The anaerobic corrosion of carbon steel in compacted bentonite exposed to naturalOpalinus Clay porewater containing native microbial populationsN. R. Smarta, B. Reddya, A. P. Rance a, D. J. Nixon a, M. Frutschib, R. Bernier-Latmanib and N. Diomidisc

aAmec Foster Wheeler, Oxfordshire, UK; bEcole Polytechnique Fédérale de Lausanne, Environmental Microbiology Laboratory, EPFL-ENAC-IIE-EML,Lausanne, Switzerland; cNagra, Wettingen, Switzerland

ABSTRACTA long-term in situ corrosion experiment is ongoing in the Mont Terri Underground ResearchLaboratory in Switzerland to (i) measure the in situ corrosion behaviour of carbon steel incompacted bentonite under simulated repository conditions, (ii) study the effect of the bentonitebuffer density on microbial activity and microbially influenced corrosion and (iii) study the effect ofwelding on the corrosion rate. Carbon steel corrosion coupons, with and without welds, weresurrounded by compacted bentonite with a range of dry densities and mounted in modulesallowing free exchange with the local anoxic groundwater. After about 20 months of exposure,corrosion coupons and bentonite were sampled. A complex corrosion product was identified,consisting predominantly of magnetite. The bentonite adjacent to the metal was finer grained,more dispersed and enriched in iron. Aerobic, anaerobic and sulphate-reducing bacteria wereidentified both in the porewater surrounding the modules and in the bentonite.

ARTICLE HISTORYReceived 14 December 2016Accepted 23 March 2017

KEYWORDSCarbon steel; anaerobic;corrosion; microbial;bentonite; waste

This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Introduction

Nagra (National Cooperative for the Disposal of RadioactiveWaste, Switzerland) is considering using carbon steel as apotential canister material for the disposal of high-level wasteand spent fuel in a deep geological repository in OpalinusClay. Bentonite clay with a dry density of 1450 kg m−3 willbe used to backfill the emplacement tunnels and will be placedaround and between the disposal canisters. An in situ cor-rosion experiment is being conducted at the Mont TerriUnderground Research Laboratory in Switzerland with theoverall long-term objective of providing measurements of thecorrosion rate of carbon steel in compacted bentonite undersimulated repository conditions. The main goals are as follows:

. Measure the in situ anaerobic corrosion rate of carbonsteel in compacted bentonite under simulated repositoryconditions, and characterise the corrosion behaviour, inorder to build confidence in canister lifetime predictions.

. Study the effect of the bentonite buffer on microbialactivity and the microbially influenced corrosion of carbonsteel.

. Study the effect of welding and post-weld heat treatmenton the corrosion rate of carbon steel.

This paper describes the set-up of the experiments andsummarises the results from a range of analyses that were car-ried out after 20 months of exposure.

Experimental

The experiment was installed in a vertical borehole in Opali-nus Clay. Stainless steel modules, containing the carbon steel

corrosion testpieces embedded in compacted bentonite with arange of controlled densities, were prepared under anoxicconditions and inserted into the borehole, which containednatural groundwater, and then sealed to maintain the long-term, low-temperature, anoxic conditions representative ofthose expected in a deep geological repository. The modulespermitted the free exchange of water with the host rock. Aset of 12 modules was initially installed in the borehole;they will be removed and analysed, and some will be replacedwith fresh modules, according to a planned schedule over a10-year period.

Bentonite preparation

Volclay MX80 bentonite from Wyoming, U.S.A. was usedthroughout the experiment. Four different conditions of ben-tonite were used, as follows:

. 1250 kg m−3, 1450 kg m−3 and 1550 kg m−3 compactedblocks: the compacted bentonite was provided by ClayTechnology, Sweden. A 100 mm diameter mould thatincorporated recesses for the corrosion coupons was man-ufactured. The bentonite was prepared to give a 95–99%degree of saturation using deionised water. It was placedin the mould, flushed with nitrogen and then placedunder vacuum during compaction, in order to minimisethe amount of residual oxygen present in the bentoniteat the start of the experiment. The blocks of bentonitewere then handled and stored in a nitrogen atmosphereuntil the modules were assembled.

. 1450 kg m−3, using a mixture of pellets and powder: thisgranular material was provided in the required density.

© 2017 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/),which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

CONTACT N. R. Smart nick.smart@amecfw.com Amec Foster Wheeler, Building 150, Harwell Oxford, Didcot, Oxfordshire OX11 0QB, UK

CORROSION ENGINEERING, SCIENCE AND TECHNOLOGY, 2017VOL. 52, NO. S1, 101–112https://doi.org/10.1080/1478422X.2017.1315233

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http://crossmark.crossref.org/dialog/?doi=10.1080/1478422X.2017.1315233&domain=pdfhttp://orcid.org/0000-0002-5946-4024http://orcid.org/0000-0002-7269-4019http://creativecommons.org/licenses/by-nc-nd/4.0/mailto:nick.smart@amecfw.comhttp://www.tandfonline.com

Preparation of corrosion coupons

Two sources of carbon steel test material were used to man-ufacture disc-shaped corrosion coupons with a diameter of20 mm and a thickness of 10 mm:

(i) A carbon steel cylinder with an electron-beam-welded lidwas manufactured. The steel was ASTM A694-08 F65(composition, wt-%: C 0.11; Mn 1.3; Si: 0.20; P 0.010; S0.002; Cr: 0.10; Ni 0.07; Mo 0.17; V 0.053; Nb 0.037; Febal.). A reduced pressure electron-beam welding processwas used (5 × 10−2 mbar pressure, helium overpressure,accelerating voltage 150 kV, maximum beam current255 mA, welding speed at full current 80 mm min−1)to perform a full penetration 14 cm deep weld, and apost-weld heat treatment (600°C for 4 h, then air-cooled)was applied before the material was used to prepare thetest coupons. Each coupon was identified as weld metal(WM) or base metal (BM), together with its position inthe weld and coupon number (e.g. WM1-1 or BM5-10).

(ii) Electron-beam-welded 516 Gr 70 carbon steel (compo-sition, wt-%: C 0.18; Mn 1.14; Si: 0.42; P 0.010; S <0.005; Cr: 0.02; Ni 0.009; Mo 0.002; Nb 0.04; Fe bal.) rec-tangular bars contained BM, a 25 mm weld (‘shallowweld’) and a 50 mm weld (‘deep weld’). The couponswere numbered DW, SW and DWBM for deep weld,shallow weld and deep weld BM, respectively, followedby a location code (e.g. DW4A).

All coupons were grit blasted with grade 120/220 alu-minium oxide powder to achieve a consistent surface rough-ness, Ra, of ∼0.5 µm and ultrasonically cleaned in acetone.Before use, the coupons were weighed to an accuracy of0.00001 g and photographed.

Assembly of corrosion test modules

The test modules were fabricated from stainless steel andwere 250 mm long with an external diameter of 126 mm.The sintered stainless steel filters had an external diameterof 106 mm, a wall thickness of 3 mm, a porosity of 30%and an average pore size of 18 µm. The housings and filtersfor each module were cleaned in acetone and deionisedwater and then the modules were assembled in an argon-filledglove box (

Analysis of corrosion couponsSEM examination and EDXA were carried out using a Hita-chi TM3000 SEM equipped with a Bruker X-ray analysis sys-tem. Raman spectroscopy used a Horiba JY LabRam Aramisconfocal Raman microscope, with an exciting laser wave-length of 532 nm. In order to determine the corrosion rateof the specimens, weight loss measurements were carriedout in the glove box, according to a standard practice [1]and Clarke’s solution (inhibited hydrochloric acid) wasused as the descaling agent. The weight loss was convertedto a corrosion rate given in units of µm/year.

Mineralogical and microchemical investigations ofbentoniteSamples of bentonite were removed in an argon-purged glovebox using a sharp stainless steel knife. For comparison, a‘background’ reference sample of visually ‘unaltered’ com-pacted MX-80 bentonite, sampled distant from the corrodedsteel, was taken from the bentonite block used in Module1. The methodology and approach used for the analyseswere similar to those used previously to study samples fromiron–bentonite interaction experiments [2,3].

Polished thin sections were prepared from sub-samples ofboth background MX-80 bentonite and reacted bentonitenear the corroded steel coupon. Low magnification imagesof whole thin sections were recorded by digitally scanningthe thin section using an Epson Perfection 1240U flatbedscanner equipped with a transmitted light (transparency)scanning attachment. The polished blocks and sectionswere then initially examined using a Zeiss Axioplan 2 opticalpetrographic (polarising) microscope, before being examinedin detail using BSEM. Element distributions in the bentonitematrix surrounding corroded steel coupons were studiedusing EDXA and energy-dispersive electron probe microana-lysis (EPMA).

Intact vacuum-dried fragments of the material, measuringapproximately 20 × 10 mm, were impregnated with epoxy-resin under vacuum in order to stabilise the material forpolished section preparation. They were then cut andpolished under propanol (to prevent reaction of the smectitewith water-based cutting fluids). The sections were finishedby polishing with a 0.45 µm diamond paste.

BSEM-EDXA and EDXA-EPMA analyses were carried outusing a FEI Company QUANTA 600 environmental SEM inconjunction with INCAMicroanalysis software. The polishedthin sections were coated with a thin (25 nm) layer of carbonby carbon evaporation under vacuum. X-ray elemental mapswere processed to show relative element concentrations usinga ‘rainbow colour scale’ (blue for none to red/white for high).

X-ray diffraction analysis of bentoniteIn order to study the bulk mineralogy of the samples and toprevent further oxidation of any Fe-bearing species, small(typically ∼10 mg) portions of material were rapidly removedusing a scalpel and ground to a fine powder. The powder wasthen deposited onto the surface of ‘zero-background’ siliconcrystal substrate X-ray diffraction (XRD) mounts using asingle drop of acetone to form a deposit with random crystalorientation. Such analyses were carried out to determine thenature of any non-clay minerals present in the samples andalso to determine the d060 spacing of any clay minerals pre-sent. Subsequently, the clay mineral assemblages of thesamples were studied by preparing oriented mounts. This

involved dispersing small (typically ∼10 mg) portions ofmaterial in deionised water using ultrasound treatment andno dispersant was added. The dispersions were then pipettedonto glass slip substrates and allowed to dry at roomtemperature.

XRD analysis was carried out using a PANalytical X’PertPro series diffractometer equipped with a cobalt-target tube,X’Celerator detector and operated at 45 kV and 40 mA. Therandom powder mounts were scanned from 4.5 to 85° 2θ at2.06° 2θ min−1. Diffraction data were initially analysedusing the PANalytical X’Pert HighScore Plus version 4.1 soft-ware coupled to the latest version of the International Centrefor Diffraction Data (ICDD) database. The basal XRD spa-cings of smectite-group minerals are particularly susceptibleto the prevailing humidity and temperature conditionswhen analysed. Therefore, in order to ensure constant con-ditions, air-dried oriented glass slip mounts were placed inan Anton Parr THC (controlled temperature and humidity)chamber attached to the diffractometer system and operatedat 50% relative humidity and 40°C. Samples were conditionedat these settings for 30 min before scanning from 2–35° 2θ at0.55° 2θ min−1. The mounts were then rescanned after glycol-solvation and after heating to 550°C for 2 h with the chamberset to 40°C and ambient humidity. In order to gain furtherinformation about the nature of the clay minerals presentin the sample, modelling of the

a concentration of 0.195 ng μL−1. The relatively low amountof DNA recovered can be attributed to a low microbialactivity in the borehole and/or the presence of clay particlesthat could have decreased the DNA extraction yield. TheDNA collected from the borehole sample was amplifiedwith universal bacterial 16S rRNA primers (28f 5′-GAGTTT GAT CNT GGC TCA G-3′ and 519r 5′GTN TTACNG CGG CKG CTG-3′), using 35 cycles with an annealingtemperature of 50°C. The PCR products were purified withMSB® Spin PCRapace (TRATEC Biomedical AG, Birkenfeld,Germany) before sequencing at the Research and TestingLaboratory in Lubbock (U.S.A.). After sequencing was per-formed, data analysis was carried out using QIIME.

The filtrate was used for chemical analysis. Hydrogen sul-phide, iron(II) and iron(III) were analysed within 24 h afterthe sampling, using a UV-2501PC spectrophotometer (Shi-madzu, Duisburg, Germany). Hydrogen sulphide (H2S, HS

−

and S2–) was measured using the Cline method, and Fe2+

and Fe3+ with the ferrozine assay. The filtered borehole pore-water was analysed for the following:

. pH using a Orion 3-Stars pH meter (Thermo Scientific,Waltham, MA, U.S.A.).

. Major anions and cations by ion chromatography, usingan ICS-3000 instrument (Dionex, Sunnyvale, CA,U.S.A.). For anions, 30 mM KOH was used as the eluent,with an IonPac AS11-HC column. The cations weremeasured with 40 mM of methanesulphonic acid as theeluent, with an IonPac CS16 column.

. Trace metals: Fe, Mn, Cr, Cu, Co, Ni, Zn, Sr, Si and Al wereanalysed using inductively coupled plasma mass spec-trometry on an Elan DRC II instrument (Perkin Elmer,Waltham, MA, U.S.A.).

Results

Weight loss measurements

The results of the weight loss measurements are summarised inFigure 2. On the basis of the weight loss measurements, it canbe seen that the corrosion rate increased in the following order:

Module 3, compacted block, 1550 kg m−3

specimen/bentonite interface. Peaks at ∼210 and 270–320 cm−1 may be indicative of the presence of haematite.

Spectra from other specimens confirmed the presence ofmagnetite and haematite corrosion products. Several sharpbands were identified at lower wavenumbers and these canbe attributed to an oxyhydroxide phase. In some cases, thespectra obtained from the dark material were dominated byfluorescence from the bentonite, suggesting that the darkmaterial forms the interface between corroded metal and ben-tonite and consists of a mixture of the two materials.

Petrographical observations

This section describes the results obtained for sample BM2-4(Module 1), but samples from other modules showed verysimilar results. The reacted bentonite immediately adjacentto the reacted carbon steel coupon was heavily stained to astrong reddish-brown to ochreous colour, but the steel sur-face was shiny and blackened. In thin section, the brownochreous staining surrounding the steel was seen to penetratethe bentonite matrix up to a sharply defined depth of ∼2 mmfrom the interface with the steel (Figure 5). The alterationhalo is visually similar to that observed in bentonite adjacentto corroded steel wires in previous experiments [2,3].

A series of small radial microfractures was observed tohave developed perpendicular to the bentonite/steel interface.High-resolution BSEM imaging also showed the developmentof discontinuous hairline microfractures parallel to the bento-nite/steel interface that link with the radial microfractures toform an anastomosing network (Figure 6). BSEM-EDXAobservations found no evidence for any secondary ironoxide corrosion products or other alteration products withinthese microfractures. Although the radial microfracturesextended up to 4 mm into the bentonite, most of the micro-fractures were confined within the brown iron-stained altera-tion zone.

BSEM-EDXA shows that the fabric of the bentoniteimmediately adjacent to the steel was altered within a zone10–20 µm wide (Figure 7), within which the bentonite fabric

was finer-grained and more dispersed or diffuse, with indi-vidual clay particles more difficult to differentiate underBSEM. In contrast, the matrix of the bentonite further awayfrom the steel/bentonite interface was coarser, with discretebentonite aggregate particles clearly visible. Furthermore,the altered zone displayed patches of bentonite that wereenriched with iron, as shown by the brighter backscattered

Figure 3. Photograph identifying different areas of the surface of specimen SW4A (Module 1, 1450 kg m−3), which were analysed with SEM and EDXA, as shown inthe example on the right hand side (a representative area of corroded metal surface).

Figure 4. Raman spectra for DW4A (Module 1, 1450 kg m−3).

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electron coefficient. Iron enrichment was also seen along theinterface between the steel and the bentonite. EDXA micro-chemical analysis and element distribution mapping of thebentonite/steel interface are shown in Figure 8. Mapping ofthe iron distribution confirms that the bentonite matrix isenriched at the contact with the steel. The iron concentrationis particularly high at the contact surface and decreases grada-tionally with distance but extends for at least 100 µm into the

matrix. Although the brown iron staining of the bentonite canbe seen visually to penetrate up to ∼2 mm into the bentonite(Figure 5), EDXA could not detect an enhanced concen-tration of iron beyond about 100 µm.

Calcium and sulphur were also found to be enriched in thebentonite in a zone about 10–20 µm wide, immediately adja-cent to the contact with the steel. The calcium and sulphur areclosely associated and correspond to the presence of a discretephase seen under BSEM that appears bright relative to thesmectite clay matrix. This is, therefore, interpreted to mostprobably be calcium sulphate (either gypsum (CaSO4·2H2O)or anhydrite (CaSO4)). The calcium sulphate was also foundto fill microfractures up to 10 µm wide penetrating into thebentonite from the interface or fill aggregate grain boundarycracks around component bentonite aggregate particles. Sili-con and aluminium appear to be reduced at the steel/bento-nite interface (Figure 8).

X-ray diffraction analysis

The results of XRD analyses are summarised in Tables 1 and2, for randomly mounted and oriented mounted samples,respectively.

‘Unaltered’ bentonitePowder XRD analyses indicate that the ‘unaltered’ grey ben-tonite, both distant from and close to the coupon alterationzone, was predominantly composed of montmorillonite.The montmorillonite in these regions displays an intensed001 peak with a mean spacing of 12.64 Å (range of 12.46–12.78 Å). The montmorillonite d00l intensities can also beused to gain information on Fe substitution by measuringthe scattering from the clay mineral octahedral sheet(Table 2). The intensity ratio of the d002 to the d003 peaksincreases, as the total number of electrons in the octahedralsheet increases. Newmod II-modelled profiles were producedfor the increasing Fe content categories: montmorillonitesenso stricto, non-ideal montmorillonite, Fe-rich montmoril-lonite and nontronite identified by Newman and Brown [7]and their d002/d003 was measured. Newmod II-modellingand a mean, measured d002/d003 value of 0.60 for the ‘unal-tered’ bentonite subsample suggest a montmorillonite sensostricto composition (∼0.27 Fe3+ per O20(OH)4). Peak widthmeasurements coupled with Newmod II-modelling suggesta mean defect-free distance of 9 layers (12.5 Å units) and asize range between 1 and 14 layers. A mean measured

Figure 6. BSEM photomicrograph of the bentonite/steel contact (BM2-4 couponremoved from top left corner, Module 1, 1450 kg m−3).

Figure 7. (a) BSEM photomicrograph of the section through the bentonite at the interface with the steel coupon (BM2-4 coupon has been removed from top leftcorner, Module 1, 1450 kg m−3) (b) detail of inset area in (a).

Figure 5. Transmitted light image of thin section (blue-dye epoxy-resin impreg-nated sample) through the bentonite/carbon steel interface (BM2-4 couponremoved, Module 1, 1450 kg m−3).

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Figure 8. BSEM photomicrograph of the steel/bentonite interface (Module 1, 1450 kg m−3: BM2-4), and corresponding EDXA microchemical maps of the same areafor iron, silicon, aluminium, calcium and sulphur. Colour concentration scale: red = high concentration, blue/black = low concentration.

Table 1. Summary of random orientation mount XRD analyses for samples taken from adjacent to the steel coupon.

Module, coupon

Subsample

Mineralogy

Montmorillonite

Code Descriptiond001(Å)

d060(Å)

Module 1, 1450 kg m−3,BM2-4

A ‘Unaltered’, distant fromalteration

Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’,cristobalite, calcite, gypsum

12.46 1.495

B Coupon alteration Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’,cristobalite, amphibole

12.77 1.496

C ‘Unaltered’, close toalteration

Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’,cristobalite

12.78 1.494

Module 2, 1250 kg m−3

BM1-4A ‘Unaltered’, distant from

alterationMontmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’,cristobalite, gypsum, zeolite

12.78 1.495

B Coupon alteration Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’, calcite 12.59 1.496C ‘Unaltered’, close to

alterationMontmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’, gypsum,pyrite

12.55 1.496

Module 3, 1550 kg m−3

BM2-1A ‘Unaltered’, distant from

alterationMontmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’, gypsum,pyrite

12.69 1.496

B Coupon alteration Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’,cristobalite, gypsum

12.75 1.496

C ‘Unaltered’, close toalteration

Montmorillonite, quartz, plagioclase feldspar, K-feldspar, ‘mica’, gypsum,pyrite

12.78 1.496

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d002/d003 value of 0.62 for the ‘unaltered’ bentonite closer tothe altered zone suggests a montmorillonite senso stricto com-position (∼0.30 Fe3+ per O20(OH)4), which is slightly moreferroan than the more distal ‘unaltered’ bentonite.

Corroded steel coupon-altered bentonite zoneXRD analyses of the stained bentonite in contact with the cor-roded steel coupons indicate that it had a generally similarmineralogy to the ‘unaltered’ bentonite. The montmorillonitein the coupon-altered bentonite subsamples displayed a meand001 peak spacing of 12.70 Å. The d002/d003 values for the ‘altered’bentonite subsamples range from 0.25 for Module 1 to 0.32 forModule 2 and 0.74 for Module 3. These values, together withNewmod II-modelling, suggest a montmorillonite senso strictocomposition for all three samples but a more ferruginous com-position (∼0.46 Fe3+ per O20(OH)4) for the Module 3 sample.

Porewater and microbial analyses

The results of the measurements of water activity are includedin Table 3 and a summary of the results of the microbialanalysis is presented in Figure 9 for anaerobic heterotrophicmicroorganisms, Figure 10 for aerobic heterotrophic

microorganisms and Figure 11 for SRB. The results of thechemical analysis of the porewater sampled from the boreholeare given in Table 4. These results clearly show that this bore-hole was maintained under anoxic conditions, as substantialconcentrations of reduced Fe and S were found.

The analyses of the porewater sample taken before exposureshowed that the great majority of the contributing genera(86%) fall within the Peptococcaceae family. Two genera dom-inate the community: Desulfurispora sp. and Desulfosporosinussp. The only reported member of the genus Desulfurispora isD. thermophile. This microorganism is able to reduce sulphate,sulphite, thiosulphate and elemental sulphur as the terminalelectron acceptor and to use H2/CO2 (80:20, v/v), alcohols, var-ious carboxylic acids and some sugars as electron donors. Itrepresents 70% of the retrieved sequences in this sample,while it is not detected in any other sampled boreholes withinthe Mont Terri facility. The second microorganism, related toDesulfosporosinus meridiei, is also likely to be a sulphate-

Table 2. Summary of oriented mount XRD analyses.

Module, coupon

Subsample Montmorillonite

Code Description Air-dry d001 (Å) Glycol 002/003

Module 1 1450 kg m−3, BM2-4 A ‘Unaltered’, distant from alteration 12.69 and shoulder at 14.67 0.61B Coupon alteration 14.83 and shoulder at 12.66 0.25C ‘Unaltered’, close to alteration 12.69 and shoulder at 14.60 0.64

Module 2, 1250 kg m−3 BM1-4 A ‘Unaltered’, distant from alteration 12.77 and shoulder at 14.50 0.57B Coupon alteration 12.74 and shoulder at 14.79 0.32C ‘Unaltered’, close to alteration 12.75 and shoulder at 14.40 0.53

Module 3, 1550 kg m−3 BM2-1 A ‘Unaltered’, distant from alteration 12.75 and shoulder at 14.38 0.63B Coupon alteration 12.61 and shoulder at 14.64 0.74C ‘Unaltered’, close to alteration 12.64 and shoulder at 14.53 0.69

Key: Glycol 002/003 = XRD d002/d003 peak intensity ratio from glycol-solvated trace, for samples taken from adjacent to the steel coupon.

Table 3. Target bentonite densities, moisture per cent and measured wateractivity for different modules.

Module

Target dry bentonitedensity beforeemplacement(kg m−3)

Per centmoisturea

Moisture %dry

weightbWater activity,

aw1 1450 24.3 32.1 0.956 (±0.0104)2 1250 28.9 40.6 0.984 (±0.027)3 1550 22.6 29.3 0.950 (±0.014)aWater weight × 100/weight of original sample.bWater weight × 100/weight of dried sample.

Figure 9. Summary of cultivation results for anaerobic heterotrophic microor-ganisms plotted as a function of measured moisture content (as % of dryweight).

Figure 10. Summary of cultivation results for aerobic heterotrophic microorgan-isms plotted as a function of measured moisture content (as % of dry weight).

Figure 11. Summary of cultivation results for SRB.

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reducing bacterium. It can use sulphate, sulphite, thiosulphate,elemental sulphur, DMSO and Fe(III) as electron acceptors inthe presence of lactate. In addition, it can use H2/CO2, acetate,pyruvate and a number of organic acids as electron donors. Adetailed analysis of the microbial ecosystem in the test boreholehas been reported elsewhere [8].

The analysis of the porewater sample taken after exposureshowed a significant change in the microbial community witha large increase in the number of Pseudomonas (87%).

Discussion

Corrosion rates

The analyses of the corrosion coupons have demonstrated thatsurface damage of the carbon steel had developed due to cor-rosion in the bentonite over a period of 1.72 years. The corrosionrates have been derived from weight loss measurements andrange from 1.21 to 3.38 µm/year. It is apparent that both bento-nite density and the initial form of the bentonite have a signifi-cant influence on the rate of corrosion, across all materials. Thecorrosion rates for all specimens in the module with the greatestblock density (Module 3, 1550 kg m−3) were lower than those inthe other modules with lower bentonite density. The influence ofthe initial bentonite form can also be seen, as the corrosion ratewas lower for all specimens in the modules containing com-pacted bentonite blocks compared to the specimens in the mod-ule containing pelletised bentonite. This is despite the fact thatthe bentonite density of one of the modules containing blockbentonite (Module 2, 1250 kg m−3) was lower than that of themodule containing pelletised bentonite (Module 1,1450 kg m−3). In contrast to the bentonite density, the effectsof the steel composition and the presence or absence of weldson the corrosion rate are minor.

Composition and morphology of corrosion product

Raman spectroscopy analysis of the specimens indicates thatthe corrosion product was predominantly sub-stoichiometric

magnetite or a mixed phase spinel (e.g. Fe3−xMxO4) and alsothat some haematite may have been present, which may havebeen a residue from the pre-test surface preparation.

SEM analysis shows a complex corrosion product that dif-fered slightly between materials and also across the surface ofthe materials, while the presence of aluminium and silicon ona number of specimens indicates the presence of bentoniteremaining on the specimens.

Analysis of bentonite–steel interaction

XRD analysisXRD analysis revealed that the ‘unaltered’, grey bentoniteboth close to and distant from the coupon alteration had asimilar composition to that previously described for MX-80[9]. The oriented XRD mount data, obtained under 40°Cand 50%RH conditions, generally indicate a relatively consist-ent ∼12.7 Å spacing of the montmorillonite air-dry d001 witha subordinate shoulder peak at ∼14.6 Å in both the‘unaltered’ and ‘altered’ bentonite samples. Exceptionally,the air-dry d001 spacings are reversed in the Module 1 samplewhere the most intense air-dry d001 is at ∼14.8 Å with ashoulder at ∼12.7 Å (Table 2). As these oriented mountswere analysed under constant humidity and temperature con-ditions, the difference in d001 spacings must be the result of achange in cation chemistry in the montmorillonite. The∼12.7 Å spacing suggests that the predominant interlayercation is monovalent (Na/K), while the ∼14.6 Å spacingsuggests divalent (Ca/Mg) cations. The reason that the XRDobservations should indicate a preponderance of divalentcations in Module 1 is unclear. In the samples from Module2 and Module 3, the ‘altered’ bentonite samples show a mean,air-dry d001 spacing of 12.68 Å with a mean, shoulder spacingof 14.72 Å, suggesting the reverse situation where monovalent(Na/K) cations predominate over divalent (Ca/Mg) cations.The mineralogical alteration revealed by detailed petro-graphic analysis of Module 1 suggests that it is broadly similarto that observed in Module 2 and Module 3, although no cal-cium mineralisation was observed in the latter.

The mean d002/d003 value for the montmorillonite in the‘unaltered’ bentonite ‘A’ and ‘C’ subsamples (see Table 2for definition of position of subsamples) of 0.61 (∼0.30 Fe3+

per O20(OH)4) compares to values of 0.25 (Module 1), 0.32(Module 2) and 0.74 (Module 3) obtained for the ‘altered’bentonite ‘B’ subsamples. Newmod II-modelling suggeststhat the montmorillonite in the coupon-altered subsamplesfrom Module 1 and Module 2 had a Fe-depleted composition(∼0 Fe3+ per O20(OH)4) compared to that found in the ‘unal-tered’ samples. Conversely, the d002/d003 values and NewmodII-modelling indicates a more Fe-rich composition (∼0.46 Fe3+

per O20(OH)4) for the montmorillonite in the coupon-alteredsubsample from Module 3. Such an increase may suggest anincrease in the total number of electrons in the octahedralsheet of the ‘altered’ bentonite, possibly as a result of Fe sub-stitution. This might indicate that Fe2+ or Fe3+ from the cor-roding steel had displaced interlayer cations in the smectite,and this had subsequently converted to form an Fe-rich octa-hedral layer. This would have the overall effect of increasingthe apparent Fe-substitution within the octahedral layer ofthe bulk of the smectite, and might provide an alternativeexplanation for the increase in d002/d003 values for the mon-tmorillonite component in the ‘altered’ bentonite in thisexperiment.

Table 4. Composition of the filtered porewater sampled from the borehole.

Component Concentration Unit

Fe(II) 76.3a μMH2S 7.4 μMCo

Petrographic and microchemical observationsThe bentonite in Module 1 (1450 kg m−3) and Module 3(1550 kg m−3) showed significant alteration of the clay fabricimmediately adjacent to the corroding steel. This altered fab-ric layer was not obvious in Module 2 (1250 kg m−3),suggesting that there may be an effect of density on thecharacteristics of the altered layer.

The ‘iron-stained’ region of the bentonite appeared todisplay a subtly different behaviour to the background ben-tonite further away from the steel. The altered layer typicallydisplayed the ready development of shrinkage cracks thatwere orientated radially and concentrically to the bento-nite/steel interface. Most of these microfractures were unmi-neralised by secondary reaction products and they areconsidered to result from drying of the bentonite duringthin section preparation. However, these shrinkage crackswere less obvious in the unaltered bentonite, which suggeststhat the iron-stained altered bentonite may have subtlydifferent shrinkage and/or swelling properties compared tothat of the original bentonite.

In some samples, somemicrofractures were observed to dis-play walls enriched by iron. This suggests that some of thesefeatures are not artefacts of sample preparation and may actu-ally have formed during corrosion of the steel, providing path-ways along which the iron migrated away from the corrodingsteel. Much of the iron staining and iron enrichment also delin-eates the original boundaries of the bentonite aggregate path-ways, which suggests these particle boundaries providedpreferential pathways for pore fluid migration, potentiallyalso including hydrogen, through bentonite.

Calcium showed a marked enrichment in the bentonitematrix immediately adjacent to the corroding steel in Module1 and Module 2. In the case of Module 1, the calcium enrich-ment was closely associated with an increased concentrationof sulphur, which suggests that calcium sulphate (possiblygypsum or anhydrite) had probably precipitated. The pres-ence of calcite and gypsum was confirmed by the XRD analy-sis. In Module 2, the calcium concentration is associated withthe precipitation of rhombohedral microcrystals of calcite.These observations closely resemble those found in previousexperiments [2,3], which indicated that subordinate Ca2+ ionsin the exchangeable cation sites of smectite in the MX-80 ben-tonite migrate towards the corroding steel surface, leading tothe precipitation of calcite and aragonite adjacent to the cor-roding metal surface. It seems likely that the concentration ofcalcium observed around the corroding steel coupons in thepresent experiment represents the early stages of a similarprocess. Module 3 appeared to be different to the other twomodules. No enrichment of calcium or sulphur was observedin the bentonite adjacent to the corroding steel in this exper-iment and no evidence was found for either calcite or calciumsulphate precipitation.

Comparison with laboratory corrosion experiments

The results from the in situ Mont Terri experiment can becompared to those from a concurrent laboratory programmeto measure the long-term corrosion rate of carbon steel [10],by measuring the rate of gas evolution. It should be noted thatthe environmental conditions for the Mont Terri specimensdiffer significantly from those used in the laboratory exper-iments, particularly with regard to the temperature of theMont Terri experiment which is approximately 15°C

compared to 60°C for the laboratory experiments, as well asthe presence of a microbial population characteristic of awater-filled borehole in Opalinus Clay rock.

Corrosion rates derived from the monitoring of hydrogenevolution in the laboratory, assuming the formation of Fe(OH)2 and Fe3O4 over durations similar to those of theMont Terri experiment, were similar. The corrosion rate oflaboratory-based experiments in bentonite saturated withsynthetic Opalinus Clay porewater over a comparableexposure period ranges from 2.65 to 3.92 µm/year, which isslightly higher than the values measured in situ (1.21–3.38 µm/year). Such corrosion rates are in general agreementwith rates reported in the literature for similar exposure con-ditions and durations [11].

Comparison of the Raman analysis carried out on speci-mens from the laboratory experiments and the in situ exper-iments shows that the corrosion product formed in the MontTerri experiments was very similar to that produced in thelaboratory experiments.

Microbial analysis

The analyses of the porewater indicate that the conditions inthis borehole (anoxic, reducing) are favourable for the growthof sulphate-reducing bacteria. The increase in the contri-bution of Pseudomonas to the community during theexposure period is largely accounted for by the decrease ina member of the Peptococcaceae family, which pertain tothe genus Desulfotomaculum. Based on the original metage-nomic study, it was determined that this organism wascapable of heterotrophic sulphate reduction, probably usingorganic matter from the breakdown of microbial necromassas well as using H2 as an electron donor [8]. The Pseudomo-nas species was thought to use hydrolysed organic matterfrom the Opalinus Clay. Its emergence as the dominantorganism suggests that the availability of H2 may havedecreased and organisms dependent on that electron donor(such as Desulfotomaculum spp.) may have given way toorganisms presumed to utilise the refractory organic matterin Opalinus Clay (such as Pseudomonas sp.).

When comparing the number of anaerobic heterotrophs(i.e. bacteria that use organic compounds as a source of car-bon and that grow in the absence of oxygen) among the ben-tonite samples from the three modules, it appears that, asexpected, Module 2, which had the lowest target dry density,exhibited the highest numbers of anaerobic heterotrophs.Modules 1 and 3, in contrast, both showed lower numbers.There is more variability in the anaerobic heterotroph num-bers in Module 1, with the next lowest density after Module 2,than in Module 3. This suggests that the density of1450 kg m−3 represents a near threshold for MX80 bentoniteinhibition of microbial activity. The threshold is probably inthe range between 1250 and 1450 kg m−3. This is in contrastto other reports for other bentonites, where varying Febexbentonite densities in the range of 1260–1620 kg m−3 resultedin very limited impact on the numbers of heterotrophicanaerobic bacteria and the threshold of aerobic bacteria wasfound [12] to be around 1500 kg m−3.

Aerobic heterotrophic bacteria, which use oxygen as anelectron acceptor, exhibit a similar behaviour as the anae-robes, with the difference that cell counts were typicallygreater for the former. There is also less variability in thecell numbers among samples from Module 1, in comparison

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with anaerobic heterotrophs. This is probably due to the het-erogeneity of the module and to the fact that individualsamples were distinct for anaerobic and aerobic enumeration.While Module 2 exhibited the largest aerobic heterotroph cellcounts, Modules 1 and 3 revealed cell counts within an orderof magnitude of each other, with Module 3 showing the largercounts. However, given the limited dynamic range of this typeof analysis, it is difficult to draw conclusions from this result.

The sampling was carried out in such a way as to allowevaluation of the impact of location of individual sampleson cell counts. There was no clear systematic trend observedfrom the data, but it appeared that the exterior location typi-cally supported higher cell numbers than the interior locationfor plate counts. The lack of a clear trend suggests that micro-organisms present in the bentonite rather than microorgan-isms entering from the borehole were present and viable inthe modules, despite the fact that the filter pore size waslarge enough (18 μm) to allow entry of microorganisms.

SRB numbers were evaluated by theMPNmethod. The SRBcounts were generally higher in the interior than the exterior ofeach subsample, particularly for Module 1. There was nospecific trend as a function of location in the subsample forthe other two modules, suggesting that the SRB originatedfrom the bentonite rather than the porewater. Surprisingly,the number of SRB was slightly higher in Module 3 (the mod-ule with the highest dry density, 1550 kg m−3) than in theother two modules. The most likely explanation for this behav-iour is the presence of spore-forming SRB and their unevendistribution in the bentonite. The SRB numbers were generallyhigher than those obtained for Febex [12].

The analyses conducted here provide only information aboutthe viability of microorganisms in bentonite. The finding, whichis in line with previous work [13], shows that viable microorgan-isms are present in bentonite and, depending on the density ofthe clay in the module, a fraction of those microorganismsremain viable. As expected, at higher densities, a smaller fractionsurvives, while at lower densities, a higher fraction survives.However, it remains unknown whether microbial activity isextant in the bentonite. The only evidence of this activity waspresented in the Engineered Barrier experiment [12]. The ques-tion of whether a specific bentonite density might precludemicrobial survival has been investigated by several groups andthe results do not provide a clear density threshold. For Febex[12], the threshold is around 1600 kg m−3, whereas for MX-80 [12], the threshold appears to be closer to 1500 kg m−3. Itwould appear from this work that the threshold for MX80may be between 1250 and 1450 kg m−3.

Conclusions

An in situ corrosion experiment has been set up and operatedin the Mont Terri Underground Research Laboratory andthree modules have been removed for detailed analysis after∼20 months of exposure. The main conclusions from thisanalysis are as follows:

(i) Greater bentonite density (for the same bentoniteform) significantly reduced the extent of corrosionfor all materials.

(ii) The corrosion rate of all materials is lower whereblocks of compacted bentonite are used in comparisonto pellet bentonite, even if the dry density of the blockbentonite is lower than the pellet bentonite.

(iii) There was little difference in corrosion rate betweenthe WM and BM samples for both steel types, for thesame bentonite density.

(iv) A complex corrosion product, which is predominantlymagnetite, has been identified.

(v) A narrow region of discoloured bentonite up to 2 mmwide exists around the surface of the steel, which prob-ably has subtly different physical properties to the bulkmaterial but contained no evidence for any secondaryiron oxide corrosion products or other alterationproducts.

(vi) Increased Fe content was identified only within100 µm of the steel surface. Within a ∼20 µm narrowalteration zone next to the metal, the bentonite wasfiner-grained and more dispersed.

(vii) The corrosion rates observed in this work are similar tothose seen for tests carried out ex situ at higher temp-eratures, while the corrosion product observed in bothexperiments was also similar.

(viii) The bentonite with the lowest density exhibited thehighest numbers of anaerobic and aerobic hetero-trophs, while SRB were most active on the interior ofthe bentonite, suggesting that the SRB originatedfrom the bentonite rather than the porewater. Itwould appear from this work that the threshold den-sity for microbial activity in MX80 may be between1250 and 1450 kg m−3.

Acknowledgements

The authors gratefully acknowledge Oxford University MaterialsCharacterisation Service for providing analytical data, TWI (The Weld-ing Institute, UK), for provision of welded carbon steel material, ClayTechnology (Sweden), for providing the compacted bentonite samples,the Swisstopo team and Solexperts (Switzerland) for on-site work atthe Mont Terri Rock Laboratory, Special Techniques (UK), for manufac-ture of the transfer flasks, Drs Anthony Milodowski and Simon Kemp(BGS, UK), for the mineralogical analysis of the bentonite, and AtomicEnergy of Canada Limited (Canada) for measurements of water activity.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

The authors gratefully acknowledge Nagra (Switzerland), Andra(France) and NWMO (Canada) for their financial support.

ORCID

A. P. Rance http://orcid.org/0000-0002-5946-4024D. J. Nixon http://orcid.org/0000-0002-7269-4019

References

[1] Standard Practice for Preparing, Cleaning, and evaluating cor-rosion test specimens, G1, ASTM, Philadelphia, PA, USA, 2011.

[2] Milodowski AE, Cave MR, Kemp SJ, et al. Mineralogical investi-gations of the interaction between iron corrosion products andbentonite from the NF-PRO Experiments (Phase 1); 2009.(Report TR-09-02, SKB).

[3] Milodowski AE, Cave MR, Kemp SJ, et al. Mineralogical investi-gations of the interaction between iron corrosion products andbentonite from the NF-PRO Experiments (Phase 2); 2009.(Report TR-09-03, SKB).

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http://orcid.org/0000-0002-5946-4024http://orcid.org/0000-0002-7269-4019

[4] Reynolds RC Jr, Reynolds III RC. Description of Newmod II™.The calculation of one dimensional X-ray diffraction patternsof mixed layered clay minerals. Crofton (MD): R.C. ReynoldsIII; 2013.

[5] Reasoner DL, Geldreich EE. A new medium for the enumerationand subculture of bacteria from potable water. Appl EnvironMicrobiol. 1985;1:1–7.

[6] Atlas RM. In: Parks LC, editor. Handbook of microbiologicalmedia. Washington (DC): CRC Press Inc; 1993.

[7] Newman ACD, Brown G. The chemical constitution of clays. In:Newman ACD, editor. Chemistry of clays and clay minerals.Mineralogical Society Monograph 6. Essex: Longman Technicaland Scientific; 1987. p. 1–128.

[8] Bagnoud A, de Bruijn I, Andersson A, et al. A minimalisticmicrobial food web in an excavated subsurface clay rock. FEMSMicrobiol Ecol. 2016;92(1):fiv138.

[9] Madsen FT. Clay mineralogical investigations related to nuclearwaste disposal. Clay Miner. 1998;33:109–129.

[10] Smart NR, Reddy B, Rance AP, et al. The anaerobic corrosion ofcarbon steel in saturated compacted bentonite in the Swiss reposi-tory concept. Corros Eng Sci Technol. 2017.

[11] King F. Corrosion of carbon steel under anaerobic conditions in arepository for SF and HLW in Opalinus Clay; October 2008.(Nagra Technical Report 08-12).

[12] Stroes-Gascoyne S, Frutschi M, Hamon C, et al. Microbiologicalanalysis of samples from the engineered barrier experiment atMont Terri Rock Laboratory, Mont Terri Technical Note 2013-56. January 2013.

[13] Stroes-Gascoyne S, Hamon CJ, Maak P, et al. The effects of thephysical properties of highly compacted smectitic clay (bentonite)on the culturability of indigenous microorganisms. Appl Clay Sci.2010;47:155–162.

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AbstractIntroductionExperimentalBentonite preparationPreparation of corrosion couponsAssembly of corrosion test modulesExposure

Removal and analysisRemoval of test modulesAnalysis of corrosion coupons and bentoniteAnalysis of corrosion couponsMineralogical and microchemical investigations of bentoniteX-ray diffraction analysis of bentonitePorewater and microbial analyses

ResultsWeight loss measurementsSEM and EDXA analysisRaman spectroscopyPetrographical observationsX-ray diffraction analysis‘Unaltered’ bentoniteCorroded steel coupon-altered bentonite zone

Porewater and microbial analyses

DiscussionCorrosion ratesComposition and morphology of corrosion productAnalysis of bentonite–steel interactionXRD analysisPetrographic and microchemical observations

Comparison with laboratory corrosion experimentsMicrobial analysis

ConclusionsAcknowledgementsDisclosure statementORCIDReferences