stress corrosion cracking investigation of copper in ... · the main conclusion from this...

$: Stress corrosion cracking investigation of copper in ... · The main conclusion from this investigation is that the slow strain tests and the SEM investigations of the fracture surfaces$
Working Report 2000-46

Stress corrosion cracking investigation of copper in groundwater

with ammonium ions

Esko Arilahti

Martin Bojinov

Kari Makela

Timo Laitinen

Timo Saario

December 2000

POSIVA OY

T6616nkatu 4, FIN-001 00 HELSINKI, FINLAND

Tel. +358-9-2280 30

Fax +358-9-2280 3719



with ammonium ions

Esko Arilahti

Martin Bojinov

Kari Makela

Time Laitinen

Time Saario

December 2000

-m MANUFACTURING TECHNOLOGY

1 (17)

A Work report

B Public research report Research report,

X confidential

Title

Stress corrosion cracking investigation of copper in ground water with ammonium ions Customer or finansing body and order date/No. Research report No.

Posiva Oy orders 9270/99/MVS and 9633/00/MVS and VAL67- 001411 Svensk Karnbdinslehantering AB order 2373 Project

Posi-2 Author(s)

Project No.

V9SU00873

Esko Arilahti, Martin Bojinov, Timo Laitinen, Kari Makela and Timo Saario

No. of pages/appendices

17 I 2+9+2

Keywords

Copper, nuclear waste, slow strain rate testing, ammonium Summary

Oxygen free, phosphorus containing copper (Cu OFP, base metal and electron beam welded material) was tested at 100 °C for susceptibility towards stress corrosion cracking in simulated groundwater and in the presence of 1, 10 and 100 mg/1 ammonium ions (NH4+). The slow strain rate tests (with o£ I ot = 10-6 s- 1

) showed fracture strain of base metal to be equal to that measured in air. The fracture strain of weld material was clearly lower than that of base metal. However, the fracture surface analysis with scanning electron microscope (SEM) showed no signs of stress corrosion cracking. The lower fracture strain of the weld material is attributed to the larger and more inhomogeneous grain size in comparison to the base metal.

The main conclusion from this investigation is that the slow strain tests and the SEM investigations of the fracture surfaces showed no signs of susceptibility to stress corrosion cracking for Cu OFP in groundwater with a maximum of 100 mg/1 ofNH4+-ions.

Date 20 November, 2000

\fL__ \IL t-__ _ Rauno Rintamaa Research Manager Distribution (customers and VTT):

Esko Arilahti Research Engineer

/:/~ Checked

Posiva Oy 1 copy, Svensk Karnbranslehantering AB 1 copy, VTT 1 copy

VTT MANUFACTURING TECHNOLOGY Materials and Structural Integrity

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S· \Z-Qb I



with ammonium ions

Esko Arilahti

Martin Bojinov

Kari Makela

Timo Laitinen

Timo Saario

VTT Manufacturing Technology

December 2000

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.

STRESS CORROSION CRACKING INVESTIGATION OF COPPER IN GROUNDWATER WITH AMMONIUM 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 small concentrations of ammonium-ions. The concentrations found in the site studies both in Sweden and Finland is 3 mg/1 at maximum.

This work was conducted to investigate the susceptibility of oxygen free phosphorus containing copper to stress corrosion cracking in simulated ground water with a maximum of 100 mgll of ammonium-ions. The simulated ground water corresponds to the near-field water chemistry calculated to prevail at the copper canister surface in the Olkiluoto ground water.

The standard slow strain rate tests (SSRT) showed fracture strain of base metal to be equal to that measured in air. The fracture strain of weld material was clearly lower than that of base metal. However, the fracture surface analysis with scanning electron microscope (SEM) showed no signs of stress corrosion cracking. The lower fracture strain of the weld material is attributed to the larger and more inhomogeneous grain size in comparison to the base metal.

The main conclusion from this investigation is that the slow strain tests and the SEM investigations of the fracture surfaces showed no signs of susceptibility to stress corrosion cracking for Cu OFP in simulated groundwater (100 °C, 14 MPa) with a maximum of 100 mg/1 ofNH4+-ions.

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

AMMONIUM-IONIEN VAIKUTUS KUPARIN JANNITYSKORROOSIOON SIMULOIDUSSA POHJA VEDESSA

TIIVISTELMA

Suomessa ja Ruotsissa kaytetty ydinpolttoaine pakataan pallografiittivaluraudasta valmistettaviin sailioihin, joiden ulkopinnalla on kuparimetallista tehty suoja. Kuparimetalli toimii valurautasailion korroosiosuojana.

Kirjallisuusselvityksen perusteella fosforimikroseostettu kupari voi olla altis jannityskorroosiolle pienia ammonium-ioni pitoisuuksia sisaltavissa vesiliuoksissa. Porareikavesista tehdyissa analyyseissa on seka Suomessa etta Ruotsissa havaittu maksimissaan 3 mg/1 pitoisuuksia ammonium-ioneja.

Tassa tutkimuksessa selvitettiin fosforimikroseostetun kuparin jannityskorroosio-alttiutta simuloidussa pohjavedessa, johon lisattiin maksimissaan 100 mg/1 ammonium-ioneja. Simuloitu pohjavesi vastaa Olkiluodon pohjaveden perusteella laskettua kuparikapselin lahikenttaan syntyvaa vesikemiaa.

Kokeet tehtiin hidasvetokokeina (SSRT-koe), joka on standardikoe jannityskorroosioalttiuden tutkimiseen. Hidasvetokokeissa havaittiin, etta perusaineen murtovenyma simuloidussa pohjavedessa tehdyissa kokeissa on hajonnan puitteissa sama kuin ilmassa tehdyissa kokeissa. Hitsiaineen murtovenyma oli huomattavasti perusainetta pienempi. Hitsiaineen raekoko oli noin kaksi kertaa perusaineen raekokoa suurempi, mika todennakoisesti aiheuttaa pienemman murtovenyman. Simuloidun pohjaveden ammonium-ioni pitoisuus ei vaikuttanut perusaineen eika hitsiaineen murtovenymaan. Kokeiden jalkeen pyyhkaisyelektronimikroskoopilla tehdyissa murtopinta-analyyseissa ei havaittu jannityskorroosiolle tyypillisili piirteita. Yhteenvetona tuloksista todetaan, etta kupari ei ole altis ammonium-ionien (pitoisuus maksimissaan 100 mg/1) aiheuttamalle jannityskorroosiolle Olkiluodon pohjavetta simuloivassa pohjavedessa loppusijoitusolosuhteita vastaavassa lampotilassa (100 °C) ja paineessa (14 MPa).

A vainsanat: Kupari, jannityskorroosio, ammonium-ioni.

1

TABLE OF CONTENTS

page

Abstract

Tiivistelma

1 INTRODUCTION .............................................................................................................. 3

2GOALS .............................................................................................................................. 4

3 RESTRICTIONS OF THE STUDY .................................................................................... 5

4 EXPERIMENTAL .............................................................................................................. 5

5 RESULTS .......................................................................................................................... 7 5.1 Stress-elongation and potential-time curves .................................................................. 7 5.2 Grain size determination .............................................................................................. 14

6 DISCUSSION ................................................................................................................... 15

7 CONCLUSIONS ............................................................................................................... 16

8 SUMMARY ...................................................................................................................... 17

9 REFERENCES .................................................................................................................. 18

2

3

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.

Literature data (Sato and Nagata, 1978; Thompson & Tracy, 1949) indicate that copper may be susceptible to SCC in presence of low concentrations of ammonium ions (NH/). The groundwater analyses from the borehole sites both in Sweden and in Finland have shown some concentration of ammonium ions. The maximum concentration has been 3 mg/1.

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 °C. During this period 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 ground water.

4

2GOALS

The goal of this investigation was to find if oxygen free phosphorus containing copper (Cu-OFP) is susceptible to SCC in final disposal vault conditions in the presence of a maximum concentration of 100 mg/1 ammonium ions.

5

3 RESTRICTIONS OF THE STUDY

The slow strain rate technique (SSRT) used is standardised (ASTM 0129-95, ISO 7 539-7), and is generally considered to be conservative with respect to revealing materials susceptibility to stress corrosion cracking. The specimens used have a smooth surface, similarly to most of the surface of the copper shield. The effect of local stress concentrations due to possible surface imperfections was not studied. In order to limit the test matrix only one strain rate was used. The strain rate should be low enough in order for the studied environmental effect to have enough time to exert an influence, but obviously in order to reduce the testing time one does not want to use a lower than

necessary strain rate. The strain rate of d%1 = 10-6 s -1 was selected based on earlier

studies conducted for a similar material in simulated groundwater (Benjamin et al., 1988).

4EXPERIMENTAL

Oxygen free phosphorus containing copper (Cu OFP) containing 99.992 wt-% Cu and 45 ppm P (Outokumpu Poricopper Oy) was used as the test material in all the experiments. The specimens were taken from a copper pipe with an electron beam weld. The cutting scheme is shown in Fig. 1. All specimens in this study were extracted from the inner surface of the tube. The length of the original tube was 1100 mm and the diameter 476/526 mm inner diameter/outer diameter (I.D./O.D).

Fig. 1. Cutting scheme for the specimens. PS = base metal, HS = weld metal.

Round SSRT specimens with a neck of diameter 5 mm and length of 30 mm were manufactured from the specimens. The specimens were polished using a 4000 grit emery paper and rinsed with MILLI-Q® water. After this the specimens were kept in an air oven at 100 °C for 48 hours in order to grow a representative oxide film.

The experiments were performed in a static AISI316 stainless steel autoclave (water volume 8000 cm3

). The exposed copper surface area was 34.3 cm2• The simulated

groundwater was prepared in a glove box under oxygen free conditions. The autoclave

6

was filled with nitrogen gas; the electrolyte was pumped into the autoclave using nitrogen gas pressure and pressurised to the test pressure (14 MPa) using nitrogen gas. After bubbling with nitrogen gas for three hours the bubbling was stopped and the test was started. An external pressure balanced AgCl/ Ag reference electrode filled with 0.1 M KCl was installed into the autoclave. Additionally, a Pd electrode saturated with hydrogen (by continuous cathodic charging at -0.05 mAcm-2) was used as a reference electrode and was assumed to behave as a Reversible Hydrogen Electrode (RHE). All the potentials in this work are reported on the standard hydrogen electrode scale (SHE).

Two different types of groundwater were used, a low and a high chloride content type. Groundwater simulates the saline near-field reference groundwater from Olkiluoto (Vuorinen & Snellman 1998). This water was modified according to the experimental conditions (100 °C). The simulated groundwater at equilibrium at 100 oc is shown in Table 1. The low chloride type had the same composition but with a chloride content of 3700 mg/1. To this basic anoxic reference groundwater the required amounts of NRt + ( 1, 10, 100 mg!L) and sulphide (1 mg!L) solutions were added. The pH of the solution was adjusted to 8.00 prior to addition of NRt + and sulphide. Also the pH of the NRt + and sulphide solutions was adjusted to pH~ 8.00 prior to addition to the simulated reference groundwater. The starting pH at 25°C was thus about 8.00. The calculated (EQ3NR!EQ6) pH at 100 oc for this solution was about 7.00 (calculations were performed by Ulla Vuorinen from VTI Chemical Technology).

The potential difference between the Pd electrode and the AgCll Ag reference electrode filled with 0.1 M KCl can be used as a measure of the pH. The calculated difference between the potential of the AgCl/ Ag reference electrode filled with 0.1 M KCl and the standard hydrogen electrode scale (SHE) is 0.211 V at T = 100 °C (E

0H+tH2 =EsHE =EAgeli Ag -0.211V). The Nemst equation relates the pHT and

potential E + 1

at 100 oc as follows H H 2

(1)

Here/H2 is the fugacity(= partial pressure) of hydrogen gas in the water. From equation (1) pHT becomes

fJ _ EsHE- EH+ 1H2

- 0.037V ·log(/H2

I O.lMPa)

p T- 0.074V (2)

The hydrogen partial pressure at the Pd electrode surface is not known precisely. Taking that the hydrogen partial pressure at the Pd electrode surface can be 14 MP a at maximum (equal to the test pressure) one can estimate that the effect of hydrogen partial pressure can be -1.1 pH units at maximum. Assuming a smaller hydrogen partial pressure of 1 MPa the effect is -0.5 pH units.

7

Table 1. Composition of the simulated high chloride reference groundwater (mg/1).

: .. Ni I{ .. : ta :. M

0.2 10066 90 815 ..

. Cs .. B .-. .. Cl ,

. .

F Br 0.2 1.7 17000 3.5 140

. ,·,

Tps · · :.:sol~ :. · . ·,.· Ba: ·I .. . .

29.4 1320 -298 ()() 0.1 2.3

The tests were performed with a load frame allowing four specimens to be tested simultaneously. The specimens were electrically insulated from the load frame using Zr02

ceramic washer rings. Cormet Ltd supplied the slow strain rate testing equipment.

SRESULTS

5.1 Stress-elongation and potential-time curves

The stress-elongation curves for each of the five test runs and the corresponding potential measurement data are shown in Figs. 2 to 11. The elongation to fracture and maximum stress data are shown in Table 2. The fracture strain of the base metal was close to that measured in air (Benjamm et al., 1988).

The water chemistry analysis both before and after the test runs is shown in Appendix 1. The water chemistry remained stable within a given test run. The only exception was sulphide (S2

) which was no more present in the analysis made after the test runs. It is proposed that the small amount of sulphide be consumed in the reactions with the autoclave walls and possibly also with copper. A small amount of Fe was found in the water after the tests. It is propable that Fe is dissolved from the autoclave walls during the test.

The fracture surface appearance is shown in Appendix 2. The appearance of the base metal specimens was that of a typical ductile fracture in all cases, except for small areas in weld metal sample H9.

8

T = 100 °e, p = 14 MPa, 3700 mg/1 er, 100 mg/1 NH/

180 ---·------·----·---------·------------·----------1 i

160

140

-120 CV ~

~ 100 ., ., 80 Q) ... -(/) 60

40

20

0

0 5 10 15 20 25 30 35 40 45 50

Elongation [%]

Fig. 2. Stress-elongation curves for base metal specimens (PSI and PS2) and weld metal specimens (HSI and HS2). Groundwater contains 3700 mg/1 Cl and IOO mg/1 NH/ .

0.2

0.1

0

~ -0.1 ~ > -0.2

~ -0.3 s ~ -0.4

-0.5

-0.6

-0.7 22.3

- ·--··-

........

~

' . 2000 23.3.

T = 100 °e, p = 14 MPa, 3700 mg/1 er, 100 mg/1 NH/

----... -----·---·---·-------····------··----... ·--·· Loading

Speci1 PS2, 1-

...-

.......... -............

000 24.3.2000 25.3.2000 26.3.2000 27.3.2000 28.3.2000

Date

----·-·--., l l !

- I Pt

I

I !

nens PS1, I S1 and HS2 I

l I

i Pd

! I I

29.3.2000 30.3 . 2000

Fig. 3. Corrosion potential of base metal specimens (PSI and PS2) and weld metal specimens (HSI and HS2), as well as potential of Pt and cathodically polarised Pd Groundwater containing 3700 mg/1 Cl and IOO mg/1 NH4+.

9

T = 100 °e, p = 14 MPa, 3700 mg/1 er, 10 mg/1 NH/

180 -------·-------·------·--·---·-----------, PS4 i

160

140

..... 120 cu D.. ~ 100 en en 80 Q)

:computer failure

... -U) 60

40

20 HS4

0

0 5 10 15 20 25 30 35 40 45 50

Elongation [%]

Fig. 4. Stress-elongation curves for base metal specimens (PS3 and PS4) and weld metal specimens (HS3 and HS4). Groundwater contains 3700 mg/1 Cl and 10 mg/1 NH4+·

0

-0.1

-0.2 w ~ -0.3

> (ij -0.4 :;; c::: .! -0.5 0

D..

-0.6

-0.7

-0.8 5.4

T = 100 °e, p = 14 MPa, 3700 mg/1 er, 10 mg/1 NH/

r-·-------·------------------...._ Loading

- .....

-~ ~· SPE

~ PS3,W

'-.. ...............

. 2000 2ooo 6.4 . 7.4.2000 8.4.2000 9.4.2000 10.4.2000 11.4.2000 12.4.2000

Date

1--·----, i i

I j

Di ! . l ._

PS4

l l

~imens i I

~3 and HS4 I !

Pd l ''\ !

I l

13.4.2000 14.4. 2000

Fig. 5. Corrosion potential of base metal specimens (PS3 and PS4) and weld metal specimens (HS3 and HS4), as well as potential of Pt and cathodically polarised Pd Groundwater containing 3700 mg/1 Cl and 10 mg/1 NH4+.

10

T = 100 °e, p = 14 MPa, 17000 mg/1 er, 100 mg/1 NH/

180

160

140

-120 ns Q.

~ 100 ., ., 80 G) ... -m 60

40

20

0

0 5 10 15 20 25 30 35 40 45 50

Elongation r.1o1

Fig. 6. Stress-elongation curves for base metal specimens (PS5 and PS6) and weld metal specimens (HS5 and HS6). Groundwater contains 17000 mg/1 Ct and 100 mg/1 NH/ .

0

-0.1

~ -0.2 ~ > "i -0.3 :w c .! £ -0.4

-0.5

-0.6

T = 100 °e, p = 14 MPa, 17000 mg/1 er, 100 mg/1 NH/

---I Loadina

r

I r l - r---p;-- I ! l

I ~ ! li._~ I

I SPE cimens PSS ! PSt , HSS and HS6 I

I --- ' I f

Pd ! 17.4.2000 18.4.2000 19.4.2000 20.4.2000 21.4.2000 22.4.2000 23.4.2000 24.4.2000 25.4.2000

Date

Fig. 7. Corrosion potential of base metal specimens (PS5 and PS6) and weld metal specimens (HS5 and HS6), as well as potential of Pt and cathodically polarised Pd Groundwater containing 17000 mg/1 Ct and 100 mg/1 NH4+.

11

T = 100 oe, p = 14 MPa, 17000 mg/1 er, 10 mg/1 NH/

180 ----------·---·-------·----·----·-·-·--·-----1 i

160

140

...... 120 ea D.. ~ 100 ., .,

80 Q) ... -fJ) 60

40 HS8

20

0

0 5 10 15 20 25 30 35 40 45 50

Elongation [%]

Fig. 8. Stress-elongation curves for base metal specimens (PS8 and PS14) and weld metal specimens (HS7 and HS8). Groundwater contains 17000 mg/1 er and 10 mg/1 NH/ .

0

-0.1

...... -0.2 w :I: fJ) > -0.3 Cij

~ -0.4 .! 0

D.. -0.5

-0.6

-0.7

T = 100 °e, p = 14 MPa, 17000 mg/1 er, 10 mg/1 NH/

-·--·--------------------------Locid~-----------·-----------1

p l f l

l l I I

j ... l l

'-.... I ----- pt l

~ l .f ~ I

t I \).1'1-. 11 SpeclmE ns t'~ts, ! .... ..._ PS14, HS'i and HSS !

l

\ a Pd l

I I

~ IL .I I . ! _._..._..

i 3.5.2000 4.5.2000 5.5.2000 6.5.2000 7.5.2000 8.5.2000 9.5.2000 10.5.2000 11.5.2000

Date

Fig. 9. Corrosion potential of base metal specimens (PS8 and PS14) and weld metal specimens (HS7 and HS8), as well as potential of Pt and cathodically polarised Pd Groundwater containing 17000 mg/1 er and 10 mg/1 NH4+.

180

160

140

.... 120 ns CL ~ 100 fl) fl) 80 G) ... ... en 60

40

20

0

0

12

T = 100 °e, p = 14 MPa, 11000 mg/1 er, 1 mg/1 NH/

·-------------------------·---,

10 20 30

Elongation [%]

40

j

!

PS11

50 60

Fig. 10. Stress-elongation curves for base metal specimens (PS10 and PS11) and weld metal specimens (HS9 and HS10). Groundwater contains 17000 mg/1 Cl and 1 mg/1 NH/ .

0

-0.1

-0.2 w ~ -0.3

> "i -0.4 ; c .! -0.5 0

CL

-0.6

-0.7

-0.8

T = 100 °e, p = 14 MPa, 17000 mg/1 er, 1 mg/1 NH4+

r·------·------._,. ___ , ________ .. ____________ ,

i Loading l

..... ! ~

,..- Pt I ....... I .,

-~ ! !

~ I '

~ ~ Specimens PS10, !

PS11, HS9 and HS10 I ., i i

"--..... Pd I l

I i

25.5.2000 26.5.2000 27.5.2000 28.5.2000 29.5.2000 30.5.2000 31.5.2000 1.6.2000 2.6.2000

Date

Fig. 11. Corrosion potential of base metal specimens (PS10 and PS11) and weld metal specimens (HS9 and HS10), as well as potential of Pt and cathodically polarised Pd Groundwater containing 17000 mg/1 Cl and 1 mg/1 NH4+.

13

Table 2. Elongation to fracture and maximum stress of the specimens.

Specimen £p,% N -2 O'MAX, mm er, mg/1 NRt+, mg/1

PS1 49.2 162 3700 100 PS2 45.9 161 3700 100 PS3 >34(x) 161 3700 10 PS4 >34(x) 161 3700 10 PS5 43.6 162 17000 100 PS6 44.8 161 17000 100 PS8 47.5 163 17000 10 PS10 50.9 163 17000 1 PS11 45.8 160 17000 1 PS14 45.4 161 17000 10 HS1 37.5 151 3700 100 HS2 30.6 150 3700 100 HS3 29.7 148 3700 10 HS4 32.0 147 3700 10 HS5 29.8 145 17000 100 HS6 21.1 135 17000 100 HS7 20.4 130 17000 10 HS8 34.2 151 17000 10 HS9 20.1 125 17000 1 HS10 31.6 144 17000 1 (x) The elongation to fracture not determined due to computer failure

14

5.2 Grain size determination

The grain sizes were determined for all the weld metal specimens and the base metal specimens PS and Pll. The cross-sectional pictures of the specimens are shown in Appendix 3. An example of the appearance of the cross-sectional pictures is shown in Fig. 12. The grains in the weld metal specimens are directional and of somewhat variable SIZe.

SAMPLE GRAIN SIZE

Jlffi

PS5 190

PS 11 190

HS1 260

HS2 350

HS3 430

HS4 350

HS5 350

HS6 350

HS7 350

HS8 350

HS9 350

HS10 350

Table 3. The grain sizes of the weld specimens and base metal specimens P5 and P 11.

Fig. 12. The cross-sectional picture of weld metal sample HS3 (16X).

15

6 DISCUSSION

The potential difference ~E = E sHE -EH+ 1 H was measured to lie between 0.5 5 V <~E < 2

0.67 V in the different test runs, see Figs 3, 5, 7, 9 and 11. The potential difference was

rather stable and no clear trend was seen in the change of ~E within a given test run.

Assuming the hydrogen partial pressure of 14 MP a and taking the average value of ~E =

0.61 V results in the estimate ofpHT=IOoc= 7.2 ±0.8 (Eq. 2). This value is close to the pH value of 7.0 estimated by calculation. This indicates that the pH of the groundwater was in neutral pH range. The fact that the ~E remained rather stable during a given test indicates that the pH did not change markedly during the tests. Polarisation of the Pdelectrode generates hydrogen on the Pd surface at a constant rate. One could argue that this would in a static autoclave result in gradual increase of the partial pressure of hydrogen. This would result in a gradual decrease in the potential of the Pt-electrode, which is obviously not true in the present case (see Figs 3,5,7,9 and 11). It is proposed that the hydrogen generated by polarising the Pd-electrode as well as from corrosion reactions at the autoclave walls diffuses out from the autoclave at a faster rate than is the generation rate. The water chemistry remained also otherwise stable within a test run. The only exception was sulphide (S2

-) which was not anymore present in the analysis made after the test runs. It is proposed that the small amount of sulphide be consumed in the reactions with the autoclave walls.

The platinum potential can be considered as a measure of the oxidative power of the ground water. Potential of Pt was quite well repeatable and varied roughly between -0.3 V < EPt < -0.1 V. This indicates that the environment was free of oxygen tp a satisfactory degree, i.e. anoxic. The equilibrium potential Ecu1cu2o is about -0.2 V at 100 °C (EPRI, 1983), while the corrosion potential Ecorr of the specimens was -0.5 V .< Ecorr < -0.35 in different test runs. This means that when the original air borne oxide is broken down the exposed bare metal has no tendency to form a new passive film. On the other hand, passive film formation has been stated to be a necessary prerequisite for stress corrosion cracking to occur in copper (Saario et al., 1999 and references therein). Thus, at such low potentials where passive film formation does not occur stress corrosion cracking of copper is not expected to occur.

The large value (>40%) and small scatter of the elongation to fracture in the stresselongation curves of the base metal specimens indicated . ductile behaviour in all environments. This was confirmed by the ductile appearance of the fracture surfaces . . Further support for this conclusion is shown in Fig. 13, where the elongation to fracture of base metal specimens is shown not to depend on the concentration of NH4 +. In case of weld metal specimens the elongation to fracture was on the average about 60% of that of the base metal and there was considerable scatter. However, the fracture surfaces did not reveal any clear signs of stress corrosion cracking., In addition, in Fig. 13 the elongation to fracture of weld metal specimens shows no dependence on the concentration ofNH4 +.

u: w

60

50

40

30

20

10

0

h

p 8

• ; • .. • ~

0 20 40

16

j• Weld material' 0 Base material

60

NH4\ mg/1

80

u

~

• • -......

•

100 120

Figure 13. The elongation to fracture results as a function of NRt +content.

The larger scatter and at least partially also the smaller values of the elongation to fracture in the weld metal in comparison with the base metal is proposed to be caused by the much larger and varying grain size of the weld metal. The larger grain size is due to the welding procedure resulting in dendrite growth in the solidification process.

7 CONCLUSIONS

The main conclusions from this investigation are

• The stress-elongation curves of Cu OFP material showed no susceptibility to stress corrosion cracking in tests performed in simulated highly saline ground water at 100 oc and 14 MPa in the presence of a maximum of 100 mg/1 ofNHt.

• The weld metal specimens showed an about 60% lower elongation to fracture than the base metal specimens. This was attributed to the much larger grain size of the weld metal specimens, resulting from the dendrite growth in the solidification process.

• The fracture surfaces showed no clear signs of stress corrosion cracking, supporting the conclusion that Cu OFP material is not susceptible to stress corrosion cracking in the environment used in the present tests.

17

SSUMMARY

This research work was carried out to investigate whether Cu OFP is susceptible to stress corrosion cracking in simulated groundwater conditions in the presence of ammonium ions (NRt). Both base metal and weld metal specimens were exposed in a slow strain rate test (SSRT) to the groundwater at 100 °C.

The fracture strain measured in the tests showed no dependence on the ammonium ion concentration. The fracture surfaces showed no evidence of stress corrosion cracking. The lower fracture strain of the weld metal specimens is proposed to be caused by the much larger and more variable grain size in the weld metal. The main conclusion derived from the results was that Cu OFP material is not susceptible to stress corrosion cracking in the simulated groundwater with the ammonium ion concentrations used in the present tests.

18

9 REFERENCES

ASTM 0129-95, Standard practise for slow strain rate testing to evaluate the susceptibility of metallic materials to environmentally assisted cracking. American Society for Testing and Materials, 1995.

Benjamin, L., Hardie, D. & Parkins, R. 1988. Stress corrosion resistance of pure coppers in ground waters and sodium nitrate solutions. Br. Corros. J., 1988, Vol. 23, No. 2, pp. 89-95.

EPRI, 1983. Computer-calculated potential pH diagrams to 300 °C, Volume 2: Handbook of diagrams. EPRI NP-3137, Electric Power Research Institute, Palo Alto, CA, USA, 1983.

EQ3NRIEQ6, Wolery, T.J., (1992) EQ3/6. A Software Package for Geochemical Mode ling of Aqueous Systems (Version 7 .0). Livermore, CA, USA: Lawrence Livermore National Laboratory. UCRL-MA-110662 PT I-IV.

ISO 7539-7, Corrosion of metals and alloys - Stress corrosion testing - Part 7: Slow strain rate testing. The International Organisation for Standardization, 1989.

Sato, S. & Nagata, K. 1978. Stress corrosion cracking of phosphorus deoxidised copper. Journal of Japan Copper and Brass Research Association, Vol. 17, No. 1, 1978, pp. 202-214. (In Japan).

Saario, T., Laitinen, T., Makela, K. and Bojinov, M. 1999. Literature survey on stress corrosion cracking of Cu in presence of nitrites, ammonia, carbonates and acetates, Posiva Working Report 99-57. Posiva Oy, October 1999.

Vuorinen, U. & Snellman , M. 1998. Finnish reference waters for solubility, sorption and diffusion studies. Posiva Working Report 98-61.

Thompson, D. & Tracy, A. 1949. Influence of composition on the stress corrosion cracking of some copper-base alloys, Trans. AIME, Vol. 185, Feb. 1949, pp. 100-106.

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RESEARCH REPORT No. VAL67- 001411

Appendix 1. Water chemistry analyses. The water chemistry analyses are shown in Tables Al-l to Al-4 below. The analyses showed that the chemistry was stable during the tests with the exception of sulphide, which was found not to be present in the analyses performed after the tests.

Table Al-l. The water analyses performed before and after the SSRT-test with specimens PS 1, PS2, HS 1 and HS2, and the water analyses performed before SSRT -test with specimens PS3, PS4, HS3 and HS4.

PSl, PS2, PSl, PS2, PS3, PS4, Detection Uncertainty HSl, HS2 HSl, HS2 HS3, HS4 limit (2 RSD %) 23.3.2000 30.3.2000 4.4.2000 mg/1 before test after test before test

Boron (B) mg/1 1,8 1,9 1,8 0,1 ±25% Calcium (Ca) mg/1 820 850 850 0,5 ± 10% Potassium (K) mg/1 91 93 93 0,5 ± 15% Magnesium (Mg) mg/1 350 360 360 0,5 ± 15% Sodium (Na) mg/1 1440 1480 1460 1 ± 10% Strontium (Sr) mg/1 30 30 30 0,1 ±20% Iron (Fe) mg/1 0,003 0,032 < 0,002 0,002 ±40% Copper (Cu) mg/1 < 0,001 0,002 < 0,001 0,001 ±40% Bromide (Bf) mg/1 150 140 140 0,1 ± 10% Chloride (Cr) mg/1 3900 3900 3700 0,1 ± 10% Fluoride (F-) mg/1 3,7 3,8 3,8 0,1 ± 10-15% Sulphide (S 2

-) mg/1 0,95 <0.02 1,00 0,02 ±20% Sulphate (S04

2-) mg/1 1400 1300 1300 0,1 ± 10%

Iodide (r) mg/1 2,2 2,3 2,2 0,01 ±30% Ammonium (NH4 +) mg/1 110 110 11 0,1 ± 10-15%

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Table Al-2. The water analyses performed before and after the SSRT-test with specimens PS5, PS6, HS5 and HS6.

PS5, PS6, HS5, PS5, PS6, HS5, Detection limit Uncertainty HS6 17.4.00 HS6 25.4.00 mg/1 (2 RSD %) before test after test

Boron (B) mg/1 1,9 2,0 0,1 ±25% Calcium (Ca) mg/1 830 820 0,5 ± 10% Potassium (K) mg/1 110 110 0,5 ± 15% Magnesium (Mg) mg/1 360 360 0,5 ± 15 o/o Sodium (Na) mg/1 11100 11300 1 ± 10% Strontium (Sr) mg/1 27 27 0,1 ±20% Iron (Fe) mg/1 < 0,002 0,49 0,002 ±20% Copper (Cu) mg/1 < 0,001 < 0,001 0,001 ±40% Bromide (Br-) mg/1 150 150 0,1 ± 10% Chloride (Cr) mg/1 17500 17800 0,1 ± 10% Fluoride (F-) mg/1 3,7 3,8 0,1 ± 10-15% Sulphide (S 2

-) mg/1 0,96 <0,02 0,02 ±20% Sulphate (Sol-) mg/1 1300 1300 0,1 ± 10% Iodide (r) mg/1 2,3 2,3 0,01 ± 30 o/o Ammonium (NH4 +) mg/1 110 110 0,1 ± 10-15%

Because of the stability of the water chemistry, as found in the analyses of the three first test runs shown in Tables Al-l and Al-2 it was decided that the analyses for the two remaining test runs are restricted to iron, copper, sulphide and ammonium. The analysis results are shown below in Tables Al-3 and Al-4.

Table Al-3. The water analyses performed before and after the SSRT -test with specimens PS8, PS14, HS7 and HS8.

PS8, PS14, PS8, PS14, Detection limit Uncertainty HS7, HS8 HS7, HS8 mg/1 (2 RSD %) before test after test

Iron (Fe) mg/1 < 0,002 0,14 0,002 ±20% Copper (Cu) mg/1 < 0,001 < 0,001 0,001 ±40% Sulphide (Sz-) mg/1 0,67 <0,02 0,02 ±20% Ammonium (NH4 +) mg/1 11 10 0,1 ± 10-15%

Table Al-4. The water analyses performed before and after the SSRT -test with specimens PS 10, PS 11, HS9 and HS 10.

PSlO, PSll, PSlO, PSll, Detection limit Uncertainty HS9, HSlO HS9, HSlO mg/1 (2 RSD %)

23.5.00 2.6.00 before test after test

Iron (Fe) mg/1 < 0,002 0,026 0,002 ±30% Copper (Cu) mgjl < 0,001 < 0,001 0,001 ±40% Sulphide (S2

-) mg/1 1,26 <0,02 0,02 ±20% Ammonium (NH4 +) mg/1 1,2 1,1 0,1 ± 10-15%

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Appendix 2. Fracture surfaces.

Scanning electron microscopy of the fracture surfaces. 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. However, in some of the weld metal samples smaller areas were also detected which could be described as "semi-ductile". An example of such appearance is shown below for weld metal specimen HS9. Further investigations would be needed in order to determine whether the cause of this type of fracture surface appearance is incomplete fusion or environmentally assisted ductile fracture. However, as the fracture strain in the SSRT -experiments showed no dependence on the concentration of ammonium-ions and as the effect of ammonium-ions was the main target of this investigation, a more detailed investigation of these details was felt unnecessary.

PS 1

PS 2 PS 2

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PS 3

PS 5

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PS 3

PS 4

PS 5

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PS 6

PS 8

PS 10

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PS 8

PS 10


PS 14

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PS 11

PS 14


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The fracture surface of weld metal specimen HS9 (3576) showed in general a ductile appearance. In some areas a "kvasi-ductile" appearance was evident. In these areas the colour of the surface was yellow, whereas the surface otherwise had a red colour typical of copper. In the centre of the specimen an area with apparently inter granular fracture was found (3577, 3578 and 3579). The other weld metal samples were mainly ductile with dimple structure.

same area as in 3580

----------------------- ----


same area as in 3580

HS 2

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ductile and dendritic like fracture

HS 1

HS 2


HS 3

HS4

HS 5

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HS 3

HS 5

·' '··


HS 6

HS7

HS 8

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HS 7

HS 8

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HS 10

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HS 10

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Appendix 3. Cross sectional pictures of the weld metal samples and base metal samples PS and P11.

The cross-sectional pictures (=X14) of the weld metal samples and the base metal samples PS5 and PS 11 below show a directionality and uneven grain size for the weld metal samples.

HS5 HS6

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PS5

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PSll

stress corrosion cracking investigation of copper in ... · the main conclusion from this...

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