Copper, nuclear waste, slow strain rate testing, ammonium. Summary. Oxygen free, phosphorus containing copper (Cu OFP, base metal and electron beam.
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
Working Report 2000-46
Stress corrosion cracking investigation of copper in groundwater with ammonium ions Esko Arilahti Martin Bojinov Kari Makela Time Laitinen Time Saario
December 2000
-m
1 (17)
MANUFACTURING TECHNOLOGY
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Title
Stress corrosion cracking investigation of copper in ground water with ammonium ions Customer or finansing body and order date/No.
Research report No.
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Esko Arilahti, Martin Bojinov, Timo Laitinen, Kari Makela and Timo Saario
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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 I = 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.
o£ ot
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
\fL__
20 November, 2000
\IL t_ __
Rauno Rintamaa Research Manager
/:/~
Esko Arilahti Research Engineer
Checked
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S· \Z-Qb I
}-t
V>.
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 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 ofNH 4+-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).
Avainsanat: 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% =10-6 s 1
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 AgCllAg 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 0 (E H+tH =EsHE =EAgeli Ag -0.211V). The Nemst equation relates the pHT and 2
potential E H + 1H at 100 oc as follows 2
(1)
Here/H2 is the fugacity(= partial pressure) of hydrogen gas in the water. From equation (1) pHT becomes fJ _ p
T-
EsHE- EH +1H
2
0.037V ·log(/H 2 I O.lMPa) 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 MPa 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 0.2 .Cs .. 0.2
I{
10066
B 1320
Cl ,
.-.
1.7 · :.: sol~ :. ·
29.4
90
..
.
. ,·,
..
..
:
ta :.
M
815 F 3.5
Br
17000 Tps · ·,.· Ba: -298 ()() 0.1
140 ·I ..
..
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 (S 2) 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/
---·------·----·---------·------------·----------1
180
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/ . T =100 °e, p
0.2 -
=14 MPa, 3700 mg/1 er, 100 mg/1 NH/
·- -··- ----... -----·---·---·-------···· ------·· ----...·--·· ----·-·- -.,
l
Loading
0.1
l !
0
~ -0.1 ~
>
-0.2
~ s
-0.3
~
-0.4 -0.5 -0.6
........
Pt
...-
23.3. 000
!
I
I lI i
..........
-............
Pd
! I
I
-0.7 22.3.2000
I
I
Speci1 nens PS1, PS2, 1- S1 and HS2
~
'
I
-
24.3.2000
25.3.2000
26.3.2000
27.3.2000
28.3.2000
29.3.2000
30.3.2000
Date
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/1NH4 +.
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 Q)
80
...
U)
:computer failure 60 40 20 HS4 0
5
0
10
15
25
20
35
30
40
50
45
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+· T =100 °e, p = 14 MPa, 3700 mg/1
er, 10 mg/1 NH/
- - - - · - - - - -Loading - - - - - - - - - - - - -1--·- - - - , 0 r-· - - ...._ -0.1
w
-0.2
ii
-
.....
~ -0.3
>
(ij :;;
I j
.
!
Di
._
PS4
l
-0.4
c:::
-~
.! -0.5 0 D..
-0.6 -0.7
~·
~
l
iI SPE ~imens PS3,W ~3 and HS4 I
!
'-.. ...............
Pd
''\
-0.8 5.4.2000
6.4.2ooo
l
7.4.2000
8.4.2000
9.4.2000
10.4.2000
11.4.2000
12.4.2000
13.4.2000
l! I
l
14.4. 2000
Date
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
5
0
10
15
25
20
Elongation
30
35
40
45
50
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/ . T = 100 °e, p = 14 MPa, 17000 mg/1
er, 100 mg/1 NH/
---
0
Loadina
r
-0.1
r
- r---p;--
~ -0.2 "i -0.3
:w c
.!
£
I ! I I
~ li._ ~
! SPE cimens PSS PSt , HSS and HS6
I
-0.4 -0.5
---
I I
Pd
-0.6 17.4.2000
! l
~
>
I Il I
18.4.2000
19.4.2000
20.4.2000
21.4.2000
22.4.2000
23.4.2000
24.4.2000
' f
! 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/1NH4 +.
11
T = 100 oe, p = 14 MPa, 17000 mg/1 er, 10 mg/1 NH/
- - - - - - - - - - · - - - · - - - - - - - · - - - - · - - - - · -·- · - - · - - - - - 1
180
i
160 140 ...... 120 ea D.. ~ 100
.,.,
-... Q)
f J)
80 60 40 HS8
20 0 10
5
0
15
20
25
30
35
40
50
45
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/ .
T = 100 °e, p = 14 MPa, 17000 mg/1 er, 10 mg/1 NH/ 0
-·--·--------------------------Lo cid~-----------·-----------1
l
p
f
l
-0.1
l
...... -0.2
l
...
w
I
:I:
>-0.3
I j
ll
'-....
-----
fJ)
Cij
~ -0.4
~ l \).1'1-.
.... ..._
.! 0
D..
.f
11
~
t
SpeclmE ns t'~ts, PS14, HS'i and HSS
-0.5
-0.6
\
I
~
Pd
a
IL
.I
I
.
_._..._..
4.5.2000
5.5.2000
II ! ! l
lI !
i
-0.7 3.5.2000
Il
pt
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/1NH4+.
12
T =100 °e, p
=14 MPa, 11000 mg/1 er, 1 mg/1 NH/
·-------------------------·---,
180
j
!
160 140 .... 120 ns CL
~ fl) fl) G)
...en...
100 80 60 40 PS11
20 0 10
0
30
20
40
60
50
Elongation [%]
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/ . T = 100 °e, p = 14 MPa, 17000 mg/1 er, 1 mg/1 NH4+ 0
r·------·- - - - - -
w
-0.2
___________ ____________ .
,
l
!
~
., ,..-
I
Pt
.......
-~
~ -0.3
>
"i -0.4 ;
c
.! -0.5 0
CL
-0.6
Specimens PS10, PS11, HS9 and HS10
~
! '!
I ii
"--.....
I
Pd
l
-0.7
I i
-0.8 25.5.2000
I !
I
~
.,~
,
i
Loading
.....
-0.1
._,.
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/1NH4+.
13
Table 2. Elongation to fracture and maximum stress of the specimens.
Nmm-2 er, mg/1 Specimen £p,% O'MAX, 49.2 162 3700 PS1 45.9 161 3700 PS2 >34(x) 161 3700 PS3 >34(x) 161 3700 PS4 162 43.6 17000 PS5 44.8 161 17000 PS6 47.5 163 17000 PS8 50.9 163 17000 PS10 160 45.8 17000 PS11 45.4 161 17000 PS14 37.5 151 3700 HS1 30.6 150 3700 HS2 29.7 148 3700 HS3 147 32.0 3700 HS4 29.8 145 HS5 17000 135 21.1 17000 HS6 20.4 130 HS7 17000 34.2 151 HS8 17000 20.1 125 17000 HS9 31.6 144 HS10 17000 (x) The elongation to fracture not determined due to computer failure
NRt+, mg/1 100 100 10 10 100 100 10 1 1 10 100 100 10 10 100 100 10 10 1 1
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 +1H
2
was measured to lie between 0.5 5 V
