Investigation of Nonmetallic Inclusions and Their Corellation

INVESTIGATION OF NONMETALLIC INCLUSIONS AND THEIR CORELLATION

TO PITTING CORROSION OF AUSTENITIC STAINLESS STEELS

A Thesis

Submitted to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

Of the Requirements for the Degree

Master of Science

Marjan Alsadat Kashfipour

December, 2015 INVESTIGATION OF NONMETALLIC INCLUSIONS AND THEIR CORELLATION

TO PITTING CORROSION OF AUSTENITIC STAINLESS STEELS

Marjan Alsadat Kashfipour

Thesis

Approved: Accepted:

Adviser Department Chair Dr. Robert Scott Lillard Dr. Michael Cheung

Interim Dean of the College Committee Member Dr. Rex D. Ramsier Dr. Rajeev Gupta

Committee Member Dean of the Graduate School Dr. Qixin Zhou Dr. Chand Midha

Date

ii ABSTRACT

Although the effect of non-metallic inclusions on pitting corrosion of stainless steel alloys have been an interesting topic during last decades, the role of the inclusions on pitting initiation and propagation is still under discussion.

In this study, different types of inclusions consisting of carbon, nitrogen, silicon, oxygen, manganese, Sulfur etc. on 304L and 303 stainless steels have been investigated using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Energy

Dispersive X-ray spectroscopy (EDX) and Focused Ion Beam (FIB) techniques.

Preliminary in-situ AFM results in 0.1 M NaCl indicate the deposition of copper on MnS inclusions at anodic potentials but not at the Open Circuit Potential (OCP). No pitting was observed at MnS inclusions or the adjacent area. Pit propagation at the MnS-matrix boundaries was observed in-situ while it was exposed to 0.1 M NaCl solution. In these experiments samples were pretreated by exposure to acidic chloride solution at the OCP.

Although it was initially appeared to be pit initiation, it was demonstrated later via SEM-

FIB analysis that the observed trenches on the surface were in fact the propagated pits below the surface which reached to the top at some point. Based on SEM-FIB cross-section observation, the MnS inclusions were passivated by a thin layer of copper, presumably as

Cu2S.

iii Further examination of other types of inclusions, showed that there was no pitting corrosion at the inclusion-matrix boundaries. This indicates that local chemistry and not the geometry is the more probable reason for the MnS-matrix boundaries to be corroded.

iv ACKNOWLEDGMENTS

I would like to express my special appreciation to my advisor Dr. Scott Lillard for his leadership, direction and encouragement throughout the entirety of this research.

Many thanks to all the thesis committee members, DR. Rajeev Gupta and Dr. Qixin

Zhou, for their valuable comments and suggestions that enhance the quality of the work.

I would like to give special thanks to my family. Words cannot express how grateful

I am to my parents and brother for all of their love and support.

v TABLE OF CONTENTS

Page

LIST OF FIGURES ...... ix

CHAPTER

I- INTRODUCTION ...... 1

II- BACK GROUND ...... 4

2.1 Mechanism of Pitting Corrosion ...... 5

2.2 Effect of Inclusions on Pitting Corrosion of Stainless Steels ...... 6

2.2.1 Various Types of Inclusions as Pitting Nucleation Sites ...... 7

2.2.1.1 Dissolution of MnS Inclusions as an Initial Step and Related Proposed Mechanisms for Pitting Corrosion of Stainless Steel Alloys ...... 8

2.2.1.1.1 Electrochemical Properties of Inclusions...... 11

2.2.1.1.1.1 In-situ, Ex-situ AFM Investigation of Inclusions ...... 12

2.2.1.2 Pit Initiation Sites at MnS Inclusions ...... 16

2.2.1.2.1 Undermine Pitting and Its relation to MnS Inclusions...... 17

2.2.1.3 The Role of Protective Films at MnS Inclusions ...... 18

2.2.1.4 Effect of Alloying Elements on Pitting Susceptibility ...... 18

III- EXPERIMENTAL PROCEDURE ...... 20

3.1 Materials and Sample Preparation ...... 21

3.2 Scanning Electron Microscope (SEM)/ Energy-Dispersive X-ray Spectroscopy (EDX) ...... 22

3.3 Atomic Force Microscopy ...... 22

vi 3.3.1 KPFM- AM Imaging ...... 23

3.3.2 AFM Fluid Imaging ...... 25

3.4 Focused Ion Beam (FIB) ...... 26

3.5 Electrochemical Tests ...... 27

3.5.1 Outside AFM ...... 27

3.5.2 Inside AFM ...... 28

IV- RESULTS AND DISSCUSSIONS ...... 29

4.1 303 Stainless Steel ...... 29

4.1.1 Cyclic Polarization Potential Curves ...... 30

4.1.2 Potentiodynamic Polarization of Copper Samples ...... 32

4.1.3 Non-Treated Samples...... 33

4.1.3.1 SEM/EDS Maps Before Polarization...... 33

4.1.3.2 AFM Maps Before Polarization ...... 34

4.1.3.3 In- Situ AFM Maps During Polarization in Solution...... 35

4.1.3.4 AFM Maps After Polarization ...... 40

4.1.3.5 SEM/EDS Maps After Polarization ...... 40

4.1.4 Pre-Treated 303 Samples ...... 42

4.1.4.1 SEM/EDS Maps Before Polarization...... 42

4.1.4.2 AFM Maps Before Polarization ...... 44

4.1.4.3 In-Situ AFM Maps During Polarization ...... 44

4.1.4.3.1 Inclusion 1 ...... 46

4.1.4.3.2 Inclusions 2, 3 and 4 ...... 47

4.1.4.4 SEM/EDS Maps After Polarization ...... 47

vii

4.1.4.5 AFM Maps After Polarization ...... 50

4.1.4.6 SEM/EDS Maps After Sonication ...... 53

4.1.4.7 Cross-Section Observation of Pits ...... 57

4.1.4.8 Behavior of MnS Inclusions at Other Polarization Potentials .68

4.1.4.9 Behavior of MnS Inclusions Before and After Treatment ...... 73

4.2 304L Stainless Steel ...... 78

4.2.1 Cyclic Polarization Potential Maps ...... 79

4.2.2 Multi-Phase Oxide Inclusions ...... 79

4.2.2.1 Silicon/Oxygen Base Inclusion ...... 80

4.2.2.2 Carbon/ Nitrogen/Oxygen/Sulfur Base Inclusion ...... 87

4.2.2.3 Carbon/Oxygen/Silicon Base Inclusion ...... 89

4.2.2.4 Carbon/ Silicon/ Nitrogen/ Oxygen ...... 91

4.2.2.5 Carbon/Silicon/Nitrogen/Nickle/Chromium/Iron Base Inclusion ...... 94

4.2.2.6 Carbon Base Inclusion ...... 96

4.2.2.7 Carbon/Nitrogen/Oxygen Base Inclusion ...... 100

V- CONCLUSION ...... 104

REFERENCES ...... 106

viii LIST OF FIGURES

Figure Page

1. Dimension Icon AFM used in this study ...... 23

2. Standard AFM cantilever holder ...... 23

3. Fluid Probe Holder ...... 25

4. EC Chuck ...... 25

5. Electrochemical set-up outside the AFM ...... 27

6. Electrochemical set-up inside the AFM ...... 28

7. Cyclic polarization potential curve of 303 stainless steel in 0.1 M NaCl ...... 30

8. Cyclic polarization potential curve of 303 stainless steel in 1 M NaCl PH 3 ...... 31

9. Overlay of CPP curves of 303 SS in 0.1 M and acidic 1 M NaCl ...... 31

10. Overlay of CPP curve of 303 SS and potentiodynamic curves of Cu in 0.1 M NaCl solution ...... 32

11. Overlay of CPP curve of 303 SS and potentiodynamic curves of Cu in acidic 1 M NaCl solution ...... 33

12. MnS inclusion SEM image on 303 SS surface ...... 34

13. EDS dot maps of inclusion in Figure 12 ...... 34

14. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b, for selected inclusion in Figure 12, before running potentiostatic test...... 35

15. In-situ AFM images of the inclusion in Figure 12 potentiostatically polarized in 0.1 M NaCl at 0.227V (SCE). Figures 1-3 were collected during OCP, and figures 4-9 belong to potentiostatic step...... 36

16. Line scan analysis of the in- situ images in Figure 1 ...... 39

17. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b of the inclusion shown in Figure 12, after potentiostatic test ...... 40

18. EDS dot maps of the inclusion in Figure 12 after polarization...... 41

19. MnS inclusion SEM image on 303 SS surface ...... 42

20. EDS dot maps of the inclusion in Figure 19 ...... 43

21. Lower magnification of the area of the inclusion in Figure 19...... 43

22. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b of the inclusion shown in Figure 19, before potentiostatic test ...... 44

23. In-situ AFM topographic map over the selected area shown in figure 19 in OCP step ...... 45

24. In-situ AFM images of the inclusion in Figure 19 potentiostatically polarised in 0.1 M NaCl at 0.227V (SCE). Figures a was taken in OCP while figures b and c were taken after 15 and 25 minutes in potentiostatic step ...... 45

25. In-situ AFM images of the inclusion number one in Figure 24. Parts a, b and c were cropped from the same parts of Figure24 ...... 46

26. Line scan analysis of Figures 25b and 24c ...... 46

27. In-situ AFM images of the inclusions two, three and four cropped from the same parts of Figure 2 ...... 47

28. SEM/ EDS dot maps of the inclusion in Figure 19 after polarization experiments (1 hour of OCP and 2 hours of potentiostatic step) ...... 48

29. EDS dot maps of manganese, sulphur and their overlay from the inclusion shown in Figure 19, after potentiostatic polarization tests ...... 49

30. EDS dot maps of copper, sulfur and their overlay from the inclusion shown in Figure 19, after potentiostatic polarization tests ...... 49

31. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b of the inclusion shown in Figure 19, after polarization (1 hour of OCP and 2 hours of potentiostatic step) ...... 50

32. Line scan analysis over the topographic and potential maps of the selected inclusion in Figure 19 after polarization experiments ...... 51

33. The overlay of after polarization EDS dot maps of copper, sulfur and manganese, on AFM topographic maps of the inclusion shown in Figure 19, after polarization tests (1 hour of OCP and 2 hours of potentiostatic step) ...... 52

34. SEM/ EDS dot maps of the inclusion in Figure 19 after running polarization test (1 hour of OCP and 2 hours of potentiostatic step Eapplied= 0.227 V vs. SCE) and sonication ...... 53

35. AFM topographic image of the inclusion after polarization tests followed by sonication ...... 54

36. The inclusion in Figure 19 in different steps of experiments ...... 55

37. Inclusion 6 in Figure 24 in different steps of the experiment ...... 56

38. Lower magnification observation over the area of the inclusion in Figure 19 after polarization tests (1 hour of OCP and 2 hours of potentiostatic step) and sonication rinsing ...... 57

39. Inclusion 1 in Figure 24 & 38 after polarization test and sonication. Red arrow shows the trench at the inclusion interface with the matrix, while white arrows show the precipitates on it ...... 58

40. FIB/ SEM technique. (a) The platinumlayer deposition was applied for surface protection and the selected inclusion is beneath this layer. (b) Early stages of FIB milling process ...... 59

41. SEM images of FIB cross- sections (a) SEM image of the cross section of the inclusion in Figure 39.(b) Higher magnification of the same area shown in part (a). The sample experienced 1 hour of OCP and 2 hours of potentiostatic step and then was ultra ...... 59

42. EDS dot maps of the cross section of the inclusion shown in Figure 39 ...... 60

43. Overlay of S and Cu EDS dot maps shown in Figure 42 confirming the presence of Cu2S shell around the inclusion after polarization and sonication ...... 61

44. (a) A random pit on the surface of pretreated sample after running polarization tests (1 hour of OCP and 2 hours of potentiostatic step). The selected MnS inclusion is shown in red rectangle. (b) SEM/ EDS dot maps of the selected inclusion in part (a). (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulfur, manganese and copper ...... 63

45. Overlay of S and Cu EDS dot maps shown in Figure 44...... 64

46. (a) A random pit on the surface of pretreated sample after running polarization tests (1 hour of OCP and 2 hours of potentiostatic step). The selected manganese sulfide inclusion is shown in red rectangle. (b) The selected inclusion in higher magnification. (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulfur, manganese and copper ...... 65

47. Overlay of S and Cu EDS dot maps shown in Figure 46...... 66

48. Overlay of SEM and EDS dot maps of inclusion shown in Figure 46 ...... 66

49. A MnS inclusion beside a random pit on the surface of pretreated sample after running polarization tests. (b) SEM/ FIB cross- section image of selected inclusion in part (a). (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulphur, manganese and copper ...... 67

50. Overlay of S and Cu EDS dot maps shown in Figure 49...... 68

51. (a) SEM image of selected MnS inclusion after pretreating the sample in acidified 1 M NaCl. (b) SEM/ EDS dot maps of the selected inclusion in part (a). (c) AFM topographic map of the inclusion in part (a), taken in air ...... 69

52. In-situ AFM images of the inclusion in Figure 50 (after pretreatment in acidified 1 M NaCl) potentiostatically polarized in 0.1 M NaCl. Figures 52-1 and 52-2 were recorded during 10 minutes of potentiostatic test at applied potential of 0.227 V (SCE)...... 70

53. SEM/ EDS dot maps and AFM topographic images of the inclusion shown in Figure 51 after polarization tests (10 minutes of potentiostatic test at applied potential of 0.227 V (SCE) followed by 20 minutes of potentiostatic experiment at applied potential...... 71

54. SEM/ EDS dot maps and FIB cross- section observation of the inclusion shown in Figure 51 after polarization tests (10 minutes of potentiostatic test at applied potential of 0.227 V (SCE) followed by 20 minutes of potentiostatic experiment at applied potential of 0.437 V (SCE)) ...... 72

55. (a) SEM image of a selected MnS inclusion before pretreatment in acidified 1 M NaCl solution. (b) After pre-treatment. (c) SEM/ FIB cross- section observation of the inclusion in part (a) ...... 74

56. (a) SEM image of a selected MnS inclusions before pretreatment in acidified 1 M NaCl. (b) After pretreatment. (c) SEM/ FIB cross- section observation of the inclusions in part (a)...... 75

57. The initiation of pitting corrosion around the left inclusion is due to presence of the solution in the crevices at the inclusion’s interfaces. Pitting propagation beneath the

xii

surface will result in lacelike pitting at this inclusion. The other MnS inclusion remains safe with no pitting at its boundaries since the solution could not reach the inherent trenches below the surface. Both MnS inclusions during potentiostatic step remains safe due to Cu2S precipitation which prevents further dissolution of the inclusions ...... 78

58. Cyclic Polarization Potential curve of 304L stainless steel in 0.1 M NaCl ...... 79

59. SEM image of Silicon/ oxygen based inclusion on 304L SS ...... 80

60. EDS dot maps of inclusion in Figure 58. This inclusion contains silicon and oxygen and is depleted of chromium, nickel, iron and manganese ...... 80

61. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b, for selected inclusion in figure 58, before running potentiostatic test in air ...... 81

62. AFM height map of Si/O based inclusion in Figure 58, after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 82

63. (a) Potential map before polarization, (b) topographic map after polarization, (c) overlay of a & b of Si/O based inclusion ...... 83

64. Silicon/ oxygen based inclusions SEM image on 304L SS ...... 83

65. EDS dot maps of inclusions of Figure 63. Beside the presence of silicon and oxygen, depletion of Mn, Ni, Fe and Cr is visible here ...... 84

66. AFM Height/ Potential maps before polarization, (a), (b) and (c) are representing respectively inclusion 1, 2 and 3 in Figure 64 ...... 84

67. (a), (b) and (c) are AFM height map after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) respectively for inclusion 1, 2 and 3 shown previously in figure 64 ...... 85

68. Before and after polarization topographic maps comparison of the silicon oxygen based inclusions in Figure 64 ...... 86

69. Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion SEM image on 304L SS ...... 87

70. EDS dot maps of the inclusion in Figure 69 ...... 88

71. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b of the Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion shown in Figure 68 before polarization test ...... 88

xiii

72. AFM height map of the Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion shown in Figure 69 after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 89

73. Carbon/ Oxygen/ Silicon based inclusion SEM image on 304L SS ...... 90

74. EDS dot maps of inclusion in Figure 73 ...... 90

75. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b of the Carbon/ Oxygen/ Silicon based inclusion shown in Figure 73, before polarization test .....91

76. (a) AFM topographic map, (b) Potential map and( c) overlay of a &b of the Carbon/ Oxygen/ Silicon based inclusion shown in Figure 73, after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 91

77. Carbon/ Oxygen/ Silicon based inclusion SEM image on 304L SS ...... 92

78. EDS dot maps of inclusion in Figure 77 ...... 92

79. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b before polarization test for carbon/ oxygen/ silicon based inclusion shown in Figure77 .93

80. AFM height map of the inclusion in Figure 77, after polarization tests. (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 93

81. Before and after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) topographic maps comparison of the carbon/ oxygen/ silicon based inclusion in Figure 77 ...... 94

82. SEM image for Carbon/ Oxygen/ Silicon based inclusion on 304L SS ...... 95

83. EDS dot maps of inclusion in Figure 82 ...... 95

84. AFM height maps comparison of the inclusion in Figure 81, before and after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 96

85. Carbon based inclusion SEM image on 304L SS ...... 97

86. EDS dot maps of inclusion in Figure 85 ...... 97

87. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b before polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) for carbon based inclusion shown in Figure 85 ...... 98

xiv 88. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE), for carbon based inclusion shown in Figure 85 ...... 98

89. AFM height maps comparison of carbon based inclusion in Figure 85 before and after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE)...... 99

90. Carbon/ Nitrogen/ Oxygen based inclusion SEM image on 304L SS ...... 100

91. EDS dot maps of the Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90 ..100

92. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b before polarization test for Carbon/ Nitrogen/ Oxygen based inclusion illustrated in Figure 90...... 101

93. AFM height map of Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90, after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 101

94. AFM height maps comparison of Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90 before and after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) ...... 102

xv CHAPTER I

INTRODUCTION

Austenitic stainless steels, similar to the other types of stainless steels are well- known due to their corrosion resistance, but pitting corrosion is their primary failure under corrosive conditions (e.g. sea water, or any other solution containing halides). Pitting corrosion is a kind of localized corrosion, in which an active site occurs in an otherwise passive surface. Thus pitting corrosion only occurs in passive metal systems. Among all different categories of the metallic alloys, austenitic stainless steels are the most pitting resistant.1

There are many different studies on nucleation sites of pitting corrosion on stainless steels, and it is accepted that in addition to the surface roughness which is affected by surface finishing and treatment, other surface defects may be the potential sites for pitting initiation. Among all different possible types of surface flaws, the non-metallic inclusions e.g. sulfide inclusions are the most notorious inhemogenities in the structure of alloys for pitting initiation.2-12

Inclusions are chemical compounds of metallic elements such as Fe, Mn, Al, Si and

Ca with nonmetallic species like O, S, C, H and N. There are two different categories of inclusions based on the source of inclusion formation, indigenous and exogenous inclusions. Indigenous inclusions forms in liquid, or solid steel as a result of chemical

reactions between dissolved elements in the steel; while exogenous inclusions derive from external sources such as furnace refractories and mold materials. Despite the small content of nonmetallic inclusions in steels, they exert significant effect on the steel properties such as tensile strength, deformability, and toughness and corrosion resistance. Therefore, it is important to study the nonmetallic inclusions to understand their influence on the properties and quality of steel products and also to estimate necessary techniques and chemical reactions in steels refining. Inclusion may act as an anode or cathode versus the alloy or create boundaries with the matrix, which is caused by different thermal expansion coefficient and cause formation of trenches at the interface of the inclusion and the alloy; thus they are considered as potential sites of pitting nucleation.4, 13

Many different explanations have been proposed about how non-metallic inclusions (especially MnS inclusions), initiate pitting corrosion in different environments.

Most of proposed theories were based on the behavior of isolated MnS inclusions with no other consideration about presence of other elements in the solution, and their possible reactions with the inclusions. Recently with incorporation of modern optical techniques such as Atomic Force Microscopy (AFM), Electrochemical Atomic Force Microscopy

(ECAFM), Scanning Tunneling Microscopy (STM) and Electronic speckle pattern interferometry (ESPI), it became possible to investigate the surface of the alloys inside the solution and under potential control which are very helpful to investigate the pitting corrosion from the very beginning steps and to understand the role of inclusions in this phenomenon in larger scale than microcell techniques.14-19

The main focus of this study was to visualize the pitting corrosion triggered by the presence of manganese sulfide inclusions in austenitic stainless steel in sodium chloride

solution using AFM technique. The recorded process was compared with reported behavior of these types of inclusions in literatures and investigation over the presence of any relation between the formed pits and the Volta potential difference (measured by Scanning Kelvin

Probe Force technique which is combined with AFM) present at that area. The secondary goal was to investigate whether other types of inclusions are present on the austenitic stainless steel surface which may be the potential sites for pitting corrosion. The other types of inclusions beside MnS have aroused attentions of possible involvement in the pitting corrosion process, due to the continued pitting occurring in low levels of MnS inclusions.

3 CHAPTER II

BACKGROUND

Pitting corrosion is a kind of localized corrosion, in which an active site occurs in an otherwise passive surface. Pitting corrosion is defined in three main steps, nucleation, metastable growth and stable growth. Nucleation is an unstable process and in many cases the pitting corrosion will stop in this step. If it survives beyond the nucleation, the pit will enter to the second step which is the metastable growth.2 Due to the increased susceptibility to pitting initiation at higher potentials, it is believed that the nucleation step is potential dependent phenomena.20 Metastable pitting refers to pits that initiate and grow for a limited time prior to the repassivation. During metastable pitting propagation step, formed pits are unstable, thus depending on some diffusion controlled factors, it may become stable or the initiated pit will be repassivated and the dissolution of the metal will be stopped.2, 21 The final stage of pitting corrosion is known as the stable pitting which is diffusion controlled through the depth of the pit as well as metastable pitting and it occurs under an autocatalytic reaction.2, 22

Cyclic polarization experiment is used in many studies to find the critical potentials of each environment. Stable pits start at potentials nobler than pitting potential Epit, at which the current density will increase sharply from passive current density. The pits growth will be continued to the repassivation potential Erep, which is lower than pitting potential with current density dropping back to passive current resulting in repassivation of created pits.

Epit and Erep are known as the characteristic parameters in pitting corrosion studies of different alloys which depends on the environment, scan rate, alloy composition and surface finishes. In the potential range between these two particular potentials, the alloy is susceptible for metastable pitting. Both metastable and stable pitting survival depend on the presence of perforated covers which can be a salt film, metal cover or a passive layer remnant. This means that these phenomena are under diffusion control. When the pit grows enough to the poit that its depth can act as diffusion barrier, it won’t need the porous cover for continuous growth.2, 21-27

2.1. Mechanism of Pitting Corrosion

For a pit to initiate, the passive film must first breakdown. There are three main mechanisms for breakdown of passive layers: passive film penetration, mechanical film breakdown and a halide adsorption mechanism. In each of these mechanisms, once the aggressive ions reache the metal beneath the protective passive layer, a high rate anodic dissolution of metal take place.21 The anodic reaction will be:

M ⇌ M2+ + 2e− (M is representing metal species)

At the beginning, released electrons from anodic dissolution of the metal will lead to OH- formation from oxygen reduction reaction (cathodic reaction by dissolved oxygen present in the solution):

− − O2 + 2H2O + 4e ⇌ 4OH

The oxygen in the pitting corrosion zone will be gradually depleted while metal dissolution still is happening. The pit zone preferentially will act as an anode and the rest of the metal

facing the bulk solution act as the cathode where the oxygen reduction takes place. The cathodic area in comparison with anodic area is much larger; this can increase the metallic dissolution, where accumulation of metallic ions makes it more electropositive in nature.

Since the concentration of metallic ions is increasing in the pit area, naturally the system will try to neutralize the M2+ ions accumulation by electromigration of counter ions present in the environment. With the presence of transferred negative ions, metallic ions and water in the pit area, some possible autocatalytic reactions will occur, such as following reactions:

- 2+ Cl + M ⇌ MCl2

+ - MCl2 + H2O ⇌M(OH)2 + 2 H + 2 Cl

2+ + M + 2H2O ⇌ M(OH)2 +2 H

Either of these reactions will increase the concentration of hydronium ions continuously which means the decrement of the pH in the pit. In high concentrations of chlorine and low value of pH, the passive layer is more susceptible for further ruptures.

2. 2. Effect of Inclusions on Pitting Corrosion of Stainless Steels

Despite the initial belief about pitting nucleation stating that this phenomenon happens at random sites of the alloy surface, there are many studies on the influence of surface inhomogenities on pitting corrosion of austenitic stainless steels. As it was explained earlier, the breakdown of passive layer is required for pitting initiation. It is believed that the local breakdown of the passivity of these alloys occurs at the sites of existing heterogeneities on the surface. As a result the inclusions play a key role in the initiation of pitting corrosion. 28-30

In this section we will review different parameters in pitting corrosion caused by inclusions and the suggested mechanisms for pitting initiation related to the inclusions in the alloys.

2. 2. 1. Various Types of Inclusions as Pitting Nucleation Sites

Streicher31 found that nonmetallic inclusions could be susceptible sites for pitting initiation in alloys. There are some other studies suggested that any type of non-metallic inclusions can be counted as a potential site for pitting nucleation and that the sulfides are especially detrimental32. This fact that manganese sulfide inclusions can cause pitting is generally accepted but about the other type of inclusions which may cause pitting nucleation, there are still some uncertainties.

It is believed that, since pitting corrosion is found in steels with low amount of sulfur in chloride solutions-where no visible MnS inclusion was found via SEM- the sulfide inclusions are not the sole reason for pitting corrosion in stainless steels.33 Different types of oxide inclusions, capable of pitting initiation were found, however it is believed that, the pitting occurring at these sites is caused by chemical reactions.34-36 The other explanation for pitting nucleation of multi oxide phase inclusions, is the presence of sulfides in their phase or at the periphery area. In fact the actual initiators are the small sulfides at or around the other inclusions.28, 37-40 As a result, although some multiphase oxide inclusions were found which could cause pitting corrosion, but oxide particles in comparison with the sulfide inclusions are counted as inert defects. In presence of MnS or mixed MnS/oxide inclusions, multi elemental oxide inclusions do not cause stable pitting.41-43

Examination of high sulfur and low sulfur stainless steels alloys revealed the large influence of sulfide inclusions on pitting corrosion of these alloys.44-46 Investigation of high purity and pretreated stainless steels in chloride solutions also reconfirmed the influence of manganese sulfide inclusions on pitting corrosion of these alloys.5, 47, 48As a result, among different types of inclusions in stainless steels, MnS inclusions are the most susceptible sulfide inclusion for pitting nucleation (except for iron sulfide).44, 45, 49

Although MnS inclusions are known for potential sites of pitting initiation, all the present manganese sulfide inclusions in stainless steel alloys will not cause pitting initiation. The lack of activity of some of the MnS inclusions can be the result of their small geometry and the orientation of them. Only the sulfide inclusions greater than 0.5 μm3 (If spherical) can create stable pitting; otherwise they will be too small to nucleate a stable pit.5, 30, 41, 42, 44, 50-56

As it was explained earlier, the detrimental effect of MnS inclusions can be the result of oxidation of these inclusions, presence of micro-crevices at their boundaries or different composition of oxide layer at these sites (creating a potential difference).4, 13

These mentioned reasons will be discussed in more details in following sections.the oxide layer

2. 2. 1. 1. Dissolution of MnS Inclusions as an Initial Step and Related Proposed

Mechanisms for Pitting Corrosion of Stainless Steel Alloys

Dissolution of inclusions will expose the bare metal underneath of the inclusion to the aggressive solution, resulting in pitting initiation. The preferential dissolution at inclusions sites is explained by higher chemical/electrochemical activities28 and/or preferential adsorption of chloride ions (changing the chemical composition and properties

of the passive layer such as its conductivity16, 55) on these sites.52, 57, 58 There are variety of proposed mechanisms for pitting initiation and propagation.

Wranglen52 proposed that, the local involved reactions in dissolution of MnS inclusions are:

Anodic reactions on sulfides:

MnS ⇌ S + Mn2+ + 2e-

- + - S +3 H2O ⇌ HSO3 + 5H + 4e

- 2- + - HSO3 + H2O ⇌ SO4 + 3H + 2e

Local cathodic reaction:

+ - 2H + 2e ⇌ H2

Chemical dissolution of manganese sulfide particles:

+ 2+ MnS + 2H ⇌ H2S + Mn

While the cathodic reaction on the matrix is:

+ - O2 + 4H +4e ⇌ 2H2O

Eklund 3 observed elemental sulfur particles on manganese sulfide inclusions after polarization. He believed that the sulfide inclusions have lower electron conductivity. He suggested another mechanism for dissolution of sulfide inclusions in the unbuffered 0.1 M

NaCl:

2+ 2- + - MnS + 4H2O ⇌ Mn + SO4 + 8H + 8e (I)

+ 2+ MnS + 2H ⇌ Mn + H2S (II)

+ - H2S ⇌ S + 2H + 2e (III)

When the inclusion was polarized, reaction (I) took place and manganese sulfide inclusions

2- 2- oxidized to SO4 . Released SO4 reduced the pH and reaction (II) was started. Released

H2S formed sulfur and hydrogen ions to continue the inclusion dissolution by reaction (II) and (III). As a result the manganese sulfide inclusion dissolved gradually.

Castle and Ke40 proposed that oxidation of MnS inclusions by the following reactions resulted in sulfur precipitations on the surface:

2+ - + - MnS + 3H2O ⇌ Mn + HSO3 + 5H + 6e

- + - HSO3 + 5H + 4e ⇌ S + 3H2O.

Lott and Alkire59, 60 proposed another mechanism with different number of electrons involved in electrochemical dissolution of MnS inclusions.

- 2+ + - 2MnS + 3H2O ⇌ S2O3 + 2Mn + 6H + 8e

+ 2+ 2H + MnS ⇌ Mn + S + H2

The dissolution of MnS inclusions resulted in exposure of bare metal beneath them to the electrolyte. Dissolution of the exposed metal resulted in local acidic environment inside the formed cavity. Whenever the aggressive solution inside the cavity reaches the critical composition, the metal repassivation will be stopped leading to stable pitting.3, 40,

46, 54, 61-67 On the other hand, the deposition of released products such as sulfide products can induce pitting by trapping the aggressive solution beneath the created film.46, 54, 67-69

Ke and Alkire70 proposed that MnS dissolution started at the rest potential, then the corrosive attack was began. As a result, the thiosulfate ions was released by the same reactions suggested by Lott and Alkire60. Both chloride and thiosulfate were required for stainless steel depassivation.

Although the proposed mechanisms showed the dissolution of MnS inclusions, but for pitting initiation, presence of chloride ions in the solution is also necessary.55, 65, 66, 71-76

Baker and Castle51 proposed that MnS dissolution in the presence of chloride ions, caused the solution in the created cavity to be concentrated in MnCl2. They suggested that,

MnCl2 deposition prevents repassivation and provides conditions which stabilize the corrosion.

2. 2. 1. 1. 1. Electrochemical Properties of Inclusions

Depending on the environment and the composition of MnS inclusions, these particles will undergo oxidation at different potential ranges.43, 49, 73, 77, 78 The oxidation of these particles can be the result of lower Volta potential (more active sites of the surface) or presence of a galvanic coupling between these particles and matrix.

Krawiec et al.79 utilized a combined set up of electrochemical microcell and scanning vibrating electrode technique (SVET), and found a galvanic coupling between bare metal beneath the MnS inclusions and the matrix. The galvanic coupling provided a driving force for pitting initiation. On the other hand, recent SVET observation suggests that during dissolution of MnS inclusions they are cathodically polarized whereas the anodic current is coming from their periphery regions. The origin of the cathodic current over the inclusion is still under discussion.78

Presence of Volta potential difference at MnS inclusions is the other proposed driving force for electrochemical dissolution of MnS inclusions.34, 80 Coupled Kelvin probe method with the AFM resulted in obtaining a direct surface potential (Volta potentials) maps with high sensitivity.

2. 2. 1. 1. 1. 1. In-situ, Ex-situ AFM Investigation of Inclusions

Atomic Force Microscopy (AFM) is a type of Scanning Probe Microscopy (SPM) with the capability of direct 3D imaging with high resolution which was invented in 1986.

AFM technique is capable of taking images from both conductive and nonconductive surfaces. This is an important advantage comparing to Scanning Tunneling Microscopy

(STM) which is another type of SPM and designed just for conductive samples and tips.

AFM uses a very sharp tip which is attached to the end of a cantilever, to scan over the sample surface. As the tip approaches to the surface, the attractive forces impel the tip to bend toward the surface, however as the cantilever brought even closer to the surface the repulsive forces push the cantilever away from the surface. These deflections are used to measure the force on the cantilever by (–k. dz) equation where k and dz are the cantilever’s spring constant and deflection respectively. The raised and lowered features on the sample surface will influence the bending of the cantilever which is measured by the position detector. As a result the accurate topographic images of the surface are generated by tracking the reflected laser light of the back of the cantilever into an array of photodiodes.81,

Beside the main capabilities of AFM for 3D imaging in air and fluid, it is useful for many other applications such as nanolithography and surface potential mapping which has been widely used in corrosion studies. 83-85

KPFM mode of AFM is able to measure the work function which is the minimum thermodynamic work required to remove an electron from a solid to a point in the vacuum outside the surface. For KPFM operation, conductive tips are needed to create a small capacitor with the surface of the sample (tip- air- sample capacitor). Since the tip and the sample are different materials, they have different work functions and once we connect them the created charges between tip and sample will result in electric force between them.

The energy in the created capacitor will be defined as:

U= ½ C. V2 (1)

Where C is the capacitance and V is potential difference. At a given bias, for a displacement of the tip along coordinate z, the force between tip and sample will be defined as:

F= - (δU/ δz) = -1/2 (δC/δz) V2 (2)

Where U stands for stored energy in the capacitor. In KPFM mode of AFM, the potential difference between tip and sample consists three components, work function difference between tip and sample (ΔΦ), an external DC bios (VDC feedback voltage) and an applied

AC bias to the tip at cantilever resonance frequency ( VAC sin(ωt)):

V= (VDC – ΔΦ) + VAC sin (ωt) (3)

If we place equation (3) in equation (2) and expand the equation for the force, after reorganization there will be three parts:

2 2 퐹 = −1/2 훿퐶/훿푧 ((푉퐷퐶 – 훥훷) + 1/2 푉퐴퐶) + 훿퐶/훿푧 (푉퐷퐶 – 훥훷) 푉퐴퐶 푠푖푛 (휔푡) +

2 1/4 (푉퐷퐶 – 훥훷) 푉퐴퐶 푐표푠(2휔푡) (4)

DC term of force causes the cantilever to bend:

2 2 FDC = −1/2 훿퐶/훿푧 ((푉퐷퐶 – 훥훷) + 1/2 푉퐴퐶) (5)

And AC term causes the oscillation of the cantilever:

2 FAC = 훿퐶/훿푧 (푉퐷퐶 – 훥훷) 푉퐴퐶 푠푖푛 (휔푡) + 1/4 (푉퐷퐶 – 훥훷) 푉퐴퐶 푐표푠(2휔푡) (6)

ω term 2ω term

In AM- KPFM mode, the ω term of FAC is only considered for measuring the work function difference and this parameter is evaluated by a nulling process. (Fω )

Fω = 훿퐶/훿푧 (푉퐷퐶 – 훥훷) 푉퐴퐶 푠푖푛 (휔푡) (7)

When DC potential difference between tip and sample (VDC – ΔΦ) is not zero in presence of an AC bias (VAC sin ωt), the electrical force Fω will cause cantilever to oscillate at its resonance frequency. Potential feedback loop by adjusting the sample bias VDC to an amount same as tip- sample work function difference ΔΦ, will make (VDC – ΔΦ) parameter zero which in fact will result in nulling cantilever oscillation amplitude at ω frequency.

Then the tip-sample work function difference as the desired parameter will be equaled to

86 applied sample bias the, VDC = ΔΦ.

Recently, SKPFM has been used in many localized corrosion studies, namely Al alloys with the related intermetallic inclusions, and surface treatment influence on pit initiation sites of stainless steels.6, 83, 86-89

Schumtz and Frankel90 implemented AFM coupled with ex situ scanning Kelvin probe force microscopy (SKPFM) to measure the Volta potential difference of the surface.

They found a linear correlation between Volta potential of pure metalsmeasured by

SKPFM, and their open circuit potential (OCP) in the solutions (water and 0.5 M NaCl).

They illustrated that higher Volta potentials from SKPFM belong to more noble alloys, while lower potentials belong to more active OCP. Based on the Schumtz and Frankel’s report, AFM potential maps provide a unique means to distinguish the more active/ noble areas with lower and higher Volta potential respectively. The relation of Volta potential and corrosion tendency has been mentioned in many literatures.34, 90, 91

Many real time observations of the pitting initiation were contributed on the isolated MnS inclusions, such as microcell techniques visualization. During in-situ AFM observation, the whole sample should be immersed in the solution and a selected part of the sample will be visualized. Since the entire sample is exposed to the solution, it causes some differences between the environmental conditions of this technique with other micro size techniques.

The AFM In- Situ investigations reveal that the corrosion products will redeposit at the inclusions and the periphery area, this phenomena may increase the aggressiveness of the trapped solution beneath this products leading to higher propagation rate of pitting corrosion at these sites. 14, 16, 20

Rynders et al.92 recorded the behavior of MnS inclusions in 304 stainless steel exposed to 0.5 M NaCl at pH = 4.2 and 50 mV overpotential for about 6 hours by AFM.

They reported that instead of pitting at MnS inclusions, there was a protrusion created on

and around the inclusion for a distance up to four times of the nominal radius of the inclusion. They believed that these bumps were the result of dissolution and re-deposition of sulfur products released from the original inclusion. On the other hand, the AFM observation in Vuillemin et al.78 study showed that pitting was initiated at the boundaries of the activated MnS inclusion. Removal of corrosion products by sonication rinsing of the sample confirmed presence of some pits in the metallic matrix as well.

In this study, beside the topographic investigation of pitting corrosion related to

MnS or other presented inclusions, we tried to find the possible relation between corroded area and the Volta potential measurement of this region.

2. 2. 1. 2. Pit Initiation Sites at MnS Inclusions

As it was mentioned earlier, the dissolution of MnS inclusions in presence of chloride ions can initiate pitting, while it should be considered that the inclusions dissolution is not uniform. Depending on the preferential dissolution sites of the inclusions, pitting initiation sites will be different.

The most preferential sites for pitting initiation at MnS inclusions was observed at the interface of these inclusions with the matrix, which will cause the creation of some crevices at these sites leading to pitting corrosion. 40, 42-44, 51, 72

Suter and Bohni12 used microcell technique to investigate the corrosion behavior of different zones of MnS inclusions. Local potentiodynamic polarization curves on different zones of a single MnS inclusion showed their pitting potential. Based on these measurements, the interface of MnS inclusion with the matrix was the weakest point and by this explanation these regions were the most susceptible part of the inclusion for pitting initiation.

Presence of chromium depletion zones, iron sulfide halo or inherent micro crevices at the boundaries (due to different thermal coefficient of the inclusion and bulk alloy) are the other theories to explain the preferential dissolution of MnS inclusions at their interfaces. 93-95

In some other studies, In-situ observations revealed that the created trenches at the interfaces of MnS inclusions grew in depth direction, and as a result, a sudden hole was appeared on the surface, making an irregular network as the lacelike pattern.3, 96

2. 2. 1. 2. 1. Undermine Pitting and Its relation to MnS Inclusions

Most of reviewed studies showed that, pits were nucleated at the boundaries of MnS inclusions. However pitting corrosion may occur under a porous cover which will cause the solution inside the pit to be more aggressive than the bulk solution, leading to stable pitting. This cover is the result of corrosion products deposition on the surface, residual passive film or a thick layer of the metal itself. It is believed that undermining pits is related to the presence of sulfide inclusions in the alloy.20, 21, 23, 24, 97-99

Frankenthal and Pickering100 showed that, there are two types of pitting corrosion on stainless steel alloys. The first type was related to inclusions propagating under the surface resulting in the lacelike pitting. The second type of pitting was not related to the inclusion and repassivated soon, making an open hole on the surface. Zhang et al.20 investigated the 304 stainless steel in 3.5 % NaCl solution; they observed that the corrosion product covers the formed pits at the very beginning stage of pitting. Newman et al.98, 99 found the formation of a lacy cover over the pit’s mouth during anodic dissolution of stainless steel alloys. They believed that at the edges of a pit, migration of chloride and metallic ions occurred so fast, thus the solution did not reach the critical concentration to

prevent repassivation. The metal will repassivate easily at the edge of pits but in deeper sites of the pit, the anodic dissolution happened continuously, resulting in undercutting the metal surface and creation of a lace cover. The created porous cover will help the solution inside the pit remains at the critical concentration of aggressive species resulting in continuous pitting propagation. They believed that salt film formation and creation of lacy cover over the pit mouth by undercutting the surface play the key parameter in stabilization of pitting propagation.

2. 2. 1. 3.The Role of Protective Films at MnS Inclusions

Pitting resistance of stainless steels is assumed to be related to the presence of protective passive layer on their surface which contains several different oxides. The passive layer is known as a neutral layer which prevents the bare metal to be in touch with the solution. However depending on the passive layer thickness, environment and alloying elements the protective act of this layer will be different.21, 101, 102

Some investigation showed that, there is no protective passive layer present on the

MnS inclusions52, 57. On the other hand some authors believed that, the thickness of the passive layer covering these inclusions is less than the rest of the surface49, thus the passive layer is weak in these points and more susceptible for rupture and pitting initiation.

2. 2. 1. 4. Effect of Alloying Elements on Pitting Susceptibility

The alloying elements, affect the composition of the passive layer and existing inclusions in the alloy. On the other hand they will be released to the solution as a result of passive oxidation. Their presence in the solution or re- deposition on the surface will affect the pitting initiation, propagation and the behavior of existing inclusions.35, 45, 53, 74, 103-117

Recently, addition of titanium and copper is used to neutralize the effect of remaining sulfur content in the stainless steels. As a result, MnS inclusions will be replaced with insoluble titanium sulfide inclusions; released H2S from dissolution of sulfide

113, 118 inclusions will be removed by the precipitation of insoluble Cu2S.

Wranglen52 reported that in copper bearing steels, copper inserted into the solution during corrosion process but later deposited as sulfide. Ke and Alkire41 reported the presence of a flower like precipitates of copper on MnS inclusions in severe corrosion conditions. They assumed that presence of this deposit slowed the dissolution rate of the inclusion and prevented the pitting initiation.

Zakipour and Leygraf119 and Daud120 proved the presence of copper sulfide precipitates by Auger Electron Spectroscopy (AES) and its hindering role on pitting corrosion. Zakipour and Leygraf119 believed that in pH range of 4 to 8, copper precipitates as copper sulfide. The ratio of atomic weight percent of copper to the sulfur (found to be

2), confirmed this hypothesis. In low pH (pH=1, mainly happen at the bottom of localized corrosion defects), copper will be deposited as metallic copper.

CHAPTER III

EXPERIMENTAL PROCEDURE

Pitting and re-passivation potentials (Epit , Erep) of 304 L and 303 stainless steel samples were measured using cyclic polarization potential test (CPP) to attain the boundaries of the applied potentials in NaCl solutions. Since SEM/EDS maps showed presence of a layer of Cu on the MnS inclusions, potentiodynamic polarization curves for anodic and cathodic branches of copper were also plotted. 303 stainless steel and copper samples went through CPP and potentiodynamic test in both 0.1 NaCl and acidic 1 M NaCl

(pH 3) solutions. Acidic NaCl solution (1 M, at pH 3) was used for pretreatment of some of 303 stainless steel samples in OCP for around 4 hours, prior to polarization tests.

Samples were polished down to 1 μm prior to CPP, potentiostatic and microscopy tests.

Scanning Electron Microscopy (SEM) and Energy Dispersive X- ray (EDX/EDS) techniques were used for locating the inclusions and characterizing their composition prior and after exposure to the solution. Topographic and potential maps of these inclusions were then collected in air using ex-situ AFM- SKPM. Combined set-up consisting of AFM and an electrochemical cell in defined potential range (Erep< Eapplied< Epit) were used for in-situ potentiostatic study to investigate the pitting/non-pitting behavior of inclusions on surface in 0.1 M NaCl solution. SEM/ EDS dot maps, ex-situ AFM- SKPM were used again to investigate the area of interest after polarization tests.

Samples that experienced pretreatment in acidified chloride solution at the OCP prior to potentiostatic tests in 0.1 M NaCl, went through FIB cross- section technique to visualize the inclusions behavior beneath the surface.

3. 1. Materials and Sample Preparation:

Samples were prepared in cylindrical and disk shapes for CPP and AFM in-situ experiments respectively. 303 and 304L samples’ composition were: 303, UNS S30300,

(nominal composition: C: 0.0460, Mn: 1.9400, Si: 0.4800, S: 0.3040, P: 0.0360, Cr:

17.5000, Ni: 8.0300, Cu: 0.5700, Mo: 0.3000, Co: 0.1500, N: 0.0400 Wt%) and 304L,

UNS S30403, (nominal composition: C: 0.0161, Cr: 18.0960, Cu: 0.5080, Mn: 1.8035, Mo:

0.3790, N: 0.0756, Ni: 8.0205, P: 0.0395, S: 0.0013 Si: 0.3875 Wt%). The cylindrical samples were embedded in epoxy resin (Epoxy-set Resin mixed with Epoxy-set hardener from Allied High Tech Products Inc.); ground through silicon carbon paper (grits 320, 400,

600, 800, 1200 Allied High Tech Products Inc.), and polished down to 1 μm using diamond suspension (9, 6, 3 and 1 μm diamond suspension Allied High Tech Products Inc.). Water and hexylene glycol based lubricant with moderate viscosity, were used respectively in grinding and polishing of the samples. These steps were followed by sonication of the surface in acetone, ethanol and DI water for 5 minutes. Commercially available NaCl salt used in electrolyte solutions was purchased from EMD Chemicals Inc. Some polished samples were pretreated in acidic 1M NaCl solution. pH in acidic sodium chloride solutions was controlled by HCl at pH= 3.

3. 2. Scanning Electron Microscope (SEM)/ Energy-Dispersive X-ray Spectroscopy

(EDX):

Scanning electron microscopy was carried out at the environmental scanning electron microscopy laboratory located at the Department of Geosciences, at the University of Akron. FEI Quanta 200 Environmental Scanning Electron Microscope with an Energy

Dispersive X-ray Spectroscopy (EDS) attachment (EDAX, QUANTA- 200/400, 132-10) was used in this study. Images were obtained under high/ low vacuum, at working distances of 10 mm.

3. 3. Atomic Force Microscopy:

AFM measurements were carried out on a Dimension Icon manufactured by

Bruker Company. Ex-situ topographic and potential maps were simultaneously taken in

KPFM mode with conductive metal-coated silicon tips (MESP tips) while; in-situ topographic maps were taken with ScanAsyst mode and ScanAsyst-Fluid+ tips. The potential distribution was measured with amplitude modulation type of KPFM (KPFM-

AM). The scan- rate and the other mapping parameters were selected due to the mode and the environment of the experiment. First generation electrochemical cell (EC) combined with AFM (EC-AFM) was used for electrochemical experiments inside the AFM. In-situ images were then analyzed and edited by Nanoscope Analysis software.

Contrary to the other authors 30a who used Ni or Pt as the reference for Volta potential maps, all the potential maps in this study are the raw data. The local potentiodynamic polarization curves which were generated in Suter and Böhni12 study on

MnS inclusions confirmed that MnS inclusions are more active than the stainless steel matrix. On the other hand the MnS inclusions in AFM potential maps appeared in dark

contrast compared to the matrix. As a result the dark contrast in the AFM potential maps represents the more active sites compared to the rest of surface.

Figure 1. Dimension Icon AFM used in this study 3. 3. 1. KPFM- AM Imaging

The probe holder for this mode was the standard AFM cantilever holder (Figure

2) and a tip with specific coating (MESP tip) was selected to measure the work function.

Probe Sitting in Cantilever Mounting Groove

Electrical Spring Loaded Mounting Socket Probe Clip

Figure 2. Standard AFM cantilever holder

Selected scan parameters:

Scan rate: 0.5 lines/ s

Scan Area: contained 512 *512 points

Feedback section:

Integral Gain: 1.0

Proportional Grain: 5.0

Interleave section:

Interleave Mode: Lift (it means that after the first topographic pass of cantilever over the surface it will lift off the surface for the second potential measuring pass)

Lift Scan Height: 100 nm

Potential (Interleave) section:

Input Igain = 1.0

Input Pgain = 5.0

The other parameters were left at their default values.

All the values mentioned here were the initial values and depending on the different situation of the experiments they might be modified.

3. 3. 2. AFM Fluid Imaging:

The Probe holder designed for imaging in the fluid is shown in Figure 3. The fluid probe holder allows the laser light reflection from the cantilever in the fluid whereas it prevents the scanner tube coming to the solution. The EC chuck which has the required terminals for electrochemical connections was used for in-situ imaging during potentiostatic tests. (Figure 4)

Figure 3.Fluid Probe holder

External temperature Spring Terminals sensor

Alignment feature in tungsten heater cover

Figure 4.EC Chuck

Depending on the desired scan size and resolution of the pictures, each parameter might be selected differently. The initial values are listed below:

Scan rate: 0.5 lines/ s

Scan Area: contained 512 *512 points

ScanAsyst Auto Control: ON by this option the software will modify the

set-point, gain, scan rate and Z limit automatically.

3. 4. Focused Ion Beam (FIB)

SEM imaging and FIB cutting were done using FEI Helios Nanolab 650 at The

Swagelok Center for Surface Analysis of Materials (SCSAM) in Case Western Reserve

University. A layer of 1 µm platinum deposition is add to the area of interest to prevent damaging from FIB (while cutting sample). The platinum deposition was first done electrons beam (E- beam) to prevent the damage in the oxide layer. For Pt deposition with

E-beam, sample was set to be perpendicular to E-beam (90°) and the Pt deposition was done using an accelerating voltage of 5 KV with a probe current of 13 nA, normally at the height of 0.3 µm. This 0.3 µm Pt layer is enough for oxide layer protection from I-beam

Pt deposition. The sample were then tilted to 52° (perpendicular to ion beam) and Pt deposition with I-beam was continued to get 1 µm platinum layer using an accelerating voltage of 3KV and a current of 0.43 nA.

The samples were then coarse milled 30 KV using current of 65 nA, 31 nA, 6.7 nA and finally polished with a current of 0.79 nA or 0.43 nA.

SEM Images were collected with ETD-Everhart-Thornley Detector using 5 kV, 0.8 nA. High resolution images were collected using TLD (through lens detector) – SE mode

2 (high magnetic mode) using 5 kV 25 pA.

3. 5. Electrochemical Tests:

All electrochemical experiments were accomplished using Gamry Reference-600 potentiostat in a three electrode set-up where platinum wire was used as the counter electrode and Ag/AgCl as the reference electrode as shown in Figure 5.

3. 5. 1. Outside AFM:

Following the Open Circuit Potential (OCP) test for 1 hour, CPP test was carried out; the potential sweep started 20 mV below OCP with scan rate of 0.167 mV/s and the total exposed area of 0.38 cm2. The experiment was stopped when potential reached 20 mV below OCP.

Figure 5.Electrochemical set-up Outside the AFM

3. 5. 2. Inside AFM:

Potentiostatic experiment with constant applied potential between Erep and Epit

(Erep< Eapplied< Epit) was conducted. (Figure 6 shows the electrochemical set up inside

AFM)

Figure 6.Electrochemical set-up inside the AFM

28 CHAPTER IV

RESULTS AND DISSCUSSIONS

In this study, In-Situ observation methods were incorporated to study the effect of different types of inclusions, containing carbon, nitrogen, silicon, oxygen, manganese and sulfur on pitting corrosion of 303 and 304L stainless steels.

4. 1. 303 Stainless Steel

As described earlier, EDS-SEM technique was used to detect the inclusions on the surface of stainless steel samples. Both spherical and needle shape manganese sulfide inclusions were observed on 303 samples via SEM. Samples were treated with two different styles, shortly described below and also in experimental section. 303 samples were grinded down with 1200 grade silicon carbide, and then polished down to 1 μm with diamond suspensions. The Epit and Erep were obtained with CPP experiments in 0.1 M NaCl electrolyte. The polished samples were then exposed to the 0.1 M sodium chloride solution and after the OCP test for one hour the sample underwent potentiostatic experiment with the applied potential of 0.227 V (SCE) .

In the second experiment, samples were prepared in the same condition as the first experiment, however they were exposed to an acidic salt solution of 1 M NaCl at pH=3 for

4 hours prior to in- Situ AFM observation. Sample was monitored with in- situ visualization while it was under 30 minutes of OCP followed by 30 minutes of potentiostatic experiment at the same potential of 0.227 V vs. SCE in 0.1 M NaCl solution.

4. 1. 1. Cyclic Polarization Potential Curves

Figures 7 and 8 show the CPP curves of 303 stainless steel in 0.1 M NaCl solution and 1 M sodium chloride solutions at pH 3 respectively. Pitting potential in 0.1 M NaCl solution was 0.339± 0.012 V and the repassivation potential was 0.026±0.050 V; whereas the pitting and repassivation potentials in acidic 1 M NaCl solution were 0.149± 0.019 V and -0.083±0.031 V respectively. The overlaid CPP curves (Figure 9) shows the huge difference between pitting potential of samples in these two different environments. 0.227

V vs. SCE was chosen as the arbitrary potential in the range of repassivation and pitting potential. It was used for potentiostatic experiments in pretreated and non- pretreated samples.

CPP curve of 303 SS in 0.1 M NaCl 0.5

0.4

0.3

0.2

0.1 E(V vs. SCE) vs. E(V 0

-0.1

-0.2 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 Log(i) A/Cm2

Figure 7. Cyclic Polarization Potential curve of 303 stainless steel in 0.1 M NaCl

CPP curve of 303 SS in 1 M NaCl 0.2 pH= 3

0.15

0.1

0.05

-0.05 E (V vs. SCE) vs. E(V -0.1

-0.15

-0.2 1.0E-10 1.0E-08 1.0E-06 1.0E-04 1.0E-02 1.0E+00 Log(i) A/cm2

Figure 8.Cyclic Polarization Potential curve of 303 stainless steel in 1 M NaCl pH 3

0.5 Overlay of CPP curves in 0.1 M and acidic 1 M 0.4 NaCl

0.3

0.2 0.1 M

0.1 acidic 1 M

0 E(V vs. SCE) vs. E(V

-0.1

-0.2 1.E-10 1.E-08 1.E-06 1.E-04 1.E-02 1.E+00 Log(i) A/Cm2

Figure 9.Overlay of CPP curves of 303 SS in 0.1 and acidic 1 M NaCl

4. 1. 2. Potentiodynamic Polarization of Copper Samples

Further SEM/EDS dot maps of MnS inclusions, after polarization tests illustrated the presence of Cu on MnS inclusion. To evaluate the proposed theories about the copper containing precipitates and their influence on passivity of MnS inclusions, Anodic and cathodic branches of potentiodynamic polarization plot of Cu was generated in NaCl solutions as well. The overlay of CPP curve of 303 samples with potentiodynamic polarization curve of Cu in 0.1 M and acidic 1 M NaCl solution are shown in Figures 10 and 11 respectively.

Overlay of 303 SS CPP curve with Cathodic and anodic branches of potentiodynamic polarization curves of Cu (0.1 M NaCl) 0.4

0.2

0 CPP of 303 SS in 0.1 -0.2 M NaCl Anodic branch of Cu -0.4

E (V vs. SCE) vs. E(V in 0.1 M NaCl

-0.6 Cathodic branch of Cu in 0.1 M NaCl -0.8 1.0E-10 1.0E-07 1.0E-04 1.0E-01 Log(i) A/Cm2

Figure 10.Overlay of CPP curve of 303 SS and potentiodynamic curves of Cu in 0.1 M NaCl solution

Overlay of 303 SS CPP curve with cathodic and anodic branches of potentiodynamic polarization curves of Cu 0.3 (1M NaCl pH 3) 0.2 CPP of 303 in acidic 0.1 solution 0 -0.1 Anodic branch of -0.2 Cu in acidic -0.3 solution -0.4

E (V vs. SCE) vs. E(V -0.5 Cathodic branch of Cu in acidic -0.6 solution -0.7 -0.8 1.0E-10 1.0E-07 1.0E-04 1.0E-01 Log(i) A/Cm2

Figure 11.Overlay of CPP curve of 303 SS and potentiodynamic curves of Cu in acidic 1

M NaCl solution.

4. 1. 3. Non-Treated Samples

Non- treated 303 stainless steel samples experienced 1 hour of OCP followed by potentiostatic experiment for around 2 hours. The in-situ investigation was conducted on samples during these steps of polarization.

4. 1. 3. 1. SEM/EDS Maps Before Polarization

The MnS inclusions first were located by SEM followed by EDS characterization.

EDS dot maps showed the composition and accumulation of elements in the area of interest. Figure 12 shows the SEM image of a typical MnS inclusion on the surface of 303 stainless steel. Due to the EDS dot maps (Figure 13), the needle like inclusion contains manganese and sulfur and is depleted of iron, nickel and chromium.

Figure 12. MnS inclusion SEM image on 303 SS surface.

Figure 13.EDS dot maps of inclusion in Figure 12.

4. 1. 3. 2. AFM Maps Before Polarization

After locating and characterizing the inclusion of interest, the topographic and potential maps of the inclusion were collected in air using AFM. Figure 14-a. 14-b and 14- c show height, potential and overlay of these two maps together respectively; while height map shows the topographic picture and potential map shows the Volta potential difference of the tip and sample in each point. Since darker spots in AFM image illustrate deeper region in height maps and more active regions in potential maps, thus it can be concluded

that the two inclusions shown in Figure 14 are more active holes in comparison to the surrounding matrix.

Figure 14. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b, for selected inclusion in Figure 12, before running potentiostatic test

4. 1. 3. 3. In- Situ AFM Maps During Polarization in Solution

The in- situ observation of the inclusion in AFM EC cell was conducted while the surface of the alloy was in contact with the 0.1 M NaCl solution. The AFM maps were collected during one hour of OCP and two hours of potentiostatic test with the applied potential of 0.227 V (SCE). Figure 15 shows the recorded in-situ AFM topographic maps of the inclusion shown in Figure 12. The first three images were recorded each for 20 minutes during OCP and the rest belong to the potentiostatic experiment.

Figure 15. In-situ AFM images of the inclusion in Figure 12 potentiostatically polarized in 0.1 M NaCl at 0.227V (SCE). Figures 1-3 were collected during OCP, and figures 4-9 belong to potentiostatic step.

It was reported in some previous studies3, 12, 36, 42, 43, 72 that MnS inclusions should experience dissolution leading to the pitting corrosion or creation of some trenches at their boundaries. In contrary, no evidence of such changes was observed in the adjacent area of the inclusion in the recorded pictures; instead, some elevation in height on the inclusion was observed. This is also in consistent with Rynders et al.92 Observation. Due to the

images in Figure 15 it seems that the creations of bumps occurred in potentiostatic step than in OCP.

Figure 16 shows the line scan analysis of in- situ images of Figure 15. The height difference between the two fixed points (blue vertical dashed lines) was measured in all recorded steps of the experiment. In all steps, same exact area on the sample (inclusion) was selected (dashed rectangles) to measure the average height of regions around the horizontal dashed line in the center.

During the OCP step, shown in Figures 16-1, 16-2 and 16-3, the change in vertical distance between the selected points were negligible (< 1 nm).

The creation of elusion on the inclusion site in the potentiostatic step (Figure 16-4_

16-9) is visible both in the AFM topographic maps and also line scan curves. The change in the vertical distance between the fixed points also reconfirmed the same results. The vertical distance in potentiostatic step of experiments, changed from 28.91 nm to -19.95 nm. Since one of the fixed points was chosen on the alloy surface with the constant height, it means that the bump created with the accumulation of precipitates in 2 hours has ~ 49 nm heights.

Vertical distance: 30.00 nm

Vertical distance: 30.23 nm

Vertical distance: 29.91 nm

Vertical distance: 28.91 nm

Vertical distance: 21.30

Vertical distance: 12.15 nm

Vertical distance: -2.88 nm

Vertical distance: -7.20 nm

Vertical distance: - 19.95

Figure 16.Line scan analysis of the in- situ images in Figure 15.

4. 1. 3. 4. AFM Maps After Polarization:

AFM maps of the sample after polarization tests (1 hour of OCP followed by 2 hours of potentiostatic experiment) are shown in Figure 17. These images illustrate that precipitates are stable. Beside the height difference which was created during immersion step, the potential map of the inclusions has changed as well. Prior to the exposure of the sample to the solution, the inclusions had lower potentials compared to the matrix, whereas they became more noble after polarization (Figure 17b). It seems that the precipitates, polarized the inclusion, resulting in brighter potential map after polarization tests.

Figure 17.(a) AFM topographic map, (b) Potential map and( c) overlay of a &b of the inclusion shown in Figure 12, after potentiostatic test.

Although the height and potential maps of the inclusions were changed, neither pitting nor detrimental effect of the inclusions was observed at the area of interest.

4. 1. 3. 5. SEM/EDS Maps After Polarization

EDS technique was used to characterize the precipitates on the inclusions. It was found that beside the presence of manganese and sulfur which were the main components of these particles, the formed layer contains copper (Figure 18). Presence of copper on the

MnS inclusions also was observed in Ke and Alkire41, Zakipour and Leygraf119 and

Daud120 studies.

Figure 18. EDS dot maps of the inclusion in Figure 12 after polarization.

Based on the overlaid CPP-potentiodynamic curves (Figure 10) and in contrary to the results of other studies 86, 87, 88, the precipitate on the MnS inclusion (acting as a protective layer) cannot be the metallic copper, since it is more active than 303 stainless steel. As a result the formed layer on this inclusion is a Cu containing complex. On the other hand, electrochemistry studies of Cu in sulfide containing solutions illustrated that,

Cu is not stable in presence of sulfide and chloride ions and will be oxidized. The Cu oxidization in sulfide and chloride containing solutions will result in formation of copper

121-123 sulfide layers (CuS/ Cu2S).

As a result, this protective copper rich layer on MnS inclusion believed to be mainly

41, 52, 118-120 Cu2S that creates differences in height and potential.

20 MnS inclusions were selected on the surface of the alloy and same characterization were exerted and no pitting of the 303 matrix was observed in or around the inclusion sites. We believe that presence of copper sulfide precipitates on the MnS inclusions in this experimental situation will prevent the inclusions dissolution.

4. 1. 4. Pre-Treated 303 Samples

In second experiment, sample were exposed to 1 M NaCl at pH 3 for 4 hours followed by 30 minutes OCP, and 30 minutes of potentiostatic experiment (Eapplied = 0.227

V vs. SCE) in 0.1 M NaCl. The AFM in- situ observation was performed in both OCP and potentiostatic steps over the selected inclusions which were located and characterized by

SEM/ EDS prior to these steps.

4. 1. 4. 1. SEM/EDS Maps Before Polarization

Following images (Figure 19 and Figure 20) show the SEM and EDS dot maps of a MnS inclusion before immersion in acidic 1 M NaCl solution. EDS dot maps show that this inclusion contains manganese and sulfur and is depleted of chromium, iron and nickel.

Figure 19. MnS inclusion SEM image on 303 SS surface

Figure 20.EDS dot maps of the inclusion in Figure 19.

The topographic image in Figure 21 was repeated over the same area with lower magnification; it shows that there are many other MnS inclusions beside the selected inclusion. The color of all inclusions were converted to green to increase the contrast and recognizablity of images.

Higher magnification

Figure 21.Lower magnification of the area of the inclusion in Figure 19

4. 1. 4. 2. AFM Maps Before Polarization Figure 22a and 22b show the AFM height and potential map before exposure to the acidic 1 M sodium chloride solution, in air. The AFM topographic image shows that the inclusion has an elevation regarding to the matrix and it is more active than the surface.

Figure 22. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b of the inclusion shown in Figure 19, before potentiostatic test.

4. 1. 4. 3. In-Situ AFM Maps During Polarization

After exposure of the sample surface to acidic solution of NaCl (1 M pH= 3), it was exposed to 0.1 M NaCl solution. The AFM maps were collected during 30 minutes of OCP and potentiostatic tests, (0.227 V vs. SCE). Figure 23 shows the in-situ AFM topographic map of the selected area while figures 24a_24c are in-situ images of different time steps overlaid on Figure 21 (MnS inclusions of the same area with green color before any immersion tests). The main inclusions (shown in Figure 22) are covered with some precipitates based on the overlaid images. Figure 24a was the last image taken in OCP period and the rest belong to the potentiostatic step. It took around 13 minutes for each picture to be completed. No changes were observed during OCP step; however the first picture during first 15 minutes in potentiostatic stage (part b), shows creation of pit- like

features at the boundaries of some of the inclusions shown in white- dashed circles. Figure

24c shows the creation of another pit at the boundaries of the inclusion number 2 (black- dashed circle) after about 25 minutes.

Figure 23.In-situ AFM topographic map over the selected area shown in figure 19 in OCP step.

Figure 24. In-situ AFM images of the inclusion in Figure 19 potentiostatically polarised in 0.1 M NaCl at 0.227V (SCE). Figures a was taken in OCP while figures b and c were taken after 15 and 25 minutes in potentiostatic step

High resolution images of the same inclusions in Figure 24 (shown in Figures 25 and 27), show formed trenches around the inclusions in more details.

4. 1. 4. 3. 1. Inclusion 1

Figure 25. In-situ AFM images of the inclusion number one in Figure 24. Parts a, b and c were cropped from the same parts of Figure24.

Figure 26. Line scan analysis of Figures 25b and 24c.

Figure 25 shows the in- situ images of inclusion number 1, while, parts a, b and c are taken from the same parts of Figure 24. Regarding to this image (Figure 25), the inclusion did not show any changes prior to the last step where some small crevices is created (since there is a color contrast at the edge which is shown by red arrows). The line scan of part c of Figure 25, confirms the creation of the trench at the edge in step c. (Figure

26)

4. 1. 4. 3. 2. Inclusions 2, 3 and 4

Figure 27. In-situ AFM images of the inclusions two, three and four cropped from the same parts of Figure 24. Figure 27, represents the sudden creation of pit- like defects around inclusions 2, 3 and 4 after OCP step. These inclusions remain safe during the OCP stage (Figure 27a). In potentiostatic step, after applying potential for 15 minutes, some sudden holes were appeared at the interface of inclusions (Figure 27b), applying potential for 12 more minutes, resulted in creation of more pits around inclusion 2 and growth of formed pit at inclusion 3 sites.

For the other inclusions shown in Figure 24, although the AFM maps after the in- situ stages, confirmed the presence of pitting corrosion at their boundaries, but there were no visible changes at their interfaces during in- situ potentiostatic experiment.

4. 1. 4. 4.SEM/EDS Maps After Polarization

After exposing the sample to 0.1 M NaCl and running OCP and potentiostatic tests,

SEM/ EDS measurements were performed on the surface to characterize the existing elements in the area of interest. SEM image shows that the inclusion is surrounded by a halo which has some contrast to the rest of matrix. It is believed that the contrast is due to dissolution occurred at these sites. In EDS maps (Figure 28), beside the presence of

manganese and sulfur which are the main components of the inclusions, copper exists as well. This is in consistent with our observation for non- treated samples.

Figure 28.SEM/ EDS dot maps of the inclusion in Figure 19 after polarization experiments (1 hour of OCP and 2 hours of potentiostatic step). Same as 0.1 NaCl solution, the overlay of 303 CPP and potentiodynamic polarization curves of Cu in acidified NaCl solution (Figure 11), illustrated that Cu is more active than the stainless steel in this situation too. As a result the metallic Cu cannot act as protective layer on MnS inclusions neither in 0.1 M NaCl nor 1 M acidified NaCl solution and this layer should be a copper containing complex as it was explained earlier.

It seems that sulfur and copper are mostly accumulated inside the inclusion boundaries, or on the surrounding matrix (for sulfur), whereas manganese dots are dispersed all over the area, including the dissolved area. To recognize the concentration of copper, sulfur and manganese in the inclusions, we overlaid the dot maps of these elements which are shown in Figure 29.

Figure 29.EDS dot maps of manganese, sulfur and their overlay from the inclusion shown in Figure 19, after potentiostatic polarization tests. Figure 29 represents manganese, sulfur and the overlaid dot maps of these elements which show how these elements are dispersed on the inclusion area. Although sulfur foot print is obvious around the inclusion but it seems that sulfur exist mostly inside the inclusion while manganese accumulates both inside and outside the inclusion, even on the dissolved area.

Figure 30 shows copper, sulfur and their overlaid image. Except for the halo of elemental sulfur around the inclusions, copper accumulation is present on the same area where sulfur was accumulated. This makes it more probable that the copper complex layer sitting on the inclusion is copper sulfide.

Figure 30.EDS dot maps of copper, sulfur and their overlay from the inclusion shown in Figure 19, after potentiostatic polarization tests.

4. 1. 4. 5. AFM Maps After Polarization:

After immersion of the sample in solution (0.1 M NaCl) and running the in- situ

experiment, it was rinsed with DI water and then the AFM height and potential maps of

the investigated inclusion in air were recorded (Figure 31). The AFM topographic map

shows the presence of some holes at the interface of the main inclusions which were

covered with some precipitates during in- situ observation.

The other inclusions such as inclusion number 2, 3 and 5 are also present in these 5 AFM maps and they experienced pitting corrosion at their interfaces as well. The potential

map shows the changes in the surface potential after immersion in the solution. Regarding

to the potential maps, it seems that the interfaces of formed pits with the matrix were active

while the pit area were noble.

Figure 31.(a) AFM topographic map, (b) Potential map and( c) overlay of a &b of the inclusion shown in Figure 19, after polarization (1 hour of OCP and 2 hours of potentiostatic step).

Cross section view of the drawn line in topographic AFM map

Cross section view of the drawn line in Potential AFM map

Figure 32.Line scan analysis over the topographic and potential maps of the selected inclusion in Figure 19 after polarization experiments.

Figure 32, contains both AFM topographic and potential maps; the scan line analysis was conducted on the same area of these pictures to make a comparison between the height and potential maps. The area between two vertical blue cursors in line scan analysis (Figure 32), shows that the interface of the pit with the matrix has lower potential than the pit wall and even the inclusion itself. It can be assumed that these sites are the most active areas around the inclusions in the solution.

To show the distribution of elements around the examined inclusions, we overlay the EDS dot map images on the recorded AFM topographic maps (shown in Figure 33).

Based on Figure 33, manganese exists on the inclusion and inside the pit,, sulfur exists inside the inclusion’s barrier and as a halo on the surrounding matrix; copper exists both inside the inclusions boundaries and formed pits.

Figure 33. The overlay of after polarization EDS dot maps of copper, sulfur and manganese, on AFM topographic maps of the inclusion shown in Figure 19, after polarization tests (1 hour of OCP and 2 hours of potentiostatic step).

4. 1. 4. 6. SEM/EDS Maps After Sonication

Since presence of some precipitates was observed in the SEM/ EDS maps after the polarization test, sample was placed in ultrasonic for 5 minutes in acetone, ethanol and DI water respectively, to make sure no unstable precipitates remains. SEM/EDS measurements (shown in Figure 34) were again exerted on the same area. The SEM image

(Figure 34) reveals that the MnS inclusions were partially removed. Besides that, surprisingly, some new pits were also appeared at the interfaces of the inclusion with the matrix. This observation shows that a pit was propagating beneath the surface and sonication removed the cover and reveals the pit mouth. Although the inclusion underwent partial dissolution, but there is still a layer of copper complex on its remnant. The AFM height map after sonication in the air reconfirms the removal of the inclusions and shows the presence of pits at the boundaries. (Figure 35)

Figure 34.SEM/ EDS dot maps of the inclusion in Figure 19 after running polarization test (1 hour of OCP and 2 hours of potentiostatic step Eapplied= 0.227 V vs. SCE) and sonication.

Figure 35.AFM topographic image of the inclusion after polarization tests followed by sonication.

Figure 36 summarizes the SEM/AFM observations over the sample in all three experimental steps explained earlier. It seems that after pretreating the samples and running potentiostatic tests on the surface, the boundaries of the inclusions underwent the dissolution; as a result they became transparent in SEM image captured after polarization step. Although the boundaries of these particles experienced dissolution, the inclusions remained safe due to presence of stable Cu2S deposits on them. Due to the appearance of new pits at the boundaries, we propose that the created pits were propagating beneath the surface and surrounded the MnS inclusions covered with Cu2S precipitates. After sonication, the weak cover and loose inclusions will be removed. Inclusions 5, 6 and 7 in

Figure 24, showed the same behavior as well. The results from inclusion 6 under same experimental conditions were shown in Figure 37.

1. Before Immersion in solution 3. After Sonication

2. After Polarization

Figure 36. The inclusion in Figure 19 in different steps of experiments.

1. Before Immersion 2. After Polarization 3. After Sonication in solution

Figure 37.Inclusion 6 in Figure 24 in different steps of the experiment.

Figure 38 shows the SEM image over the same area, after sonication. Several small pits can be found inside the white dashed circle similar to the lacelike pitting corrosion.

For more accurate investigation over this area, FIB technique was used to visualize the cross section of the pitting sites.

Figure 38. Lower magnification observation over the area of the inclusion in Figure 19 after polarization tests (1 hour of OCP and 2 hours of potentiostatic step) and sonication rinsing.

4. 1. 4. 7. Cross-Section Observation of Pits

SEM/FIB cross-sectional analysis was used to visualize whether the created pits are connected to each other beneath the surface. The SEM image of the inclusion 1 (in Figure

38) is shown in Figure 39, and the presence of the trench which was shown in Figure 25 was observed (shown by red arrow). The white precipitates on the inclusion surface may be formed as the result of copper sulfide deposition or they may be elemental sulfur as

Eklund3 has proposed before.

Figure 39.Inclusion 1 in Figure 24 & 38 after polarization test and sonication. Red arrow shows the trench at the inclusion interface with the matrix, while white arrows show the precipitates on it.

A platinum layer was deposited on the inclusion for surface protection as shown in

Figure 40. A cross section was then milled across this area to obtain information about the subsurface pitting morphology. Figure 40b shows the early stages of the FIB milling process before milling reaches the final platinum deposited area, which shows the pits are connected beneath the surface.

Figure 40. FIB/ SEM technique. (a) The platinumlayer deposition was applied for surface protection and the selected inclusion is beneath this layer. (b) Early stages of FIB milling process. Figures 41a, 41b and 42 show the final FIB cross section image of the inclusion, its high magnification and EDS dot maps respectively.

Figure 41. SEM images of FIB cross- sections (a) SEM image of the cross section of the inclusion in Figure 39.(b) Higher magnification of the same area shown in part (a). The sample experienced 1 hour of OCP and 2 hours of potentiostatic step and then was ultra

Figure 42. EDS dot maps of the cross section of the inclusion shown in Figure 39.

The cross-section EDS dot maps show the presence of copper on the inclusion beneath the surface, indicates that solution has reached to this area. Figure 43, represents the overlay of S and Cu EDS dot maps of cross-section view of inclusion shown in Figure

42. Figure 43 confirms presence of Cu2S as a crust around MnS inclusions.

Figure 43. Overlay of S and Cu EDS dot maps shown in Figure 42 confirming the presence of Cu2S shell around the inclusion after polarization and sonication.

It seems that, a subsurface pit was created and propagated in both lateral and depth directions. Once the pit reached the inclusion, its propagation toward the surface, initiated the trench formation; however the MnS inclusion remained intact in the presence of Cu2S precipitates.

Some random pits on the pretreated sample experienced with potentiostatic test, were selected to evaluate this proposal. The MnS inclusions around the selected pits were investigated using FIB cross-section technique. Some of MnS inclusions had trenches around them and/or they were connected to the big pit subsurface while some of them were intact (shown in Figures 44, 44 and 49). The overlay of S and Cu in Figures 45, 47 EDS dot maps of cross-section view of all these inclusions confirm the presence of Cu2S shell around the MnS inclusions. The Mns inclusion in Figure 50 did not experience corrosion and Cu2S is only present on its surface.

In Figure 49-50, It is clear that, although a big undermining pit is propagating close to the selected inclusion, there are no ditches at the interface of the inclusion with the matrix. Based on Figure 50, copper sulfide covered only the top surface of inclusion,

suggesting that solution was not in contact with the inclusion beneath the surface. For the selected inclusion, neither pretreatment nor applied potential were successful in creating trenches at its boundaries with the alloy.

c) Platinum Depositio n Trenches around the inclusion

Figure 44.(a) A random pit on the surface of pretreated sample after running polarization tests (1 hour of OCP and 2 hours of potentiostatic step). The selected MnS inclusion is shown in red rectangle. (b) SEM/ EDS dot maps of the selected inclusion in part (a). (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulfur, manganese and copper

Figure 45. Overlay of S and Cu EDS dot maps shown in Figure 44.

a b ) )

c Platinum ) Depositio n

The Pit beneath the Surface

Figure 46. (a) A random pit on the surface of pretreated sample after running polarization tests (1 hour of OCP and 2 hours of potentiostatic step). The selected manganese sulfide inclusion is shown in red rectangle. (b) The selected inclusion in higher magnification. (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulfur, manganese and copper.

Figure 47.Overlay of S and Cu EDS dot maps shown in Figure 46.

Figure 48. Overlay of SEM and EDS dot maps of inclusion shown in Figure 46

a) b) a) b) Platinum Deposition

MnS inclusions

c) c)

Figure 49. A MnS inclusion beside a random pit on the surface of pretreated sample after running polarization tests. (b) SEM/ FIB cross- section image of selected inclusion in part (a). (c) SEM/FIB cross-section and EDS dot maps over the same inclusion confirming the presence of sulfur, manganese and copper.

Figure 50. Overlay of S and Cu EDS dot maps shown in Figure 49.

4. 1. 4. 8. Behavior of MnS Inclusions at Other Polarization Potentials

The SEM/ FIB cross section observation was conducted again, over the pretreated sample that underwent 10 minute of potentiostatic experiment at 0.227 V (SCE) followed by 20 minutes of 0.437 V (SCE) in 0.1 M NaCl solution. The in- situ AFM images were also recorded during OCP and potentiostatic tests.

Figure 51a shows the SEM image of the selected inclusion after immersion in acidic sodium chloride solution (1M NaCl at pH=3). This inclusion was selected because it has color contrast compared to the other MnS inclusions, shown by white arrows. Figure 51b shows a higher magnification of the chosen inclusion and its EDS dot maps. It shows that there is no copper accumulation on the inclusion after immersion in acidic sodium chloride solution. Figure 51c is the AFM topographic map in air after exposure to acidic solution.

Figure 52 illustrates the in- situ AFM images during potentiostatic tests. Figures

52-1 and 52-2 were recorded during 0.227 V (SCE) applied potential in 10 minutes and the rest of the images were taken during 20 minutes of 0.437 V (SCE) applied potential. The in- situ images show no changes at the inclusion or its surrounding area, except for the

changes in the height due to copper sulfide deposition. The SEM/ EDS dot maps of the inclusion after polarization (Figure 53) confirms the accumulation of copper sulfide on the inclusion while there is no visible trenches or pits at the boundaries of the inclusion or in the adjacent area.

a) b)

Figure 51. (a) SEM image of selected MnS inclusion after pretreating the sample in acidified 1 M NaCl. (b) SEM/ EDS dot maps of the selected inclusion in part (a). (c) AFM topographic map of the inclusion in part (a), taken in air.

Figure 52. In-situ AFM images of the inclusion in Figure 50 (after pretreatment in acidified 1 M NaCl) potentiostatically polarized in 0.1 M NaCl . Figures 52-1 and 52-2 were recorded during 10 minutes of potentiostatic test at applied potential of 0.227 V (SCE)

a) b)

Figure 53. SEM/ EDS dot maps and AFM topographic images of the inclusion shown in Figure 51 after polarization tests (10 minutes of potentiostatic test at applied potential of 0.227 V (SCE) followed by 20 minutes of potentiostatic experiment at applied potential

Platinum Deposition

Trenches around the inclusion

Figure 54. SEM/ EDS dot maps and FIB cross- section observation of the inclusion shown in Figure 51 after polarization tests (10 minutes of potentiostatic test at applied potential of 0.227 V (SCE) followed by 20 minutes of potentiostatic experiment at applied potential of 0.437 V (SCE))

The SEM observation of the FIB cross section of the selected inclusion (Figure 54) confirms the presence of pitting corrosion around the inclusion as micro crevices beneath the surface. The foot print of pitting corrosion around the MnS inclusion was not observed on the surface during AFM mapping. As a result, the copper deposition or the remnant of metallic foil is covering the initiation of this process.

4. 1. 4. 9. Behavior of MnS Inclusions Before and After Treatment

To find the source of the trenches at the boundaries of the inclusions, some random

MnS inclusions were selected and visualized before and after pretreatment in acidic sodium chloride solution (1M NaCl at pH=3).

Figures 55a, 55b, 55c and Figure 56a, 56b, 56c show the MnS inclusions, prior to the exposure to acidic sodium chloride solution, after immersion and FIB cross- section images of the same inclusions respectively.

In Figure 55c, just inclusion 1 (shown by red rectangle in Figure 55a and 55b) had a micro- crevice at its barrier with matrix. The existing trench, which may be created in the cooling process of alloy or in pretreatment period did not come along all the inclusion’s boundary; on the other hand, there was no visible creviced at the boundaries of inclusion 3 as well.

a) b)

Trench around Platinum MnS the Deposition inclusion inclusion 1

Figure 55. (a) SEM image of a selected MnS inclusion before pretreatment in acidified 1 M NaCl solution. (b) after pretreatment. (c) SEM/ FIB cross- section observation of the inclusion in part (a).

a) b)

c) Platinum Deposition

MnS inclusion Trenches 1 around the 3 inclusion

Trenches around 2 the inclusion s

Figure 56. (a) SEM image of a selected MnS inclusions before pretreatment in acidified 1 M NaCl. (b) After pretreatment. (c) SEM/ FIB cross- section observation of the inclusions in part (a).

Inclusions 1 and 3 in Figure 56c (shown inside the red rectangle in Figures 56a and

56b) both have micro-crevices at their interface with the matrix. Inclusion 2 is present beneath the surface and although it was not in touch with the electrolyte, some ditches are present at its left side boundaries.

4. 1. 4. 10. Summary

We succeeded to record the pitting formation at the interface of some manganese sulfide inclusions on pretreated sample during potentiostatic test. Although it initially appeared to be pit initiation and very similar to the pitting initiation which was illustrated in many studies, with further FIB observations it was demonstrated that the sudden holes at the barriers of the inclusions (recorded during in- situ visualization) were actually the results of a big undermining pit which became visible when it reached the surface while the inclusions are still protected by copper sulfide depositions.

Further investigation of manganese sulfide inclusions on the pretreated surface reveals that the undermining pitting corrosion is most likely initiated at the interfaces of the manganese sulfide inclusions and the matrix beneath the surface. It results in creation of micro size trenches at inclusion boundaries which are not visible on the surface. The created cracks tend to propagate in depth direction more than lateral direction, and whenever they reach to the surface, they will create lacelike pitting. Although these potential sites for pitting initiation were observed at the manganese sulfide inclusions barriers, but it does not mean that all manganese sulfide inclusions on the surface can create such ditches at their boundaries. We observed some manganese sulfide inclusions in the same situations which were completely safe with no crevices at their interfaces with the

matrix. These exceptions are due to the inherent presence of some crevices at the boundaries (due to the thermal coefficient difference of these particles and matrix).41, 50, 57

Figure 57 shows the summary of pitting initiation and propagation at the boundaries of MnS sulfide inclusions. 1- In the first step of this mechanism the MnS itself undergoes chemical dissolution. 2- At this stage Cu deposition as Cu2S begins. If the deposition rate is faster than the dissolution rate then the MnS inclusion Passivates and no further attack is observed, however 3- some inclusions have inherent trenches at their boundaries that the dissolution of the inclusion results in solution to reaches these crevices. As a result the trapped solution in these crevices will cause corrosion beneath the surface while the inclusion itself is safe due to presence of Cu2S precipitates. 4- Finally when the undermining corrosion reaches the surface its finger print will be observable on the sample surface.

2 3

4 Figure 57. The initiation of pitting corrosion around the left inclusion is due to presence of the solution in the crevices at the inclusion’s interfaces. Pitting propagation beneath the surface will result in lacelike pitting at this inclusion. The other MnS inclusion remains safe with no pitting at its boundaries since the solution could not reach the inherent trenches below the surface. Both MnS inclusions during potentiostatic step remains safe due to Cu2S precipitation which prevents further dissolution of the inclusions. In the next section, the different types of inclusions on 304L stainless steel were investigated with the same experimental procedure which was conducted on 303 samples.

4. 2. 304L Stainless Steel:

The effects of different types of inclusions on pitting corrosion of 304L stainless steels and their correlation to pit creation and Volta potential were studied. It was observed that all inclusions (except for silicon oxide inclusions), were partially dissolved in solution under experimental conditions , as a result no defects were found neither on matrix nor matrix-inclusion interfaces; whereas silicon oxide particles underwent partial dissolution which results in formation of holes in the inclusion area.

4. 2. 1. Cyclic Polarization Potential Maps:

Pitting and repassivation potential, Epit and Erep, of 304L stainless steel was measured from the cyclic polarization potential (CPP) map (shown in Figure 55),

Epit=0.351± 0.025 V (vs. SCE) and Erep=0.0271 ± 0.010 V (vs SCE). As mentioned earlier, an arbitrary potential between pitting and repassivation potentials was selected and used for potentiostatic experiments (Epit< Eapplied< Erep). Eapplied=+ 0.227 V vs. SCE was applied on 304L for one hour.

CPP curve of 304L SS in 0.1 M NaCl 0.6

0.5

0.4

0.3

0.2

0.1 E( V SCE) vs. V E( 0

-0.1

-0.2 1.0E-11 1.0E-09 1.0E-07 1.0E-05 1.0E-03 1.0E-01 Log(i) A/ Cm2

Figure 58. Cyclic Polarization Potential curve of 304L stainless steel in 0.1 M NaCl

4. 2. 2. Multi-Phase Oxide Inclusions

Most of the found inclusions on 304L stainless steel were made up of different components such as nitrogen, oxygen, silicon, sulphur and carbon.

4. 2. 2. 1. Silicon/Oxygen Base Inclusion

Combinations of SEM and EDS techniques were used to identify and characterize the inclusions on the surface of the stainless steel. The dot map data of the selected inclusion shown in Figure 58, confirmed that this inclusion contains silicon and oxygen and it is depleted of chromium, nickel, iron and manganese.

Figure 59. SEM image of Silicon/ oxygen based inclusion on 304L SS.

Figure 60. EDS dot maps of inclusion in Figure 58. This inclusion contains silicon and oxygen and is depleted of chromium, nickel, iron and manganese. Figure 60a, 60b and 60c show the AFM topographic image, potential map and the overlay of them for the selected inclusion shown in Figure 58 before immersing the sample

in the solution (0.1 M NaCl). In general brighter areas indicate elevation due to the matrix or nobler potentials in AFM topographic and potential maps; whereas darker regions represent deeper topographical areas or more active potential sections. Since the darker areas are representing more active potentials, it is expected that more active areas are more susceptible for corrosion.

Figure 61. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b, for selected inclusion in figure 58, before running potentiostatic test in air.

The AFM topographic map indicates that the inclusion is consisting of some elevated and depressed regions surrounded by matrix (Figure 60a). The potential map of this inclusion illustrates a non-uniform, noble inclusion area, with the highest observed potential in the center of the inclusion surrounded by active matrix (Figure 60b). With the overlaid picture it is easier to see the correlation between the topographic and potential maps (Figure 60c)

The sample went through 1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE. The AFM topographic map (Figure 61) shows the surface of the sample after last two steps. Figure 60 shows that some parts of the inclusion

underwent dissolution, whereas there is no defect on the periphery area. Dissolved areas are highlighted by dashed circles.

Figure 62. AFM height map of Si/O based inclusion in Figure 58, after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE)

It is believed that the inclusion went through chemical dissolution, while the matrix experienced no corrosion. Partial dissolution of the selected inclusion did not result in pitting, but rather resulted in creation of pit-resembling structures. The detrimental effects of silicon oxide inclusions is in consistent with Freiman’s 35 study, whereas they believed these inclusions are susceptible sites for pitting corrosion.

Figure 62a and 62b show the potential and topographic maps of the selected inclusion before and after the polarization respectively, while Figure 62c is the result of overlaying them, shows that the dissolved areas are all in the inclusion region with nobler potential comparing to the matrix.

Figure 63. (a) potential map before polarization, (b) topographic map after polarization, (c) overlay of a & b of Si/O based inclusion.

Inclusions illustrated in Figure 63 are similar in chemical composition to the inclusion of the Figure 58, and they both contain elevated levels of oxygen and silicon due to EDS maps. These three inclusions were analyzed using AFM prior to potentiostatic polarization, and found to be more topographically depressed compared to the matrix and also have more active boundaries (Figure 65).

Figure 64. Silicon/ oxygen based inclusions SEM image on 304L SS.

Figure 65. EDS dot maps of inclusions of Figure 63. Beside the presence of silicon and oxygen, depletion of Mn, Ni, Fe and Cr is visible here.

Figure 66 shows the AFM height and potential maps of inclusions 1,2 and 3 in in

Figure 64 before polarization tests.

Figure 66. AFM Height/ Potential maps before polarization, (a), (b) and (c) are representing respectively inclusion 1, 2 and 3 in Figure 64.

Figure 67 shows the AFM maps of the inclusions after polarization, in air.

Figure 67. (a), (b) and (c) are AFM height map after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) respectively for inclusion 1, 2 and 3 shown previously in figure 64.

Before Polarization After Polarization

Figure 68. Before and after polarization topographic maps comparison of the silicon oxygen based inclusions in Figure 64.

Although the inclusions experienced partial chemical dissolution , but no defects were observed on the matrix around them, shown in Figure 67. The only visible defects are the result of partial chemical dissolution of the inclusions inside their boundaries. In Figure

68 for better understanding of the differences made during the polarization tests, the recorded pictures in both steps are brought together,for each inclusion and the changed areas are shown by dashed circles.

4. 2. 2. 2. Carbon/ Nitrogen/Oxygen/Sulfur Base Inclusion

Figure 70 shows the EDS results related to the selected inclusion of Figure 69, it was observed that this inclusion contains mainly carbon, nitrogen, sulfur and oxygen.

Figures 71 and 72 depict the AFM pictures of the inclusion selected in Figure 69, before and after polarization respectively. It was found that the inclusion appears as a bump on the surface and its potential is close to the matrix (Figure 71). It should be noted that the darker square area arround the inclusion in the potential map (Figure 71b) is caused by

SEM, so its potential difference is artificial.

Figure 69. Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion SEM image on 304L SS.

87 Figure 70. EDS dot maps of the inclusion in Figure 69.

Figure 71. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b of the Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion shown in Figure 68 before polarization test.

88 Figure 72 shows the topographic map of carbon/ nitrogen/ oxygen/ sulphur base inclusion after polarization test. According to this figure, the upper part of the inclusion (shown in dashed circle) experienced prefential dissolution and it seems that dissolution products sit around the inclusion after the polarizationand, there were no other visible changes observed around the inclusion.

Figure 72. AFM height map of the Carbon/ Nitrogen/ Oxygen/ Sulfur based inclusion shown in Figure 69 after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE)

4. 2. 2. 3. Carbon/Oxygen/Silicon Base Inclusion

The carbon, oxygen, silicon based inclusion was detected using EDS techniqes

(shown in Figure 74) it was observed in AFM topographic and potential maps that this inclusion is elevated in height regardin to the matrix and has different potential distribuation. Figure 75 and 76 show the AFM maps prior and after the polarization test in air. Comparing the selected area (dashed circles) shows the partial dissolution around the inclusion.

89 Figure 73. Carbon/ Oxygen/ Silicon based inclusion SEM image on 304L SS.

Figure 74. EDS dot maps of inclusion in Figure 73.

Figure 75. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b of the Carbon/ Oxygen/ Silicon based inclusion shown in Figure 73, before polarization test.

Figure 76. (a) AFM topographic map, (b) Potential map and( c) overlay of a &b of the Carbon/ Oxygen/ Silicon based inclusion shown in Figure 73, after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) Although clear changes was observed over time (Figure 75 and 76) but similar to the other studied inclusions, the matrix remained safe and dissolution of inclusion did not cause any flaws to the matrix.

4. 2. 2. 4. Carbon/ Silicon/ Nitrogen/ Oxygen

The EDS maps for this type of inclusion in Figure 78, shows that this inclusion -similar to the previous one contains carbon, oxygen and silicon and has depletion in the same elements.

Figure 79 is the AFM maps of this inclusion before being subjected to polarization test, in air.

It shows that this inclusion appeared as a proturusion on the surface and relying on its potential map, it is more active at its left interface with the matrix and adjacent regions.

Figure 77. Carbon/ Oxygen/ Silicon based inclusion SEM image on 304L SS.

Figure 78. EDS dot maps of inclusion in Figure 77.

Figure 79. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b before polarization test for carbon/ oxygen/ silicon based inclusion shown in Figure 77.

Figure 80. AFM height map of the inclusion in Figure 77, after polarization tests. (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE)

No severe changes was observed for the inlucion shown in Figure 77 after the polarization tests. Figure 81 compares the topographic maps of the inclusion after polarization and before that. The partial chemical dissolution is shown in the dashed circle. Like all previuos

inclusions, although there is partial changes among the inclusion itself but the surrounding surface, faces no defects.

Figure 81. Before and after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) topographic maps comparison of the carbon/ oxygen/ silicon based inclusion in Figure 77. 4. 2. 2. 5. Carbon/Silicon/Nitrogen/Nickle/Chromium/Iron Base Inclusion

The inclusion demonstrated in Figure 81 has carbon, silicon, nitrogen, nickle, chromium and iron in its composition based on EDS data shown in Figure 82.

Figure 82. SEM image for Carbon/ Oxygen/ Silicon based inclusion on 304L SS.

Figure 83. EDS dot maps of inclusion in Figure 82.

Figure 84 reperesnts the AFM topographic maps of the inclusion shown in Figure 82 before and after polarization tests. Although the inclusion underwent serious dissolution but the surrounding matrix is safe from being corroded.

Before Polarization After Polarization

Figure 84. AFM height maps comparison of the inclusion in Figure 81, before and after polarization (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE)

4. 2. 2. 6. Carbon Base Inclusion

The inclusion which is present in Figure 85, is a carbon base inclusion and it has just carbon in its composition. It underwent the same experimental conditions as the previously mentioned types of inclusion. The AFM images were recorded in different steps and are illustrated in Figures 87 and 88.

Figure 85. Carbon based inclusion SEM image on 304L SS.

Figure 86. EDS dot maps of inclusion in Figure 85.

Figure 87. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b before polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE) for carbon based inclusion shown in Figure 85.

Figure 88. (a) AFM topographic map, (b) Potential map and (c) overlay of a &b after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE), for carbon based inclusion shown in Figure 85.

The height maps of the inclusion before and after polarization are shown in Figure

89. A horizental lines that were drawn on the same area of the inclusion on both maps

(before and after polarization) showed that ~3 um decrease in the size of the inclusion as a result of chemical dissolution. The chemical dissolution from the z axis was also observed from the cross section topographic maps of the selected horizontal line over the inclusion.

Before Polarization After Polarization

Cross section of selected area by line Cross section of selected area by line

Horizontal distance Horizontal between cursors: distance between 29.354 μm cursors: 26.03 μm Figure 89. AFM height maps comparison of carbon based inclusion in Figure 85 before and after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE).

4. 2. 2. 7. Carbon/Nitrogen/Oxygen Base Inclusion

Figure 90. Carbon/ Nitrogen/ Oxygen based inclusion SEM image on 304L SS.

Figure 91. EDS dot maps of the Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90.

Illustrated inclusion in Figure 90, is a carbon/ nitrogen/ oxygen based particle based on EDS experiment. Similar to other types of inclusions, topographic and potential maps was obtained prior and after the polarization (shown in Figure 92 and 93 respectively).

The horizental distance measurments between inclusion boundaries showed that the

100

inclusion’s size decreased, however there is no other defects on the surrounding matrix.(

Figure 94)

Figure 92. (a) AFM topographic map, (b) Potential map and (c) overlay of a&b before polarization test for Carbon/ Nitrogen/ Oxygen based inclusion illustrated in Figure 90.

Figure 93. AFM height map of Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90, after polarization test (1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE).

101

Before Polarization After Polarization

Cross section of selected area by Cross section of selected area by

Horizontal Horizontal distance between distance between cursors: 10.729 cursors: 13.804

Figure 94. AFM height maps comparison of Carbon/ Nitrogen/ Oxygen based inclusion in Figure 90 before and after polarization test(1 hour OCP, followed by 1 hour potentiostatic polarization at 0.227 V vs. SCE). The behavior of seven different types of multi oxide inclusions of 304L stainless steel was studied under 0.227 V vs SCE in NaCl (0.1 M) solution. AFM visualization illustrated that, even though these inclusions underwent partial chemical dissolution and their size was decreased, but the matrix did not get damaged in most cases (in contrary to the MnS inclusions). Silicon oxide base inclusions were the only type of multi oxide

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inclusion which their dissolution created pit resembling features inside the inclusions boundaries.

As a result, although some investigators have proposed that MnS boundary is more susceptible to pitting corrosion due to formation of a crevice at these locations, based on generated data of other types of inclusions (no pitting at the inclusion/matrix boundary), local chemistry and not the geometry is more likey the reason for pitting corrsion at MnS boundaries.

103

CHAPTER IV

CONCLUSION

There have been numerous studies to understand the effect of different types of inclusions on pitting behavior of stainless steel alloys.

In this study, we investigated the effect of different types of inclusions on pitting corrosion of austenitic stainless steels and also their correlation between pit creation and

Volta potential in chloride solution. The in- situ AFM maps inside the solutions were also recorded to follow the effect of polarization on the pitting initiation.

Initial results in 0.1 M NaCl solution, recorded copper deposition on MnS inclusions only at anodic potentials and not at OCP. This deposition continued throughout the experiment to a thickness off ~ 49 nm. I was proposed that Cu (II) entered solution as a result of passive oxidation or possibly metastable elsewhere on the sample. EDS data later, indicated that this deposition was not metallic copper, rather Cu2S.

Although pit propagation at MnS/ matrix boundaries was imaged during exposure to NaCl solution, these samples were pretreated by exposure to acidic chloride solution at the OCP. Though it initially appeared to be pit initiation, further SEM- FIB analysis demonstrated that, the propagation was occurring below the surface in a larger covered pit, propagating toward the surface. This result may explain why some investigators have concluded that smaller MnS inclusions are immune to corrosion and larger inclusions whose major axis is oriented perpendicular to the surface are more susceptible to pitting.

104 SEM-FIB analysis showed that during pit propagation at the boundaries of MnS inclusions with matrix, the inclusions were immune to damage. SEM-FIB combined with

EDS analysis showed that the MnS inclusions were passivated by a thin layer of Cu presumably as Cu2S.

Finally, while some investigators have proposed that the MnS boundary is more susceptible to pitting corrosion, due to the formation of a crevice at this location, we have examined other types of inclusions and found no pitting corrosion at the inclusion/matrix boundary. This indicates that local chemistry and not the geometry is the more likely reason for attack at the MnS/matrix boundary.

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