Journal of Hazardous Materials 285 (2015) 464–473

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Corrosion of high-level radioactive waste iron-canisters in contact with bentonite Stephan Kaufhold a,∗ , Achim Walter Hassel b,c , Daniel Sanders b , Reiner Dohrmann a,d a

BGR, Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, D-40237 Düsseldorf, Germany Institute for Chemical Technology of Inorganic Materials, Johannes Kepler University Linz, Altenberger Straße 69, 4040 Linz, Austria d LBEG, Landesamt für Bergbau, Energie und Geologie, Stilleweg 2, D-30655 Hannover, Germany b c

h i g h l i g h t s

g r a p h i c a l

• At the iron bentonite interface a 1:1

Corrosion at the bentonite iron interface proceeds unaerobically with formation of an 1:1 Fe silicate mineral. A series of exposure tests with different types of bentonites showed that Na–bentonites are slightly less corrosive than Ca–bentonites and highly charges smectites are less corrosive compared to low charged ones. The formation of a patina was observed in some cases and has to be investigated further.

Fe layer silicate forms upon corrosion. • A series of iron–bentonite corrosion products showed slightly less corrosion for Na-rich and high-charged bentonites. • In some tests the formation of a patina was observed consisting of Fe–silicate, which has to be investigated further.

a r t i c l e

i n f o

Article history: Received 11 July 2014 Received in revised form 9 September 2014 Accepted 29 October 2014 Available online 15 December 2014 Keywords: High-level radioactive waste canister Bentonite Iron Corrosion Bentonite–iron-interaction

a b s t r a c t

a b s t r a c t Several countries favor the encapsulation of high-level radioactive waste (HLRW) in iron or steel canisters surrounded by highly compacted bentonite. In the present study the corrosion of iron in contact with different bentonites was investigated. The corrosion product was a 1:1 Fe layer silicate already described in literature (sometimes referred to as berthierine). Seven exposition test series (60 ◦ C, 5 months) showed slightly less corrosion for the Na–bentonites compared to the Ca–bentonites. Two independent exposition tests with iron pellets and 38 different bentonites clearly proved the role of the layer charge density of the swelling clay minerals (smectites). Bentonites with high charged smectites are less corrosive than bentonites dominated by low charged ones. The type of counterion is additionally important because it determines the density of the gel and hence the solid/liquid ratio at the contact to the canister. The present study proves that the integrity of the multibarrier-system is seriously affected by the choice of the bentonite buffer encasing the metal canisters in most of the concepts. In some tests the formation of a patina was observed consisting of Fe–silicate. Up to now it is not clear why and how the patina formed. It, however, may be relevant as a corrosion inhibitor. © 2014 Elsevier B.V. All rights reserved.

1. Introduction ∗ Corresponding author. Tel.: +49 5116432765. E-mail address: [email protected] (S. Kaufhold). http://dx.doi.org/10.1016/j.jhazmat.2014.10.056 0304-3894/© 2014 Elsevier B.V. All rights reserved.

Various countries favor the encapsulation of high-level radioactive waste (HLRW) in iron or steel canisters surrounded by highly

Table 1 Comparison of the experimental results and proposed corrosion mechanism of recent selected studies. Refs.

Model/results Experiment

Tmax / tmax ◦

1

Formation of new Fe/Mg-layer silicates (chlorite, saponite), zeolites, quartz Formation of new phases depends on di/trioctahedral smectite, exchangeable cations, pH Formation of magnetite and analcime, smectite alteration (amongst others) different reaction products, CEC deand increase

Dissolution − precipitation + solid state reaction

[8]

Fe powder/plate in MX80 + water

300 C/9 months

[Ref.] Lantenois et al. (2005)

[9]

Fe powder with suspensions of different smectites

80 ◦ C/1.5–4 months

[Ref.] Wilson et al. (2006)

[10]

Fe powder with Kunipia F suspension

80–250 ◦ C/3 months

[Ref.] Perronnet et al. (2007)

[11]

Fe powder with suspensions of different smectites

80 ◦ C/3 months

[Ref.] Carlson et al. (2007)

[12]

Carbon steel wires embedded in compacted MX80

50 ◦ C/approximately 2 years

[Ref.] Xia et al. (2005)

[13]

[Ref.] Osacky´ et al. (2010)

[14]

Diffusion experiment with and without Fe in contact to bentonite Fe powder with 7 bentonites

[Ref.] Osacky´ et al. (2009)

[15]

5 Slovak bentonites

[Ref.] Chuanhe et al. (2011)

[16]

Numerical model with sorption and kinetics of corrosion and mt precipitation

[Ref.] Savage etr al. (2010)

[17]

QPAC including nucleation, growth, precursor cannibalisation, and Ostwald ripening

[Ref.] Wersin and Birgersson (2014)

[18]

Kinetically based reactive transport model and thermodynamic database

75 ◦ C/35 days

60 ◦ C/120 days

Reduction of structural Fe but no secondary phases observed Color change

Formation of berthierine-like phase, splitting of layers Formation of Fe-oxohydroxides

Corrosion of metal Fe results from its interaction with protons liberated from the clay surface = Fe2+ forms TEM indicates: loss of tetrahedral sheet = isolated 7 A layers reduction of structural Fe = increase of LCD = uptake of protons = increase of pH

3

Fe2+ migrates into interlayer and from there into octahedral layer

The new octahedral sheet and old tetrahedral sheet do not fit together anymore = separation

The 7 A units are supposed to be the precursor of berthierine and/or chlorite alkalinity causes smectite destabilization = formation of Si–Al–Fe gels

Gas induced cracks

Corrosion rate 0.1 ␮m/a

Fe-rich bentonites react more than others

Splitting of layers may play a role

Fe2+ from corrosion migrates into gel = formation of 7 or 14 A non swellable clay minerals

Smectite rearrangement rather than dissolution was found Taking kinetics into Surface account reduce complexation corrosion rate affects pH less than the Fe concentration Alteration of clay the solid alteration Berthierine by Fe-rich fluids sequence is dominating solid may proceed via an corrosion product, magnetite → Ostwald step may be replaced cronstedtite → sequence with time by berthiersiderite ine → chlorite Only a few cm next to the Fe source were affected; large impact of microbial sulphate reduction which may lead to increase in pH or other indirect processes is deduced. Low smectite content resulted in less corrosion

465

[Ref.] Guillaume et al. (2003)

2

S. Kaufhold et al. / Journal of Hazardous Materials 285 (2015) 464–473

Observation

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S. Kaufhold et al. / Journal of Hazardous Materials 285 (2015) 464–473

Table 2 Basic properties of the bentonite B06 (data from [16–18]). Sample

B06

B06

/wt.% /wt.% /wt.%

93 7 0.1

Chemical composition (XRF) SiO2 /wt.% TiO2 /wt.% /wt.% Al2 O3 /wt.% Fe2 O3

BET Specific surface area

/m2 /g

43

MnO MgO CaO

/wt.% /wt.% /wt.%

0.0 3.5 1.5

Alkylammonium method Layer charge density

/eq/FU

0.33

Na2 O K2 O P2 O5

/wt.% /wt.% /wt.%

0.5 0.5 0.2

CEC (Cu-trien method) Na+ K+ Mg2+ Ca2+ sum cations CEC sum-CEC

/meq/100g /meq/100g /meq/100g /meq/100g /meq/100g /meq/100g /meq/100g

18 2 40 53 113 104 10

LOI Sum

/wt.% /wt.%

18.7 99.7

Mineralogical composition Montmorillonite Feldspar Calcite (from LECO)

compacted bentonite as a geotechnical barrier. Favorable bentonite properties are swelling and sealing capacity and radionuclide retention of either natural [1], organophilized bentonite [2], or thermally treated material [3]. Bentonite, therefore, is extensively investigated with respect to its role in metal corrosion processes (mostly Fe but also Cu [4]). The corrosion of iron (or steel) in contact with clays is also relevant for buried pipelines [5]. Moreover, corrosion products may alter the required bentonite properties such as swelling and adsorption capacity. Different corrosion products are reported as a result of many different iron-clay corrosion experiments. Ironoxohydroxides, among them, are common products of a first aerobic corrosion in which all oxygen available in the system is consumed. This process is followed by anaerobic corrosion which is supposed to determine the long term behavior of the canister clay system. Smectites generally are transferred to 7 Å (700 pm) phases (often referred to as berthierine) or Fe–smectites at lower temperatures (150 ◦ C), respectively [6]. The formation of either berthierine or Fe–smectite also depends on the amount of available oxygen (low fO2 (with fO2 being the fugacity of molecular oxygen) values favor the formation of berthierine; [7]). Distinguishing between Fe–smectites and chlorites (or serpentines) is particularly important with respect to the performance of a HLRW repository because chlorites and serpentines (in contrast to smectites) are non swellable clay minerals. The transformation of smectite (even of Fe–smectite) to chlorite or serpentine, therefore, would significantly affect the properties of the geotechnical barrier. The safety of a HLRW repository has to be assessed for up to 1 Ma (106 years). A typical study may take 1 year and therefore, needs an extreme extrapolation. The performance of a repository, therefore, has to be foreseen based on numerical models. For this, all relevant processes have to be identified. It is therefore, essential to understand the actual corrosion mechanisms of metallic iron in contact with clays. On the one hand a lot of studies only report on the identification of corrosion products and on the other hand quite different models are published. Some important studies are compared in Table 1 and discussed in the following. Based on corrosion experiments conducted at 50 ◦ C Carlson et al. [12] report on the significant increase of the Fe-content along with the decrease of the cation exchange capacity (CEC) but no crystalline (XRD-detectable) alteration products could be found which most probably can be explained by the low exposure temperature.

LECO data (elemental analysis) Ctotal /wt.% Corg /wt.% Canor /wt.% Stotal /wt.%

52.8 0.7 18.0 3.4

0.04 0.03 0.01 0.01

However, no corrosion model could be developed apart from the fact that cation exchange is supposed to play a role or represents an initial step. The corrosion and/or bentonite alteration products identified in the studies conducted at higher temperature are in accordance with the summary given before. In contrast to the others, Wilson et al. [10] report on the formation of magnetite which is probably caused by a higher oxygen concentration compared to the other experiments. However, Wilson et al. [10] observed isolated 7 Å (700 pm) layers after the experiment which are believed to represent the basic units of berthierine and/or chlorites and Guillaume et al. [8] propose a mixture of dissolution-precipitation processes and solid state reactions. More detailed corrosion mechanisms which could be used for modelling long-term corrosion are given by Lantenois et al. [9] and Perronnet et al. [11]. Perronnet et al. [11] propose a 3-step model: structural Fe is reduced by corrosion increasing the layer charge density (LCD) which is at least partially balanced by protons which in turn increases the pH. In a second step the increased alkalinity results in smectite destabilization and the formation of a Si–Al–Fe gel. The ongoing Fe2+ diffusion into the gel finally induces the formation of 7 or 14 Å (700 or 1400 pm) Fe-rich layer silicates. The corrosion model proposed by Lantaneois et al. [9] is based on the migration of Fe2+ from the interlayer to the octahedral sheet. More specifically, corrosion is initiated by protons liberated from the clay surface. The increased charge deficiency is balanced by Fe2+ first approaching the smectite edges and finally entering the octahedral sheet via the interlayer. The modified octahedral sheet contains trioctahedral domains and, therefore, does not fit anymore with the unchanged tetrahedral sheet. According to Balko et al. [19] smectites prevent the formation of a passive layer by adsorption of the species formed upon corrosion. Most of the studies focused on a few bentonites only, which hardly allows for comprehensive conclusions. However, Osacky´ et al. [14] selected seven different Na-exchanged materials which allowed comparing the effect of structural Fe and LCD. He found Fe-rich bentonites to be more corrosive. Also Osacky´ et al. [15] found the smectite content to play a role. Since the detection of the corrosion products more studies about modelling the Fe–bentonite interaction are published [16–18]. The mechanisms leading to different corrosivities of bentonites are, however, not fully understood, yet. The differences of the selected corrosion models explained above (Table 1), suggest that still some open questions have to be answered. One of the key problems in all

S. Kaufhold et al. / Journal of Hazardous Materials 285 (2015) 464–473

other studies is that a few different bentonites were considered. The comparison of the corrosion rates determined with four different bentonites does not allow to identify the reason because of the huge variability of bentonites. Therefore, the aim of the present study is to compare the corrosivity of a set of well characterized bentonites and to try to improve the understanding of the corrosion mechanism by identifying the corrosion determining parameters. 2. Materials and methods 2.1. Sample selection The exposition tests were performed using the sample set described by Kaufhold and Dohrmann [20], Kaufhold et al. [21], and Ufer et al. [22]. This sample set consists of 38 different bentonites and 2 illite/smectite clays (assigned B01–B40). Characterization of these materials is provided elsewhere [16–18]. The identification of the corrosion products (XRD reflection around 7 Å (700 pm)) is difficult if kaolinite and/or chlorite are accessory minerals. Therefore, for the investigation of the corrosion product bentonite B 06 was selected which neither contains kaolinite nor chlorite but exclusively smectite and feldspar. The basic properties of this material are given in Table 2. The mineralogical composition was determined by Ufer et al. [22], the specific surface area was determined by N2 adsorption (Micromeritics Gemini III 2375 surface area analyzer; 300 mg sample mass), the layer charge density of the smectite was determined using the alkylammonium method [19,20], the CEC was determined by the Cutrien method [21], the chemical composition was measured by XRF (PANalytical Axios and PW2400 spectrometer), and the C- and S-contents were measured using a C–S-analyzer (LECO CS-444). 2.2. Investigation of run products The run products were characterized by XRD and ESEM-EDX. For the detailed clay mineralogical investigation of the run products oriented aggregates of the

Corrosion of high-level radioactive waste iron-canisters in contact with bentonite.

Several countries favor the encapsulation of high-level radioactive waste (HLRW) in iron or steel canisters surrounded by highly compacted bentonite. ...
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