LOW ALLOY STEEL SUSCEPTIBILITY TO STRESS CORROSION CRACKING IN HYDRAULIC FRACTURING ENVIRONMENT

Transcription

1 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 By Ezechukwu J. Anyanwu Dayton, OH May, 2014

2 LOW ALLOY STEEL SUSCEPTIBILITY TO STRESS CORROSION CRACKING IN HYDRAULIC FRACTURING ENVIRONMENT Name: Anyanwu, Ezechukwu John APPROVED BY: Douglas C. Hansen, Ph.D. Advisory Committee Chairman Research Advisor and Professor Chemical and Materials Engineering Sean C. Brossia, Ph.D. Research Advisor Senior Vice President and Senior Principal Engineer DYCE USA Robert J. Wilkens, Ph.D., P.E. Committee Member Professor Chemical and Materials Engineering John G. Weber, Ph.D. Associate Dean School of Engineering Tony E. Saliba, Ph.D. Dean, School of Engineering and Wilke Distinguished Professor ii

3 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

4 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 Na 2 CO 3 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 Na 2 CO 3 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. iv

5 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, v

6 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. vi

7 TABLE OF CONTENTS ABSTRACT iii ACKNOWLEDGEMENTS v TABLE OF CONTENTS... vii LIST OF FIGURES.. x LIST OF TABLES. xv NOMENCLATURE.. xvi CHAPTER INTRODUCTION Background Fracturing Fluid Chemistry Literature Review Hypotheses 9 vii

8 CHAPTER MATERIALS AND METHODS Test Materials and Sample Preparation Electrochemical Testing in Environment with Varying Chloride Ion Concentration Electrochemical Testing in Simulated Fracturing Fluid Solution Post Test Analysis Stress Tests in Simulated Fracturing Fluid Solution Metallographic Analysis CHAPTER RESULTS Electrochemistry in Environment with Varying Chloride Ion Concentration Electrochemistry in Simulated Fracturing Fluid Environment Slow Strain Rate Test Measurement (SSRT) in Fracturing Fluid Environment Crack Microstructure Slow Strain Rate Tests at Potentials Close to the E corr Long Open Circuit Potential (OCP) Measurement.. 47 viii

9 3.7 Static Load Test Post Test Analysis 52 CHAPTER DISCUSSION Effect of Chloride Ion Concentration on the Passivation Behavior of AISI Electrochemical Behavior of AISI 4340 in Simulated Fracturing Fluid Solution Relating Test Environment to Field Condition Static Load Test Below and Above Yield Surface Film Analysis Conclusion REFERENCES.. 77 ix

10 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 Figure 4: Cyclic potentiodynamic polarization curve for AISI 4340 test in nitrate/chloride environment Figure 5: Sample after testing in 3.7M NaNO 3 solution Figure 6: Sample after testing in 3.7M NaCl & 3.7M solution NaNO 3 28 Figure 7: Sample after testing in 1M NaCl & 3.7M solution NaNO 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 Figure 10: Difference between the breakdown and free corrosion potential in the different chloride ion concentration environment Figure 11: Sample after testing in 1M NaCl solution x

11 Figure 12: Sample after testing in 0.1M NaCl solution Figure 13: Sample after testing in 0.01M NaCl solution Figure 14: Sample after testing in 0.001M NaCl solution Figure 15: Comparison of CPP curves in solution 2 and solution 3 at ph 7-10 and temperature 50 C Figure 16: CPP curves in solution 3 at ph 7-10 and temperature 25 C Figure 17: E corr 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 Figure 19: Slow strain rate chart in air, solution 1 and solution Figure 20: Slow strain rate chart in air, solution 1 and solution 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 xi

12 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 Figure 26: Microstructure image of cracks on AISI 4340 sample after SSRT in solution 3 ph 7 environment Figure 27: Slow strain rate test results in solution 3 at ph 7at varying potentiostatic hold 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 Figure 30: Sample after SSRT in solution 3 ph 7 at -600mV vs. SCE potentiostatic hold Figure 31: Overlay of OCP measurement of AISI 4340 in solution 3 ph 7 environment for 20 hrs and 108 hrs Figure 32: Displacement measurement at stress held below yield strength of material in solution 3 at ph xii

13 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 Figure 35: Image of sample after test at stress hold below yield strength in solution 3 at ph Figure 36: Image of sample after test at stress hold above yield strength in solution 3 at ph Figure 37: Potentiostatic test of AISI 4340 material in solution 3 ph 7 and solution 2 ph 7 environment at -400mV vs SCE 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.. 53 Figure 40: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at ph 7 (Test 1) xiii

14 Figure 41: AES depth profile (3keV Ar+) of oxide film on AISI 4340 in solution 2 at ph 7 (Repeat Test) 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) 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: 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) Figure 48: Dependence of pitting potential (E p ) on the activity of Cl- (a Cl -) in solution xiv

15 LIST OF TABLES Table I: The composition and mechanical property of AISI Table II: Solution chemistry of simulated fracturing fluid Table III: Data summary from CPP curve for AISI 4340 test in nitrate/chloride environment xv

16 NOMENCLATURE E corr Corrosion Potential (mv) E p Pitting Potential (mv) E Rep 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 (%) E F Elongation at failure (in./in.) E PL Elongation at proportional limit (in./in.) L I Initial gauge length (in.) (usually 1 in.) σ F Stress at failure (lbs/in 2 ) σ PL Stress at proportional limit (lbs/in 2 ) xvi

17 D i Initial diameter (in.) D f Final diameter (in.) a Cl - 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

18 IR Ohmic Resistance AISI American Iron and Steel Institute βa Anodic Tafel Slope (ΔmV/ΔIog i) βc Cathodic Tafel Slope (ΔmV/ΔIog i) R p Polarization Resistance (Ohms) I corr Corrosion Potential (amps) xviii

19 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 psi) that the geologic structure cracks or fractures 1. 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 fractures 1. While the hydraulic fracturing fluid is typically more than 99% water, other components are used to perform different specialized functions during the fracturing process 2. 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 losses 3. The variety of fracturing solution which could be used and the insufficient reported information about the fracturing fluid chemistry make it 1

20 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 fluids 4. 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 completed 5. 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 fracture 1. Chemicals used during the fracturing process are a vital component to a successful well completion 6.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 process 7. 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 specifications 6. Chemicals are mixed in very low concentration with water and 2

21 make up less than 1% of the total job volume 6,7. 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 crosslinkers 4. 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 additives 2. 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. 3

22 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 alloy 8. Such passive films, however, are often susceptible to localized breakdown such as pitting, which results in accelerated dissolution of the underlying metal 8. Pitting of a given material depends strongly on the presence of aggressive species in the environment, such as chloride ions, and a sufficient oxidizing potential 9. Pitting occurs when portions of the metal surface lose their passivity and dissolve rapidly 9. 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 contaminant 8. The level of interference of the chloride ion with passivity depends on the stability of the salt film formed 8. Chloride ions play an essential role in one of the most destructive type of corrosion; localized corrosion 10. Chloride also is an anion of a strong acid and many metals, iron inclusive, exhibit considerable solubility in chloride solutions 8. The presence of oxidizing agents such as oxygen and hydrogen in a chloridecontaining environment is extremely detrimental, and will further enhance localized corrosion 8. The minimum anodic potential needed for pitting to occur on a metal in a particular environment is known as the critical pitting potential 11. However, the potential at which pit growth or crevice corrosion will cease is known as the repassivation potential 12. The critical pitting potential shifts to less positive values as the chloride concentration 4

23 increases, giving rise to pits forming at a less positive potential 11. The critical potential required for pitting to occur varies with the logarithm of the bulk chloride concentration 8. Most researchers agree that the first step to pitting corrosion is adsorption of chloride anions on the passive film 10. 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 occur 13. 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 halides 10. 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 observed 13. 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

24 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 concentration 14. Valeria and Christopher investigated the passive films formed by carbon steel, chromium steel and high speed steel (HSS) in bicarbonate and chloride environments 15. They also found that these passive films behaved like n-type semiconductors, showing that the passive film properties are dominated by iron 15. 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 film 16. 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 cracks 16. 6

25 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 pipe 17. 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 SCC 3. The cracks frequently initiate at surface flaws that either preexist or are formed during service by corrosion, wear or other processes 18. SCC can be initiated from the bottom of a pit either by a dissolution process or mechanical process 19. With the dissolution mechanism, SCC generally requires a dissolution rate of at least 10 times higher in the depth direction than in the lateral direction 19. 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 phenomena 9. 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 stress 9. 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 SCC 20. However, the accumulation of metal cations, in this case, Fe 2+ 7

26 ions within a stress-corrosion crack, can hydrolyze to form hydrogen ions leading to an acidified local environment within the crack tip 11. 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 embrittlement 11. It is well accepted that the mechanism of high ph SCC of steel involves anodic dissolution for crack initiation and propagation 3. 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 steel 3. 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 mode 3. Contreras et al 3 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 metal 3. 8

27 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. 9

28 - 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 (nearneutral) 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. 10

29 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. 11

30 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 12

31 Figure 2: Cylindrical sample for electrochemical measurement Table I: The composition and mechanical property of AISI 4340 YS (MPa) UTS (MPa) Elongation (%) C (%) Si (%) Mn (%) P (%) S (%) Ni (%) Cr (%) Mo (%) Al (%) AISI 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

32 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 50 o C 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; M NaOH in 1M NaCl solution M NaOH in 0.1M NaCl solution M NaOH in 0.01M NaCl solution and M 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

33 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 standard 21. 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. 15

34 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. 16

35 Table II: Solution chemistry of simulated fracturing fluid Solution Chemistry Na 2 SO 4 NaCl CH 3 COOH Na 2 CO 3 NaOH Solution 1 ph of 5.6±0.3 1ppm 4ppm ppm - - Solution 2 ph 7 1ppm 4ppm ppm ppm Solution 2 ph 8 1ppm 4ppm ppm ppm Solution 2 ph 9 1ppm 4ppm ppm ppm Solution 2 ph 10 1ppm 4ppm ppm ppm Solution 3 ph 7 1ppm 4ppm ppm 0.9ppm - Solution 3 ph 8 1ppm 4ppm ppm 1.537ppm - Solution 3 ph 9 1ppm 4ppm ppm 2.913ppm - Solution 3 ph 10 1ppm 4ppm ppm 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 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. 17

36 Potentiostatic Measurements By observing the curves obtained from the CPP testing of the material, a potential which is above the repassivation potential (E rep ) and below the break down potential (E p ) 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. 18

37 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 surface 22. 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 -80 o C 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 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 (Na 2 CO 3 ph adjusting agent) were prepared and used for the stress tests. 19

38 Solution 2 and Solution 3 with ph ranging from 7-10 were prepared using NaOH and Na 2 CO 3 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. 20

39 Figure 3: Slow Strain Rate Test (SSRT) cell assembly The test cell containing the electrolyte and the test sample was heated to 50 o C 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

40 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 acid 23. 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, Na 2 CO 3 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. 22

41 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/in 2 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

42 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. 24

43 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 al 24 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 50 o C. The ph of the solution was adjusted with 0.05M sodium hydroxide solution and was deaerated with nitrogen gas. Figure 4 shows 25

44 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 E corr, βa, βc and R p values read from the Tafel fit are summarized in Table III. The Stern-Geary 11 equation was used to calculate I corr from the anodic and cathodic slopes and the polarization resistance. The calculated I corr is further divided by the immersed surface area of the sample to obtain the corrosion current density i corr. (1) R p is also said to be the ratio of the electrode over potential and the net change in current 11. Table III shows the summary of the CPP curves in Figure 4. 26

45 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 (E p ) and repassivation (E rep ) potentials respectively. The value for the E p, reading from the curve is -373mV vs. SCE while the E rep from the curve is 468mV vs. SCE. 27

46 Table III: Data summary from CPP curve for AISI 4340 test in nitrate/chloride environment Solution Composition 3.7M NaNO 3 ph 9 3.7M NaNO 3 1M NaCl ph 9 3.7M NaNO 3 3.7M NaCl ph 9 3.7M NaCl ph 9 E corr a c Polarization Resistance R p (mv vs. SCE) (ΔmV/ΔIog i) (ΔmV/ΔIog i) (Ohm-cm 2 ) (A/cm 2 ) (mv) E E-08 96mV E E-08 No obvious i corr E p - E rep E E-05 No obvious E E-06 No obvious E p E p E p The images of samples after testing in the different environment are shown in Figure 5 to Figure mm 0.5mm Figure 5: Sample after testing in 3.7M NaNO 3 solution Figure 6: Sample after testing in 3.7M NaCl & 3.7M solution NaNO 3 28

47 0.5mm 1mm Figure 7: Sample after testing in 1M NaCl & 3.7M solution NaNO 3 Figure 8: Sample after testing in 3.7M NaCl 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. 29

48 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 (E p ) 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 (E rep ) 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 (E p ) and free corrosion potential (E corr ) is shown clearly in Figure

49 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 mm 0.5mm Figure 11: Sample after testing in 1M NaCl solution. Figure 12: Sample after testing in 0.1M NaCl solution. 31

50 0.5mm Figure 13: Sample after testing in 0.01M NaCl solution. Figure 14: Sample after testing in 0.001M 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 Na 2 CO 3 ph adjusting agent) cyclic polarization measurements were done to observe if there is any change in the free corrosion potential (E corr ) and polarization resistance (R p ) 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. 32

51 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. 33

52 Figure 16: CPP curves in solution 3 at ph 7-10 and temperature 25 C The free corrosion potential (E corr ) and polarization resistance (R p ) of AISI 4340 material in solution 3 at 50 C and 25 C were compared for the ph ranging from Figures 17 and 18 show the graphical representation of this comparison. 34

53 Figure 17: E corr 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 35

54 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 environment 12. 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 (Na 2 CO 3 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

55 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 37

56 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 G The percentage reduction in area (%RA) was calculated using the following expression 25 : ( ) (2) Where D f and D i 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 expression 25 ; 38

57 [ ( ) ( )] (3) Where EP = Plastic strain to failure (%) E F = Elongation at failure (in./in.) E PL = Elongation at proportional limit (in./in.) L I = Initial gauge length (in.) (usually 1 in.) σ F = Stress at failure (lbs/in 2 ) σ PL = Stress at proportional limit (lbs/in 2 ) A summary of the calculated results for each of the tests conducted are shown graphically in Figures 21, 22 and

58 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 40

59 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

60 Figure 24: SEM image of AISI 4340 sample after testing in solution 2 ph 7 environment 42

61 Figure 25: SEM image of AISI 4340 sample after testing in solution 3 ph 7 environment 43

62 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 µm 50µm 50µm 50µm 50µm 50µm Figure 26: Microstructure image of cracks on AISI 4340 sample after SSRT in solution 3 ph 7 environment 44

63 3.5 Slow Strain Rate Tests at Potentials Close to the E corr 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

64 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. 46

65 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

66 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. 48

67 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

68 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

69 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. 51

70 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. 52

71 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 53