Influence of Microstructure on the Absorption of Tritium Into Gold-Plated 316 Stainless Steel

Influence of Microstructure on the Absorption of Tritium into Gold-Plated 316 Stainless Steel

Values <1 indicate reduced absorption in gold-plated samples 200 nm Vendor #1

Vendor #1 NL Vendor #1 Vendor #1 Vendor #2 (hard Au) Vendor #2 (hot Au) Relative inventory 200 nm

0 5 10 15 20 25 Sample number

M. Sharpe, C Fagan, and W. T. Shmayda Tritium Focus Group-Sandia University of Rochester Albuquerque, NM Laboratory for Laser Energetics 22–25 October 2018

1 Summary High-integrity gold layers reduce tritium absorption into 316 stainless steel (SS316)

• High-integrity gold layers on SS316 are challenging to obtain commercially –– vendors follow MIL-DTL-45204 • Greater than 25% reduction in tritium inventory is not observed for gold-plated SS316

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2 Gold layers on national lab samples contain deep valleys that may reach the substrate ) m 9 –

10 120 # 80 4.5 ) 40 m 3.0 n

z axis ( 0 5 1.5 4 3 2 1 0 x axis ( y axis ( nm) Au thickness ~ 100 nm Fe Au Fe Cr Cr Ni

Ni 1.6 3.2 4.8 6.4 X-ray energy (keV) 4 5 6 7 8 9 200 nm X-ray energy (keV)

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3 Samples from Vendor #1 have smooth surfaces with sporadic pinholes

Gold-plating specification MIL-DTL-45204C Type I ($99.7% Au) # ) 2 Grade A (HK 90 kgf mm )

1 nm 10 nm

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4 Samples from Vendor #1 have porous gold layers

200 nm Ni Au

Au (0.8 nm)

Ni (6 nm)

1 nm 0.8 1.6 2.4 3.2 4.0 4.8 Substrate X-ray energy (keV)

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5 Samples from Vendor #2 show complete gold layers with different roughness

“Hard” gold “Hot” gold MIL-DTL-45204 MIL-DTL-45204 $ ) 2 # ) 2 Grade D (HK 201 kgf mm ) Grade A (HK 90 kgf mm ) Type II (99.0% Au) Types I and III ($99.7% Au)

200 nm 200 nm

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6 High-resolution STEM of “hot” gold reveals high-porosity gold layers

1 nm

SS316

Ni Au (~1.5 nm) 500 nm 100 nm

E28054 STEM: scanning transmission electron microscopy

7 SEM cross section of “hard” gold reveals large grains in both gold and nickel layers

200 nm SS316

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8 XPS analysis confirms thick, pure gold layers for “hard” and “hot” gold

18,000 16,000 14,000 ) 12,000 O 1s 10,000 C 1s • Lack of O 1s 8,000 post-sputter Intensity indicates 6,000 arbitrary units

( no hydroxide 4,000 or metal oxide 2,000 0 1200 1000 800 600 400 200 0 Binding energy (eV)

No evidence of SS316 lines indicate a Au layer > 10 nm.

E27919 XPS: x-ray photoelectron spectroscopy

9 Surface activity was measured by immersing the coupon in either ZnCl2, water, or 15% surfactant

“Hot” gold sample in • ZnCl used for water 2 non-plated SS316 –– solution liberates adsorbed tritium* • Surfactant or water used H+ ZnCl2 mechanism H+ H+ for Au-plated samples – – Cl Cl– Cl– Cl– Cl– Cl– Cl– – surfactant mixture Zn2+ Cl– mildly etches SS316 Zn2+ Zn Zn Zn OH OH OH OH OH OH O O O O O O Surface oxide film Surface oxide film Substrate Substrate

E27921 * M. D. Sharpe et al., Fusion Eng. Des. 130, 76 (2018).

10 The total tritium inventory was influenced by the integrity of the gold layer

Source Observation Values <1 indicate reduced absorption National Rough layers in gold-plated samples lab (NL) Valleys reach substrate Porous layers Vendor #1 Vendor #1 Sporadic large pinholes Vendor #1 Porous layers Vendor #2 NL Vendor #1 Different roughness Vendor #1 Vendor #2 (hard Au) Vendor #2 Mean relative Relative inventory Gold type (hot Au) inventory “Hard” 0.97!0.22 “Hot” 0.75!0.17 0 5 10 15 20 25 Sample number

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11 Surface inventory appears to be influenced by roughness for “hard” and “hot” gold

3.5 Hard Au • Surfactant removes more (surfactant) activity than water 3.0

) –– etching?

mCi 2.5 hard Au surfactant ^ h = 18. 2.0 hot Au surfactant Non-plated Hard Au ^ h Hot Au ( surfactant ) Hot Au ( RF - 10 shots ) SS316 (ZnCl2) (water) hard Au water 1.5 ^ h = 19. hot Au water ^ h 1.0 Hot Au (water) Surface activity ( 0.5

0.0 1 2 3 4 5 6 7 8 9 10 11 Sample number

E27986 RF: radio frequency

12 Summary/Conclusions High-integrity gold layers reduce tritium absorption into 316 stainless steel (SS316)

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13 Supplemental material

14 The void fraction in gold layers from General Plating was estimated to be 0.01%

• Volume etched ~ 69 nm3 Mean!v = 451!14 nm # voids = 80 • Void volume ~ 0.01% of surface

• Assuming 1 atm of T2 gas

Counts present in all voids –– activity = 0.43 nCi

20 30 40 50 60 70 80 90 Void diameter (nm)

200 nm

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