such passive films, however, are often susceptible to localized breakdown such as pitting, which results in accelerated dissolution of the underlying metal8. Pitting of a given material depends strongly on the presence of aggressive species in the environment, such as chloride ions, and a sufficient oxidizing potential9. Pitting occurs when portions of the metal surface lose their passivity and dissolve rapidly9.

Chloride is a relatively small anion with a high diffusivity and electronegativity, which by forming a salt film on the metal surface interferes with passivation and is ubiquitous as a contaminant8. The level of interference of the chloride ion with passivity depends on the stability of the salt film formed8. Chloride ions play an essential role in one of the most destructive type of corrosion; localized corrosion10. Chloride also is an anion of a strong acid and many metals, iron inclusive, exhibit considerable solubility in chloride solutions8. The presence of oxidizing agents such as oxygen and hydrogen in a chloride- containing environment is extremely detrimental, and will further enhance localized corrosion8.

The minimum anodic potential needed for pitting to occur on a metal in a particular environment is known as the critical pitting potential11. However, the potential at which pit growth or crevice corrosion will cease is known as the repassivation potential12. The critical pitting potential shifts to less positive values as the chloride concentration

4 increases, giving rise to pits forming at a less positive potential11. The critical potential required for pitting to occur varies with the logarithm of the bulk chloride concentration8.

Most researchers agree that the first step to pitting corrosion is adsorption of chloride anions on the passive film10. Several studies have been done on the interaction of chloride iron with the passive film of iron with the use of such techniques as Auger electron spectroscopy (AES), secondary ion mass spectrometry (SIMS) and X- ray photoelectron spectroscopy (XPS)10. Auger and SIMS measurement have shown that films formed in chloride containing borate solution at a pH of 8.4 do not contain Cl-, even at potentials and Cl- concentration where pitting occur13. This indicates that Cl- incorporation into the oxide is not a precursor or a cause of pit initiation. Studies have also shown that the oxide thickness in Cl- and Br- containing solution is the same as the film thickness in the absence of these halides10.

Chin et al.13 studied the optical properties of the passive film on iron using in situ ellipsometric spectroscopy. Extensive efforts were made to detect any spectroscopic changes of the passive film occurring before pitting. The breakdown processes were studied by injecting chloride ions in the form of 2.5N NaCl solution after measuring the passive film spectrum. No spectral changes of the passive film were observed prior to the pitting of the electrode. No changes due to ad- or absorption of Cl- ions were observed13.

This also suggests that the structure of the passive film was not altered by the introduction of the chloride ion.

Li et al.14 in their study of the influence of temperature, chloride ion and chromium element on the electronic property of passive film formed on carbon steel in

5 bicarbonate/carbonate buffer solution found that the passive film formed showed an n- type semi conductive character. Their EIS results showed that the transfer impedance and diffusion impedance decrease with increasing solution temperature, with the addition of chromium into carbon and with increasing the chloride ion concentration. They concluded that the corrosion protection effect of the passive film on the substrate decreases with increasing solution temperature, chromium content of the carbon steel material and increase in the chloride ion concentration14. Valeria and Christopher investigated the passive films formed by carbon steel, chromium steel and high speed steel (HSS) in bicarbonate and chloride environments15. They also found that these passive films behaved like n-type semiconductors, showing that the passive film properties are dominated by iron15.

Tongson et al.16 examined the mechanism of breakdown of passive films on iron in borate buffer solution at pH of 8.4 caused by chloride ions. Their finding was in contradiction to the published results of other authors that have performed similar investigations. XPS, SIMS and ISS measurement of the systems used in the electrochemical work were studied and Cl- ions were detected up to the metal/metal oxide interface with the local concentration highest in the outer layer of the film16. They also determined that the peak of the concentration of Cl- ion at the time corresponded with the breakdown time of the passive film. The authors assumed that adsorption of Cl- ion on the passive film surface lowered the interfacial tension at the film surface interface, which resulted in the formation of cracks16.

Stress Corrosion Cracking in Different Environments

Research reveals that SCC is engendered by a number of factors, of which two main contributors are stress in the pipe steel and a particular type of environments in contact with the pipe17. For example, environments with high pH conditions result in the intergranular form of SCC and environments with near neutral pH condition tends to result in the transgranular form of SCC3. The cracks frequently initiate at surface flaws that either preexist or are formed during service by corrosion, wear or other processes18.

SCC can be initiated from the bottom of a pit either by a dissolution process or mechanical process19. With the dissolution mechanism, SCC generally requires a dissolution rate of at least 10 times higher in the depth direction than in the lateral direction19. With the mechanical SCC initiation process, micro-cracks are initiated at the weakest link site in the hydrostatic zone ahead of a notch tip (which might be the bottom of a pit)19.

General theories regarding mechanism of SCC initially supported one of two fundamental considerations: anodic dissolution or hydrogen-related phenomena9. There has been an effort made to show which has the obvious predominant controlling mechanism; studies have shown that the factors controlling the SCC process are much more complex and likely to be unique with regards to alloy composition, metallurgical condition, chemical environment, electrochemical state and state of mechanical stress9. In cases where crack growth is a result of localized dissolution processes, potent solutions will need to promote a critical balance between activity and passivity since a highly active condition will result in uniform corrosion, while a completely passive condition will not lead to SCC20. However, the accumulation of metal cations, in this case, Fe2+

7 ions within a stress-corrosion crack, can hydrolyze to form hydrogen ions leading to an acidified local environment within the crack tip11. Thus the hydrogen ions produced within the crack can be reduced to form hydrogen atoms which absorbs on the metal surface. Some of these hydrogen atoms migrate into the stressed regions ahead of the crack tips and can promote the growth of the stress corrosion cracks by the process known as hydrogen embrittlement11.

It is well accepted that the mechanism of high pH SCC of steel involves anodic dissolution for crack initiation and propagation3. In contrast, it has been suggested that the low pH SCC is associated with the dissolution of the crack tip and sides, accompanied by the ingress of hydrogen in the steel3. Studies have shown that applying different potentials affects the mechanism of SCC process that occurs. With a shift in the applied potential in the negative direction, the SCC of steel changes from an anodic dissolution mechanism to a hydrogen base mechanism resulting in a transgranular cracking mode3.

Contreras et al3 studied the mechanical and environmental effects on SCC of low carbon pipeline steel in a soil environment. The degree of susceptibility was assessed by the differences in the behavior of the mechanical properties of the material in tests conducted in a specific environment from that obtained from tests conducted in the controlled environment. Among the conclusions made based on their result was that the SCC susceptibility increases with an increasing strength of the steel. It was also stated that specimens tested in soil solutions and applying potentials of -400mV vs open circuit potential (OCP) showed a transgranular fracture as a resulted of hydrogen ingress into the metal3.

The present work concentrates on the study of the behavior of low alloy steel and it susceptibility to stress corrosion cracking using the hydraulic fracturing environment as a case study. While there has been a lot of work done on the mode of crack propagation of high strength steel and pipelines under various conditions, there has been little work on the effect of pH and pH adjusting agents on the susceptibility of AISI 4340 to SCC. An extensive study of the effects of solution pH, pH adjusting agent, stress and chloride concentration on the SCC susceptibility of low alloy steel as it relates to hydraulic fracturing environment is the main aim of this work.

1.4 Hypotheses

The role of pH, chloride ion concentration and pH adjusting agent in the initiation and propagation of SCC of AISI 4340 will be evaluated by testing the following hypotheses;

- Hydrogen embrittlement often takes place at low pH localized at the pit base.

Therefore the crack mode observed in a low pH environment should be hydrogen

embrittlement and give rise to transgranular cracks. As a result of this, the crack

propagation at static loads after crack initiation would proceed due to the ingress

of hydrogen atom.

- Localized corrosion results from breakdown of the protective oxide layer due to

the aggressive nature of the chloride ions in the solution. Pit formation and

propagation decrease by reducing the concentration of the chloride ion

concentration. This can be seen by the trend in the difference between the

breakdown potential and the free corrosion potential as a function of chloride ion

concentration in solution.

- pH plays an important role in the susceptibility of low carbon steel materials.

Using two pH adjusting agents, which use two different mechanisms to reduce the

hydrogen ion concentration in solution, results in different cracking behavior of

the metal, especially at low pH.

The first hypothesis was tested by running slow strain rate measurements in a low (near- neutral) pH environment and characterizing the cracks by doing a metallographic analysis on the cracks. A transgranular form of crack propagation especially at low pH environment was used as an indication for hydrogen assisted cracking. Also the test material, low alloy steel AISI 4340, at a low pH environment, was kept on static load in order to observe the crack propagation in cases where cracks had initiated. This was done to examine if the cracks will propagate by observing the trend in the recorded current.

The second hypothesis was tested by running cyclic polarization curves on samples of the test material, AISI 4340, in an environment where passivation is occurring. Chloride ion concentration was gradually increased and the difference between the pitting and repassivation potential was observed at the different chloride ion concentrations. Two environments were used for this test, a sodium nitrate environment which is known to promote passivity and a very high pH environment which is also known to promote passivity of this material.

Slow strain rate testing and SEM analysis were used to test the third hypothesis. Auger electron spectroscopic analysis was used to investigate the nature of the oxide film on the stressed and unstressed material in the most aggressive environment.

CHAPTER 2

MATERIALS AND METHODS

2.1 Test Materials and Sample Preparation

Tensile specimen and cylindrical specimen of AISI 4340 material cut from a single plate

(with dimensions 500” x 14 ½” x 24 ½”) were used to perform slow strain rate and electrochemical testing respectively. The geometry of the tensile specimens and the cylindrical specimen are shown in Figure 1 and Figure 2 respectively. The composition and mechanical properties of the AISI 4340 steel used in testing are listed in Table I. All the AISI 4340 samples used for this testing were supplied by metal samples (located at

Munford, Alabama). The mode of material production was ensured to be the same for all batches of materials used for this research. All samples were inspected to ensure that they all had the same dimensions.

All the samples used for testing were prepared the same way. Samples were polished to

800 grit (starting from more coarse paper of 400 grit and 600 grit) using silicon paper to get a uniform surface finish. After polishing, the samples were sonicated for five minutes each in acetone, isopropanol and methanol and dried in a stream of nitrogen gas. Samples were used for testing within 5 minutes of completing the cleaning process in all the tests conducted.

A Gamry reference 600 potentiostat was used for all electrochemical measurements in this research. Prior to testing, the instrument was calibrated. A Saturated Calomel

Electrode (SCE) was used as the reference electrode for all electrochemical measurements. The potential of the reference electrode was checked against an archive reference electrode (sometimes called master reference electrode) and the difference of less than 5mV was ensured before the testing reference electrode was used in any measurement. Either a graphite rod or a platinized palladium wire was used as a counter electrode for all the electrochemical measurements done. The already polished and cleaned AISI 4340 test sample was used as the working electrode.

Figure 1: Tensile specimen with dimensional measurement

Figure 2: Cylindrical sample for electrochemical measurement

Table I: The composition and mechanical property of AISI 4340

YS UTS Elongation C Si Mn P S Ni Cr Mo Al (MPa) (MPa) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%)

AISI 308 590 21 0.42 0.26 0.81 0.009 0.002 1.80 0.83 0.24 0.036 4340

2.2 Electrochemical Testing in Environment with Varying Chloride Ion

Concentration

Test Solution Preparation

Solutions of sodium nitrate and sodium chloride were prepared using deionized water and reagent-grade chemicals and used to perform the first set of tests. These tests were performed in different solutions where the mole fraction of sodium chloride in solution with sodium nitrate was reduced from 1 to 0. Each solution was prepared and then mixed in a particular ration in order to achieve the desired mole concentration of chloride in solution. The pH of the test solution was adjusted to 9 by adding drops of 0.05M sodium

13 hydroxide solution. After preparation, the test solution was purged with dry nitrogen gas directly from a nitrogen tank for at least 6 hours and heated up to 50oC before the sample was immersed. The solution was continuously deaerated condition during testing by constant nitrogen gas purging through the solution.

In the second set of experiments conducted, the pitting behavior of AISI 4340 steel exposed to alkaline solution of sodium chloride was investigated. Four different solutions of sodium chloride were prepared in which the concentration of sodium chloride in solution was reduced by a factor of 10 starting from a solution containing 1M concentration of NaCl (1M, 0.1M, 0.01M, 0.001M NaCl solutions). The pH of the solution was adjusted to pH of 12 with 0.5 M sodium hydroxide. The final concentration of NaOH in the different sodium chloride solutions after adjusting the pH was;

- 0.01830M NaOH in 1M NaCl solution

- 0.012676M NaOH in 0.1M NaCl solution

- 0.01076M NaOH in 0.01M NaCl solution and

- 0.00980M NaOH in 0.001M NaCl solution.

The prepared solution was purged for at least 6 hours after preparation and heated up to

50°C before the test was started. The solution was kept in a deaerated condition during the period of the test.

An open circuit potential (OCP) measurement was conducted before any electrochemical measurement was started to ensure that the film formation on the sample surface had reached a steady state. OCP measures the free corrosion potential of AISI 4340 sample

14 over time. In this work, the steady state for OCP measurement was defined as when there is a small change in potential of the sample in solution with respect to time which is less than or equal to +/-5mV/hr. This was done in order to have a uniform sample surface before polarization.

Cyclic Potentiodynamic Polarization (CPP) Measurement

This test technique measures the current as the AISI 4340 sample is polarized by steadily applying potential in the anodic direction. This potentiodynamic sweep was started at a cathodic potential of -50mV or more vs OCP to an anodic potential and reversed at the same rate until it reaches the starting potential of -50mV vs OCP (or more negative potential than the starting potential). The effect of the changing chloride ion concentration on the pitting behavior of the sample in solution was characterized using the cyclic potentiodynamic polarization test technique. The nature of the potentiodynamic polarization curve in all environments was observed for the occurrence or non-occurrence of a breakdown potential and a repassivation potential. The scan rate for all the potentiodynamic polarization measurements was 0.6V/hr (±5%) according to the ASTM

G5 standard21.

After the test, the samples were cleaned with acetone, isopropanol and methanol as described previously and dried under a stream of nitrogen gas. The pictures of the samples were taken and documented.

2.3 Electrochemical Testing in Simulated Fracturing Fluid Solution

Test Solution Preparation

The test solution was prepared in order to simulate the fracturing fluid. In simulating the hydraulic fracturing fluid, the following factors were considered:

- Dilute nature of the solution

- Chloride ion concentration

- Sulfate ion concentration

- Acetate ion concentration

- pH adjusting agents

The solutions were categorized into three different groups as follows;

Solution 1 composition: This is also known as the stock solution (the solution pH was not adjusted).

Solution 2 composition: Solution pH was adjusted with 0.05M sodium hydroxide solution. The effect of sodium hydroxide as a pH adjusting agent on the susceptibility of the AISI 4340 material to localized corrosion was examined.

Solution 3 composition: The solution pH was adjusted with 0.05M sodium carbonate.

This is usually the main pH adjusting agent used in the field.

Table II shows the solution chemistry for all the simulated fracturing fluid solutions used for analysis. All solutions were deaerated before the test was started and also while the test was being conducted.

Table II: Solution chemistry of simulated fracturing fluid

Solution Chemistry Na2SO4 NaCl CH3COOH Na2CO3 NaOH

Solution 1 pH of 5.6±0.3 1ppm 4ppm 0.1464ppm - -

Solution 2 pH 7 1ppm 4ppm 0.1464ppm - 0.30ppm

Solution 2 pH 8 1ppm 4ppm 0.1464ppm - 0.42ppm

Solution 2 pH 9 1ppm 4ppm 0.1464ppm - 1.50ppm

Solution 2 pH 10 1ppm 4ppm 0.1464ppm - 10.76ppm

Solution 3 pH 7 1ppm 4ppm 0.1464ppm 0.9ppm -

Solution 3 pH 8 1ppm 4ppm 0.1464ppm 1.537ppm -

Solution 3 pH 9 1ppm 4ppm 0.1464ppm 2.913ppm -

Solution 3 pH 10 1ppm 4ppm 0.1464ppm 17.22ppm -

Cyclic Potentiodynamic Polarization (CPP) Measurement

The same parameters were used in the electrochemical testing in environments with varying chloride ion concentration as were used in these measurements. The results from these tests were used to establish the effect of pH and the pH adjusting agent on the corrosion behavior of AISI 4340. The electrochemical behavior of C4340 material in these environments was characterized by observing the changes in the open circuit potential, corrosion current density (current at zero overpotential), pitting potential and repassivation potential (where pitting occurs) as the solution variables are changed.

Potentiostatic Measurements

By observing the curves obtained from the CPP testing of the material, a potential which is above the repassivation potential (Erep) and below the break down potential (Ep) was applied on the sample. The material shows maximum susceptibility to localized form of corrosion in this potential region (between the breakdown potential and the repassivation potential)12. The current is monitored while the potential is applied on this sample for 15 hours. The two purposes of this test are:

- To form a protective film on the sample surface for the purpose of running

post-test analysis to study the nature of the oxide film formed.

- To observe the difference in the corrosion rate of C4340 in different

environments at the same potential by measuring the current density in these

two environments. The difference in the measured current density, at the

selected potential, was used to compare the corrosion rate of this material in

the various electrolyte environments.

2.4 Post Test Analysis

Energy-dispersive X-ray Spectroscopy EDS

This analytical technique was used for the chemical characterization of the oxide film formed on these samples after potentiostatic tests. The samples were extracted from the test solution and dried under a stream of nitrogen gas and put inside a glass jar covered with a lid. An EDS/SEM analysis was conducted on the sample. This analysis was performed on four different locations on the sample concentrating around where surface defects were noticed.

Auger Electron Spectroscopy (AES)

AES is a surface-sensitive technique that can, in general, detect all elements except hydrogen present at levels > 0.5 atom % within ~3 nm of a sample surface22. This technique was used to analyze the areas of the sample covered with oxide film. After the samples were subjected to a potentiostatic hold for over 15 hours, they were quickly extracted and put into a vial containing liquid nitrogen and covered with a lid. The vial was stored inside a dewar containing liquid nitrogen and stored inside a -80oC freezer to avoid any further surface reaction of the sample with the environment. The AES analysis was conducted on both the stressed and unstressed samples (slow strain rate test samples) and was performed using a Varian 981-2707 Auger electron spectrometer. The incident electron beam voltage was 5 keV and the electron beam was rastered over an area ~0.1 mm x ~0.1 mm. Additionally, AES was combined with argon ion-sputtering to generate a profile of sample composition as a function of sputter time (or depth, if the sputter rate is known). The AES surface scans were recorded on 3 locations on each sample. In addition to the surface scans, an AES depth profile was recorded on each sample using the integrated Eurovac 3 keV ion gun. For the purpose of having a reference for depth presentation, the sputter rate of 14nm/min was measured on a chemical vapor deposition

(CVD) silicon nitride film under the conditions that were used for these depth profiles.

2.5 Stress Tests in Simulated Fracturing Fluid Solution

Test Solution Preparation

Solution 1 (unadjusted stock solution), Solution 2 (NaOH pH adjusting agent) and solution 3 (Na2CO3 pH adjusting agent) were prepared and used for the stress tests.

Solution 2 and Solution 3 with pH ranging from 7-10 were prepared using NaOH and

Na2CO3 as the pH adjusting agents respectively. These solutions were prepared with the same concentration of the individual additives as shown in Table II.

Methodology

Two forms of stress testing methods were used.

- Slow strain rate testing (SSRT) and

- Static load test

Slow Strain Rate Testing (SSRT)

Before the test was started, the overall length, the gauge mark length and the gauge diameter of the sample were noted. The sample was polished and cleaned as indicated in the Test Material and Sample Preparation section above. In order to reduce the solution resistance (IR) effect, the saturated calomel reference electrode (SCE) was placed in a

Luggin probe containing 8M solution of potassium chloride. A platinized niobium wire or a graphite rod was used as the counter electrode (CE). In cases where the platinized niobium wire was used as the CE, the wire was constructed to be in a concentric circle geometry surrounding the working electrode without making any form of contact with it.

An arrangement of the test cell used for the stress testing is shown in Figure 3 below.

Figure 3: Slow Strain Rate Test (SSRT) cell assembly

The test cell containing the electrolyte and the test sample was heated to 50oC and maintained at that temperature with the aid of a heat tape and a thermocouple. The thermocouple was covered with a shrinking insulating tube to prevent any interaction with the solution which might influence on the potential reading. An open circuit measurement was performed while the sample was under a preload load of 75 lbm until a steady state potential reading was recorded. All the slow strain measurements in all the environments were conducted at a potential hold of -400mV vs SCE. This potential was selected for all the stress tests because it is above the repassivation potential in all the test environments. The potential of -400mV vs SCE was passed through the sample for at least one hour to ensure that a surface oxide film as well as surface defects, if any, had sufficiently formed on the test metal before slow strain measurements were started. The

21 sample was strained at a rate of 5E-07in/in-sec. A slow strain rate was used for testing to ensure maximum effect of the environment on the sample. The straining of the sample continued until failure occurred.

After testing, the sample was cleaned by sonication with a rodine solution in cases where

AES analysis of the sample after failure was not required. Rodine solution was prepared by mixing deionized water, hydrochloric acid and rodine concentrate in a ratio of

7.5:1:1.2 respectively. This rodine solution provides a good method of cleaning the corroded metal surface as well as prevents the corrosion of the base metal by the acid23.

After cleaning, the sample was examined under a scanning electron microscope for cracks and other forms of localized attack. This test was conducted for the different pH adjusting agents, Na2CO3 and NaOH, with the pH ranging from 7-10 respectively. The slow strain rate test was performed in the different prepared test solutions and the time to failure, the percentage elongation, the reduction in area, the ultimate tensile strength and the percentage plastic elongation were recorded. After detecting the solution pH in which the cracking behavior of AISI 4340 was significantly different between the solutions with the respective pH adjusting agents, an AES analysis was performed around the fractured area on the failed samples.

Using these techniques, the solution pH and pH adjusting agent in which AISI 4340 appeared to be most susceptible to SCC was therefore established. Additional slow strain rate test measurements were conducted in this solution (where AISI 4340 showed the highest susceptibility) at more negative potentials until crack initiation and propagation were noticed to stop.

To further link the obtained results to field conditions, a long open circuit potential measurement was conducted on an unstressed AISI 4340 sample. The purpose of this test was to examine the trend in the open circuit potential and observe if it would approach the potential where cracking was observed to still occur.

Static Load Test

This test was conducted on the AISI 4340 material in the environment in which the test sample showed the highest susceptibility to SCC based on the results obtained from the

SSRT measurement and the SEM of the sample after testing. The tensile strain samples used in running this test had the same geometry as the sample used in the slow strain rate tests. The sample was polished and cleaned as indicated in the Test Material and Sample

Preparation section above. The open circuit potential of this sample was observed while under a preload of 6112lbm/in2 until a steady state is reached. The sample was held at a potential of -400mV vs SCE for at least an hour to ensure that the oxide film was sufficiently formed on the sample before the loading started. In one of the tests conducted, the straining of the test sample was stopped at a point above the yield of the material. The trend of the current measured as a function of time was observed. As cracks initiate and propagate, new metal surfaces emerge and will corrode faster than areas already covered with the oxide film. Therefore, the trend in the measured current was used as an indication of crack propagation. The displacement of this sample and the change in the load at that particular stress hold was also recorded. Conclusions on whether cracks were initiated and propagated were reached based on the visual examination of the samples and the trends being observed in these three parameters (i.e., current, displacement and change in load). On the second test that was conducted, the

23 loading on the sample was stopped at a point below the yield of the material in the same environment. The current reading was observed and the trend was used to indicate the crack behavior of this sample under the load. The displacement and change in the load on the sample was also measured and the trend was also used to ascertain the initiation and propagation of cracks.

2.6 Metallographic Analysis

Metallographic analysis was performed on test samples which showed the most cracks when analyzed under the SEM. This was done by sectioning the sample to separate the uncracked area with the use of an electric cutter. The cracked section of the sample was embedded into epoxy using a standard metallurgical mold. The epoxy was prepared by mixing equal amount of epoxy resin and its hardener. The epoxy with the sample was allowed to set overnight. In order to have a cross sectional view of the cracks, the sample was ground using an automatic grinding wheel until the crack tips were exposed. With the aid of a polishing cloth and a 1 micron diamond paste, the sample was polished to a surface finish of 1 micron. The sample surface was etched using a nital solution which is a solution containing 95% of ethanol and 5% of concentrated nitric acid. The etched sample was observed under the metallurgical microscope to capture the mode of crack propagation.

CHAPTER 3

RESULTS

3.1 Electrochemistry in Environment with Varying Chloride Ion Concentration

Electrochemical experiments were conducted in order to understand the effect of chloride ion on pitting susceptibility of AISI 4340 in an environment aggressive enough to form pitting. Raji et al24 studied the corrosion behavior of carbon steel in sodium nitrate solution. In their study it was shown that carbon steel undergoes localized corrosion in sodium nitrate solution and this starts to occur at 700ppm concentration of sodium nitrate and above. They showed that with changing solution temperature and/or nitrate concentration, the corrosion morphology of carbon steel changes (i.e. from stress corrosion cracking into general corrosion).

In this experiment performed to understand the effect of chloride ion on the susceptibility of AISI 4340 material to localized corrosion, the concentration of sodium nitrate was kept at 3.7M. The concentration of the sodium nitrate solution was kept above the 700ppm limit where carbon steel is known to undergo localized corrosion. The mole fraction of sodium chloride in solution with sodium nitrate for the different solutions was 1, 0.5, 0.2 and 0 at pH of 9 and temperature of 50oC. The pH of the solution was adjusted with

0.05M sodium hydroxide solution and was deaerated with nitrogen gas. Figure 4 shows

25 the overlayed cyclic potentiodynamic polarization curves of the four different tests carried out. With the help of Tafel fit function on Echem analyst software (software manufactured by Gamry Instruments, Inc. Westminster, PA), a Tafel fit was performed at

120mV above (anodic slope, βa) and 120mV below (cathodic slope, βc) the open circuit measurement. The portion selected for the Tafel fit falls within the activation polarization portion of the curve. The Ecorr, βa, βc and Rp values read from the Tafel fit are

11 summarized in Table III. The Stern-Geary equation was used to calculate Icorr from the anodic and cathodic slopes and the polarization resistance. The calculated Icorr is further divided by the immersed surface area of the sample to obtain the corrosion current density icorr.

(1)

Rp is also said to be the ratio of the electrode over potential and the net change in current11.

Table III shows the summary of the CPP curves in Figure 4.

Figure 4: Cyclic potentiodynamic polarization curve for AISI 4340 test in nitrate/chloride environment.

The dotted lines in Figure 4 shows the points on the CPP curves obtained in 3.7M sodium nitrate solution, which correspond to the pitting (Ep) and repassivation (Erep) potentials respectively. The value for the Ep, reading from the curve is -373mV vs. SCE while the

Erep from the curve is 468mV vs. SCE.

Table III: Data summary from CPP curve for AISI 4340 test in nitrate/chloride environment

Solution Ecorr a c Polarization icorr Ep - Erep

Composition Resistance Rp (mV vs. SCE) (ΔmV/ΔIog i) (ΔmV/ΔIog i) (Ohm-cm2) (A/cm2) (mV)

3.7M NaNO3 -471 34.7 25.2 3.80E+08 1.67E-08 96mV pH 9

3.7M NaNO3 -496 43.9 109.7 3.25E+08 4.20E-08 No 1M NaCl obvious

pH 9 Ep

3.7M NaNO3 -571 12.5 22 2.53E+05 1.37E-05 No 3.7M NaCl obvious

pH 9 Ep 3.7M NaCl -740 54 82.6 4.06E+06 3.50E-06 No pH 9 obvious

The images of samples after testing in the different environment are shown in Figure 5 to

Figure 8.

0.5mm 0.5mm

Figure 5: Sample after testing in 3.7M NaNO 3 Figure 6: Sample after testing in 3.7M NaCl & solution 3.7M solution NaNO3

1mm 0.5mm

Figure 7: Sample after testing in 1M NaCl Figure 8: Sample after testing in 3.7M NaCl & 3.7M solution NaNO3

In the second set of experiments conducted, the pitting behavior of AISI 4340 steel exposed to alkaline solutions of sodium chloride was investigated. At a pH as high as 12, a temperature of 50°C and in solutions with varying concentration of sodium chloride, the difference in the pitting potential with reference to repassivation potential was observed and measured. The pH of the solution was adjusted with 0.5M sodium hydroxide and was kept under a deaerated condition before and during the test. The overlayed semi-log plot of the current density versus the potential for the four different concentrations of sodium chloride is shown in Figure 9.

Figure 9: Cyclic potentiodynamic polarization curve for AISI 4340 test in varying chloride ion concentration

The polarization curves in Figure 9 shows more clearly the pitting (Ep) potential on the forward scans. The reverse polarization scan did not intersect at any point with the forward scan which made the reading of the repassivation potential (Erep) difficult. The trend in the pitting potential with respect to chloride concentration gives information on how the chloride ion in solution affects the stability of the passive film formed in the pH environment. The different between the pitting potential (Ep) and free corrosion potential

(Ecorr) is shown clearly in Figure 10.

Figure 10: Difference between the breakdown and free corrosion potential in the different

chloride ion concentration environment

Pictures of the samples after testing were taken to further highlight the difference in the

susceptibility of this material to localized corrosion in the different test environments.

The images of these samples are shown in Figures 11 – 14.

0.5mm 0.5mm

Figure 11: Sample after testing in 1M NaCl Figure 12: Sample after testing in 0.1M solution . NaCl solution.

0.5mm

Figure 13: Sample after testing in 0.01M Figure 14: Sample after testing in 0.001M NaCl solution. NaCl solution.

3.2 Electrochemistry in Simulated Fracturing Fluid Environment

The effect of the pH adjusting agent on the corrosion behavior of unstressed AISI 4340 material was studied using the cyclic potentiodynamic polarization method. These electrochemical measurements were made from pH range of 7-10 for each of the pH adjusting agents and were compared. In solution 3 (with Na2CO3 pH adjusting agent) cyclic polarization measurements were done to observe if there is any change in the free corrosion potential (Ecorr) and polarization resistance (Rp) trend from pH 7- pH 10. A comparison of the cyclic polarization measurements made in solution 2 and solution 3 environments at 50C is presented in Figure 15. More tests were carried out in solution 3 environment at a temperature of 25C. Figure 16 shows the cyclic polarization result obtained from this measurement.

Figure 15: Comparison of CPP curves in solution 2 and solution 3 at pH 7-10 and temperature 50C

The arrows on the curve indicate the direction of the potentiodynamic scan.

Figure 16: CPP curves in solution 3 at pH 7-10 and temperature 25C

The free corrosion potential (Ecorr) and polarization resistance (Rp) of AISI 4340 material in solution 3 at 50C and 25C were compared for the pH ranging from 7-10. Figures 17 and 18 show the graphical representation of this comparison.

Figure 17: Ecorr summary in solution 3 with varying pH and at temperature 25C and 50C

Figure 18: Polarization resistance summary in solution 3 with varying pH and at temperature 25C and 50C

Studying and comparing the susceptibility of the AISI 4340 in environments where these two pH adjusting agents are used is the reason for conducting these electrochemical measurements. Observing these curves and taking note of any breakdown potential which might occur in any of these environments were used as an indication of possible susceptibility of AISI 4340 to localized corrosion in that particular environment.

3.3 Slow Strain Rate Test Measurement (SSRT) in Fracturing Fluid Environment

After careful examination of all the measured CPP curves in Figure 15, especially for environments where there was indication of passive film breakdown, it was observed that the potential of -400mV vs SCE falls between the breakdown potential and the repassivation potential for this material. This region (between the breakdown potential and the repassivation potential) is often times regarded as one where there is maximum localized corrosion susceptibility of the test material in the environment12. Based on this assessment, the slow strain rate tests were conducted at a potential of -400mV vs SCE

(which is 350mV above the open circuit) in the same solution as that used in the electrochemical test environments. SSRT was conducted in the following environment;

- Air (control test)

- Solution 1 (pH of solution unadjusted and typically 5.6)

- Solution 2 (NaOH pH adjusting agent) pH 7 – pH 10

- Solution 3 (Na2CO3 pH adjusting agent) pH 7 – pH 10

The same environmental conditions of temperature and solution oxygen content were maintained as they were in the electrochemical tests. The sample was kept in solution under a preload of 75lbm while the open circuit of the cell was observed until a steady

36 potential reading was reached. A potential of -400mV vs. SCE was applied on the cell for one hour before stress was applied. The strain rate used for all the SSRT was 5.0E-

07in./sec.

Figure 19: Slow strain rate chart in air, solution 1 and solution 2

Figure 20: Slow strain rate chart in air, solution 1 and solution 3

The evaluation of SCC susceptibility through SSRT was expressed in terms of the percentage reduction in area (%RA), the time to failure and the percentage plastic elongation (%EP) according to ASTM G12925.

The percentage reduction in area (%RA) was calculated using the following expression25:

( ) (2)

Where Df and Di are the final and initial diameter of the tensile specimen respectively have units in inches.

The percentage plastic elongation can be calculated using the following expression25;

[ ( ) ( )] (3)

Where

EP = Plastic strain to failure (%)

EF = Elongation at failure (in./in.)

EPL = Elongation at proportional limit (in./in.)

LI = Initial gauge length (in.) (usually 1 in.)

2 σF = Stress at failure (lbs/in )

2 σPL = Stress at proportional limit (lbs/in )

A summary of the calculated results for each of the tests conducted are shown graphically in Figures 21, 22 and 23.

Figure 21: Chart showing the % reduction of AISI 4340 in various test environments

Figure 22: Chart showing the time to failure of AISI 4340 in various test environments

Figure 23: Chart showing the % plastic elongation of AISI 4340 in various test environments

The labels in red in Figures 21 to 23 indicate the environments in which there was a difference in the cracking behavior of the test sample after testing.

The data series with the red labels in Figures 21, 22 and 23 shows the average of two measurements conducted and the arrow bars on them represents the standard deviation of these measurements around the mean.

After testing in solution 2 and solution 3 environments, samples were analyzed using the scanning electron microscopy. Observed SEM images suggested that AISI 4340 behaved differently between each other under stress in solution 2 pH 7 and solution 3 pH 7 environments. The SEM images of the AISI 4340 material in these two environments are shown in the Figures 24 and 25.

Figure 24: SEM image of AISI 4340 sample after testing in solution 2 pH 7 environment

Figure 25: SEM image of AISI 4340 sample after testing in solution 3 pH 7 environment

3.4 Crack Microstructure

The microstructure image of the cracks which were formed on AISI 4340 slow strain sample after testing are shown in Figure 26.

50µm 50µm

Figure 26: Microstructure image of cracks on AISI 4340 sample after SSRT in solution 3 pH 7 environment

3.5 Slow Strain Rate Tests at Potentials Close to the Ecorr

More slow strain rate measurements were performed at potentials of -500mV vs. SCE and -600mV vs. SCE. The chart in Figure 27 shows the overlayed stress vs. strain measurement of the slow strain rate test performed in solution 3 pH 7 environment at a potential of -400mV, -500mV and -600mV vs. SCE.

Figure 27: Slow strain rate test results in solution 3 at pH 7 at varying potentiostatic hold.

Picture of samples after testing were taken and examined to note the potential where the stress corrosion cracks on the sample stopped. The image of the samples tested in solution 3 pH 7 environment at -400mV, -500mV and -600mV vs. SCE are shown in

Figure 28, Figure 29 and Figure 30.

Figure 28: Sample after SSRT in solution 3 pH 7 at -400mV vs. SCE potentiostatic hold.

0.5mm

Figure 29: Sample after SSRT in solution 3 pH 7 at -500mV vs. SCE potentiostatic hold.

0.5mm 0.5mm

Figure 30: Sample after SSRT in solution 3 pH 7 at -600mV vs. SCE potentiostatic hold.

3.6 Long Open Circuit Potential (OCP) Measurement

The open circuit measurement of AISI 4340 in solution 3 pH 7 environment was carried out for 20 hours and for 108 hours respectively. The overlay of these OCP measurements is shown in Figure 31 below.

Figure 31: Overlay of OCP measurement of AISI 4340 in solution 3 pH 7 environment for 20 hrs and 108 hrs.

3.7 Static Load Test

As earlier stated, this test was conducted in solution 3 pH 7 environment while the sample was held at a potential of -400mV vs. SCE. The static load test was conducted at two stress points: above the yield strength of the material and below the yield strength of

47 the material. The displacement measurements for stress held below the sample yield and stress hold above the sample yield are shown in Figure 32 and 33 respectively.

Figure 32: Displacement measurement at stress held below yield strength of material in solution 3 at pH 7.

Figure 33: Displacement measurement at stress held above yield strength of material in solution 3 at pH 7.

The current reading of the stress hold tests was taken. An overlay of the current readings at stress held below and above the yield strength of the material is shown in Figure 34.

Figure 34: Overlay of current reading at stress held above and below the yield strength of AISI 4340 in solution 3 at pH 7.

The image of the sample after static test at stress hold below material yield strength is shown in Figure 35 while the image of the sample after static test at stress hold above material yield strength is shown in Figure 36.

0.5mm

Figure 35: Image of sample after test at stress hold below yield strength in solution 3 at pH 7.

Figure 36: Image of sample after test at stress hold above yield strength in solution 3 at pH 7.

3.8 Post Test Analysis

An AES analysis was carried out on the stress and unstressed sample at a potential hold of -400mV vs SCE while the oxide film of the unstressed sample analyzed using SEM.

In carrying out a potentiostatic test, a potential of -400mV vs. SCE was applied on AISI

4340 material in solution 3 pH 7 and solution 2 pH 7 solutions respectively for 16 hours.

The current reading was plotted against time and is shown in Figure 37.

Figure 37: Potentiostatic test of AISI 4340 material in solution 3 pH 7 and solution 2 pH 7 environments at -400mV vs SCE.

The SEM image of the oxide film in solution 3 (pH 7) and solution 2 (pH 7) environment are shown in Figure 38 and Figure 39, respectively.

Figure 38: SEM image of oxide film formed on AISI 4340 after

potentiostatic hold of -400mV vs. SCE for 16hrs in solution 3 pH 7 environment.

Figure 39: SEM image of oxide film formed on AISI 4340 after potentiostatic hold of -400mV vs. SCE for 16hrs in solution 2

pH 7 environment.

The AES depth profile of AISI 4340 sample after formation of oxide film was conducted in duplicates for the sake of reproducibility. Figure 40 shows the depth profile of AISI

4340 after formation of oxide film in solution 2 (pH 7) environment while Figure 41 shows the depth profile of the repeated test.

Figure 40: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at pH 7 (Test 1).

Figure 41: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at pH 7 (repeat test).

Figure 42 shows the depth profile of AISI 4340 after formation of oxide film in solution 3

(pH 7) environment and Figure 43 shows the depth profile of the repeated test in the same environment.

Figure 42: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 3 at pH 7 (Test 1).

Figure 43: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 3 at pH 7 (repeat test).

The AES depth profile around the failed region of stressed AISI 4340 sample in solution

2 pH 7 environment is shown in Figure 44. As in the AES depth profile of the oxide film on the unstressed AISI 4340 sample, the repeated AES depth profile around the failed region of the stressed AISI 4340 sample in solution 2 (pH 7) environment was conducted.

The depth profile of the repeat experiment is shown in Figure 45.

It is worth mentioning that chlorides are decomposed by the ion beam that was used for sputtering and so the chloride level is higher than indicated in the profiles.

Figure 44: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 2 at pH 7 (Test 1).

Figure 45: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 2 at pH 7 (Repeat test).

Figure 46 shows the AES depth profile around the failed region of the stressed AISI 4340 sample in solution 3 (pH 7) environment while Figure 47 shows the AES depth profile around the failed region of the stressed AISI 4340 sample in a repeat experiment.

Figure 46: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 3 at pH 7 (Test 1).

Figure 47: AES depth profile around fracture area of AISI 4340 SSRT sample in solution 3 at pH 7 (Repeat test).

CHAPTER 4

DISCUSSION

4.1 Effect of Chloride Ion Concentration on the Passivation Behavior of AISI 4340

It is well known that oxidizing anions such as chromates, nitrites and nitrates, are capable of passivating steel in the absence of oxygen26. The association between sodium nitrate solution and localized corrosion has been widely discussed24. While some authors agree that sodium nitrate mitigates localized corrosion, others report that localized corrosion is induced by sodium nitrate24. In the work done by Anderko et al12, it was shown that nitrate ions show strong inhibiting properties where pitting potential substantially increases with an increase in nitrate concentration. In the results shown in Figure 4, AISI

4340 is observed to passivate the most when immersed in 3.7M sodium nitrate solution.

However, the ability of AISI 4340 to form a passivating film in solution was greatly reduced as the mole concentration of the chloride ion in solution was increased.

Considering that chloride ions are relatively small anions with high diffusivity, the increase in chloride ion concentration would penetrate the oxide film and form an iron

(III) chloride with the substrate metal, which would undergo an active dissolution. While the aggressive specie (chloride ions) will form metal complexes with Fe ion which will dissolve in the active state, inhibitory species such as the nitrate ion will contribute to the formation of oxides which will induce passivity27. As shown in the summary in Table III,

AISI 4340 showed a clear break down potential in 3.7M sodium nitrate solution while in the other solutions where chloride ions were introduced no obvious break down potential was observed. Introduction of a high concentration chloride ions results in the competitive formation and adsorption of the oxide film and salt film on the metal substrate. The experimental result presented in Figure 4 agrees with the model developed by Anderko et al27 which assumes that at any given instant, the oxide layer covers a certain fraction of the metal surface. This fraction is increased as repassivation is approached. The dissolution rate of the metal under the oxide film (pure nitrate solution) is lower than at the metal-halide interface and corresponds to the passive dissolution rate12.

Several theories have been developed to relate the pitting potential to the activity of an aggressive solution species12. These theories are based on different physical concepts such as the thermodynamic stability of pits, transport and coalescence of vacancies in the passive film or irreversible thermodynamic stability of pit nuclei12. Although these physical concepts are different, it has been shown that there is a simple linear dependence of the pitting potential on the activity of an aggressive ion in solution12. This relationship is given by12:

- Ep = a + b ln aCl (4)

Where,

Ep is the pitting potential a and b are constants and

- - aCl is the activity of the Cl in solution.

The above model was applied to the test result obtained in Figure 9. The activity of the

Cl- in solution was obtained using the equation below28;

A = ɣ [C] (5)

Where ɣ is the activity coefficient and [C] is the concentration of Cl-.

The activity coefficient is typically assumed to be equal to 1 especially in dilute solution28. Therefore the activity of the Cl- in solution is equal to the concentration of Cl- in solution.

The pitting potential of AISI 4340 from the cyclic polarization curves was plotted against the natural log of the activity of the chloride ion in each solution and shown in Figure 50.

The plot in Figure 48 shows a clear linear dependence of the pitting potential on the chloride ion activity in solution which agrees with equation (4).

- - Figure 48: Dependence of pitting potential (Ep) on the activity of Cl (aCl ) in solution

The passivation phenomena of a given material may be influenced by pH and the presence of aggressive or inhibitive species in the solution29. With the solution pH in the above test (Figure 9 and Figure 48) kept constant at 12, an increase in the concentration of aggressive species (Cl-) in the solution would expectedly affect the ability of AISI

4340 to form stable passive films. This effect on the stability of the passive film affects its protective properties and will result in a change in the pitting potential. Adsorption of aggressive ions (in this case Cl-) on the passive film always precedes pit nucleation and propagation30. The same substitutional adsorption mechanism process of water molecules adsorbed on the passive film by Cl- in solution proposed in the work done by

Petek et al30 is adopted for the test with results shown in shown in Figure 48.

(6)

The increased passive current density in Figure 9 may be due to the conversion of

31 Fe(OH)2 to a soluble FeOCl by the following reaction :

(7)

Results of past work have also shown that the pitting potential of a passive metal is dependent on the chloride ion concentration by the following equation31:

(8)

Comparing equation (8) with equation (4) we can see that

(9)

Where n is the number of moles of chlorides

n1 is the number of moles of electrons transferred in the cell reaction in equation (7)

R is gas constant (8.314 J/K-mol)

F is Faradays constant (96,485C/mol)

Inserting the values for b, R, T and F into equation (9) will give;

(10)

Substituting n1 with 1 in equation (10) according to the reaction in equation (7) we have; n = 3.85moles ≈ 4 moles of Cl-

This shows that in the dissolution of one surface Fe(OH)2 during the pitting of AISI 4340 in a chloride solution, four chloride ion are involved according to the following reaction:

(11)

4.2 Electrochemical Behavior of AISI 4340 in Simulated Fracturing Fluid Solution

Effect of pH on Free Corrosion Potential (Ecorr)

Figures 17 and 18 show the summary of the results obtained from the cyclic polarization measurement carried out in solution 3 (Na2CO3 pH adjusting agent) from pH 7 to pH 10 at 50C and 25C, respectively. Using the reaction mechanism leading to the formation

32 of FeCO3 as described in the work done by Alves et al , the following reaction is used to explain the formation of the carbonate film;

(12)

(13)

The reaction in equation (13) is dependent on the pH and HC is favored by the high

HC concentration.

With the observed Ecorr trend of AISI 4340 in solution 3 over the pH range of 7 to 10 in deaerated condition shown in Figure 17, the dominant cathodic reaction can be said to be the reduction of hydrogen ion according to the following:

(14)

The hydrogen formed in equations 14 and the dominant anodic reaction is the oxidation

2+ of Fe to Fe which leads to the formation of FeCO3 in the presence of a bicarbonate ion according to equation 13 above. The complete net ionic reaction is suggested to be:

(15)

By using sodium carbonate as the pH adjusting agent, a buffer will be formed. The pH of the solution is increased due to the formation of the bicarbonate ion according to the forward reaction in equation (16). At open circuit potential, the net rate of reaction is equal to zero and the cathodic and the anodic reaction are said to be at

equilibrium. However, with the increase in the concentration of , the release of more hydrogen ion into the solution according to the anodic reaction in equation (13) is favored. This increase in the concentration of the hydrogen ion shifts the equilibrium at

open circuit towards the cathode. Therefore, at even higher concentration of

(higher pH), the equilibrium shifts even further towards the cathode which results in a more cathodic Ecorr value as shown in the data in Figure 17.

(16)

Figure 15 also contains the cyclic polarization curves of AISI 4340 in the solution 2 environment with solution pH ranging from 7-10 and solution temperature of 50C. With the exception of pH 10, there was no statistically significant difference in the nature of the polarization curves obtained in the other solution 2 pH range of 7-9. It is known that at temperatures above ambient, the corrosion rate of low alloy steel is greater and is

68 accompanied by the risk of caustic stress corrosion cracking (CSCC). However, at low concentration of below 50% NaOH solution, carbon steel is considered to be safe up to a temperature of 82C. These concentration and temperature conditions are less severe in solution 2 environment and thus are within the region of safety. At solution pH of 10, the

AISI 4340 test sample exhibited a passive behavior which was not seen in the lower solution 2 pH environments.

Effect of Temperature on Polarization Resistance (Rp)

The solution temperature does not affect the trend in the Ecorr values in the test solution over the pH range of 7-10. However, for each pH, with the exception of pH 7, the Ecorr values are more cathodic at 25C than at 50C. This suggests that the increase in temperature shift the equilibrium of the reaction in equation (16) towards the left. This shift in the reaction equilibrium results in a relatively lower concentration of bicarbonate ions being present in solution at 50C. This causes the Ecorr value at 50C to be more positive than that at 25C.

El-Anadouli et al33, studied the effect of temperature on the polarization resistance for a corrosion reaction under pure activation control. The analysis was performed in terms of the Stern-Geary equation in which polarization resistance (Rp) was related to temperature using the following equation taking 25C as a reference point33:

[ ( )] (17)

Where

is the polarization resistance at 25C (ohm),

is the activation energy in kJ/mol

T is the temperature in kelvin and

is the polarization resistance at the temperature above 25C

It can be seen from equation (12) that as the temperature increases, the numerator increases linearly while the denominator increases exponentially and hence decreases.

The experimental results represented in Figure 18 agree with equation (12). For all the test solution pH (pH 7 – pH 10), the polarization resistance of AISI 4340 material decreased with an increase in the solution temperature.

It can be also be deduced from the CPP curves in Figures 15 and 16 that AISI 4340 showed a greater tendency of forming a passive film as the pH of the solution increased.

It is can also be seen that a potential of -400mV vs SCE is above the repassivation potential in the cyclic polarization curve obtained in all the environments as shown in

Figures 15 and Figure 16. This was the potential the test material was held at while stress was applied in the slow strain rate test.

Slow Strain Rate Test (SSRT) Measurements in Solution 1, Solution 2 and Solution 3

Environment

The visual examination of the sample after slow strain measurements showed no difference in the cracking behavior of the sample except in solutions at pH of 7.

However, the trend in the graph shown in Figures 21, 22 and 23 suggests that at a high pH, using NaOH as the pH adjusting agent results in a more aggressive environment while using Na2CO3 as the pH adjusting agent proves to be aggressive on the metal at low

70 pH solutions. Based on the SEM analysis of the sample, severe cracks were observed to occur on AISI 4340 after slow strain rate testing in solution 3 (pH 7) environment, none of such cracks were observed to occur on the test material after testing in the solution 2 pH 7 environment. This difference in corrosion behavior of this material was not evident in other solutions at higher pH values. Figure 24 and Figure 25 shows the SEM image of the sample after testing in solution 2 at pH of 7 and solution 3 at pH of 7 respectively. It was also observed from the SEM images of the samples after testing in all the environments that the cracks observed in solution 3 at pH 7 (Figure 25) are the most severe. Images in Figure 25 show that the nature of cracks formed on AISI 4340 material in solution 3 at pH 7 are transgranular in nature. The formation of transgranular cracks are likely due to the stress concentration at the crack tip as a result of presence of reduced hydrogen atom which can occur more prevalently in this environment. Transgranular

SCC is typically associated with dilute, neutral pH environment in contract with the steel surface19. The result obtained from the electrochemical testing of AISI 4340 material in solution 3 also supports the results obtained from the slow strain rate measurements and the static load test at a pH of 7.

4.3 Relating Test Environment to Field Condition

Since pipelines used in the hydraulic fracturing operation are not held at a potential of

-400mV (vs. SCE) while in service, it is important to ascertain if AISI 4340 material is susceptible to SCC at a potential equivalent to the open circuit potential of the pipeline steel. For this reason slow strain rate measurements were conducted at more negative potentials (closer to the OCP) and the corrosion behavior of the sample after testing was observed. Figures 28, 29 and 30 show the image of the sample after testing in solution 3

71 pH 7 at -400mV vs SCE, -500mV vs SCE and -600mV vs SCE respectively. It can be seen from the images that samples held at potentials of -600mV (vs. SCE) and -500mV

(vs. SCE) had the same strain but exhibited different cracking behavior. While the sample held at a potential of -600mV (vs. SCE) did not crack, the sample held at -500mV (vs.

SCE) had several cracks formed on it.

Due the result obtained from the slow strain rate measurement carried out at potentials close to the Ecorr, a long open circuit potential measurement was done for 20 hours and

108 hours respectively. Both curves represented in Figure 31 showed the same change in potential trend. The open circuit potential (OCP) was increasingly negative and then reversed in the direction towards a more positive potential. The OCP after 40 hours reached a steady state at a potential of -500mV vs SCE. At this potential of -500mV (vs.

SCE) in solution 3 pH 7 environment, stress corrosion cracking occurred on AISI 4340 as shown in Figure 29. This shows that AISI 4340 material may be susceptible to stress corrosion crack under field conditions.

4.4 Static Load Test Below and Above Yield

The current reading is used to measure the corrosion rate of a sample in solution. Increase in exposed surface area results in increase in the recorded current. This was used as an indication for crack initiation and propagation. The visual study of the sample after testing and the sample displacement (which is the same as the extension of the test material) were also used to support the inference drawn from the trend in the current reading. The increasing displacement at the stress level above the yield strength of the material as shown in Figure 31 as well as the increasing current density at this stress level is an indication that there is an initiation and propagation of SCC on the test sample. The

72 current reading recorded at the stress level below the yield of the material shows that no crack initiation or propagation occurred. The image of the sample after the test shown in

Figure 35 and Figure 36 supports the displacement and currents results shown in the

Figures 32, 33 and 34. While stress corrosion cracking occurred on the sample subjected to stress above the yield strength of the material, no evidence of SCC was observed on the sample subjected to stress below the yield strength. This shows that the test material,

AISI 4340, may only be susceptible to SCC only at a stress level above the yield strength of the material. It can be said that the material only shows marginal susceptibility to

SCC.

4.5 Surface Film Analysis

There was no statistically significant difference in the current reading measured while holding the sample at -400 mv (vs. SCE) as shown in Figure 37. However, the morphology of the oxide looked different according to the images shown in Figure 38 and Figure 39. More cracks were observed in the oxide film on the carbonate film than on the hydroxide film.

The set of depth profile results of the unstressed AISI 4340 samples in sodium hydroxide and sodium carbonate are shown in Figures 40 through 44. The depth profile of oxygen indicates that the oxide film formed in the sodium carbonate environment is thicker than the oxide film formed in sodium hydroxide environment. At a depth of 50nm the approximate oxygen percent is 25 for the oxide film in the carbonate environment (Figure

42 and Figure 43) while the approximate oxygen percent is ranges from 12 to18 in the hydroxide environment (Figure 40 and Figure 41). To further highlight the difference in the thickness of the oxide film in the two environments, it can be seen that the

73 approximate atom percent of the iron substrate under the hydroxide film at 200nm depth is above 80% while that in carbonate environment is below 80%. No chlorides were detected on the passive film form on the unstressed test sample.

The depth profiles of the oxide film around the fractured area of the stressed test sample in solution 3 and solution 2 are different from that obtained on the unstressed sample. The approximate atom percent of the chloride ions detected in the oxide film in the two test solutions were different. The flattening of the Fe depth profile at a depth of between

100nm and 150nm shows that the electron beam is having a direct contact with the substrate metal. While the oxide film formed on the sample in the solution 2 environment at 150nm depth for the two tests conducted contains an average of 10% of the approximate chloride, the oxide film formed on the sample in the tests performed in solution 3 contains about 5% for the second test. No chloride was detected in the passive film on AISI 4340 in the first test performed in solution 3 at pH 7. The high concentration of chloride still remaining in the oxide film in the solution 2 environment is a strong indication that the scale is an iron chloride scale. This explains the difference in the nature of the oxide film formed in solution 2 and solution 3 at pH of 7.

4.6 Conclusion

The test results support the earlier stated hypotheses and are summarized as follows:

Hypothesis 1: Transgranular form of SCC occurs at low pH in an environment aggressive enough to initiate cracks.

Results: Transgranular form of SCC was revealed by the metallographic analysis of the crack formed on AISI 4340 in solution having a pH of 7 and Na2CO3 as the pH adjusting

74 agent. The cracks formed in this environment propagated only at stress levels above the material yield.

Hypothesis 2: The susceptibility of AISI 4340 to localized corrosion (pitting) reduces with a decrease in chloride ion concentration in solution.

Result: The CPP measurements of AISI 4340 in solution with adjusted chloride ion concentration at pH of 12 showed that the potential where pitting occurs reduces (closer to the OCP) as the chloride ion concentration in solution increases. At a chloride ion concentration as high as 1M, AISI 4340 undergoes an active dissolution as observed in

CPP measurement in the nitrate/chloride solution.

Hypothesis 3: The pH adjusting agent used will affect the SCC susceptibility of the AISI

4340 material.

Result: The composition of the passive films formed on AISI 4340 in the solution with the two pH adjusting agents is the proposed reason for the observed difference in the

SCC susceptibility as shown by the results from the AES analysis. The SEM image of the test sample after SSRT test in solution using the two pH adjusting agents showed that

AISI 4340 exhibited more SCC susceptible at near neutral pH solutions with Na2CO3 used as the pH adjusting agent. The SSRT suggests that it is safer to used NaOH as the pH adjusting agent at near neutral solutions while at higher pH solutions, it is safer to use

Na2CO3 as the pH adjusting agent.

Future Work

Conclusions were made on the possible composition of the passive film form in the solutions the pH was adjusted to 7 with the different pH adjusting agents. The difference in the thickness of the oxide film as well as the possible presence of FeOCl complex which dissolves actively as the possible cause of the difference in the SCC behavior of the material was inferred from the AES depth profile. A future work on the XPS analysis on the samples that were analyzed by AES will give information on how the chlorine and oxygen are bonded and which atoms they are bonded to. The information obtained could further validate or invalidate the earlier stated propositions.

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