Stress Corrosion Cracking of Ni-Fe-Cr Alloys Relevant to Nuclear Power Plants

261 Pages·2015·15.73 MB·English

by  Suraj Persaud

Stress Corrosion Cracking of Ni-Fe-Cr Alloys
Relevant to Nuclear Power Plants

by
Suraj Persaud
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Suraj Persaud 2015
Stress Corrosion Cracking of Ni-Fe-Cr Alloys
Relevant to Nuclear Power Plants
Suraj Persaud
Doctor of Philosophy
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2015
Abstract
Stress corrosion cracking (SCC) of Ni-Fe-Cr alloys and weld metals was investigated in simulated
environments representative of high temperature water used in the primary and secondary circuits of
nuclear power plants. The mechanism of primary water SCC (PWSCC) was studied in Alloys 600, 690,
800 and Alloy 82 dissimilar metal welds using the internal oxidation model as a guide. Initial experiments
were carried out in a 480 °C hydrogenated steam environment considered to simulate high temperature
reducing primary water. Ni alloys underwent classical internal oxidation intragranularly resulting in the
expulsion of the solvent metal, Ni, to the surface. Selective intergranular oxidation of Cr in Alloy 600
resulted in embrittlement, while other alloys were resistant owing to their increased Cr contents. Atom
probe tomography was used to determine the short-circuit diffusion path used for Ni expulsion at a sub-
nanometer scale, which was concluded to be oxide-metal interfaces. Further exposures of Alloys 600
and 800 were done in 315 °C simulated primary water and intergranular oxidation tendency was
comparable to 480 °C hydrogenated steam. Secondary side work involved SCC experiments and
electrochemical measurements, which were done at 315 °C in acid sulfate solutions. Alloy 800 C-rings
were found to undergo acid sulfate SCC (AcSCC) to a depth of up to 300 µm in 0.55 M sulfate solution at
pH 4.3. A focused-ion beam was used to extract a crack tip from a C-ring and high resolution analytical
electron microscopy revealed a duplex oxide structure and the presence of sulfur. Electrochemical
measurements were taken on Ni alloys to complement crack tip analysis; sulfate was concluded to be the
aggressive anion in mixed sulfate and chloride systems. Results from electrochemical measurements and
crack tip analysis suggested a slip dissolution-type mechanism to explain AcSCC in Ni alloys.
ii
Acknowledgments
The author would like to thank his supervisor, Professor Roger Newman, for his guidance and
support; in particular, the freedom provided to pursue personal research interests and the
exposure at national and international conferences was greatly appreciated. This research was
funded by The University Network of Excellence in Nuclear Engineering (UNENE) and The
National Sciences and Engineering Research Council of Canada (NSERC).
3-D APT and electron microscopy analysis on Alloy 600 and Alloy 800 were carried out at the
Canadian Centre for Electron Microscopy (CCEM) at McMaster University; collaboration was
done with Professor Gianluigi Botton whose help was greatly appreciated. FIB extraction was
carried out by Julia Huang and Travis Casagrande. In particular, extraction of a crack tip using
a FIB would have not have been possible without the impressive skill and precision of Julia
Huang. STEM, EDX, and EELS were carried out by Dr. Andreas Korinek. 3-D APT work was
done by Dr. Brian Langelier who it was a pleasure to collaborate with.
480 °C hydrogenated steam experiments and AES depth analysis were performed at Surface
Science Western (SSW) with the help of Dr. Sridhar Ramamurthy; the allocation of time to
accommodate several of these experiments was greatly appreciated. FIB-SEM work for Alloy
690 and Alloy 82 dissimilar metal welds were carried out by Dr. Todd Simpson at the
Nanofabrication Facility at Western University. 315 °C primary water experiments were
performed in the H5 loop at Canadian Nuclear Laboratories (CNL); many thanks to Dr. Jared
Smith for organizing and allowing these experiments to proceed.
The author would like to thank his laboratory colleagues who made graduate studies a
memorable experience. A special thanks to Dr. Anatolie Carcea who was an excellent
laboratory instructor and friend; in particular, assistance with autoclave experiments, fruitful
discussions, and the significant amount of time spent teaching was greatly appreciated.
Finally, the author would like to thank his family for their support; in particular, his wife,
Mobareka Masood, for her encouragement and sacrifice.

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Preface
The author was born on November 23, 1987 in Toronto, Ontario. He grew up in Mississauga,
Ontario and originally aspired to be a lawyer for the majority of his youth.
In 2005, the author started his undergraduate studies at the University of Toronto and graduated
in 2009 with a Bachelor of Applied Sciences from the Department of Chemical Engineering and
Applied Chemistry. During his undergraduate studies he spent a significant amount of time
working at AMEC NSS, a nuclear engineering firm, in the Asset Management and Degradation
of Materials division.
Upon completion of undergraduate studies, the author immediately pursued graduate research
at the University of Toronto under the supervision of Professor Roger Newman in 2009. He
graduated in 2011 with a Master of Applied Sciences from the Department of Chemical
Engineering and Applied Chemistry; his thesis was entitled: ―Indications of Stress Corrosion
Cracking Resistance in Alloy 82 Dissimilar Metal Welds in Simulated Primary Water
Environments‖.
The author started his doctoral studies in 2011 under the supervision of Professor Roger
Newman at the University of Toronto, with the goal of extending his masters research and
diversifying his area of expertise. An Ontario Graduate Scholarship was used to fund a portion
of his doctoral research. In addition, several prizes were accumulated for his work at national
and international conferences. Throughout his doctoral studies he has published or submitted
3 full conference papers and 9 articles in high quality journals. His doctoral thesis is entitled:
―Stress Corrosion Cracking of Ni-Fe-Cr Alloys Relevant to Nuclear Power Plants‖.

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Dedicated to Mobareka Masood
v
Table of Contents
Acknowledgments ...................................................................................................................... iii
Preface ....................................................................................................................................... iv
Table of Contents ....................................................................................................................... vi
List of Tables .............................................................................................................................. ix
List of Figures ............................................................................................................................. xi
List of Appendices .................................................................................................................. xxiii
List of Acronyms ..................................................................................................................... xxiv
Chapter 1 Literature Review ....................................................................................................... 1
1.1 Chemistry and Metallurgy of Ni Alloys in Nuclear Plants ................................................. 1
1.1.1 Microstructure ..................................................................................................... 2
1.1.2 Ni Alloy and Weld Metal Compositions ................................................................ 2
1.1.3 Ni-Fe-Cr Ternary Phase Diagram ........................................................................ 4
1.1.4 Heat Treatment ................................................................................................... 5
1.1.4.1 Annealing .............................................................................................. 5
1.1.4.2 Thermal Treatment ................................................................................ 6
1.1.5 Weld Metal Dilution due to Mixing ....................................................................... 8
1.2 Primary Water Stress Corrosion Cracking (PWSCC) ...................................................... 9
1.2.1 Primary Water Chemistry .................................................................................... 9
1.2.2 Effect of Temperature........................................................................................ 10
1.2.3 Effect of pH and Dissolved Hydrogen/Potential ................................................. 13
1.2.4 Effect of Alloy Composition ............................................................................... 15
1.2.4.1 Alloy 600 ............................................................................................. 16
1.2.4.2 Alloy 690 and 800 ................................................................................ 19
1.2.4.3 Dissimilar Ni Alloy Metal Welds ........................................................... 24
1.2.5 Effect of Cold Work ........................................................................................... 26
1.2.6 Effect of Microstructure ..................................................................................... 29
1.2.7 Potential PWSCC Mechanisms ......................................................................... 31
1.2.7.1 Slip Dissolution .................................................................................... 32
1.2.7.2 Enhanced Surface Mobility Theory ...................................................... 33
1.2.7.3 Hydrogen Embrittlement ...................................................................... 35
1.2.7.4 Internal Oxidation ................................................................................ 36
1.3 The Internal Oxidation Model ........................................................................................ 39
1.3.1 Kinetics of Internal Oxidation ............................................................................. 40
1.3.2 Relief of Internal Stress from Volume Expansion ............................................... 42
1.3.3 Transition from Internal to External Oxide ......................................................... 45
1.3.4 Effect of Temperature........................................................................................ 47
1.3.5 Influence of Surface Cold Work ......................................................................... 48
1.3.6 Influence of Hydrogen on Diffusion .................................................................... 49
1.3.7 Low Temperature Internal Oxidation ................................................................. 51
1.3.7.1 Criticisms of the Internal Oxidation Model............................................ 54
1.4 Acid Sulfate Stress Corrosion Cracking (AcSCC) ......................................................... 56
1.4.1 Secondary Side Water Chemistry...................................................................... 57
1.4.2 Pourbaix Diagrams ............................................................................................ 58
1.4.3 Effect of pH ....................................................................................................... 60
1.4.4 Effect of Sulfate Concentration .......................................................................... 61
1.4.5 Alloy Composition ............................................................................................. 62
1.4.6 Effect of Potential .............................................................................................. 62
1.4.7 Effect of Temperature........................................................................................ 63
1.4.8 Acid Sulfate Solution Chemistry at High Temperature ....................................... 64
vi
1.4.8.1 Solubility Limitations ............................................................................ 65
1.4.8.2 Suppression of Bisulfate Dissociation .................................................. 66
1.4.9 Magnetite Formation ......................................................................................... 67
1.4.10 Reduced Sulfur Species .................................................................................... 68
1.4.10.1 Sulfur-Accelerated Corrosion Mechanisms .......................................... 69
1.4.11 Possible Mechanism of AcSCC ......................................................................... 71
Chapter 2 Introduction .............................................................................................................. 72
Chapter 3 Experimental Methods and Analytical Techniques ................................................... 75
3.1 480 °C Hydrogenated Steam Experiments ................................................................... 75
3.1.1 Materials and Sample Preparation .................................................................... 75
3.1.2 Environment Conditions .................................................................................... 78
3.1.3 Equipment and Procedures ............................................................................... 79
3.2 315 °C Primary Water Experiments .............................................................................. 80
3.2.1 Materials and Sample Preparation .................................................................... 80
3.2.2 Experimental Conditions and Equipment ........................................................... 81
3.3 AcSCC Experiments ..................................................................................................... 83
3.3.1 Materials ........................................................................................................... 83
3.3.2 SCC Experiments .............................................................................................. 83
3.3.2.1 Sample Preparation ............................................................................. 83
3.3.2.2 Equipment Description ........................................................................ 84
3.3.3 Electrochemical Procedures .............................................................................. 85
3.3.3.1 Sample Preparation ............................................................................. 85
3.3.3.2 Equipment and Procedures ................................................................. 85
3.3.3.3 Acid Sulfate Solutions ......................................................................... 86
3.4 High Resolution Analytical Techniques and Equipment ................................................ 87
3.4.1 Scanning Electron Microscopy (SEM) ............................................................... 87
3.4.1.1 Alloy 600 in 480 °C Hydrogenated Steam ........................................... 87
3.4.1.2 Alloy 800 in 480 °C Hydrogenated Steam ........................................... 87
3.4.1.3 Alloy 690 in 480 °C Hydrogenated Steam ........................................... 88
3.4.1.4 Alloy 82 Weld Metal in 480 °C Hydrogenated Steam ........................... 88
3.4.1.5 Ni-Fe-Cr Alloys in 315 °C Primary Water ............................................. 88
3.4.2 Focused Ion Beam (FIB) Milling ........................................................................ 89
3.4.2.1 Conventional FIB Lift-out ..................................................................... 89
3.4.2.2 FIB Trenching and EDX ....................................................................... 90
3.4.2.3 FIB Crack Tip Extraction ...................................................................... 90
3.4.3 Transmission Electron Microscopy (TEM) ......................................................... 93
3.4.4 X-ray Diffraction (XRD) ...................................................................................... 94
3.4.5 Atom Probe Tomography (APT) ........................................................................ 94
3.4.5.1 APT Sample Preparation ..................................................................... 95
Chapter 4 Investigating Oxidation and SCC in Ni-Fe-Cr Alloys Exposed to Simulated
Primary Water Environments ............................................................................................... 97
4.1 Internal Oxidation of Alloy 600 in 480 °C Hydrogenated Steam .................................... 97
4.1.1 Surface Analysis ............................................................................................... 97
4.1.2 FIB Extraction ................................................................................................... 99
4.1.3 Chemical Composition Analysis ...................................................................... 100
4.1.3.1 Alloy 600SA ....................................................................................... 100
4.1.3.2 Alloy 600TT ....................................................................................... 107
4.1.4 Summary ......................................................................................................... 113
4.2 High Resolution Characterization of Internal Oxidation in Alloy 600 using Atom Probe
Tomography ............................................................................................................... 114
4.2.1 Intergranular Oxidation .................................................................................... 114
4.2.1.1 Alloy 600SA ....................................................................................... 114
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4.2.1.2 Alloy 600TT ....................................................................................... 119
4.2.2 Intragranular Oxidation .................................................................................... 123
4.2.3 Summary ......................................................................................................... 128
4.3 Oxidation Tendency of Alloy 800 in 480 °C Hydrogenated Steam ............................... 129
4.3.1 SEM Surface Imaging and EDX ...................................................................... 129
4.3.2 FIB Cross-section Extraction ........................................................................... 130
4.3.3 Grain Boundary TEM Analysis ........................................................................ 133
4.3.4 Intragranular Internal Oxidation TEM Analysis ................................................ 137
4.3.5 Summary ......................................................................................................... 143
4.4 Oxidation Tendency of Alloy 690 in 480 °C Hydrogenated Steam ............................... 144
4.4.1 Optical Microscopy and Diffraction .................................................................. 144
4.4.2 FIB Sectioning and EDX Mapping ................................................................... 147
4.4.3 Summary ......................................................................................................... 154
4.5 The Effect of Ni Alloy Weld Chemistry on Oxidation Tendency ................................... 155
4.5.1 Weld-Metal Chemistry ..................................................................................... 155
4.5.2 Oxidation Behaviour ........................................................................................ 157
4.5.2.1 Surface Analysis and Depth Profiling ................................................. 157
4.5.2.2 SEM Imaging and EDX Analysis after FIB Sectioning ....................... 160
4.5.3 Discussion ....................................................................................................... 163
4.5.4 Summary ......................................................................................................... 167
4.6 High Resolution Analysis of Oxidation in Ni-Fe-Cr Alloys after Exposure to 315 °C
Primary Water ............................................................................................................. 168
4.6.1 Oxidation in Alloy 600...................................................................................... 168
4.6.2 Oxidation in Alloy 800...................................................................................... 172
4.6.3 Discussion ....................................................................................................... 177
4.6.4 Summary ......................................................................................................... 183
Chapter 5 Investigating Acid Sulfate Stress Corrosion Cracking in Ni-Fe-Cr Alloys ............... 184
5.1 Analytical Electron Microscopy of an AcSCC Crack Tip in Alloy 800 ........................... 184
5.1.1 EDX Elemental Mapping an AcSCC Crack Tip in Alloy 800............................. 184
5.1.2 Electron Energy-Loss Spectroscopy ................................................................ 187
5.1.3 Summary ......................................................................................................... 190
5.2 Electrochemistry of Ni-Fe-Cr Alloys in Acid Sulfate Environments .............................. 191
5.2.1 Effect of Sulfate ............................................................................................... 191
5.2.2 Effect of Fe2+ Ions ........................................................................................... 196
5.2.3 Sulfate Reduction ............................................................................................ 198
5.2.4 Further Discussion .......................................................................................... 199
5.2.5 Summary ......................................................................................................... 202
Chapter 6 Conclusions ........................................................................................................... 203
6.1 Oxidation and Embrittlement in Ni-Fe-Cr Alloys after Exposure to Simulated Primary
Water Environments ................................................................................................... 203
6.2 Acid Sulfate Stress Corrosion Cracking in Ni-Fe-Cr Alloys .......................................... 205
Chapter 7 Future Work ........................................................................................................... 207
7.1 Primary Water Stress Corrosion Cracking in Ni-Fe-Cr Alloys ...................................... 207
7.2 Acid Sulfate Stress Corrosion Cracking in Ni-Fe-Cr Alloys .......................................... 208
References ............................................................................................................................. 209
Appendix A: Diffusion Data for Cr and O in Ni and Ni Alloys ................................................... 230
Appendix B: Calculations for Environmental Conditions in 480 °C Hydrogenated Steam
Exposures ......................................................................................................................... 232

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List of Tables
Table 1: Nominal compositions of Ni alloys and weld metals used in nuclear plants (in wt. %)
[7,8]. ........................................................................................................................................... 3
Table 2: Composition of Alloys 600, 690, and 800 purchased from Rolled Alloys Inc. (in at. %) 75
Table 3: Nominal compositions of Alloy 82, Alloy 600 and carbon steel (in wt. %). ................... 76
Table 4: The H5 loop simulated CANDU primary water conditions. .......................................... 82
Table 5: Composition of Alloy 800 and Alloy 600 tubing purchased from Rolled Alloys Inc. (in
wt. %). ...................................................................................................................................... 83
Table 6: List of solutions used for electrochemical work. .......................................................... 86
Table 7: The results from EDX spectra on Ni nodules on the surface of Alloy 600SA samples
exposed to hydrogenated steam at 480 °C. The areas of analysis are shown in Figure 71. ... 101
Table 8: APT measured composition from the grain boundary oxide for 600SA shown in Figure
85. .......................................................................................................................................... 116
Table 9: APT measured composition from the grain boundary oxide for 600TT shown in Figure
89. .......................................................................................................................................... 121
Table 10: APT measured compositions from the regions in Figure 91. ................................... 125
Table 11: EDX analysis on points 1, 2, and 3 in Figure 95 (in wt. %). ..................................... 130
Table 12: Quantified EELS spectra composition analysis on points labeled in Figure 101 (in at.
%). ......................................................................................................................................... 136
Table 13: Quantified EELS spectra composition analysis on points 1 to 6 identified in Figure 103
(in at. %). ................................................................................................................................ 139
Table 14: EDX analysis done on the surface of Alloy 690 after exposure to hydrogenated steam
at 480 °C and 1 bar with conditions maintained below the Ni/NiO equilibrium (in at. %). ........ 147
Table 15: EDX point analysis done on the layer and nodules between the weld root and crown.
............................................................................................................................................... 159
Table 16: EDX results for the point spectra indicated in Figure 120 in wt. %. ......................... 162
Table 17: EDX point analysis taken on the surface and in the trench below the Ni layer in wt. %.
............................................................................................................................................... 163
Table 18: Quantified EELS spectra composition analysis on points labeled in Figure 132 (in at.
%). ......................................................................................................................................... 176
Table 19: Quantified EELS spectra composition analysis on points labeled in Figure 134 (in at.
%). ......................................................................................................................................... 177
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Table A-1: Diffusion coefficient (D) data for Cr in Ni, Ni-Cr alloys, or Alloy 600 ....................... 230
Table A-2: Diffusion coefficient (D) data for O in Ni, Ni-Cr alloys, or Alloy 600 ........................ 231
Table A-3: Low temperature D for O in Ni, Ni-Cr alloys, or Alloy 600 ...................................... 231
Table B-1: Constants used in Equation B-4 to calculate Gibbs standard free energy [41]....... 233

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