stress corrosion cracking investigation of copper in ... oy fi-27160 olkiluoto, finland tel...

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May 2006

Work ing Repor t 2006 -18

Stress Corrosion Cracking Investigation of Copperin Groundwater with Acetate Ions

May 2006

Working Reports contain information on work in progress

or pending completion.

The conclusions and viewpoints presented in the report

are those of author(s) and do not necessarily

coincide with those of Posiva.

Pet r i K innunen

VTT Indus t r i a l Sys tems

Work ing Report 2006 -18

Stress Corrosion Cracking Investigation of Copperin Groundwater with Acetate Ions

STRESS CORROSION CRACKING OF COPPER IN GROUNDWATER WITH ACETATE IONS

ABSTRACT

In Sweden and Finland the spent nuclear fuel is planned to be encapsulated in spheroidal graphite cast iron canisters that have an outer shield made of copper. The copper shield is responsible for the corrosion protection of the canister.

Based on literature study oxygen free phosphorus containing copper (Cu OFP) can be susceptible to stress corrosion cracking in presence of water with acetate-ions. The work described in this report was launched to investigate the effect of acetate-ions (in the concentration range 1 mg/l to 100 mg/l) on susceptibility of CuOFP to stress corrosion cracking. Experiments conducted in anoxic highly saline and saline simulated groundwater included slow strain rate tests complemented by fractography and voltammetry. Tests were performed with both CuOFP base metal and EB-weld metal samples. The test temperature was 100oC and the pressure 14 MPa, simulating the pressure expected due to the sum of the hydrostatic pressure and the compacted bentonite swelling pressure in a KBS-3 type repository at a depth of about 500 m in crystalline bedrock. Additionally, Auger-spectroscopy studies were performed to investigate the possibility of phosphorus segregation onto grain boundaries or other interfaces.

The elongation to fracture values in the slow strain rate tests were comparable to those in air and showed no dependence on the acetate-ion concentration. The fractography of the samples showed a dimple like appearance of the fracture surfaces in all cases. Both of these results indicate that neither CuOFP base metal nor the EB-weld metal is susceptible to SCC in the environments in question. The voltammetry results in highly saline groundwater further supported this conclusion. The Auger – spectroscopy results (at the resolution limit of about 0.1 at%) showed no evidence for phosphorus segregation in either CuOFP base metal or the EB-weld metal.

Keywords: Copper, stress corrosion cracking, acetate-ion.

ASETAATTI-IONIEN VAIKUTUS KUPARIN JÄNNITYSKORROOSIOON SIMULOIDUSSA POHJAVEDESSÄ

TIIVISTELMÄ

Suomessa ja Ruotsissa käytetty ydinpolttoaine pakataan pallografiittivaluraudasta valmistettaviin säiliöihin, joiden ulkopinnalla on kuparimetallista tehty suoja. Kuparimetalli toimii valurautasäiliön korroosiosuojana.

Kirjallisuusselvityksen perusteella fosforimikroseostettu kupari voi olla altis jännityskorroosiolle asetaatti-ioneja sisältävissä vesiliuoksissa. Tässä tutkimuksessa selvitettiin fosforimikroseostetun kuparin jännityskorroosio-alttiutta simuloidussa pohjavedessä, johon lisättiin 1 - 100 mg/l asetaatti-ioneja.

Kokeet tehtiin hapettomassa suolaisessa ja erittäin suolaisessa simuloidussa pohjavedessä hidasvetokokeina (SSRT-koe), joiden tuloksia täydennettiin murtopintatutkimuksella ja voltammetria-mittauksin. Kokeissa tutkittiin sekä hitsi- että perusainetta. Koelämpötila oli 100oC ja koepaine 14 MPa, joka paine vastaa hydrostaattisen paineen ja kompaktoidun bentoniitin paisumispaineen yhteenlaskettua arvoa KBS-3 tyyppisessä loppusijoitustilassa noin 500 m syvyydellä peruskalliossa. Lisäksi tutkimuksessa selvitettiin Auger-spektroskopian avulla fosforin mahdollista suotautumista raerajoille tai muille pinnoille.

Hidasvetokokeissa havaittiin, että murtovenymä simuloidussa pohjavedessä tehdyissä kokeissa on hajonnan puitteissa sama kuin ilmassa tehdyissä kokeissa, eikä murtovenymän todettu riippuvan asetaatti-ionipitoisuudesta. Murtopinta-analyysissä havaittiin sitkeälle murtumalle tyypillinen pinnan ulkonäkö kaikissa tapauksissa. Molemmat näistä havainnoista viittaavat siihen, että CuOFP perus- tai hitsiaine ei ole altis jännityskorroosiolle tässä työssä tutkituissa ympäristöissä. Myös voltammetria-tulokset tukevat tätä johtopäätöstä. Auger-spektrosopiatuloksissa ei havaittu CuOFP perus- tai hitsiaineessa fosforin suotautumista.

Avainsanat: Kupari, jännityskorroosio, asetaatti-ioni.

1

TABLE OF CONTENTS

ABSTRACT

TIIVISTELMÄ

1 INTRODUCTION................................................................................................. 2

2 GOALS ................................................................................................................ 3

3 DESCRIPTION OF THE TARGET AND THE METHODS .................................. 4

3.1 SSRT - experiments ............................................................................................ 43.2 Surface analytical studies.................................................................................... 53.3 Voltammetry ........................................................................................................ 6

4 RESULTS AND DISCUSSION............................................................................ 7

4.1 Slow strain rate test results ................................................................................. 74.2 Fracture surface investigations ......................................................................... 134.3 Voltammetry ...................................................................................................... 144.4 Auger-spectroscopy results............................................................................... 16

5 CONCLUSIONS ................................................................................................ 22

6 SUMMARY ........................................................................................................ 23

REFERENCES ............................................................................................................. 24

Appendix 1. Fractography............................................................................................. 25

Appendix 2. Voltammetry.............................................................................................. 31

2

1 INTRODUCTION

In Sweden and Finland the spent nuclear fuel is planned to be encapsulated in spheroidal graphite cast iron canisters that have an outer shield made of copper. The copper shield is responsible for the corrosion protection of the canister.

After the cast iron insert with the spent fuel has been loaded inside the copper canister and it has been sealed, the canister will stay in an intermediate storage waiting for the transport to the final disposal vault. During this period, which may last for several months, the outer surface will heat up and stay at approximately 100 oC, the canister is exposed to air and an air borne oxide will grow on the surface. This has to be taken into account when preparing the specimens for tests in simulated groundwater.

Acetate-ions at high concentrations (CAc 30000 mg/l) have been found to cause stress corrosion cracking (SCC) in copper under oxidising conditions (Escalante and Kruger, 1971). The acetogen group of microorganisms produces acetate from one carbon organic compounds or from hydrogen and carbon dioxide. The methanogen group of microorganisms on the other hand produces methane from one carbon organic compounds and from acetate or from hydrogen and carbon dioxide. Acetogens (acetate producing bacteria) have been found in several boreholes in the Olkiluoto area at depths between 324-866 m. The most probable number (MPN) was found to be 15 MPNml-1

for autotrophic acetogens and about 7000 MPNml-1 for the heterotrophic acetogens (Pedersen, 2000). The corresponding numbers for the methanogens were about 5 MPNml-1 and 1 MPNml-1, respectively. The concentration of dissolved organic carbon in deep groundwater sampled from Olkiluoto has generally been in the range of 4-14 mg/l (Pitkänen et al., 2004). Acetogens produce acetate-ions (CH3COO-) e.g. through the reaction

4H2 + 2CO2 ---> CH3COO- + H+ + 2H2O (1)

A competitive reaction catalysed by the methanogens produces methane e.g. as the reaction product 4H2 + CO2 ---> CH4 + 2H2O (2)

Large amount of methane has been found especially in the deep saline groundwater at Olkiluoto (Pitkänen et al., 2004). Also hydrogen has been found in the deep saline groundwater. Based on this it is likely that the methanogens are dominating in the final disposal vault environment and accordingly the concentration of acetates produced by the acetogens is expected to stay rather low.

3

2 GOALS

The main goal of this study was to investigate the susceptibility of CuOFP to stress corrosion cracking in anoxic groundwater in the presence of acetate-ions. This study also targeted in finding whether phosphorus, used as a minor alloying element in CuOFP will segregate and possibly concentrate on grain boundaries or other interfaces when the material is exposed to 100oC temperature and corroding conditions in groundwater.

4

3 DESCRIPTION OF THE TARGET AND THE METHODS

The composition of the highly saline simulated groundwater (HSSGW) used in this work is shown in Table 1. This highly saline groundwater simulates the near-field saline groundwater composition proposed by Vuorinen and Snellman (Vuorinen and Snellman 1998). A more dilute saline simulated groundwater (SSGW) in Table 1 was used to simulate conditions were the role of general corrosion would be less dominating. The electrolytes were prepared from p.a. chemicals and MILLI-Q water. Acetate was added as NaCH3COO. The pH of the HSSGW solution after preparation and addition of acetate was about 7.5. It is estimated that at the temperature of 100oC the pH of this solution was about 7.0. The pH of the SSGW solution was fixed after addition of acetate at 8.0 at the beginning of the test by adding NaOH. During the experiment potentials were monitored with a AgCl/Ag external pressure balanced reference electrode. Potential of a Pt electrode was monitored and taken as the potential of water (redox potential, Eh). Any in-leakage of oxygen was presumed to be revealed by a clear increase of the potential of the Pt electrode. Tests were performed in an AISI 316 L stainless steel autoclave with a volume of about 10 dm3.

Table 1. Composition of the saline (SSGW) and highly saline (HSSGW) simulated

groundwater (mg/l).

Cl- SO42- Mg2+ Ca2+ Na2+ K+ HCO3

-

Saline

SSGW

8000 - - 1900 2900 - -

Highlysaline

HSSGW

53800 1200 700 9900 22700 190 4.8

The test temperature was 100oC. The autoclave was pressurised to 14 MPa with nitrogen and dissolved oxygen was removed using nitrogen gas bubbling with 5N (99.999%) nitrogen gas further purified with an Oxisorb R 200 nitrogen purification system. Tests were performed in highly saline simulated groundwater (HSSGW) using three different acetate concentrations, i.e. 1 mg/l, 10 mg/l and 100 mg/l and in the saline simulated groundwater (SSGW) with the acetate concentration of 100 mg/l.

3.1 SSRT - experiments

The slow strain rate tests (SSRT) were performed in the simulated groundwater with two different Cl- concentrations (SSGW = 8000 mg/l and HSSGW = 53800 mg/l) and with a relevant copper alloy (CuOFP) delivered by Posiva. The received material blocks were investigated with 100% radiography to make sure that they did not contain any manufacturing defects of detectable size. SSRT specimens with gauge length of 26 mm and gauge diameter of 5 mm were manufactured. The specimens were pre-oxidised in

5

air at 100 oC for two days before starting the test in the autoclave. The SSRT strain rate used was 10-6 s-1.

With each acetate concentration level two weld and two base material specimens was tested simultaneously. Corrosion potential of each of the four specimens was monitored. The test matrix is shown in Table 2.

Table 2. Number of specimens tested. Three test runs, four SSRT specimens at the same

time (Ac = CH3COO-).

CuOFP material EB-weld Base

HSSGW (53800 mg/l Cl-) + 1 mg/l Ac 2 specs 2 specs



SSGW (8000 mg/l Cl-) + 100 mg/l Ac 2 specs 2 specs

3.2 Surface analytical studies

After the SSRT tests were terminated the specimens were removed from the autoclave and the fracture surfaces were examined by scanning electron microscopy (SEM) to identify possible features indicating stress corrosion cracking.

In CuOFP some 40…60 ppm of phosphorus is used as an alloying element. Samples (20 mm x 20 mm x 5 mm) were manufactured from the material blocks (both base and weld material) for exposure tests. Samples were polished on one side to 4000 grit SiC grade. One set (a weld material and a base material sample) was left untreated, one set was exposed to 100oC air for 48 hours and one set was first exposed to 100oC air for 48 hours and then for about 200 hrs to the highly saline simulated groundwater with 100 mg/l acetate-ions at 100oC and 14 MPa. The exposure samples were examined with Auger electron spectroscopy for possible segregation of phosphorus during the CuOFP manufacturing and welding process or during the exposure to either 100oC air or to highly saline simulated groundwater at 100oC. The samples were cleaned ultrasonically to remove any detachable contamination. The Auger spectroscopy was performed with Perkin Elmer PHI model 610 (electron beam energy 5 keV, beam current 100 nA). Measurements were performed at several locations within a given sample and using different sampling areas (diameter from a micrometer to a millimetre). Ion bombardment was performed with Ar-ions using 3 keV energy and 2.5 mA current. Depending on the sputtered area the sputtering rate was 5-10 nm/min. The sputtering times varied from a few minutes to two hours.

6

3.3 Voltammetry

Potentiokinetic polarisation curves have been successfully used (Parkins and Holroyd, 1982) to predict the susceptibility of brass to SCC e.g. in 0.1 M (5900 mg/l) acetate. This method is based on the assumption that the SCC of brass in this environment propagates with the anodic dissolution mechanism. In such a case the passivation properties of the crack sides (represented by a slow sweep rate) versus the passivation properties of the crack tip (represented by a fast sweep rate) are expected to be remarkably different within the potential range at which the material is susceptible to SCC. In other words, it is expected that in that range of potentials, the current densities in a fast sweep should exceed considerably those in a slow sweep. An additional somewhat arbitrary criterion adopted by Parkins and Holroyd is that the current density in the fast sweep exceeds 1 mA cm-2, which is considered reasonable for active anodic dissolution at the crack tip (no appreciable repassivation).

In this study, polarisation curves were measured in the highly saline simulated groundwater with 53800 mg/l Cl- at the three different acetate levels using two different sweep rates, 10 mV/min and 5 V/min. The data was analysed as to the difference in the magnitude of the current density for the two sweep rates as well as to the absolute magnitude of the current density achieved.

A conventional three-electrode arrangement was used featuring a Pt wire as a counter electrode and a AgCl/Ag(0.1 M KCl) electrode as a reference. All of the potentials in this report have been converted and are given in the standard hydrogen electrode (SHE) scale. The diameter 5 mm electrodes were embedded in PTFE specimen holders, mechanically polished with emery paper up to 4000 grade and rinsed with MILLI-Qwater before the measurements. The potentiokinetic polarization curves were measured using a Solartron 1287 potentiostat controlled by CorrWare software (Scribner Associates).

7

4 RESULTS AND DISCUSSION

4.1 Slow strain rate test results

The slow strain rate test results in the highly saline groundwater are shown in Figs. 1 and 2 for the base material samples and the EB-weld material samples, respectively. The elongation to fracture in base material samples was about 52%, which is in the same range as found before (Arilahti et al., 2000). The elongation to fracture was roughly the same irrespective of the acetate concentration, except for one sample tested in 10 mg/l of acetate which showed an elongation to fracture of 41%. However, as seen in the stress-elongation curve, this sample also shows a markedly higher yield stress than the rest of the samples. This indicates that the sample has been exposed to some amount of cold work prior to testing (most probably during the specimen manufacture), which then is naturally seen as a lower than normal elongation to fracture.

In case of the EB-weld samples the elongation to fracture was 25% … 31%, which is in the same range as found before (Arilahti et al., 2000). No dependence of elongation to fracture on acetate concentration could be found, Fig. 2. In 100 mg/l acetate solution only one weld material test result is shown. The other weld sample in this solution suffered from failure of the test equipment fixture and thus the result is not shown.

Potentials of the SSRT test samples, Pt-electrode and the stainless steel body of the autoclave are shown in Fig 3 to 5. The first test run had 100 mg/l, the second test 10 mg/l and the third test run 1 mg/l acetate-ions in the highly saline solution. Prior to the first test run (with 100 mg/l acetate-ions) the autoclave body had been oxidised for several weeks in pressurised water reactor (PWR) environment at 320oC, where a protective magnetite-type spinel oxide is formed. After that the autoclave was washed in a recirculation loop with ion pure water until the outlet water showed a conductivity of less than 0.15 Scm-1. Thus the tests were started with an autoclave that would leach out a minimum amount of Fe, Ni or Cr ions.

Potentials of the SSRT test samples, that of the Pt-electrode and of the autoclave body were almost equal in all test runs, Figs 3 to 5. The potential of the hydrogen line (i.e. equilibrium potential of the H2/H

+ -reaction) for this particular pH and temperature (pH (100oC) 7.0) is about -0.52 VSHE. The equilibrium potential of CuO/Cu2O is about +0.22 VSHE and that of Cu2O/Cu is about -0.18 VSHE (EPRI, 1983).

In the first run with 100 mg/l acetate-ions the potentials of the autoclave body, the pre-oxidised CuOFP samples and the Pt-electrode potential showed a potential of about -0.2 VSHE, which is slightly higher than the hydrogen line (probably a mixed potential partially controlled by the presence of the Cu2O/CuO pre-oxidised film on the surface of the samples and dissolved Cu2+ - containing Cu-Cl -complexes in the solution). The period of voltammetric measurements is clearly seen as disturbance in the measured potential values, although the SSRT samples were not polarised. This may be due to formation of local electric fields inside the autoclave.

8

In the second run with 10 mg/l acetate-ions the potentials were roughly 0.2 V higher than in the previous run with 100 mg/l acetate-ions, close to 0 VSHE. In the third run with 1 mg/l acetate-ions, the potentials were yet higher, close to +0.2 VSHE. The fact that the CuOFP- and Pt-potentials coincide indicates that the potentials are dominated by a redox-active species in the solution. It is worth mentioning here that the autoclave body was not specially treated between the three successive test runs so that any dissolved copper containing species adsorbed on the autoclave walls in one test were at least partially carried on to the next test run. As discussed previously by Bojinov and Mäkelä (Bojinov and Mäkelä, 2003) copper may dissolve in chloride containing solutions and form Cu-Cl - complexes (including Cu(II)) which are able to operate as redox-agents keeping the potential of Cu at a higher level than would be expected based on the dissolved oxygen level only. The fact that the potentials of the autoclave body, CuOFP samples and the Pt-electrode in the last run are close to the CuO/Cu2O equilibrium potential further supports the hypotheses that it is the Cu(II) in the solution acting as a redox-species which is controlling the potentials.

In the saline simulated groundwater (8000 mg/l Cl-) with 100 mg/l acetate-ions the stress-elongation behaviour was rather similar to the highly saline simulated groundwater, Figs 6 and 7. The elongation to fracture in base material samples was the same for both two samples, about 43%, which is slightly lower than in found for the highly saline groundwater (Fig. 1) and before (Arilahti et al., 2000). In case of the EB-weld samples the elongation to fracture was also the same for both two samples, about 23%, which is at the lower end of the range found for the highly saline groundwater (Fig. 2) and before (Arilahti et al., 2000).

Potentials of the SSRT test samples, Pt-electrode and the stainless steel body of the autoclave in the saline simulated groundwater (8000 mg/l Cl-) with 100 mg/l acetate-ions are shown in Fig 8. The autoclave had been cleaned by glass pebble blasting after the tests in the highly saline groundwater and then oxidised in air at 100oC for 48 hrs. The potentials were close to each other, indicating once again the presence of a redox-active species, presumably dissolved copper-chloride complexes as discussed above.

9

CuOFP Base, HSSGW, 100oC, 14 MPa

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60

Elongation / %

Str

es

s / N

/mm

2

1 mg/l

1 mg/l

10 mg/l

10 mg/l

100 mg/l

100 mg/l

Figure 1. Stress-elongation curves of CuOFP base material samples as exposed to

varying concentrations of acetate-ions in highly saline simulated groundwater

(HSSGW).

CuOFP Weld, HSSGW, 100oC, 14 MPa

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30 35

Elongation / %

Str

es

s / N

/mm

2

1 mg/l

1 mg/l

10 mg/l

10 mg/l

100 mg/l

Figure 2. Stress-elongation curves of CuOFP EB-weld material samples as exposed to

varying concentrations of acetate-ions in highly saline simulated groundwater

(HSSGW).

10

CuOFP, 100oC, 14 MPa, HSSGW + 100 mg/l Ac

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200 250

Time / hrs

Po

ten

tial / V

SH

E SSRT 1

SSRT 2

SSRT 3

SSRT 4

Pt

SS Body

SSRT - test period

Period of voltammetricmeasurements

Figure 3. Potentials of the four SSRT samples, Pt-electrode and AISI 316 L stainless

steel autoclave body as a function of time in the test with 100 mg/l acetate-ions in highly

saline simulated groundwater (HSSGW).


-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 50 100 150 200 250

Time / hrs

Po

ten

tial / V

SH

E SSRT 1

SSRT 2

SSRT 3

SSRT 4

Pt

SS Body

SSRT - test period

Period of voltammetricmeasurements

End of test,oxygen in




11


-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 20 40 60 80 100 120 140 160 180

Time / hrs

Po

ten

tial / V

SH

E SSRT 1

SSRT 2

SSRT 3

SSRT 4

Pt

SS Body

SSRT - test period




CuOFP Base, SSGW, 100oC, 14 MPa

0

20

40

60

80

100

120

140

160

180

0 5 10 15 20 25 30 35 40 45 50

Elongation [%]

Str

ess [

MP

a]

P12

P13

Figure 6. Stress-elongation curves of CuOFP base material samples as exposed to 100

mg/l concentration of acetate-ions in saline simulated groundwater (SSGW).

12

CuOFP Weld, SSGW, 100oC, 14 MPa

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25

Elongation [%]

Str

ess [

MP

a]

H4

H5

Figure 7. Stress-elongation curves of CuOFP EB-weld material samples as exposed to

100 mg/l concentration of acetate-ions in saline simulated groundwater (SSGW).

CuOFP, 100oC, 14 MPa, SSGW + 100 mg/l Ac

-0.50

-0.40

-0.30

-0.20

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0 50 100 150 200 250 300

Time / hrs

Po

ten

tial / V

SH

E SSRT 1

SSRT 2

SSRT 3

SSRT 4

Pt

SS Body

SSRT - test period

Start N2 bubbling, oxygen removal

End of test,oxygen in


steel autoclave body as a function of time in the test with 100 mg/l acetate-ions in saline

simulated groundwater (SSGW).

13

4.2 Fracture surface investigations

Fracture surface examination by scanning electron microscope (SEM) showed a dimple type surface appearance typical of a ductile fracture (examples are shown in Figs 9 and 10 for the HSSGW and the SSGW, respectively) in all specimens. The dimples are formed around inclusions and precipitates in the matrix. In case of CuOFP these inclusions may be oxygen-containing compounds which were formed during the manufacturing process. When the amount of plastic deformation during the SSRT test increases first the interface between the inclusions and the surrounding matrix breaks and a void is formed around the inclusion. This void grows in volume under further straining, and is finally exposed to the environment after the fracture strain is exceeded and the sample breaks into two. The inclusions in the centre of the void typically dissolve and are not any more detected in fracture surface analysis. Because the formation of the voids is associated with a large amount of plastic strain their appearance on the fracture surface can be taken as evidence of a ductile material behaviour. In case of susceptibility to SCC the fracture surface appearance would be expected to be totally different, consisting of planar areas, possibly associated with exposed grain boundaries and a total lack of dimples.

The fractography of all the specimens is shown in Appendix 1.

H6 Weld metal, 100 mg/l Ac, 40x H6 Weld metal, 100 mg/l Ac, 500x

Figure 9. SEM fractography of the H6 weld metal sample after slow strain rate test in

highly saline simulated groundwater with 100 mg/l acetate-ions. The surface shows a

dimple structure typical of a ductile failure.

14


Figure 10. SEM fractography of the H4 weld metal sample after slow strain rate test in

saline simulated groundwater with 100 mg/l acetate-ions. The surface shows a dimple

structure typical of a ductile failure.

4.3 Voltammetry

Examples of the voltammetry results for both base metal and weld metal in highly saline simulated groundwater with 10 mg/l acetate-ions are shown in Figs 11 and 12. Voltammetry was only performed in the highly saline groundwater. All the voltammetry results are compiled in Appendix 2.

As seen in Figs 11 and 12, the Parkins and Holroyd criterion of the current density in the fast sweep exceeding 1 mA cm-2 is fulfilled only for the base metal sample. In case of the weld metal sample, Fig. 12, the current density in the fast sweep falls below 1 mA/cm2 for almost all potentials. Moreover, for the weld metal sample the current densities in the slow and fast sweep are rather close to each other. At positive potentials there is no crossover of the current density curves in either material. The same trends were found for all acetate-ion concentrations as shown by the data in Appendix 2. Additionally, there is a trend that the higher the acetate-ion concentration the lower the current density. These data indicate that based on the Parkins and Holroyd approach CuOFP is not susceptible to SCC in simulated groundwater in the presence of 1 to 100 mg/l acetate-ions. Another way of interpreting the results would be to argue that the Parkins and Holroyd approach is not applicable for CuOFP in the present environments.

15

HSSGW + 10 mg/l Ac, base metal sample P9

0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2

1st sweep 0.01 V/min

2nd sweep 0.01 V/min

1st sweep 5 V/min

2nd sweep 5 V/min

3rd sweep 5 V/min

Figure 11. Potential – Current density –curves for base metal sample P9 in highly

saline simulated groundwater (HSSGW) with 10 mg/l acetate-ions, 100oC and 14 MPa.

HSSGW + 10 mg/l Ac, weld metal sample H12

0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2



1st sweep 5 V/min

2nd sweep 5 V/min

Figure 12. Potential – Current density –curves for weld metal sample H12 in highly

saline simulated groundwater (HSSGW) with 10 mg/l acetate-ions, 100oC and 14 MPa.

16

4.4 Auger-spectroscopy results

Fig. 13 shows the Auger-spectra of the base metal sample P9-C exposed first to air at 100oC for 48 hrs and then to HSSGW + 100 mg/l Ac at 100oC and 14 MPa for about 200 hrs. This area was rich in sulphur, probably caused by adsorption of sulphate from the solution. Fig. 14 shows the same area after sputtering to a depth of about 140 nm. No evidence of phosphorus was found in either the outer surface layer or deeper inside the surface. Fig. 15 shows an example of spectra from an area less rich in sulphur with evidence for calcium and chloride, also believed to have adsorbed from the solution.

Fig. 16 shows the Auger – spectra of weld metal sample H9-A exposed only to laboratory air, Fig. 17 that of weld metal sample H9-B exposed to laboratory air at 100oC for 48 hrs and Figs 18 and 19 those of weld metal sample H10-C exposed first to laboratory air at 100oC for 48 hrs and then for about 200 hrs to highly saline simulated groundwater (HSSGW) with 100 mg/l acetate-ions at 100oC and 14 MPa. Some sulphur contamination has occurred even for the sample exposed only to laboratory air (Fig. 16), and more pronouncedly so for the sample exposed to 100oC air (Fig. 17). The source of this sulphur contamination (and in Fig. 17 also of chloride) is unclear, but could be caused by segregation of sulphur from the copper material onto the surface.

The main finding in Auger spectroscopy was that no evidence of phosphorus was found on any of the samples studied. As the resolution limit of the Auger device used is about 0.1 at% in case of phosphorus it can be concluded that phosphorus is not remarkably segregated to grain boundaries or other interfaces in either base metal or weld metal of CuOFP.

17

Figure 13. Auger-spectra of the base metal sample exposed first to air at 100oC for 48

hrs and then to HSSGW + 100 mg/l Ac at 100oC and 14 MPa for about 200 hrs. An

area with high sulphur concentration.

0 200 400 600 800 1000 1200

Cl

S

Base metal sample P9-C

Cu Cu

CuCuOC

Kinetic Energy [eV]

18

Figure 14. The same area as in Fig. 9 but after sputtering for about 140 nm.

Figure 15. Auger-spectra of the base metal sample exposed first to air at 100oC for 48

hrs and then to HSSGW + 100 mg/l Ac at 100oC and 14 MPa for about 200 hrs. An

area with less sulphur showing also chloride and calcium.

0 200 400 600 800 1000 1200

S


Cu Cu

CuCuOC

Kinetic Energy [eV]

0 200 400 600 800 1000 1200

CaCl

S


Cu

Cu

CuCu

O

C

Kinetic Energy [eV]

19

Figure 16. Auger-spectra of the weld metal sample H9-A exposed to laboratory air at

room temperature.

Figure 17. Auger-spectra of the weld metal sample H9-B exposed to air at 100oC for 48

hrs.

0 200 400 600 800 1000 1200

S

Cu

Weld sample H9-A

Cu

Cu

Cu

N

OC

Kinetic Energy [eV]

0 200 400 600 800 1000 1200

Cl

Cu

Weld sample H9-B, 48 h 100oC, air

S

Cu Cu

Cu

OC

Kinetic Energy [eV]

20

Figure 18. Weld sample H10-C after exposure first 48 hrs in air at 100oC and then

about 200 hrs in highly saline simulated groundwater (HSSGW) with 100 mg/l acetate-

ions, 100oC, 14 MPa. Auger-profile of 1-3 nm surface layer.

0 200 400 600 800 1000 1200

SCu

Weld sample H10-C

Cu

Cu

O

Cl

Cu

C

Kinetic Energy [eV]

21

Figure 19. Weld sample H10-C after exposure first 48 hrs in air at 100oC and then

about 200 hrs in highly saline simulated groundwater with 100 mg/l acetate-ions,

100oC, 14 MPa. Auger-profile after sputtering to 140 nm.

0 200 400 600 800 1000 1200

SCu

Weld sample H10-C

Cu

CuO

Cu

C

Kinetic Energy [eV]

22

5 CONCLUSIONS

Based on this investigation the following conclusions can be made:

The SSRT test results and the post-test SEM investigations showed no evidence of susceptibility to SCC in CuOFP base or weld metal in the environment in question, i.e. highly saline or saline simulated groundwater with 1 to 100 mg/l acetate-ions at 100oC and 14 MPa.

The voltammetric studies conducted in the highly saline groundwater further support the SSRT and SEM results in that based on the approach suggested by Parkins and Holroyd CuOFP (base or weld metal) should not be susceptible to SCC in the range of environments studied in this investigation.

Cu(II) dissolved in solution may act as a redox agent controlling the potential of CuOFP.

The Auger spectroscopy showed no evidence of phosphorus segregation on grain boundaries or other interfaces in either CuOFP base metal or weld metal.

23

6 SUMMARY

The work described in this report was launched to investigate the effect of acetate-ions (in the concentration range 1 mg/l to 100 mg/l) on susceptibility of CuOFP to stress corrosion cracking. Experiments conducted in anoxic highly saline (53800 mg/l Cl-) and saline (8000 mg/l Cl-) simulated groundwater included slow strain rate tests complemented by fractography and voltammetry. Tests were performed with both CuOFP base metal and EB-weld metal samples. The test temperature was 100oC and the pressure 14 MPa, simulating the pressure expected to arise as a sum of the hydrostatic pressure and the bentonite swelling pressure. Additionally, Auger-spectroscopy studies were performed to investigate the possibility of phosphorus segregation onto grain boundaries or other interfaces.

The elongation to fracture values in the slow strain rate tests were comparable to those in air and showed no dependence on the acetate-ion concentration. The fractography of the samples showed a dimple like appearance of the fracture surfaces in all cases. Both of these results indicate that neither CuOFP base metal nor the EB-weld metal is susceptible to SCC in the environments in question. The voltammetry results from tests conducted in the highly saline groundwater further supported this conclusion. The Auger – spectroscopy results (at the resolution limit of about 0.1 at%) showed no evidence for phosphorus segregation in either CuOFP base metal or the EB-weld metal.

24

REFERENCES

Arilahti, E. Bojinov, M., Mäkelä, K., Laitinen, T. and Saario, T., Stress corrosion cracking investigation of copper in groundwater with ammonium ions. Posiva Working Report 2000-46.

Benjamin, L., Hardie, D. & Parkins, R. 1988. Stress corrosion resistance of pure coppers in groundwaters and sodium nitrate solutions. Br. Corros. J., 1988, Vol. 23, No. 2, pp. 89-95. Pedersen, K., Microbial processes in radioctive waste disposal. SKB Technical Report TR-00-04, 2000.

Bojinov, M. and Mäkelä, K., Corrosion of copper in anoxic 1M NaCl solution. Posiva Working Report 2003-45.

EPRI 1983. Computer-calculated potential pH diagrams to 300oC. Volume 2: Handbook of diagrams. EPRI NP-3137, Vol. 2, June 1983.

Escalante, E. & Kruger, J. 1971. Stress corrosion cracking of pure copper, Journal of the Electrochemical Society ,Vol.118, No.7, 1971, pp. 1062-1066.

Kotelnikova, S. & Pedersen, K., Distribution and activity of methanogens and homoacetogens in deep granitic aquifers at Äspö Hard Rock Laboratory, Sweden. FEMS Microbiology Ecology, Vol. 26, 1998, pp. 121-134.

Parkins, R. & Holroyd, N. 1982. Stress corrosion cracking of 70/30 brass in acetate, formate, tartrate and hydroxide solutions. Corrosion, Vol. 38, No. 5, 1982, pp. 245-255.

Pedersen, K., Microbial processes in radioctive waste disposal. SKB Technical Report TR-00-04, 2000.

Pitkänen, P., Partamies, S. & Luukkonen, A. 2004. Hydrogeochemical interpretation of baseline groundwater conditions at the Olkiluoto site. Helsinki, Finland: Posiva Oy. 159 p. Posiva-2003-07. ISBN 951-652-121-5.

U. Vuorinen ja M. Snellman, Finnish reference waters for solubility, sorption and diffusion studies. Posiva Working report 98-61, p. 26.

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RESEARCH REPORT No. TUO75-055733

Appendix 1. Fractography

The fracture surface of all base metal specimens showed fully ductile appearance with heavy plastic deformation and dimple structure. Also the weld metal fracture surfaces were ductile.

Highly saline simulated groundwater with 1 mg/l acetate-ions.

P10 Base metal, 1 mg/l Ac, 50x PS 10 Base metal, 1 mg/l Ac, 500x

P14 Base metal, 1 mg/l Ac, 35x P14 Base metal, 1 mg/l Ac, 500x

26 (5)




Highly saline simulated groundwater with 10 mg/l acetate-ions.


27 (5)





Highly saline simulated groundwater with 100 mg/l acetate ions.

28 (5)





Saline simulated groundwater with 100 mg/l acetate ions.

29 (5)





30 (5)



31 (3)


Appendix 2. Voltammetry

The voltammetry results (i.e. potential – current density curves) in highly saline SGW are shown in Figs A2-1 to A2-6.

SGW + 1 mg/l Ac, base metal P9

0.001

0.01

0.1

1

10

100

-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5 0.7

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2



3rd sweep 0.01 V/min

1st sweep 5 V/min

2nd sweep 5 V/min

Figure A2-1. Potential – Current density –curves for base metal sample P9 in highly

saline simulated groundwater with 1 mg/l acetate-ions, 100oC and 14 MPa.


0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2

Potential vs. SHE

Cu

rren

t d

en

sit

y /

mA

cm-2



1st sweep 5 V/min

2nd sweep 5 V/min

3rd sweep 5 V/min



32 (3)



0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2

1st sweep 5 V/min

2nd sweep 5 V/min

3rd sweep 5 V/min





SGW + 1 mg/l Ac, weld metal H12

0.001

0.01

0.1

1

10

100

-0.7 -0.5 -0.3 -0.1 0.1 0.3 0.5

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2



1st sweep 5 V/min

2nd sweep 5 V/min

3rd sweep 5 V/min

Figure A2-4. Potential – Current density –curves for weld metal sample H12 in highly


33 (3)



0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2



1st sweep 5 V/min

2nd sweep 5 V/min




0.001

0.01

0.1

1

10

100

-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2

Potential vs. SHE

Cu

rren

t d

en

sit

y / m

Acm

-2



1st sweep 5 V/min

2nd sweep 5 V/min

3rd sweep 5 V/min



stress corrosion cracking investigation of copper in ... oy fi-27160 olkiluoto, finland tel...

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