Working
Report
2008-46
Interactions Between Copper Corrosion
Products and MX-80 Bentonite
Torbjörn
Carlsson
December
POSIVA
OY
Olkiluoto
FI-27160 EURAJOKI, FINLAND
Tel
+358-2-8372 31
Fax +358-2-8372 3709
2008
Working
Report
2008-46
Interactions Between Copper Corrosion
Products and MX-80 Bentonite
Torbjörn
Carlsson
VTT
December
2008
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.
INTERACTIONS BETWEEN COPPER CORROSION PRODUCTS AND MX-80
BENTONITE
ABSTRACT
The report presents results from a study of the possible interaction between copper
corrosion products and MX-80 bentonite under conditions that might occur in a final
repository for spent nuclear fuel in Finland. The first part of the report describes the
results from a literature survey, the objective of which was to identify some relevant
corrosion products that might form when copper corrodes in wet MX-80 bentonite. On
the basis of the literature survey, atacamite and a green copper corrosion product
produced in-house were used for experimental studies.
Experiments were performed with both soft and compacted MX-80. The soft samples
consisted of water-saturated MX-80 mixed with CuCl2 solutions of various
concentrations. The samples were kept under anaerobic conditions at ambient room
temperature or at 750C for 330 days. Porewater samples were then squeezed from the
samples and analysed.
Compacted MX-80 samples were stored under anaerobic conditions and kept in contact
with an NaCl solution. The samples were kept at room temperature and 75 ºC for 2.9
years and then analysed. The presence of either atacamite or the green copper corrosion
product on the plates did not have any notable effects on the porewater chemistry.
However, the Cu concentration profiles indicated that the corrosion products did
dissolve, and then diffused into the surrounding bentonite. Concentration profiles were
found to be roughly the same, irrespective of whether the samples had been stored at
room temperature or at 75 ºC.
Key words: bentonite, copper corrosion product, copper bentonite interaction
VUOROVAIKUTUKSET PURISTETUN MX-80 BENTONIITIN JA KUPARIN
KORROOSIOTUOTTEIDEN VÄLILLÄ
TIIVISTELMÄ
Raportissa esitetään tutkimustulokset vuorovaikutuksista puristetun MX-80 bentoniitin
ja kuparin korroosiotuotteiden välillä, kun olosuhteet vastaavat niitä, jotka voivat vallita
ydinjätteen loppusijoitustilassa Suomessa. Raportin alussa esitetään tulokset
kirjallisuustutkimuksesta, jonka tarkoituksena oli määritellä korroosiotuotteet, jotka
voivat muodostua kuparin korrodoituessa märässä MX-80 bentoniitissa.
Kirjallisuustutkimuksen perusteella atakamiitti ja itse valmistettu, vihreä kuparin
korroosiotuote valittiin käytettäväksi kokeissa.
Kokeet tehtiin sekä pehmeäillä että puristetuilla bentoniittinäytteillä. Pehmeät näytteet
koostuivat MX-80 bentoniitista, johon oli sekoitettu eri pitoisia CuCl2-liuoksia. Näytteet
pidettiin hapettomissa olosuhteissa huoneen lämpötilassa ja 75 °C:n lämpötilassa 330
päivän ajan. Sen jälkeen puristettiin huokosvesinäytteet, joiden kemiallinen koostumus
määritettiin.
Puristetut MX-80 bentoniittinäytteet pidettiin hapettomissa olosuhteissa kosketuksessa
NaCl-liuoksen kanssa. Näytteet pidettiin huoneen lämpötilassa ja 75 °C:ssa 2.9 vuoden
ajan, jonka jälkeen ne analysoitiin. Korroosiotuotteiden läsnäololla ei ollut havaittavaa
vaikutusta huokosvesikemiaan. Kuparin pitoisuusjakauma kuitenkin viittaa siihen, että
korroosiotuotteet ovat liuenneet ja sitten diffundoituneet ympäröivään bentoniitiin.
Profiilit olivat suunnilleen samanlaiset riippumatta siitä olivatko näytteet olleet huoneen
vai 75 oC:n lämpötilassa.
Avainsanat: bentoniitti, kuparikorroosiotuote, kupari bentoniitti vuorovaikutukset
1
TABLE OF CONTENTS
ABSTRACT
TIIVISTELMÄ
FOREWORD
1
INTRODUCTION ……………………………………………………………………….3
2
LITERATURE SURVEY: CORROSION PRODUCTS OF COPPER... ................ 5
2.1
2.2
2.3
2.4
2.5
2.6
2.7
Expected conditions at the Olkiluoto vault.……………….…........ ............... 5
Copper corrosion prior to water saturation………………………………… ....6
Copper corrosion after disposal……………………………………................ 7
Copper corrosion during water saturation…………………………………. ... 7
Copper corrosion after water saturation………………………. .................... 8
Substitutes for copper corrosion products………………………………….... 9
Conclusions concerning experimental design ........................................... .9
3
MODELLING ………………………...…………………… .................................... 10
4
EXPERIMENTAL …...…..………...…..…………………..…….………….. ........... 11
4.1
4.2
5
RESULTS AND DISCUSSION..………...…..…………………..…….………….... 14
5.1
5.2
6
Soft bentonite .………………….…………..………………………….. ......... 11
Compacted bentonite….…………………………………………………….. . 11
Experiments with soft bentonite …………………………………………… ... 14
Experiments with compacted bentonite ….……………………………….... 18
SUMMARY .………...…..…………………..…….………….. ............................... 22
REFERENCES……………………………………………………………………………… 24
2
FOREWORD
This study is part of the NF PRO project within the Sixth Framework Programme of the
European Commission (Integrated Project NF-PRO, Contract No. FI6W-CT-200302389). This subproject was co-funded by European Commission and Posiva Oy.
3
1
INTRODUCTION
The canisters containing the spent nuclear fuel in the Finnish repository are planned to
be made of iron that is protected by a copper coating. It is expected that the main factor
that may cause canister failure is corrosion. Initially, the canister corrosion is expected
to involve reactions between the copper and the materials in its vicinity (groundwater,
MX-80 bentonite, gases and/or bacteria). At later stages, the corrosion may also involve
the inner iron canister.
In case corrosion reactions occur, corrosion products will form and start to interact with
the surrounding MX-80 bentonite. It is presently not known to what extent the
interaction between copper corrosion products and bentonite might change the
functioning of the bentonite in the repository.
The objectives of the study performed at VTT were to:
1) Identify, by means of a limited literature study, the copper corrosion products
that may form under repository conditions.
2) Design, on the basis of the results from the literature study, a number of
interaction experiments (one year and three years).
3) Determine the interaction between MX-80 bentonite and the chosen copper
corrosion products.
Geochemical modelling (PHREEQC) was used to predict the chemical behaviour of
copper corrosion products in the samples. The results indicated that no copper
compounds would precipitate and that copper was merely present as Cu2+ under the
experimental conditions.
The interactions between MX-80 bentonite and the copper corrosion product(s) were
experimentally studied in small cells under anaerobic conditions. Two types of
experiments were performed:
1) Soft water-saturated MX-80 was homogeneously mixed with copper corrosion
product(s) and stored at room temperature and at 75 °C in closed cells under anoxic
conditions. The effect of background electrolyte was studied by adding NaCl to half
of the samples. The cells were disassembled after 330 days. Pore water was then
squeezed from the samples and analyzed chemically, while dried MX-80 samples
were analysed by XRD.
2) Compacted water-saturated MX-80 samples containing copper corrosion products
were placed in small cells and equilibrated with an external NaCl solution. The
conditions were the same as described above, i.e. anaerobic conditions and two
temperatures (RT and 75 ºC). The duration of the experiment was 2.9 years. The
opening of the cell was followed by chemical analyses of the squeezed pore water
and the MX-80 bentonite.
4
Briefly, the results indicate that under conditions close to those in the experiments,
interactions between bentonite and copper corrosion products can, more or less, be
regarded as interactions between dissolved Cu2+ ions and bentonite.
5
2
LITERATURE SURVEY: CORROSION PRODUCTS OF COPPER
This section contains the main results from a literature survey by T. Kaunisto and T. Laitinen, VTT.
The final storage of nuclear waste includes the encapsulation of spent nuclear fuel in
copper canisters. Strictly speaking, the canisters should be described as copper-coated
iron canisters, but the work described here focuses only on the interaction between
copper and bentonite.
A limited literature survey was made in order to collect necessary background
information. The objectives of the literature study were to:
1) Define the corrosion processes that are important under the conditions expected in
Olkiluoto. (The conditions vary with time with regard to redox conditions, water
saturation, salinity, pH, and temperature, etc.)
2) Define the corrosion products that might be formed under various conditions.
3) Suggest corrosion products that could be used in the experiments.
2.1 Expected conditions at the Olkiluoto vault
King et al. [1] have summarized the ranges of groundwater and bentonite pore-water
constituents estimated for different times, as given in Tables 1 and 2. The division into
different times is based on the different stages of the wetting of the bentonite.
Table 1. Repository groundwater conditions at three stages; at closure, the first
hundred years, and the first ten thousand years [1].
Constituent
Unit
pH
redox
DIC*
ClNa+
Ca2+
Mg2+
K+
SO42HSNH4+
CH4 (g)
H2
DOC**
mV
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
mol/l
*
**
At closure,
infiltration into
unsaturated
bentonite
6-8
oxic to -400
(0.1-16.4)·10-3
(0.1-6.2)·10-1
(0.1-2.8)·10-1
(0.3-1.5)·10-1
(0.4-1.0)·10-2
(1.3-7.7)·10-4
(0-6.3)·10-3
(0-3.0)·10-4
<5.5·10-6
< 4.5·10-6
< 2.2·10-5
< 1.7·10-4
DIC = dissolved inorganic carbon
DOC = dissolved organic carbon
After closure and
saturation (up to
100 years)
After closure up to
10000 years)
7-8
-150 to -308
(0.5.10)·10-3
(0.2-1.6)·10-1
(0.02-9.1)·10-2
(0.03-0.2)·10-1
(0.4-1.0)·10-2
(1.3-7.7)·10-4
(0-5.8)·10-3
(0-3.0)·10-4
(0.03-1.7)·10-4
0.4·10-2
< 4.4·10-6
< 8.3·10-4
7-9
-200 to -300
(0.1-7)·10-3
(0.06-4.2)·10-1
(0.04-2.2)·10-1
(0.005-1.0)·10-1
(0.004-1.0)·10-2
(0.5-5.1)·10-4
(0-5.2)·10-3
(0-0.9)·10-4
< 0.6·10-4
(0.004-17.9)·10-3
<2.2·10-5
< 1.7·10-4
6
Table 2. Conditions in the water phase in contact with the corroding copper at three
stages; at closure, the first hundred years, and the first ten thousand years [1].
Constituent
Unit
Infiltrating
Pore water in
Pore water after
groundwater at
saturated
closure up to
closure
bentonite (up to
10000 years
100 years)
pH
6-8
7-9
7-9
redox
mV
oxic to -250
-150 to -250
-200 to -280
-4
DIC
mol/l
(0.02-1.6)·10
no estimate
no estimate
Clmol/l
(0.3-6.2)·10-1
(0.3-6.2)·10-1
(0.06-4.2)·10-1
Na+
mol/l
(0.2-2.8)·10-1
(3-5)·10-1
(3-4)·10-1
Ca2+
mol/l
(0.3-1.5)·10-1
(0.4-4.0)·10-2
(0.4-4.0)·10-2
-3
-1
2SO4
mol/l
(0-5.2)·10
1.4·10
1.4·10-1
HSmol/l
(0-0.9)·10-4
(0-3)·10-4
(0-0.9)·10-4
NH4+
mol/l
<5.5·10-6
(0.03-1.7)·10-4
<5.5·10-5
-4*
<1.7·10
CH4 (g)
mol/l
<4.5·10-6
<4.5·10-3
(0.004-17.9)·10-3
-2**
<2.7·10
*) constituent value in the case of marine water, **) constituent value in the case of saline water
The values given in Tables 1 and 2 above indicate that the conditions are initially oxic,
i.e. oxygen is present and may lead to some oxidation processes, while a change to
anoxic conditions takes place owing to the reduction of oxygen, probably as a result of
microbial processes.
2.2 Copper corrosion prior to water saturation
This stage of copper corrosion is attributable to the corroding effect of the atmosphere
to which the copper canister is exposed before coming into contact with the
groundwater. After encapsulation, the copper canisters will be transported to the
repository. The time period between the encapsulation and the final disposal is
estimated to be from several weeks to 2 years, and during this time the canisters are
more or less in indoor atmosphere at room temperature.
The atmospheric conditions in the storage may be close to urban atmosphere, although
the repository itself is situated in a rural atmosphere. The canisters will be stored in
well-ventilated areas and sheltered from sea salt spray.
The corrosion of copper in atmospheric exposure is possible when the relative humidity
of air is high enough to form a water film on the surface. The cathodic reaction is the
reduction of oxygen. The critical relative humidity depends on the surface conditions,
but it is generally 50-70 %. Soluble pollutants will dissolve in the moisture film. SO2
will form HSO3- , which can oxidize to sulphate and NO2 will normally absorb in the
moisture film as HNO3. If the corrosion products are protective, the corrosion rates will
decrease.
7
The corrosion products of copper are mainly copper oxides like cuprous oxide and
above it, copper hydroxides containing other anions. In the open atmosphere in
positions sheltered from rain, the sulphates like posnjakite, brochantite and antlerite as
well as the chlorides nantokite and atacamite have been found on the copper surface
after an exposure period of 8 years. The relative amount of antlerite on sheltered copper
strongly depends on the sulphur dioxide and ozone concentrations, but the ratio of
brochantite depends on the humidity and chloride. On sheltered copper, posnjakite is
frequently found at urban sites, which is expressed by a negative correlation with ozone.
In the field exposures of 4 years, also nitrates were found on the copper specimens [2].
The indoor relative humidity can be above the critical humidity, at least during the
summer months, but the high temperature of the canister surface compared to the
surrounding atmosphere will keep the surface drier than normal. There are only few
studies concerning the atmospheric corrosion of copper at elevated temperatures, and
some of them give quite different models for kinetics. However, the total oxide
thickness predicted is 0.03-0.07 µm at 100 °C after a few years of exposure. The
atmospheric pollutants accelerate the oxidation rate by a factor of 3-8 [1].
The storing time is not expected to have an effect on the service life of the canister. The
most likely corrosion product will be copper oxide, but also copper sulphates, chlorides
and nitrates may form depending on the atmospheric pollutants present.
2.3 Copper corrosion after disposal
When the canister has been installed into the bedrock and the free space has been filled
with bentonite, the conditions are no longer controlled. The initial saturation degree of
bentonite is 85 %. The heat of the canister surface (max 90 °C) will lead to
redistribution of the moisture in the bentonite so that the water ratio close to the canister
will be about 10 % and the relative humidity about 50 %. Atmospheric corrosion is
possible in these conditions.
During the period of the unsaturated phase, the corrosion will most probably be uniform
and the corrosion product layer will comprise a compact layer of Cu2O covered by basic
Cu(II) salts. No pitting corrosion is expected to occur [1].
2.4 Copper corrosion during water saturation
This stage includes the wetting of the compacted bentonite surrounding the holes for the
canisters and that of the backfill mixture of bentonite and crushed rock. After the
canisters have been surrounded with compacted bentonite and the tunnels have been
backfilled with a mixture of bentonite and crushed rock, the type and rate of corrosion
of the copper canisters depends on the wetting process leading to the final watersaturated conditions. Groundwater coming into the repository is brackish or saline. The
full saturation and homogenization of bentonite is expected to occur in 6 to 35 years,
even though much longer periods have been suggested, depending on the rate of water
penetration to the deposition hole. Reactions between copper and bentonite pore water
are expected only in fully saturated bentonite.
8
As the bentonite swells, the microbes will become embedded by the clay. It is assumed
that, in saturated bentonite, no microbial processes will occur and only solution-mineral
processes are possible. All the available oxygen will be dissolved in the pore water at
increased temperature (60 °C) and hydrostatic pressure. The oxygen in the pores will be
consumed in corrosion reactions or oxidizing reactions of minerals in the clay, and the
conditions will turn anoxic. As a result of the bacterial activity and reactions with iron
compounds, the environment will become reducing.
During the wetting process the buffer will probably not swell homogeneously, and there
will be local contact places of the canister and bentonite. These contact points are
potential starting points for pitting corrosion, especially when there is oxygen available.
During the swelling of the bentonite, the gaps will close so that the contact points will
move and the corrosion will appear uneven with corrosion pits.
Depending on the groundwater quality, several corrosion products may form, for
example Cu2O and Cu2CO3(OH)2 (malachite), which have been formed in the oxidizing
condition, and basic cupric chloride (CuCl2·3Cu(OH)2) [1]. The conditions have been
interpreted to be oxic during the formation of the products mentioned above.
Sulphate-reducing bacteria may survive at the beginning of the disposal even though the
temperature of the canister surface is high (90 °C). It is expected that the low
availability of water will make the conditions very difficult for SRB to survive, but
without a careful investigation the microbiological influences cannot be excluded.
2.5 Copper corrosion after water saturation
As a result of the water saturation and the evolution of the environment, the corrosion
behaviour of copper will change with time. However, the conditions will gradually
attain a steady state that is associated with a less aggressive environment than that at the
initial stage. The corrosion behaviour of copper will evolve from an initial period of
relatively fast uniform corrosion accompanied by possible localized corrosion to a longterm steady-state condition of a low rate of uniform corrosion with little or no localized
attack.
During the corrosion after water saturation, the mass transport to and from the canister
surface proceeds by diffusion in the aqueous phase and not in the gas phase. The oxygen
trapped in the pores of the bentonite first creates oxic conditions but is then consumed
by the corrosion of the canister, reactions with mineral impurities and sulphide on the
clay and microbial activity. This process will finally lead to the establishment of anoxic
conditions. Additionally, the decay heat from the spent fuel inside the canister will first
maintain the high temperature at the canister surface, but gradually the temperature will
decrease, simultaneously with the change from oxic to anoxic conditions.
As stated above, upon saturation the canister surface will be covered by a duplex layer
consisting of an inner layer of Cu2O and an outer layer of basic Cu(II) salts such as
Cu2CO3(OH)2 (malachite) and CuCl2·3Cu(OH)2 (atacamite), depending on the relative
concentrations of CO32- and Cl- ions in the pore water. When the bicarbonate content of
the groundwater exceeds 600 ppm, Cu2CO3(OH)2 can be assumed to dominate over
9
CuCl2·3Cu(OH)2. Based on the review by King et al. [1], no significant changes in the
composition of the corrosion film take place even after the change of the conditions to
anoxic. In the presence of sulphide ions, the formation of such films as Cu2O/Cu2S in
reducing conditions is also possible.
2.6 Substitutes for copper corrosion productsl
The reports of King et al. [1, 3] contain an extensive survey on the repository conditions
as well as on the corrosion processes taking place on the copper canister surface.
However, it has not been possible to connect sufficiently the discussion on the corrosion
processes and on the possible corrosion products with the discussion on the repository
conditions. Although good reasons for this exist, for instance, because the exact
conditions are not exactly known, this makes the assessment of the most relevant
corrosion product slightly arbitrary. Kaunisto and Laitinen consider that additional
calculations on the thermodynamic stability of the most likely products in repository
conditions are needed in order to give a more definite proposal.
Based on the insufficient information that was at their disposal, Kaunisto and Laitinen
considered CuCl2·3Cu(OH)2, i.e. atacamite, to be the most relevant corrosion product.
2.7 Conclusions concerning experimental designl
Owing to the uncertainty related to the definition of the most relevant corrosion product,
Kaunisto and Laitinen proposed an alternative approach: Instead of choosing one
corrosion product of copper and adding it to the experiment, they recommended that
several pieces of copper metal be added to the experiment. When such pieces of copper
material are in contact with the compacted bentonite in conditions approaching the real
conditions in the repository vault, the bentonite surrounding the Cu metal will be
influenced in exactly the way it would be in real conditions. This would result in the
most relevant understanding of the effect of copper corrosion on bentonite.
More versatile information of the effect of the corrosion products of copper on bentonite
can be obtained, if some of the copper samples in the experiment are treated in such a
way that the desired corrosion products have already formed on their surface before the
start of the test.
10
3
MODELLING
The relative ease by which Eh is measured in various systems is accompanied by wellknown difficulties in its interpretation. Ríos-Mendoza et al. (2003) recently recognized
this in the following way: “There are many publications regarding redox-measurements
in natural aqueous systems. However, the values obtained have not been easy to
interpret and it has not been possible to explain whether the pE measured characterizes
all the redox system (e.g., Morris and Stumm 1967, Whitfield 1969, 1974, Stumm 1978,
Champ et al 1979, Bricker 1982, Peiffer et al. 1992). Hence, to make quantitative
thermodynamic interpretations of pE would mean knowing all the redox pairs detected
by the sensor and their respective concentrations, which would not be very practical.”
11
4
EXPERIMENTAL
The interaction between potential copper corrosion products and MX-80 bentonite
under anoxic conditions was studied in two types of samples:
1) Soft water-saturated MX-80 mixed with CuCl2 solution.
Duration of study: 330 days.
2) Compacted water-saturated MX-80 containing small Cu-plates or a solid copper
compound (atacamite).
Duration of study: 2.9 years.
The samples were kept under N2. The effect of temperature was studied by storing the
samples at room temperature and at 75 ºC.
4.1 Soft samples
Soft MX-80 samples were prepared by mixing MX-80 with CuCl2-solutions having Cuconcentrations between 1·10-7 and 1·10-4 M. Half of the solutions were made by mixing
proper amounts of CuCl2 with de-ionized water, while the other half of the solutions
contained in addition 0.1 M NaCl as a background electrolyte. The dry density of the
MX-80 samples was 0.5 g/cm3. These samples (plus blanks to which only de-ionized
water had been added) were prepared in a glove-box under N2. Two identical sets of
samples were made; one was kept in the glove-box for anaerobic storage at ambient
room temperature, the other was put into small steel autoclaves and transferred into a
water bath outside the glove-box. The temperature of the water bath was 75 ºC.
4.2 Compacted samples
The experiments with compacted MX-80 contained solid copper corrosion products in
contact with the MX-80. Two types of solids were used: grains of atacamite, or copper
oxychloride (from ChemService), and copper plates with corroded surfaces.
The copper plates contained green corrosion products that had been produced in
accordance with draft standard ISO/DIS 16151 ('Corrosion of metals and alloys Accelerated cyclic tests with exposure to acidified salt spray, "dry" and "wet"
conditions'). The treatment involved cyclic exposure of the copper specimens to a water
solution and to drying conditions. The water solution contained sodium chloride, nitric
acid and sulphuric acid, and the pH value of the solution was about 3.5 [4]. Figure 1
shows an example of a corroded copper plate prepared in the above manner.
12
Figure 1. Copper plate with corrosion products on the surface. Dimensions: thickness
0.9 mm, surface 10·10 mm.
Compacted MX-80 samples were prepared by placing either Cu-plates or granules of
copper oxychloride in loose MX-80 powder and compacting to a dry density of 1.4-15 g/cm3. The compacted, initially dry MX-80 samples were kept in cylindrical copper
vessels, which were submerged into de-ionized water in an outer vessel. Sinters at the
end of the copper cylinders allowed the MX-80 to absorb water from the outside, see
Figure 2. Those samples that were kept at an elevated temperature (75°C) were
subsequently moved, in a glove-box under N2, to air-tight steel autoclaves, see Figure 3,
and kept in a heat controlled water-bath for 2.9 years. After
Gas phase
External solution
Sinter
Bentonite
Cu corrosion product
Figure 2. Vessels with compacted MX-80 bentonite containing solid copper corrosion
products. Left: Sample with atacamite grains. Right: Sample with four layers of
corroded copper plates.
13
O-ring
200 mm
Teflon vessel
Steel autoclave
80 mm
Figure 3. Schematic drawing of a steel autoclave containing a compacted MX-80
sample of the type shown in Figure 2. Not to scale.
this time the samples were disassembled under N2. The chemical composition of
external solutions were analysed, see Table 4, and the compacted samples were
analysed with SEM/EDS.
14
5
RESULTS AND DISCUSSION
5.1 Experiments with soft bentonite
The experiments with soft bentonite (MX-80) were terminated after 330 days. All
autoclaves, which contained samples that had been heated to 75ºC, were transferred
from the waterbath to a glovebox and opened under N2. Porewater was then squeezed
from these samples and from those that had been stored at 25°C. The chemistry of the
porewater was subsequently analysed, see Table 3.
The analytical results do not indicate any dramatic changes in the samples during the
330 days. The Table 3 shows that the concentrations of the major ions (Na+, K+, Mg2+,
Ca2+, Cl- and SO42+) were about the same in all samples irrespective of whether the
samples had been heated or not. The alkalinity, however, seemed to be influenced by
the heating; samples that had been heated exhibited higher alkalinity than those that had
been stored at room temperature, see Figure 4.
The effect of adding a background electrolyte (0.1 M NaCl) lead, possibly via ionexchange reactions with the montmorillonite component in MX-80, to expected
increases of K+, Mg2+, and Ca2+ concentrations in the porewater.
The Cu2+ concentrations in the porewaters were in most cases below the detection limit.
The few results that actually were obtained, see Figure 4, do not allow of any certain
conclusions. Since the added amounts of Cu2+ (added as CuCl2) was small, in
comparison to the total amount of ions in the bentonite, it was not expected that the
added Cu might have any impact on the macro-chemistry of the system. The diagrams
in Figure 5 confirm this by showing that the concentrations of the macro-ions were
independent of the total amount of copper added, within the copper concentration range
used (0 - 1 10-4 M).
7
Total alkalinity (mM)
Cu conc. (M)
5.0×10 -6
4.0×10 -6
3.0×10 -6
2.0×10 -6
1.0×10 -6
0
25
[Cu] (M)
75
6
5
4
3
2
1
0
25
25
25
25
75
75
75
75
75
o
T ( C)
Figure 4. The effects of temperature and background electrolyte (NaCl) on Cu
concentration in porewater (left) and total alkalinity (right). Unfilled and filled bars
represent 0 and 0.1 M background electrolyte concentrations, respectively. The low
copper concentrations are uncertain due to analytical difficulties.
15
15
Table 3. Squeezed pore water compositions in soft MX-80 samples after 330 days. The contents of Na, Mg, Ca and K were determined by inductively
coupled plasma atomic emission spectrometry (ICP-AES), the contents of Cu were determined by graphite furnace atomic absorption spectrometry
(GFAAS) and the contents of Cl- and SO42- were determined by ion chromatography (IC).
Experimental conditions
Squeezed porewater
Sample
Solution
Temp.
Na
Mg
Ca
K
Cu
ClSO42Total
alkalinity
[Cu]
[NaCl]
(ºC)
(M)
(M)
(M)
(M)
(M)
(M)
(M)
(mmol/L)
(M)
(M)
1
0
0
RT
5.79 10-2
3.50 10-4
8.98 10-4
4.09 10-4 < 7.87 10-8
1.44 10-3
2.79 10-2
1.00
-7
-2
-4
-4
-4
-8
-3
-2
2
1 10
0
RT
5.00 10
3.09 10
6.74 10
3.58 10
< 7.87 10
1.66 10
2.51 10
1.85
3
1 10-6
0
RT
4.96 10-2
2.92 10-4
7.49 10-4
4.09 10-4
4.09 10-7
1.27 10-3
2.50 10-2
0.33
-5
4
1 10
0
RT
6.8
-4
-2
-4
-3
-4
-7
-3
-2
5
1 10
0
RT
7.31 10
3.41 10
1.22 10
7.16 10
2.99 10
2.99 10
3.76 10
1.41
-1
-1
-3
-3
-3
-8
-1
-2
6
0
1 10
RT
1.68 10
2.06 10
4.74 10
1.43 10
< 7.87 10
1.21 10
2.96 10
0.70
7
1 10-7
1 10-1
RT
1.54 10-1
2.14 10-3
4.24 10-3
1.20 10-3 < 7.87 10-8
1.06 10-1
2.73 10-2
1.70
-6
-1
-1
-3
-3
-3
-7
-1
-2
8
1 10
1 10
RT
1.62 10
2.06 10
5.24 10
1.38 10
8.50 10
1.12 10
2.89 10
0.75
9
1 10-5
1 10-1
RT
-4
-1
-1
-3
-3
-3
-8
-1
-2
10
1 10
1 10
RT
1.58 10
1.97 10
5.24 10
1.33 10
< 7.87 10
1.08 10
2.91 10
1.05
-2
-4
-4
-4
-8
-3
-2
11
0
0
75
6.39 10
3.70 10
5.49 10
6.14 10
< 7.87 10
1.35 10
2.89 10
6.35
12
1 10-7
0
75
6.26 10-2
3.99 10-4
5.49 10-4
5.88 10-4 < 7.87 10-8
1.61 10-3
2.86 10-2
6.16
-6
-2
-4
-4
-4
-6
-3
-2
13
1 10
0
75
6.92 10
3.95 10
3.24 10
7.16 10
4.72 10
1.47 10
3.19 10
5.89
-5
-2
-4
-4
-4
-7
-3
-2
14
1 10
0
75
7.35 10
3.62 10
4.24 10
7.16 10
< 1.57 10
1.69 10
3.38 10
5.35
15
1 10-4
0
75
6.87 10-2
4.53 10-4
6.74 10-4
7.42 10-4 < 7.87 10-8
1.61 10-3
3.21 10-2
6.37
-1
-1
-3
-3
-3
-8
-1
-2
16
0
1 10
75
1.71 10
2.51 10
5.99 10
1.43 10
< 7.87 10
1.15 10
3.02 10
2.60
-7
-1
-1
-3
-3
-3
-8
-1
-2
17
1 10
1 10
75
1.81 10
2.26 10
5.24 10
1.64 10
< 7.87 10
1.21 10
3.19 10
1.98
18
1 10-6
1 10-1
75
1.78 10-1
2.30 10-3
4.74 10-3
1.66 10-3
8.34 10-7
1.48 10-1
3.81 10-2
4.20
-5
-1
-1
-3
-3
-3
-6
-1
-2
19
1 10
1 10
75
1.91 10
2.39 10
6.74 10
1.79 10
3.93 10
1.27 10
3.52 10
1.94
20
1 10-4
1 10-1
75
1.82 10-1
2.18 10-3
1.34 10-3
1.64 10-3 < 7.87 10-8
1.21 10-1
3.32 10-2
3.03
16
Na+
K+
0.25
0.002
Conc. (M)
Conc. (M)
0.20
0.15
0.10
0.001
0.05
10 -7
10 -6
10 -5
10 -4
0.000
10 -8
10 -3
Ca2+
0.002
0.0050
0.001
10 -6
10 -5
10 -4
10 -3
0.0000
10 -8
10 -7
10 -6
10 -5
[Cu]Tot (M)
[Cu]Tot (M)
Cl-
SO42-
10 -3
10 -4
10 -3
0.05
Conc. (M)
Conc. (M)
10 -4
0.0025
0.2
0.1
0.0
10 -8
10 -5
Mg2+
0.0075
10 -7
10 -6
[Cu]Tot (M)
0.003
0.000
10 -8
10 -7
[Cu]Tot (M)
Conc. (M)
Conc. (M)
0.00
10 -8
10 -7
10 -6
10 -5
10 -4
10 -3
0.04
?
0.03
0.02
10 -8
[Cu]Tot (M)
10 -7
10 -6
10 -5
10 -4
10 -3
[Cu]Tot (M)
0 M NaCl, 25°C,
0.1 M NaCl, 25°C,
Background sample, 25°C
0 M NaCl, 75°C,
0.1 M NaCl, 25°C,
Background sample, 25°C
Figure 5. Measured concentrations of common ions in squeezed porewater of soft
bentonite samples vs. added Cu concentration. Squeezing took place after 330 days. The
data represents four combinations of temperature and NaCl concentration. Background
samples consisted of MX-80 mixed with deionized water.
17
Figure 6. Typical XRD-diffractogram for dried soft MX-80 samples with an initial
CuCl2 concentration of 1 10-5 M. The presence of Cu could not be confirmed.
XRD was used in order to check whether the presence of Cu in the bentonite samples
could have any effects on, e.g., the inter-lamellar spacing of the montmorillonite or the
types of solids present. Five samples were chosen for this task, one of which (sample 11
in Table 3) contained no CuCl2 and served as a background. The other samples (samples
4, 9, 14, and 19 in Table 3) represented samples with relatively high Cu2+
concentrations (1 10-5 M). The samples were dried and subsequently examined. The
obtained five diffractograms all looked the same, which indicated that the Cu2+ was
present in too small quantities to have any detectable physico-chemical effects. A
typical diffractogram is seen in Figure 6.
18
5.2 Experiments with compacted samples
The experiments with compacted MX-80 bentonite samples were terminated after 2.9
years. The chemistry of the external solutions changed during this time from an 0.5 M
NaCl solution to a solution with several cations and anions, see Table 4. The changes
were caused by simple ion transport between the bentonite and the external solution.
The long duration of the experiment made it reasonable to expect that the external
solutions were in equilibrium with the corresponding bentonite porewater, and,
consequently, that the copper corrosion products in the samples were in contact with
porewaters having compositions close to those given in Table 4.
The dry densities of the samples were calculated from the gravimetric water content
(determined after drying at 110°C for 24 h) and assuming specific densities of 1.0 and
2.75 g/cm3 for water and MX-80 bentonite, respectively. The scatter in the density data is
small and insignificant for the present purposes. pH in the external solutions was
measured with an IrOx pH electrode against an Ag/AgCl reference electrode and found to
have values between 8.1 and 9.3, see Table 4.
The compacted MX-80 samples contained two types of solid Cu corrosion products;
corroded Cu plates and solid atacamite grains. The possible chemical interaction
between the corrosion products and the bentonite required that the latter were dissolved
and, thus, that the copper most probably was present as Cu2+ ions. Interactions between
the Cu2+ ions and the various bentonite components might then, a priori, include, e.g., 1)
precipitation (with anions in the porewater), 2) adsorption on outer montmorillonite
surfaces, 3) diffusion into interlamellar montmorillonite spaces, or 4) diffusion in the
free water between the bentonite particles.
In order to determine whether these ions could diffuse into the bentonite, compacted
samples were analysed using SEM (FEI XL 30 ESEM) and an energy dispersive X-ray
analyser (Thermo Noran System Six). The MX-80 samples that had been kept at 75 °C
Table 4. Chemical composition of external solutions in contact with compacted MX-80
bentonite after 2.9 years. The contents of Na, Mg, Ca and K were determined by
inductively coupled plasma atomic emission spectrometry (ICP-AES), the contents of
Cu were determined by graphite furnace atomic absorption spectrometry (GFAAS) and
the contents of Cl- and SO42- were determined by ion chromatography (IC).
Experimental conditions
Sample
MX-80 Blank
MX-80 Blank
MX-80 Cu oxy
MX-80 Cu oxy
MX-80 4 plates
MX-80 4 plates
Dry
density
(g/cm3)
1.44
1.30
1.53
1.43
1.44
1.43
External solution
Temp.
Na
Mg
Ca
(°C)
(mg/L)
(mg/L)
(mg/L)
RT
75
RT
75
RT
75
640
850
610
1040
600
950
1.7
2.7
1.0
3.9
1.1
3.4
5.7
12
5.4
17
4.1
13
K
Cl-
SO42-
(mg/L)
(mg/L)
(mg/L)
41
17
51
21
17
45
47
31
68
200
78
130
1050
1210
1010
1310
930
1230
pH
8.8-8.9
9.3
9.0
8.2
8.7
8.1
19
Figure 7. Compacted MX-80 bentonite with Cu coupons. The coupons were removed
prior to the elemental analyses. The letters A and B shows the areas analysed.
Figure 8. EDS diagram of compacted water-saturated MX-80 bentonite showing copper
0.2 mm away from the atacamite grain.
were mounted in epoxy and analysed with regard to the Cu concentration in the
bentonite closest to the copper materials, see Figure 7. The analytical results were given
as EDS diagrams, which showed the elemental concentrations at chosen spots in the
sample. The detection limit of the copper was low and estimated to be about 0.01
weight percent. A typical EDS diagram is seen in Figure 8, which was obtained in a
MX-80 bentonite sample containing atacamite grains. The Cu concentration profiles
were constructed by combining the information from several EDS diagrams. Figure 9
shows concentration profiles in the vicinity of two different atacamite grains in a
compacted sample that had been heated to 75 °C for 2.9 years. It is clearly indicated,
that some atacamite has dissolved, and that Cu thereby has been released. Both profiles
show clearly elevated Cu concentrations at distances up to about 0.8 mm. At distances
further away, both profiles drop to values close to or equal to zero concentration. The
reason for the clearly different amplitudes at the shorter distances is not known.
20
12
Cu (weight%)
10
8
6
4
2
0
0.0
0.5
1.0
1.5
2.0
Distance (mm)
Figure 9. Cu concentration vs. distance from an atacamite grain in compacted watersaturated MX-80. The two graphs indicate a relatively high scatter in the Cu
concentrations in the vicinity of the grains. The graphs were taken in samples that had
been heated at 75 °C for 2.9 years under anaerobic conditions.
Cu (weight%)
1.0
0.5
0.0
0
250
500
750
1000
Distance (µ
µm)
Figure 10. Copper concentration vs. distance from copper corrosion products on
surfaces of Cu plates. The plates were anaerobically stored in compacted watersaturated MX-80 bentonite for 2.9 years at ambient room temperature (black graphs)
and at 75 °C (red graphs).
Copper concentration profiles were also found in samples with plates having corrosion
products on their surfaces. Figure 10 shows nine concentration profiles obtained in
different samples stored either at room temperature or at 75 °C. However, the
concentrations were clearly lower in this case as compared to the above case with
atacamite. The heating of the samples does not seem to have had any influence on the
concentration profiles; these are practically the same in the heated and unheated
samples.
21
The experiments with compacted MX-80 bentonite involved only four samples
containing copper corrosion products and two blank samples. The limited amount of
data collected in this study does not reveal unambiguously the nature of the interaction
between copper corrosion products and MX-80 bentonite. However, the data in Table 4
indicate that the presence of the atacamite or the corrosion products on the plates did not
have any measurable effects on the porewater chemistry, since the samples with the
corrosion products exhibited practically the same pore water composition as the blank
samples. At the same time, the Cu concentration profiles in Figures 9 and 10 show that
the corrosion products do dissolve, and that, most probably, Cu2+ ions diffuse into the
surrounding bentonite. The concentration profiles in Figure 10 are roughly the same,
irrespective of whether the samples were stored at room temperature or at 75oC, which
suggests that the temperature had little effect on the diffusion of copper in the bentonite.
A comparison between the concentration profiles in Figures 9 and 10, indicate that the
copper concentrations at short distances were somewhat higher in the samples with
atacamite as compared to the samples with copper plates. A simple tentative explanation
for this is that atacamite dissolved faster than the corrosion products on the surface of
the copper plate. In both cases, the copper was found to diffuse roughly the same
distance (about 1 mm) into the bentonite during the 2.9 year long experiment.
22
6
SUMMARY
This report presents results from a study of the possible interaction between copper
corrosion products and MX-80 bentonite under conditions that might occur in a final
repository for spent nuclear fuel in Finland. The first part of the study included a
literature survey, the objective of which was to identify some relevant corrosion
products that might form when copper corrodes in wet MX-80 bentonite.
The information collected in the literature survey suggested that commercially available
atacamite, CuCl2·3Cu(OH)2, would be relevant as a copper corrosion product to be used
in subsequent interaction experiments. In addition, the experiments involved of green
corrosion products that had been produced in accordance with draft standard ISO/DIS
16151 on small copper plates.
Geochemical modelling (PHREEQC) was used to predict the chemical behaviour of
copper corrosion products in the samples. The results indicated that no copper
compounds would precipitate and that copper was present as Cu2+ under the
experimental conditions.
The experiments were performed with both soft and compacted MX-80 using different
experimental setups. The soft samples consisted of water-saturated MX-80 that had
been mixed with CuCl2 solutions of various concentrations and then been stored in
closed vessels. Half of the samples also contained NaCl as a background electrolyte.
The samples were kept under anaerobic conditions at ambient room temperature or at
750C for 330 days. Porewater aliquots were then squeezed from the samples and
analysed. The macro-chemistry of the porewater behaved as expected in the sense that
the addition of NaCl to the samples clearly increased the concentrations of Mg, K, and
Ca in the porewater, while the effect on the SO4-concentration was insignificant. The
effect of increasing the temperature from room temperature to 75 ºC seems to have a
rather small impact on the concentrations of the macro-components. The total alkalinity
was higher in the samples to which NaCl had been added as compared to the
background samples. The alkalinity also seemed to be somewhat higher in the samples
that had been heated.
The compacted MX-80 samples were stored under anaerobic conditions in vessels that
were submerged in an external NaCl solution. The vessels were equipped with sinters
that allowed contact between the bentonite and the external solution. The samples were
kept at room temperature and 75 ºC for 2.9 years and then analysed.
The presence of the atacamite or the corrosion products on the plates did not have any
measurable effects on the porewater chemistry, since the samples with the corrosion
products exhibited practically the same pore water composition as the blank samples. At
the same time, the Cu concentration profiles indicate that the corrosion products do
dissolve, and that, most probably, dissolved copper diffuses as Cu2+ ions into the
surrounding bentonite. Concentration profiles were found to be roughly the same,
irrespective of whether the samples had been stored at room temperature or at 75 ºC.
This suggests that the temperature had little effect on the diffusion of copper in the
23
bentonite in these cases. The extension of the concentration profiles suggested that
copper from atacamite and surfaces of the plates, diffused less than 1 mm during the
course of the experiment.
It might be argued that the work presented here deals with copper corrosion products
that may not necessarily represent the most realistic ones under all real repository
conditions. The copper products used (atacamite and an in-house made copper corrosion
product on the surface of a copper plate) were subjectively chosen and the results may
not necessarily be applicable to all types of copper corrosion products in contact with
bentonite. However, the results indicate that under conditions close to those in the
experiments, interactions between bentonite and copper corrosion products can, more or
less, be regarded as interactions between dissolved Cu2+ ions and bentonite. The results
are therefore not be restricted to the two corrosion products used in this study, but may
also be applicable to similar cases where copper corrosion products interact with
bentonite.
24
REFERENCES
F. King, L. Ahonen, C. Taxén, U. Vuorinen and L. Werme (2001): Copper corrosion
under expected conditions in a deep geologic repository, SKB TR-01-23.
UN/ECE International Co-operative Programme on Effects on Materials, Including
Historic and Cultural Monuments. Report No 33: Results from XRD analysis of
copper corrosion products. A. Krätschmer & B. Stöckle, B
F. King (2002): Corrosion of copper in alkaline chloride environments, SKB TR 0225.
T. Kaunisto (2004). Private communication.