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IVBcrofilnis lateniai^oiial
8612390
Lee, Jong-Kwon
STRESS CORROSION CRACKING AND PITTING OF SENSITIZED TYPE 304 STAINLESS STEEL IN CHLORIDE SOLUTIONS CONTAINING SULFUR SPECIES AT TEMPERATURES FROM 50 TO 200 DEGREES C
The Ohio State University Ph.D. 1986
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University Microfiims International
STRESS CORROSION CRACKING AND PITTING
OF SENSITIZED TYPE 304 STAINLESS STEEL
IN CHLORIDE SOLUTIONS CONTAINING SULFUR SPECIES
AT TEMPERATURES FROM 50 TO 200°C
DISSERTATION
Presented 1n Partial Fulfillment of the Requirments for
the Degree Doctor of Philosophy in the Graduate
School of the Ohio State University
By
Jong-Kwon, Lee, B.S, M.S.
*****
The Ohio State University
1986
Reading Committee:
Prof. Susan Smialowska Approved By
Prof. John P. Hirth
Prof. Bryan E. Wilde Adviser Department of Metallurgical Engineering To my parents To my wife
11 ACKNOWLEDGEMENTS
I wish to express my deep appreciation to professor S. Smialowska for her advice and encouragement during the course of this study. I would like to thank Professor 0. P. Hirth and B. E. Wilde who served as members of reading committee for this dissertation. I am also Indebted to Dr. G. Cragnolino for sincere discussion and guidance. My special appreciation must be given to V. Jaganathan for correcting my English. I also wish to acknowledge the encourgement from colleagues; M. Y. Lee, C. 0. Park, H. Betrabet, J. R. Park and S. H.
Shim. I wish to thank the following workers for their assltance; R. B. Farrar, C. Macdonald, L. Otenberger, D. Murfleld, T. Biggert and S.
Euans. Financial support from the Electric Power Research Institute Is also gratefully appreciated. Also, I would like to thank Korea
In stitu te of Machinery and Metals and Korea Science and Engineering Foundation for their encouragement and support. Finally, I'd like to express my appreciation to my parents, my wife, my brother and sisters.
ill VITA
June 1, 1955 ...... Born - Daegu, Korea
1977 ...... B.S., MetalTurglca Engineering, Seoul National University, Seoul, Korea 1979 ...... M.S. m aterials Science Korea Advanced In stitu te of Science, Seoul, Korea
1979 - 1982 ...... Research Scientist, Korea Institute of
Machinery and Metals, Changwon, Korea 1982 - 1985 ...... Gradute Research Associate, Department of Metallurgical Engineering, The Ohio State
University, Columbus, Ohio
FIELD OF STUDY
Major Field: Metallurgical Engineering
Studies in Corrosion: Professors S. Smialowska, D. D. Macdonald, R. A. Rapp and F. H. Beck
Studies in Physical Metallurgy: Professors G. W. Powell, G. Meyrick, P. G. Shewmon and J, P. Hirth Studies in Chemical Metallurgy: Professors G. St. Pierre and W. Johnson
Studies in Mechanical Metallurgy: Professors R. G. Hoagland and R. Wagoner Studies in Electron Microscope: W. Clark
iv TABLE OF CONTENTS
DEDICATION ...... il ACKNOWLEDGEMENT ...... H i VITA ...... iv
TABLE OF CONTENTS ...... v
LIST OF TABLES ...... ix LIST OF FIGURES ...... xi CHAPTER I. INTRODUCTION ...... 1
II. LITERATURE SURVEY ...... 3
2.1 Introduction ...... 3 2.2 Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel ...... 3 2.2.1 Stress Corrosion Cracking in Chloride Solution . 3 2.2.1.1 Effect of Chloride Ion Concentration .. 4
2.2.1.2 Critical Potential ...... 7
2.2.1.3 Effect of pH ...... 15 2.2.2 Effect of Dissolved Oxygen Content in Water on
the Stress Corrosion Cracking Susceptibility ... 18
2.2.3 Effect of Sulfur Species on the Stress Corrosion Cracking Susceptibility ...... 29 2.2.3.1 Solution ...... 29
2.2.3.2 Effect of Potential ...... 44 2.2.3.3 Effect of pH ...... 49
V 2.3 Pitting Corrosion ...... 53 2.3.1 Chloride Environment ...... 53 2.3.1.1 Chloride Concentration ...... 53 2.3.1.2 Temperature ...... 58
2.3.1.3 pH ...... 62 2.3.1.4 Effect of Anion Addition ...... 65
2.3.2 Solution Containing Sulfur Species ...... 69
2.4 Chemistry of Sulfur Species ...... 76 2.4.1 Potential-pH Diagram ...... 76
2.4.2 Oxidation of Sulfide ...... 83
I II . EXPERIMENTAL PROCEDURE ...... 97 3.1 Materials ...... 97 3.2 Solutions ...... 99
3.3 Equipment ...... 99 3.3.1 Slow Strain Rate Test ...... 99 3.3.2 Electrochemical Measurement ...... 107
3.4 Experimental Procedure ...... I ll
3.4.1 Specimen Preparation ...... I l l 3.4.2 Solution Preparation ...... I ll 3.4.3 Slow Strain Rate Test with Measurement of
Corrosion Potential ...... 113 3.4.4 Measurements of Polarization Curves ...... 115 3.4.5 Potentiostatic P itting Potential Measurement . . . 116
IV. RESULT...... 118 Vi 4.1 Slow Strain Rate Tests ...... 118 4.1.1 Slow Strain Rate Tests In Air ...... 127 4.1.2 Slow Strain Rate Tests In Air-Saturated Solution Containing 104 ppm Cl" ...... 138
4.1.3 Slow Strain Rate Tests In Air-Saturated Solution
Containing 104 ppm Cl” and D ifferent Concentration of Sulfide ...... 138 4.1.4 Effect of Sulfide Ion In Air-Saturated Solution
with Chloride Ion ...... 161 4.1.5 Effect of Sulfide Ion In the Solution Containing 0.2 ppm Og and 104 ppm Cl ...... 165
4.1.6 Effect of Thiosulfate Ion In the Solution
Containing 8 ppm Og and 104 ppm Cl ...... 169
4.2 Anodic Polarization Studies ...... 175 4.3 Pitting Potential Studies from the Potential-Time Curves ...... 185 V. DISCUSSION ...... 196
5.1 in the Environment without Sulfur Species ...... 196 5.1.1 Stress Corrosion Cracking In Air-Saturated
Pure W ater...... 186
5.1.2 Effect of Chloride Ion In Air-Saturated Water .. 198 5.1.3 Relationship between Pitting and Stress Corrosion Cracking In 104 ppm Chloride Solution
v11 5.2 In the Solution Containing Sulfur Species ...... 200 5.2.1 Effect of Sulfur Species on Stress Corrosion
Cracking ...... 200 5.2.2 Effect of Sulfur Species on Pitting ...... 202 5.2.3 Relation of Stress Corrosion Cracking and Pitting In the Solution Containing Sulfur Species ...... 205 5.2.4 Effect of Dissolved Oxygen In the Solution
Containing 104 ppm Chloride and Sulfide Ion . . . . 206 5.2.5 Effect of Chloride Ion In Air-Saturated Solution
Containing Sulfide ...... 207
5.3 Effect of pH ...... 210 5.4 Mechanistic Study ...... 215 VI. CONCLUSION ...... 222
REFERENCE ...... 224 APPENDIX ...... 231
vlll LIST OF TABLES
Table Page 1. Chemical Analysis of Samans Solution Containing 0.38 moles/liter Polythlonic Acid After Oxygen
Bubbling for Different Periods During Experiment ...... 36 2. Minimum Content of Chloride Ion to Inlatlate Pitting
In Various Steel ...... 55
3. Summary of Kinetic Data for Oxygenation of Reduced Sulfur Species at pH 7.55 ...... 89 4. Observed Production Distribution for Catalyzed and
Noncatalyzed Reactions ...... 91 5. Summary Reaction Products Observed In Investigations of Oxygenation of Reduced Sulfur Species ...... 94 6. Factory Product Specification of Type 304 Stainless Steel Supplied by A1 Tech Specialty Steel Corporation ...... 98 7. Composition and pH a t Room Temperature ...... 100
8. Representative Values, Eg^E^T) - #Eggg for the External Ag/AgCl Reference as a Function of IT and Concentration
of KCL ...... 109
9. Experimental Results of SSRT for Sensitized Type 304 Stainless Steel ...... 128 10. Fracture Morphology of Sensitized Type 304 Stainless Steel ...... 135 Ix 11. EDS Analysis of the Corrosion Product on the Specimen Tested In 8 ppm Og + 104 ppm 01 + 40 ppm S“ a t 200*0 . . . . 160
12. Table of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*0 to 200*0 In the Deaerated Solution Containing 104 ppm Chloride .... 179
13. Table of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*0 to 200*0 In the Deaerated 104 ppm Chloride Solution with
40 ppm Sulfide ...... 181
14. Table of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*0 to 200*0 In the Deaerated 104 ppm Chloride Solution with
70 ppm Thiosulfate ...... 184 15. Calculated pH Values of the Solution at Elevated
Temperature ...... 211
16. Solution Analysis during Slow Strain Rate T est ...... 216 17. Dissociation Constant and pH Values of Pure Water at Elevated Temperature ...... 232
18. Activity Coefficients, Dissociation Constants and Calculated pHs at Elevated Temperatures for
Thiosulfate Solution ...... 236
19. Activity Coefficients, Dissociation Constants and Calculated pHs at Elevated Temperatures for Sulfate Solution ...... 237 LIST OF FIGURES
Figure Page 1. Effect of C1~ concentration on reduction of area (ROA) and ultimate tensile strength (UTS) of Type 304 stainless steel
In 35% (NH^lgSO* solutions a t 104"C ...... 6 2. The effects of oxygen and chloride on the SCO of austenltic stainless steels In high temperature water ...... 8
3. Effect of potential on the time to failure of sensitized (SO hour at 650*0 Type 304 stainless steel In NaCl solutions
of different concentrations at 100*C ...... 9
4. Normalized elongation to failure ^®f,scc^ef,Duct11e^ vs. potential for sensitized Type 304 stainless steel In 0.01 M
NaCl solution at temperatures ranging from 250*0 to 100*0 ...... 12 5. Potentlal-temperature diagram showing regions of different modes of failure ...... 13 6. The effect of chloride concentration on stress corrosion cracking of sensitized alloy In 100 ppm sulfate solution ...... 14 7. The effect of pH on the strain to crack Initiation at 288*0
for various types of Impurities at a 10 "5/cm concentration ... 17
8. Effect of dissolved oxygen on cracking times ...... 19 9. Effect of oxygen concentration on the IGSOO of sensitized
Type 304 stainless steel (from one heat) In 288*0 water at
two applied stress level ...... 20 x1 10. Estimated crack growth rate vs. dissolved oxygen content, = 10"® /sec...... 22
11. Variation of IGSCC susceptibility of sensitized Type 304
stainless steel In pure water for various oxygen/temperature
combinations ...... 23 12. Corrosion potentials of Type 304 vs. dissolved oxygen ...... 24
13. The effect of dissolved oxygen concentration on the corrosion potential of Type 304 stainless steel In high purity water at 274*0 ...... 26
14. Ratio of time to failu re In 0.01 M NagSO^ and high purity water at 250*0 as a function of potential to that In argon
and anodic polarization curve for sensitized Type 304 a t 250*0 ...... 28 15. Variation of average crack propagation rate of sensitized 304 stainless steel In 0.01 M NagSO^ at 97 + 2 0 and In water/oxygen at 100*0 ...... 30 16. Effect of tetrathlonic acid concentralon on the time to failure of sensitized (4 hr at 650*0) Type 304 stainless
steel at room temperature ...... 32 17. Effect of polythlonic acid concentration on stress corrosion
life of sensitized 304 austenltic stainless steel ...... 33 18. Elongation to failure vs. concentration of sulfur for
sensitized Type 304 stainless steel In air saturated boric acid solution containing Na2S^0g at room temperature ...... 37 x11 19. Schematic showing anticipated effect of the addition of
another anion on the rate of thiosulfate SCC ...... 40
20. Dependence of crack velocity In CER tests on temperature for
sensitized Inconel 600 In air-saturated 1.3% H^BOg + 0.7 ppm sulfur as sodium thiosulfate ...... 42 21. Effect of potential on the time to failure of sensitized (10 hr at 650*0 Type 304 stainless steel In polythlonic acid solution at room temperature ...... 45
22. Elongation to failure vs potential curves for sensitized
Type 304 stainless steel In deaerated boric acid solulon containing 0.01 M NagSgOg at room temperature ...... 46 23. Potent1al-pH diagram for Fe-S-HgO at 298 K, not considering
S0^^~. All equilibria Involving dissolved species are drawn for unit activity; equilibria Involving HgS are for 1 atmosphere gaseous HgS ...... 48 24. Effect of polarization on stress corrosion behavior of sensitized AISI 304,sta1nless steel U-bend specimens In Samans solution containing 0.38 mol/L of polythlonic acid 50
25. Variation of repassivation rate parameter with potential for Iron-9 Cr-10 N1 alloy In 0.5 M NagSgOg adjusted to various pH
with sulfuric acid ...... 52 26. Corrosion morphologies observed for U-bend specimens of
Type 304 steel In HgSO^/NaCl solutions ...... 56 27. Effect of chloride Ion activity on steady-state critical
potential for pitting, 25*0 ...... 56 x lll 28. Effect of chloride concentrations(Z) on the pitting potentials of steels at 20*C ...... 57 29. Effect of time at constant potential on observed critical potential for pitting In 1 N NaCl, 25*0 ...... 57
30. Effect of temperature on critical potential for pitting 0.1 N NaCl ...... 59
31. Breakdown potential of 430, 304 and 316 ste els a t 30 to 90*C In 3% NaCl solutions ...... 59 32. Effect of temperature on the pitting potential of steels In 32 NaCl solution ...... 60 33. Passivity breakdown potentials, measured under potentlodynamic and potentiostatic conditions,
as a function of temperature for sensitized Type 304 stainless steel In 0.01 M NaCl solution ...... 61 34. Pitting potentials of Type 304 stainless steel on
unstrained and continuously strained specimens as a function of temperature ...... 63 35. Effect of pH on critical potential for pitting In 0.1 N NaCl, 25"C...... 64 36. Dependence of the breakdown potential upon pH for 430, 304
and 316 steels In 32 NaCl ...... 66
37. Activity of S0^^~ or C10^“ required to Inhibit pitting as a function of Cl" activity, 25'C ...... 67 38. Activity of OH" or NOg" required to Inhibit pitting
as a function of Cl" activity, 25*C ...... 68 xlv 39. Effect of sulphate concentratlon(X) on the pitting potential of steels In 32 NaCl solution ...... 68
40. Effect of temperature and NaCl concentration on
pitting potentials of AF 22 In aqueous NaCl solutions
containing HgS ...... 70 41. Effect of temperature and NaCl concentration on pitting potentials of AF 22 In aqueous NaCl solutions ...... 71 42. Pitting potential data for Type 304 steel In 0.25 M NaCl
with additions of sulfur compounds ...... 73
43. Potent1al-pH diagram for the stable equilibria of the system sulphur-water, a t 25'C ...... 77 44. Potential-pH metastable equilibrium diagram for the system
S-HgO a t 25'C and 1 a tm ...... 78 45. Potentlal/pH diagram for Fe-HgO-S system at 25'C ...... 79 46. Potent1al/pH diagram for Fe-HgO-S system at lOO'C ...... 80 47. Potential/pH diagram for Fe-HgO-S system at 150'C ...... 81 48. Potentlal/pH diagram for Fe-HgO-S system at 200'C ...... 82 ?«• 49. Semllog plot of optical density of SgOg Ions against time . . . 84 50. Relation of concentration to tim e ...... 84 51. Reaction of oxygen with hydrogen sulfide In 3.52 sodium
chloride solution. HgS = 200 ppm, T = 25 C ...... 86 52. Reaction pathway of oxygenation of sulfide ...... 88 53. Model predictions vs. experimental data In the sulfide
oxygenation reaction. pH = 7.55 ...... 88
XV 54. Development of turbidity and autocata ly sis. Sodium su lfite added from 20 mln on; su lfite :su lfid e = 0.75. 70 "M sulfide
added to each te s t aliquot ...... 92
55. Temperature-pH diagram for solutions 0.1 .M In total monosulflde and 0.01 M In total zerovalent sulfur ...... 95
56. Schematic diagram of SSRT apparatus ...... 102
57. Cross-sectional view of the te s t vessel showing the spatial arrangement of fittings, pressure seals, specimen,
counter electrode and other ancillary parts ...... 103
58. Straining Device ...... 105 59. The Ag/AgCl external reference electrode assembly ...... 110 60. Dimension of specimen, a) for stress corrosion cracking
b) for electrochemical study ...... 112 61. Schematic diagram of experimental procedure ...... 114 62. Nominal stress vs. elongation curves for sensitized type
304 stainless steel In air saturated solutions with various anions at 50*0 ...... 119
63. Nominal stress vs. elongation curves for sensitized type
304 stainless steel In air saturated solutions with various anions at 100*0 ...... 120 64. Nominal stress vs. elongation curves for sensitized type
304 stainless steel In air saturated solutions with
various anions at 150*0 ...... 121 xvl 65. Nominal stress vs. elongation curves for sensitized type
304 stainless steel In air saturated solutions with
various anions a t 200"C ...... 122 66. Nominal stress vs. elongation curves for sensitized type 304 stainless steel In the solutions containing 0.2 ppm Og with various anions at lOO'C ...... 123
67. Nominal stress vs. elongation curves for sensitized type 304 stainless steel In the solutions containing 0.2 ppm Og with Various anions a t 150'C ...... 124 68. Nominal stress vs. elongation curves for sensitized type 304 stainless steel In the solutions containing 0.2 ppm Og
with various anions at 200*0 ...... 125
69. SEN fractograph of sensitized Type 304 stainless steel tested In air at 50*0 ...... 131
70. SEN fractographs of sensitized Type 304 stainless steel
tested In air at 100*0 ...... 132 71. SEN fractographs of sensitized Type 304 stainless steel tested In air at 150*0 ...... 133
72. SEN fractographs of sensitized Type 304 stainless steel tested In air at 200*0 ...... 134
73. Time-to-fallure vs. sulfide Ion concentration In air-saturated 104 ppm 01" solution at various temperatures .... 139
74. Iggg vs. sulfide Ion concentration In air-saturated 104 ppm 01" solution at various temperatures ...... 140
xvll 75. SEN fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution at 50*0 .. 142
76. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution containing
10 ppm S" a t 50*C ...... 143 77. SEM fractograph of sensitized Type 304 stainless steel
tested In air-saturated 104 ppm Cl" solution containing 40 ppm a t 50*C ...... 145 78. SEN fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution at 100*C . 146
79. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution containing 5 ppm s ’" a t 100*C ...... 147 80. SEM fractograph of sensitized Type 304 stainless steel
tested In air-saturated 104 ppm Cl" solution containing
10 ppm S“ a t lOO'C ...... 148 81. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution containing
40 ppm s'' a t lOO'C ...... 150 82. SEM fractograph of sensitized Type 304 stainless steel
tested In air-saturated 104 ppm Cl" solution at 150*C . 153
83. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution containing
10 ppm S“ a t 150*C ...... 154 xvl 11 84. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl” solution containing 40 ppm s ' a t 150'C ...... 155
85. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm C1~ solution at 200*0 ...... 157 86. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm C1~ solution containing 10 ppm S“ a t 200'C ...... 158
87. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl” solution containing 40 ppm s ' a t 200'C ...... 159
88. Tlme-to-fallure vs. sulfide Ion concentration In
air-saturated water at various temperatures ...... 162 89. SEM fractograph of sensitized Type 304 stainless steel
tested In air-saturated pure water at 200'C ...... 163 90. SEM fractograph of sensitized Type 304 stainless steel tested In air-saturated solution containing 40 ppm s ' a t 2 00'C ...... 164
91. T1me-to-fa11ure vs. sulfide Ion concentration In 0.2 ppm Og + 104 ppm Cl" solution at various temperatures ...... 166
92. SEM fractograph of sensitized Type 304 stainless steel
tested In 0.2 ppm Og + 104 ppm Cl" solution at 150*C ...... 167 93. SEM fractograph of sensitized Type 304 stainless steel
tested In 0.2 ppm Og + 104 ppm Cl" solution at 200'C ...... 168 xlx 94. Tlme-to-fallure vs. thiosulfate ion concentration in air-saturated 104 ppm Cl" solution ...... 170 95. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOg* a t 50'C ...... 171 96. SEN fractograph of sensitized Type 304 stainless steel
tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOg^ a t lOO'C ...... 172 97. SEN fractograph of sensitized Type 304 stainless steel
tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOj" a t 150"C ...... 173 98. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOg" a t 200"C ...... 174 99. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing
120 ppm S0^“ a t 200"C ...... 176 100. Polarization curves of sensitized Type 304 stainless steel
tested in deaerated 104 ppm Cl" solution at various temperatures ...... 178
101. Polarization curves of sensitized Type 304 stainless steel
tested in deaerated 104 ppm Cl" + 40 ppm S~ solution at various temperatures ...... 180
XX 102. Polarization curves of sensitized Type 304 stainless steel tested In deaerated 104 ppm Cl' + 70 ppm SgO^" solution at various temperatures ...... 183
103. Current-time curves tested In deaerated 104 ppm Cl' solution a t 50'C ...... 186 104. Current-time cifrves tested In deaerated 104 ppm Cl' solution
a t lOO'C ...... 187 105. Current-time curves tested In deaerated 104 ppm Cl' solution a t 150"C ...... 188
106. Current-time curves tested In deaerated 104 ppm Cl' solution a t 200"C...... 189 107. Corrosion p otential, Ep^ and Ep^ values obtained In deaerated 104 ppm C l' solution ...... 191
108. Corrosion poten tial, Ep^ and Ep^ values obtained In
deaerated 104 ppm C l' + 40 ppm solution ...... 193
109. Corrosion potential, Ep^ and Ep^ values obtained In deaerated 104 ppm C l' + 70 ppm SgOg solution ...... 194 110. Effect of chloride Ion on the normalized t1me-to-fa11ure
In air saturated water...... 197 111. Potential-tem perature diagram showing region of IGSCC
In 104 ppm chloride solution ...... 199
112. Variations of normalized t1me-to-fa11ure as a function of temperature In 104 ppm chloride solution
with or without sulfur species ...... 201
xxl 113. Potential-tem perature diagram showing domain of IGSCC In 104 ppm chloride solution containing various sulfur species ...... 203
114. Variations of Ep^ and as a function of temperature In 104 ppm chloride solution with or without sulfur species ... 204 115. Corrosion rates cf SAE 1020 mild steel from te sts I, II
and III ...... 209
116. Normalized tlm e-to-fallure as a function of pHj at various temperatures ...... 212 117. Normalized tlm e-to-fallure In the chloride solution ...... 214 118. Temperature-composltlon diagram for the Iron-sulphur system ... 218
xxll CHAPTER I INTRODUCTION
Stainless steel. In spite of its excellent general corrosion resistance, has been limited in its wide application due to localized corrosion problems, such as stress corrosion cracking!SCO and pitting corrosion. Materials used in the nuclear industry are especially prone to see and pitting.
sec occurs in oxygenated pure water containing a small amount of impurities and pitting occurs when a small concentration of chloride ion is present together with oxygen. Sulfur compounds are very often found in water as impurities. Recently, it was suggested that sulfur compounds present in water, along with chloride anions, might be particularly detrimental.
Therefore, sulfur-bearing environments have recently become a subject of practical interest after several nuclear accidents. Since common steels or stainless steels contain sulfide inclusions which are easily dissolved in aqueous solutions, most aqueous solutions contain unavoidable sulfide ions in solution. Sulfur species can also come from the ion exchange resin used in the processing of the water. It is
speculated that the sulfide ion transforms into the metastable thiosulfate ion. The thiosulfate!used as sprays in the containment building in case of a nuclear accident) can accidentally leak into the
primary coolant system. Thiosulfate ions have a detrimental effect on
1 2 the see of steels and nickel base alloys even at the ppm level. Therefore, environments containing sulfur species have been studied recently. However, most of the studies were performed a t ambient temperatures. There has been no work dealing with the action of sulfur species In SCC and pitting corrosion of metals at temperatures higher than lOO'C. This work Is aimed at evaluating the effect of sulfide and thiosulfate Ions In alkaline or neutral solutions on the stress corrosion cracking and the pitting corrosion of sensitized Type 304 stainless steel at temperatures ranging from 50 to 200*C. This dissertation Is divided Into five main sections. First, the
Literature Survey Includes prior work on the effect of sulfur species on the stress corrosion cracking and pitting corrosion, and sulfur chemistry. The second section describes the type of material, apparatus and procedures used In these experiments. In the third section, the obtained results are presented. In the fourth section, the results are analyzed and mechanisms for stresss corrosion cracking and pitting In the aqueous solution containing sulfur species are proposed. Finally, the conclusions are given. CHAPTER II
LITERATURE SURVEY
2.1 Introduction Stainless steel is known to be vulnerable to localized corrosion,
i.e., stress corrosion, pitting etc.. Therefore, the factors affecting stress corrosion cracking and pitting have been extensively studied. In this chapter, those factors will be discussed in three sections. Firstly, the stress corrosion cracking susceptibility of sensitized Type 304 stainless steel is surveyed at room and at elevated temperatures.
Effects of chloride ion, dissolved oxygen and various sulfur species will be discussed. Information about corrosion in high concentration chloride environments will be excluded since it is not relevant to this study. Only corrosion in the solution containing low concentration of chloridedess than 1000 ppm) will be discussed. The second section will deal with the pitting corrosion of Type 304 stainless steel in chloride
solutions with and without sulfur species. The third section will discuss sulfur chemistry. The potential-pH diagram of the sulfur system
also will be included in this section.
2.2 Stress Corrosion Cracking of Sensitized Type 304 Stainless Steel
2.2.1 Stress Corrosion Cracking in Chloride Solution
It is well known that chloride is one of the most aggressive species to cause stress corrosion cracking. The effect of chloride ion 3 on stress corrosion cracking susceptibility was extensively studied at low temperature as well as at high temperature. However, most of the
work was confined to high chloride concentration studies such as in
boiling MgClg solutions and several reviews concerning this topic were publishedd, 2). The effects of concentration, critical potential and pH on the stress corrosion cracking will here be discussed. The effect
of internal factors, such as heat treatment, applied stress and strain
rate will be excluded. The susceptibility to stress corrosion cracking was assessed
usually by the critical cracking potential(Egg^) or the critical concentration of aggressive anions. Critical cracking potential was
defined as the lowest potential necessary to promote stress corrosion cracking of the material in a given environment. Critical concentration
means the lowest concentration of aggressive ions which cause stress
corrosion cracking and below which material is immune to stress corrosion cracking.
2.2.1.1 Effect of Chloride Ion Concentration Generally, the stress corrosion cracking susceptibility tends to increase as the chloride concentration increases. Thus, many studies determined the lowest concentration of chloride to cause stress
corrosion cracking. Scharfstein and BrindleyO) studied the effect of chloride ion in
sensitized 304 stainless steel at 74 - 93“C using stressed U-bend specimens. IGSCC was found in neutral water containing as low as 5 ppm
of chloride, whereas no cracking was found in the absence of chloride ion. The oxygen concentration of the solution was 2.8 and 0.9 ppm at 74 and 93"C, respectively. However, there was no difference in the susceptibility of 304 stainless steel to stress corrosion cracking with the change of temperature due to the counterbalancing effect of oxygen content in these ranges, whereas further increase of oxygen concentration to 16 ppm greatly increased the attack. It was found that the size of cracks decreased as the chloride concentration was decreased. Numerous p its were also found on a ll the specimens tested in é chloride solutions.
The critical concentration of chloride for stress corrosion cracking of quench-annealed 304 stainless steel was studied by Tong and
Swartz(4) in 35 solution containing various concentrations of chloride a t 104'C. As opposed to the sensitized ste e l, mainly TGSCC was observed. The c ritic a l concentration obtained by SSRT was between 1000
- 2000 ppm Cl", below which there was relatively little difference in ultimate tensile strength with the decrease of chloride concentration, as shown in Figure 1. The critical concentration for stress corrosion
cracking of annealed steel was significantly higher than that of sensitized steel. Although stress corrosion cracking susceptibility was closely related to the pitting initiation potential obtained from polarization curves, the formation of pits on the specimen was not
reported.
The effect of 01“ ion was also studied at high temperatures. Hishida and Nakada{5) investigated the effect of the concentration of
chloride ions on stress corrosion cracking of welded Type 304 stainless steel in air-saturated water at 286*0. In the slow strain rate te s ts , time-to-failure sharply decreased with increasing chloride 60 •Ultimate Tensile Strength A Reduction ot Area 70 cn d 60 2 ÿ 50 -5 40 1
30
0 1000 2000 3000 4000 5000 6000 7000 Chloride Concentration (ppm)
Figure 1. Effect of Cl concentration on reduction of area (ROA) and ultimate tensile strength(UTS) of Type 304 stainless steel in 35% (NH^lgSO^ solutions at 104*0 (4) concentration. Pits were observed if stress corrosion cracking occurred. Similar resu lts were reported by Andresen and Duquette(6), too. However, stress corrosion cracking of welded Type 304 stainless steel did not occur in deaerated water containing up to 1000 ppm Cl~ unless an anodic potential is applied(7).
Gordon(8) constructed a diagram showing the dependence between concentration of oxygen and concentration of chloride at 290'C. The domain of SCC was plotted on this by compiling the data of other inverstigators (Figure 2). From this figure, it is clear that oxygen content plays a more dominant role on the stress corrosion cracking of austenitic stainless steel than the concentration of chloride. The presence of some oxygen is a prerequisite for stress corrosion cracking to occur; 20,000 ppm of chloride did not produce cracking in an oxygen-free environment. However, the result of stress corrosion cracking studies are not consistent at and below 0.3 ppm Og level. This is probably the result of different materials and different techniques used for testin g .
2.2.1.2 Critical Potential .
Herbsleb(9) observed IGSCC of heavily sensitized 19 Cr - 10 Hi stainless steel in NaCl solutions ranging in concentration from 10 * to 0.5 M NaCl at their boiling temperature(around lOO'C) above a certain critical potential. The results of their constant load test are shown in Figure 3. As seen in the figure, the critical cracking potential increases with decreasing concentration of chloride ions. In 0.5 M NaCl solution, is -0.1 while it shifts to 0.4 V^, in 10"* M NaCl solution. At potentials more positive than these critical cracking V V
▼ ▼ vv
O- . 0 “ V
O ta o#i O" #0
▼ me
Figure 2. The effects of oxygen and chloride on the SCC of austenitic
stainless steels in high temperature water (8). Type 304 Stainless Steel Sensitized 50 hr. at 650®C T = IOO®C «/) O - I.7 5 0 o ,2 3 10^ O x: O-Intergranular crocks O'Transgranular cracks Q> Q )-Inter and trans- â 10^ gronulor cracks £ O No Cl concentration 0> £ K) • 0.5M o 0,1 M A lO'^M V lO'^M o io "4m i— Pitting potentials | jâ L liS n - 0.2 - 0.1 0.1 0.2 0.3 0 .4 0.5 0.6 0.7 Potential
Figure 3. Effect of potential on the time to failure of sensitized (50 hour at 650*0 Type 304 stainless steel in NaCl solutions of
different concentrations at 100*0 (9). to 10
potentials, the time-to-failure decreased with increasing potential and the specimens were preferentially attacked by pitting corrosion. The critical cracking potentials were almost the same as or slightly lower
than the pittin g potentials measured on the unstressed specimens as given on the abscissa. Considering that stressed specimens were pitted at more negative potentials than unstressed specimens, it seems that critical cracking potentials are the same as pit nucléation potentials.
They assumed that stress corrosion cracking is preceded by pitting in the chloride solutions. Ford and Silverman(lO) studied the effect of potential of
sensitized Type 304 stainless steel in 0.01 M NaCl around 100*C. The critical cracking potential was 0.1 Y^, which agrees reasonably well with Herbsleb's resultsO). The critical cracking potential of sensitized Type 304 stainless steel at high temperature was extensively investigated(6, 7, 11, 12, 13).
Andresen and Duquette(6, 7) studied the c ritic a l corrosion A potential for cracking in aerated 100 ppm Cl" solution at 290"C by controlling the oxygen content. They found that the critical cracking potential in aerated solutions is identical to that in deaerated chloride solution measured potentiostatically. It indicates that the primary effect of oxygen in chloride water is to shift the corrosion potential into the susceptible region for cracking. The critical potential in sensitized Type 304 stainless steel was between -300 to
-350 mV^, which corresponds to the corrosion potential in pure water containing 20 to 40 ppb Og. They found the same c ritic a l cracking 11 potential in chloride solutions and in pure water. The critical cracking potential of sensitized Type 304 stainless steel was extensively studied by Cragnolino et a l.(13) and Lin et a l.(12). They performed SSRT in 0.01 M NaCl solution over a wide temperatures ranging from 100 to 250*0. As shown in Figure 4, the critical cracking potential decreased progressively with increasing temperature. At 100*0, IGSOO initiated on pits at potentials above
0.075 V^. At the highest potential tested, 0.3 V|^. the specimen failed not by intergranular corrosion but by pitting. At temperatures above 150*0, a mixed mode of TGSOO and IGSOO was observed a t potentials equal to or higher than 0.0 V^. In the potential region between c ritic a l cracking potential and pitting potential, only IGSOO was observed. Pure
IGSOO occurs over a wide potential range at high temperatures, and the susceptible potential range expands with increasing temperature. The critical cracking potential at 250*0 is -0.3 Vj^, above which time-to-failure decreases. Based on these SSRT result, domains of various fracture morphology were determined as a function of potential and temperature(Figure 5).
Similar resu lts were obtained by Duquette and Poznansky(14) at 290*0. A sensitized Type 304 stainless steel cracked in S3R test at the potentials above -0.3 Vy in 100 ppm 01’ + 100 ppm S0^~ solution. In addition, it was reported that the critical cracking potential of sensitized stainless steel was almost independent of the chloride concentration when the chloride was added from 1.8 ppm to 100 ppm to the
100 ppm sulfate solution(Figure 6). However, the maximum stress at which the specimen failed decreased above the c ritic a l cracking 12
250*C 200"C 100
d 80
60
40
N 20
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 POTENTIAL (Vh )
Figure 4. Normalized elongation to failu re (^fscc^^T.Ductile* potential for sensitized Type 304 stainless steel in 0.01 M
NaCl solution at temperatures ranging from 250°C to 100“G
(12). 13
0 .3 deep-s pitting— »stiallow
0.2 o pitting | TGSCC
IGSCC IGSCC
z -0.1
IGSCC Ü. -0 .2
-0 .3
DUCTILE -0 .4
100 150 200 2 5 0 T(°C)
Figure 5. Potential-temperature diagram showing regions of different
modes of failu re. (X = IGSCC, o = IGSCC + pitting, A =
p ittin g , D = IGSCC + TGSCC, *= TGSCC,4# = ductile) (12). 14
400
i 300 S % w K W 200 ? EFFECT OF CHLORIDE CONCENTRATION X4 ON SCC OF SENSITIZED 304 S/S IN 100ppm so ; AOUEOUS SOLUTION 100 - ® IGppmCf' • lOOppmCI'
-800 -600 -400 - 200 0 200 400 600 800 POTENTIAL (mv) ( VS. OOIN Aq/AgCI, 290*C)
Figure 6. The effect of chloride concentration on stress corrosion
cracking of sensitized alloy in 100 ppm sulfate soluion (14). 15
potential as the chloride concentration Increased. Based on these findings. It was suggested that chloride contributes to the process of crack propagation by participating In anodic reactions at the grain boundaries. 2.2.1.3 Effect of pH
SchafsteInO) performed U-bend te sts on sensitized Type 304
stainless steel at 85"C In 10 ppm Cl" solution. Stress corrosion cracking was not found a t a pH of 8.8, while 70 - 1002 of the specimens failed In a neutral solution. Griess and Creek(15) reported that an Increase In pH from 4.5 to 9.3 by addition of NaOH remarkably decreased the Intergranular stress corrosion cracking susceptibility. The test was performed In air saturated 0.28 M boric ac1d(1n1t1al pH = 4.5) containing chloride Ion.
Truman(16) Investigated the pH effect on the stress corrosion cracking of mill-annealed 304 stainless steel In NaCl solution by using a constant strain test. The pH of the solution was adjusted by either HCl or NaOH. It was found that the stress corrosion cracking susceptibility Increased with decreasing pH. In 500 ppm Cl" at 80'C and In 100 ppm Cl" a t lOO'C, specimen cracked only a t pH 2, while no cracking was observed a t pHs 7 and 12. Also, In 1000 ppm Cl" around lOO’C, the tlme-to-fallure decreased as the pH decreased. In 500 ppm Cl" at the boiling temperature, the most aggressive effect was observed In the solution with pH 2, followed by pH 7. The specimen was not cracked In the same solution at pH 12 In 13500 hours. From these results, It Is clear that transgranular crack propagation Is accelerated by lowering the pH. 16
Neumann and Gr1ess(17) reported that the change of pH did not Influence the transgranular stress corrosion cracking susceptibility of Type 347 stainless steel within the pH range of 2.8 - 10.5(measured at room temperature). In their experiment, the time required to develop at le a st one crack at 300*C was between 100 and 200 hours, independent of pH when U-bend specimens were exposed to the solution containing 1200 ppm Og and 100 ppm Cl” . AndresendS) found that pH affects the stress corrosion cracking of sensitized(500*0, 24 hours) Type 304 stain less steel at 288*0; The decrease of pH accelerates the crack initiation. As shown in Figure 7, the addition of NagSO^ and NaOH, which increased the pH of the solution, impedes the crack initiation relative to pure water. The effect of chloride ion can be neglected since stress corrosion cracking susceptibility was independent of chloride ion concentration at temperatures higher than 250*0. Also, i t was found th at crack initiation was not dependent on solution conductivity in neutral solutions whereas crack propagation rates increased with increasing conductivity. Although the author suggested that crack propagation rate linearly increased with conductivity, close examination of the results revealed that the average crack propagation rates increased with the increase of conductivity up to 1 pS/cm for any type of impurity at temperature above 250*C, but further increase of conductivity has no considerable effect on crack propagation rate. It was concluded that the stress corrosion cracking susceptibility depends predominantly on the pH of the solution at high temperatures. 17
A37 1------lO/tS/cm IMPURITY NaOH 304SS WELD+500*C/24h CERT 3.3xlO-%S * A30 N02SO4 6 200ppb OXYGEN W AI3 52 PURE WATERj A40 z 5 NoCf,A39 • NG2CO3 'NGHSO4 A27 A 20 _ 55/is/cm H2SO4 , S 3 A22 7 c -HCf+ ce 53xlO"yS o Ali A26, H2 SO4 0 6 8 PH288°C Figure 7. The effect of pH on the strain to crack initiation at 288“C for various types of impurities at a 10 pS/cm concentration (18). 18 2.2.2 Effect of Dissolved Oxygen Content In Water on the Stress Corrosion Cracking Susceptibility The effect of dissolved oxygen content on the stress corrosion cracking susceptibility was extensively studied mainly at high temperatures around 280*C. It Is well known that stress corrosion cracking of sensitized Type 304 stainless steel occurs only In the presence of dissolved oxygen In high temperature water(19 - 22). Recently, two reviews have been publ1shed(23, 24) on the SCC of sensitized 304 stainless steel in a wide temperature range. Generally, two different experimental methods. I.e., constant load and slow strain rate tests, were employed to study the effect of oxygen In high temperature pure water or water containing Impurities. Berry, et a l.(19) conducted constant load tests In pure water containing 0.2 to 100 ppm O 2 at 288*C and their results are sunmarlzed In Figure 8. As shown In Figure 8, an Increase of dissolved oxygen In high temperature water reduces the tlme-to-fallure of sensitized Type 304 stainless steel. More work was performed by Clarke, et a l.(20) at the same temperature. Their results are shown In Figure 9. The tlme-to-fallure decreased with Increasing oxygen concentration at oxygen concentration ranging from 0.2 ppm to 600 ppm except at medium oxygen level. In the medium range, tlme-to-fallure was almost constant regardless of the oxygen content with the stress of 48 ksl. At a lower stress of 35 ksl, maximum susceptibility was observed In the solution containing 8 ppm Og, which Is equivalent to air-saturated pure water. The effect of oxygen content was also Investigated In slow strain rate testsdO, 21). Welch(21) compared the crack propagation rate as a 19 3Sm LOAD BWR WATER 288 C TYPE TYPE 304 316 O SENSITIZED 7 HR AT 621 C ■ SENSITIZED 24 HR AT 621 C I NO FAILURE z oUI 1 .0 g 438 DAYS ^ NORMAL OPERATING ' RANGE «0 * * 0 100 200 300 TIME TO CRACK. DAYS Figure 8. Effect of dissolved oxygen on cracking times (19), 20 Median Foilwr# Time, t^,(h) 3SKti 4 8 k«i OLa. 1.0 0.1 J.4 •o'* Reciprocal Folluro Time, l/f,, (h Figure 9. Effect of oxygen concentration on the IGSCC of sensitized Type 304 stain less steel (from one heat) in 288®C water at two applied stress levels (20). 21 function of dissolved oxygen concentration at various temperatures (Figure 10). At temperatures above 250"C, IGSCC was observed and the crack propagation rate increased as the Og content increased from 0.1 to 32 ppm. However, below 0.1 ppm Og* TGSCC was observed and the crack propagation rate was independent of the oxygen concentration. At 200"C, IGSCC was observed in pure water with 0.8 to 32 ppm Og, while TGSCC was present below 0.8 ppm Og to 0.01 ppm Og. In addition, the transition concentration of oxygen between TGSCC and IGSCC tends to decrease with increasing temperature. It is noteworthy to mention that TGSCC was observed at 150'C with the oxygen concentration as low as 0.01 ppm. Such transgranular cracking above 150"C has been reported only in slow strain rate tests(19, 21). The effect of oxygen content in pure water was studied over the wide range of temperature by Ford, e t a i . (10). The compiled data are shown in Figure 11. IGSCC always occurred at temperature above 150"C if the dissolved oxygen content is higher than 0.1 ppm. At 100*C, the oxygen concentrations which showed IGSCC are from 0.8 to 7 ppm. p Moreover, IGSCC was observed at temperatures as low as 50*C in slow strain rate tests provided that the oxygen content was in the range of 1 to 3 ppm. The critical temperature of stress corrosion cracking in air saturated pure water is approximately 120"C, which agrees well with White e t a i . (25) and Staehle(26). Systematic studies of the effect of dissolved oxygen concentration on the corrosion potential was done by Lee(27). The result are shown in Figure 12. The results show a definite decrease of corrosion potential with decreasing oxygen content. At 288"C, the corrosion potential -7 10 Composite Plot - Estimated Crock Growth Rote vs Dissolved Oxygen Content 304 Stainless Steel, € =10" /Second 290°C. Sen 304 10" - o o ■o 200% , Sen 304 Mode 250*C, Sen 304 Crocking i ! 200% , Sen 304 ^29 0% , Sen 304 250»C, QA 304 150%, Sen 304 ld'° 10^ 10" Dissolved Oxygen, ppb Figure 10. Estimated crack growth rate vs. dissolved oxygen content,6 = ro ro id "®/sec (21). 23 T TT Type 304 Stoinless Steel Sensitized 24hr of 650 “C High purify wafer Ê = 2.1 «0.06^ 8 ,7 Z ^ .7 X _ - 0.17 IGSCC 0.15 1,4 # < 0 .0 6 3.0 X 3.7^ 1.5 4.0 1.76 <0.06 • XX X X , 2.2 10 CRACKING AT 2.49 — V<0.2*l0'7cm sec X 2.3 X I.4X - 1 Jd 50 100 150 20 0 250 300 TEMPERATURE K Figure 11. Variation of IGSCC susceptibility'6f Sensitized Type 304 stainless steel in pure water for various oxygen/temperature combinations. (•) denotes no cracking observed; (X) denotes IGSCC observed at an average crack propagation rate giben by -7 subscript number in cm/s x 10 . hatched area denotes uncertainty of exact position of boundary line (10). 600 "I I I I" ! I T LEGEND Type 304 Stainless Steel 400 o I00=c ▲ I50°C A 2 0 0*0 ------g O I 200 • 250*0 > □ 288*0 E o 0 c a> o 100°C r 200=0 g -2 0 0 tn OL. S -400 i/ . - q - " 288°C -or XT -600 -800 I 10 100 1,000 10,000 Dissolved Oxygen Concentrotion, ppb N5 Figure 12. Corrosion potentials of Type 304 vs. dissolved oxygen (27). 25 gradually changed from 0 to -600 mV^ as the oxygen content decreased from 8 to 0.01 ppm. At 100*0, the corrosion potential ranged from 200 to -400 mV^. However, the corrosion potential decreased rapidly at certain oxygen concentration level. The rapid change of corrosion potential occurred below 10 ppb Og a t 100*0, while at 288*0, i t occurred at much higher oxygen content, i.e ., 100 to 1000 ppb Og. This indicates th a t the rapid change region of oxygen concentration decreased as the temperature decreased. Indig and Mcllree(22) also studied the relationship between corrosion potential of Type 304 stainless steel and oxygen content at 274*0, which are shown in Figure 13. Their result shows similar relationship between oxygen contents and corrosion potentials as in Figure 12, The authors emphasized that the redox potential on platinum electrode showed similar sigmoidal behavior as in the case of the corrosion potential of Type 304 stainless steel. This indicates that the corrosion potential is governed by the reduction of hydrogen ions a t very low oxygen contents, while it is governed by the reduction of * oxygen a t high oxygen content(24). Based on this hypothesis, IGSCC was studied under applied potential instead of changing dissolved oxygen content. Assuming that sulfate ion does not affect the stress corrosion cracking of steels, sodium sulfate was used as auxiliary electrolyte to increase the conductivity of the solution. Many researchers studied the SCC of sensitized Type 304 stainless steel under applied potentials(10, 22, 28). Cowan and Kaznoff(28) performed constant load experiment under controlled potential(0.32 V^) 26 -200 Ui J " -400 0 to 10,000 ppb 0^ Rot urn from I0,000ppb Oy Run L A .A RunM -«00 O- -«00 10 to* W Og ppb (Input-aa^vOutput-HttA) Figure 13. The effect of dissolved oxygen concentration on the corrosion potential of Type 304 stainless steel in high purity water at 274'C (22). 27 In dilute sodium sulfate solution at 288"C and found similar IGSCC behavior as In the pure water containing dissolved oxygen which resulted In the same potential as In the case of the applied experiment. Indig and Mcllree(22) Investigated the critical cracking potential In 0.01 N NagSO^ at 274*C by SSRT. The critical potential of sensitized 304 stainless steel was -400 mVy, corresponding to 50 ppb of oxygen. That value agrees reasonably well with Ford et al(29), who found IGSCC at 100 ppb of oxygen concentration at the same temperature (Figure 11). Cragnolino et a l.(30, 31) also studied the critical cracking potential at 250'C In the same solution. Their results are shown in Figure 14. The obtained cracking potential In sensitized 304 stainless steel was -300 mV^, which showed good agreement with Indig's result(22). Similar work was done by Fujiwara e t a l . (32) a t 285'C In deaerated 0.01 N NagSOg using the constant load method. The critical cracking potential obtained was -150 mV with respect to SCE(saturated calomel electrode), corresponding to the oxygen content of 1 ppm which Is 150 mV higher than that of Cragnolino et al.(31). It Is obvious that the * difference between these authors resu lts from the different experimental techniques. It Is known that slow strain rate test causes more aggressive conditions than the constant load or deflection tests(33, 34). In other words, the slow strain rate te s t can Induce IGSCC In a less aggressive environment In comparison with the constant load test. Hence, higher critical oxygen concentration or higher .rltlcal potential Is necessary to obtain stress corrosion cracking In constant load tests. Ford and Sllvermann(lO) Investigated the crack propagation rates In deaerated 0.01 M NagSO^ at lOO'C under potentlostatic control. Their 28 10' I I I ...... I I Sen Type 304 Stainless Steel 0 0 1 M N O g S O ^ T=250**C a 10 00 *E < 0 75 Q>c e,o" 0503. I3 o 0 2 5 Deareoted 001 M Na,SO, At Applied Potenfiol _ Air Saturated 10 - O.IIVI NOjSO^ At Open Circuit O Dearea ted H%0 - Ec & • Air Saturated HjO -08 -06 -04 -02 0 0 2 0 4 Potenfiol, Figure 14. Ratio of time to failure in 0.01 M NagSO^ and high purity water at 250°C as a function of potential to that in argon and anodic polarization curve for sensitized Type 304 at 250"C (31). 29 results are shown in Figure 15, which includes those obtained in oxygenated water at same temperature. As shown in Figure 15, in both sulfate solution and pure water, IGSCC occurred in the same range of potentials, i.e ., 150 to 220 mV^. Herbsleb(9) observed IGSCC in 0.01 M NagSO^ at the same temperature but the critical cracking potential was more anodic than that of Ford and Silverman by 100 mV. 2.2.3 Effect of Sulfur Species on the stress corrosion cracking Susceptibility Not only dissolved oxygen and chloride, but sulfur species can cause or influence stress corrosion cracking of sensitized stainless steel. Hence, in this section, the effects of various sulfur species!i.e., polthionates, thiosulfate, sulfite, sulfide and sulfate ions) and their concentrations on the stress corrosion cracking susceptibility at room temperature and at elevated temperatures will be discussed. Then, the effect of potential and pH on stress corrosion cracking in the presence of sulfur species will be discussed. 2.2.3.1 Solution A Polythionate: Since the 1950's, sulphur species were recognized as highly aggressive agents in producing stress corrosion cracking of sensitized stainless steel at room temperature. Many researchers performed SCC experiments on different alloys in the solution containing sulfur species at room temperature. Dravnieks and Samans(35), reported that the presence of polythionic acid causes IGSCC of sensitized Type 304 stainless steel at room temperature. The polythionic acid solution!so called Wackenroder solution) was prepared by passing sulfur dioxide gas into distilled 30 icr' O0IMN«2S04.pH2sS8 4300 98'C . . >‘ 4200 4100 •100 10** I0"‘ INTERCRANULAR CRACK PROPAGATION RATE, c m t * Figure 15. Variation of average crack propagation rate of sensitized 304 stainless steel in 0.01 M NagSO^ at 97 + 2 C and in water/oxygen at 100*C. Note two different strain rates used-but similar potential range in both soluitons, where high susceptibility is observed (10). 31 water and then bubbling hydrogen sulfide Into i t . According to ASTM G-35-73, the pH of this solution is about 1.0 to 1.5. Figure 16 shows time to failure of 304 stainless steel as a function of tetrathionic acid concentration in diluted polythionic acid solution. Even at low concentration, less than 0.001 M HgS^OgfpH 5), sensitized Type 304 stainless steel failed in about 30 minutes under constant load condition. The aggressive effect of polythionic acid was confirmed by Piehl(36), Brophy(37) and Ahmad e t a l . (38). Piehl(36) observed IGSCC of sensitized Type 304 stainless steel in polythionic acid at 95*C where U-bend specimens were broken instantly. Brophy(37) also studied stress corrosion cracking of various sensitized and annealed austenitic stainless steel in Wackenroder solution by using U-bend test. Only sensitized Type 304 stainless steel was cracked in one day in polythionic acid(pH 1.5), whereas annealed 304 stainless steel and sensitized or annealed 304 L, 321 and 347 stainless steel did not show any cracking. This indicates that IGSCC was due to anodic dissolution of the chromium depleted areas along grain boundaries promoted by the presence of sulfur species or chemisorbed sulfur at the crack tip. The presence of oxygen in polythionic acid reduced the time-to-failure from two days to one day. Also, in the 1 % tetrathionate sclution(pH 5.3), IGSCC occurred in 18 hours. Recently, Ahmad et a l.(38) studied the effect of polythionic acid concentration on stress corrosion cracking of sensitized Type 304 stainless steel. Figure 17 shows that the stress corrosion life decreased rapidly with increasing the concentration of polythionic acid 32 pH = 5 Type 3 0 4 Stainless Steel 30 Sensitized, 4 hours at 650®C Polythionic Acid (T=7 ksi = 4 8 MPa <0 *—o o E I- 0 0.001 0.01 0.1 0 .5 Approximate Molarity of Tetrathionic Acid Figure 16. Effect of tetrathionic acid concentralon on the time to failure of sensitized (4 hr at 650*0 Type 304 stainless steel at room temperature (35). 33 IfO 140 > fOO a O9 i 40 4 0 20 CO #00 HO ICO 220 MO to o 040 cco "C ConctntroHon of polythionic ocid m tOAont «oluilon (motMflltro) kk T Figure 17. Effect of polythionic acid concentration on stress corrosion life of sensitized 304 austenitic stainless steel (38). 34 from 0.03 to 0.18 M/liter and a further Increase up to 0.38 M/liter has a small effect on the time to failure. They obtained a much longer time-to-fallure than Dravnieks and Samans(35) since a less severe experimental technique,U-bend, was empolyed. Brophy(37) studied the stress corrosion cracking of sensitized Type 304 stainless steel In tetrathionate solution. The time to cracking at 25*C In 1 % tetrathionate solution was 18 days while In 0.5 % tetrathionate solution spedment failed after 16 days at 80'C. However, the effect of temperature could not be quantitatively assessed since a different concentration was employed In two tests. It was Interesting to note that no cracking was found In mixed 0.5 % tetrathionate + 25 % NH^Cl solut1on(pH 3.6) at 80"C, while cracking was observed In 0.5 % tetrath1onate(pH 5.0) as well as 25 * NH^CKpH 2.6) solutions at that temperature. Wackenroder solut1on(S^0g^") Is an extremely complex mixture of sulfur-oxygen-hydrogen compounds, mainly trithlonic, tetrathionic, pentathlonic and hexathlonic acids In varying concentrations. On preparing Wackenroder solution tetrathionic acid Is first formed; the other thionic acids are then formed by side react1ons(39). Since sulfur compounds show such a complex chemical behavior, the question raised was one regarding the Identity of the species responsible for cracking In Wackenroder solution. P1ehl(36) suggested sulfurous acid as an aggressive agent In polythionate solution after detecting SOj^" polarographlcally and demonstrating the occurrence of stress corrosion cracking In 7.7 * MgSO^ at 94*C. 35 BrophyO?) compared the time-to-faflure of U-bend in polythionic acid with that in sulfurous acid and demonstrated the shorter time-to-failure in polythionic acid by two orders of magnitude. It was concluded that the active species responsible for cracking in polythionic acid is not sulfurous acid but tetrathionic acid, even though sulfur species other than polythionate can induce IGSCC. This was later confirmed by Ahmad et al.(38). The analysis of polythionic acid by gravimetric method, before and after the U-bend test, showed significant changes only in the concentration of tetrathionic acid. The concentration change in different sulfur species with time are presented in Table 1. In the separate test, IGSCC was found only in the solution containing tetrathionate among the various synthetic thionates. Thiosulfate: The highly aggressive effect of thiosulfate ions on the stress corrosion cracking susceptibility of sensitized Type 304 stainless steel was reported by Isaac et a l.(40). The minimum concentration of thiosulfate which causes cracking was about 0.1 ppm(6 x 10 ^ M NagSgOg) in air saturated solution by using SSRT with a strain rate of lO"® S"^. With 0.18 M boric acid added, higher concentrationsd ppm) of thiosulfate are required to enhance stress corrosion cracking. The increase of the concentration of thiosulfate ion also increased the time-to-failure for stress corrosion cracking. Dhawale(41) showed the highly aggressive effect of thiosulfate ions in an air-saturated 13,000 ppm(0.21 M) boric acid solution with pH equal to 4.5 by using SSRT with a strain rate of 10"® S“^. The critical concentration for thiosulfate was as low as 10"® M NagSgOg, while that for tetrathionate was 5 x lO"® M (Figure 18). Above these critical Table 1 Chemical Analysis of Wackenroder solution Containing 0.38 m oles/liter Polythionic Acid After Oxygen Bubbling for Different Periods During Experiment (38). . Constituents^!) SI Duration Olthlonic Trithlonic Tetrathionic Pentathlonic Hexathlonic Sul f urous Sul furlc Ho. (minutes) acid acid acid acid acid acid acid 1. Before 3.6 X 10"^ 2.2 X 10"* 2.1 X 10"! 7.0 X 10"* 3.7 X 10"* 2.5 1.7 the te s t 2. 60 3.6 X 10"^ 2.0 X 10"* 2.3 X 10"! 7.1 X 10"* 3.4 X 10"* 2.4 1.8 3. 120 3.4 X 10"* 1.2 X 10"* 2.6 X 10"! 7 .2 X 10"* 3.0 X 10"* 2.0 1.9 4. 180 3.1 X 10"^ 0.4 X 10"* 3.2 X 10"! 7.8 X 10"* 2.6 X 10"* 1.5 1.9 5. 210 2.6 X 10"* 0.1 X 10"* 1.1 X 10"! 7.6 X 10"* 2.0 X 10"* 1.3 2.0 6. 300 8.4 X 10"3 1.7 X 10"* 4.5 X 10"* 5.5 X 10"* 1.5 X 10"* 1.2 2.2 ( 1) The concentration of thionic acids is given In moles/liter while the concentration of sulfurous and sulfuric acids is given in a percentage. wO' 37 TYPE304 Stainless Steel Sensitized 2hrs. at 650 V idfiOOppm HjBOg 100 i 2 5 80 11. c o 60 o 0 » c o UJ 40 20 Log. Cone. S ,g Atom / 1 Solution Figure 18. Elongation to failure vs. concentration of sulfur for sensitized Type 304 stainless steel in air saturated boric acid solution containing Na^S^Og at room temperature (41). 38 concentrations, the elongation to failure decreases linearly with logarithm of sulfur concentration. As is seen in the figure, above the critical concentration, the stress corrosion cracking susceptibility is independent of the identity of the anion. Moreover, Dhawale(41) found that quench-annealed Type 304 stainless steel as well as heat treated Type 304 L stainless steel are not susceptible to IGSCC in thiosulfate solutions, as in polythionate solutions. Horowitz(42) studied the behavior of 9* Ni - 7* Cr steel which simulates the grain boundary of sensitized Type 304 stainless steel at room temperature in air-saturated acetate buffer solution(0.9 M CHgCOONa + 0.1 M CHgCOOH) with and without thiosulfate ion. He found that anodic peak current on voltammetric scans was promoted by the addition of 0.001 M NagSgOg in the acetate buffer solution. In addition, the increase in the anodic peak was demonstrated after simply dipping the steel in the thiosulfate solution. Based on these findings, it was suggested that the adsorption of thiosulfate promotes anodic dissolution, leading to IGSCC. The effect of added anions in thiosulfate solutions on the stress corrosion cracking process was studied by many authors. Isaacs et a l .(40) reported that the addition of 0.18 M H^BOj to 0.1 ppm NagSgOg solution inhibits, to some degree, the stress corrosion cracking at room temperature. Addition of 6 x 10“^ M NagSO^ in 6 x lO"* M NagSgOg solution decreases the crack propagation rate greatly(43). The ratio of [SO^ ]/CSgOg ] = 20 was required for complete inhibition of stress corrosion cracking. It was suggested that accumulation of large amount of sulfate within the cracks caused passivation of the crack tip by 39 preventing electromlgration of thiosulfate ions. Horowitz(42) also found, after investigating anodic dissolution current and time-to-elongation of SSRT, that the addition of 0.05 M NOg", PO^Z", cOgZ- to deaerated 0.001 M NagSgOg inhibits IGSCC of 7 % Cr - 0.5 % Mo steel, whereas NOg", Cl", SO^^" do not have a considerable effect on stress corrosion cracking. This result disagrees with that of Mewman(43). The absence of the inhibiting effect of sulfate ions in Horowitz's result can be caused by the low ratio of [SOg ]/[SgOg "]. Also, sodium sulfide was reported(42) to act as an inhibitor for the action of thiosulfate, implying that thiosulfate is not acting simply as a source of an iron sulfide film. In another study, Newman et a l.(44) reported that the addition of lithium hydroxide inhibits the stress corrosion cracking of sensitized Inconel 600 in 1.3 % boric acid solution containing lO"® M thiosulfate ion with pH 5 - 8, where HB^Oy" is the predominant ion. The inhibition mechanism is explained in Figure 19. In the presence of an excess of borate ion, the enriched thiosulfate solution is dissipated by diffusion and the concentration of thiosulfate in the crack is low. As a result, the NiO passive film confers protection against the aggressive environment. Although it is expected that at elevated temperature the cracking of sensitized Type 304 stainless steel will be accelerated, there are few data of the effect of sulfur species at temperaturea higher than room temperature. Recently, Newman et a l.(44) studied stress corrosion cracking of sensitized(621"C for 18 hours) Inconel 600 at temperatures ranging from 40 N iO X ' iog(S,oi') lo g (X * ) log(X *) log(S.O} I St«ody>«lott see Afttr X agdilien Crock orrofi kut k*fer« crock orroct Figure 19. Schematic diagram showing anticipated effect of the addition of another anion on the rate of thiosulfate SCC (44). 41 22 to 95*C. The mean crack velocity in air-saturated 1.3 * H^BOg + 0.7 ppm sulfur(as a sodium thiosulfate) was presented as a function of temperature(Figure 20). Crack propagating velocity was seen to increase with increasing temperature. The levelling out of the velocity at temperatures above 60*0 was due to decreasing oxygen content in the solution. However, the data in thiosulfate at temperature above 100*0 is not available. Sulfide; Heller and Prescott(45) studied the effect of sulfide ion at ambient temperature on the stress corrosion cracking of sensitized Type 304 stainless steel in the constant load test. They found severe pitting and extensive intergranular cracking in water saturated with HgS and air(pH 4.4) without chloride ion. Cracks and severe pitting were also found in annealed condition but the predominant fracture morphology was transgranular cracking. However, the cracks in annealed steel were six times more shallow than those in sensitized condition. Similar work done by Herbsleb and Poepperling(46) on annealed 22 Or - 6 Hi -3 Mo duplex stainless(AF 22) steel, lead to contradictory results. Using the bend test in H2$(3.2 g/liter - 1.8 g/liter) saturated 0.9 M - 4.3 M NaOl solution with pH 3, they found transgranular stress corrosion cracking with severe pitting. Stress corrosion cracking was not detected in an aqueous solution containing 2.5 g/liter HgS in the absence of chloride at the same temperature. This indicates that the chloride ion was essential to induce stress corrosion cracking in an HgS environment. According to (46), chloride ion in itially induces formation of pits. Asphahani(47) performed SCC tests on annealed and cold worked Type 42 if» E c 8 .0 kcol mol > H 0.8 O 3 ÜJ > 0.6 O < (r ü z 0.4 < w z o o 0.2 2.6 2.8 3.0 3.2 3.4 1000 (K-l) Figure 20. Dependence of crack velocity in CER tests on temperature for sensitized Inconel 600 in air-saturated 1.3% ^^BOg + 0.7 ppm sulfur as sodium thiosulfate (44). 43 316 stainless steel using C-rIng specimens In aqueous environments containing HgS, COg and Cl" Ion. He found that the specimens were cracked In 5 % NaCl solution acidified by 0.1 * CHgCOOH saturated with HgS at 25*0. Cracking occurred up to 177*0. However, a further Increase of temperature to 199*0 Inhibited stress corrosion cracking. Similar behavior was reported for AF-22, duplex stainless steel and 20 Type high nickel stainless steelOS N1 - 20 Or - 4 Mo - 1.5 Ou stainless steel). The time-to-failure Increased by a factor of four In the solution containing 18 * NaOl + 9 * OaOlg saturated with OOg + HgS mixed gas when the temperature Increased from 121*0 to 177*0. The reason for this temperature dependence of SCO was not explained by the author. Recently, another evidence of the Inhibiting effect of sulfide Ions was reported by Cragnolino and Sm1alowska(48). The addition of 50 ppm S“ to 0.01 M NagSO^ solution Increased elongat1on-to-failure of 304 stainless steel by a factor of two at 250*0. In another test, a similar Inhibiting effect was found when 200 ppm of sulfide was added to 0.01 M NagSO^ and the pH was adjusted to 9.0. The authors suggested that protective films of Iron sulfide Increased the reslstnce of steel to stress corrosion cracking. The pH of the solution containing NagS was adjusted to the same pH(6.7) as the sodium sulfate solution by the addition of HgSO^ at room temperature. However, the pH at the temperature of Interest should be corrected on the basis of the dissociation of water and hydrolysis of sodium sulfate. Since the second dissociation constant of sulfuric acid Is very low at high temperature, pH In the solution containing NagS + HgSO^ Is more alkaline at high temperatures than In the sodium sulfate solution, even though 44 they have the same value at room temperature. Considering the pH change at high temperature, i t is evident that the inhibiting effect did not result from iron sulfide but resulted from a high pH. 2.2.3.2 Effect of Potential The effect of potential on stress corrosion cracking of sensitized Type 304 stainless steel in Wackenroder solution was studied by Matsushima(49). As shown in Figure 21, IGSCC was observed in the potential range of -160 to 440 mV^(-400 to 200 mV vs. SCE). It should be emphasized that the highest susceptibility was obtained near the corrosion potential, around 140 mV^t-lOO mV vs. SCE). Later, almost the same results were reported by Kowaka and Kudo(50). The effect of potential was also investigated by Isaacs et a l.(40) in thiosufate solution. It was found that the critical cracking potential in 6 x 10 * M(100 ppm) S^Og^" + 0.18 M HgBOg was roughly -0.5 V vs. SCE, above which the rate of cracking increased with increasing potential up to 0.5 V vs. SCE(0.74 Y^). Dhawale(41) found the potential region inducing IGSCC in deaerated 0.21 M HgBOg containing 0.01 M NagSgOgCpH 4.5), was from -0.2 to 0.2 V^fFigure 22). The potential range of maximum susceptibility was about 0.1 - 0.2 V^, which corresponds to the corrosion potential in air-saturated solution. Despite the different thiosulfate concentration, the critical cracking potential agrees well with Isaacs et al's result. It should be emphasized that different from the results of Isaacs et a l.(40), only ductile failure was obtained at potentials above 0.3 V^. According to (41), at potentials above 0.3 V^, thiosulfate was oxidized to other innocuous sulfur oxyanions. 45 - Type 3 0 4 Stainless Steel _ Sensitized, 10 hours at 650°C Polythionic Acid 0.6 0.4 0.2 corrosion ipotentioi - 0.2 -0.4 5 10 Time to Failure, hr Figure 21. Effect of potential on the time to failure of sensitized (10 hr at 650*0 Type 304 stainless steel In polythionic acid solution at room temperature (49). 46 TYPE304Stainless Steel Sensitized 2hrs at650 °C OOtMNagSgOs, I3,000ppm H3 BO3 Room Temperature è=lxlO’ŸS n= 80 -200 0 200 400 Potential, mV„ Figure 22. Elongation to failure vs potential curves for sensitized Type 304 stainless steel in deaerated boric acid soluion containing 0.01 M Na^SgO^ at room temperature (41). 47 presumably, HSO^” Ions. The redox potential of SgO^^'/HSOg^” is roughly 230 mVy, assuming equiconcentration of both species(51). After IGSCC of 304 stainless steel, a yellow deposit of sulfur along cracks was detected first by Matsushima(49) in polythionic acid, and by Zucchi et al.(52) in tetrathionate solution and later by Dhawale(41) in thiosulfate solution. But, the presence of sulfur was limited to the area close to the mouth of cracks. Cragnolino(25) explained the formation of sulfur by the following reaction: S^OgZ" + 12 H* + 10 e“ ----- 4 S + 6 HgO [1] Taking 0.1 M tetrathionate and pH = 1, the estimated redox potential for this reaction is 339 mV^. The estimation of redox potential, which was only 100 mV higher than the corrosion potential measured by forementioned authors, indicates that the formation of elemental sulfur is possible. In the potential-pH diagram generated by superimposing the Fe-HgO and the metastable S-HgO diagram, the cracking ranges in three experiments are compared in Figure 23: A: Matsushima(49) in polythionic acid B: Dhawale et a l.(41, 53) in borated thiosulfate solution(pH 3 of crack tip was assumed) C: Newman et a l.(43) in 0.5 M thiosulfate solution(pH 3 was assumed at crack tip). The correlation with the stability domain for Fe^'*’ + S indicates that the formation of atomic sulfur promoted IGSCC, presumably by adsorption. Ahmad et al.(54) reported that the potential range inducing stress corrosion cracking in 304 stainless steel is dependent on the polythionic acid concentration in the range of 0.03 M/liter to 0.38 M/liter, the range decreasing with decreasing concentration of 48 ♦ ♦ V F e 2 Û j 1 i >*25°î\HÎsOÿ Fe j Oj ♦ HSOj 0-4 bi FeS - 0 4 -0» fe ♦ Hs; Figure 23. Potential-pH diagram for Fe-S-HgO at 298 K, net considering 2- SO^ . Ail equilibria involving dissolved species are drawn for unit activity; equilibria involving H^S are for 1 atmosphere gaseous HgS. p"(FeS) = -97.7 kJ mol’ ^; other values from Pourbaix "Atlas". Severe cracking ranges are indicated by arrows; A:(ref. 49), B(refs. 41, 53), C:(ref. 43) (43). 49 polythionic acid. Maximum susceptibility was obtained in 0.38 M/liter polythionic acid solution at potential of -1 to 0.25 V vs. SCE, while minimum susceptibility occurred in 0.03 M/liter in the potential range of -0.36 to 0.2 V SCE. Moderate susceptibility to stress corrosion cracking of 304 stainless steel was observed in 0.18 M/liter polythionic acid within -0.38 to 0.2 V vs. SCE. It should be mentioned that stress corrosion cracking was observed at corrosion potentials, which are -0.43, -0.38 and -0.36 V vs. SCE for polythionic concentrations of 0.03, 0.18, 0.38 M/liter, respectively. As shown in Figure 24, stress corrosion cracking on cathodic polarization was observed only in 0.38 M/liter polythionic acid, which suggested that cracking can occur by hydrogen embrittlement. In these experiment, the formation of sulfur on the surface was not found, but precipitation of colloidal sulfur was observed in the solution at high anodic polarization. There are some contradictory explanations concerning the action of elemental sulfur. As pointed out by Dhawale et a l . (41), chemisorbed sulfur significantly enhances the stress corrosion cracking by increasing dissolution rate. On the other hand, it was reported that elemental sufur may inhibit stress corrosion cracking by preventing electromigration of aggressive ion(55). 2.2.3.3 Effect of pH Dravnieks and Samans(35) showed that the time-to-failure in polythionic acid is greatly dependent on the pH of the solution . In Figure 11, time-to-failure increased roughly by a factor of five as the pH increased from 1 to 5. Brophy(37) confirmed the pH effect on SCC of 304 stainless steel in 50 «fwF (41 hourt I 14 NF,(CCCI H f . TF AT FCC . » h o u rt (CCCI Fa Failure or tpaciman NF a No failure or apeclmen (CCCI a Changea in chemical 20 compoalrion or the aolutlon TF AT FCC «Time 10 Tellure al free corroilon condition 2«* -1000 -100 -600 0 4-200 4-400 4-600 4600 4(000 Cathodic• — — Anodic polarization polarization Potential,! SCE l, mv Figure 2 4 a Effect of polarization on stress corrosion behavior of sensitized AI SI 3 0 4 stainless steel U-bend specimens in Samans solution containing Q a 3 8 mol/L of polythionic acid ( 5 4 ) a 51 1 i synthetic tetrathionate solution(pH 5.3) using the U-bend test. The acidification of tetrathionate solution to pH 1.5 causes failure of the specimen in one hour, whereas at pH 5.3 failure occurs after 18 days. Zucchi et a l.(52) found stress corrosion cracking in near neutral solutionCpH 6.4) containing as low as 2 ppm tetrathionate. However, maximum susceptibility to stress corrosion cracking occurred at pH 3 in the solution with tetrathionate concentration ranging from 0.017 M to 3.3 X 10~® M. This pH dependence is reflected in NACE standard RP-01-70(56), which recommends alkaline washing(2 wt * sodium carbonateCsoda ash) with pH greater than 9) to protect austenitic stainless steel from polythionic acid stress corrosion cracking. Similar pH effect was also found in thiosulfate solutions. Newman et a l.(43) studied the pH effect in 0.5 M Na2$202 on the repassivation rate in terms of charge density by assuming that currents are closely associated with dissolution within the cracks. It was found that the charge density increased as pH decreased from pH 9.1 to 3.0(Figure 25). A major increase in the peak charge density occurred at pH below 3.6, where retarded repassivation became evident. This trend is consistent with the result of Griess and Bacarella(57). They did not find stress corrosion cracking of welded Type 304 stainless steel in 0.15 m NaOH + 0.28 M HgBOg + 0.064 M NagSgO^ solution of pH 9.3 by using U-bends at temperature ranging from 66 to 149°C. Another study was done by Bandy et a l.(58) on sensitized Inconel 600, using the U-bend test. Rapid cracking was observed in 0.1 M thiosulfate + 1.3 % boric acid at pH 3, whereas no crack was found in a neutral 0.1 M thiosulfate solution. 52 ü 2 5 0 u 15 0 >- 10 0 U 5 0 —400—200 0 200 400 POTENTIAL (mV SCE) Figure 25. Variation of repassivation rate parameter with potential for iron-9 Cr-10 Ni alloy in 0.5 M Na^S^Og adjusted to various pH with sulfuric acid. o...pH 9.0; + ...7.0; * ...4.4; •...3 .6 ; X ...3.0 (43). 53 2.3 Pitting Corrosion Pitting corrosion is one of the most dangerous and common types of attack of metals in aqueous solutions. Usually, the tendency of a metal or alloy toward pitting is assessed by(59) (1) determination of the breakdown potential (2) determination of the minimum concentration of aggressive ions such as Cl~ in solution that causes pitting (3) measurement of the distribution of pits, i.e., depth and width, in a suitable standard solution In this section, the effect of Cl" concentration, pH, temperature and anion on the breakdown potential will be explained. The effect of sulfur species on the pitting corrosion of Type 304 stainless steel will be included in the final part. 2.3.1 Chloride Environment 2.3.1.1 Chloride Concentration Mazza and Greene(60) investigated corrosion morphology for U-bend specimens of Type 304 stainless steel in various solutions with different ratios of HgSO^/NaCl. Figure 26 shows the corrosion domains at open circuit potential. No critical concentration of chloride for pitting was found. Corrosion morphology changed from pitting to fissuring and cracking when the concentration of NaCl was higher than 0.05 N NaCl. The addition of HgSO^ does not increase the domain of pitting, but if the concentration of HgSO^ is larger than 2.5 N, uneven general corrosion take place. 54 On the contrary, Stolica(61) reported the minimum content of Cl" needed to initiate pitting in different steels when exposed to 0.5 M sulfuric acid (Table 2). It is clearly shown that the addition of Or increased the resistance to pitting. The critical concentration of chloride in Type 304 stainless steel is 0.1 M in the presence of 0.5 M sulfuric acid. Leckie and Uhlig(62) studied the effect of 01" concentration on the pitting potential of Type 304 stainless steel in neutral NaCl solutions at room temperature by using a step-by-step polarization technique with a scan rate corresponding to 10 mV/min. Figure 27 shows their results. Pitting potential linearly decreased from 0.35 to 0.26 Vy when the activity of chloride changed from 0.01 to 0.1. The relationship between pitting potential(V) and Cl" concentration(M) is as follows: Ep = -0.088 logCCl"] + 0.168 [2] Man and Gabe(63) also studied the pitting potential of 304 stainless steel as a function of chloride concentration at 20*C potentiodynamically with a scan rate of 1 mV/sec. It was found that pitting potential decreased from 0.66 V(^ to 0.34 Vy as the Cl" concentration increased from 1*(0.17 M) to 5% NaCl(0.85 M) (Figure 28). However, further increase of concentration to 10 % NaCl(1.7M) did not influence the pitting potential. Despite the similar dependence of pitting potential on chloride concentration within the corresponding concentration range used by Leckie and Uhlig(62), higher pitting potentials by 0.2 to 0.4 V were reported. Obviously, the shift of pitting potential into anodic direction is due to the fast potential scan rate in a Leckie and Uhlig's experiments. Pitting potential 55 Table 2. Minimum content of chloride ion to initiate pitting in various steel (61). Material Cl" (M) Fe 0.0003 Fe - 5.6 Cr 0.017 11.6 Cr 0.069 20 Cr 0.1 24.5 Cr 1.0 29.5 Cr 1.0 18.6 Cr - 9.9 Ni 0.1 56 IGA Uneven J General j Uneven Generol CO Pitting NNoCl Figure 26. Corrosion morphologies observed for U-bend specimens of Type 304 steel in HgSO^/NaCl solutions. Here, no applied potential was utilized (60). MOLAR CONG. OF HoU 0.01 0.05 0.1 0.5 035 030 020 015 001 005 Ol 0 5 ACTIVITY OF a * Figure 27. Effect of chloride Ion activity on steady-state critical potential for pitting, 25*0 (62). 57 0.6 # I 0.4 316 3I7L. 0. 304 CtvlQrid* conctntratton, % Figure 28. Effect of chloride concentrations (%) on the pitting potentials of steels at 20*C (63). (O 1 1.....1 1 1T T '1 1 1■ ■ 1 l ‘“ '■ 3 0-35 - Noble 2 0.30 — 0 0 2 5 1 < 0.20 I ^ 0.15 .1 1__l. 1 1 1 II1 1. . 1 1 o 2 3 4 5 6 7 8 910 20 30 40 ® TIME AT EACH 5 0 mv STEP (MIN) Figure 29. Effect of time at constant potential on observed critical potential for pitting in 1 N NaCl, 25'C (62). 58 Increased from 0.18 Vy to 0.30 as the scan rate increased from 1.7 mV/min to 20 mV/min as shown in Figure 29. 2.3.1.2 Temperature Leckie and Uhlig(62) investigated pitting potential of Type 304 stainless steel in 0.1 N NaCl solution as a function of temperature ranging from 0 to 50*0. The results are shown in Figure 30. It was found that pitting potential does not change significantly above room temperature, but as the temperature decreased to below room temperature, pitting potential increased remarkably. The pitting potential was 0.92 Vy at 0*0, showing more than a 0.5 V increase compared to that at 25*0. Another study of the effect of temperature on pitting potential was performed at temperatures ranging from 30 to 100*0 in 3 % NaOl solution(64). As shown in Figure 31, the pitting potential decreased linearly from 0.3 to 0.1 with increasing temperature from 30 to 100*0 . Man and Gabe(63) investigated the effect of temperature on different steels in 3 % NaOl solution at temperature from 10 to 90*0 by potentiodynamic tests with a scan rate of 1 mV s"^. The results are shown in Figure 32, showing the different temperature dependence for different steels. It is believed that the different temperature dependence of pitting potential is caused by different microstructure and solution concentration, etc. A extensive study of the temperature effect on the pitting potential of Type 304 stainless steel was done by Lin et a l.(12) in 0.01 M NaOl solution in temperatures ranging from 100 to 250*0. Figure 33 shows pit initiation potentials determined under potentiodynamic 59 0.9 Z 0 8 o 07 5 0.5 o. 04 H 03 ) 30 40 50 60 70 TEMPERATURE "C Figure 30. Effect of temperature on critical potential for pitting 0.1 N NaCl (62). +0.5 +0.4 +0.3 316 +0.2 304 +0.1 430 30 40 SO 60 70 80 90 100 «I Figure 31. Breakdown potential of 430, 304 and 316 steels at 30 to 90 C in 3* NaCl solutions (64). 60 0.3 I O S a 316 a. 3I7L - 0.1 -O.E 20 40 60 80 too Solution tsmporoturo, *C Figure 32. Effect of temperature on the pitting potential of steels In 3% NaCl solution (63). 61 Type 3 0 4 Stainless Steel 05 Sensitized !2Hrs. a t6 5 0 X 0 . 0 ! M N a C t ^ OA O Pofenltodynomic Dofo A Potentiostotic Data Ô 5 03 6 0 02 1 O— CD 0 I -0 I _L 50 100 150 200 250 300 Temperature, *C Figure 33. Passivity breakdown potentials, measured under potentiodynamic and potentlostatic conditions, as a function of temperature for sensitized Type 304 stainless steel In 0.01 M NaCl soltulon (12). 62 conditions with a scan rate 5 mV/min and also under potentiostatic conditions. Pitting potentials obtained by these two methods show similar dependency on temperature, but the potentiodynamic pitting potential exhibited a value approximately higher by 0.2 V. Pitting potential decreased by about 0.18 V with increasing temperature from 100 to 150"C, but at temperatures higher than 150"C, i t remained almost constant. It should be emphasized that pitting potential decreased by 0.07 V under straining at lOO'C, while the same values were observed at temperature above 150*C. The result obtained in 100 ppm Cl~ solution at temperatures from 25 to 300°C exhibited almost the same temperature dependence(lB). In Figure 34, pitting potentials!scan rate 0.1 V/hour) for unstrained samples are shown. They rapidly decreased with increasing temperature until around 200*C. However, strained specimens with a strain rate of 6.7 X lO”"® min"^ had 0.15 V lower pitting potential at 150"C. Above 200*C, there is essentially no difference between strained and unstrained specimens. Despite the similar temperature dependency of pitting potential, the pitting potentials reported in the literature exhibit a large deviation. Presumably, the difference in scan rate, heat treatment, chloride concentration and sulfide distribution(65) and morphology might cause deviations. 2.3.1.3 pH Leckie and Uhlig(62) studied the effect of pH on pitting potential of 304 stainless steel by a potentiostatic step-by-step polarization technique. Figure 35 shows their results. The critical 63 O T &> 800 OJ TYPE 3 0 4 STAINLESS STEEL 100 ppm c r As NaCl 3: CO 600 0.10 V/HR SCAN RATE # UNSTRAINED * 6.7 X 10" 5 min”' Eg H- O 400 OLü 0_ Q= CC O O ZO 200 UJ h-O S OS UJ -200 1 ± 1 100 200 300 TEMPERATURE 'C Figure 34. Pitting potentials of Type 304 stainless steel on unstrained and continuously strained specimens as a function of temperature. Above 200“C, there is essentially no difference between tests in which the passive film is lift intact and tests in which the bare surface is exposed by dynamic straining (18). 64 Noble V) 0 9 w 0.8 .J 0 7 P 0 .6 - 2 0 5 Q3 Figure 35. Effect of pH on critical potential for pitting in 0.1 N NaCl, 25*0 (62). 65 potential is the same in 0.1 N NaCl ranging in pH from 1 to 7. However, in solutions with pH 7 - 10, the critical potential was more noble than in neutral or acidic solutions. With further increase of the pH above 10, the apparent pitting potential decreases when it was determined from the increase of current. However, the increase of current do not correspond to pitting but to attainment of the transpassive state with dissolution of Cr and formation of CrO^ as follows. Cr + 4HgO ------CrO^Z- + sh"^ + 6e“ [3] The equilibrium potential for above equation decreases with a slope of -0.08 V/pH unit as pH increases. The observed slope of pitting potential -0.09 V/pH unit, shows good agreement with the calculated slope of the above anodic reaction. Smialowska(59) reported the pitting potential of Type 304 stainless steel in 3 % NaCl. As shown in Figure 36, pitting potential increased about 10 mV per unit pH in the pH range from 2 to 11.5 for Type 304 and 430 stainless steels. 2.3.1.4 Effect of Anion Addition Leckie and Uhlig(62) studied the inhibiting effect of various anions when added to chloride solution. The effect of the addition of SO^^ , CIO^", OH , NOg” is summarized in Figure 37 and 38. The minimum anion activities necessary to inhibit pitting of Type 304 stainless steel are as follows; logCCl”] = 1.62 logCOH’] + 1.84 [4] = 1.88 logCNO-"] + 1.18 [5] = 0.85 logCSO^Z-] - 0.05 [ 6] 66 +0.85 +0.65 316 +0.45 304 +0.25 • 430 +0.05 1 3 5 9 11 pH7 Figure 36. Dependence of the breakdown potential upon pH for 430, 304 and 316 steels in 3% NaCl (64). 67 0 5 I 0.10 O 05 >- t- > Pitting Inhibition h- 0.01 —pitting Intiibition 0 .0 0 5 0.001 0 .0 0 5 0.01 0 0 5 0.1 0 5 1.0 ACTIVITY OF SO4 or (%0; Figure 37. Activity of SO^^’ or CIO^" required to Inhibit pitting as a function of 01“ activity, 25'C (62). 68 0H“ 5.0 no ; Pitting IntilW tion Ü 0 .5 Ü. O >- h* > Pitting / Intiibition 0 .0 5 0.01 0.01 0 .0 5 0.1 0 .5 1.0 ACTIVITY OF NOj OR OH" Figure 38. Activity of OH or NO^ required to inhibit pitting as a function of Cl” activity, 25*0 (62). 3I7L S > 316 o 304 a. Swiphot# eoncantratiofl, % Figure 39. Effect of sulphate concentration (%) on the pitting potential of steels in 3* NaCl soluion (63). 69 = 0.83 logCClO^'] - 0.44 [7] From these equations, the efficiency of inhibition for [Cl”] >0.1 decreases in the order: OH” > NO3” > SO^^” > CIO4” It is believed that pitting is inhibited by competitive adsorption of impurity anions and aggressive ions such as Cl” . Man and Gabe(63) studied the effect of anions on the pitting resistance of different steels in 3% NaCl solution. Figure 39 shows the effect of sulfate ion on the pitting potential. The increase in sulfate ions increased the pitting potential, but a limiting pitting potential, of 0.74 Vy was reached at concentrations above 7 * NagSO^. The addition of NOg^ , oh” , C O ^ ions also inhibited pitting corrosion, but the addition of these over an equimolar concentration of chloride did not show any additional inhibiting effect. Some sulfur species also strongly influence the pitting corrosion in chloride solution. This will be described in section 2.3.2. 2.3.2 Solution Containing Sulfur Species , Herbsleb and Poepperling(46) investigated pitting potentials of 22Cr-5Ni-3Mo duplex stainless steels in various concentrations of NaCl solutions saturated with HgS at temperatures from 20 to 80*C and their results are shown in Figure 40. As a reference, the pitting potentials obtained in the same NaCl solution without HgS are shown in Figure 41. Like those in NaCl solution, pitting potentials decreased with increasing temperature or concentration of NaCl in HgS saturated solution. However, pitting potentials are significantly lower in the 70 NaCt conctntrathn, M 2 0 0 Q- -O ■ - - 6 0 0 V- •7 8 0 0 potmtiodynamic dironopotantiostatie nMaturammts nMasuramanta ^ fraa oorrotkm potantial (20 Cl Figure 40. Effect of temperature and NaCl concentration on pitting potentials of AF 22 in aqueous NaCl solutions containing HgS (46). 71 î a e NaCl conctntrathn, M Figuré 41. Effect of temperature and NaCl concentration on pitting potentials of AF 22 in aqueous NaCl solutions. o...20*C; D...50*C; A...80*C; free corrosion potential a ambient temperature (46). 72 presence of HgS. This tendency is more pronounced at room temperature than at 50 and 80*C. At 20*C, the depression of pit potential by HgS is more than 1 V, whereas 0.2 - 0.5 V depression was observed at 50 and 80*C. It was also found that pitting potential showed a less dependency on temperature and concentration of NaCl in H^S saturated solution in comparison with plain NaCl solution. Newman(55) extensively studied the effect of sulfur species on pitting corrosion at room temperature. Pitting potentials were measured potentiodynamically with a scan rate 0.2 mV s’ ^ for Type 304 stainless steel in 0.25 M NaCl solution containing various concentration of sulfur species!pH 5.0 < pH < 6.5) from 0 to 2 molar. The results are shown in Figure 42. Additions of 0.01 to 0.02 M NagSgOg lowered the pitting potential by more than 0.3 V, while additions of more than 0.5 M NaigSgOg inhibited pitting. KSCN shows similar but less marked effects, while increasing NagS additions up to 0.1 M continuously decreased the pitting potential. Additions of NagS^Og up to 0.05 M promoted pitting, but addition of 0.01 M NagSOg had no effect. The acidified thiosulfate solution(by addition of hydrochloric acid) decreased pitting potential by 0.1 V. As in the case of stress corrosion cracking, pitting corrosion susceptibility of stainless steel in thiosulfate solution is associated , with efficient generation of sulfur at the crack tip(55). The inhibiting effect of high thiosulfate concentration is due to preferential electromigration of SgOg^" into pit nuclei, preventing chloride accumulation and probably retarding acidification due to reactions such as 73 I I f - // 2o6 ^...... / q i X I f 1 0 0 - -100 -2 0 0 fî X X -3 -2 - I 0 LOG (SULFUR SPECIES CONCENTRATION,M) Figure 42. Pitting potential data for Type 304 steel in 0.25 M NaCl with additions of sulfur compounds. The dotted line at + 280 mV represents the 0.2 mV s pitting potential with no additions. Potentiodynamic pitting potentials are shown for NagSgOg and Na^S^Og with no pH adjustment (X and ■ respectively) and for the following with 5.0 < pH < 6.5: NagSgOg (+), KSCN (#), HgS (o). Scratch pitting potentials are indicated by 's '. Bracketed points are ^50 mV; all others are reproducible to < 10 mV. Addition of 0.01 M NagSOg had no effect on the pitting potential (55). 74 SgOgZ" + 6H* + 4e" = 2S + SHgO. [ 8] However, HgS does not electromigrate and therefore increasing concentrations above 0.01 M accelerate chloride accumulation and acidification. The addition of thiosulfate of 0.005 M and 0.001 M did not show an appreciable effect, while a large effect was observed in 0.01 M thiosulfate solution. This sharp threshold concentration corresponds to that below which an adsorbed sulfur layer of sufficient coverage cannot be produced at the base of a flaw in the passive film before the film begins to heal. They found thiosulfate alone has no noticeable aggressive effect on the passive film and therefore probably exerts aggressive effect on the bare metal surface following chloride induced film breakdown. Garner(64) found pitting of Type 304 stainless steel in 10 ppm thiosulfate and 100 ppm sulfate solution(pH 4.5 at 50*0 in the absence of chloride solution. Such pitting was also found to occur readily at pH 7. The importance of this finding is the fact that pits can be initiated in environments not containing chloride. In the solution containing 100 ppm 30^^" + 10 ppm SgOjipH 4.5) at 50*C, the pitting potential of sensitized(650*C, 1 hour) Type 304 stainless steel was -0.3 V vs. Ag/AgCl. On the other hand, in the solution containing 100 ppm 30^ + 200 ppm Cl(no SgOg ) at the same pH and temperature, the pitting potential was shifted anodically by 0.4 V, i.e., 0.1 V vs. Ag/AgCl. In addition, the simultaneous presence of Cl" and SgO^^" shows a synergistic effect to promote pitting. The pitting potential decreased by 0.1 V in comparison to the solution containing Cl" and 2- S^Og separately. In the presence of 20 ppm Cl , pitting was found in 75 the solution with the ratio in the range 1.6 to 58. The addition of up to 200 ppm NagSO^, AlgfSO^)^ or 500 ppm of NaCl into white solution(20 ppm Cl” , 100 ppm S0^^“, pH 4.5 at 50"C) has negligible effect on pitting corrosion, whereas the addtion of NagS or ^®2^2®4 promotes pitting. However, the simultaneous presence of thiosulfate and chloride in white water did not always initiate pitting, only the presence of 3 - 75 ppm SgOg^ promotes pitting. Below or above this range, pitting did not occur. Recently, pitting corrosion of Fe-19Cr-10Ni alloy was studied in thiosulfate solutions by Newman( 66). Pits were found only in the presence of both sulfate and thiosulfate ion, whereas pits were not initiated in pure sulfate or in pure thiosulfate alone. Pitting was confined to a range of potential from -0.325 to -0.1 V vs. SCE(-0.085 - 0.14 V^) in the solution containing 0.25 M NagSO^ and 0.025 M NagSgO^. An interesting feature of pitting in the NagSO^ + NagSgO^ solution is the presence of upper potential limit, above which pitting does not occur.' The upper potential limit of the pitting probably corresponds to that at which adsorbed sulfur can no longer be generated to activate the dissolution of metal in the pit. Assuming that the concentration of HSOg” and SgOg^” are equal to 0.01 M, the calculated redox potential of S20s^”/HS03“ at pH'7 is 0.08 Y^(51), which agrees reasonably with the upper limit of pitting potential(0.14 V^). 76 2.4 Chemistry of Sulfur Species 2.4.1 Potential - pH Diagram The chemistry of sulfur is very complex and the stability of sulfur species varies with redox potential and pH of the solution. Moreover, the solution temperature greatly affects not only the stability of sulfur species but its reaction rate as well. Valensi(Sl) constructed a potential-pH diagram for sulfur-water system at 25*0, which is shown in Figure 43. The metastable equilibrium diagram(67) is shown in Figure 44, in which the domains of some of metastable sulfur ions are included. As shown in these figures, the predominant species present between pH 3 and 6 is HgS. Above pH 6, HgS, - 2- - HS and S species are present, but HS is predominant. The potential-pH diagrams at high temperature were constructed based on a 'Correspondence Principle'( 68). Biernat and Robins(69, 70) completed the stability diagrams for S-HgO, Fe-HgO and Fe-HgO-S systems at temperatures ranging 25 - 300*0. The stability diagrams for Fe-S-HgO at 25, 100, 150 and 200*0 are shown in Figure 45 - 48. They have used the non-conventional standard state for the electrode potential(i.e., E^She = 0 V at 25*0) . Therefore, the reported potential should be shifted so that = 0 V at any temperature. Later, Chen and Theus(71) constructed the quaternary stability diagram of Fe-S-Ol-HgO and Ni-S-Ol-HgO systems for 25, 100 and 200*0. It was found that the addition of chloride do not change the stability domain in neutral and alkaline solution. Hence, the combined quaternary Fe-S-Ol-HgO stability diagrams do not give a significant amount of additional information than two separate diagrams(Fe-Ol-HgO and 77 -2-1 0 I 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 HS--2 10 11 12 13 14 IS ..16 Figure 43. Potential-pH diagram for the stable equilibria of the system sulphur-water, at 25*C. The part of this diagram lying outside the line log C = 0 refers to solutions which are not saturated with solid sulphur but which contain 1 g-at /1 (32 g/1) of sulphur, dissolved in the forms HgS + HS + S + HSO^”” + SO^ + SgOg"". The part inside this line refers to solutions saturated with solid sulphur (51). 78 2.0 a s 0 4 so! -04 04 MS' - 1.2 -16 pH Figure 44. Potentiai-pH metastable equilibrium diagram for the system 2-, S-HgO at 25*C and 1 atm. Dithionite (SgO^ ), dithionate (SgOg^ ), trithionate (SgO^^ ), and sulphate (SO^^ ) are r included. Total concentration = 0.2 g S/1 H^O (67). 79 .3* HSO, As SO4 V -. (B) FeOH I 19a 22a F«S; 15a 2O0 F«(OH). a-FeS 22c 20b 20c HS ,2- -IS PH Figure 45. Potential/pH diagram for Fe-HgO-S system at 25*C. ——« -----, water stability (A) and (B); ------dissolved iron species; ------S-HgO system; ------, involving solid iron species (70). 80 . . i .J F . ^ X ' ^ - FtOH* HF,0 ; Figure 46. Potential/pH diagram for Fe-HgO-S system at lOO'C. Key as Figure 45 (70). 31 FaOH I6d .2 + EKI ISO HS" 23d^Fej pH Figure 47. Potential/pH diagram for Fe-H^O-S system at 150“C. Key as Figure 45 (70). 82 Fe^OH) m i \ ' r - | - \ lFeOH‘1 HF«Or- Figure 48. Potential/pH diagram for Fe-HgO-S system at 200"C. Key as Figure 45 (70). 83 Fe-S-HgO). 2.4.2 Sulfur Chemistry The chemistry of sulfur species has been studied for a long time. Several reviews have been published on the chemistry of sulphur species(39, 72). The kinetics of oxidation of sulfide was studied by numerous researchers(73 - 85) at ambient temperatures. Avrahami and Golding(73) studied the oxidation of the sulfide ion over the pH range 11 - 14 at 25 - 55*0. In this pH range, HS" concentration is nearly constant and dominant, whereas the S"^ concentration is low and proportional to the OH" concentration. However, the oxidation rate was enhanced by increasing the pH, temperature and oxygen ratio. The oxidation of HS” occurs according to following partial reactions; 2 HS" + 3 Og — 2 SOgZ" + 2 H^ [9] 2 SO3 + Og — 2 SO^Z- [10] 2 SO3 + 2 HS" + Og — 2 Sg 03^" + 2 OH" [11] 2 SgOgZ" + Og — 2 SO^Z- + 2 S [12] In the obove reactions, Reaction[9] is the slowest and hence rate-determining. Since ReactionClO] and [11] proceed quickly , no SOg^ could be detected. Therefore, the final productobtained was thiosulfate, sulfate and occasionally, elemental sulfur. However, the concentration of thiosulfate was reported to decrease with time as shown in Figure 49. This indicates that thiosulfate is an intermediate species in the oxidation reaction of HS". A similar study has been done in sea water of pH 7.5 at 10"C by 4 0 0 84 I3 200 % 100 a 60 c ■D I I 20 o 100 200 300 400 500 Tim* (hr.) Figure 49. Semilog plot of optical density of ions against time (73). 60 X. 40 40 60 80 Figure 50. Relation of concentration to time. Total sulfide (•), thiosulfate (A), sulfite (■), and sulfate (♦) concentration. Initial concentration ratio (CgozCgg) was 4. Sulfate concentration was estimated by difference (74). 85 Cline and R1chards(74). The concentration variation vs. time is presented in Figure 50, where the rate constants for oxygen and total sulfide concentration are 1.5 x lO"^ and 0.5 x 10”^(pg - atoms/liter)*(hr)"^, respectively. The major reaction products were thiosulfate and sulfate, with sulfite occurring as an unstable but fairly long-lived intermediate product. However, no elemental sulfur was detected. Judging from Figure 50, sulfite appeared to be stable in the presence of oxygen for at least 50 hours. This result is contradictory to the extrapolation of Fuller and Crist's result(75) which showed a higher oxidation rate of sulfite by 100 times , i.e ., rapid decrease of sulfite concentration, at nearly the same pH in pure water. The difference in results could be related to a different solution composition; sea water contains various metal ions, which can act as catalyzers of sulfide oxidation. Also, the presence of the oxidation products of reduced sulfur species can change the ratio of concentration of different reaction products as pointed out by Avrahami et al.(73). Snavely and Blount(76) measured the rate of oxidation of H 2SC200 ppm) in 3.5% NaCl solution at room temperature. As shown in Figure 51, the reaction occurs at a significant rate only at high pH, suggesting that S“ is involved in this reaction. This result implies that the reaction of dissolved Og with HgS is autocatalytic and requires the build-up of an intermediate species which is unstable at low pH. However, they could not describe the nature of the intermediate and conjectured a polysulfide. In their study, elemental sulfur was suggested as the final product according to the following equations; 86 • 0 4.9 to to to Figure 51. Reaction of oxygen with hydrogen sulfide in 3.5* sodium chloride solution. HgS = 200 ppm, T = 25 C (76). Curve pH 0 2.2 X 6.5 11.5 87 $2- + Og X [13] X + s ^ “ X s f " [ 1 4 ] Xsf" + 1/2 Og + HgO X + sP + 20H’ [15] where X is an intermediate. After oxidation of sulfide in the pH range of 5.5 to 11.5, Chen and Gupta(77) and Chen and Morris{78) found elemental sulfur, polysulfide thioslfate and sulfate with a small quantity of sulfite. It was reported that the rate of elemental sulfur formation decreased with increasing pH. At pH higher than 8, no sulfur was found unless excessive amounts or thiosulfate and sulfide are present. The proposed reaction scheme(Figure 52) shows that sulfide is slowly oxidized to sulfur which then combines with the remaining sulfide to form polysulfides, i.e., sulfide undergoes a sulfide-sulfur-polysulfide cycle. The initial oxidation of sulfide to sulfur was suggested as the rate determining step. A more comprehensive study was done by O'Brien and Birkner(79) at pHs of 4, 7.5 and 10 in dilute NagS solution. In the reaction products, they found sulfite, thiosulfate and sulfate ions. + 3/2 Og(aq) S0^“ [16] 2sf" + 3/2 Og(aq) [17] SO?" + 1/2 Og(aq) SO^^” [18] S^’ + 20g(aq) — S o f [19] The obtained kinetic data and concentration of products are shown in Table 3 and Figure 53, respectively. It was reported the [S^~]/[Og] ratio, and pH and total sulfide concentration in the solution affects the distribution of individual 88 ♦HS S- ♦0, Choin ftoction so; so: s,o; so: SfSO! so: ♦HS Figure 52. Reaction pathway of oxygenation of sulfide (78) e.o 2.4 10.0 2.0 o X . ^ o •*» o o s .0 ,S (-S lf DATA n K 6 0 - 1.2 rj A.90,**0ATA Q DATA S A 3 2 4.0 • — .PRCOICTEO CONCENTRATIONS,.^ 0.8 O A " J j p K 3 0 , : 20 A 0.4 KT . 1______L.------1------L 100 200 300 400 900 TIME (MINUTE*) Figure 53. Model predictions vs. experimental data in the sulfide oxygenation reaction. pH = 7.55 (79). 89 Table 3. Summary of Kinetic Data for Oxygenation of Reduced Sulfur Species at pH 7.55 (79). initial concn. - 1 -1 M X 10^ Rate constants, M min Expt S2- k »2 ^1 h ^3 y 1 1.10 1.02 1.12 0.226 10.110 1.56 1.96 2 1.18 0.93 1.37 0.331 0.204 1.25 2.06 3 1.04 1.07 1.75 0.553 0.411 1.42 1.95 4 1.17 0.85 1.97 0.521 0.220 2.51 1.70 5 1.21 1.21 0.97 0.207 0.093 1.28 1.86 90 reaction products: a high ratio, along with a total sulfur concentration greater than 10”^ M, leads to the formation of sulfur, whereas the low ratio favors the formation of sulfite, thiosulfate and sulfate. In the pH range of 7.5 to 11, the distribution of reaction products is not affected by the pH. However, at pH lower than 6, sulfate is the predominant product. If S^" concentration is lower than 10"^M and if the pH is above 7, sulfite, thiosulfate and sulfate are favored. Hoffmann and Lim(80) studied the effect of impurities present in the solution which act as catalyzers in the oxidation of sulfide. They concluded that those impurities are the cause for the lack of reproducibility in previous studies. The reaction product and concentrations in the presence of Co*^, Ni*^ and Cu*^ ions are shown in Table 4. It is obvious that a small concentration of catalyzer!as low as lO"^ M) greatly changes the reaction products and their distribution at same pH. It is noteworthy that polysulfides are formed in the absence of catalyzer, which is consistant with Chen and Gupta's result(78). Weres et a l.(84) found that the oxidation process of sulfide at 45"C(pH 7.8) is autocatalytic, i.e., at first, the reaction rate is slow but increases rapidly with time, reaching steady state in an hour. However, small amounts of sulfite or thiosulfate inhibit the development of the autocatalytic reaction!Figure 54), destroying the sulfur chain molecules by converting their zerovalent sulfur to thiosulfate. It was +2 +2 found that the presence of Co and Ni had strong catalytic potency, 2+ 2- but Fe has a weaker effect. The reaction product was primarily SgOg with elemental sulfur and small amount of SO^^” and showing a 91 Table 4. Observed Production Distributions For Catalyzed and Noncatalyzed Reactions (80). [catalyst], M products mol % as S of Sy ^ _ 2- 2- 2- CuTSP: 5 X 10"® V s • S®3 ' V 37%, 3%, 60% 2- 2- 2- Ni ISP: 5 X 10“® W • “ 3 • “ 4 51%, 4%, 44% 2- 2- 2- CoTSP; >10"? ®8* * ^ 3 * ^®4 32%, 11%, 1%, 56% 2- 2- 2- CoTSP: <10"7 34%, 2%, 64% W • SO3 ' V EDTA: lOT® ^8’ ^4 • ^5 ■ V s ’ V s ^ '- ^ 3 ^ ' - V [Og]^ = 10"3 M, [HST] = 10"3 M, pH = 8.8, p =0.4 M, T = 25"C ISP : tetrasulfophthalocyanine 92 IO 50 CM O' u. ■’ □o 50 100 ISO Time (tnifv) Figure 54. Development of turbidity and autocatalysis. Sodium sulfite added from 20 min on; sulfite:sulfide = 0.75. 70 pM sulfide added to each test aliquot. "Fraction HgS remaining after 15 s" is the fraction of that 70pM which remains after 15 seconds reaction time. If a large fraction remains after 15 s, this indicates slow reaction and vice versa (84). 93 good agreement with Hoffmann and L1m(80). The reaction products th at have been reported in the literature are summarized in Table 5(79). Despite numerous attempts to determine the kinetics and reaction products for oxidation of sulfide, most of those results show considerable disagreement. However, all the data point to the formation of thiosulfate and sulfate as a result of oxidation of sulfide at room temperature. The formation of elemental sulfur and/or sulfite is not detected in every study. It is difficult to rationalize these divergent re su lts. These could be the resu lt of differen t chemical composition of the solution: (1) different pH and (2) [sf'J/COg] ratio(70) or (3) contents of various metal ions, which may act as a catalyzers. (4) presence of oxidized sulfur species such as sulfite or thiosulfate before starting reaction, which can retard the sulfide oxygenation rate(84, 85) and change the distribution of reaction products(71, 72). The general features of sulfide oxidation at high temperature or effect of chloride on sulfide oxidation are not sufficiently studied in the literature. Giggenbach( 86) studied the reaction of elemental sulfur in deaerated near-neutral aqueous sulfide solution at temperatures ranging 20 to 240*C. The reaction rate strongly depends on temprature and pH, increasing with increase of temperature and pH. The stability domain for sulfur species are presented in Figure 55 as a function of temperature and pH. At pH lower than neutral, the reaction of elemental sulfur can be described as follows. nS + SH" +(1-P)0H" = HpS^sP'Z tfl-plHgO [20] 94 Table 5. Summary of Reaction Products Observed in Investigations of Oxygenation of Reduced Sulfur Species ( 79). Investigator pH Solutlon [S'^l/COg: Product obsd Chen and Morris (78) 6-12 Controlled 0.06-1.25 s '^ so-2. s \ Chen and Gupta (77) 0.08-8 Avrahami and Golding 11-14 Controlled 0.08-0.67 .S^(occaslonally). (73) Cline and R1chards(74) 7.8 Seawater 0.125-0.5 s o j^ SgO^z. so;2 Skoplntsev et a i.(81) 8.2 Seawater 0.2-8.0 SO’2. SgO'^ DemlrJIan (82) 7, 8.6 Controlled 0.03-5.0 S°. S0-2. SjO-^. < Alferova and Titova 9-13 Controlled 20 S0;2. SgO;Z. SO’2 (83) O'Brien and Birkner 4-10.7 Controlled 1.0-1.37 SO-2. SgO;2, S0;2 (79) Hoffmann and.11m (80) 8.8 Controlled 1.0 S°, s“^, SOj^, Weres e t a l . (84) 7.8 Controlled SgOg ■ S. SO^t 95 •c . 10% 2 % 2 5 0 m ^2.,-010 "’«O) 200 thiosulfote 150 aulfioes potysültde-thlosulfate \ équilibration slow 10O stable — 5 0 eiementol polysulfides sutlur metostcAle 8 9 10 13 PH Figure 55. Temperature-pH diagram for solutions 0.1 M in total monosulfide and 0.01 M in total zerovalent sulfur. Dashed lines delineate conditions where 2, 10, 50% of the total initial zerovalent sulfur are present in the form of polysulfide. The dotted line shows variation in pH with temperature, for a SHzHgS buffer ratio of unity ( 86). 96 where P is the number of protons attached to the species and n = 1 to 4. The resulting polysulfide solutions contain an approximately equimolar mixture of tetra- and pentasulfide ions with hydropolysulfide ions lik ely to be present only in very small concentrations. These polysulfides ions dissociate into radicals, or S^", and disproportionate into sulfide and thiosulfate. Above 150"C, the rates of dissociation and disproportionation become significant. Polysulfide ions are thermodynamically stable with respect to this disporportionation up to 240"C in near neutral solution. In acidic solutions, zerovalent sulfur is in equilibrium with hydrogen sulfide and bisulfate or sulfate at around 200"C. 4S + 4H2O SHgS + HSO," + [21] At pH above 8, polysulfide ions become metastable even at room temperature and thiosulfate ions are predominant at temperatures below 250"C. 4S + 40H = SgOgZ- + 2SH“ + HgO [22] Above 250*C, thiosulfate again disporportionates according to SgOgZ" + OH" = + SH". [23] The final products in both cases are sulfur species in the oxidation states -2 and 6. It was suggested that sulfur in a +4 state is unlikely to be present in significant amounts under these experimental conditions. Sulfurous acid and sulfur dioxide disproportionated rapidly to sulfur and sulfuric acid, whereas sulfite disproportionated to sulfide and sulfate. CHAPTER III EXPERIMENTAL PROCEDURE To study the effect of sulfide ion and thiosulfate Ion on pitting corrosion and the stress corrosion cracking In Type 304 stainless steel, anodic polarization studies and slow strain rate test were done. Scanning electron microscopy was also carried out. This section will be divided Into four parts. First, the materials tested will be described. In second p art, the composition of te s t solutions will be provided and In the third part, equipment for the SSR tests and for the electrochemical measurement will be presented. In the la s t p a rt, the experimental procedures for the slow strain rate tests and the electrochemical studies will be explained. 3.1 Materials The materials used In this study was Type 304 stainless steel and the chemical composition and mechanical properties of the test sample Is given In Table 6 . A cylindrical rod of 6.35 mm(l/4 Inch) diameter was supplied by A1 Tech Specialty Steel Corporation In a cold drawn and annealed condition according to ASTM Specification A-580-80A. Each 36 cm(14 Inch) length te s t specimen rod was cut from th is 360cm long stock and machined. After degreasing with methyl alcohol, the specimens were solution-annealed at 1050*0 for 1 hour under an argon gas atmosphere followed by a water quench. The dimensions of the specimens for the 97 98 Table 6. Factory Product Specification of Type 304 Stainless Steel Supplied by AL Tech Specialty Steel Corporation. Chemical Composition (*) c Mn SI PS N1 Cr Mo Cu 0.07 1.10 0.52 0.03 0.02 8.00 18.2 0.5 0.46 Physical Properties Tensile KSI Yield KSI Elongation % RA % Hardness 112 77.0 44.0 73 BR 241 * Magnetic permeability : less than 1.02 * Macro etch te s t : O.K * Intergranular corrosion test : O.K * Bend te s t : O.K * Material free from mercury contamination Material free from continuous carbide network * Solution annealed at 1900*F - minimum and water quenched 99 different experiments are described in section 3.4. Prior to solution annealing, the specimens are degreased and rinsed with methyl alcohol and distilled water. A sensitization treatment was subsequently done in order to get the highest susceptibility to SCC. The sensitizing treatment was done at 650*C for 12 hours in air argon gas atmosphere followed by a water quench. 3.2 Solution The compositions of the solutions employed for the SCC tests as well as the electrochemical tests are listed in Table 7. As is evident in Table 7, most of the tests were conducted in chloride solutions, while a few were done without chloride to examine the effect of chloride ions. The chloride concentration used was 104 ppm, which is equivalent to 3 X lO”^ mole sodium chloride. Two levels of oxygen content were employed for SSRT - 8 ppm and 0.2 ppm. 8 ppm Og corresponds to air-saturated conditions and 0.2 ppm Og corresponds to the Og content due to radiolysis in nuclear power reactors. For the pitting study, only deaerated solution was used. 3.3 Equipment 3.3.1 Slow Strain Rate Tests Slow strain rate tests(SSRT) were employed to study the effect of sulfur species on stress corrosion cracking of stainless steel since they are known to be a more severe test to determine SCC susceptibility than conventional constant strain or constant load tests. The advantages over other experimental techniques are as follows(33). 100 Table 7. Composition of Solution and PH a t Room Temperature. Solutlon (ppm) pH (at R.T.) c r s'' 104 0 0 0.2 6 - 7 104 10 0 0.2 10.0 - 10.4 104 40 0 0.2 10.8 - 11.4 0 0 0 8 6 - 7 SSRT 0 10 0 8 10.0 - 10.4 te s t 0 40 0 8 10.8 - 11.4 0 0 70 8 7 - 7.5 104 0 0 8 6 - 7 104 10 0 8 10.0 - 10.4 104 40 0 8 10.8 - 11.4 » 104 0 70 8 7 - 7.5 P ittin g 104 0 0 0 6 - 7 te s t 104 40 0 0 10.7 - 11.4 ' 104 0 70 0 7 - 7.5 101 ( 1) short testing time (2) reduction of incubation time for crack nucléation (3) rapid screening of environment and metal combinations which re su lt in SCC. The main drawback of th is technique, however, is the difficulty in relating SCC susceptibility measured in the laboratory with actual performance under service conditions. Slow strain rate tests conducted at high temperatures up to 250"C required a pressurized system to maintain a liquid single phase in the test cell. To avoid the depletion of active species and dissolved oxygen in the cell, a recirculating solution system was adopted. Figure 56 shows the schematic diagram of the recirculating system designed to operate up to 2000 psi. The system consists of a pressure vessel, load cell, a straining device, a high pressure pump, heater and preheater, a back pressure regulator, heat exchanger and a solution reservoir. These units will be described in this section. * Pressure Vessel A Type 316 stainless steel autoclave was used in th is study. Figure 57 shows the cross sectional view of the pressure vessel which is designed to use national pipe threads(NPT)rather than weld joints which might be susceptible to SCC(87). As shown in the Figure 57, a thermocouple and Luggin probe were located close to the gage section to control the temperature and potential in the cell, respectively. Specially designed PTFE fittin g s were used between the pressure vessel and the ends of the specimen for two reasons. In SSRT at high temperature and pressure, i t is required to strain the specimen while maintaining high pressure in the autoclave. The low frictional and high 6 Figure 56. Schematic diagram of SSRT apparatus. PG: pressure gage RF: reference electrode TC: thermocouple BR: back pressure regulator Recorder LC: load cell PS: potentiostat H ; heat exchanger PP: high pressure pump O PH; pre-heataer AC: accumulator ro C : cooler 00: dissolved oxygen measuring stage 103 ■Tensile Specimen Conax EGT-I87-A-T Swogelog Cajon 316- I6-R3-8 I" NPT MPG-I87-A-T Outlet for Counter Elective Lead Wire ^Outlet Swogelog Lead Wire with Shrinkoble Teflon Cover 1/4' NPT 1/8" NPT MPG-I87-A-T ' r = r S ^ r \ . ^ Thermocouple ''Reference Electrode (Conox Fitting EOT- Counter Electrode 125-A-T) -Insuloting Teflon -316 Vessel,1/2" Well 1/8 NPT- ’Cp=i_J-r" Inlet Swogelog \J 1/2 NPT- EGT-I87-A Conox Fitting ■Teflon Seal (Port of Fitting) Figure 57. Cross-sectional view of the test vessel showing the spatial arrangement of fittings, pressure seals, specimen, counter electrode and other ancillary parts (87) . 104 thermal expansion characteristics of PTFE satisfies the above requirement. The second purpose is to insulate the working electrode(specimen) from the autoclave. To ensure a uniform temperature distribution in the cell, the preheated solution is supplied at the bottom of the autoclave and ejected from the top. * Straining Device and Load Cell The straining device is similar to a plain tensile testing machine except for the extremely low crosshead speeds employed. The constant, slow strain rate was obtained by means of a series of reduction gears powered by a constant speed motor of 1800 r.p.m . as shown in Figure 58. In slow strain rate tests, the selection of the optimum strain rate is very important. If the strain rate is too high, ductile fracture will occur due to the lack of time for the corrosion reactions to occur. On the other hand, SCC cannot occur at too low strain rate, either, as exposed bare surfaces will be passivated before further stress is applied. For many systems, the sensitive range of strain rate is known 10~^ to 10"®/sec (88). Based on this, the strain rate was fixed at 2.2 xlO"® s”^. The ten sile load on the specimen was measured by a load cell (BLH-T3P1) with a maximum capacity of 3000 lb , purchased from BLH electronics. The output potential of the load cell was read on a strip chart recorder (HP 7132A). * High Pressure Pump To avoid the vaporization of the solution at the high temperatures employed, it is required to pressurize the system over the liquid-vapor equilibrium pressure. A 'Pulsafeeder microflo' metering pump (L-20-S) was used to pressurize and recirculate the solution from the solution 105 1 0 * 0 c a i M0VK81E CAIWIACE a u ANO s n c M tN .SCKEW 0«V E MECHANISM c o n s t a n t SfEED SOUACE Consisting of: an electric motor, & reducing gears. Figure 58. Straining Device( 87) 106 reservoir to the test cell. The flow rate was 1 liter/hr. * Accumulator and Back Pressure Regulator An accumulator was installed to minimize pressure fluctuations in the autoclave during pressurizing. The nitrogen gas filled accumulator was purchased from Greer Hydraulics, Inc.. The optimum gas pressure was half of the working pressure. A back pressure regulator was employed to adjust the operating pressure of the system. * Solution Reservoir The dissolved oxygen content in the solution container was controlled by purging with the nitrogen-oxygen mixture or argon. During this study, the design of the solution reservoir was changed. The first container was made of Ni 200. However, i t was found that the highly aggressive chloride and sulfide ions cause severe pitting all over the inner surface of the tank. Ferrous, ferric and nickel ions are known to have catalytic effects on sulfide oxidation to various oxysulfides, as described in section 2.4. A blue sulfur compound, which was believed to be a nickel and sulfur compound, was found around the p its. Therefore, it was decided to change the reservoir to an inert glass bottle of 15 liter capacity with a Neoprene stopper. In the glass solution reservoir, the blue sulfur compound was not observed. * Heater and Heat Exchanger The custom-made 750 Watts band heater, supplied by E M equipment company, surrounded the test cell to control the temperature in the cell. A heat exchanger was installed to increase the heating efficiency. It was fabricated with two seamless Type 316 stainless steel tubes. The outer tube had a diameter of 12.7 mm and the inner 107 tube had a diameter of 6.35 mm. The fresh cold solution in the inner tube was heated by the counter-current of hot water in the outer tube. Fiber glass tapes were wound over the band heater, autoclave, heat exchanger and other hot piping to minimize heat losses. 3.3.2 Electrochemical Measurement To study the electrochemical behavior of stainless steel, three experiments were carried out. (1) Measurements of corrosion potentials in SSRT (2) Anodic polarization behavior (Potentiodynamic pitting potentials are also obtained from polarizations curves) (3) Measurements of potentiostatic p ittin g potentials For these experiments, the pressure vessel and pressurizing system as described in section 3.3.1 were used. To apply the potential on the specimen, a Model 350 Corrosion Measurement System(manufactured by EG G Princeton Applied Research) was employed. For the applied potential experiment, the wall of the autoclave (Type 316 stainless steel) was used as counter electrode. The reference electrode used is explained separately. * Reference Electorde A major problem in high temperature electrochemical studies in aqueous systems is a choice of a suitable reference electrode. For a good performance of the reference electrode, the following is recommended (89). (1) The potential should be constant and remain stable for the duration of the experiment. (2) The reference electrode should be compatible with the solution 108 used. (3) If the reference electrode is to be used for thermodynamic purposes, it is necessary for it to behave thermodynamically. An external Ag-AgCl/0.1 mol KCl reference electrode was chosen on the basis of the above recommendations, especially for its high stability at elevated temperatures. The conversion data for the silv er-silv er chloride electrode to the standard hydrogen electrode were obtained by D.D. Macdonald(90). The calibrated potential vs. the standard hydrogen electrode is given by = AEgbs AE^g/AgCl^T) " AEyh [24] where AE^^g is the measured potentials, (T) is the isothermal potential of the silver-silver chloride electrode vs. the standard hydrogen electrode a t the temperature T and AEj^ is the potential difference of silver-silver chloride electrode at two different temperatures. The values for the correction parameter AE^gy^gQ-|(T) - AEjb are given in Table 8 as a function of temperature and concentration "of KCl. For example, 0.211 volts are added to the observed value at 100"C if the concentration of KCl is 0.1 Mol. Figure 59 shows the design of the Ag-AgCl reference electrode. A silver wire anodized in 0.1 mol HCl solution at 24 hours was housed in a 6.35 mm diameter flexible PTFE tube at one end. The other end of the tube established a liquid junction with the test solution through a porous zirconia plug. A copper cooling tube wound over the Type 316 stainless steel which supports the flexible PTFE tube maintained the temperature of silver wire a t room temperature. The open c irc u it potential of the specimen with respect to the reference electrode was measured and calibrated to 109 Table 8. Representative Values, " ^^OBS External Ag/AgCl Reference as a Function of AT and Concentration of KCl(90). KCl Temperature ("O Concentration 50 100 150 200 250 300 (mole kg”^) (VOLTS) 0.005 0.332 0.281 0.228 0.162 0.071 0.056 0.010 0.318 0.268 0.215 0.154 0.082 0.007 0.025 0.298 0.249 0.193 0.130 0.058 0.024 0.051 0.279 0.229 0.174 0.112 0.043 0.037 0.102 0.262 0.211 0.158 0.100 0.035 0.039 0.252 0.239 0.191 0.137 0.073 0.003 0.094 0.505 0.225 0.176 0.119 0,052 0.027 0.119 110 Metal Hex Cap Metal Disc Type 316 SS Back Ferrule Teflon Ag/AgCI Wire Holder Ag/Ag Cl Wire Teflon Front Ferrule Swogelok Reducing Union Teflon Tube Coiled Copper Tubing for Water Cooling of Electrode Type 316 SS Tube Swogelok Connector w/ 1/4" NPT Nipple ^ Teflon Tubing (1/4" I.D.) -Teflon Holder for Zirconia — Zirconia Plug Figure 59. The Ag/AgCl external reference electrode assembly. I l l the hydrogen scale according to Table 8. 3.4 Experimental Procedure 3.4.1 Specimen Preparation The dimensions of the specimens are shown in Figure 60(a) and (b). As shown in Figure 60(a), specimens for SSRT were machined as ten sile specimens with a gage section of 3.18 mm(l /8 inch) diameter and 12.7 mm(l/2 inch) length. Cylindrical specimens with 6.35 mm(l/4 inch) diameter and 15 mm length were used for electrochemical studies as shown in Figure 60(b). One end of the specimen was threaded to connect to a specimen holder. After machining, specimens were heat-treated as described in Section 3.1. Specimens were then ground on a lathe with silicon carbide emery paper up to grit No. 600. The gage section was reground at right angles to the previous scratches with grit No. 600 to make the surface free of coarse scratches. The surface was thoroughly cleaned with acetone and rinsed in distilled water. 3.4.2 Solution Preparation Test solutions were prepared with compositions specified in Table 7. The necessary amount of reagent grade sodium chloride (NaCl), sodium sulfide (Na^S.OHgO) or sodium th io sulfate (NagSgOg.SHgO) was weighed and dissolved in 10 liters of double distilled water. In order to control the dissolved oxygen content, mixed gas or prepurified argon gas was sparged in the solution reservoir until the dissolved Og content reached the specified value in Table 7. The gas composition was 21% Og + 79% Ng and 0.5% Og + 99.5% Ng for 8 ppm and 0.2 ppm dissolved oxygen 0.635 cm - 28 Fine Thread 0.318 cm I R 0.635 era 0.635 cm - 1 ---- G.L. T- "1,27 cnH 36.0 cm it- 5 - 40 Female 0.318 cm Thread s b) k— 4 0.635 cm Figure 60. Dimension of specimen, a) stre ss corrosion cracking ro b) electrochemical study. 113 content, respectively. Usually, this process took 12 hours or more. Self-filling ampoules, containing indigo-carmine in an acidic solution, manufactured by Chemetrics, Inc., were employed to measure dissolved oxygen levels colorimetrically. In the solution containing sulfide, it was difficult to read dissolved oxygen content due to interference effects. Therefore the oxygen content was checked before the addition of sulfide ion. Sulfide concentration was measured by the methylene blue method using Type S Chemets containing N, N-dinethyl-p-phenylenediamine in the ampoule under vacuum. The range of measurement is 0.1 - 10 ppm sulfide. For the analysis of thiosulfate ions, disposable Titret(Type T-S, supplied from Chemitrics) ampoules was used. This measuring method is based on the lodate-Iodide titration method. The measuring range is 5 - 50 ppm. 3,4.3 Slow Strain rate Test with Measurement of Corrosion Potential Figure 61 shows the outline of experimental procedure for SSR te s ts . Ù) To avoid unnecessary consumption of the solution, only the gage section is exposed to the solution while other areas are covered by PTFE tape. Prior to mounting the specimen in the te s t vessel, PTFE fittings were inserted at both ends of the cell. (2) After changing the KCl solution in the external reference electrode, the potential was checked with respect to a calomel electrode before every te s t. The measured value was 44 ± 5 mV. If the potential was not in the range of 44 ± SmV, the anodized silver wire was polished and reanodized. After the reference electrode and specimen were put in 114 SPECIMEN 304 SS MACHINING Tensile Specimen ANNEALING 1050'C, 1 hr In Air SENSITIZATION 650"C, 12 hr In Air POLISHING S.S.R.T. è = 2.2 X 10“®/sec [Cl"] = 104 ppm Og = 0. 2, 8 ppm T = 50, 100, 150, 200"C Impurities: 10 or 40 ppm Sulfide, 70 ppm thiosulfate ANALYSIS T1me-to-fa11ure Potential Fractography Figure 61. Schematic Diagram of Experimental Procedure. 115 place in the vessel, both ends of the specimen were fastened by Conax threads. (3) The specimen was stabilized in the te s t solution at the specified temperature till it reaches a constant corrosion potential. It usually took 24 hours. Tests were conducted at temperatures ranging from 50"C to 200*0. (4) Prior to straining, the specimens were preloaded to 18 Kg to remove the relaxation from the system. During straining, the applied load and open circuit potential with respect to Ag-AgCl electrode were recorded. (5) The time-to-failure, total elongation, normalized tim e-to-failure(t^/t^^^) and were the parameters used to determine the SCC susceptibility. A lower value of t^, elongation and normalized time-to-failure indicate higher susceptibility to SCC. The fracture surface and lateral surface were examined by a scanning electron microscope!HITACHI S-510 or JEOL JSM 35), to identify fracture morphology. Some selected specimens were analyzed by Energy Dispersion Spectroscopy to identify the corrosion product. 3.4.4 Polarization Curves Polarization curves were analysed to study anodic behavior of Type 304 stainless steel and to evaluate pitting potentials. The detailed procedures are as follows. (1) The prepared specimen as shown in Figure 60(b) was connected to PTFE coated specimen holder by means of a threaded tap. To avoid crevice corrosion, a PTFE washer was inserted between the specimen and holder. 116 (2) The specimen was mounted in the test vessel and sealed with PTFE fitting at one end. The other end was closed with dummy specimen. (3) After the specimen stabilized in solution at the specified temperature, the potential is scanned from a potential 250 mV lower than corrosion potential. The employed scan rate was 1 mV/sec. The wall of the autoclave was used as the counter electrode. (4) The current flow was recorded and stored in memory automatically in the Model 350 Corrosion Measurement system and plotted as a function of applied potentials. (5) The potentiodynamic pitting potential and cathodic and anodic Tafel slopes were evaluated from the polarization curves. The corrosion current was obtained by extrapolation of the cathodic Tafel slopes to the corrosion potential. If the cathodic Tafel slope was not well defined, the anodic Tafel sloped was used to measure corrosion current. However, in some cases, neither of them was available. 3.4.5 Potentiostatic Pitting Corrosion Measurement Potentiostatic studies are more time consuming but give reliable pit initiation potential than potentiodynamic tests. Therefore, potentiostatic p itting corrosion measurements were performed in the same environment as in the polarization study. (1) The prepared specimen as shown in Figure 60(b) was connected to a PTFE coated specimen holder by means of a threaded tap. To avoid crevice corrosion, a PTFE washer was inserted between the specimen and holder. (2) The specimen was mounted in the te s t vessel and sealed with PTFE fitting at one end. The other end was closed with dummy specimen. 117 (3) After achieving stable open circuit potential at the specified temperature, a constant anodic potential was applied for 2 hours or more. The starting potential was 200 mV lower than the pitting potential measured potentiodynamically. The current response as a function of time was recorded on the X-Y recorder. (4) If the current decreased with time, the applied potential was increased in step of 50 mV. This increase was repeated till the current was seen to increase with time. Since the current increase with time was not noticeable at high temperatures, the specimen was inspected by an optical binocular(30X) for visible signs of pitting. The pitting potential was determined as that potential at which current increased with time and at which pitting was observed visually. CHAPTER IV RESULT 4.1 Slow Strain Rate Tests Slow strain rate tests on sensitized Type 304 stainless steels were conducted in solutions with varying concentrations of chloride and sulphur species to examine the effect of these species on the stress corrosion cracking susceptibility of the steel. The experimental variables were temperature, dissolved oxygen content, concentration of sulfur species and chloride ion concentration. The resu lts are shown in Figures 62 - 65 which show nominal stress vs. elongation curves in air saturated water with and without chloride ion and sulfur species at 50, 100, 150 and 200*0, respectively. The nominal stress vs. elongation curves obtained in the same solution containing 0.2 ppm Og at different temperatures are presented in Figures 66 - 68. As a reference, resu lts of slow strain rate te s ts in a ir are also sAown. All the curves show a similar shape. It is noteworthy that specimens with shorter time-to-failure have a lower fracture strength. Important experimental parameters such as time-to-failure, total elongation, elongations at maximum load, ultimate tensile stress and corrosion potential, are summarized in Table 9. To evaluate the scatter in experimental results, several tests were repeated and the data are included in Table 9. As shown in the table, most of the repeated tests show relatively good reproducibility. For example, the experiments 118 119 Strain, % 20 4 0 6 0 8 0 6 0 0 5 0 0 4 0 0 to ^ 3 0 0 Sensitized 200 3 0 4 SS 8 ppm 2 Û 100 3 T=50 20 4 0 6 0 80 100 Time, hr Figure 62. Nominal stress vs. elongation curves for sensitized type 304 stainless steel in air saturated solutions with various anions at 50*C. 1. 104 Cl + 40 S" 5. 10 S 2. 104 Cl + 10 s ' 6. 40 S“ 3. 104 Cl + 70 SgO' 7. 104 Cl" 4. Air 8. Pure Water 120 Strain, % 20 4 0 6 0 0 6 0 80 5 0 0 4 0 0 3 0 0 .5 200 Sensitized Type 3043^ 8ppm 2O 100 T ^ f O O ^ C 4 0 6 0 8 0 100 Time, hr Figure, 63. Nominal stress vs. elongation curves for sensitized type 304 stainless steel in air saturated solutions with various anions at 100*C. 1. 104 Cl" + 40 ppm S' 5. 10 ppm S 2. 104 Cl" + 10 ppm s'" 6. Air 3. 104 Cl" + 70 SgOg 7. 40 ppm S' 4. Pure Water 8. 104 ppm CT 121 Strain, % 2 0 4 0 6 0 6 0 0 Sensitized Type 304. S3 5 0 0 8ppm 2 Ü T =150^0 . 4 0 0 300 200 100 20 4 0 6 0 8 0 Time, hr Figure 64. Nominal stress vs. elongation curves for sensitized type 304 stainless steel in air saturated solutions with various anions at 150"C. 1. 104 ppm Cl + 70 ppm S^Oj 5. Air 2. 104 ppm Cl" 6. 10 ppm S" 3. 104 ppm Cl" + 10 ppm S" 7. 104 ppm Cl + 40 ppm S" 4. 40 ppm S~ 8. Pure Water 122 Strain, % 20 40 60 Sensitized Type 304S3 8 ppm 2Ü T=200*>C \ = 200 60 80 Time, hr Figure 65. Nominal stress vs. elongation curves for sensitized type 304 stainless steel in air saturated solutions with various anions a t 200°C. 1. 104 ppm Cl' 5. 104 ppm Cl + 40 ppm S' 2. Pure Water 6. Air 3. 104 ppm Cl" + 70 ppm SgO^ 7. 40 ppm S' 4. 104 ppm Cl" + 10 ppm S" 123 Strain, % 20 4 0 6 0 8 0 600 Â 300 .5 200 Sensitized Type 3 0 4 33- 0.2ppm Û2 T= 100*^0 4 0 6 0 8 0 100 Tim e, hr Figure 66. Nominal s tr e s s vs. elongation curves fo r se n sitiz e d type 304 stainless steel in the solutions containing 0.2 ppm Og with various anions at 100"C. 1. 104 ppm Cl" + 40 ppm S" 2. 104 ppm Cl" + 10 ppm S” 3. Air 104 ppm Cl“ + 5 ppm S" 4. 104 ppm Cl" 124 Strain, % 20 4 0 60 6 0 0 T ...... 1 1 Sensitized Type 304S3 5 0 0 ~ 0.2ppm Op __ S. _ 7=150^0 ^ ------4 0 0 \ ^ - 3 0 0 00I i '2^^ - o I .5 200 1 E 1 o 1 ^ 100 • — ' 1 ...... 20 4 0 6 0 8 0 Time, hr Figure 67. Nominal s tre s s vs. elongation curves for sen sitiz e d type 304 stainless steel in the solutions containing 0.2 ppm Og with Various anions at 150“C. 1. 104 ppm Cl" 3. Air 2. 104 ppm Cl + 10 ppm S 4. 104 ppm Cl” + 40 ppm S" 125 Strain, % 4 0 6 0 6 0 0 Sensitized Type 3 0 4 S3 5 0 0 0.2 ppm 2O T=2 0 0 ^ 0 4 0 0 300 c 200 100 20 4 0 6 0 80 Time, hr Figure 68. Nominal stress vs. elongation curves for sensitized type 304 stainless steel in the solutions containing 0.2 ppm Og with various anions at 200°C. 1. 104 ppm Cl" 2. 104 ppm Cl" + 10 ppm S" 3. Air 126 performed in the solution containing 8 ppm and 104 ppm Cl" a t 200*0 were repeated four times. The times-to-failure were 8.2, 9.5, 9.2 and 6.0 hours. The average value was 8.225 and the standard deviation(S) 1.584. Times-to-failure of these specimens fall within +20% of the average value except one, which has a 30* error. Most of the other repeated results show errors less than 5*. However, all experiments were not repeated because of the long duration of each test. Several parameters can be used to assess the suscep tib ility of stainless steel to stress corrosion cracking. They are: (1) Time-to-failure(t^) (2) Ratio of the time-to-failure in the corrosive environment to the time-to-failure in air - normalized time-to-failure(Ryyp) (3) Ratio of the ultimate tensile stress in the corrosive environment to that in air(Ryjj) (4) Ratio of the final cross sectional area to the initial value <"a > (5) Ratio of fracture areaiR^^^): Ratio of see area to final total cross sectional area (1 + en) *^UTS where e^ : strain at maximum load in air e^: strain at maximum load in corrosive environment ^UTS* tensile stress in air ultimate tensile stress in the corrosive environment 127 ^ ||Tc( ^ " Gf) (7) Ips = 1 - ’---STF” iîF. [26] e®^*": total strain in air These parameters provide a useful means of comparing and ranking various environments. A higher susceptibility to SCC is reflected as a 1. decrease in t^ 2. decrease in 3. decrease in R^yg 4. increase in R^ 5. decrease in R^^^ 6. increase in 1^^^ (88) 7. increase in Ipg (21) In this study , ty and Rjjp are employed as SCC parameters. These values are compiled in Table 9. Regardless of which parameter is used, the trend showing the suceptibility to SCC is the same. 4.1.1 Slow Strain Rate Test in Air To obtain baseline or reference data, slow strain rate tests were performed in air at temperatures ranging from 50 to 200*C. These results are also shown in Table 9. Time-to-failure decreases as temperature increases, being 93, 83, 66 and 63 hours at temperatures of 50, 100, 150 and 200"C, respectively. The fracture surfaces of these specimens are shown in Figures 69 - 72; they reveal ductile fracture. The fractograph at 50*C shows large cracks along the diameter of the fracture surface at low magnification. However, the predominant feature 128 Table 9. Experimental Results of SSRT for Sensitized Type 304 Stainless Steel. Temp Solution(ppm) ^UTS ®f t f Cc t f ^UTS "n ^scc CO “2 Cl S" S^Og" (MPa){*)(hr) (mVy) t^ (a ir) (*) (%) (%) 50 Air 548 74 93.1 100 100 62 8 104 571 78 98.6 32 106 104 67 -0.07 8 104 10 501 38 48.6 2 52 91 35 0.31 8 104 40 482 31 39 -8 42. 88 28 0.44 8 571 84 106.5 220 114 104 71 -0.09 8 10 592 78 98.4 132 106 108 71 -0.12 8 40 591' 80 101.3 2 109 108 71 - 0.12 8 104 70 539 51 64.4 112 69 98 50 0.1 100 Air 501 66 83.4 53 8 104 497 69 87 141 104 99 57 - 0.02 .8 104 5 504 64 81.2 121 97 100.6 57 -0.03 8 104 10 279 18 23.1 135 28 56 16 1.36 8 104 40 299 15 19.1 11 23 60 12 1.29 8 104 40 361 20 25.5 -19 31 72 17 0.81 8 496 60 75.9 71 91 99 50 0.03 8 10 499 62 78.2 161 94 100 48 0.04 8 10 498 60 75.2 111 90 99 48 0.04 8 40 497 67 84.5 21 101 99 55 0.001 8 104 70 468 44 55.2 51 66 93 40 0.17 129 Table 9. (continued) Temp. Solution(ppm) ^ T S Ec ^UTS % ^scc CO °2 Cl s" (MPa)(%)(hr) (mV^) t^ (a ir) (%) (*) (*) 100 0.2 104 511 66 83.5 -105 100 102 56 -0.04 0.2 104 481 71 89.8 51 108 96 57 0.015 0.2 104 5 502 65 82.5 -29 99 100 56 - 0.02 0.2 104 10 493 67 84.1 -349 101 98 56 0 0.2 104 10 492 68 85.8 -279 103 98 55 0.005 0.2 104 40 486 66 83.7 -369 100 97 55 0.02 0.2 10 494 70 88.1 -509 106 98 55 0.001 150 Air 474 53 66.4 42 8 104 442 48 61.1 36 92 93 40 0.09 8 104 10 454 50 63.7 -122 96 96 43 0.04 104 40 476 55 69.3 -262 104 100 46 -0.03 8 466 55 69.7 204 105 98 44 0.003 8 10 465 54 67.6 93 102 98 44 0.005 8 40 457 51 64.3 -442 97 96 40 0.05 8 40 455 52 66.0 -392 99 96 42 0.04 8 40 ' 472 54 68.8 -502 104 100 44 0.01 8 104 70 358 21 26.4 -2 40 76 18 0.59 130 Table 9. (continued) Temp. Solution(ppm) ^UTS t f Ec t f ^UTS ®n ^scc CO Cl °2 s" S2O3" (MPa)(%)(hr) (mV^) (*) (%) (%) 150 0.2 104 460 44 55.4 -122 83 97 42 0.03 0.2 104 10 472 55 69.5 -622 105 100 43 -0.003 0.2 104 10 467 51 64.6 -662 97 99 46 0.013 0.2 104 40 468 56 71.3 -627 107 99 43 0.006 200 Air 475 50 62.6 40 8 104 188 6 8.2 -20 13 40 6 2.34 8 104 173 8 9.5 4 15 36 4 2.68 8 104 209 8 9.2 -20 15 44 5 2.03 8 104 233 5 6.0 0 10 49 3 1.77 8 104 10 405 27 34.5 -350 54 85 26 0.303 ,8 104 40 451 40 50.9 -380 80 95 39 0.06 8 335 15 19 196 30 71 12 0.77 8 40 474 36 46 -400 73 100 43 0.02 8 104 70 257 16 20 -130 32 54 12 1.31 8 104 120 SO4 213 8 10.2 110 16 45 6 1.94 0.2 104 460 44 56 -315 88 97 39 0.04 0.2 104 10 472 56 70.7 -570 112 99 47 -0.04 250 8 309 19 23.5 135 8 295 18 22.5 160 131 a) Figure 69. SEN fractograph of sensitized Type 304 stainless steel tested in air at 50®C. 132 I Figure 70. SEN fractograph of sensitized Type 304 stainless steel tested in air at 100*C. 133 Figure 71. SEN fractograph of sensitized Type 304 stainless steel tested in air at 150®C. 134 ■¥. 'W ,:' ■‘ 1 'V' > .* y Figure 72. SEN fractograph of sensitized Type 304 stainless steel tested in air at 200®C. 135 Table 10. Fracture Morpholygy of Sensitized Type 304 Stainless Steel Temp. Solution(ppm) Fracture Remarks ★ CO Og Cl S“ SgOg morphology 50 Air 8 104 D 8 104 10 0.7 IG 8 104 40 IG 8 D 8 10 D 8 40 D 8 104 70 0.7 IG numerous secondary cracks 100 Air D .8 104 D 8 104 5 0.1 IG 8 104 10 0.8 IG numerous secondary cracks 8 104 40 0.95 IG several big cracks and pits 8 D 8 10 D 8 40 D 8 104 70 mixed IG and TG 136 Table 10. (continued) Temp. Solution(ppm) Fracture Remarks * ( " 0 Og Cl S" S2O3 morphology 100 0.2 104 D 0.2 104 5 D 0.2 104 10 D 0.2 104 40 D 0.2 10 D 150 Air 8 104 0.7 grain boundary separation 8 104 10 0.5 grain boundary separation 8 104 40 0.1 grain boundary separation 8 D 8 10 D 8 40 0 8 104 70 IG one big secondary crack and several small secondary cracks 0.2 104 0.9 IG numerous shallow secondary cracks 0.2 104 10 D 0.2 104 40 D 137 Table 10. (continued) Temp. Solution(ppm) Fracture Remarks * CO «2 Cl S" SgOg morphology 200 Air 8 104 IG several secondary cracks 8 104 10 0.9 IG numerous secondary cracks 8 104 40 0.7 IG numerous shallow secondary cracks 8 0.7 IG big secondary cracks 8 40 0.7 IG 8 104 70 0.95 IG numerous secondary cracks 8 104 120 SO4 IG several secondary cracks 0.2 104 0.2 IG numerous secondary cracks 0.2 104 10 D 250 8 0.9 IG large secondary cracks * digit indicates the ratio of area of intergranular stress corrosion cracking over total area of fracture surface IG; intergranular stress corrosion cracking TG: transgranular stress corrosion cracking 1 3 8 in the fracture surface is the failure by microcoalescence of voids in the grains as shown in Figure 69(b). Similar fracture morphology has been reported by Chen et al.(91) for 304 stainless steel tested by the slow strain rate technique in glycerine. 4.1.2 Slow Strain Rate Tests in Air-Saturated Solution Containing 104 ppm Cl" Sensitized Type 304 stainless steel was tested in air-saturated solutions containing 104 ppm chloride at temperatures ranging from 50 to 200*C. The time-to-failure decreased with increasing temperature. They are 99, 87, 61 and 8 hours at 50, 100, 150 and 200'C, respectively. 4.1.3 Slow Strain Rate Tests in Air-Saturated Solutions Containing 104 ppm Cl" and Different Concentration of Sulfide The variation of time-to-failure as a function of sulfide ion concentration is shown in Figure 73, which also includes the results of the test in air for comparison. By using as the parameter. Figure 73 was replotted as Figure 74. From both figures it is seen that susceptibility to SCC can be presented equally well by the time-to-failure or by 1^^^ as the index. The effect of sulfide ion on the SCC susceptibility was strongly dependent on the temperature of the solution. At temperatures below 100*C, the addition of sulfide accelerated SCC, but inhibited it at temperatures above 150'C. At 50*C, time-to-fail lire decreased from 99 hours to 39 hours as the sulfide ion concentration was increased from 0 to 40 ppm. Fractographs of specimens tested in air-saturated 104 ppm chloride solutions 139 Sulfur Concentration, x 10"^M 0 0 .5 1.0 1.25 T Sensitized Type 3043S -(^pm0g+i04ppmCi’‘ osqoc - 8 0 \ o ioo®c \ A I50®C \ □ 200®C £ ' w E 10 20 3 0 Sulfide Concentration, ppm Figure 73. Time-to-failure vs. sulfide ion concentration in air-saturated 104 ppm Cl" solution at various temperatures. 140 Sulfur Concentration,X 10'®M 0 0 .5 1.0 1.25 g Sensitized Type 304 8ppm Og +i04ppmCi' 2.0 0 5 0 “C O 100®C A I50®C □ 200°C H 1.0 0 i i 0 10 20 3 0 4 0 Sulfide Concentration, ppm Figure 74. vs. sulfide ion concentration in air-saturated 104 ppm Cl~ solution at various temperatures. 141 containing various concentrations of sulfide ion are shown in Figures 75 - 77. In Figure 75, the fracture surface shows an apparent intergranular feature over 30* of fracture surface at low magnification. However, a higher magnification in Figure 75(b) revealed predominantly ductile fracture. Fractographs in Figure 76 and 77 show the increasing tendency to IGSCC as the concentration of sulfide increases. Figure 76 also shows a dimple with grain separation(Figure(b)) but typical intergranular cracking in Figure(c) over 40* of fracture surface. A sharp transition is discernible between these two fracture morphologies in Figure 76(d), where corrosion products are present. In Figure 77, intergranular cracking is predominant over the fracture surface, but no secondary cracks were found on the side of the gage section. At 100*C, sulfide ions show a more pronounced effect on IGSCC susceptibility than at 50°C. Time-to-failure decreased from 87 to 19 hours, as the sulfide ion concentration was increased from 0 to 40 ppm. Figures 78 - 81 show the transition in fracture morphology from ductile to completely intergranular, with increasing sulfide concentration. In the solution containing 5 ppm S", 10* of the fracture surface shows IGSCC as in Figures 79(a) and(b). Samples tested in 10 ppm sulfide solution showed IGSCC over 80 * of the fracture surface(Figure 80(b)). The central part showed ductile features(Figure 80(c)) whereas, on the side surface, numerous secondary cracks were developed. In Figure 81(a),(b) and(c) corresponding to specimens tested in 40 ppm S" solutions, the whole area was dominated by IGSCC. Moreover, big pits are seen along the secondary intergranular cracks, as shown in Figure 81(d). Pronounced intergranular corrosion was observed inside the pit 400uni a) b) Figure 75. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution at 50®C. ro a) b) Figure 76. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution containing 10 ppm S at 50*0. c) d) Figure 76. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution containing 10 ppm S” at 50*0. a) b) Figure 77. SEN fractograph of sensitized Type 304 stainless steel tested In air-saturated 104 ppm Cl" solution containing 40 ppm S” at 50*C. -P» a) b) Figure 78. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution at 100*C. -pfc a% 1 1mm 3000 7 2.5K 100urn a) b) Figure 79. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 5 ppm S° 100*0. 500pm a) b) Figure 80. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 10 ppm S“ at 100®C. c) d) Figure 80. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution containing 10 ppm S at 100®C. kO SOOum a) b) c) Figure 81. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 40 ppm S" at 100®C, tn o m d) el Figure 81, SEM fractogreph of sensitized Type 304 stainless steel tested In alr-aaturated 104 ppm Cl" solution containing 40 ppm S" at 100*C. 152 at high magnification(Figure 81(e)). At 150*C, the addition of sulfide slightly inhibits SCC. The time-to-failure increases from 61 to 70 hours when the amount of sulfide ion increases from 0 to 40 ppm. Fractographs(Figures 82 - 84) are also consistent with the observed times-to-failure. I.e., the proportion of IGSCC area decreases with the increase in sulfide ion concentration. In Figure 82(a), intergranular corrosion is observed over 80* of the surface. As the sulfide concentration increases from 10 to 40 ppm, the proportion of IGSCC area decreases from 50 to 20*, as shown in Figure 83(a) and 84(a). High magnification fractograghs of the IGSCC area in Figures 82(b), 83(b) and 84(b) reveal that dimples are present on the intergranularly separated grain surfaces. Similar fracture morphologies have been reported by Bruemmer e t a l . (92) in the borated solutions containing 15 ppm chloride at 32°C in slow strain rate tests. Interestingly, typical IGSCC was reported when the specimen was tested with constant load in the same environment. It was considered that the intergranular path of fracture was a secondary path which formed after individual grains deformed and fractured by a coalescence of microvoids. The secondary intergranular fracture surfaces probably formed from the stress and strain states resulting from plastic instability(92). However, the occurence of IGSCC in the constant load test clearly indicates that this structure is related to IGSCC. It can be concluded th a t th is morphology appears only in the marginally aggressive environments combined with slow strain rate te s t. Further increase of temperature to 200*C showed remarkable inhibiting effects of sulfide ions. The time-to-failure increased from I 400um a) b) Figure 82. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl“ solution at 150*0. wcn I 1 400%m a) b) Figure 83. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 10 ppm S" at 150*C. 4kcn I------1 400um 20ym a) b) Figure 84. SEM fractofraph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm 01” solution containing 40 ppm 3° at 150*0, C Jl tn 1 5 6 9 to 51 hours when 40 ppm of sulfide ion was added. This corresponds to an increase of 5.5 times as compared to the time-to-failure of the specimen tested without any sulfide. Figures 85 - 87 show the SEM fractographs of specimens tested in 104 ppm Cl” + 8 ppm Og with 0, 10 and 40 ppm S", respectively. The fracture surface of the sample tested without any sulfide shows pure IGSCC in Figure 85(a) and(b). Several secondary cracks are also seen in Figure 85(c). The area of IGSCC on fracture surface decreased gradually from 100 to 70% when sulfide ion was increased up to 40 ppm as shown in Figures 86 and 87. The addition of sulfide also changed the distribution of secondary cracks,(Figures 85(c), 86(c) and 87(c)). The morphology changes from a few large cracks to numerous shallow cracks on the addition of sulfide ions to the solution. A large amount of white corrosion product is also seen in Figure 87. This corrosion product contained a large concentration of sulphur as determined by EDS analysis - Table 11. The inhibiting effect of sulfide above 150"C agrees well with the results of Cragnolino and Smialowska(48), where the addition of sulfide to 0.01 M NagSO^ solution at 250*C increased the time-to-failure by a factor of two. The simultaneous presence of chloride and sulfide with 8 ppm Og dramatically changed the SCC susceptibility with changing temperature. It was considered necessary to examine the effect of sulfide anions alone on the SCC susceptibility. For this purpose, a series of experiments were performed as follows; Firstly, the SSRT was done in air-saturated solution containing sulfide without any chloride ions. bOQum a) b) c) Figure 85. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution at 200*C. oi 500ym a) b) c) Figure 86. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution containing 10 ppm S” at 200*C, CJl 00 oOOym a) b) c) Figure 87. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl' solution containing 40 ppm S" at 200°C. cn 160 Table 11. EDS Analysis of the Corrosion Product on the Specimen Tested in 8 ppm Og + 104 ppm Cl’ + 40 ppm S^ at 200*0. element * wt % S 2.19 01 0.25 Or 11.38 Fe 79.67 Ni 3.27 Ou 3.25 161 Then, similar experiments were designed in 104 ppm Cl" solutions with controlled sulfide concentration but with the dissolved oxygen concentration reduced to 0.2 ppm. Finally, experiments with sulfide ion substituted by thiosulfate ions were conducted. 4.1.4 Effect of Sulfide Ion in Air-Saturated Solution without Chloride Ion The variation in time-to-failure as a function of sulfide ion concentration is shown in Figure 88. Compared to the time-to-failure values obtained in a ir, time-to-failure in the sulfide solutions in the absence of chloride was almost the same from 50 to 150*C. At 50°C, the times-to-failure were 106, 98 and 101 hours as the sulfide concentration increased up to 40 ppm by addition of NagS. Similarly, at lOO'C and 150"C, the times-to-failure were in the range of 76 - 85 and 70 - 64 hours, respectively, when 40 ppm of sulfide was added. The fracture surfaces of the specimens tested under these experimental conditions show ductile failure. At 200*C, however, the 304 Type stainless steel was susceptible to SCC in air-saturated pure water with a time-to-failure of 17 hours. When 40 ppm S“ was added, the time-to-failure was increased from 17 to 46 hours. Figure 89(a) shows the fractograph of the specimen tested in 8 ppm Og pure water at 200*C, which exhibits 70% of IGSCC over the fracture surface. A few large secondary cracks on the side surface are also seen in Figure 89(b). Figure 90 shows the fractograph of the specimen tested in the presence of 40 ppm S", which also reveals 70% IGSCC. 162 Sulfur Concentration, xlO“^M 0.5 1.0 1.25 [^s/t/zed Type304SS^ppm Og 100 8 0 tn 6 0 2 c 0 is 5 0 4 0 O 5C)°C o ioo°c ■ 2 0 A I50®C □ 200X 1 1 L 10 2 0 3 0 4 0 Sulfide Concentration, ppm Figure 88. Time-to-failure vs. sulfide Ion concentration In air-saturated water at various temperatures. a) b) Figure 89. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated pure water at 200*C. S M » 1 00um a) b) Figure 90. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated solution containing 40 ppm S” at 200*C. or 165 Based on these findings, it can be concluded that sulfide, in the presence of 8 ppm oxygen, has no influence on failure mode and time-to-failure at the temperatures below lOO'C, but it does inhibit SCC at 200*C. 4.1.5 Effect of Sulfide Ion in the Solution Containing 0.2 ppm Og and 104 ppm Cl. Figure 91 shows time-to-failure as a function of sulfide concentration at temperatures ranging from 100 to 200*C in the solution containing 104 ppm Cl" and 0.2 ppm Og. At 100"C, time-to-failure did not change with the addition of sulfide. Fractographs of those specimens show complete ductile failure. At 150“C, sensitized Type 304 stainless steel is susceptible to SCC in water with 104 ppm Cl" containg 0.2 ppm Og. As in the experiment in air saturated solution, the addition of 10 ppm sulfide ion inhibits SCC, i.e., time-to-failure increases from 55 to 70. A further increase to 40 ppm in sulfide concentration did not show considerable difference in time-to-failure. Figure 92 shows the fracture surface tested in 0.2 ppm Og + 104 ppm Cl solution at 150"C. The fracture surface shows predominantly intergranular fracture. Numerous secondary cracks are present on the side surface. The specimen tested in the presence of sulfide ion in the same solution as above and at the same temperature showed completely ductile failure. At 200"C, sensitized Type 304 stainless steel showed IGSCC in the solution containing 0.2 ppm Og + 104 ppm Cl". The presence of sulfide ion inhibits SCC. The time-to-failure increased from 56 to 71 hours with the addition of 10 ppm sulfide. Figure 93 show the fractograph of the specimen tested in the solution 166 Sulfur Concentration, x 10"® M 0 0 .5 1.0 1.25 Sensitized Type 304S3 100 - 0.2ppm Of + i04ppm Ci~ 8 0 # o — 6 0 c o 4 0 I s Air O IOO®C A 150% 2 0 □ 200% 1 10 2 0 3 0 4 0 Sulfide Concentration ,ppm Figure 91. Time-to-failure vs. sulfide ion concentration in 0.2 ppm 0 + 104 ppm Cl" solution at various temperatures. Figure 92. SEM fractograph of sensitized Type 304 stainless steel tested in 0.2 ppm Og + 104 ppm Cl” solution at 150®C. a \ Figure 93, SEM fractograph of sensitized Type 304 stainless steel tested in 0.2 ppm Og + 104 ppm Cl" solution at 200*0, a% 00 169 containing 0.2 ppm Og and 104 Cl” at 200*0, which exhibits 20 % of IGSCC over the fracture surface with numerous shallow secondary cracks on the side surface. 4.1.6 Effect of Thiosulfate Ion In the Solution Containing 8 ppm Og and 104 ppm Cl" To relate the effect of sulfide Ion on SCC susceptibility with that of thiosulfate Ion, 70 ppm of thiosulfate Ion was added In the air-saturated 104 ppm Cl" solution. This corresponds to the same sulfur concentration as In 40 ppm S“(sulfur concentration of 1.25 x lO"^ M). Figure 94 shows the change of time-to-failure with thiosulfate additions to the air saturated solution containing 104 ppm Cl" at temperature ranging from 50 to 200*C. In the temperature range from 50 to 150*C, thiosulfate Ions accelerated IGSCC, but Inhibited It at 200*C. At 50*C, time-to-failure decreased from 99 to 64 hours as 70 ppm thiosulfate was added. At 100"C, It decreased from 87 to 55 hours and at 150*C It decreased from 61 to 26 hours. Interestingly, the magnitude of the decrease In time-to-failure with the addition of thiosulfate Ion appeared to be almost the same at these three temperatures. I.e., a decrease of around 35 hours. Figures 95 - 97 show the fractographs of the specimens tested In the presence of thiosulfate Ion at temperatures between 50 to 150“C. The fractograph of the specimen tested at 50*C shows 70* IGSCC over the fracture surface and numerous shallow secondary cracks(F1gure 95). At the higher testing temperature of 100*C, the fractograph!Figure 96) shows largely transgranular features along with Intergranular cracking. The fractograph!Figure 97) of the specimen tested at 150"C shows 170 Sulfur Concentration, x 10“’ M 0 0 .5 1.0 1.25 Sensitized Type 304S3 _ ^ pm +i04ppm Ci‘ <> sqoc 8 0 O IOO°C (O L. A I50°C JC □ 200X 6 0 2? c o £ 4 0 I if Air 2 0 “Q 0 7 0 Ttiiosulfate Concentration, ppm Figure 94. Time-to-failure vs. thiosulfate ion concentration in air-saturated 104 ppm 01“ solution. m Figure 95, SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOg" at 50*0. m a) b) Figure 96, SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 70 ppm SgOg at 100*C. ro Figure 97. SEN fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl’ solution containing 70 ppm SgO^* at 150*C, w Figure 98. SEM fractograph of sensitized Type 304 stainless steel tested in air-saturated 104 ppm Cl” solution containing 70 ppm SgOg” at 200°C. 175 predominantly intergranular morphology with a large secondary crack on the side surface. At 200*C, the addition of thiosulfate inhibits SCC as in the case of the solution containing sulfide ion. The addition of 70 ppm thiosulfate increased the time-to-failure from 9 to 20 hours. Figure 98 shows the fractograph examined by SEM. The predominant fracture morphology is intergranular cracking and numerous secondary cracks are observed all over the side surface. In addition, the effect 120 ppm of sulfate ion on SCC susceptibility was investigated at 200*C. The results are shown in Table 9. The time-to-failure was 10.2 hours, which indicates a slightly inhibiting or negligible effect of sulfate on SCC in air saturated chloride solution at 200"C. Figure 99(a) and(b) shows the fractrue surface at low and high magnification, revealing typical intergranular cracking . Several secondary cracks are seen in Figure 99(c). 4.2 Anodic Polarization Studies To study the effect of sulfur species on pitting corrosion of Type 304 stainless steel, anodic polarization studies were conducted in the deaerated chloride solution containing sulfide and thiosulfate ion at temperatures ranging from 50 to 200*C. For comparison purposes, the polarization behavior of sensitized Type 304 stainless steel was investigated at temperatures ranging from SO to 200*C in the deaerated neutral solution containing 104 ppm Cl”. The results of these measurements are shown in Figure 100. They show that the corrosion potential decreases with increasing temperature. Figure 99. SEM fractograph of sensitized 304 stainless steel tested in air-saturated 104 ppm Cl" solution containing 120 ppm S0^~ at 200*C, 177 With increase in potential from the corrosion potential, all polarization curves exhibit a large passive region, the width of which reached 400 - 600 mV in magnitude. Further increase in potential shows an abrupt increase in current , which is due to the breakdown of the passive film. Numerous pits were found all over the specimen after the polarization experiments. Potentials at which current significantly increases were denoted Ep^. The pit initiation potentials at each temperature are presented in Table 12, which also includes the corrosion potentials and polarization parameters such as anodic and cathodic Tafel slopes. The pitting potentials are 450, 286, 105 and 80 mVy at 50, 100, 150 and 200“C, respectively. It is noteworthy that the decrease of pitting potential(Ep^j) from 150"C to 200*C was only 25 mV, compared to a 170 mV decrease from 50 to lOO'C. The measured corrosion currents!i^) tends to increase with increasing temperature, from 0.2 pA/cm at 50"C to 3.16 pA/cm^ at 200*C. The cathodic Tafel slope was 0.045 V/Decade at 50"C, and increased to 0.74 V/Decade at 150"C and 200*C. The anodic Tafel slope was larger than the cathodic Tafel slope. To study the effect of sulfide ion on the polarization behavior, similar experiments were carried out with the addition of 40 ppm S", which result in alkaline pH's ranging from 10.8 to 11.4 at room temperature. All other experimental conditions were the same as in the previous polarization study. The polarization curves are shown in Figure 101. After the active-passive transition, a secondary peak was developed with a high current followed by breakdown of the passive film. It is clearly shown that passive current and corrosion current increased as temperature increased. The corrosion current increased by roughly 178 ICX)°C Sensitized Type 304S3 200® C _ Deaerafed i0 4 ppm c r -10 -0.5 05 Potential, V, Figure 100. Polarization curves of sensitized Type 304 stainless steel tested in deaerated 104 ppm Cl" solution at various temperatures. 179 Table 12. Tables of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*C to 200*0 in the Deaerated Solution Containing 104 ppm Chloride. 50*C lOO'C 150*C 200"C Eg(mV) -166 -347 -404 -440 Epd(mV) 450 286 105 80 Eps(mV) 362-412 261-311 58-158 50-100 ig(pA/cmf) 0.201 1.13 3.16 Bj.(V/Dec) 0.045 0.074 0.074 BjiV/Dec) 0.110 0.100 * : Corrosion Potential c * ^pd * Initiation Potential measured in Polarization Curve * E : Pit Initiation Potential measured Potentiostatically * ig : Corrosion Current * Bg : Cathodic Tafel Slope * gg : Anodic Tafel Slope 180 Sensitized Type 304 S3 Deaerated 2 0 0 ° ^ IOO®C _ 104 ppm Ci’+ 4 0 ppm ' S ***** i n - 1.0 -0.5 0.5 Potential, V, Figure 101. Polarization curves of sensitized Type 304 stainless steel tested in deaerated 104 ppm Cl + 40 ppm S solution at various temperatures. 181 Table 13. Tables of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*0 to 200*0 in the Deaerated 104 ppm Chloride Solution with 40 ppm Sulfide. 50*C lOO'C 150'C 200"C Eg(mV) -308 -673 -725 -777 Epd(mV) 377 310 216 216 Eps(mV) 362-412 211-261 158-258 150-250 igfpA/cmf) 0.444 2.762 3.621 gg(V/Dec) 0.042 0.084 gg(V/Dec) 0.124 0.116 * Eg : Corrosion Potential * Epj : Pit Initiation Potential measured in Polarization Curve * ^ps * Initiation Potential measured Potentiostatically * ig : Corrosion Current * gg : Cathodic Tafel Slope * gg : Anodic Tafel Slope 182 one order of magnitude when temperature was increased from 50 to 200*C. It should be noted that the magnitude of corrosion current(ig) in the presence of sulfide was larger than those obtained in the absence of sulfide. As given in Table 13, the corrosion current was more than two times greater than that in the solution without sulfide ion at 50 and 150"C. At 200*C, the corrosion current was higher even though the difference is not so significant as at temperatures below 150*C. On the other hand, the corrosion potential and pitting potential decreased with increasing temperature. The measured pitting potentials were 377, 310 and 216 mV^ at 50, 100 and 150*C, respectively. Above 150"C, a limiting value was reached, similar to those tested in the solution without sulfide. Polarization curves were also obtained in deaerated 104 ppm Cl~ solution with 70 ppm thiosulfate ion addition. The concentration of sulphur in the 70 ppm thiosulfate solution is equivalent to that in 40 ppm sulfide in terms of total sulfur concentration!1.25 x 10"^ m). The pH of the thiosulfate solution at room temperature was between 7.0 - 7.5. ' The obtained curves are shown in Figure 102. At 50 and lOO'C, a passive region is present and the current increase at the breakdown potential is remarkable. However, at 150*C and 200*C, the passive region is not clear and the breakdown potential can not be precisely determined. Numerous pits were also found all over the specimen. The measured pitting potential and polarization parameters are shown in Table 14. The pitting potentials obtained from the polarization curve are 478, 261, 84 and 395 mVy at 50, 100, 150 and 183 Senitized Type 304 SS _ Deaerated t04ppm C r+ 70ppm 200® Q -10 - 0 5 0.5 Potential, V. Figure 102. Polarization curves of sensitized Type 304 stainless steel tested in deaerated 104 ppm Cl~ + 70 ppm solution at various temperatures. 184 Table 14. Tables of Corrosion Potential, Pitting Potential and Polarization Parameters at Temperature from 50*0 to 200*0 In the Deaerated 104 ppm Chloride Solution with 70 ppm Thiosulfate. 50*C lOO'C 150*0 200*C Eg(mV) -86 -181 -214 -292 478 261 84 395 Ep^imV) 212-262 111-161 8-58 100-150 ig(pA/cnf) 0.155 2.445 6.225 3.16 Bg{V/Dec) 0.158 0.079 0.147 Bj(V/Oec) 0.137 0.118 0.124 * Eg : Corrosion Potential * Epj : P it In itia tio n Potential measured in Polarization Curve * Epj : P it In itia tio n Potential measured P oten tio statically * ig : Corrosion Current * 6g : Cathodic Tafel Slope * : Anodic Tafel Slope 185 200*C, respectively. An interesting finding is the fact that the pitting potential decreased gradually with increasing temperature till 150"C, but subsequently increased with increasing temperature from 150'C to 2G0'C. At the same time, corrosion current increase till 150'C, but decreased at higher temperture. Compared to the value in the sulfide solution, a lower corrosion current was obtained in the studied temperature range except at 150'C. At 150'C, the corrosion current was more than two times larger than that in the sulfide solution. 4.3 Pitting Potential Studies from the Potential-Time Curves Since the pitting potentials can not always be precisely evaluated from polarization curves, pitting potential studies from potential-time curves at different constant potentials were performed to obtain more reliable data. The current-time curves from specimens tested in deaerated 104 ppm Cl" solution a t temperature from 50 to 200'C are shown in Figure 103 - 106. In Figure 103, current decreased with time at potentials below 362 mVy, whereas a considerable increase in current was observed at 412 mVy. Therefore, the pitting potential was within the potential range of 362 - 412 mVj^ at 50'C. At high temperatures, above lOO'C, significant current-increase with time is not observed. However, when pitting occurs, the current was higher than in the case when it did not . Hence, after each measurement, the specimen was inspected by means of an optical binocular at 30 X for further confirmation. The pitting potentials so determined were between 261 - 311, 58 - 158 and 50 - 100 mV^ at 100, 150 and 200'C, respectively. This indicates that pitting potentials decrease with TT «I Sensitized Type 304S3 E Deaerated 104 ppm Ci * < Q 0 2 r T ^ S O ^ C E / u > c a> 0 0.01 r 1w 4l2mVH 3 o 262m VH 362mVH l62mVH Q =b 3 4 5 8 Time, xlO* sec Figure 103. Current-time curves tested in deaerated 104 ppm Cl" solution a t 50*0. COa> I ~ I I I Sensitized Type 304 S3 M _ Deaerated i04ppm Ct' I 0.4 < k T^iOO^C Sllm Vy E (/) c O 261 mVn "5 0.2 7 2 llmVH 2k_ ./ 3 O ISImVH / eim V n ± 3 4 5 8 Time, x 10* sec Figure 104. Current-time curves tested in deaerated 104 ppm Cl” solution a t lOO'C. CO ISSmVn I5 8 m \^ Sensitized Type304S3 Deaerafed i04ppm Ci' 3 O T=i50<’C 58mVy J ' i f 1 i I Z L i 3 4 5 8 Time, xIO® sec Figure 105. Current-time curves tested in deaerated 104 ppm CT solution 00 at 150'C. 00 T r 1.0 «1 Sensitized Type 3 0 4 S 3 E Deaerated i04ppm Ci" < 7^200 V E (/> c 0) 0.5 o c *.-50mVH 0) lOOmVH o OmVH * r y > - 5 cSOmVn - - {7! 0 3 0 3 4 5 8 Time, xlO® sec Figure 106. Current-time curves tested in deaerated 104 ppm Cl" solution a t 200*0. \o0 0 190 increasing temperature, but reach a limiting value at temperatures above 150"C. Pit initiation potentials measured potentiodynamically(Epj) and potentiostatically(Epg) in chloride solution are compared in Table 12 and in Figure 107. The values of Ep^ agree very well with Ep^ values except at 50"C, where the deviation is about 50 mV. The corrosion potential in deaerated chloride solution also decreased from -166 mV to -440 mV as the temperature increased from 50 to 200*C. However, i t should be noted that E^ a t 50"C was very sensitive to the dissolved oxygen content in the solution. In this experiment, the variation in E^ in deaerated chloride solution was -18 mV^ — 288 mVH at 50“C, while dissolved oxygen was below 10 ppb. According to the Figure 12, E^ mainly changed a t the oxygen content below 10 ppb a t low temperature. At 100°C, E^ decreased by 300 mV when oxygen content decreased from 10 ppb to 5 ppb, while it was almost the same at 200"C in the same oxygen range. This indicates that the changes in Eg below 10 ppb Og become greater as the temperature decreases. Hence, the change in Eg below 10 ppb Og a t 50“C is greater than or almost same as that at 100*C. This large variation in Eg makes it difficult to compare the corrosion potential in different solutions with and without thiosulfate because of the severe effects of Og level variation on Eg in the different solutions. A comparison of corrosion potential was therefore made by measuring Eg before and after adding thiosulfate ions to a chloride solution. This eliminated the effect of oxygen level on Eg. The addition of thiosulfate results in an increase of Eg by 80 mV, therefore. Eg in the solution containing 104 ppm Cl” + 70 ppm SgOg^” solution a t 50*0 was approximated by adding 80 mV to the 1 9 1 5 0 0 -5 0 0 Sensitized Type 304 S3 Deaerated 104ppm Ci' -1000 50 100 150 200 Temp, *C Figure 107. Corrosion potential, E and E . values obtained in ps po deaerated 104 ppm Cl” solution. 192 corrosion potential in the solution containing 104 ppm Cl" alone. The pitting potentials of Type 304 stainless steel were also evaluated in the presence of sulfide and thiosulfate, but their current-time curves are not presented as they have similar shapes as those obtained in chloride solution. Figure 108 shows the pitting potentials and corrosion potentials obtained in deaerated 104 ppm 01 + 40 ppm S" solution. A relatively good correspondence was found between the two p ittin g potentials, i . e . , those evaluated from anodic polarization curves and those from potential-time curves. The potentials decreased with increasing temperature. It should be noted that the rate of decrease is slower in the presence of sulfide. Compared to the 104 ppm Cl" solution, the corrosion potentials in the presence of 40 ppm S“ show a lower value, ranging from -308 to -777 mV^ at temperatures from 50 to 200*C. The pH of the solution at high temperature was calculated from the dissociation constant of water. The values for the dissociation constant of water are shown in Table 17. Tfie pit nucléation potentials obtained from potential-time curves in the presence of thiosulfate as a function of temperature are shown in Table 14 and Figure 109. The pitting potentials obtained from anodic polarization curves show significant deviation from those obtained po ten tio statically . The maximum deviation was 250 mV a t 50 and 200"C. At 150'C, they showed almost the same value. However, the temperature dependence of pitting potentials are comparable inspite of the different experimental techniques employed. P itting potentials decreased from 230 to 30 mV^ as the temperature increased from 50 to 150'C, but further 1 9 3 500 Sensitized Type 304 SS Deaerated i04ppmCr+40ppm S ' -5 0 0 • • -1000 5 0 100 50 200 Temp, *C figure 108. Corrosion potential, Ep^ and -pd values obtained in deaerated 104 ppm Cl’ + 40 ppm S“ solution. 194 5 0 0 PD PS -5 0 0 Sensitized Type 304 S3 Deaerafed 104 ppm Cr-h TOppm2 OS -1000 50 100 150 2 0 0 Temp, ®C Figure 109. Corrosion potential, Ep^ and Ep^ values obtained in deaerated 104 ppm Cl" + 70 ppm SgOg solution. 195 Increase of temperature to 200'C showed a sharp reversal in the pitting potential which increased to 130 mV^. The corrosion potentials measured in th is solution were higher than those measured in the previous two solutions throughout the temperature range of interest and the values ranged from -86 at 50'C to -292 mV^ a t 200'C. CHAPTER V DISCUSSION 5.1 Environment without Sulfur Species 5.1.1 Stress Corrosion Cracking In Air Saturated Pure Water The results of slow strain rate tests In air saturated pure water show that SCC occurs only at 200'C, as shown In Table 10. At temperatures below 150'C, complete ductile fracture Is found. The absence of SCC at temperatures lower than 150'C agrees well with the results of White et a l.(93) and other researchers. Ford and Pov1ch(29), however, reported IGSCC at 150'C In air saturated high purity water at a strain rate equal to 2.1 x 10“^ s"^. The SCC susceptibility of 304 stainless steel In Ford and Povlch's experiment Is probably the result of the high degree of sensitization In their samples. 5.1.2 Effect of Chloride Ion In Air Saturated Water The effect of chloride Ions on SCC Is shown In Figure 110. In the'presence of 104 ppm chloride, SCC occurs at 150'C as well as 200'C. Furthermore, the fracture surface shows 100% IGSCC In the chloride solution In contrast to 70% IGSCC In the absence of chloride Ion at 200'C. This Indicates that chloride Ions enhance SCC In air-saturated water at 200'C, and Induce SCC at 150'C. The effect of chloride on SCC at temperatures below lOO'C Is negligible. Enhancement of stress corrosion cracking In chloride solutions Is probably caused by easy crack Initiation because chloride Ions damage 196 197 T A' 1.0 i L t • H-o *3 0 .5 Sensitized Type304S3 g dppmQg o 104 ppm Cl k A without c r 0 1 I 50 100 150 200 Temperature (®C) Figure 110. Effect of chloride ion on the time-to-failure in air saturated water. 198 the passive film and in the presence of chloride Ion, metal at the crack tip dissolves and repassivation of crack tip does not occur. 5.1.3 Relationship between Pitting and SCC In 104 ppm Chloride Solution Figure 111 shows the domain of IGSCC In a potential“temperature diagram, taken from Lin et a l.(12). Their results are shown as dotted lines In the figure. In this figure, solid lines represents the result obtained In the present work. The occurrence of C IGSCC Is dependent on the potential and temperature. However, In our experiment, IGSCC occurs at potentials lower than those of Lin et al.. The IGSCC domain has a larger area than those of Lin et al.. This seems to be the result of the different chloride concentration and the different degree of sensitization. The studied steel contains higher concentration of carbon than steel used by Lin e t a l.. In th is figure, p ittin g potentials measured on the strain free specimen are Included(marked A). The estimated p itting potentials In this study are higher than those of Lin et al., because the concentration of chloride Is lower. In fact, a three times more dilute chloride solut1on(0.003 M MaCl) was employed In this study than that used In Lin e t al.(0.01 M NaCl). It should be emphasized th at all the corrosion potentials measured In slow strain rate te sts are lower than the estimated p ittin g potentials measured on unstrained specimens. Even taking Into consideration the fact that the pitting potential Is lower on strained than unstrained spec1men(about 70 mV at 100*0(12), the measured corrosion potential Is still lower than pitting potential. This fact Is consistent with our observation that no pits are formed on 199 Sensitized Type304SS 5 0 0 - 104 ppm C r 0 - Linetal. c f Ductite IGSCC . o Ductile Failure - 5 0 0 - ■ Intergrcnular Crocking A Pitting Potential 50 100 150 200 Temperature (®C) Figure 111. Potential-temperature diagram showing region of IGSCC in 104 ppm chloride solution. 200 the specimen In slow strain rate te s ts . 5.2 Solution containing Sulfur Species 5.2.1 Effect of Sulfur Species on Stress Corrosion Cracking The results of slow strain rate tests with or without sulfur species are summarized In Figure 112 as a function of temperature. As mentioned earlier, the susceptibility to SCC of sensitized Type 304 stainless steel In air saturated chloride solutions Increases with Increasing temperature In the temperature range studied. In the case of chloride containing solutions, IGSCC Is observed only at 150'C and 200"C. However, In the solution containing sulfur species, IGSCC occurs at temperatures as low as 50*C. The addition of 70 ppm of thiosulfate Ions to the chloride solution considerably decreases time to failure with Increase In temperature, but at 200"C, the time to failure Is higher In the presence of thiosulfate than without. On the other hand. In the presence of sulfide Ions In the chloride solution, significant SCC occurs a t 50 and lOO'C. But, a t 150"C, the presence of sulfide Ions decreases the SCC susceptibility. Thus, In the solution containing 40 ppm sulfide Ion at 150"C, SCC Is completely Inhibited. At 200"C, In the presence of sulfide Ion, IGSCC occurs again, but Is less pronounced than In the chloride solution without sulfide Ion. At temperatures lower than lOO'C, the specimen tested In the sulfide solution shows shorter time-to-failure than In the thiosulfate solution. However, at temperatures higher than 150"C, SCC 201 Sensitized Type304S3 i04ppm Cr+ dppm Og 1.0 0 .5 □ No Sulfur Species A 4 0 ppm S" o 7 0 ppm S 2 O3 50 100 150 200 Temperature (®C) Figure 112. Variations of normalized time-to-failure as a function of temperature in 104 ppm chloride solution with or without sulfur species. 202 susceptibility In the presence of thiosulfate Ion Is higher than that In the presence of sulfide Ion In the same air-saturated 104 ppm chloride solution. To summarize, sulfide Ions show Inhibiting effect at temperatures above 150'C, whereas thiosulfate Ions show Inhibiting effect only at 200*0. Below lOO'C, both anions Induce IGSCC. Figure 113 shows the damain of IGSCC In a potential-temperature diagram obtained In the 104 ppm chloride solution containing various sulfur species. In th is figure, IGSCC Is denoted by solid marks and ductile failure Is denoted by open marks. Pitting potentials In the 40 ppm sulfide containing solution are also presented. Hydrogen evolution potentials In the solution with pHs corresponding to those of 40 ppm sulfide solution are represented in the figure. The boundary between IGSCC and ductile failu re Is shown. The domain of IGSCC In the presence of sulfur species Is larger than In chloride solut1ons{dotted line Indicates the domain of IGSCC obtained In the chloride solution without any sulfur species). Hence, a t 50 and lOO'C, IGSCC Is found a t the open circuit potentials In the presence of sulfide and thiosulfate while only ductile failure Is found In the chloride solution at the same values of p o ten tial. 5.2.2 Effect of Sulfur Species on Pitting The temperature dependence of pitting potentials In the three different solutions are compared In Figure 114. Pitting potentials obtained In the 104 ppm chloride solution containing 40 ppm sulfide Ion are lower than those In the absence of sulfide Ions at temperatures below lOO'C. At temperatures above 150'C, the addition of sulfide Ion 203 Sensitized Type 3 0 4 S S 5 0 0 - i04ppm Cr+ Su/fur Species 3. o I Oppm S] A 4 0 p p m S" □ 70ppm SgOj f ■Ep 0 - a> i - 5 0 0 - 100 150 200 Temperature (®C ) Figure 113. Potential-temperature diagram showing domain of IGSCC in 104 ppm chloride solution containing various sulfur species. Ep and H^/Hg indicate pit nucléation potential and hydrogen evolution potentials in the presence of 40 ppm sulfide, respectively. Open marks indicate ductile failure and black marks indicate IGSCC. Dotted line indicates the domain of IGSCC obtained in the chloride solution without any sulfur species. 204 r 0 no S Species 5 0 0 - EpitjA 4 0 p p m S ° TOppmSjOj" ^ O no S Species Ec *{ A 40ppm S - 7 0 ppm S 2 O3" E I -5 0 0 - Sensitized Type304 SS Deaerafed i04ppm Ci' -1000 J______I______I_____ 5 0 100 150 200 Temp, *C Figure 114. Variations of Ep^ and as a function of temperature irv 104 ppm chloride solution with or without sulfur species. 205 to the chloride solution impedes p ittin g , resulting in more noble pitting potentials than those in the chloride solution. On the other hand, in the presence of thiosulfate ions(70 ppm), pitting potentials are lower 100 to 150 mV than those in the chloride solution at temperatures below 150'C. However, a t 200*0, the presence of thiosulfate ions inhibit the nucléation of pits in the chloride, solution. In conclusion, both thiosulfate and sulfide ion accelerate pitting corrosion at low temperatures and retard it at high temperatures in chloride solutions. However, pitting was impeded by thiosulfate ions at 200*0 while i t was impeded already by sulfide ions at 150*0. The second important difference is that the pitting potentials in sulfide solution are always more noble than those in thiosulfate solutions. 5.2.3 Relation of SCO and Pitting in the Solution containing Sulfur Species Comparing Figures 112 and 114, it is seen that temperature dependence of 304 stainless steel to SCO and pitting corrosion shows similar trends; At 50 and 100*0, the presence of either thiosulfate or sulfide ion in the chloride solution promotes SCO and pitting. At 150*0, the presence of thiosulfate ion accelerates SCO and pitting but the presence of sulfide ion inhibits the SCO and pitting. At 200*0, the presence of bothe sulfur species inhibits SCO and pitting. The only different action of sulfur species on SCO and pitting corrosion is that stainless steel is more susceptible to SCO in sulfide environment than in the thiosulfate solution at temperatures lower than 100*0, whereas it is less susceptible to pitting in the sulfide environment than in the 206 thiosulfate solution at the same temperatures. Although temperature dependence of pittin g corrosion and SCC shows such a similarity In the solution containing sulfur species, only the specimen tested In the air saturated chloride solution containing 40 ppm sulfide solution(Figures 81(d) and (e)), a few pits are developed. However, It Is not clear If the formation of pits lead to IGSCC as previously reported(13) or crevice condition Inside the crack facilitates the formation of pits. The corrosion potential of the specimen In SSRT was around 0 mV^ In the a ir saturated solution containing 104 ppm Cl" + 40 ppm S" at lOO'C. Since the pitting potential obtained in the same solution at the same temperature lies between 211 and 261 mVy, corrosion potential of SCC test Is more than 200 mV lower than pitting potential. Assuming that pitting potential decreases by 70 mV under straining as reported by Lin et al.(12), corrosion potential Is still 150 mV lower than the pitting potential. Therefore, It can be concluded that SCC leads to the formation of pits since pit cannnot be nucleated at such a low corrosion potential; Crack, containing aggressive solution, promotes the p it formation. 5.2.4 Effect of Dissolved Oxygen In the Solution containing 104 ppm Chloride and Sulfide Ion By comparing Figures 73 and 91, the effect of dissolved oxygen on SCC In the solution containing 104 ppm Cl" and sulfide lonsdO or 40 ppm) can be seen. It Is clear that In the solution containing 104 ppm Cl" and 40 ppm S^" and 8 ppm Og, sulfide Ions produce SCC at temperatures below lOO'C but Inhibit SCC at temperatures above 150*C. On the other hand. In a solution containing a lesser amount of 207 oxygen(0.2 ppm Og), the presence of sulfide does not produce SCC at temperature below lOO'C, and Inhibits I t above 150'C. It can be concluded that 8 ppm of dissolved oxygen Is required to obtain SCC at low temperatures. At low concentration of oxygen, SCC does not occur because corrosion potentials are low and more negative than critical cracking potentials. Sulfide Ions Inhibit SCC at temperatures above 150'C at both oxygen levels. This Implies that SCC In chloride solutions containing SH" Is controlled by a different mechanism at high and low tempratures; this will be discussed later. 5.2.5 Effect of Chloride Ion In Air Saturated Solution containing Sulfide By comparing Figures 73 and 88, the effect of chloride Ions In a ir saturated water containing sulfide Ions Is seen. As mentioned earlier, the presence of sulfide Ions In air saturated chloride solutions cause SCC a t temperatures below lOO'C, but Inhibit I t above 150'C. However, In the absence of chloride Ions, sulfide Ions do not cause SCC. This does not agree with the result of Heller and Prescott(45). In their experiments, IGSCC was found on sensitized Type 304 stainless steel In HgS saturated water with or without chloride Ion at room temperature. The different ph of the solution Is probably the reason of this result. On the other hand, Meyer et al.(95) reported that the corrosion rate of 1020 steel was Increased In the presence of chloride In HgS saturated water, while It decreased In the absence of chloride Ions. They suggested that the different film structure lead to the different corrosion rate In HgS saturated solution. Using X-ray diffraction techniques, they observed that In the brine solution. 208 hydrogen sulfide produced a less protective mackInawlte (or kanslte, FOgSg) tarnish followed by macklnawlte scale. On the contrary. In the saturated hydrogen sulfide solution without chloride, the macklnawlte tarnish formed Initially was first converted to a macklnawlte scale and then became covered by protective pyrrhotlte and pyrite scales(Figure 115). The poor protective property of the macklnawlte film has been was confirmed by many authors(107, 108, 109). I t has to be emphasized that even though Meyer e t a l . 's results were found for carbon ste e l, the corrosion resistance of sensitized stainless steel Is comparable to that of low alloy steels or carbon steel(43, 104). Hence, their results can be used for Interpretation of resu lts obtained In th is work. I t seems, therefore, that the non-protective sulfide fllm(macklnawlte) produced on the grain boundaries on the stainless steel causes SCC In the presence of chloride at low temperatures. In addition, chloride Ions locally acidify the solution at the crack tip by hydrolysis as follows(95); M^Cr + HgO ------MOH + H^Cr [28] The Increased concentration of chloride and acidity accelerate the dissolution rate and consequently promote cracking with the aid of sulfide Ions. It Is worth to note that as the concentration of sulfide Ions Increases, the susceptibility to SCC also Increases, but the corrosion potential decreases. Therefore, SCC caused by hydrogen has to be taken Into consideration. In fact, high strength steel s(96 - 99) and nickel base alloysdOO, 101) have been reported to be susceptible to hydrogen embrittlement In 209 MO — — I— r - 1 r ISO - tEGCNO TEST 1 0 M,S - OISTILLEO **TE* TEST K « H ,S - BRINE MO- TEST m O H |S - CO, - BRINE BRANCH CORROSION PRODUCT I RANSITE TARNISH t kansite scale J PYRRHOTITE . PYRITE SCALE I I - 2 WO 110 no ISO MO ISO Figure 115. Corrosion rates of SAE 1020 mild steel from tests I, II and III (94). 210 sulfide solutions. Moreover, the inhibiting effect of sulfide ions at around 200"C seems to support the mechanism of hydrogen embrittlement, because the susceptibility to hydrogen embrittlement usually decreases with increasing temperature. However, in an alkaline pH, the fraction of HgS is very low(102). At pH 11, the dominant species is HS~ followed by S", while the HgS fraction is extremely low; it is less than 0.1% of total sulfide (i.e., HgS concentration in 40 ppm sulfide solution is 0.04 ppm HgS). As shown in Figure 113, the corrosion potentials of the specimen showing IGSCC are s t i ll much higher than hydrogen evolution potentials in the sulfide solution. Such a low concentration of HgS and high corrosion potential does not seem to involve hydrogen embrittlement. 5.3 Effect of pH The pHs of the solutions measured a t room temperature is given in Table 7. However, at elevated temperatures, the pH of the solution is different from room temperature. The pHs at elevated temperatures are calculated based on the water dissociation and acid dissociation constants. The procedure of thermodynamical calculation of the pH values at elevated temperatures is given in Appendix and their calculated values are given in Table 15. The normalized times-to-failure are shown in Figure 116 as a function of the calculated pH in the chloride solution containing various anions at temperatures ranging from 50 to 200*C. At 50 and lOO'C, SCC susceptibility increases as pH increases. At 150'C, as pH increases, SCC susceptibility increases, showing maximum 2 1 1 Table 15. The calculated pH values of the solutions at elevated temperatures. pH Temp Pure 10 ppm 40 ppm 120 ppm 70 ppm (*C) water NagS NagS NOgSO,* NagSgOg 25 7.0 10.49 11.09 7.00 7.00 SO 6.63 • 9.76 10.36 6.66 6.64 100 6.13 8.75 9.36 6.27 6.21. ISO 5.82 8.14 8.74 6.20 6.13 200 5.64 7.76 8.37 6.34 6.33 2S0 5.57 7.63 8.23 300 5.70 7.89 * 8.49 * Naumov e t a l . (227) 212 O No Sulfur Species 0 10 ppm S" - A 70 ppm SgOg V 4 0 ppm S" □ 120 ppm SO 4 0.5 Figure 116. Normalized tim e-to-failure as a function of pHj a t various temperatures. 213 susceptm ty to SCC at around pH 6, and then it decreases with further Increase of pH. AT 200"C, SCC susceptibility decreases with the increase of pH. The same pH dependence of SCC as at 200"C was reported by AndresendS) at 288"C. Examining figure 116, i t can be remarked that below pH 7, thiosulfate ions cause SCC in chloride solutions at temperatures from 50 to 150'C. In chloride solutions without thiosulfate , SCC does not occur in this temperature range except at 150'C. At the same pH range, SCC occurs at 200'C in chloride solution with or without thiosullfate. PHs, higher than 7, are obtained only in sulfide solution. In this pH range, SCC is dependent on temperature. Severe SCC is observed at 50 and lOO'C, but at 200*C, slight SCC is observed and at 150'C, marginal SCC occurs. In the solution containing low dissolved oxygen, SCC susceptibility shows the same pH dependency, at 200'C, but SCC occurs only in a low pH range (Figure 117). The results of the specimens tested in the absence of chloride are shown in the figure as open marks. The time-to-failure obtained in pure water is longer than that in the presence of chloride at the same pH by a factor of 2-3. However, in the solution containing 40 ppm sulfide, the time-to-failure obtained in the absence of chloride is longer than that in the presence of chloride. It is considered that chloride ion has considerable effect on SCC susceptibility in neutral solutions at 200'C, but has no effect in alkaline solutions at the same temperature. 214 1.5 Sensitized Type 304 S3 i04ppm cr, 200X 1.0 ^Q2ppm02 o w - 0 .5 # No Sulfur Species ♦ 10 ppm S" ■ 120 ppm SO 4 ▼ 40 ppm S" ▲ 7 0 ppm S 2 O3 0 8 10 II p H 200 *C Figure 117. Normalized tlme-to-fallure In the chloride solution containing 8 ppm Og and 0.2 ppm Og as a function of pH^oo'C" 215 5.4 Mechanistic Study To explain the peculiar behavior of time-to-failure vs. pH at different temperatures in chloride solutions containing sulfur species, i t is necessary to take into consideration the behavior of these sulfur species in oxygenated aqueous solutions. The effect of these sulfur species on properties of steel and on active dissolution of steel should also be considered. It is known that thiosulfate is produced as an intermediate species during the oxidation of sulfide(73 - 85). In this study, the formation of thiosulfate ion in the oxygenated sulfide solution was proved by direct measurement using titrimmetric analysis of the solutions and indirectly from the shape of anodic polarization curves. The results of sulfide and thiosulfate analysis before and after the slow strain rate tests are shown in Table 16. In the sulfide solution at any temperature, SgOg" was detected. The concentration of thiosulfate produced was dependent on the testing time and temperature; i t decreases with increasing testing temperature and time. At all the temperatures studied, sulfide ions are completely consumed in 24 hours in the air-saturated solution; presumably, they transform partly to thiosulfate and partly to iron sulfide. On the polarization curves of steel measured in the sulfide solution(Figure 101), a second peak appears while i t does not appear when measurements are performed in chloride solutions. This peak can be related to the formation of thiosulfate ions, according to reaction: 2HS" + SHgO ------SgOg^ + BH'^ + 8e“ [29] The equilibrium potentials for reaction [29] according to Biernat and 216 Table 16. Solution Analysis during Slow Strain Rate Test. Temp Initial Concentration Time Final Concentration CO (ppm) (hr) (ppm) SgOg' s ' Cl" Og ^2°3 ^ 50 70 104 8 70 >50 0 SCC 150 70 104 8 40 >50 0 SCC 50 10 8 4 11 2 D 90 11 0 100 10 8 0 12 5 D 24 12.5 0.5 48 9.7 0 100 40 104 8 27 35 0 SCC 100 40 0.2 24 N.A. 3 D 48 N.A. 0 150 40 8 21 40 0 D 47 32 0 80 17.5 0 200 40 8 55 9 0 SCC N.A. : not analyzed 217 Rob1ns(69) are as follows; E(25"C) = 0.201 - 0.059 pH + 0.0074 log([S203^"]/[HS"]^) [30] E(IOO'C) = 0.237 - 0.074 pH + 0.0093 logCCSgOj^'l/CHS"]^) [31] E(150*0 = 0.247 - 0.084 pH + 0.0105 log([S203^"]/[HS’ ]^) [32] E(200*C) = 0.247 - 0.094 pH + 0.0117 log([S203^"]/[HS"]^) [33] Assuming that the concentrations of HS’ and $ 203^" is equal to 10"^ M and using the calculated pH at elevated temperatures in Table 15, the equilibrium potentials are; E(25*C) = -0.432 V E(IOO'C) = -0.431 V E(150*C) = -0.452 V E(200*C) = -0.508 V Considering that a high scan rate (1 mV/sec) was employed, the shifts of the peak potentials in the positive direction is expected. Taking this fact into consideration, the calculated equilibrium potentials agree reasonably well with the potentials at which increae of current corresponding to the second peak occurs. Despite the presence of thiosulfate in sulfide solution at all studied temperatures, i t follows that the effect on SCC is connected with the initial presence of sulfide in the solution. It seems that at this initial stage, sulfide reacts with metal as well as with oxygen. Therefore, sulfide films are formed at the initial stage of SCC. Depending upon temperature, this film can be protective or non-protective. Figure 118 shows the temperature-composition diagram for iron-sulfur system(105). As shown in Figure 118, non-protective low tcmpL 150 hexagonal pyrrhotlte monoclinic pyrrhotlte i plus pyrite ; pyrite plus monocBntcsulohur i 100 TEMP, I O) ipyrlte 50 m onocinic pyrite plus pyrrhotlte orthorhomblc plus FC3 S4 sulphur ATOMIC PERCENT IRON Figure 118. Temperature-composition diagram for the Iron-sulphur system (105). H-* CO 219 macklnawlte Is not stable at temperatures above 140*C. Thus, above 140"C, the stable Iron sulfide is troilite(stoichiometric pyrrhotite), which is known to be the most protective sulfide(105, 106)(see chapter 5.2.5). All the measurements of the protectiveness of iron sulfide in the literature were performed in acidic solution. However, i t seems that the same protective properties of film can be expected in alkaline solutions as well. Therefore, the different susceptibility of steel to SCC in sulfide solution at different temperatures can be explained by propeties of protective film formed above and below 150'C. The presence of non-protective mackinawite is the reason for severe SCC of steel at 50 and lOO'C. As was mentioned before, different properties of the film at pH above 7 are responsible for the behavior of steel in the chloride solution containing sulfide anions. The firs t step in SCC is again the breakdown of film by Cl” ions, which occurs much more easily on the surface covered by mackinawite. The initiation of cracks is more easier on mackinawite than on an oxide film. The role of the sulfide anion is to promote SCC at 50 and lOO'C. At higher temperatures, a protective film which cannot be broken easily at high pH is formed. A lower pH ,of the solution is obtained in chloride solutions with or without thiosulfate. In these solutions, no iron sulfide film is formed on the metal surface. The metal is covered by an oxide film. The necessary condition for SCC in alkaline solutions containing sulfur species is the presence of chloride. At a certain critical potential, chloride anions break the film and dissolve bare metal and initiate 220 localized corrosion. As a result of hydrolysis, a low pH is built up, causing the growth of pits(cracks). Because sulfur anions enhance SCC, they have to either enhance breakdown of the film or inhibition of repassivation. According to data in literature{42, 43, 66), the aggressive effect of thiosulfate ion is due to enhancement of the active anodic dissolution of steel. The first step of this process is adsorption of thiosulfate on the metal surface. The thiosulfate ion is derived from the sulfate ion by the replacement of one ligand oxygen atom by a sulfur atom in the oxidation number -2 (whereas the central sulfur atom - as also in sulfuric acid - has an oxidation number of + 6). This sulfur atom(valency of -2) is adsorbed on the surface film. The thiosulfate anions also adsorb on the vare metal. Hence, they enhance dissolution and impede repassivation. It was reported that the ratio of metal atoms dissolved to thiosulfate ions consumed correspond to 100 or 400 to 1. In other words, the effect of the thiosulfate was catalytic. A small amount of thiosulfate catalyzed the dissolution of a large amount of iron (42). As shown in Table 13 and 14, in our studies, i t was found that the corrosion currents obtained in the chloride solution containing sulfur species is higher than in the absence of sulfur species. To explain the small inhibiting effect of thiosulfate anion on SCC at 200"C, the probable inhibiting ability of the sulfate ion was taken into consideration. Sulfate ions form as the final product of thiosulfate oxidation. Newman(43) observed that the addition of NagSO^ to the NagSgOg solution inhibited SCC of sensitized stainless steel at room temperature. It was suggested that accumulation of large amount of 221 sulfate within the cracks caused passivation of the crack tip by preventing electromigration of thiosulfate ions. If this mechanism occurs at high temperatures too and a part of thiosulfate is oxidized to sulfate, a similar inhibiting phenomenon should be expected. The concentration of sulfate obtained by oxidation of 70 ppm of thiosulfate is too low to affect SCC. Therefore, to maximize the sulfate effect, SSRT was performed in air-saturated solution containing 104 ppm Cl" and 120 ppm S0^^~. It was found that sulfate ion does not inhibit SCC at 200'C. As shown in Table 15, pH of thiosulfate solution(pH 6.3) is higher than that of chloride solution(pH 5.6) at 200'C. This difference in pH between chloride and thiosulfate solution implies that inhibiting effect of thiosulfate ion is [resemt at pH than at low. The action of sulfur species in pitting corrosion is the same as in SCC. It means that thiosulfate ions act by increasing the dissolution of metal inside the pit, and making repassivation more difficult. Pitting behavior in a solution containing sulfur species has the same temperature dependence as see behavior. CHAPTER VI CONCLUSION The important findings in this study are listed as follcws; (1) Sensitized Type 304 stainless steel is susceptible to stress corrosion cracking in air saturated pure water at 200"C. At temperatures below 150'C, stress corrosion cracking does not occur. (2) Stress corrosion cracking occurs in 104 ppm chloride solution containing either 8 ppm Og or 0.2 ppm Og at 150'C or above. At temperature below lOO'C, stress corrosion cracking does not occur. (3) The presence of sulfide ions promotes stress corrosion cracking of sensitized Type 304 stainless steel in air saturated 104 ppm chloride solution at 50 and lOO'C. At 150'C or above, the presence of sulfide ions in the air saturated 104 ppm chloride solution inhibits stress corrosion cracking. (4) The sulfide ion does not promote stress corrosion cracking at temperatures below 150'C in air saturated pure water, but inhibits stress corrosion cracking at 200'C. (5) Sulfide ions do not promote stress corrosion cracking at lOO'C in the 104 ppm chloride solution containing 0.2 ppm Og. But, it inhibits stress corrosion cracking at 150'C and above in the same solution. (6) Thiosulfate ion promote stress corrosion cracking at 150'C or above in air saturated 104 ppm chloride solution, but inhibit stress 222 223 corrosion cracking at 200*C in air saturated 104 ppm chloride solution. (7) Sensitized Type 304 stainless steel is more resistant to stress corrosion cracking in the solution containing thiosulfate ions than in solution containing sulfide ions at temperatures below lOO’C if chloride is present in the solution. However, at 150*C or above, their behavior is reversed. (8) The presence of sulfide ions in the 104 ppm chloride solution enhances pitting corrosion of sensitized Type 104 stainless steel at temperatures below 100*0, but inhibits pitting corrosion at temperatures above 150*0. 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Sulfide Solutions In this study, two sulfide concentrations were employed, 10 ppm and 40 ppm, which are equivalent to 3.12 x 10 * and 1.25 x 1 0 M, in terms of sulfur, respectively. To calculate the pHs at elevated temepratures, followings are assumed; 231 232 Table 17. The Dissociation Constant and pH Values of Pure Water at Elevated Temperatures. Temp Kw pH 25 1 .0 0 8 X 1 0 " 7 .0 0 50 5.47 X 10"1* 6 .6 3 100 5.50 X 10"13 6 .1 3 -12 15 0 2 .2 9 X 10 5 .8 2 -12 200 5 .3 7 X 10 5 .6 4 2 5 0 7 .4 1 X 1 0 - 1 2 5 .5 7 3 0 0 4.07 X 10"12 5 .7 0 233 (1) Sodium sulfide is completely hydrolyzed + HgO ------OH’ + HS" [ 3 4 ] HS" + 2 Og ------HSO4’ [ 3 5 ] (2) Concentration of OH" dominates the pH of the solution and the effect of HSO4" and Cl" on pH are negligible. (3) Hydroxide is present as a completely dissociated form in a dilute solution and its concentration is constant at the all studied tmeperatures. Then, pH is given Kw pH = -lo g [40] [OH"] For example, the pH in 10’^ M NaOH and 0.05 M NagSO^ at 200*C is pH = -log -----5“ --— = 9.27 10"^ A good agreement with the pH(9.3) measured by Tsuruta and Macdonald (111) in the same solution and temperature proves above assumptionsare reasonable. F;or 10 ppm sulfide (3.12 x lO"* M) solution at 200"C, pH = -log ------T- = 7.764 3 . 1 2 X 1 0 "* Similarly, the pH values at 50, 100 and 150*C were calculated from equation[40] and are given in Table 15. 3. Thiosulfate Solution The used concentration of thiosulfate ion was 6.24 x 10"* M. Assuming first dissociation constant for HgSgOg — HSgOg" + H^ is 234 infinity, the following reactions are in equilibrium: HSgOg" —--— + SgOgZ- [41] HgO — --— + OH" [42] where K and Kw are dissociation constants for HSgOg" and water. Then, ^ HS203" ^^HS203"‘ ^1^ Kw = a • a gy- [44] where = Y o ^ f = Y^^+ and = Y ^^O S ^- The charge neutrality condition leads to: Cwa* * " ^OH" " ^ ^5203^" " ^HS203" " ° ^^S] The mass balance is expressed ^S203^" ^ ^HS203" ^ [*G] where [S] = total thiosulfate concentration in Mole/liter. From equations (43) (44) (45) (46) 3 2 ''I (3^^) + (9^+) • (yj[S] + K—) - (9^+) * Kw - Kw*K — =0 [47] ^2 ^2 For the computation of the activity coefficients, y^ and Xg, modified Debye-Huckel equation was used (103). log Vj = -Z^d(T/(1 + l.sff) [48] where I is the ionic strength, Z is the charge of the ion and D is the Debye-Huckel slope. I is given by: I = 1 I C jZ/ [49] The calculated y^ and Xg are given in Table 18, which also includes the dissociation constant of HSgOg" (103). 235 The pHs of the sulfate solution at elevated temperatures were calculated in similar method for thiosulfate solution. The employed concentration of sulfate ion was 1.25 x 10”^ M. The dissociation constants(103) and calculated pHs are included in Table 19, 236 Table 18. Activity Coefficients, Dissociation Constant and Calculated pHs at Elevated Temperatures for Thiosulfate Solution. Temp T. Tz %1 pH 25 0 .9 3 6 0 .7 6 8 1.12 X 10"2 7 .0 0 0 5 0 0 .9 3 3 0 .7 5 7 5.12 X 10"3 6 .6 6 4 100 0 .9 2 4 0 .7 2 9 1.00 X 10"3 6 .2 6 8 1 5 0 0 .9 1 1 0 .6 9 0 1 .8 2 X lo T * 6 .1 9 8 200 0 .8 9 3 0 .6 3 7 3.09 X lO'S 6 .3 4 4 dissociation constant for HSgO^ 237 Table 19. Activity Coefficients, Dissociation Constant and Calculated pHs at Elevated Temperatures for Sulfate Solution. Temp Vi Yg *2 pH 25 0 .9 5 3 0 .8 2 6 1 .9 1 X 1 0 " 2 6 .9 9 9 5 0 0 .9 5 1 0 .8 1 8 7 .4 1 X 1 0 - 3 6 .6 4 3 100 0 .9 4 4 0 .7 9 5 1.05 X 10"3 6.211 1 5 0 0 .9 3 5 0 .7 6 4 1.45 X lOT* 6 .1 3 4 200 0 .9 2 1 0 .7 2 1 1 .8 6 X 1 0 ‘ ® 6 .3 3 0 dissociation constant for HSO, ■H* + S0^^“