copp er corrosion and its im plication s for the kbs-3 con ... · polycryst. 0.28g zirconium...

Scientific symposium New insights into the repository’s engineered barriers Stockholm, 20 –21 November 2013

Copper corrosion and its implications for the KBS-3 concept

Peter Szakálos and Gunnar Hultquist

Surface and Corrosion Science at the Royal Institute of Technology, Sweden

Hydrogen-containing copper oxide relevant to the KBS 3-model

SKB assumes that the oxide Cu2O is the most

stable oxide both at high and low temperatures.

This assumption must be questioned since below approximately 250oC a hydrogen-containing oxide (including

hydroxide) is formed from water without assistance of molecular oxygen and only above this temperature ”pure”

Cu2O/CuO is formed.

GH November 2013

Part 1

0

1E-08

2E-08

3E-08

4E-08

0 200 400 600 800 1000Temperature, oC

H2, mbar

Ground Yttrium polycryst. 0.07 g

Nickel polycryst. 1.1g

Ground Copper monocryst. 9.9g

Gold polycryst.

0.28g

Zirconium polycryst. 1.2 g

Measured outgassing after 12 hours at respective temperature

Hydrogen in Cu and other metals after more than ten years in ambient air By means of the thermal desorption spectroscopy (TDS) method not only is the total hydrogen

content in a sample determined but also the hydrogen bond strenght is indicated.

GH November 2013

• All metals exept gold contain hydrogen • Hydrogen is desorbed at the highest temperature from yttrium

Part 1

Desorption (outgassing) of hydrogen after different Cu-exposures

Increased temperature of 50oC every 12 hours. Measurement in mass spectrometer, MS, after 2 hours of desorption

GH November 2013

• Only oxide formed at 350oC remains tarnished (in green). • A main release of hydrogen takes place near 250oC when sample

is exposed for extended time near room-temperature (in red).

Part 1

0.1mm Cu-foil after 19 000 hours in the absence of O2

SEM 1µm l----l

SEM 1 µm l-----------l

0.1 mm Cu-foil after 3 minutes at 500oC in ambient air

As recieved 0.1 mm Cu-foil

SIMS

Characterisation of solid reaction product after two different exposures

0 2 4 6 Depth, µm

• More hydrogen in O2 -free exposure

Part 1

10-2

10-4

10-6

Normalised H-count rate

20 140 105 Time, hours

GH November 2013

0.5 gram 2.3 gram

250oC

H2 mbar

TDS

Hydrogen content in different samples of 0.1 mm Cu-foils

Not tarnished 0.33 wtppm: As received + 2 years in ambient air at room-temperature

0.23 wt ppm: As received + 2 years in ambient air at room-temerature + electrolyte pol. 9 min. (High hydrogen content in surface)

0.81 wt ppm: As received + 1 bar H2 for 120 hours at room-temperature

� 0.01 wt ppm: Outgassed up to 600oC + 3 minutes in air at room-temperature

Tarnished

0.035 wt ppm: Outgassed and 3 minutes in air at 500oC 0.90 wt ppm: As received + 22 000 hours in water without molecular oxygen near room-temperature + 1 year in air at room-temperature

• Uptake of hydrogen in Cu during long time exposure in water without molecular oxygen.

Part 1 GH November 2013

l: Below the temperature of approximately 250oC: (stability of CuOH+stability of Cu-H) ≥ stability of H2O ll: Above the temperature of approximately 250oC: (stability of CuOH+stability of Cu-H) � stability of H2O Overall summary of part 1 and a fundamental question

The copper canister in KBS-3 will be held below 250oC and therefore thorough knowledge of hydrogen-containing oxides and what this means for the chemical and physical properties are needed before the use of the KBS-3 method.

Example of such thorough knowledge is the answer to the question: What is the hydrogen content required in the copper metal and in the gas phase respectively to avoid formation of a hydrogen-containing oxide by water ?

GH November 2013 Part 1

Acknowledgement: Swedish Radiation Safety Authority is acknowledged for financial support Questions regarding Part 1 and a copy of this part: [email protected]

Some analysis of 50 mm thick Cu after 7 years, 500 m underground near Oskarshamn

(Äspö HRL, the prototype repository)

• The Cu-block (top) was provided by SKB for analysis at NRC, Ottawa and NUS, Singapore

Part 2 GH+PS November 2013

Hydrogen vs. depth

0

0,005

0,01

0,015

0,02

0,025

0,03

-5 0 5 10 15 20 25 30 35 40 45 50 55 60depth, mm

ratio

, 63C

uH/63

Cu

not exposed Cu-foil, no O in measurement

OH vs. depth

0

0,1

0,2

0,3

0,4

0 1 2 3 4 5 6depth, mm

ratio

, OH

/63C

u

not exposed Cu-foil

1.36 mm during the 7 years exposure makes a OH-penetration of 250 years. This estimate is based on a constant penetration rate and a wall-thickness of 50 mm.

1.36 mm penetration

• 1.36 mm penetration during the 7 years exposure makes a penetration through the 50 mm Cu-thickness in 250 years based on a constant penetration rate.

• Hydrogen is higher throughout the 50 mm Cu-thickness relative to hydrogen in an unexposed Cu-foil.

GH+PS November 2013 Profiles from SIMS-measurement of 50 mm thick Cu

after 7 years, 500 m underground near Oskarshamn (Äspö HRL)

Part 2

What has happened after the international copper corrosion hearing in 2009?

- Several laboratories confirm high H2-pressures, KTH, Studsvik AB, Aalto University, Microbial Analytics Sweden AB.

- A solid copper corrosion product is detected after one year exposure in pure anoxic water under controlled conditions (KTH)

- Theoretical (ab initio) surface calculations show that formation of CuOH can explain the high hydrogen pressure, around 1 mbar, detected in the system Cu/H2O. (A. Rosengren and A. Belonoshko)

- Copper corrosion in the real repository environment is much more severe than estimated by the solely theoretical KBS-3 model.

- Hydrogen embrittlement in copper seems to be a more serious problem than anticipated in 2009. “ The copper manifests a remarkable sensitivity to hydrogen in constant load tests.” 1 (H. Hänninens talk)

- The influence by irradiation on copper corrosion (C. Leygrafs talk)

(Regarding copper corrosion in anoxic water)

1) Yagodzinskyy, Y. ; Malitckii, E. ; Saukkonen, T. ; H�nninen, H. “ Hydrogen-enhanced creep and cracking of oxygen-free phosphorus-doped copper” Scripta Materialia, 2012, Vol.67(12), pp.931-934

P. Szakálos, November 20, 2013

Copper corrosion studies in anoxic water after 2009 Several laboratories has confirmd high H2-pressures, KTH, Studsvik AB, Aalto University, Microbial Analytics Sweden AB, Göteborg. Contradictive results has been obtained by Uppsala University (UU). However, their results has been influenced by high hydrogen activities: i) The pre-treatment of the copper samples in UU resulted in ten times higher hydrogen content than in the copper quality used by SKB and than in the copper quality used by us at KTH. ii) All stainless steel components in our equipment were of a grade aimed for UHV-applications. The stainless steel quality in the UU-equipment was not and in the reference group meeting October 3, it was shown that unusual high rates of hydrogen outgassing was detected from the equipment.


Copper corrosion studies in gas tight glass vials (Microbial Analytics, Göteborg) Special vials (test tubes), developed by the biomedical industry for studying anaerobic microbiology, were used to study copper corrosion in water without dissolved oxygen.

Experiment N7: Vials with copper, water and nitrogen gas. Incubator at 70°C with N2-atmosphere.

”The experiments are reproducable” and ”there are consequently no doubts that H2 can emit from copper immersed in anoxic water.”

SKB TR-13-13 “ Development of a method for the study of H2 gas emission in sealed compartments containing canister copper immersed in O2-free water” Karsten Pedersen et al. Microbial Analytics Sweden AB

-Important results since the study confirms that the hydrogen originates from a copper-water reaction (not from stainless steels or other metals). -The study confirms that the equilibrium hydrogen pressure at 70°C is in the mbar-range. This value should be compared with the thermodynamically expected value of 10-9 mbar for a pure copper oxide (Cu2O), i.e. a factor of one billion in difference. -The study confirms that when the oxygen dissolved in water is consumed, hydrogen production starts. -The study confirms that the H2-production continues when H2 was removed in the same rate order as was observed before H2 removal. This is important since it excludes a surface reaction as an explanatory theory.

Copper corrosion studies in glass vials (cont.)


Copper reacts with water molecules under formation of a solid corrosion product

Anoxic copper corrosion in pure water at 50°C, 1 year. (Sample taken directly from water and incerted in the SEM-chamber)

Unpublished results, but similar findings (SEM-micrographs) are published in SSM-report 2013-07

Electron backscatter diffraction (EBSD)


1) A.B. Belonoshko & A. Rosengren (2012): A possible mechanism of copper corrosion in anoxic water, Philosophical Magazine, 92:36, 4618-4627

2) Belonoshko, A. B. ; Rosengren, Anders “ Ab Initio Study of Water Interaction with a Cu Surface” Langmuir, 2010, Vol.26(21), pp.16267-16270

3) Kim, J. Y. et al, ” Reduction of CuO and Cu2O with H2: H embedding and kinetic effects in the formation of suboxides” , Journal of the American Chemical Society, 2003, Vol.125(35), pp.10684-92

Theoretical model for copper corrosion by water based on ab-initio calculations1,2

If hydroxide from the copper surface diffuses into a copper grain boundary, it can dissociate to O and H atoms. Hydrogen and oxygen will diffuse independently of each other. Hydrogen is predicted to ”escape” relatively fast, either into the metal and eventually get trapped or out to the surface and associate to molecular hydrogen. The theoretically predicted equilibrium hydrogen pressure is in the same order of magnitude as the experimental results, i.e. in the mbar-range. The free oxygen atoms in the grain boundaries are expected to react with copper under Cu2O-formation. A formed Cu2O-crystal can not be reduced by hydrogen unless the temperature is as high as 300°C 3


From section ” Underground Nuclear Waste Containment”

One (theoretical) study suggests that under such (repository) conditions uniform corrosion of oxygen-free copper would only amount to 1.1 mm in 106 years (1 nm/yr).9

Experimental results indicate that the clay may reduce the corrosion rate by about a factor of ten over that in bulk solution, although these results suggests a corrosion rate of about 1 µm/yr.10

ASM Handbook: Copper and Copper Alloys

What is said about the KBS-3 model in the ”Copper Bible?”

ASM Handbook references: Comment: All published studies in Äspö HRL confirms the ASM Handbook statement; The LOT studies, The MiniCan studies and The Prototype Repository studies. The copper corrosion rates are always in the µm/yr-range.


The KBS-3 model:

- Copper dissolution as Cu2+ and CuCl2- - The bentonite surface act as a sink for these ions. Different

crystalline corrosion products precipitate on the bentonite (oxides, hydroxides, carbonates, sulphates, sulfides and complex mixtures thereof).

- The chloride ions help to activate the corrosion process but are not nessesary ” consumed” , i.e. not included in the corrosion end products.

- The bentonite and copper ” barriers” break down each other and the corrosion continues with short diffusion distances

The copper corrosion processes are calculated by diffusion through an buffer in an ” ideal state” , water saturated and pressurised. The only expected corrosion product under normal repository conditions is copper sulfide.

Reality:

The theoretical KBS-3 sulfide diffusion model has no support by experimental data. SSM 2012:17 P. Szak�los and S. Seetharaman ” Corrosion of copper canister” P. Szakálos, November 20, 2013

Total salt content in Forsmark groundwater is around 0.95 wt-%, 16 kilo salt/deposition hole/year one cubic meter of salt or 2500 kg may precipitate in such a deposition hole within 100 years.

Ground water evaporation and salt precipitation ” The sauna effect” -Consequence

Only a few metals can resist chlorine- and sulfur salt deposition during heating and water evaporation, with or without oxygen. In the industry, palladium alloyed titanium or tantalum is used in such environments.

12-15°C

50-100°C

There is a thermodynamic force, driving moisture/ steam from the deposition hole to the colder tunnel, thus accumulating salt in the buffer and on the canister

SSM 2012:17 P. Szak�los and S. Seetharaman ” Corrosion of copper canister” P. Szakálos, November 20, 2013

Corrosion in a proposed Forsmark repository during 105 years according to our estimate

Blue = lower estimate Red = higher estimate

5 cm

SSM 2012:17 P. Szak�los and S. Seetharaman ” Corrosion of copper canister”


- Copper really corrodes by pure water in absence of dissolved oxygen.

- Hydrogen gas pressures has been measured and confirmed by other laboratories to be in the mbar-range, i.e. 109 times higher than thermodynamically expected by formation of a pure copper oxide.

- Ab-initio calculations support the theory that the driving force for anoxic copper corrosion is governed by CuOH-formation and that the equilibrium hydrogen pressure is in the mbar-range.

- CuOH is most likely a precursor to the corrosion end product Cu2O.

- Copper corrosion by pure water offers an explanation why copper corrodes much faster (not controlled by sulfide formation) in a repository environment than predicted by the KBS-3 model.

- The simple sulfide diffusion model in KBS-3 predicts a corrosion rate of 1-2 nm/year. Estimates of the copper corrosion rates in a repository at the Forsmark site are in the range of 1-300 µm/year depending on temperature and local environment in the deposition hole.

Conclusions


copp er corrosion and its im plication s for the kbs-3 con ... · polycryst. 0.28g zirconium...

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