The Effect of Surface Modifications on Tritium Adsorption and Absorption by Stainless Steel 316

C. Fagan1,2, M. Sharpe1 W.T Shmayda1, W.U Schröder21Laboratory for Laser Energetics2University of Rochester Departments of Physics and Chemistry

37th Tritium Focus Group MeetingRochester, NY

25-27 October 2016

Tota

l Act

ivity

(MB

q)

As Received EP2 EP3 0.2 M HNO3 4 M HNO3

1

Surface conditions strongly influence the total inventory of tritium in the metal sample

– Modifications to stainless steel surfaces alter the total tritium uptake

- 40% reduction by Polishing/Electropolishing- 20% reduction with surface coatings of TiN or CrN- 100% increase with nitric acid treatments- Several coatings of gold were ineffective in reducing tritium

uptake

– No simple correlation observed between surface roughness and total inventory

– Significant quantities of the tritium inventory are located on the surface

2

Tritium must first transfer through the adsorbed water layer before diffusing into the bulk metal

Hydroxyl layer (chemisorbed)

TritonHydrogen

Metal lattice

Ice-like layers (chemisorbed)

Liquid-like layers(physisorbed)

Hydrogen isotopesOxygenMetal atomHydrogen bondFull chemical bonds

Theil, P.A.; Madey, T.E. Surface Science Reports 1987,7,211-385

3

Alterations to the metal surface alter the hydroxide concentration

Hydroxyl layer alterations may correlate with different surface tritium inventories

Ozeki, Y.; Hatano, Y.; Taniguchi, H.; Matsuyama, M. Fusion Sci. Tech. 2011, 1499-1502. *

O1s spectra*

H2O

OH

4

Samples are charged with tritium at room temperature for 24 hours

Loading Pressure (Torr) %T

550 ± 10 57 ± 2

Surface Modifications• Mechanical Polish• Electropolished• HNO3 Treatments• Electroplated Gold• TiN/CrN5.1 x 1.9 x 0.3 cm

5

Total tritium inventories in the samples are measured with temperature programed thermal desorption

The liquid scintillation counter measures in situ in real time

6

Nitric acid treatments increased the total quantity of absorbed tritium relative to untreated surfaces

𝑨𝑹𝑭 = (𝑨𝑨𝑹 − 𝑨𝑵𝑨𝑨𝑹

)

* L. Boulange-Petermann, B. Baroux, and M.-N. Bellon-Fontaine, J. Adhes. Sci. Technol. 7, 221 (1993).

4 M Acid

0.2 M Acid

EP2 EP3

ARF 118% 99% 37% 33%

Nitric acid treatments enhanced tritium uptake by activating the surface

Acid treatment intended to create hydrophobic or hydrophilic surfaces

Tota

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As Received EP2 EP3 0.2 M HNO3 4 M HNO3

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Mechanical polishing appears to reduces the total tritium inventory

AR Polished EP

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(MB

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• Compared to unmodified (AR), polishing reduces the total tritium• electropolishing has no effect

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XPS data shows increased Cr and Fe concentrations on surfaces treated with nitric acid

XPS data from H. Peebles, Sandia N.L.

No evident correlation between surface composition and total tritium

2: T. Hirabayashi, et al , J. Nuc. Mattter., Vol. 127, Issue 2, 1985, 187-192

• Previous studies have suggest that increased Cr content leads to lower tritium inventories1,2

• we see a competing effect

1: Ozeki, Y.; Hatano, Y.; Taniguchi, H.; Matsuyama, M. Fusion Sci. Tech. 2011, 1499-1502.

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Electroplating gold on the surface of stainless steel does not suppress tritium uptake

• Increasing the thickness of the gold layer had no influence on tritium uptake

0.8 µm

1.7 µm

8.3 µm

Gold coatedUntreated

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Mechanical polishing appears to consistently reduce tritium uptake

• Batch to batch variability is not understood but reduced tritium uptake due to polishing is evident in both batches

• Polished samples were mechanically buffed with grit paper until a Ra (3-4) was achieved

Batch #1 Batch #2

Untreated

1.7 µm goldPolished

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(MB

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0.5 µm 0.3 µm

TiN and CrN surface coatings reduce tritium uptake compared to untreated surfaces

Polished TiN CrN

ARF 42% 27% 30%

Is it the polishing or the coatings that reduce the tritium uptake?

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There does not appear to be a correlation between surface roughness and tritium uptake

EPPolish 3Polish 8Polish 12As Received

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There appears to be a correlation between tritium uptake and the quantity of water present on a surface

Hydroxyl layer (chemisorbed)

Metal lattice

Ice-like layers (chemisorbed)

𝐓𝐡𝐞𝐫𝐞𝐢𝐬𝐚𝐩𝐨𝐭𝐞𝐧𝐭𝐢𝐚𝐥𝐭𝐨𝐚𝐜𝐜𝐨𝐦𝐦𝐨𝐝𝐚𝐭𝐞𝐥𝐚𝐫𝐠𝐞𝐚𝐦𝐨𝐮𝐧𝐭𝐬𝐨𝐟𝐭𝐫𝐢𝐭𝐢𝐮𝐦𝐢𝐧𝐰𝐚𝐭𝐞𝐫: 𝟏𝟎𝟔𝒎𝒐𝒍𝑯𝒎𝟑

7.4x10-8 moles of HTO per monolayer ~ 40 MBq/layer (1.1 mCi/layer)

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The quantity of adsorbed water (Q) was estimated from the total activity desorbed from the sample

• The quantity of surface water is varied until the predicted total activity matches the measured total activity

• assuming tritium can only redistribute between the surface and the bulk

• Predicted surface water inventory compares favorably with published data

Con

cent

ratio

n of

ad

sorb

ed w

ater

(µ

mol

/m2 )

Expected minimum (21.8)One monolayer (16.6)

*

As Received EP2 EP3 0.2 M HNO3 4 M HNO3

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The fraction of the tritium on the surface can be estimated from the concentration of surface water (Q)

The initial fraction of tritium on the surface is > 40%

Surface Monolayers of H2O

Surface Activity (MBq)

AR 1.3 32.02

EP 0.7 17.5

Acid 3.3 77

Frac

tion

of tr

itium

on

the

surf

ace

As Received EP HNO3 Treatments

16

Surface conditions strongly influence the total inventory of tritium in the metal sample

– Modifications to stainless steel surfaces alter the total tritium uptake

- 40% reduction by Polishing/Electropolishing- 30% reduction with surface coatings of TiN or CrN- 100% increase with nitric acid treatments- Several coatings of gold were ineffective in reducing tritium

uptake

– No simple correlation observed between surface roughness and total inventory

– Significant quantities of the tritium inventory are located on the surface

17

Two equilibrium processes control tritium migration into the metal lattice

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