Dealloying and Formation of Nanoporosity in Noble-Metal Alloys · ii Dealloying and Formation of...
Transcript of Dealloying and Formation of Nanoporosity in Noble-Metal Alloys · ii Dealloying and Formation of...
Dealloying and Formation of Nanoporosity
in Noble-Metal Alloys
by
Mariusz Albert Bryk
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Chemical Engineering and Applied Chemistry
University of Toronto
© Copyright by Mariusz Albert Bryk (2012)
ii
Dealloying and Formation of Nanoporosity
in Noble-Metal Alloys
Mariusz Albert Bryk
Master of Applied Sciences
Department of Chemical Engineering and Applied Chemistry
University of Toronto
2012
Abstract
Nanoporosity formation by selective dissolution of Ag-5 at. pct Au in perchloric acid has
been investigated with regards to the mechanism of stress-corrosion cracking (SCC),
film-induced cleavage in particular. It has been proven that dealloying of silver-gold
systems containing low concentration of gold leads to the formation of a three
dimensional nanoporous layer and that it can be carried out in a broad range of potentials
and concentrations of a dealloying solution. Therefore, stress-corrosion cracking
observed in these alloys may be caused, initiated or at least accompanied by the
formation of nanoporosity resulting from dealloying. These results will have impact on
the fabrication of cheaper nanomaterials where there is required large surface to volume
ratio with gold as the outermost layer. Understanding the role of dealloying will also help
us to design new materials of higher resistance against stress-corrosion cracking.
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Acknowledgements
I would like to express my sincere gratitude to Professor Roger Newman for the
supervision of the whole project and giving me this incredible opportunity to become a
member of his research group. His suggestions and guidance have been invaluable and
helped me to become a better scientist. I would also like to thank Professor Roger
Newman for accepting me as a PhD student.
Special thanks towards Dorota Artymowicz for the SEM sessions that we were doing
together and Anatolie Carcea for explaining to me how one may quickly organize and
effectively operate with computer softwares designed for studying corrosion. In some
cases doing my experiments was solely possible owing to his personal engagement.
Finally, I would like to thank all my colleagues for their assistance and help that I have
received from them during the project.
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Table of Contents
Introduction ...................................................................................................................................... 1
1 Dealloying ................................................................................................................................ 2
2 The dealloying critical potential ............................................................................................... 5
3 Nanoporous metals ................................................................................................................... 9
4 Stress-corrosion cracking (SCC) ............................................................................................ 12
4.1 The surface mobility SCC mechanism ......................................................................... 14
4.2 The film-induced cleavage SCC mechanism ................................................................ 16
4.3 Short summary of the two SCC mechanisms ............................................................... 21
5 Objectives ............................................................................................................................... 22
5.1 Single-shot fracture experiments .................................................................................. 22
5.2 Electrochemistry ........................................................................................................... 23
5.3 Digital simulations ........................................................................................................ 23
6 Experimental .......................................................................................................................... 24
6.1 Materials ....................................................................................................................... 24
6.2 Samples preparation ...................................................................................................... 24
6.3 Electrochemical cell ...................................................................................................... 25
6.4 Mounting samples ......................................................................................................... 26
7 Results and Discussion ........................................................................................................... 28
7.1 Formation of nanoporosity by dealloying of Ag-5 at. pct Au ....................................... 28
7.2 The composition of a nanoporous layer formed on Ag-5 at. pct Au ............................ 29
7.3 The growth of nanoporous layers on Ag-5 at. pct Au ................................................... 30
7.4 The anodic behaviour of Ag-5 at. pct in perchloric acid .............................................. 38
7.5 Fracture experiments..................................................................................................... 41
7.6 Premature cracks and coarsening of a nanoporous layer .............................................. 41
7.7 Fracture tests ................................................................................................................. 43
8 Conclusions ............................................................................................................................ 65
9 Further work ........................................................................................................................... 67
10 References ......................................................................................................................... 68
11 Appendix ........................................................................................................................... 70
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List of Figures
Figure 1.1. Illustration of the anodic behaviour of binary alloys by Pickering (2). ............ 2
Figure 1.2. A schematic showing a cross-section of ligaments formed on the surface (2). 4
Figure 2.1. Possible misinterpretation of the critical potential by extrapolation. ................ 5
Figure 3.1. Transmission electron micrograph of Ag50 Au50 exposed to 50 % HNO3 (1).10
Figure 3.2. The growth of a gold island by the surface disordering (1). ........................... 10
Figure 3.3. Porosity evolution during selective oxidation proposed by Erlebacher (3).
Reproduced by permission of The Electrochemical Society. ............................................ 11
Figure 4.1. Crack propagation in the presence of an ionic contaminant by Galvele (19). 14
Figure 4.2. An illustration of the distances which atoms have to travel by Galvele (20).. 15
Figure 4.3. An illustration of film-induced cleavage generated during dealloying.
(Adapted from Ref.18.)...................................................................................................... 17
Figure 4.4. A cross-sectional view of a sample that was bent while maintaining the
dealloying potential (24). ................................................................................................... 18
Figure 4.5. Fracture surface after application of strain: (a) brittle and (b) ductile (25)..... 19
Figure 4.6. Crack front striations on the fracture surface of a copper single crystal (26). 20
Figure 4.7. Correlation of acoustic events with current for TG cracks in α-brass (28). .... 21
Figure 6.1. Electrochemical cell used in experiments: WE – working electrode, RE –
reference electrode, CE – counter electrode. ..................................................................... 25
Figure 6.2. Mounting a sample for a single-shot fracture test. .......................................... 26
Figure 6.3. An illustration of the electrochemical cell used in single-shot fracture tests:
WE – working electrode, RE - reference electrode, CE – counter electrode. ................... 27
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Figure 7.1. Nanoporous layer obtained by dealloying of Ag95Au5 dealloyed at E = 170
mV in 0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t
= 35,000 s. ......................................................................................................................... 28
Figure 7.2. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 5 mA/cm
2, t = 628 s. Sample fractured
in air. Marker 50 µm. ......................................................................................................... 29
Figure 7.3. EDX performed on the fracture surface of Ag95Au5 dealloyed at E = 300 mV
in 0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 5 mA/cm
2, t = 628
s. Sample fractured in air. Marker 50 µm. ......................................................................... 29
Figure 7.4. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.48 mA/cm
2, t = 772 s. .... 30
Figure 7.5. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.55 mA/cm
2, t = 532 s. .... 31
Figure 7.6. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 7.2 mA/cm
2, t = 410 s. ...... 31
Figure 7.7. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 2 C/cm2, i = 7.0 mA/cm
2, t = 296 s. ...... 32
Figure 7.8. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.8 mA/cm
2, t = 128 s. ...... 32
Figure 7.9. The observed and theoretical thickness of nanoporous layers obtained by
dealloying of Ag95 Au5 in 0.1M HClO4 at E = 300 mV. ................................................... 33
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Figure 7.10. Changes in capacitance versus charge passed during dealloying of Ag95 Au5
in ten consecutive steps at different potentials in 0.1 M HClO4 at room temperature (33).
........................................................................................................................................... 34
Figure 7.11. The development of relative area versus charge passed during dealloying of
Ag95 Au5 in ten consecutive steps at different potentials in 0.1 M HClO4 at room
temperature. (33)................................................................................................................ 35
Figure 7.12. Changes in capacitance versus charge passed during dealloying of Ag95 Au5
in ten consecutive steps at different potentials in 0.1 M HClO4 ........................................ 35
Figure 7.13. The development of relative area versus charge passed during dealloying of
Ag95 Au5 in ten consecutive steps at different potentials in 0.1 M HClO4 ........................ 36
Figure 7.14. Potentials used for potentiostatic dealloying of Ag95Au5 and Ag77Au23
performed in ten consecutive steps in 0.1 M HClO4 at room temperature. ....................... 37
Figure 7.15. Changes in capacitance versus charge passed during dealloying of Ag95Au5
and Ag77Au23 in ten consecutive steps at different potentials in 0.1 M HClO4. ................ 37
Figure 7.16. The development of relative area versus charge passed during dealloying of
Ag95Au5 and Ag77Au23 in ten consecutive steps at different potentials in 0.1 M HClO4. . 38
Figure 7.17. Current densities observed during dealloying Ag95Au5 and Ag77Au23.......... 38
Figure 7.18. Polarization curves performed on Ag95 Au5 and Ag77 Au23 in 0.1 M HClO4 at
room temperature. Scan rate 0.5 mV/s. IR drop corrected. ............................................... 39
Figure 7.19. Polarization curves performed on Ag95 Au5 and Ag77 Au23 in 1.0 M HClO4 at
room temperature. Scan rate 0.5 mV/s. IR drop corrected. ............................................... 39
Figure 7.20. Potentiodynamic curves performed on Ag95 Au5 in HClO4 of different
aeration degree at room temperature. Scan rate 0.5 mV/s. IR drop corrected. .................. 40
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Figure 7.21. Potentiodynamic curves performed on Ag95Au5 in HClO4 of different
concentration at room temperature. Scan rate 0.5 mV/s. IR drop corrected. .................... 40
Figure 7.22. Premature cracks on the surface after dealloying of Ag95Au5 at room
temperature in 0.1 M HClO4 at E = 300 mV vs. MSE. Marker 200 µm. .......................... 41
Figure 7.23.Ag95Au5 surface free from premature cracks after dealloying at room
temperature in 0.1 M HClO4 at E = 170 mV vs. MSE. Marker 20 µm. ............................ 42
Figure 7.24. Nanoporous layer after dealloying of Ag95Au5 in 0.1 M HClO4 for t = 125 s
at E = 170 mV. Charge passed Q = 50 mC/cm2. ............................................................... 42
Figure 7.25. Nanoporous layer after dealloying of Ag95Au5 in 0.1 M HClO4 for t = 2528 s
at E = 170 mV. Charge passed Q = 0.5 C/cm2. ................................................................. 43
Figure 7.26. Crack depth versus thickness of a nanoporous layer for Ag95Au5 dealloyed in
0.1M perchloric acid at 300 mV. Samples fractured beyond the dealloying solution....... 44
Figure 7.27 Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 200 A/cm
2, t = 15,000 s. Sample
fractured in the dealloying solution at E = -300 mV vs. MSE. ......................................... 45
Figure 7.28. Magnification of the fracture surface of Ag95Au5 dealloyed at E = 170 mV in
0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 200 A/cm
2, t =
15,000 s. Sample fractured in the dealloying solution at E = -300 mV vs. MSE. ............. 45
Figure 7.29. Grain boundary corrosion on the fracture surface of Ag95Au5 dealloyed at E
= 170 mV in 0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 200
A/cm2, t = 15,000 s. Sample fractured in the dealloying solution at E = -300 mV. ........ 46
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Figure 7.30. Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s. Sample
fractured in the dealloying solution at the positive potential after dealloying. ................. 46
Figure 7.31. Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s. Sample
fractured in the dealloying solution at the positive potential after dealloying. ................. 47
Figure 7.32. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s.
Sample fractured in the dealloying solution at the positive potential after dealloying. .... 47
Figure 7.33. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s. Sample
transferred and fractured in deionised water right after dealloying. .................................. 48
Figure 7.34. Close-up of the fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1
M HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s.
Sample transferred and fractured in deionised water right after dealloying. ..................... 48
Figure 7.35. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s. ..... 49
Figure 7.36. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s. ..... 49
Figure 7.37. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s ...... 50
Figure 7.38. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s ...... 50
x
Figure 7.39. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. Sample
transferred and fractured in deionised water right after dealloying. .................................. 51
Figure 7.40. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. ..... 51
Figure 7.41. Spot 2 - fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. ..... 52
Figure 7.42. Spot 3 - fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. ..... 52
Figure 7.43. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. ..................... 53
Figure 7.44. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in
air. ...................................................................................................................................... 53
Figure 7.45. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in
air. ...................................................................................................................................... 54
Figure 7.46. Close-up of the fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1
M HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 54
Figure 7.47. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 55
xi
Figure 7.48. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 55
Figure 7.49. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 56
Figure 7.50. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 56
Figure 7.51. Spot 5 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s.
Fractured in air. .................................................................................................................. 57
Figure 7.52. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 57
Figure 7.53. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 58
Figure 7.54. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 58
xii
Figure 7.55. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 59
Figure 7.56. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 59
Figure 7.57. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After
dealloying sample rinsed in deionised water and fractured in air. .................................... 60
Figure 7.58. Side surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at RT.
Charge passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s. After dealloying the sample
was rinsed in deionised water, ethanol and dried in air. Fractured in air. ......................... 61
Figure 7.59. Side surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at RT.
Charge passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s. After dealloying the sample
was rinsed in deionised water, ethanol and dried in air. Fractured in air. ......................... 61
Figure 7.60. Spot 1 - Nanoporous layer on the side surface of Ag95Au5 dealloyed at E =
170 mV in 0.1 M HClO4 at RT. Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s. ................... 62
Figure 7.61. Spot 2 – Grain boundary surface of Ag95Au5 dealloyed at E = 170 mV in 0.1
M HClO4 at RT. Charge passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s.................... 62
Figure 7.62. Potentiodynamic curves of Ag77Au22Pt1 and Ag77Au21Pt2 in 0.1 M HClO4.
Scan rate 0.5 mV/s. IR drop corrected. Ag77Au23 plays a role of a reference. .................. 63
xiii
Figure 7.63. Fracture surface. Nanoporous layer obtained from dealloying of Ag77Au23 at
60oC in 0.1 M HClO4 at E = 520 mV. Q = 1.2 C/cm
2, i = 10 mA/cm
2, t = 120 sec. Sample
fractured in air. Fracture surface. ....................................................................................... 64
Figure 7.64. Fracture surface. Nanoporous layer obtained from dealloying of Ag77Au22Pt1
at 60oC in 0.77 M HClO4 at E = 520 mV. Q = 2 C/cm
2, i = 8 mA/cm
2, t = 240 sec.
Sample fractured in the dealloying solution. Fracture surface. ......................................... 64
Figure 11.1. Polarization curves performed on Ag95 Au5 in 0.1 M and 1.0 M HClO4 at
room temperature. Scan rate 0.5 mV/s. IR drop corrected. ............................................... 70
Figure 11.2. Changes in capacitance versus charge passed during dealloying of Ag95 Au5
in ten consecutive steps at different potentials in 0.1 M HClO4 at room temperature. One
side of the sample exposed to the dealloying solution. ..................................................... 71
Figure 11.3. Changes in capacitance versus charge passed during dealloying of Ag95 Au5
in ten consecutive steps at different potentials in 0.1 M HClO4 at room temperature. Both
sides of the sample exposed to the dealloying solution. .................................................... 71
Figure 11.4. Fracturing device: A – electrochemical cell, B – mobile part made of
syringes. ............................................................................................................................. 72
Figure 11.5. Mobile part: C - the syringe tube, D – piston, E – small syringe with the
sample mounted in. ............................................................................................................ 73
Figure 11.6. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 120 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step
Q = 600 mC/cm2. ............................................................................................................... 73
xiv
Figure 11.7. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 170 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step
Q = 600 mC/cm2. ............................................................................................................... 74
Figure 11.8. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 220 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step
Q = 600 mC/cm2. ............................................................................................................... 74
Figure 11.9. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 300 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step
Q = 600 mC/cm2. ............................................................................................................... 75
Figure 11.10. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 400 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step
Q = 600 mC/cm2. ............................................................................................................... 75
Figure 11.11. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 550 mV for Ag77 Au23 in 0.1M HClO4. Charge passed in each
step Q = 0.6 C/cm2. ............................................................................................................ 76
Figure 11.12. Changes in the imaginary part of impedance after dealloying in ten
consecutive steps at E = 650 mV for Ag77 Au23 in 0.1M HClO4. Charge passed in each
step Q = 0.6 C/cm2. ............................................................................................................ 76
1
Introduction
There is growing interest in nanoporous metals for which we are finding nowadays more and
more useful applications e.g., production of sensors or filtration purposes. Nanoporous
structures may be produced in different methods. The simplest way to do that is free corrosion
in a highly concentrated oxidizing agent or the application of electrochemical techniques (1, 2).
The properties of nanoporous metals fabricated by dealloying are remarkable, for instance,
nanoporous gold obtained from dealloying of silver–gold alloys possesses impressive surface to
volume ratio (3). Moreover, after dealloying the size of ligaments can be tuned by annealing.
Since in case of silver-gold alloys the ligaments are covered with gold, such a nanoporous
material can also find various applications in catalysis. Until now, however, not much attention
has been devoted to silver-gold alloys containing low percentage of gold, for example, Ag-5 at.
pct Au which is investigated in this research. This is because it was believed that these systems
would not undergo dealloying. Apart from obvious benefits of using silver-gold alloys
containing low percentage of gold, there is another important aspect correlated to the formation
of nanoporosity. There is a general agreement that dealloying processes can be the cause of
stress-corrosion cracking (SCC) (4, 5). Thus, the ability of an alloy to undergo dealloying may
be the key characteristic for assessing whether or not one may expect stress-corrosion cracking
to occur in a given alloy. Although silver-gold alloys are not typical construction materials used
in the industry but due to their chemical and physical properties they constitute a perfect object
for studying the mechanism of stress-corrosion cracking, the understanding of which will
broaden our knowledge on corrosion and help us to limit or prevent this corrosion attack in
construction materials.
2
1 Dealloying
Selective dissolution of one constituent from an alloy has been practised for centuries. First
applications of this process date back to the Indians of pre-Columbian Central America who
developed the method of depletion gilding (1). The mechanism of dealloying became recently
particularly interesting due to the fact that selective dissolution of the more active metal of an
alloy leads not only to the enrichment of the alloy surface in the more noble metal but also,
under certain conditions, to the formation of a nanoporous layer on the surface.
A good introduction to the selective dissolution phenomenon is works carried out by Pickering
(2, 6) who investigated the electrochemical behaviour of binary alloys of the general formula
AnB(1-n), where A refers to the less noble metal. Pickering investigated binary alloys using
potentiodynamic polarization and noted that the polarization curves for different alloys
exhibited certain similarities. He singled out their common features and marked them as
particular regions, as shown in Figure 1.1.
Figure 1.1. Illustration of the anodic behaviour of binary alloys by Pickering (2).
Region (a) refers to the part of a curve which exhibits passivation-like character, similarly to
the passive region of metals that undergo electrochemical passivation on the surface.
3
The passivation-like state in region (a) is due to the surface enrichment in the more noble
metal. This prevents further dissolution of the less noble metal and visibly restricts the
dissolution current. Because of that, this part does not noticeably depend on potential. Region
(b) is a potential-dependent increasing current region. The sudden change in current results
from the selective dissolution of the less noble metal. Similarly to region (b), region (c) is also
potential-dependent with this difference that only the initial dissolution is selective in character.
The two metals dissolve in this region. Region (d) is solely due to the constant dissolution of
the less noble metal. The accumulation of the more noble metal on the surface is either not
sufficient or a nanoporous layer formed is non-adherent. Region (d) is typical for pure metals or
alloys which are very rich in the more active metal. Analyzing the difference between the
standard reduction potentials of metals and the mole fraction of the more noble metal (NB),
Pickering noted a strong dependence between the two and the anodic behaviour of an alloy. He
divided the process of selective dissolution into two different types: Type I dissolution -
containing regions (a) and (b), and Type II dissolution - made up of regions (a) and (c).
Pickering concluded that the tendency increases for the Type I dissolution along with
increasing difference between the standard reduction potentials of the two metals. The tendency
for the Type II dissolution increases with the increase in the mole fraction of the more noble
metal. Thus, there must be the highest concentration for the more noble metal in a given binary
alloy, where such a transition between the two types of dissolution behaviour occurs. Pickering
denoted this threshold as (NB*). The presence of dealloying thresholds was confirmed
experimentally by Sieradzki et al. (7) who proposed explanations for the lack of agreement
between the conventional percolation theory and the observed dealloying thresholds in alloys,
including silver-gold systems (8). Recently, an attempt was made to explain the experimental
value of parting limit in silver-gold alloys using high density percolation theory (9).
4
Not much was devoted by Pickering towards the determination of a potential at which the
transition between regions of the passivation-like state and the selective dissolution takes place.
It was, however, proposed that a potential at which the transition occurs was the critical
potential (Ec) and that further increase in polarization would result in the generation of porosity
on the surface. In case of an alloy containing gold and a less noble metal, for instance, silver, a
nanoporous structure formed will consist of ligaments made up from two different phases: a
layer of pure gold on the outside and a gold-enriched alloy inside the ligaments, as shown in
Figure 1.2.
Figure 1.2. A schematic showing a cross-section of ligaments formed on the surface (2).
Since the formation of nanoporosity leads inherently to the development of surface area,
nanoporous materials obtained by selective dissolution can find various applications, for
instance, as catalysts or high surface area electrodes for advanced applications. Another reason
for which the understanding of dealloying processes has become important is that these
processes play an important role in corrosion and may be partially or even solely responsible
for the destruction of construction materials.
5
2 The dealloying critical potential
It has been generally accepted that different reactions leading to corrosion of alloys can be
initiated or even entirely governed by dealloying processes e.g., as in the case of stress-
corrosion cracking (SCC) in Cu-Zn or Cu-Au alloys (10-12). For obvious reasons, the
determination of a potential at which dealloying occurs has attracted a great deal of attention.
As we mentioned in the first part, a selective dissolution of the less noble metal gives sudden
rise to the anodic current preceded by the passivation-like state. The dissolution of the less
noble metal will continue with the increase in polarization as long as the less noble metal is
“available” in the alloy. The determination of a dealloying critical potential may be done in
different ways. Using the method of potentiodynamic polarization one needs to locate the onset
of the rapidly growing anodic current and extrapolate it to the baseline current (Figure 2.1).
Figure 2.1. Possible misinterpretation of the critical potential by extrapolation.
At this point, one may want to ask whether the method of dynamic polarization is the best
approach for the determination of Ec? It appears not to be. Figure 2.1 shows that reading off the
value corresponding to the critical potential may be quite ambiguous due to the fact that the
6
change between regions (a) and (b) is never sharp and that, as often occurs in transition points,
the onset of the change may depend on several parameters. These will be discussed below.
In potentiodynamic polarization, scan rate is one of those fundamental parameters that must be
set before each experiment and is normally expressed in millivolts per second. Sieradzki et al.
(13) examined several silver-gold alloys in perchloric acid investigating how sweep rate and
the composition of a solution may affect the appearance of a critical potential. Starting with the
Gibbs-Thomson effect, which relates the effect of curvature on chemical potential, they derived
an equation for a critical potential of a given binary alloy of the composition ApBg (g = 1 - p),
where (B) is the more noble metal. Sieradzki et al. proved the critical potential to be a function
of both the composition of an alloy and the composition of a dealloying solution.
]))2(
(2
ln[2
)2(
2)ln( 2
2
2
g
g
aJ
ND
nqnq
k
g
g
aa
nq
TkEE
o
SSB
A
BoH
crit n Eq. 1
where:
Eo - the standard reduction potential [V], kB - Boltzmann’s constant [J/K], q – unit charge [C],
n – number of electrons, a – lattice parameter [m], Ns – the number of atoms per unit area, Ds –
surface diffusivity [m2/s], δ – dissolution potential of an alloy [V], Ω - atomic volume [m
3], γ –
isotropic solid/electrolyte interfacial free energy [J/m2], Jo – exchange current density [A/m
2],
T – temperature [K].
Sieradzki et al. (13) started with the premise that, similarly to other kinetically controlled
processes such like the brittle-ductile transition in solids, the change from the passive-like state
to the selective oxidation of the less noble metal was rate dependent. They experimentally
demonstrated that the critical potential was a function of sweep rate.
7
Basically, there are a limited number of methods available by means of which one could
precisely determine the value of a critical potential for a given alloy in a given solution. In the
past, the simplest approach was to take this potential value as the critical one, at which the
current density is i = 1 mA/cm2 (13). Unfortunately, both the older method and the method of
potentiodynamic polarization have been proven to be an overestimation.
A great deal of work was done by Dursun et al. (14) who investigated dealloying of silver-gold
systems in 0.1M perchloric acid. On the contrary to what had been believed in the past, they
observed the formation of porosity well below the empirical critical potential. The novelty of
their approach was the application of potential hold experiments in the vicinity of the critical
potential. It was noted that the dealloying current eventually became nearly constant for the
samples potentiostatically polarized above (Ec), whereas samples polarized below the
dealloying potential displayed current continually decaying with time. An important conclusion
coming from this work is that the determination of the dealloying critical potential by its
extrapolation from a potentiodynamic curve may contain a significant error. The dealloying
critical potential can be overestimated even by a hundred millivolts. In addition, the method of
potentiostatic polarization, owing to its nature, eliminates the influence from sweep rate which
affects the shape of a polarization curve. In the light of what has been said, determining a
critical potential by potentiostatic polarization appears to be more precise than doing it in the
potentiodynamic fashion, because the necessity of extrapolation may itself be very subjective
and thus, apart from other issues, produce a large error. One should notice, however, that
potentiostatic experiments can also pose a serious problem since, as it is with every experiment,
this method requires time and time is a very important parameter in the kinetically controlled
processes. The influence from time on current fluctuations in potentiostatic experiments were
further investigated by Dursun et al. (15) who proved that short-time experiments can produce
8
very illusive effects because the observed current may give an impression of its decaying
character, whilst if the same experiment lasted longer, the investigator could notice the
development of a steady-state current and conclude that we are above the critical potential.
Apart from such parameters like scan rate or the nature of a given method (15), precise
determination of a critical potential may be problematic in the presence of particular species in
a dealloying environment. Dursun et al. (16) also examined the influence of halides and
demonstrated that the dealloying critical potential for silver-gold alloys was sensitive to their
presence. They proved that the addition of halides could affect the size of ligaments and
volume of pores in the resultant nanoporous layer, for instance, its porosity in the solution
containing iodide was almost ten times larger than in the absence of this ion.
As we may see, the presence of a critical potential (Ec) is the key characteristic of various
dealloying systems. If the concentration of the more noble component in an alloy is NB ≤ NB*,
a rapid growth in current density will mark the transition from a passive-like state to the region
of selective dissolution. The location of a critical potential may depend on the following
parameters: the difference between the standard reduction potentials of the constituents (ΔEo),
the mole fraction of the more noble metal (NB), sweep rate in potentiodynamic polarization,
time in potentiostatic experiments, the composition of an alloy and the content of a dealloying
environment.
9
3 Nanoporous metals
The application of high-porosity materials is now rapidly gaining interest due to their
functionality and unique chemical and physical properties (3, 17). Nanoporous structures may
be created in different forms, for instance, bulk or thin films. Although there are other ways in
which nanoporous materials can be produced, dealloying is still most commonly used
technique. Nanomaterials obtained by dealloying exhibit large surface to volume ratio and
remain uniform on the macroscopic scale. Moreover, the size of ligaments and pores can be
controlled and tailored specifically for a given application, for instance, ligaments can be
coarsen by annealing to the required size ranging from nanometers to micrometers. There are
four basic features that must be met for an alloy to be turned into a nanoporous material: 1)
significantly large difference in the standard reduction potentials between or among (in case of
ternary alloys) the components, 2) the mole fraction of the more noble constituent cannot be too
high, 3) the material must have homogenous structure prior to dealloying, 4) surface diffusivity
of the more noble metal must be sufficiently high (3).
Generally, binary systems used for the generation of nanoporous structures should possess
similar physical properties between the constituents such as: good solubility of the metals
across a broad range of concentrations used in the production of a given alloy, similar thermal
expansion or homogeneity. Because of that, most commonly used for this purpose alloys are
silver-gold, copper-gold or other similar systems that meet the requirements mentioned above.
It should also be noted that the number of available well understood and easily applicable
dealloying techniques greatly contributes to the popularity of a particular alloy.
Historically, Forty was probably one of the first scientists, if not the first one, who proposed the
surface diffusion mechanism for the formation of porosity caused by the selective oxidation of
10
the less noble metal. Forty used in his experiments high purity single crystals of silver-gold
containing equal concentrations of the two elements. The samples were exposed to 50 % nitric
acid for 30 and 60 seconds of free corrosion (1).
Figure 3.1. Transmission electron micrograph of Ag50 Au50 exposed to 50 % HNO3 (1).
Forty noted that selective dissolution of silver led to the generation of gold-rich regions on the
surface. Annealing these samples at 720 K caused the islands to merge and grow. On the basis
of his experimentally made observations he proposed the mechanism of porosity formation
during dealloying.
Figure 3.2. The growth of a gold island by the surface disordering (1).
Based on the kinetic Monte Carlo algorithm (KMC) (8), Erlebacher performed digital
simulations of dealloying (MESOSIM) and proposed that the formation of a nanoporous
structure by selective dealloying occurs in particular steps (3), as shown in Figure 3.3.
0.1 µm
11
Figure 3.3. Porosity evolution during selective oxidation proposed by Erlebacher (3).
Reproduced by permission of The Electrochemical Society.
According to this kinetic model, the evolution of porosity on the surface exposed to a
dealloying environment begins with stripping of atoms of the less noble metal (a). The
formation and coarsening of islands of the more noble metal (b) occurs as the second step. This
process is followed by stripping atoms of the less noble metal from the next layer (c) and the
accumulation of the more noble atoms at the hills (d). Undercutting of the hills gives the typical
shape to the ligaments (e). The nucleation of new noble clusters ends the process (f).
Coarsening will continue while the electrolyte penetrates the surface.
Although the simulations proposed by Erlebacher are based on certain simplifications, for
instance, the model does not include reduction processes such as redeposition of the less noble
metal on the anode, it has been accepted that the simulations mimic very well several aspects of
nanoporosity formation in real systems. Moreover, owing to the nature of a digital simulation,
one can obtain good insight in what happens on the surface during dealloying which, for
obvious reasons, cannot be observed in situ.
Apart from the digital simulations, which aim was to visualize the growth of nanoporosity,
Erlebacher performed simulations of potentiodynamic and potentiostatic curves. Comparison of
the simulated curves with other observations revealed that: 1) the empirical critical potential
12
does not separate the transition between passivation-like region from the formation of
nanoporosity, 2) the intrinsic critical potential does not depend on the sweep rate, 3) both the
intrinsic critical potential and empirical critical potential follow the same trend with the
composition of an alloy, 4) the intrinsic critical potential falls before the empirical critical
potential, 5) surface diffusion plays several roles during dealloying (3).
4 Stress-corrosion cracking (SCC)
Stress-corrosion cracking is one the most dangerous corrosion attacks that leads to a sudden
destruction of construction materials. Such problems are commonly known and occur in a
variety of industries: shipbuilding, aircraft, mining or facilities such as underground
transmissions pipelines. The prevention against SCC requires both the appropriate design of a
construction and its constant monitoring. During this corrosion process the material, which a
given construction is made of, may not show any visible features on the surface and at the same
time it can be filled with microscopic cracks inside. Thus, a sudden catastrophic failure may
occur “without warning” and the damage identification may not be easy for a casual inspection.
In order for a material to undergo stress-corrosion cracking, a combination of three different
factors must take place: 1) susceptible material, 2) presence of stress, 3) corrosive environment.
It is often misunderstood that only metals, alloys in particular, can undergo this type of
corrosion. Stress-corrosion cracking may also occur in polymers when exposed to corrosive
chemicals. Similarly to metals, the attack is confined to particular types of polymers in
particular environments. Thus, the chemical environment causing stress-corrosion cracking in a
given material may not have this adverse effect on other materials exposed to the same
environment.
13
Although the phenomenon of SCC has been studied for many decades, certain aspects still
remain unclear, for instance, the exact mechanism of stress-corrosion cracking. Generally, we
can distinguish five types of stress-corrosion cracking (18):
Type A – occurs by intergranular slip-dissolution
Type B – passive systems; cracking is induced by Cl-
Type C – surface dealloying
Type D – caused by films other than oxides or dealloyed layers
Type E – hydrogen embrittlement
For a given occurrence of SCC, a number of different models might be proposed as plausible
explanations. Some of them could be ruled out on thermodynamic grounds and thus they
cannot be perceived and applied as a universal model that would account for all intergranular
and transgranular examples of cracking in construction materials. As it was mentioned earlier,
stress (both residual and applied) is one of the three main factors contributing to this corrosion
attack. A metal subjected to a tensile strain may release a part of this stress by the motion of
dislocations (glide, creep) in the bulk material. The internal tensions caused by an external
force can also be relaxed by the generation of cracks inside the material.
Several mechanisms of stress-corrosion cracking have been proposed over the years but two of
them deserve special attention, namely the surface mobility model (SM) proposed by Galvele
(19, 20) and the film-induced cleavage (FIC) model of Sieradzki and Newman (4, 5), partially
originating from early works of Edeleanu and Forty (21). The two models will be briefly
discussed in the next two chapters.
14
4.1 The surface mobility SCC mechanism
The surface mobility SCC mechanism has been proposed by the late José R. Galvele and is
based on the assumption that a high stress present at the tip of a crack may locally create a
vacancy deficient region (20). According to the SM mechanism, the driving force for the crack
propagation will be the surface vacancy movement towards the tip of a crack, where vacancies
are captured or, equivalently, by the flow of adatoms in the opposite direction. The movement
of surface vacancies is possible due to the action from a corrosive environment which is
thought to be the cause for the formation of low melting point species on the walls of a crack. It
is suggested that such contaminants are not only responsible for increasing the surface mobility
but they may also contribute to the appearance of surface vacancies required in this SCC
mechanism. The movement of adatoms occurs in the first atomic layers and only these are
assumed to be taking part in the propagation of a crack (20), as demonstrated in the figure
below.
Figure 4.1. Crack propagation in the presence of an ionic contaminant by Galvele (19).
Following the above assumptions, a crack propagates an atomic distance after the stressed
lattice has captured a vacancy. Thus, the diffusion of vacancies (or adatoms) on the surface will
15
be the rate controlling process. Taking this into account, Galvele proposed the following
equation for the crack propagation rate (19).
]1)[exp(...3
kT
a
L
Drpc S Eq. 2
where:
c.p.r. - crack propagation rate [m/s], Ds - surface diffusion coefficient [m2/s], L – diffusion
distance of vacancies [m], σ – elastic surface stress at the tip of a crack [N/m2], a – the atomic
size [m], k – Boltzmann’s constant [J/K], T- temperature [K].
Figure 4.2. An illustration of the distances which atoms have to travel by Galvele (20).
In general, Galvele forms the following postulates for the SM mechanism (20):
sufficiently high surface mobility is created by the environment
SCC occurs at the temperature lower than 0.5 Tm
only elastic stress is relevant
surface vacancies are captured by the tip of a crack
16
4.2 The film-induced cleavage SCC mechanism
In the great majority of instances, corrosion reactions between a metal and a corrosive
environment occur at the interface between the two phases or in the vicinity of the interface.
Corrosion products might exist in different chemical forms such like oxides, salts etc. It has
been proven in the past that, under certain conditions, corrosion products may exhibit very
specific mechanical properties (22, 23). Various examples of stress-corrosion cracking failures
have shown over years the presence of common features in the materials destroyed this way,
for instance, the appearance of brittleness (5). It has been suggested that these failures could
result from cleavage-like events introduced by a thin film formed on the surface of a metal. It
should be noted here that this brittle-like character of a fracture has also been observed in
materials that exhibit inherent ductility (5). By contrast to other explanations of SCC failures
(18), Sieradzki and Newman developed a concept (4) according to which the mechanical
properties of a nanoporous structure could be the cause of SCC in metal/solution systems.
Following this assumption, a nanoporous layer formed on the surface in the process of selective
dissolution can inject micro-cracks into the “healthy” part of the material untouched by
corrosion. It was suggested that a minimal thickness of a nanoporous layer is required for this
type of fracture to occur (4, 5). On the contrary to the surface mobility SCC concept, which
assumes a continuous character of a fracture, Sieradzki and Newman have proved that a
cleavage event may occur in a series of consecutive steps (4) and will penetrate the material
until its arrest. The arrest might be due to several reasons such like crack bifurcation at a triple
point, exhaustion etc. For the reasons that were discussed previously, selective dealloying will
function here as the driving force because it can be repeated after each cleavage event. If the
conditions remain unchanged, that is, the stress and corrosive environment are still present, the
whole process of selective dissolution and crack propagation may happen again and will
17
proceed this way until the crack has entirely penetrated the material. An illustration of this
process is shown below.
Figure 4.3. An illustration of film-induced cleavage generated during dealloying. (Adapted
from Ref.18.)
Several different experiments have been carried out in the past when investigating a variety of
systems susceptible to stress-corrosion cracking. In the film-induced cleavage model, the most
important aspect lies in its basic assumption, that is, corrosion reactions leading to the
generation of nanoporosity can be separated from the action of stress. Following this
assumption, after a nanoporous layer has been formed on the metal surface, a sudden
application of stress can “trigger” the nanoporous layer to inject a crack into the uncorroded
part of the substrate. By doing it with the proper strain rate and at the appropriate potential,
other SCC mechanisms could simply be ruled out owing to the fact that the reactions, which are
thought to be the cause of these mechanisms, would need more time to occur.
The above aspects were investigated by Kelly et al. (24) who carried out experiments on silver-
gold alloy (Ag-20 at. pct Au) in 1M HClO4 applying stress under different conditions to the
pre-dealloyed samples. The purpose of this work was to confirm the ability of nanoporous
layers to generate a brittle cleavage into the “healthy” part of material. The single-shot fracture
experiments were performed in and out of the dealloying solution and it has been successfully
stress-corrosion crack
nanoporous layer brittle crack
plastic blunting
crack-arrest mark (striation)
18
proven that only under particular conditions the mechanical properties of a nanoporous layer
allowed a crack to reach a sufficient velocity and penetrate the uncorroded substrate.
Figure 4.4. A cross-sectional view of a sample that was bent while maintaining the dealloying
potential (24).
The tendency for a brittle fracture disappeared in the case of samples fractured in air. This
suggests that nanoporous layers are very susceptible to ageing which can reverse the
embrittlement. Kelly et al. (24) reported that both intergranular (IG) and transgranular (TG)
cracks were observed in the investigated samples. This stays in accordance to the observations
previously made by Newman et al. who did similar experiments on alpha-brass (Cu-35 wt.%
Zn) in cuprous ammonia solution (25) prior to the experiments on Ag-20 at. pct Au. Apart from
obvious differences regarding the composition of the alloys and dealloying environments, the
significant distinction between these two investigations is that the tests performed on alpha-
brass were performed without application of a potential, that is, by free corrosion of the metal
for up to 100 minutes. After the exposure, several procedures were undertaken before the
samples were strained. Brittle fractures were observed only in two cases; in the samples
strained right after dealloying and these ones which were transferred to liquid nitrogen and
strained to fail right after, as shown in Figure 4.5.
19
Figure 4.5. Fracture surface after application of strain: (a) brittle and (b) ductile (25).
Identically to the experiments on the silver-gold alloy, rapid ageing of nanoporous layers was
observed in alpha-brass. Tests with shorter immersion (10 minutes) produced similar effects
with this difference that the brittleness observed was not this pronounced as it was in the case
of long immersion tests (100 minutes).
The presence of transgranular SCC events in different copper-zinc systems was very well
documented over the past years. However, one may ask here a question whether it would be
possible for SCC to occur in pure copper. Sieradzki et al. (26) examined the behaviour of pure
copper single crystals exposed to 1 M NaNO2 and demonstrated a detailed fractographic study
on that. The novelty of this study was the combined application of single crystals and potential.
The samples were polarized (E = 0.0 V vs. SCE) during slow strain rate tests.
The striations seen on the fracture surface of a copper single crystal, as tiny horizontal lines
(Figure 4.6), suggest that the crack growth could be cleavage-like in its character because the
striations are parallel to the crack front. This brittle and cleavage-like appearance of cracks is
similar to the previously observed in Cu-Zn alloys (25, 27) and indicates that the crack
propagation may have the same or a similar fracture mechanism.
20
Figure 4.6. Crack front striations on the fracture surface of a copper single crystal (26).
Apart from various single-shot fracture experiments on pre-dealloyed samples followed by
microscopic assessment, transgranular stress-corrosion cracking in Cu-Zn systems was also
investigated by the analysis of acoustic emission that may accompany SCC events. The main
idea behind these experiments is that if stress-corrosion cracking propagates in a given material
as a series of short cleavage-like events, it is possible that a single event produces enough of
acoustic emission which can be amplified, recorded and correlated with the corresponding
increase in the anodic current. Similarly to the works done but by Pugh et al., Newman and
Sieradzki (28) carried out an investigation whose purpose was to demonstrate the discontinuous
character of transgranular events in α-brass. The tests were performed in 1M sodium nitrate
with potential held at E = 0 V vs. SCE. The test electrodes were fractured using a tensile
machine. The results of the acoustic emission were compared with the current variations
proving the discontinuous character of transgranular SCC in α-brass, as shown in Figure 4.7.
Scanning electron microscopy performed on the investigated samples also confirmed the
intermittent progress of cracks in this alloy (28).
21
Figure 4.7. Correlation of acoustic events with current for TG cracks in α-brass (28).
4.3 Short summary of the two SCC mechanisms
In the light of what has been discussed above, one may notice that there are certain similarities
and differences between the two SCC mechanisms. Both models assume that the environment
plays a fundamental role, that is, it initiates the process and increases the dissolution current. As
regards the film-induced cleavage model, this mechanism requires the composition of an alloy
to be in the appropriate ratio. A nanoporous layer can only be formed on the surface, if the
percentage of the more noble component is high enough but does not exceed its maximum
either (NB*). If it does, the two constituents will be oxidized (at higher potentials) without the
formation of nanoporosity. On the other hand, nanoporosity will not be generated when the
concentration of the more noble component is too low, that is, NB < 3% (2). Note that its
presence in the minimum concentration is strongly required for covering the ligaments during
dealloying. By contrast, the SM model assumes no correlation between SCC and dealloying,
thus the range of compositions which an alloy may stay in is much broader. Following the SM
model assumptions, the propagation of cracks is continuous, whereas in the film-induced
cleavage model, the crack progress is assumed to be a series of short cleavage-like steps.
22
5 Objectives
The main objective of this research is to investigate the mechanism of stress-corrosion cracking
in metals, film-induced cleavage in particular. Following film-induced cleavage assumptions,
the action of stress and anodic dissolution are thought to be discrete aspects that can be
separated in time (29, 30). Because film-induced cleavage is correlated with dealloying, the
formation of nanoporosity will constitute an integral part of this research. By contrast to what
has been suggested by Galvele (31, 32), we believe that formation of nanoporosity in silver-
gold alloys containing low concentration of gold is possible and therefore dealloying may
accompany or be the cause of stress-corrosion cracking observed in these alloys. For the
purpose of this investigation, binary and ternary noble-metal alloys such as silver-gold and
silver-gold-platinum have been chosen. The investigation is divided into three different parts:
1) electrochemistry, 2) single-shot fracture tests and 3) digital modelling. The latter issue,
although not performed by the present researcher, will constitute an attempt to correlate the
dealloying behaviour of the alloys in real conditions with those modelled digitally (33).
5.1 Single-shot fracture experiments
The purpose of single-shot fracture tests is to find the conditions under which film-induced
cleavage occurs. These experiments will be performed with help of a lab-constructed device by
means of which it will be possible to fracture a sample at any time of experiment, that is, when
the physical properties of a nanoporous layer formed have met the assumed requirements. The
device will allow on fracturing samples by the application of a sudden strain, in or out of a
dealloying solution, at room or elevated temperature. Samples fractured in this way will be
examined with scanning electron microscopy (SEM) and the following aspects will be
investigated: crack depth, thickness of a nanoporous layer, intergranular stress-corrosion
23
cracking (IGSCC), transgranular stress-corrosion cracking (TGSCC) and grain boundary
corrosion
5.2 Electrochemistry
The aim of this part is to answer how electrochemical conditions e.g., a potential applied,
dealloying time, temperature or the concentration of a dealloying solution, can affect the
following features of a nanoporous layer: 1) the size of ligaments, 2) development of surface
area versus charge passed, 3) thickness of a nanoporous layer versus charge passed.
Since the formation of nanoporosity occurs in the region of selective dissolution, the
experiments will need to be carried out above the critical potential. This requires its value to be
established in the first instance.
5.3 Digital simulations
In this particular part of the research, the present student will contribute by performing required
electrochemical experiments (33). The digital modelling will be carried out using the modified
version of the software MESOSIM. We will attempt to correlate the dealloying behaviour of a
particular noble alloy with Kinetic Monte Carlo simulations of the dealloying process.
Specifically, the issue of retained residual material in ligaments and the development of surface
area during dealloying will be under consideration. The results will then be applied in further
quantifications.
24
6 Experimental
The anodic behaviour of Ag-Au alloys has been investigated in perchloric acid of different
concentrations. The present researcher did also an attempt to examine Ag-Au-Pt alloys, but due
to the amount of work which these alloys required, only a few significant results were obtained.
6.1 Materials
High purity binary alloys (99.99 %) of the compositions Ag-5 at. pct Au and Ag-23 at. pct Au
were provided by Goodfellow Metals in the form of thin foil 100 µm thick. Ternary alloys
(99.95 %) of the following compositions: Ag77Au22Pt1, Ag77Au21Pt2, Ag77Au19Pt4 were
supplied by Ames Laboratory in the form of rolled strips approximately 3 cm x 30 cm. Their
thickness varied from 100 µm to 300 µm.
6.2 Samples preparation
Prior to use in the electrochemical cell, the materials were cut into rectangular pieces, 10 mm x
3 mm (for single-shot fracture tests) and 5mm x 6mm (other purposes), abraded manually with
Buehler 2500 grit sand paper, polished with a diamond paste (1 m) and thoroughly rinsed with
deionised water and acetone. Samples prepared this way were ultrasonically cleaned up in
ethanol and annealed in two different ways: at 900oC for one hour in air and for two hours in
argon with 2.5% of hydrogen. After annealing, the samples annealed in air were furnace cooled
and water quenched after the temperature had dropped to approximately 400oC. The samples
annealed in the gas were furnace cooled until the temperature had dropped below 180oC and
water quenched. Care was taken so as not to apply any cold work to the annealed samples
during the preparations.
25
6.3 Electrochemical cell
The three electrode electrochemical cell consisted of a 500 mL glass container with the average
volume of a solution 200 mL.
Figure 6.1. Electrochemical cell used in experiments: WE – working electrode, RE – reference
electrode, CE – counter electrode.
In all electrochemical investigations, Gamry 600 and Gamry Femtostat potentiostats were
employed. Single-shot fracture tests were carried out in a lab-made device that has been
designed by the present researcher. All the solutions of perchloric acid were prepared from
stock chemicals and deionised water of the specific resistivity 18 MΩ-cm obtained from a
Barnstead Nanopure system. Experiments were carried out at room temperature. A saturated
mercury/mercurous sulphate electrode (MSE) was used as a reference electrode (E = 640 mV
versus the standard hydrogen electrode SHE). In order to limit any interactions with the
sulphate ion, during all experiments the reference electrode was always residing in a double
junction. All the potentials in this work are given versus the MSE electrode. A five centimetres
long platinum wire was used as a counter electrode. Before each experiment the platinum wire
was cleaned up electrochemically for 5 minutes in 0.25 M HNO3. In some cases a dealloying
solution was deaerated by purging it for 30 minutes with nitrogen.
WE RE N2 CE
26
6.4 Mounting samples
Depending on the type of an experiment, samples were mounted in three different ways.
Samples destined for potentiodynamic experiments were covered with lacquer (Microshield) so
that only one surface area was exposed to the solution. This was done to eliminate interference
from the edges which normally tend to corrode at higher rates than flat areas. Another type of
mounting was employed for samples that were to be fractured after dealloying (Figure 6.2).
These were soldered to a copper wire and covered with lacquer in such a way that only the
central part of the sample was exposed to the dealloying solution. Copper wire was used to
form a small loop attached to the sample, the purpose of which was to block the sample during
fracturing it by a short and rapid pull-up. The loop (0.1 - 0.12 g) was also covered with lacquer.
Figure 6.2. Mounting a sample for a single-shot fracture test.
Figure 6.3 illustrates the way samples were fractured in all single-shot fracture tests. Since each
experiment was carried out on a thin (100 µm) and very soft material, fracturing samples
required doing it in a sufficiently reliable way, that is, without bending the sample prior to the
fracture. A 60 mL syringe was adapted for this purpose. Holes were drilled in the syringe tube
in order to provide a free flow of the solution. A deep hole made in the centre of the piston
allowed on embedding samples in an easy way. An insulated piece of a metal drill was led
through the loop each time after loading the sample in the tube. The piston was kept
immobilized over each experiment until the final pull-up breaking a sample in two, after which
loop sample joint coated copper wire
27
a sample was transferred to deionised water. Since straining samples was always done
manually, the average time required for fracturing a sample was assumed as t = 0.5 s.
Figure 6.3. An illustration of the electrochemical cell used in single-shot fracture tests: WE –
working electrode, RE - reference electrode, CE – counter electrode.
Occasionally, samples used for any other purposes than the ones described above, were
prepared just by soldering the sample to a copper wire and insulating the joint with lacquer.
The wire was coated with heat-shrink tubing providing sufficient protection.
RE CE N2
WE
28
7 Results and Discussion
7.1 Formation of nanoporosity by dealloying of Ag-5 at. pct Au
According to percolation theory (33), in order to form a connected random structure, the critical
volume fraction should be at least 16%. Following this assumption, and taking into account the
composition of the investigated alloy, the formation of a 3D structure will only be possible if
the nanoporous layer contains the residual silver, which has not been removed during
dealloying. In other words, if the whole silver is dissolved, a nanoporous layer will not be
formed due to the insufficient concentration of gold in the alloy. Figure 7.1 presents a
nanoporous layer obtained by dealloying of Ag-5 at. pct Au in 0.1M HClO4.
Figure 7.1. Nanoporous layer obtained by dealloying of Ag95Au5 dealloyed at E = 170 mV in
0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s.
The above result has significant implications. The formation of nanoporosity by dealloying of
alloys containing low percentage of the more noble metal can be achieved. Thus, a dealloying
based mechanism of stress-corrosion cracking is possible in Ag-5 at. pct Au. This is in
contradiction to what was suggested by Galvele (31, 32).
Side surface
Fracture surface
Thickness
29
7.2 The composition of a nanoporous layer formed on Ag-5 at. pct Au
The composition of a nanoporous layer has been determined with SEM/EDX performed on the
fracture surface, as shown in Figure 7.2.
Figure 7.2. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 5 mA/cm
2, t = 628 s. Sample fractured in air.
Marker 50 µm.
Figure 7.3. EDX performed on the fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1
M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 5 mA/cm
2, t = 628 s. Sample
fractured in air. Marker 50 µm.
Ag 83%
Ag 95%
30
The above result confirms the assumption regarding the expected high concentration of the
residual silver in ligaments. This fact can be very important from a synthetic point of view
because nanoporous layers fabricated from silver-gold alloys containing low percentage of gold
will be cheaper than those obtained by dealloying of Ag77Au23. Moreover, rich in silver
ligaments covered with a thin layer of gold may display similar properties as gold rich Ag-Au
alloys and could successfully be used in catalytic processes.
7.3 The growth of nanoporous layers on Ag-5 at. pct Au
In order to assess how the thickness of a nanoporous layer depends on charge passed, there was
performed a series of potentiostatic experiments at E = 300 mV during which different amounts
of charge were passed. The results of these experiments are presented in the next five figures.
All the current densities presented under the figures have been averaged.
Figure 7.4. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.48 mA/cm
2, t = 772 s.
31
Figure 7.5. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.55 mA/cm
2, t = 532 s.
Figure 7.6 and Figure 7.7 show the continual reduction in the observed thickness of a
nanoporous layer along with smaller amounts of charge passed during dealloying.
Figure 7.6. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 7.2 mA/cm
2, t = 410 s.
32
Figure 7.7. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 2 C/cm2, i = 7.0 mA/cm
2, t = 296 s.
Figure 7.8. Nanoporous layer obtained by dealloying of Ag95Au5 at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 1 C/cm2, i = 7.8 mA/cm
2, t = 128 s.
The conclusion coming from the above presented figures is that the thickness of a nanoporous
layer depends on the charge passed. We need to bear in mind, however, that the samples were
transferred from one environment to another during the preparations. This could lead to certain
physical changes. Thus, it was necessary to illustrate a theoretical thickness of a nanoporous
33
layer and compare it with the observed thickness (Figure 7.9). A theoretical thickness was
calculated by making two different assumptions: 1) the percentage of residual silver in the
ligaments is Ag = 83 % (as determined with EDX), 2) there is no residual silver in the
ligaments (the nanoporous layer consists of gold). The calculations have been done using the
following equation (33).
][AgFd
qMH
Ag
Ag Eq. 3
where:
q – charge passed [C/cm2], MAg – molar mass of silver, dAg – density of silver, F – Faraday’s
constant, [Ag] – the original content of silver in the alloy, - the fraction of available silver
removed during dealloying.
Figure 7.9. The observed and theoretical thickness of nanoporous layers obtained by dealloying
of Ag95 Au5 in 0.1M HClO4 at E = 300 mV.
34
The aim of further experiments was to investigate the development of surface area in
nanoporous layers during their formation. An assumption was made that dealloying at different
potentials would take different amount of time for passing the same charge. Since nanoporous
layers undergo physical changes such as coarsening or collapse, it seemed to be possible that
these changes could be observed by indirect measurement of capacitance. Thus, for the purpose
of these experiments, there were selected five potentials: 120 mV, 170 mV, 220 mV, 300 mV
and 400mV. Electrochemical impedance spectroscopy (EIS) was employed because this
method has been proven to be capable of measuring surface area of nanoporous layers (34).
Every potentiostatic experiment consisted of the same sequence: dealloying and EIS
measurement. During each step, equal amount of charge Q = 0.6 C/cm2 was passed. Impedance
was measured at constant potential E = 70 mV. The results of all the potentiostatic experiments
are shown in Figure 7.10 and Figure 7.11. See the Appendix for the raw impedance data.
Figure 7.10. Changes in capacitance versus charge passed during dealloying of Ag95 Au5 in ten
consecutive steps at different potentials in 0.1 M HClO4 at room temperature (33).
7.55 E-4 [F/cm2]
35
Figure 7.11. The development of relative area versus charge passed during dealloying of Ag95
Au5 in ten consecutive steps at different potentials in 0.1 M HClO4 at room temperature. (33)
Figure 7.12 and Figure 7.13 present the above results in a logarithmic scale.
Figure 7.12. Changes in capacitance versus charge passed during dealloying of Ag95 Au5 in ten
consecutive steps at different potentials in 0.1 M HClO4
Area increased 20 times
36
Figure 7.13. The development of relative area versus charge passed during dealloying of Ag95
Au5 in ten consecutive steps at different potentials in 0.1 M HClO4
The results demonstrated in Figures 7.10 and Figure 7.11 show that there is the direct
correlation between the changes in capacitance and surface area. In the case of the highest
dealloying potential, the developed surface area was 1700 larger than the initial one. Regardless
of the applied potentials, no decrease in capacitance was observed. This proves that, under the
applied conditions, dealloying of Ag-5 at. pct Au results in the continual development of
surface area and that the application of electrochemical impedance spectroscopy is possible for
the characterization of nanoporous layers during their formation. The reproducibility of the
above experiments is displayed in Figure 11.2 and Figure 11.3 in the Appendix. One needs to
keep in mind that in case of potentiostatic experiments performed at lower potentials, long
exposure of a nanoporous layer to the dealloying solution will lead to coarsening of ligaments.
This could be minimized by carrying out potentiostatic experiments in a dealloying solution of
higher concentrations. As it will be shown in the next chapter (Figure 7.21), current density
increases along with the concentration of a dealloying solution.
37
The development of surface area during dealloying of Ag-5 at. pct Au in 0.1M perchloric acid,
has been compared with Ag-23 at. pct Au. Since the empirical critical potentials for Ag-5 at.
pct Au and Ag-23 at. pct Au are different (Figure 7.18), the assumption was made that in order
to quantify the two alloys, one must stay in the same distance from the critical potential of Ag-5
at. pct Au and Ag-23 at. pct Au. The below table presents the potentials applied.
Alloy The empirical critical
potential vs. MSE
Potential applied I Potential applied II
Ag-5 at. pct Au Ec = 50 mV E = 300 mV E = 400mV
Ag-23 at. pct Au Ec = 300 mV E = 550 mV E = 650mV
Figure 7.14. Potentials used for potentiostatic dealloying of Ag95Au5 and Ag77Au23 performed
in ten consecutive steps in 0.1 M HClO4 at room temperature.
The results of these experiments are shown in Figure 7.15 and Figure 7.16.
Figure 7.15. Changes in capacitance versus charge passed during dealloying of Ag95Au5 and
Ag77Au23 in ten consecutive steps at different potentials in 0.1 M HClO4.
38
Figure 7.16. The development of relative area versus charge passed during dealloying of
Ag95Au5 and Ag77Au23 in ten consecutive steps at different potentials in 0.1 M HClO4.
Changes in capacitance observed during dealloying of Ag-23 at. pct Au (Figure 7.15) indicate
that the development of surface area is significantly different compared to Ag-5 at. pct Au, as
shown in Figure 7.16. The table below shows the average current density vs. potential applied.
Ag-5 at. pct Au E = 300 mV i = 9.0 mA/cm2
Ag-5 at. pct Au E = 400 mV i = 20 mA/cm2
Ag-23 at. pct Au E = 500 mV i = 5.4 mA/cm2
Ag-23 at. pct Au E = 650 mV i = 17.5 mA/cm2
Figure 7.17. Current densities observed during dealloying Ag95Au5 and Ag77Au23.
7.4 The anodic behaviour of Ag-5 at. pct in perchloric acid
The anodic behaviour of Ag-5 at. pct Au has been investigated in perchloric acid. The obtained
potentiodynamic curves, Figure 7.18 and Figure 7.19, confirm the typical shift in a critical
potential in relation to the composition of a binary alloy. This was reported in the past (2, 15).
39
Figure 7.18. Polarization curves performed on Ag95 Au5 and Ag77 Au23 in 0.1 M HClO4 at room
temperature. Scan rate 0.5 mV/s. IR drop corrected.
Figure 7.19. Polarization curves performed on Ag95 Au5 and Ag77 Au23 in 1.0 M HClO4 at room
temperature. Scan rate 0.5 mV/s. IR drop corrected.
The empirical critical potential for Ag-5 at. pct Au was further investigated in 0.1M perchloric
acid of different degree of aeration and concentration, as shown in Figure 7.20 and Figure 7.21.
40
Figure 7.20. Potentiodynamic curves performed on Ag95 Au5 in HClO4 of different aeration
degree at room temperature. Scan rate 0.5 mV/s. IR drop corrected.
Figure 7.21. Potentiodynamic curves performed on Ag95Au5 in HClO4 of different
concentration at room temperature. Scan rate 0.5 mV/s. IR drop corrected.
All the potentiodynamic experiments displayed excellent reproducibility. Similar curves
performed on Ag-5 at. pct Au in 1.0 M and 0.1 M HClO4 are presented in the Appendix.
41
7.5 Fracture experiments
Prior to single-shot fracture tests, there was performed a series of experiments whose aim was
to examine physical properties of nanoporous layers. Following the assumptions of film-
induced cleavage, a nanoporous layer formed on the surface must be of the appropriate
thickness in order to trigger a brittle crack. The presence of premature cracks in the nanoporous
layer or coarsening of the layer could have adverse effects in these experiments. Due to the
above, only potentials giving sufficiently high rate of dealloying were used.
7.6 Premature cracks and coarsening of a nanoporous layer
Figure 7.22 and Figure 7.23 show the presence of premature cracks on the side surface and
their absence after dealloying performed at two different potentials. This fact was taken into
account during fracture tests. Figure 7.22 also confirms the observations that were reported in
the past (35, 36).
Figure 7.22. Premature cracks on the surface after dealloying of Ag95Au5 at room temperature
in 0.1 M HClO4 at E = 300 mV vs. MSE. Marker 200 µm.
42
Figure 7.23.Ag95Au5 surface free from premature cracks after dealloying at room temperature
in 0.1 M HClO4 at E = 170 mV vs. MSE. Marker 20 µm.
Closer examination of nanoporous layers revealed coarsening of ligaments and its strong
dependence on time, as shown in Figure 7.24 and Figure 7.25.
Figure 7.24. Nanoporous layer after dealloying of Ag95Au5 in 0.1 M HClO4 for t = 125 s at E =
170 mV. Charge passed Q = 50 mC/cm2.
Note that the ligaments seen in Figure 7.24 are almost three times smaller than the ligaments
shown in Figure 7.25. This is due to the shorter time of dealloying not smaller charge passed.
43
Figure 7.25. Nanoporous layer after dealloying of Ag95Au5 in 0.1 M HClO4 for t = 2528 s at E
= 170 mV. Charge passed Q = 0.5 C/cm2.
7.7 Fracture tests
The main goal of single-shot fracture tests was to find the conditions under which transgranular
or intergranular film-induced cleavage could be generated. For this purpose, there was
performed a series of experiments in which both charge passed and potential applied were
controlled. Generally, each such an experiment consisted of the following steps:
dealloying a sample at a given potential
fracturing the sample after dealloying
SEM imaging
Fracturing samples after dealloying was performed in three different environments:
1) in the dealloying solution (at the positive or negative potential), 2) in deionised water and
3) in air.
There has been identified a linear relation between the thickness of a dealloyed layer and crack
depth for Ag-5 at. pct Au at E = 300 mV, as shown in Figure 7.26.
44
Figure 7.26. Crack depth versus thickness of a nanoporous layer for Ag95Au5 dealloyed in 0.1M
perchloric acid at 300 mV. Samples fractured beyond the dealloying solution.
Since dealloying at higher potentials had produced premature cracks (Figure 7.22), it was
assumed that single-shot fracture tests should be performed on the material pre-dealloyed at
lower potentials, neglecting the fact that dealloying at lower potentials inherently extends time
of the process and introduces coarsening. Based on the results presented in Figures 7.4 to 7.8,
the amount of charge passed, Q = 3 C/cm2, was assumed to be high enough for generating
sufficiently thick nanoporous layer that could trigger a brittle crack. Several fracture tests were
performed during which samples were fractured at the negative potential (E = -300 mV vs.
MSE). This was done in order to eliminate or limit faradaic reactions that could result in
superficial dealloying of the freshly revealed fracture surface. The negative polarization was
usually done from the dealloying potential in one step, after which short time (5-7 s) was given
for the cathodic current to saturate at the constant level. Similar fracture experiments to these
performed at E = -300 mV were also performed at E = -150 mV. Identical results were
observed in both cases. An example of such experiments is shown in Figures 7.27 to 7.29.
45
Figure 7.27 Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 200 A/cm
2, t = 15,000 s. Sample fractured in the
dealloying solution at E = -300 mV vs. MSE.
Figure 7.28 shows a magnification of the area marked with the rectangle in Figure 7.27.
Figure 7.28. Magnification of the fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1
M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 200 A/cm
2, t = 15,000 s.
Sample fractured in the dealloying solution at E = -300 mV vs. MSE.
One may have an impression that the fracture surface seen in Figure 7.27 has a brittle character
with only a tiny ductile region visible in the centre of the sample. Higher magnification of the
46
brittle part, however, revealed the presence of features that could be a deposition or superficial
dealloying on the fracture surface, as shown in Figure 7.29.
Figure 7.29. Grain boundary corrosion on the fracture surface of Ag95Au5 dealloyed at E = 170
mV in 0.1 M HClO4 at room temperature. Charge passed Q = 3 C/cm2, i = 200 A/cm
2, t =
15,000 s. Sample fractured in the dealloying solution at E = -300 mV.
Since the negative polarization could cause re-deposition of silver, fracture tests were also
performed at the positive potential. The results are shown in Figures 7.30 to 7.32.
Figure 7.30. Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s. Sample fractured in the
dealloying solution at the positive potential after dealloying.
47
Figure 7.31 shows the magnification of the area marked in Figure 7.30.
Figure 7.31. Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s. Sample fractured in the
dealloying solution at the positive potential after dealloying.
Figure 7.32 presents the magnification of the spot 1 marked in Figure 7.31.
Figure 7.32. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 100 A/cm
2, t = 35,000 s. Sample fractured
in the dealloying solution at the positive potential after dealloying.
Because fracture tests performed in a dealloying environment inherently involve sudden
exposure of fresh areas of the tested material to the dealloying solution, other single-shot
1
48
fracture tests on pre-dealloyed samples were performed beyond the dealloying solution. This
was done to eliminate the presence of solution that could cause corrosion after the fracture.
Figure 7.33. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s. Sample transferred and
fractured in deionised water right after dealloying.
Figure 7.34 shows the deepest crack found on the sample demonstrated in the figure above. The
fracture surface was examined with SEM at four different spots, as shown below.
Figure 7.34. Close-up of the fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s. Sample
transferred and fractured in deionised water right after dealloying.
1
2
3 4
49
The following four figures will present magnifications of the four spots marked in Figure 7.34.
Note that the depth of pits on the surface decreases along with the distance from the dealloyed
layer on the surface. This suggests that corrosion along grain boundaries has a gradual
character. Figure 7.38 presents corrosion-free surface found on the investigated fracture
surface.
Figure 7.35. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s.
Figure 7.36. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s.
50
Figure 7.37. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s
Figure 7.38 shows the magnification of the spot 4 marked in Figure 7.34.
Figure 7.38. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 5 C/cm2, i = 6.5 mA/cm
2, t = 772 s
Similarly to the results presented above, Figure 7.39 shows a fracture test performed on a pre-
dealloyed sample. The only difference is smaller amount of charge passed (Q = 3 C/cm2). The
nanoporous layer obtained by passing the above charge was assumed to be thick enough
(Figure 7.6) to trigger a brittle crack.
51
Figure 7.39. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s. Sample transferred and
fractured in deionised water right after dealloying.
The next three figures will demonstrate magnifications of the spots marked in Figure 7.39.
Figure 7.40 shows superficial dealloying on the brittle part of the fracture surface. Since this
area of the sample was exposed to the dealloying solution for longer time than the deeper part,
one may expect more intense coarsening of ligaments.
Figure 7.40. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s.
1
2
3
52
Figure 7.41. Spot 2 - fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s.
Figure 7.42. Spot 3 - fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s.
Figure 7.43 demonstrates magnification of the rectangular area marked in Figure 7.42.
Similarly to Figure 7.38, the fracture surface is free from corrosion. This indicates that the
penetration of the material by dealloying processes has reached its maximum depth.
53
Figure 7.43. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 3 C/cm2, i = 7.0 mA/cm
2, t = 410 s.
On the basis of the results presented so far, one can make a conclusion that in some cases
corrosion seen on the fracture surface could result from reactions caused by the dealloying
solution transferred to DI water together with the sample. The next set of experiments was
performed to eliminate this possibility (Figure 7.44 to 7.51). Before fracturing in air, the
samples were rinsed with DI water after dealloying and then fractured.
Figure 7.44. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
54
Figure 7.45 demonstrates the deepest crack found on the sample presented above.
Figure 7.45. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
The next figures will show magnifications of the five spots marked in Figure 7.46. Note the
preferential attack of dealloying along the grain boundary.
Figure 7.46. Close-up of the fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M
HClO4 at room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured
in air.
1
2
3
4 5
Uniform progress
of dealloying
Preferential progress
of dealloying
55
Closer examination of the spots marked in Figure 7.46 also showed surface dealloying. The
degree of grain boundary corrosion varied along with the distance from the side surface.
Figure 7.47. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
Figure 7.48. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
In some instances corrosion pits were found to be significantly deeper at the ductile part of the
fracture surface. This is seen in Figure 7.47 and Figure 7.50. The difference in the depth of pits
56
and degree of dealloying may be due to the limited access of the solution and different time
during which a given part of the sample was exposed to the solution during dealloying.
Figure 7.49. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
Figure 7.50. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
Also in this case, corrosion-free surface area was found, as shown in Figure 7.51. This means
that the material has been dealloyed along the grain boundaries only to some depth.
57
Figure 7.51. Spot 5 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 4 C/cm2, i = 7.2 mA/cm
2, t = 532 s. Fractured in air.
Similarly to the results demonstrated above, Figure 7.52 to Figure 7.57 will show a fracture test
performed on pre-dealloyed samples fractured in air, after dealloying performed at E =300 mV
during which smaller amount of charge was passed. This was done to limit the penetration of
grain boundaries.
Figure 7.52. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying sample
rinsed in deionised water and fractured in air.
58
Figure 7.53 shows the magnification of the rectangle marked in Figure 7.52 and four marked
spots which magnifications are presented in Figures 7.54 to 7.57.
Figure 7.53. Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at room
temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying sample
rinsed in deionised water and fractured in air.
Figure 7.54. Spot 1 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying
sample rinsed in deionised water and fractured in air.
1
2
3
4
59
Figure 7.55. Spot 2 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying
sample rinsed in deionised water and fractured in air.
Figure 7.56. Spot 3 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying
sample rinsed in deionised water and fractured in air.
60
Figure 7.57. Spot 4 - Fracture surface of Ag95Au5 dealloyed at E = 300 mV in 0.1 M HClO4 at
room temperature. Charge passed Q = 1 C/cm2, i = 7.3 mA/cm
2, t = 128 s. After dealloying
sample rinsed in deionised water and fractured in air.
The results demonstrated until now showed three different environments in which samples
were fractured, that is, in the dealloying solution, in DI water and in air. Since the smallest
amount of charge passed was Q = 1 C/cm2, a conclusion was made that only very shallow
dealloying could limit the propagation of corrosion processes along grain boundaries. For this
purpose, there was performed an experiment in which the sample was dealloyed at the potential
which never caused premature cracks. Compared with all the presented results, the amount of
charge passed in this experiment was significantly smaller (Q = 50 mC/cm2). After dealloying,
the sample was transferred from the dealloying solution to deionised water, rinsed with ethanol
and dried in air (Figure 7.58 to Figure 7.61). Although the fracture surface exhibited only
ductile features, the applied strain revealed grain boundaries which were examined. In this case
crack depth to the thickness of the nanoporous layer was beyond expectations.
61
Figure 7.58. Side surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at RT. Charge
passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s. After dealloying the sample was rinsed in
deionised water, ethanol and dried in air. Fractured in air.
Figure 7.59 shows the magnification of the rectangle marked in Figure 7.58.
Figure 7.59. Side surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M HClO4 at RT. Charge
passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s. After dealloying the sample was rinsed in
deionised water, ethanol and dried in air. Fractured in air.
The next two figures show magnifications of the spots 1 and 2, marked in Figure 7.59.
1
2
62
Figure 7.60. Spot 1 - Nanoporous layer on the side surface of Ag95Au5 dealloyed at E = 170
mV in 0.1 M HClO4 at RT. Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s.
As it was in any other case, examining the side of the grain revealed rough surface with pits.
Figure 7.61. Spot 2 – Grain boundary surface of Ag95Au5 dealloyed at E = 170 mV in 0.1 M
HClO4 at RT. Charge passed Q = 50 mC/cm2, i = 360 A/cm
2, t = 125 s.
Figure 7.61 ends the results of all the fracture tests performed on Ag-5 at. pct Au in perchloric
acid. There has not been found any particular relation between the ways in which samples were
fractured and the depth of grain boundary penetration. This differs from the results reported in
the past on the behaviour of silver-gold alloys containing higher concentration of gold (24, 29).
63
The next three figures will present initial results of the experiments done on the ternary alloys
that also constitute a part of this research. Figure 7.62 shows potentiodynamic curves
performed on Ag77Au22Pt1 and Ag77Au21Pt2 in 0.1 M HClO4. The material containing 4% of
platinum has not yet been well examined. The main purpose of this investigation is to test
whether small additions of platinum introduced to Ag77Au23 at the expense of gold, may have
an impact on stress-corrosion cracking. This is based on the assumption that lower surface
diffusivity of platinum may slow down the diffusion of gold during dealloying and affect the
formation of nanoporosity.
Figure 7.62. Potentiodynamic curves of Ag77Au22Pt1 and Ag77Au21Pt2 in 0.1 M HClO4. Scan
rate 0.5 mV/s. IR drop corrected. Ag77Au23 plays a role of a reference.
The reproducibility of the results obtained from potentiodynamic curves performed on the
ternary alloys left a bit to desire and will be investigated in the future. Long annealing, up to 50
hours, did not improve the reproducibility. Its lack was probably due to the inhomogeneity of
the material. Figure 7.64 presents the very initial results of potentiostatic dealloying of the
64
ternary alloys and show the difference in the size of ligaments between Ag77Au23 and
Ag77Au22Pt1.
Figure 7.63. Fracture surface. Nanoporous layer obtained from dealloying of Ag77Au23 at 60oC
in 0.1 M HClO4 at E = 520 mV. Q = 1.2 C/cm2, i = 10 mA/cm
2, t = 120 sec. Sample fractured
in air. Fracture surface.
Figure 7.64. Fracture surface. Nanoporous layer obtained from dealloying of Ag77Au22Pt1 at
60oC in 0.77 M HClO4 at E = 520 mV. Q = 2 C/cm
2, i = 8 mA/cm
2, t = 240 sec. Sample
fractured in the dealloying solution. Fracture surface.
65
8 Conclusions
The results obtained from dealloying of Ag-5 at. pct Au in perchloric acid have proven that it is
possible to generate a three dimensional nanoporous layer by dealloying of silver-gold systems
containing low concentration of gold (Figure 7.4 to Figure 7.8). Dealloying of these systems
can be carried out in a broad range of potentials and concentrations of a dealloying solution, as
shown in Figure 7.18 and Figure 7.19. Therefore, stress-corrosion cracking observed in these
alloys may be caused, initiated or at least accompanied by the formation of nanoporosity
resulting from dealloying. This stays in contradiction to Galvele’s suggestions (31, 32).
Moreover, since the formation of 3D structures is possible in Ag-5 at. pct Au, it might be
possible that other silver-gold alloys of lower gold concentration than 5 %, could also be
successfully dealloyed.
The thickness of a nanoporous layer formed depends on the charge passed (Figures 7.4 to 7.8).
The composition of nanoporous layers, determined with EDX, revealed surprisingly large
concentration of the residual silver in ligaments, as shown in Figure 7.3. The observed
thickness of nanoporous layers is smaller than it could be expected from the calculations
(Figure 7.9). The difference may result from the collapse of the layer but it also seems to be
possible that a part of charge passed may be consumed in other dealloying processes that are
not taken into account, for instance, dealloying may penetrate the bulk material in narrow
channels via grain boundaries.
During dealloying nanoporous layers formed on Ag-5 at. pct Au undergo physical changes
leading to coarsening (Figure 7.24 and Figure 7.25). Precise estimation of the degree of
coarsening may be difficult since, after dealloying, samples were transferred several times from
one environment to another during the preparations (DI water, and ethanol etc). It should also
66
be noted that SEM imaging requires samples to reside in vacuum where surface diffusion may
be different to those in liquid solutions and air.
The influence of oxygen dissolved in a dealloying solution (Figure 7.20) is negligible and does
not affect the appearance of the empirical critical potential of Ag-5 at. pct Au in 0.1M HClO4.
The concentration of a dealloying solution, however, does affect the anodic current which
increases along with the increase in the concentration of a solution (Figure 7.21). The empirical
critical potential can be determined for Ag-5 at. pct Au (Figure 7.18 and Figure 7.19) and
changes along with the concentration of the more noble metal, as reported in the past (2, 15).
Although the conditions under which film-induced cleavage occurs have not yet been found, a
great deal of work was done on studying grain boundary corrosion and, specifically, on finding
the answer how to eliminate or limit this process as it was found to be significantly interfering.
In order to do it, two different ways of fracturing samples have been examined, that is,
fracturing in the dealloying solution (at positive or negative potential), and fracturing beyond
the dealloying solution (DI water and air). No relation has been found between the crack depth
and the ways in which Ag-5 at. pct Au was fractured. The depth of a crack depended only on
the charge passed. Since all the samples were fractured manually, the rate of the applied strain
(approximately t = 0.5 s) could be too low and will have to be reconsidered in further single-
shot fracture experiments.
The presence of premature cracks on the surface has been confirmed (Figure 7.22). Premature
cracks may also negatively affect the generation of film-induced cleavage. Their formation and
influence will further be investigated.
There has been observed an excellent reproducibility of all potentiodynamic and potentiostatic
experiments performed on Ag-5 at. pct Au in the range of potentials of interest.
67
The fact that silver rich Ag-Au alloys undergo dealloying will have a significant impact on the
way nanoporous layer may be fabricated and used, for instance, as cheaper catalysts than
nanoporous gold fabricated from gold rich Ag-Au alloys.
9 Further work
Since experiments on polycrystalline materials inherently involve grain boundaries, further
investigation of film-induced cleavage in silver-gold systems will be divided into two different
parts during which one type of film-induced cleavage will be investigated at a time: 1)
transgranular film-induced cleavage (TG-FIC), 2) intergranular film-induced cleavage (IG-
FIC). In order to study TG-FIC, further single-shot fracture tests will be carried out on
monocrystals or samples of sufficiently large grains because it will eliminate grain boundaries.
IG-FIC will be studied on a given grain boundary using bicrystals. Before fracture tests, EBSD
will be performed in order to determine the type of a grain boundary. Both monocrystals and
bicrystals will be produced by the application of small amounts of cold work (up to 5%) and
long annealing at 900oC in argon plus 2.5% H2. These conditions have been already checked
and proved to produce excellent results. Prior to any of the above experiments, for a given
alloy, the highest dealloying potential causing no premature cracks will be determined. Because
the generation of film-induced cleavage requires a nanoporous layer to be sufficiently thick and
formed as quickly as possible, new conditions for a dealloying solution will be employed such
as higher concentration of perchloric acid and elevated temperatures.
68
10 References
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69
17. J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov and K. Sieradzki, Nature, 410, 450
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11 Appendix
Figure 11.1 shows the reproducibility of potentiodynamic curves performed on Ag-5 at. pct Au
in 0.1 M and 1.0 M HClO4 at room temperature. Compared with the potentiodynamic curves
presented in Figure 7.21, the below curves were obtained by potentiodynamic polarization of
the samples exposed to the dealloying solution from each side. This was done on purpose to
confirm that the changes in current density will be the same, regardless of the way samples are
exposed, that is, both sides or one side only.
Figure 11.1. Polarization curves performed on Ag95 Au5 in 0.1 M and 1.0 M HClO4 at room
temperature. Scan rate 0.5 mV/s. IR drop corrected.
Figure 11.2 and Figure 11.3 show the reproducibility of the potentiostatic experiments
performed on Ag-5 at. pct Au at E = 400 mV and E = 300 mV in 0.1M HClO4 for the samples
exposed from both sides and one side only.
71
Figure 11.2. Changes in capacitance versus charge passed during dealloying of Ag95 Au5 in ten
consecutive steps at different potentials in 0.1 M HClO4 at room temperature. One side of the
sample exposed to the dealloying solution.
Figure 11.3. Changes in capacitance versus charge passed during dealloying of Ag95 Au5 in ten
consecutive steps at different potentials in 0.1 M HClO4 at room temperature. Both sides of the
sample exposed to the dealloying solution.
72
The below image demonstrates the lab-made device designed by the present student. It was
used in all single-shot fracture experiments. The device consists of two main parts:
electrochemical cell and a mobile part. The cell was an adapted glasslock container normally
used for storage of sandwiches. Four holes were drilled in the lid of the container, one for the
mobile part and the three smaller ones for reference electrode (kept in a double junction),
counter electrode and the glass tube supplying nitrogen. The lid is separated from the glass with
a gasket. The gasket seals off the cell very well and prevents nitrogen escaping during
deaeration of a dealloying solution. The whole provided a very convenient and quick way of
loading samples at will and immediate start of an experiment without any delay. It was
especially required during experiments at elevated temperatures. The copper wire sticking out
from the mobile part connects the sample with the potentiostat.
Figure 11.4. Fracturing device: A – electrochemical cell, B – mobile part made of syringes.
The mobile part consists of two syringes of different sizes (Figure 11.5). The main part was
made of a largish 60 mL syringe. Holes were drilled in the syringe tube in order to provide an
undisturbed exchange of a dealloying solution. The hole seen in the piston has the same
diameter as the diameter of the little syringe. A piece of wood led through the syringe piston
A
B
73
blocks the piston against moving during loading the mobile part into the container. It also
protects the sample against bending or any accidental application of cold work.
Figure 11.5. Mobile part: C - the syringe tube, D – piston, E – small syringe with the sample
mounted in.
The set of figures below shows changes in the imaginary part of impedance versus frequency
for the potentiostatic experiments performed on Ag-5 at. pct Au and Ag-23 at. pct Au.
Figure 11.6. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 120 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step Q = 600 mC/cm2.
C D E
74
Figure 11.7. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 170 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step Q = 600 mC/cm2.
Figure 11.8. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 220 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step Q = 600 mC/cm2.
75
Figure 11.9. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 300 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step Q = 600 mC/cm2.
Figure 11.10. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 400 mV for Ag95 Au5 in 0.1M HClO4. Charge passed in each step Q = 600 mC/cm2.
76
Figure 11.11. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 550 mV for Ag77 Au23 in 0.1M HClO4. Charge passed in each step Q = 0.6 C/cm2.
Figure 11.12. Changes in the imaginary part of impedance after dealloying in ten consecutive
steps at E = 650 mV for Ag77 Au23 in 0.1M HClO4. Charge passed in each step Q = 0.6 C/cm2.