The effect of reaction product layers on copper corrosion in repository conditions

Transcription

1 The effect of reaction product layers on copper corrosion in repository conditions Jari Aromaa, Alexander Chernyaev, Vesa Lindroos, Atte Tenitz Aalto University Hydrometallurgy and Corrosion Research Group

2 Contents Introduction Environments and corrosion rates Copper corrosion mechanisms Oxide films and corrosion Corrosion tests Copper corrosion products Characterization tests Summary 2

3 Introduction The interior of the final disposal canister is made of cast iron to resist the mechanical stresses. The outer part of the canister is made of nominally 49 mm thick copper. The purpose of copper is to protect the cast iron insert and the fuel assemblies from the groundwater. anister 3

4 Introduction Canister ensures containment of the spent nuclear fuel, safety depends on mechanical strength of the cast iron insert and corrosion resistance of copper. Buffer contributes to mechanical, geochemical and hydrogeological conditions that are predictable and favourable to the canister, and protect canisters from external processes that could compromise the safety of complete containment. Deposition tunnel backfill contributes to favourable and predictable mechanical, geochemical and hydrogeological conditions for the buffer and canisters. Source: Posiva Report

5 Introduction Canister material is phosphorus-alloyed oxygen-free copper with ppm P. Nominal thickness of cylinder is 49 mm, bottom and lid 50 mm. Outer surface defects accepted if 35 mm copper thickness remains tons of copper for each canister. Outer surface area m 2. Casting, hot forming and machining of tube, bottom and lid. Electron beam welding (EBW) or friction stir welding (FSW). 5

6 Introduction The alternative manufacturing methods are pierce and draw, and extrusion. Starting from up to 16 ton billet. The pierce and draw process makes an integral base and only the lid requires welding. Both the lid and base must be welded if the copper tube is manufactured by extrusion. Source: Posiva Report

7 Introduction From the perspective of corrosion behaviour, the important characteristics of the initial state of the canister include: Nature and thickness of the surface film (oxide) on the copper overpack Nature and concentration of the surface contaminants Canister dimensions (corrosion allowance) and mechanical properties Maximum weld defect size Mechanical damage and cold work Level of residual stress Residual water and gas content. Source: Posiva Report

8 Introduction Hot processing causes oxidation of copper surface, these films are removed by pickling before non-destructive tests for quality control. The canisters components will be cleaned before assembly, mainly to remove cutting fluid residues, but also other impurities. The canister components are typically at room temperature during the spent fuel encapsulation, but canister surface temperature has been estimated to be ºC, maximum 100 ºC. Source: Posiva Reports &

9 Introduction The canister surface will have an oxide layer when it is put into the disposal hole. The aims of the REPCOR project are: Produce oxide films on the surface of clean copper Characterize the oxide film composition and thickness Measure the effect of oxide film on corrosion rate in different corrosive environments. 9

10 Environments and corrosion rates The corrosion loads for the canister are: atmospheric corrosion during manufacturing and storage aerobic corrosion in the deposition hole inner corrosion due to radiolysis of residual water outer corrosion due to radiolysis of external water localised corrosion (mainly due to Cl - ) general corrosion due to sulphide ions from groundwater, buffer and backfill (including microbially-induced corrosion). Source: Posiva Report

11 Environments and corrosion rates Corrosive environments during the encapsulation and deposition process: 1. Canister manufacturing and assembly, atmospheric corrosion at ambient temperature. 2. Encapsulation, atmospheric corrosion in indoors, urban or underground environment, elevated surface temperature. 3. Canister in unsaturated bentonite, up to 100 years, atmospheric and/or immersion, high redox, high temperature. 4. Canister in saturated bentonite, porewater replaced by ground water, up to years, immersion, redox and temperature decreasing. 11

12 Environments and corrosion rates The atmospheric corrosion of the copper shell during the storage time before emplacement is estimated to be a couple of months at most It is considered negligible in spite of the elevated temperature of about ºC in the storage facility. A layer of copper oxide with a thickness of a few tens to a few hundreds of nanometres will form on the canister surface. Even if the storage time would extend up to 2 years, the total corrosion attack would be less that 1 mm. Source: Posiva Report

13 Environments and corrosion rates Posiva Report gives (max.) pollutant levels as SO mg/m 3, NO 2 75 mg/m 3, NH 3 <20 mg/m 3 and H 2 S <3 mg/m 3. Time scale few weeks. Corrosion rates are based on literature: In urban atmosphere 6-27 nm/a at 20 o C and nm/a at 50 o C. The total corrosion attack even after two years storage will be less than 1 mm. The most likely corrosion product will be copper oxide, not stated whether Cu 2 O, CuO or both. 13

14 Environments and corrosion rates The corrosivity of the atmosphere depends on impurities and time of wetness. Corrosion rate decreases with time. Category, ISO 9223, ISO st year, mm/a 10 years, mm/a 30 years, mm/a C1 <0.1 <0.05 <0.03 C C C C CX

15 Environments and corrosion rates The emplaced canister will be covered by an air-formed oxide with a thickness of a few tens to a few hundreds of nanometres. The canister wall is expected to reach up to 100 ºC. RH is 50-80%, enough to cause condensation. The corrosion depth ranges from 90 nm in case of oxidation of the surface to 90 mm in case of corrosion under a condensation film. Maximum possible loss of thickness is estimated to be less than 1 mm based on available oxygen. Source: Posiva Report

16 Environments and corrosion rates Corrosion during bentonite saturation can lead to localized corrosion due to uneven water saturation and bentonite swelling. Areas with lower oxygen supply will become anodic. Saturation phase will take several years. Immersion in groundwater/pore water Evaporation and condensation of water causing atmospheric corrosion. Dry deposits becoming moist and dissolving again can lead to very concentrated solutions. 16

17 Environments and corrosion rates Corrosion analysis after water saturation phase assumes mass transport of species to and from the canister surface by diffusion through water-filled pores and unlimited water on the surface. General change from warm and oxic to cool and anoxic conditions. First increase in Cl - concentration from groundwater infiltration to bentonite, later Cl - decreases. Corrosion is controlled first by oxygen and later by sulfide compounds. 17

18 Environments and corrosion rates Constituent Baseline At closure, infiltration into unsaturated bentonite After closure and saturation, up to 100 years After closure up to years ph Redox, mv to oxic Cl -, mg/l SO 4 2-, mg/l < DIC, mmol/l < Na +, mg/l Ca 2+, mg/l Mg 2+, mg/l K +, mg/l Posiva Report , Potential hydrogeochemical conditions at Olkiluoto site 18

19 Environments and corrosion rates Constituent Infiltrating groundwater at closure After closure and saturation, up to 100 years ph After closure up to years Redox, mv -250 to oxic Cl -, mg/l SO 4 2-, mg/l DIC, mmol/l Na +, mg/l Ca 2+, mg/l HS -, mmol/l Posiva Report , Estimated bentonite porewater evolution at Olkiluoto site 19

20 Environments and corrosion rates After emplacement of the capsule in the disposal hole atmosphere can no longer be controlled. Posiva Report states that if oxygen supply is unlimited, corrosion rate is mm/a. Dominant corrosion product most likely Cu 2 O. Assuming all oxygen in a disposal hole is used in copper corrosion, the maximum even thinning would be 300 mm. Based on models, corrosion rate for oxic conditions is 7 mm/a. Aspö HRL results <100 nm/a at 75 o C, corrosion product Cu 2 (OH) 3 Cl, possibly also Cu 2 O. (Taxén, Mat. Res. Soc. Symp. Proc. Vol. 807, 2004 ) 20

21 Environments and corrosion rates Corrosion rates during bentonite saturation have been reported using different bentonite-solution setups and methods. Posiva Report references give 3 mm/a, 7 mm/a for oxic conditions, from mm/a down to 1 mm/a with increasing chloride, and mm/a. SKB Report R mm/a, typically mm/a. 21

22 Environments and corrosion rates Corrosion rates after bentonite saturation and compaction. Posiva Report references give 0.06 mm after 6000 years, from less than 6 mm to about 30 mm in years, and mm after 10 5 years. SKB Report TR gives mm/a at o C and 1.72 mm/a at 130 o C. Kinetic models by King et al. presented in Posiva Report show that corrosion depth is less than 0.1 mm and corrosion virtually stops after years. 22

23 Copper corrosion mechanisms Atmospheric corrosion is electrochemical with dissolved oxygen as oxidant. 4 Cu + 2 H 2 O + O 2 = 4 Cu OH - 4 Cu + O 2 = 2 Cu 2 O In atmospheric corrosion the corrosion rate is controlled by mass transfer of oxidant to the surface. The first product will be Cu 2 O that can react to other compounds depending on the environment. In typical atmospheres this will be seen as copper surface turning to brown or black due to Cu 2 O and then formation of the colorful patina layer. 23

24 Copper corrosion mechanisms Source: Krätschmer et al. Corrosion Science 44(2002), 3. 24

25 Copper corrosion mechanisms Electrochemical corrosion follows mixed potential theory. An oxidant in the system reacts cathodically and this in turn starts the anodic dissolution of copper. The surface films on copper can affect the rates of electrochemical reactions. 25

26 Copper corrosion mechanisms The electrochemical corrosion of copper follows mixed potential theory. Increase in [Cl - ] will increase the driving force. Increase in [O 2 ] will increase cathodic reaction rate. Both will increase corrosion rate. Source: Posiva Report

27 Copper corrosion mechanisms A steady-state model of general corrosion with varying [Cl - ] and [O 2 ] shows i corr from 10-2 ma/cm 2 in oxic to 10-5 ma/cm 2 in lowoxygen system. Roughly 0.1 mm/y in oxic and 0.1 nm/y in low-oxygen. Source: Posiva Report

28 Copper corrosion mechanisms Copper Corrosion Model in oxygen containing compacted buffer material by King et al. Source: Posiva Report

29 Copper corrosion mechanisms Copper Sulphide Model by King et al. Source: Posiva Report

30 Copper corrosion mechanisms Copper corrosion by formation of Cu-Cl complexes 2 Cu + 4 Cl H 2 O = 2 CuCl OH - + H 2 2 Cu + 6 Cl H 2 O = 2 CuCl OH - + H 2 Copper corrosion under oxygen-free systems by auto-ionisation of water: 2 Cu + H 2 O Cu 2 O + H 2 30

31 Copper corrosion mechanisms Low equilibrium constant k values, not probable reactions. Calculated with HSC 8.1 Reaction Equations module. See also King & Lilja, CEST 46(2011) 2, p

32 Oxide films and corrosion Two corrosion test series and two oxide film characterization series. First corrosion test series by Mr. Vesa Lindroos Second corrosion test series by Mr. Atte Tenitz First oxide film series by Atte Tenitz Second oxide film series by Mr. Alexander Chernyaev ongoing

33 Oxide films and corrosion Immersion tests to measure corrosion rates. Glass reactor with glycol heater jacket Porvoon Lasilaite QCM sensor and potentials SRS QCM 200 for mass change and Cu potential measurements, Pt counter electrode, SCE reference Hanna Instruments ph, DO and EC probes, WTW [Cu²+] probe Acrylic reactor lid CNC-machined to fit all of the probes and gas inlet Sealable with original reactor clamp 33

34 Oxide films and corrosion 34

35 Oxide films and corrosion Short time immersion tests, up to 16 hours. OL-SR synthetic groundwater acidified to ph = 4 with nitrogen bubbling. DO levels typically 2.5 ppm (room temp.) and 0.5 ppm (50 ºC) before test. 35

36 Oxide films and corrosion 36

37 Oxide films and corrosion 37

38 Oxide films and corrosion Fresh copper, OL-SR, ph = 4, 25 ºC, 191±103 mm/a. Fresh copper, OL-SR, ph = 4, 50 ºC, 985±60 mm/a. Oxidized 16 h 25 ºC, OL-SR 25 ºC, ph = 4, 920±170 mm/a. Oxidized 16 h 25 ºC, OL-SR 50 ºC, ph = 4, 485±165 mm/a. Oxidized 16 h 50 ºC, OL-SR 25 ºC, ph = 4, 155±85 mm/a. Oxidized 16 h 50 ºC, OL-SR 50 ºC, ph = 4, 295±130 mm/a. Oxidized 16 h 90 ºC, OL-SR 25 ºC, ph = 4, 380±240 mm/a. Oxidized 16 h 90 ºC, OL-SR 50 ºC, ph = 4, 390±200 mm/a. 38

39 Oxide films and corrosion The base solution was OL-SR synthetic ground water. OL-SR water total dissolved solids is ppm, more concentrated solutions were ppm and ppm. ph-levels 4, 6 and 8. Temperatures 40, 60 and80 o C. Air or nitrogen purging during the 24 hour test. Electrodeposited copper on QCM crystals First series for copper, second for copper oxidized at 90 ºC for 7 days. 39

40 Oxide films and corrosion 0 Copper, S = ppm, T = 45 C, ph = 4, air. Mass loss, microg/ cm y = x R² = Time, s 40

41 Oxide films and corrosion Copper mm/a = [Cl] C gas [Cl] gas C gas Oxidized mm/a = C gas- 22,80 C gas S = salinity ppm, T = temperature ºC, gas: air = 1, N 2 = 0 41

42 Oxide films and corrosion MLR model measured vs. predicted thinning mm/year. 42

43 Oxide films and corrosion Corrosion rates in room temperature OL-SR water under nitrogen purging were 1-2 mm/a for copper and oxidized copper, several hundred mm/a in hot, oxic solution. Corrosion rates were high and variation was large Copper μm/a Oxidized copper μm/a Copper in nitrogen purged 1,4-91 μm/a Oxidized copper in nitrogen purges 2,6-141 μm/a Large variation in aerated conditions. Increase in aeration increases corrosion rate. Increasing ph decreases corrosion rate. 43

44 Copper corrosion products Copper exposed to air containing moisture and pollutants will form various corrosion products on surface. Cuprous oxide (Cu 2 O) forms at an early stage of the corrosion process. Further oxidation produces new layer of cupric oxide (CuO) that grows preferentially over the Cu 2 O layer. Eventually, corrosion products on copper change to a stable patina, whose composition depends on the anions in the environment. 44

45 Copper corrosion products The outer patina layer is usually precipitated basic Cu(II) compounds such as: Atacamite or Paratacamite Cu 2 (OH) 3 Cl (chloride compound) Brochantite Cu 4 (OH) 6 SO 4 (sulfate compound) Posnjakite Cu 4 (OH) 6 SO 4 H 2 O (sulfate compound) Malachite Cu 2 (OH) 2 CO 3 (carbonate compound) Copper sulfides, mainly Cu 2 S, can form on copper surfaces in atmosphere containing H 2 S or in a solution containing S 2- ions. 45

46 Copper corrosion products Copper oxides can be reduced in very alkaline solution. The oxides can be separated as long as they are not too thick. The examples are for approximately 1 mm films and 1 cm 2 samples. Scan rate 1 mv/s. S. Nakayama et al. J.Elchem. Soc.,

47 Copper corrosion products Electrochemical reduction of copper compounds: Cu 2 O + H 2 O + 2 e - 2 Cu + 2 OH - CuO + H 2 O + 2 e - Cu + 2 OH - Using the charge from reduction tests the thickness of oxide film can be calculated using Faradays law and oxide density. 47

48 Copper corrosion products Cu-OF copper samples embedded in epoxy. Polishing with 1200 grit sandpaper, immersion in 10% citric acid for 2 minutes three times with rinsing between dips, followed by rinsing with distilled water and ethanol, and dried with hot air. Electrochemically deposited copper on QCM crystals. Rinsing and drying after deposition. 48

49 Copper corrosion products I (A) s1 s4 A1 A2 A1 = Cu 2 O A2 = CuO C1 = CuO C2 = Cu 2 O C4 = unknown, appeared in some very oxidized samples C2 C1 C E (V vs. Ag/AgCl) 49

50 Copper corrosion products Background current density 0.5 ma/cm 2, no reduction peaks. 50

51 Copper corrosion products Cu-OF molded in epoxy and QCM crystals Fresh or oxidized at ºC for 1, 3 or 7 days. Immersion in air or nitrogen purged OL-SR groundwater at 40, 60 or 80 ºC. Samples in air for 1-3 days nm Cu 2 O. Samples oxidized in oven 100 nm Cu 2 O. 51

52 Copper corrosion products Oxidation (days) Thickness (nm) Sample Oven Immersion T = 40 C T = 60 C T = 80 C Oxide Crystal Cu 2 O Cu 2 O Cu 2 O Copper CuO/Cu 2 O CuO/Cu 2 O Cu 2 O 52

53 Copper corrosion products Cu 2 O was the dominant oxide in almost all tests. Small amounts (10 nm) of CuO reduced in the tests with water temperature 40 ºC and the samples were oxidized in oven for one and three days. No CuO was reduced on samples which were oxidized in oven for seven days before immersion test. 53

54 Copper corrosion products I (A) Oxidation products on copper after 24 h immersion in aerated OL-SR groundwater at 40 ºC s2-t2 s2-t1 s3-t E (V vs. Ag/AgCl) 54

55 Copper corrosion products 0.00 I (A) Oxidation products on copper after 24 h immersion in aerated OL-SR groundwater at 60 ºC s1-t1 s3-t2 s5-t3 s4-t3 s3-t E (V vs. Ag/AgCl) 55

56 Copper corrosion products I (A) C3 C2 s4-t1 s5-t1 s4-t2 s5-t2 s6-t2 s1-t3 Oxidation products on copper after 24 h immersion in aerated OL-SR groundwater vapour at 60 ºC E (V vs. Ag/AgCl) 56

57 Copper corrosion products I (A) s4-t1 s5-t1 s6-t1 s1-t2 s5-t3 s6-t3 Oxidation products on copper first oxidized at 90 ºC for 7 days and then 24 h immersion in aerated OL-SR groundwater at 60 ºC E (V vs. Ag/AgCl) 57

58 Copper corrosion products I (A) C1 C2 A1 s5-t1 s6-t1 s1-t2 s2-t2 s4-t3 Oxidation products on copper first oxidized at 160 ºC for 24 hours and then 24 h immersion in aerated OL-SR groundwater at 60 ºC E (V vs. Ag/AgCl) 58

59 Copper corrosion products Film thickness (µm) C 24 h oven 120 C 48 h oven 160 C 24 h oven C 24 h SGW 60 C 24 h SGW 90 C 7 d oven + 60 C 24 h SGW C 24 h oven + 60 C 24 h SGW 60 C 24 h in SGW vapor

60 Summary In the corrosion rate analyses the estimated corrosion hasdecreased during last 20 years. The estimated general corrosion rates are in the range 1-3 mm/a, often less than 1 mm/a. Deep pitting is not considered probable. The combined effect of different stages on loss of thickness is usually maximum few millimeters for 10 5 or 10 6 years. Some situations can result in high corrosion rates, but these are expected to be short. 60

61 Summary Short time tests to estimate the effect of environment and oxide layer by using QCM tend to produce corrosion rates that are often much too high compared to long-term studies. Especially the hot, acid and aerated systems tend to result in high and varying corrosion rates. The oxidized samples lose weight faster than clean copper surfaces. The thickness of the copper oxide layer shows very strong variation from less than 1 mm to tens of mm. 61

62 References POSIVA , An Update of the State-of-the-art Report on the Corrosion of Copper Under Expected Conditions in a Deep Geologic Repository. Fraser King, Christina Lilja, Karsten Pedersen, Petteri Pitkänen & Marjut Vähänen, July 2012, 246 p. POSIVA , Canister Design Heikki Raiko, April 2013, 156 p. POSIVA , Canister Production Line Design, Production and Initial State of the Canister. Heikki Raiko, Barbara Pastina, Tiina Jalonen, Leena Nolvi, Jorma Pitkänen & Timo Salonen, December 2012, 174 p. POSIVA , Safety Case for the Disposal of Spent Nuclear Fuel at Olkiluoto FEP Screening and Processing. Posiva Oy, January 2014, 148 p. SKB TR-13-17, LOT A2 test parcel. Compilation of copper data in the LOT A2 test parcel. Paul Wersin, December 2013, 28 p. Atmospheric Corrosion of Copper 450 Metres Underground. Results From Three Years Exposure in the Äspö HRL. Claes Taxén, MRS Proceedings, 807, doi: /proc Scientific basis for corrosion of copper in water and implications for canister lifetimes. Fraser King & Christina Lilja. CEST 46(2011) 2, pp