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Real-time corrosion monitoring system under in situ conditions of crystalline groundwater

Leena CARPÉN1, Pauliina RAJALA2, Tuomo KINNUNEN3

1VTT Technical Research Centre of Finland, Materials Performance, Kemistintie 3, FI-02044

VTT, Finland, leena.carpen@vtt.fi 2VTT Technical Research Centre of Finland, Materials Performance, Kemistintie 3, FI-02044

VTT, Finland, pauliina.rajala@vtt.fi 3VTT Technical Research Centre of Finland, Materials Performance, Kemistintie 3, FI-02044

VTT, Finland, tuomo.kinnunen@vtt.fi Abstract An in situ corrosion monitoring system was developed to monitor real-time corrosion rates of two stainless steel grades (EN 1.4301 and EN 1.4404). The corrosion monitoring system relied on linear polarization resistant (LPR) method. Besides the electrochemical coupons, the monitoring system contained gravimetric specimens to confirm the results of electrochemical measurements. The results of this study can be used when evaluating risks of the microbially induced corrosion of metallic materials in the underground radioactive waste repository. Low and intermediate level radioactive waste (LLW and ILW) is produced during the operation, maintenance and decommissioning of nuclear power plants. The metallic waste includes pipes, valves, filters and tools, the majority of which is made of various stainless steels and carbon steel. In Finland at Olkiluoto nuclear power plant site, the LLW and ILW have been disposed of in a geological repository located at the depth of 60 of 95 m below surface since 1992. Surprisingly, higher corrosion rates were detected in the molybdenum contained stainless steel type EN 1.4404 compared to the lower alloyed type EN 1.4301. This indicates possible role of microbial activity in corrosion. Microbial community from groundwater has been earlier analyzed and a vast bacterial and archaeal community was discovered. The microbial community involved microbial groups linked frequently to MIC. Here, thick biofilm was detected on surfaces of both steel grades. Results during the exposure of two years are presented and discussed here. Keywords: stainless steel; nuclear waste repository; MIC Introduction Contaminated low and intermediate level radioactive waste (LLW and ILW) is produced during the operation, maintenance and decommissioning of nuclear power plants. The metallic waste includes pipes, valves, filters and tools, the majority of which is made of various stainless steels and carbon steel. In Finland at Olkiluoto nuclear power plant site, the LLW and ILW has been since 1992 transferred to a geological repository (at the depth of 60 to 95 m below surface) where it will be disposed into concrete silos. The groundwater at the repository depth is anoxic and brackish, with the current inflow into the silos being 39 L min-1 [1]. A durability of hundreds of years is required for a repository system containing low and intermediate waste. Knowledge of the evolution and changes occurring in the repository site are essential when estimating the long-term safety of the final repository. The changes in microbial and chemical processes resulting from, or affecting, the degradation of waste placed in the repository may significantly influence radionuclide speciation [2]. In anoxic water, the corrosion rate of carbon and stainless steels is typically low, unless microbial activity exists in the surrounding environment. It is known that there are a variety of microbes; bacteria and archaea at the bedrock ground water of the disposal site [3]. Microorganisms may facilitate the corrosion

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processes that challenge the long-term stability of the repository. Microorganisms may develop biofilms on the surfaces of metallic materials and microbial activity within the formed biofilms may alter the conditions under the biofilm or modify the kinetics of cathodic and/or anodic corrosion reactions as well as the chemistry of any protective layer. Microbial community from groundwater at this site has been earlier analyzed and a vast bacterial and archaeal community was discovered [3]. The microbial community involved microbial groups linked frequently to MIC [4,5]. Because of corrosion of metallic waste, the repository geochemistry and transport of radionuclides may be altered and radioactive nuclides can be released into groundwater and further transferred to areas near the repository. Attached bacteria and biofilm may adsorb radionuclides and immobilize them, while planktonic bacteria with adsorbed radionuclides may mobilize the radionuclides [6]. An in situ corrosion monitoring system was developed to monitor real-time corrosion rates of two stainless steel grades (EN 1.4301 and EN 1.4404). The corrosion monitoring system relied on linear polarization resistant (LPR) method. Preliminary results of two years exposure are presented here. Experimental set-up An in situ corrosion monitoring system was developed to monitor open circuit potential and corrosion rates of two stainless steel grades in real-time (EN 1.4301 and EN 1.4404), Table 1. The corrosion monitoring system relied on linear polarization resistant (LPR) method. Each monitoring system had three square-shaped specimens of size 10 mm x 10 mm for electrochemical measurements and six rectangular specimens of size of 71 mm x 27 mm for gravimetric measurements, surface characterization and microbiological studies. The surfaces of the latter specimens were in as received condition while the specimens for electrochemical measurements were ground to 600 grit finish. Prior to the experiments, all specimens were cleaned with distilled water and ethanol and finally air-dried. The coupons for gravimetric measurements were weighed to the accuracy of 0.01 mg (Metler AT261 delta range), after which they were sterilized at 160°C for 3 hours. Electrochemical coupons were sterilized in 70 % ethanol. For the electrochemical measurements two platinum electrodes were assembled in each of monitoring system to work as counter electrode and for monitoring the redox-potential of the environment. A commercial AgAgCl reference electrode (TF0221, Metal Samples Company) was used as a reference electrode for the open circuit potential monitoring. Table 1. Composition of the test materials.

Steel C Si Mn S P Cr Ni Mo Cu Al W V Ti Co Nb

EN1.4301

0.035 0.44 1.54 0.003 0.021 18.2 8.38 0.09 0.17 <0.003 0.02 0.055 0.002 0.16 0.005

EN1.4404

0.030 0.54 1.47 0.002 0.026 16.8 10.4 2.54 0.24 0.003 0.03 0.050 0.022 0.16 0.005

Figure 1 describes the assembly of the monitoring rack.

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Figure 1. Schematic representation of the monitoring rack (left), assembled rack for electrochemical coupons (middle) and coupons for gravimetric and surface analysis (right). Monitoring racks as well as racks for gravimetric and microbiological coupons were installed in two drill holes at the disposal site of ILW and LLW (Teollisuuden Voima Oyj, VLJ-cave, Olkiluoto, western Finland, 61°14′13″N 21°26′27″E) for two years’ time. The chemical composition of the ground water of each drill hole was analyzed while assembling the racks and again when removing the devices from drill holes after two years. Electrochemical monitoring Open circuit potential (OCP) of one electrochemical coupon of each material was recorded regularly in a data logger. The second electrochemical coupon was used as a working electrode and third coupon as a reference electrode for the linear polarization measurements (LPR). LPR measurements were performed from -20 mV to 20 mV with respect to EOCP using the scan rate of 0.167 mV s-1. Polarization resistance, Rp, is inversely proportional to the corrosion current as follows [7]: Rp = B (1) icorr Where icorr is the corrosion current and B is the proportionality constant obtained by: B = ba bc (2) 2.3(ba +bc) Here, ba and bc are Tafel constants defined on the basis of earlier Tafel measurements conducted with same materials in ground water environment of laboratory experiments [4,5]. To verify the used Tafel constants Tafel plots were measured four times during the second year using small polarization of ± 30 mV with respect to OCP to avoid changing the surface properties too much since these coupons were used also for LPR measurements. Corrosion rate was calculated from the corrosion current according to Faraday’s law. All the measurements were performed with Reference 600TM potentiostat (Gamry Instruments) using DC105 DC Corrosion software. Counter electrode was platinum.

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Gravimetric and surface characterization The monitoring racks were removed from the drill holes and all the specimens were detached immediately after the removal at the site. The gravimetric coupons were immersed quickly in ethanol, air dried and stored in a desiccator before weight measurements. Each coupon was photograph and chosen coupons were examined using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). For the gravimetric analysis the specimens were weighed, cleaned with brush and pickled according to the standard ISO 8407/C3.1. To determine the mass loss of the base metal when removing the corrosion products, a replicate non-exposed control specimen was cleaned by the same procedure being used on the test specimens. The mass losses were determined and from the mass losses the average corrosion rates (µm/a) were calculated as follows:

Corrosion rate = (K ´ W) (3) (A ´ T ´ D)

K = constant = 0.365 ´ 104 W = mass loss in mg T = time in exposure in days A = area in cm2 D = density in g (cm3)-1 After the weight loss determinations, the coupons were examined under a stereomicroscope to verify the type of possible corrosion. Microbiological studies The coupons for FE-SEM analysis to detect the biofilm were immersed in phosphate (0.1 M, pH 7.2) buffered 2.5% glutaraldehyde for fixation of presumed biofilm. After 16 h the coupons were rinsed with phosphate buffer three times. Dehydration was carried out with ethanol series, followed by hexamethyldisilazane. Results and discussion The recorded open circuit potentials of stainless steels in each drill hole are shown in Fig. 2 and 3.

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Figure 2. OCP of two stainless steels (EN 1.4301(304) and 1.4404 (316)) and platinum (Pt) in a drill hole A.

Figure 3. OCP of two stainless steels (EN 1.4301(304) and 1.4404 (316)) and platinum (Pt) in a drill hole B.

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From the OCP measurements it can be seen that in both drill holes all the potentials had decreased to -430…-460 (drill hole A) and -475…-500 (B) during the first 20 days of exposure. After that the potentials started to increase and again decrease and in the drill hole A being quite stable and close to -450 mV between days 200 and 550 except for one month period around days 300 where all the potentials dropped and that of stainless steel EN 1.4301 dropped most (Fig. 2). After 550 days all the potentials dropped again and the lowest values being around -550 mV vs. Ag/AgCl. In drill hole B, the potentials of both grades of stainless steels as well as that of platinum were more stable and closer to each other during the whole exposure period. After the decrease noticed in the beginning there was and increase and then again small decrease and the potentials stayed around -430 … -450 mV vs Ag/AgCl during most of the exposure time after 150 days. There was an increase of potentials around the day 600 but the potentials returned back to the previous levels quite fast, Fig. 3. The corrosion rates calculated from the LPR measurements are shown in Fig. 4. In the beginning of the exposure time, the corrosion rates in both stainless steels in both drill holes were similar and very small; less than 1 µm a-1. But after 200 days of exposure the corrosion rates of grade EN 1.4404 (316) started to increase in both drill holes while those of less molybdenum containing grade EN 1.4301 (304) specimens in both drill holes stayed at very low levels (less than 0.5 µm a-1) until there was a notable increase in drill hole A after 600 days of exposure, Fig. 4. This increase was simultaneous with an increase of corrosion rate seen with stainless steel grade 1.4301 in the same drill hole. The highest occasional corrosion rate of grade 1.4404 was 9 µm a-1 in drill hole A and around 3 µm a-1 in drill hole B accordingly. However, most of the exposure time the corrosion rates in drill hole B had been higher for both stainless steels except the last hundred days. The observations from the electrochemical measurements correspond well with our earlier results obtained from shorter laboratory scale experiments simulating the drill hole conditions [4].

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Fig. 4. Corrosion rates of stainless steels based on the linear polarization measurements in two drill holes at the disposal site of low and intermediate level nuclear waste. Cumulative corrosion rates were determined by weight loss of coupons. The weight losses of both types of stainless steels were below the detection limit, i.e., within the detection limit of the scales. After the exposure, the surfaces of chosen specimens were studied with SEM and deposits on the surfaces were analyzed with EDS. By visual observation, the stainless steel specimens were quite clean and only small areas of deposits were detected. With EDS high amounts of sulfur (S, up to 24.2 w-%) was detected on the surface of stainless steel EN 1.4404 immersed in drill hole B. Occasionally also manganese (Mn) was enriched in the deposits of type 1.4404 (up to 4.3 w-% in drill hole B and 6.5 w-% in A accordingly). In drill hole B exposed specimen of type 1.4301 contained locally sulfur (up to 4.9 w-%). The weight loss coupons were also examined with SEM to detect possible indications of pitting. Since these specimens had the surface as received, it is rather difficult to distinguish small pits from the small defects naturally occurring in as received surfaces. However, some indications of incipient possible pits were detected in type EN 1.4404 stainless steels, Fig. 5. Despite being seemingly clean after exposure, the SEM examination of surfaces revealed biofilm on the surfaces, Fig. 6. Biofilm formed unevenly distributed patches on surfaces of both stainless steel grades. Microbial colonization was detected especially on grain boundaries and other places were surface was discontinuous. Visually the microbial community in both drill holes and on both stainless steel grades was similar. The diversity and number of microbial community remains to be clarified using molecular biological methods.

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The water in the drill holes differ somehow each other. Drill hole B contains more chloride (1160-808 mg/L, measured during the exposure), sulfate (194-163 mg/L) and calcium (170 -115 mg/L) than water in drill hole A where chloride varied from 380 to 762 mg/L, sulfate 117-142 and calcium from 48 to 94 mg/L. The amount of sulfide was ≤ 0.1 mg/ L in both drill holes and pH varied from 7.8 to 8.2 (drill hole A) and 7.8 to 8.0 (drill hole B).

Figure 5. Incipient low pits in stainless steel grade EN 1.4404 (316) exposed in a) drill hole A and b) drill hole B.

Figure 6. Microorganisms colonizing surfaces of stainless steel EN 1.4301 (304) and EN 1.4404 (316) Summary An in situ corrosion monitoring system was developed to monitor open circuit potential and corrosion rates of two stainless steel grades in real-time (EN 1.4301 and EN 1.4404). Open

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circuit potentials and linear polarization resistance were monitored during the two years exposure. The open circuit potentials of both stainless steels and platinum showed that the conditions in both drill holes are reducing. The conditions seem to be more stable in the drill hole B compared to that of A. Corrosion rates calculated from the LPR measurements were low most of the exposure time. However, in drill hole A these instantaneous corrosion rates notably increased around 600 days of exposure even though before that the corrosion rates in drill hole B had been higher. Surprisingly, the instantaneous corrosion rates of stainless steel EN 1.4404 (316) were higher than those of stainless steel EN 1.4301 (304). Cumulative corrosion rates calculated from the weight loss coupons were below the detection limits in all cases. Even though no notable general corrosion could be detected, some incipient pitting could be seen in stainless steel type EN 1.4404 coupons. It should be noted that to understand the corrosion and related processes during the whole repository timescale of several hundreds of years, longer experiment times could be used. References [1] Nuclear Waste Management of the Olkiluoto and Loviisa Power plants, 2010. [2] J. Small, M. Nykyri, M. Helin, U. Hovi, T. Sarlin, and M. Itävaara, Appl.

Geochemistry. 23 (2008) 1383–1418. [3] P. Rajala, M. Bomberg, M. Vepsäläinen, and L. Carpén, , Biofouling. 33 (2017). [4] P. Rajala, E. Huttunen-Saarivirta, M. Bomberg, and L. Carpén, Corrosion2017 C2017-

9359. [5] L. Carpén, P. Rajala, M. Vepsäläinen, M. Bomberg, in: Eurocorr, 2013: p. O-1589. [6] C. Anderson, A. Johnsson, H. Moll, and K. Pedersen, Geomicrobiol. J. 28 (2011) 540–

561. [7] R. Winston Revie, ed., Uhlig’s corrosion handbook, 3rd Editio, Wiley, 2011.