Working Report 99-57

Literature suNey on stress corrosion cracking of Cu in presence of nitrites, ammonia. carbonates and acetates

Time Saario

Time Laitinen

Kari Makela

Martin Bojinov

October 1999

POSIVA OY

Mikonkatu 15 A, FIN-001 00 HELSINKI, FINLAND

Tel. +358-9-2280 30

Fax +358-9-2280 3719

Working Report 99-57

literature survey on stress corrosion cracking of Cu in presence of nitrites. ammonia. carbonates and acetates

Timo Saario

Timo Laitinen

Kari Makela

Martin Bojinov

VTT Manufacturing Technology

October 1999

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.

-t1'r V ALMISTUSTEKNIIKKA

Posiva Oy Margit Snellman Mikonkatu 15 001 00 HELSINKI

24.09.1999

Kirjallisuusselvitys kuparin jannityskorroosiosta

Viite: Tilauksenne 9594/99/MVS

Hyva Margit,

1 (1)

Lahetan ohessa raportin "Literature survey on stress corrosion cracking of Cu in presence of nitrites, ammonia, carbonates and acetates" Posivan raporttimuotoon tehdyn version.

VTI V ALMISTUSTEKNIIKKA Voimalaitosten materiaalitekniikk:a

Parhain terveisin

Timo Saario

Kemistintie 3, Espoo PL 1704 02044 VTI

J

Puh.vaihde (09) 4561 Faksi (09) 456 7002, (09) 456 5875 WWW:http://www.vtt.fi/manu/

Working Report 99-57

Literature survey on stress corrosion cracking of Cu in presence of nitrites, ammonia, carbonates and acetates

Timo Saario

Timo Laitinen

Kari Makela

Martin Bojinov

October 1999

LITERATURE SURVEY ON STRESS CORROSION CRACKING OF CU IN PRESENCE OF NITRITES, AMMONIA, CARBONATES AND ACETATES

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.

General corrosion of copper is not expected to be the limiting factor in the waste repository environment when estimating the lifetime of the canister construction. However, different forms of localised corrosion, i.e. pitting, stress corrosion cracking (SCC), or environmentally assisted creep fracture may cause premature failure of the copper shield. This literature study was made to identify the boundary conditions for SCC of copper in presence of nitrites, ammonia, carbonates and acetates. The boundary conditions looked for included the critical concentration of the species, the potential range where the particular species has been found to cause sec and the threshold stress level required for SCC to occur. The boundary conditions were compared with the corresponding parameters known to exist in the final disposal vault conditions.

Based on the literature study nitrites and carbonates are not expected to cause a threat for the integrity of the copper shell. The concentration of ammonia found in the site studies at Hastholmen and Olkiluoto is of the range of the concentration found to be critical in some laboratory tests. The potential range where SCC of copper in ammoniacal environments was reported to occur is about the same or slightly higher than the highest potential expected to prevail in the disposal vault. However, most of the referenced test results were gained in a test environment, which does not correspond to the expected disposal vault condition. It is suggested that in order to exclude the possibility of SCC caused by ammonia an experimental program may be needed in which the boundary conditions are verified in an environment closely simulating the expected disposal vault conditions. Based on this study acetates are known to cause SCC in pure copper. Because of the limited number of investigations on this issue boundary conditions SCC caused by acetates could not be made. Also in this case experimental verification may be needed.

Keywords: copper, stress corrosion cracking, nitrite, ammonia, carbonate, acetate

NITRIITIN, AMMONIAKIN, KARBONAATIN JA ASETAATIN VAIKUTUS KUPARIN JANNITVSKORROOSIOON- KIRJALLISUUSSELVITVS

TIIVISTELMA

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

Kuparin yleinen korroosio ei muodostu sailiorakenteen elinikaa rajoittavaksi tekijaksi ydinjatteen loppusijoitustilassa. Paikalliset korroosiomuodot, s.o. pistekorroosio, jannityskorroosio tai ympariston avustama viruminen, ovat vauriomuotoja, joita pitaa tarkastella tarkemmin. Tama kirjallisuusselvitys on tehty niiden reunaehtojen selvitta­miseksi, joiden vallitessa kuparin jannityskorroosio voi mahdollisesti esiintya nitriitti-, ammoniakki-, karbonaatti- ja asetaattipitoisessa pohjavedessa. Reunaehtoja ovat ko. aineiden kriittinen konsentraatio, potentiaalialue, jossa ko. aineiden on todettu aiheuttavan jannityskorroosiota kuparissa, seka kynnysjannityksen arvo, joka taytyy ylittaa, jotta jannityskorroosiota voi esiintya. Naita reunaehtoja on verrattu vastaaviin loppusijoitusolosuhteissa vallitseviin ko. muuttujien arvoihin.

Kirjallisuusselvityksen perusteella nitriittien ja karbonaattien ei odoteta muodostavan minkaanlaista uhkaa sailiorakenteelle. Hastholmenin ja Olkiluodon pohjavedesta mitatut ammoniakkipitoisuudet ovat samaa luokkaa kuin minka on todettu aiheuttavan jannitys­korroosiota kuparissa laboratoriokokeissa. Jannityskorroosio esiintyy ammoniakki­ymparistoissa kuitenkin lahes samassa tai vain jonkin verran korkeammissa potentiaa­leissa kuin mita paikkatutkimuksissa on todettu yleensa esiintyvan pohjavedessa. Suuri osa ammoniakkiymparistoissa tehdyista kokeista on tehty koejarjestelylla, joka ei vastaa odotettavissa olevaa loppusijoitusolosuhdetta. Tasta syysta tulosten edustavuus on kyseenalainen ja on suositeltavaa, etta ammoniakin aiheuttaman jannityskorroosioriskin mahdollisuuden eliminoimiseksi tulisi suorittaa reunaehtojen kokeellinen verifiointi koeymparistossa, joka on mahdollisimman Hihella odotettavissa olevaa loppusijoitus­olosuhdetta. Taman kirjallisuusselvityksen perusteella asetaattien tiedetaan aiheuttavan jannityskorroosiota puhtaassa kuparissa. Myos asetaattien kohdalla kokeellinen verifiointi on suositeltavaa silHi edellytykselHi, etta asetaatti-konsentraatio pohjavedessa on merkittava.

Avainsanat: kupari, jannityskorroosio, nitriitti, ammoniakki, karbonaatti, asetaatti

-------------------------------------~-

1

TABLE OF CONTENTS

Abstract

Tiivistelma

1 INTRODUCTION 2

2 DISPOSAL CONDITIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3 EFFECT OF DIFFERENT SPECIES ON SCC OF COPPER . . . . . . . . . . . . . . . 5

3.1 Nitrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3.2 Ammonia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3.3 Carbonates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.4 Acetates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 2

4 SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5 REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

-----------------------------------------------------------------~-

2

1 INTRODUCTION

Posiva Oy is responsible for the fmal disposal of the spent nuclear fuel from Olkiluoto and Loviisa nuclear power plants. The preliminary site studies were performed in five sites during 1987-1992. Detailed site studies at Kuhmo Romuvaara, Aanekoski Kivetty and Eurajoki Olkiluoto sites are performed in 1993-2000. Those at Loviisa Hastholmen will be made in 1997-2000. The location of the fmal disposal site will be selected in 2000.

In the present design spent nuclear fuel is packed in a canister made of spheroidal graphite cast iron. The cast iron canister has an outer shield made of copper. The copper shield is responsible for the corrosion protection of the canister.

The goal of this investigation was to make a literature study on the ability of ammonia, nitrites, carbonates and acetate to expose copper to stress corrosion cracking. The study concentrates especially on the concentration and potential range where the detrimental effect of these anions occurs.

3

2 DISPOSAL CONDITIONS

In the analysis of the groundwater composition small amounts of ammonium ions (NH4 +) has been found. The maximum concentrations have been about 3 mgll at the Hastholmen site and 1.1 mgll at the Olkiluoto site (Anttila et al. 1999a, 1999b ). Ammonium has been mainly found in the upper part of the bedrock at the depth of 100-200 m, in the brackish groundwater with the ancient Litorina Sea water influence. Ammonium, as well as nitrites and nitrates, is known to expose copper to stress corrosion cracking. The nitrite and nitrate concentrations in some single samples from Olkiluoto site have been 0.01 mgll N02 and 0.2-0.3 mgll N03. The corresponding concentrations at Hastholmen site have been 0.01 mgll N02 and 0.03-0.1 mgll N03 also in some single samples. Mostly nitrite and nitrate concentrations are below detection limits (0.01 mgll) as would be expected in deep reducing groundwater and the reported analytical results are associated with uncertainties due to analytical problems. Carbonate, which is known to expose copper at least to pitting corrosion, has been found at Olkiluoto site in a maximum concentration of 400 mgll and at Hastholmen site in a maximum concentration of 250 mg/1 in the brackish groundwater in the upper part of the bedrock. In the saline water at the depth below about 500-600 m the corresponding concentrations have been found to be 20-50 mgll. Acetogens have been observed in the groundwaters of the investigation sites, these may form acetate (Haveman et al. 1998). Acetates may be consumed as energy sources in methanogenesis or by iron reducing or sulphate reducing bacteria (Pedersen 1997).

During the transient phase oxidising conditions will prevail for some hundreds of years after the closure of the repository. The undisturbed natural redox conditions of the groundwaters at depth of all the Finnish investigation sites including Olkiluoto and Hastholmen sites has been estimated to be from -0.2 VsHE to -0.3 VsHE, (at ambient temperature) depending on the pH (e.g. Anttila et al. 1999a). Because of the uncertainties related to the measurement of the redox-potential this estimate is based not only on the measured values but also on theoretical studies and reaction path calculations with EQ3/EQ6 also by taking into account a number of parameters effecting the redox-potential, such as the concentration of sulphides, iron, dissolved hydrogen and methane. Also included in the estimate is the effect of fracture mineralogy e.g. the presence of pyrite on fractures.

In the near-field of the spent-fuel canister, bentonite is expected to bring about changes to the chemistry of the groundwater equilibrated with it. The chemical changes brought about depend to a great deal on the type of groundwater, fresh, brackish saline and brine. For saline groundwaters, such as those occurring at depth in Olkiluoto and Hastholmen, main observed changes has been related to pH buffering, ion-exchange (possibly also including NH4 +), bicarbonate concentration (calcite dissolution), sulphate (gypsum dissolution), and increase in ionic strength (Vuorinen & Snellman 1998). Bentonite is also expected to affect the redox-potential (e.g. oxygen consumption by pyrite).

4

During the manufacturing period and after placing the spent nuclear fuel in the canister, the canister remains exposed to air for a longer time. Partly this time is spent at room temperature and partly at around 100 °C. The oxide growing on the copper surface during this period may reach the thickness of several micrometers (W erme 1998). When placed in the disposal vault and after the oxygen remaining in the disposal vault has been consumed the oxide film will tend to reduce. As long as the oxide film stays unreduced, the potential of the copper shield will stay at a higher level than what would be expected based on the measured redox-potential values of the undisturbed natural groundwater. This may have to be considered when estimating the effect of potential on stress corrosion cracking of copper. The kinetics of the oxide reduction process is not known.

Stress corrosion cracking can occur only if the material is under a sufficiently high tensile stress. The expected momentary maximum stress levels from e.g. canister handling would be normally of the order of 15 to 35 MPa (Raiko & Salo 1999), but forced straining beyond yield could occur locally due to external pressure loads in the repository. The canister shell base material is in hot-formed condition and the EB-welds are in cast condition. The yield strength of this kind of copper is about 50 MP a in room temperature and the ultimate strength more than 200 MPa.

The canisters reach their maximum temperature of about 90 °C in the repository within 20 years after the disposal. Now we presume that simultaneously when reaching the maximum temperature the canister overpack is deformed due to creep or plastic deformation under the external pressure load. As a result of these deformations all the radial 1 mm gap between the cylindrical overpack and insert is closed. The copper overpack will be deformed until a full contact is reached on all surfaces between the overpack and the iron insert due to the possibly very slowly increasing external pressure load. The actual maximum local strain in the copper overpack will be about 2% according to earlier calculations when the 1 mm radial gap between the overpack and the insert is forced to close due to creep (or plastic) deformation caused by external pressure load (Raiko & Salo 1999). This 2% strain corresponds to some 80 MPa stress level in hot-formed copper.

The local (positive, tension) stress component in corner area of the copper shell thus exceeds the copper yield strength (50 MPa) on the surface. However, the stress distribution over the wall thickness is such that that most of the wall thickness is in compression state. In addition, this kind of residual stresses are displacement controlled, in other words, the residual stresses are relaxed, if there is cracking or creeping.

The postulated cracking can not penetrate all the wall, because the crack growth is stopped when the tension stress is relaxed in the area and the crack front reaches the compressive stress area. In spite of possible local crack initiation and growth, the remaining wall thickness (more than half of the nominal wall) will be thick enough for

corrosion allowance.

5

3 EFFECT OF DIFFERENT SPECIES ON SCC OF COPPER

3.1 Nitrites

Nitrites are known to cause stress corrosion cracking in pure copper. However, stress corrosion cracking occurs only above some minimum concentration of nitrites, above some minimum stress level and within a range of potentials. This allows one to look for the windows in these parameters inside which the stress corrosion cracking phenomena may occur. In 1 M NaN02 at room temperature copper was found in slow strain rate tests (SSRT) to be immune to SCC below the potential of -0.13 VscE (Yu & Parkins 1987), Fig. 1. This potential is equal to +0.12 V sHE· At potentials higher than about +0.2 V SCE extensive general corrosion occurred. The critical potential for sec to occur was found to be insensitive to variation in pH up to pH = 12. Rosborg and Svensson (1994) studied Cu-OFP in 0.3 M NaN02 at room temperature using the SSRT technique and found the critical potential for SCC to be about -0.1 V seE· In the work of Aaltonen et al. (1987) stress corrosion cracking occurred in 0.3 M NaN02 at 80 °C at -0.05 V seE, which was the lowest potential studied.

4( w er < z -z 0 t= 0 ::J c w er

80 • ······················-· ---~-Cu ~mmune 1\ I. I \··· .. · Copper . / m J

I. ,.. ·~: Cf

immune 60 Brass

40

I ~ ~~ f

I 0 '

··.····.··~l,o • I : Brass .· .. . • • •. ·. J I ' I I I I .. ·.·.·.·. . . .

j. I ................... I ~Q. •. (J·O~~...._,__.

20

0 -1.0 -{).8 .. 0.2 -0. 1 0 0~ 1 0.2 0,4

POTENTIAL V (see)

Fig. 1. Reduction in area to fracture in slow strain rate tests ( 1.8 x 10-6s-1) of a-brass

and copper in 1 M NaN02, pH 9, (Yu & Parkins 1987). The lower the reduction in area, the less ductile copper is and the more susceptible it is to sec.

E z :I . Cl) Cl) UJ a: t­U)

6

250--------------------------~~-----

10 20 30 40 50 60

%ELONGATION

Fig. 2. Susceptibility to stress corrosion cracking (as reduction in elongation to fracture in SSRT test) as a function of potential. OFHC copper at room temperature in 1 M NaN02 (Benjamin et al. 1988).

RA, ~)f.

Fig. 3. Effects of sodium nitrite concentration and potential upon reduction in area at fracture, RA, of OFHC (Oxygen free high conductivity) and PDO (Phosphorus deoxidised) copper (Benjamin et al. 1988).

7

Fig. 2 (Benjamin et al. 1988) shows the effect of potential on elongation to fracture in slow strain rate tests. The smaller the elongation to fracture, the less ductile the material behaviour and the larger the effect of stress corrosion cracking. The elongation to fracture at -0.1 VscE was comparable to that in air (59 %), which indicates that at this potential (or lower) copper was not susceptible to stress corrosion cracking. Thus SCC occurs at potentials where Cu(II) is stable. Benjamin et al. (1988) showed that at room temperature the minimum amount of NaN02 needed for SCC to occur is higher than 0.001 M and that the lower the concentration the higher is the critical potential for SCC, Fig. 3. They also showed that in 0.6 M NaN02 increasing the temperature to 80 °C tended to diminish the susceptibility of copper to stress corrosion cracking.

In order for SCC to occur some threshold level of stress needs to be exceeded. In case of nitrites Salmond and Atrens (1992) found that in I M NaN02 at room temperature the threshold stress for copper was 120 MPa.

Based on the results from the above referenced works nitrites do not cause SCC in copper at concentrations below 0.001 M, which is equal to 69 mgll, or at potentials lower than +0.1 V SHE in near neutral solutions. The reported concentration of nitrites in some groundwater samples at the Hastholmen and Olkiluoto sites was 0.01 mgll (associated with large uncertainties). This is roughly four decades lower than the critical concentration of 69 mg/1. It is thus clear that SCC of copper can not be caused by such a small concentration of nitrites. Additionally, the critical potential which has to be exceeded for SCC of copper to occur in nitrite containing environment, +0.1 V SHE, is high (a potential where Cu(II) is stable is required), in comparison with the redox­potentials (and thereby also corrosion potential of copper) measured and predicted to prevail in the ground water of the disposal vault environment.

3.2 Ammonia

Ammonia is known to cause SCC in copper and copper alloys. Several investigations have indicated that sec in ammonia solutions occurs only if an adherent "tarnishing" Cu20 film is formed on the surface (Bertocci & Pugh 1984; Bertocci et al. 1984; Suzuki & Hisamatsu 1981 ). Suzuki and Hisamatsu (1981) showed that this is also the case at 70 °C in 0.05 M NH40H (1750 mgll) under a constant load equal to 66 % of ultimate tensile strength. These results indicate that the lowest potential at which SCC of copper in ammonia can occur is the Cu20/Cu - oxidation potential. This potential is given (Pourbaix 1974) in SHE scale at room temperature as

E Cu20/Cu (25 °C) = 0.471- 0.0591pH (1)

and at 80 °C (EPRI 1983) as

E Cu20/Cu (80 °C) = 0.49 - 0.075pH (2)

8

At pH of 8 the potential E Cu20/Cu (25 °C) = -0.002 V sHE, and the potential E Cu20/Cu

(80 °C) = -0.11 V SHE·

Sato and Nagata ( 1978) investigated susceptibility of copper to SCC in ammoniacal environments with a test set up in which ammonia was carried by continuous flow of moist air through the test chamber. This is a common test method in ammoniacal environments. They found that the susceptibility of Cu OFP to SCC depends on the concentration of phosphorus in the alloy. The alloy is not susceptible to SCC even at very high loads (200 N/mm2

) at phosphorus concentration of less than 80 ppm, Fig. 4. For a Cu-300ppm P alloy no detrimental effect was found at ammonia level of 0.5 mgll, while a clear indication ofSCC was found already at ammonia level of5 mg/1, Fig. 5.

Sato and Nagata (1978) based on McLean's equilibrium segregation theory proposed that the phosphorus content on the grain boundaries would become about 100 times higher than the bulk content. This was evidenced by a clear dependence of the SCC susceptibility on grain size, Fig. 6. The larger the grain size, the higher is the susceptibility to SCC. This result indicates that the part in the copper container structure that requires most attention regarding sec risk by ammonia are the weld zones, where large grain sizes can not be excluded due to manufacturing practices.

20

~

~ ~ 0 0 0 ~Jt ,. .)( ~ ,.. ~ 0 No ruJ:?ture in ~ ~· 20.000 hrs

'" /.. )( Ruptured

" v.: - 0 0 0 ~- ~ X 'l l. ~

0\._ ll ll )( ll

"' ~ - 0 0 0 l'

\ 'l.-- 0 0 0 a o ~"}:/a/ ,"-0J'W/& .. . 0 0 00 0 0 • ..

0 t j( ..L ...L __._ l . ""- _.. 1 1

0 0.003 0.01 0.1 Phosphorus content(%)

Fig. 4. The effect of P content of Cu-P alloys on the time to rupture in ammonia under various applied tensile stress. Ammonia concentration 48.5 mg!l and humidity 98 % (Sato & Nagata 1978).

9

An average person produces 300 to 600 mmol urea per day (Source: www 1999). One urea molecule decomposes in water producing two molecules of ammonia, NH3 • Thus, on the average a person is expected to produce roughly one mole of ammonia per day. One mole of ammonia weighs 1 7 g. This amount of ammonia is sufficient to increase the concentration of ammonia in about 17000 litres of water to the critical level of 1 mgll. Thus it is clear that proper measures have to be taken to exclude contamination of the disposal vault environment by urea during the construction period.

'2 g Cl) L.. ::::J

a. ::::J L..

.8 Cl)

E F

4 10

103

2 • 10

5

t (not ruptured) A. __ ...... _ ------..t. A

0.5 mg/1

~- S.Omg/1

~~ 48.5mg/l '

108 mg/1

10 15 20 25 30

Applied tensile stress (kg/mm2)

Fig. 5. The effect of ammonia concentration content of Cu-P alloys on the time to rupture in ammonia under various applied tensile stress. Ammonia concentration 48.5 mg/1 and humidity 98% (Sato & Nagata 1978).

-r:::::: g ~ 3 a 10 2 .8 G)

E F

2

tt

10

f not ruptured

5kg!nun2

A

A

•

10 11------~--~~-.--~._~~~~~~~ 0.01 0.015 0.1 1

Grain size (mm)

Fig. 6. The effect of grain size of a Cu - 300 ppm P alloy on the time to rupture in ammonia under several tensile stress levels. Ammonia concentration 48.5 mg/l and humidity 98% (Sato & Nagata 1978).

The alloy which is planned to be used for the construction of the copper shell has 40-60 ppm of phosphorus (Raiko & Salo 1999). This is slightly below the critical limit of 80 ppm found by Sato and Nagata (1978). The requirement of the presence of a Cu20 film on the surface sets the lowest potential at which SCC by ammonia can occur at E = -0.035 VsHE at pH= 7 and at -0.185 VsHE at pH= 9 (at T = 80 °C). These values are almost the same or slightly higher than the redox-potentials (and thereby also the corrosion potential of copper) estimated to prevail in the undisturbed groundwater ( -300 to -200 m V, at ambient temperature) at depth after the short transient period when the potential may be higher. The smallest concentration of ammonia causing SCC in phosphorus deoxidised copper has been found to lie between 0.5 mg/1 and 5 mg/1. This is comparable with the concentration of ammonia, maximum 1.1 mg/1 and 3 mgll, found at the Olkiluoto and Hastholmen sites, respectively.

The threshold stress level for SCC to occur in ammoniacal environments is roughly 40 MPa (see Figs 4, 5 and 6).

11

In another work (Thompson & Tracy 1971) copper with different phosphorus concentrations was studied at 35 °C in a gaseous atmosphere with 80 % air, 16 % ammonia and 4 % water vapour. In this highly oxidising environment the minimum concentration of phosphorus to cause SCC was found to be 40 ppm (0.004 %). The alloys with lower concentration of phosphorus did not crack and no intergranular attack was found after a four-week test period. However, the reported general corrosion rate was as high as 0.84 mm/year (0.033 inch per year).

Based on the above referenced studies, SCC of phosphorus deoxidised copper by ammonia occurs at almost the same or slightly higher potentials and for slightly higher phosphorus concentration of copper than are typical for the planned construction. The concentration of ammonia found at the Olkiluoto and Hastholmen sites is close to the lower limit of ammonia causing SCC in copper. However, most of the referenced test results were gained with a test procedure (gaseous environment) which does not correspond to the expected disposal vault condition. Thus, the referenced results may not be directly applicable to the aqueous environment expected to prevail in the disposal vault. Because of the several variables that are involved in the ammonia caused SCC in copper and because of the above mentioned uncertainty related to the test environment commonly used, a defmitive clearance for the present design-environment combination cannot be given. The resistance of phosphorus deoxidised copper to SCC by ammonia should be verified by an experimental program performed in conditions as close to the predicted fmal disposal vault environment as possible. The effect of the decrease in ammonium content due to possible ion exchange with bentonite needs also to be addressed.

3.3 Carbonates

Carbonates are known to cause pitting in copper (Sanchez et al. 1990; Drogowska et al. 1993; Ribotta et al. 1995). Pitting of copper in the presence of carbonates occurs exclusively at high positive potentials, where Cu(II) is stable. However, no references were found to works showing that carbonates could cause SCC in copper. In their work Benjamin et al. (1988) added at room temperature simultaneously a higher concentration of carbonate, C03

2-, (500 mg/1) and bicarbonate, HC03-, (500 mgll) into simulated

ground water, which already contained 123 mgll of bicarbonate. No effect on SCC of copper was found in SSR T tests performed at free corrosion potential, at 0 V seE and at +0.1 VscE· Aaltonen et al. (1987) in their study found no difference in the ductile behaviour of copper when tested with the SSRT technique at 80 °C in glycerin, m distilled water and in simulated ground water containing 123 mg/1 ofbicarbonate.

Based on the above referenced works carbonates and bicarbonates do not promote SCC in copper at least for concentrations below of about 1000 mgll. The possible effect of bentonite on the level of the bicarbonate needs to be considered. Carbonates were reported to cause pitting corrosion in copper, but only at high positive potentials, which are not likely to exist in the disposal vault.

12

The maximum level of bicarbonate reported for the ground water at 0 lkiluoto is in the range of 400 mgll and the highest level at Hastholmen is about 200 mgll, which is well below the critical level of 1000 mgll. At both sites the bicarbonate concentration decreases with salinity and depth. In the saline groundwater the reported level is about 10-30 mg/1.

3.4 Acetates

Copper is known to suffer from stress corrosion cracking in cupric acetate containing environments (Escalante & Kruger 1971; Parkins & Holroyd 1982). Escalante and Kruger ( 1971) studied 99.9 % copper wire and large grain 99.999 % copper using constant load specimens in 0.05 N cupric acetate (:= 8900 mgll) environment at room

temperature. Their tests were performed at tensile stresses exceeding the yield stress in an oxygen saturated solution, i.e. at high positive potentials where Cu(II) is stable. The 99.9 % copper cracked in as short a time as 18 hours while the large grain 99.999 % copper cracked in 88 hours.

Parkins and Holroyd (1982) investigated SCC of brasses in 0.1 M to 1 M sodium acetate at 22 °C. In 0.1 M solution cracking was found in a pH range from 6 to 12. Cracking was found to occur at potentials higher than about 0.15 VscE with increasing pH tending to increase the critical potential. SCC was reported to occur both in the potential range where Cu(I) and Cu(II) are stable. As these results were measured for a 70/30 brass and not for pure copper, the results may be of indicative value only.

As discussed earlier (section 2) acetogens have been observed in the groundwaters from all the Finnish investigation sites (Haveman et al. 1998). As for other microbes acetate production by bacteria is not likely to occur in the near-field (bentonite) due to the limiting low water activity (Pedersen 1997).

13

4 SUMMARY AND CONCLUSIONS

Based on this literature study nitrites and carbonates are not expected to cause a threat for the integrity of the copper shell. The concentration of nitrites found at the sites is several decades lower than the critical concentration needed for the occurrence of SCC. Carbonates were reported to cause pitting corrosion but not SCC. In both cases, for nitrites and carbonates, the harmful effects were reported to take place exclusively at high positive potentials, which are not expected to prevail in the disposal vault.

The reported potential range where SCC of copper occurs in ammoniacal environments is about the same or slightly higher than the highest potential expected to prevail in the disposal vault. The concentration of ammonia found in the site studies at Hastholmen and Olkiluoto is of the range of the concentration found to be critical in some laboratory tests. However, most of the referenced test results were gained with a test environment, which does not correspond to the expected disposal vault condition e.g. moist air, gaseous environment, and high ammonium content. It is suggested that in order to exclude the possibility of sec caused by ammonia an experimental program may be needed in which the boundary conditions are verified in an environment closely simulating the expected disposal vault conditions. The experimental program would consist of slow strain rate tests (SSRT) performed in a relevant reference ground water (see e.g. Vuorinen & Snellman 1998) at 80 °C using a strain rate of e.g. 1 o-6 s-1

• Tests should be performed using e.g. three different ammonium concentrations, in the range from 1 mg/1 to 100 mg/1 and for two different grain sizes, representative of base material and welded material. For statistical reliability at least three specimens should be tested with each set of variables.

Based on this study acetates are known to cause SCC in copper. Because of the limited number of investigations on this issue boundary conditions sec caused by acetates could not be made. Also in this case experimental verification may be needed.

Common to all cases of SCC in copper is a prerequisite of presence of a film on the surface. For nitrites the film is a Cu(II) film forming at very high positive potentials, while for ammonia the film is a Cu(I) film which forms at slightly lower potentials, still higher than the potentials expected to prevail in the disposal vault. For acetates it is uncertain whether the film on pure copper has to be a Cu(II) or a Cu(I) film, although brasses have been found to suffer from SCC in acetates in the potential range of Cu(I). In the case of carbonates which were not reported to cause SCC but only pitting, the pitting was reported to occur in the potential range where Cu(II) is stable.

The threshold stress to be exceeded for SCC to occur was reported to be 40 MPa in ammoniacal environments and 120 MPa in nitrite containing environments. The magnitude of the maximum positive strain (2 %) in hot-formed copper corresponds to about 80 MPa stress level, which is higher than the reported threshold stress level in ammoniacal environments. However, there may be no risk for penetrating SCC growth,

14

because the tensile stressed areas of the shell wall are local and the whole wall thickness is never tensile stressed. The postulated cracking can not penetrate all the wall, because the crack growth is stopped when the tension stress is relaxed in the area and the crack front reaches the compressive stress area. In spite of possible local crack initiation and growth, the remaining wall thickness (more than half of the nominal wall) will be thick enough for corrosion allowance.

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