Corrosion of copper in final disposal environments. Jari Aromaa Aalto Unversity Hydrometallurgy and Corrosion Research Group

Transcription

1 Corrosion of copper in final disposal environments Jari Aromaa Aalto Unversity Hydrometallurgy and Corrosion Research Group

2 Contents The concept Corrosive environments Copper corrosion mechanisms Copper corrosion thermodynamics Corrosion rates Some what-if scenarios Summary 2

3 The concept The multiple release barriers principle; fuel state, canister, bentonite barrier and bedrock. 3

4 The concept The KBS-3 concept: 1. Final disposal canister 2. Bentonite buffer 3. Tunnel backfill 4. Bedrock 4

5 The concept 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 50 mm thick copper, 47.5 mm taken as minimum. The purpose of copper is to protect the cast iron insert and the fuel assemblies from the groundwater. anister 5

6 The concept 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

7 Corrosive environments Corrosive environments during the encapsulation and deposition process: Canister manufacturing, atmospheric corrosion at ambient temperature, indoors, rural, urban? Encapsulation, atmospheric corrosion in indoors, urban or underground environment, elevated surface temperature. Canister in unsaturated bentonite, up to 100 years, atmospheric and/or immersion, high redox, high temperature. Canister in saturated bentonite, porewater replaced by ground water, up to years, immersion, redox and temperature decreasing. 7

8 Corrosive environments After encapsulation the canister is subject to atmospheric corrosion. The time scale is likely few weeks. Atmosphere is mostly indoors or urban. Underground atmosphere before setting in disposal hole? Temperature from ambient up to 100 o C. ISO 9223 classifications: C2 Low during storage, mm/a C4 High or C5 Very High underground, mm/a or mm/a 8

9 Corrosive environments 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. 9

10 Corrosive environments Corrosion analysis of 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 controlled by first oxygen and later by sulfide compounds. 10

11 Corrosive environments 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 11

12 Corrosive environments 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 12

13 Corrosive environments ph: Low and high ph maintain stabile soluble species and active corrosion. Neutral ph will lead to deposition of corrosion product films, but the films are not necessarily protective. Redox-potential: Describes the oxidizing or reducing conditions of the environment. It is linked to corrosion mechanisms but not on corrosion rates. Oxygen: Main oxidant in oxic phase, increase in [O 2 ] will increase cathodic reaction rate and thus corrosion rate. Sulfides: Main oxidant in anoxic phase. Corrosion rate increases with increasing [HS - ], will form non-protective sulfide films. 13

14 Corrosive environments Chloride: Cu(II) in low salinity, Cu(I) in high salinity. Corrosion rate increases slightly with increasing salinity in water, but decreases strongly in bentonite. Sulphate: Can affect corrosion film formation together with chlorides and dissolved inorganic carbon. Can affect HS - equlibrium. Ammonia: Can promote SCC, but usually concentration is considered too low to cause corrosion. Dissolved cations: Ca 2+ can deposit as gypsum, Fe 3+ can act as oxidant 14

15 Copper corrosion mechanisms Copper is one of the few metals found in metallic state in the nature. In 1840's pieces of copper weighing tens on tons were found underground in Great Lakes area. 15

16 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

17 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

18 Copper corrosion mechanisms Corrosion monitoring in bentonite saline water system. Corrosion rates decreased with time by an order of magnitude to less than 1 mm/y. Source: SKB Report R

19 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. 19

20 Copper corrosion mechanisms Source: Krätschmer et al. Corrosion Science 44(2002), 3. 20

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

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

23 Copper corrosion thermodynamics Thermodynamics gives tools to estimate, which reactions might be possible under certain conditions. Thermodynamics uses equilibrium states based on userdefined assumptions on temperature, reacting species and their concentrations, activity constants etc. Thermodynamic calculations do not provide information about reaction rates. The results of thermodynamic calculations might be dependent on the used software and databases. 23

24 Copper corrosion thermodynamics 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 autoionisation of water: 2 Cu + H 2 O Cu 2 O + H 2 24

25 Copper corrosion thermodynamics Low equilibrium constant k values, not probable reactions. Calculated with HSC 8.1 Reaction Equations module. 25

26 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 25 o C. a(cu 2+ ) = 10-6 [Cl - ] = 10-3 to 1 mol/dm 3. 26

27 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 50 o C. [Cu 2+ ] = 10-6 mol/kg, [Cl - ] = 1 mol/kg. Calculated with HSC 8.1 EpH module. 27

28 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 50 o C. [Cu 2+ ] = 10-2 mol/kg, [Cl - ] = 1 mol/kg. Calculated with HSC 8.1 EpH module. 28

29 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 90 o C. [Cu 2+ ] = 10-6 mol/kg, [Cl - ] = 1 mol/kg. Calculated with HSC 8.1 EpH module. 29

30 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 90 o C. [Cu 2+ ] = 10-2 mol/kg, [Cl - ] = 1 mol/kg. Calculated with HSC 8.1 EpH module. 30

31 Copper corrosion thermodynamics Cu-Cl-H 2 O system at T = 90 o C. [Cu 2+ ] = 10-6 mol/kg, [Cl - ] = 10-2 mol/kg. Calculated with HSC 8.1 EpH module. 31

32 Corrosion rates The wall thickness of the copper canister is 50 mm, and this sets a limit of 0.5 µm/year for the maximum general corrosion rate for expected lifetime of years. The general corrosion rate can not be taken as constant. Corrosion rate will usually decrease with time even if the environment is constant. Formation of deposits can cause pitting corrosion or underdeposit corrosion. The effect of pitting corrosion is often estimated by Pitting Factor that is the ratio of pit depth to thinning due to general corrosion. Pitting Factors 2 to 5 have been used in lifetime estimates. 32

33 Corrosion rates Corrosion or oxidation of the surface after encapsulation: Posiva Report states 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 rate 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. 33

34 Corrosion rates After emplacement of the capsule in the disposal hole atmosphere can no longer be controlled. Posiva Report states that in the repository RH = 80% and canister wall temperature is up to 90 o C. 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 ) 34

35 General corrosion rates Corrosion rates during bentonite saturation have been reported in many studies 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. 35

36 General 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. 36

37 Localized corrosion The Posiva report (revised ) lists following forms of localized corrosion: galvanic corrosion pitting corrosion crevice corrosion stress corrosion cracking (SCC) microbially influenced corrosion (initiating uniform corrosion and SCC) 37

38 Localized corrosion No galvanic corrosion expected for intact canister. Pitting corrosion is possible when copper is covered by duplex layer of Cu 2 O and either CuO or Cu(OH) 2, corrosion happens in weak points of the film. Crevice corrosion can be possible under deposits and can cause damages similar to pits or roughened surface. Stress corrosion cracking could be initiated by nitrogen and ammonia, but their levels are considered too low to cause SCC. 38

39 What-if scenarios Posiva Reports and refer to several different estimates on canister lifetime: Predictions from Sweden & Finland, Canada, Japan and Switzerland. The corrosion forms considered are general corrosion and pitting. Sweden & Finland: General corrosion mm and pitting mm in 10 6 years, SR-Can (2006) gives general corrosion 4 mm in 10 5 years and 0.05 mm roughening. Canada: General corrosion mm and pitting mm in 10 6 years. Japan: General corrosion 9-13 mm, pitting 2-26 mm in 10 3 years. Switzerland: General corrosion <1 mm, pitting <0.1-5 mm in 10 5 years. 39

40 What-if scenarios gas phase Corrosion in gas phase in disposal hole Corrosion rate of copper in the atmosphere is a few mm/year in the beginning, decreasing to 0.1 mm/year in 100 years. In bentonite clay corrosion rate is 2-3 mm/year. In saline groundwater corrosion rate is 2-20 mm/year. Vapour side corrosion of condenser tubes in desalination, up to 0.5 mg/cm 2 /day 200 mm/year. Weight change tests with Quartz Crystal Microbalance capable to detect 18 ng/cm 2 weight change. 40

41 What-if scenarios gas phase m / mg/cm Copper in air above distilled water T = 80 o C T = 60 o C T = 40 o C t / min 41

42 What-if scenarios gas phase In tests were run using Allard water the weight changes under nitrogen purging were ng/cm 2 /s. Under air or oxygen purging the average weight change rate was from to almost -2 ng/cm 2 /s (70 mm/year). The largest weight change rates were ng/cm 2 /s at 80 C in air. Under oxic conditions the thinning with Allard water was more than three times faster than with distilled water. 42

43 What-if scenarios water composition Extreme changes in water composition were tested using OL-SR synthetic groundwater with three TDS levels. OL-SR synthetic ground water salinity is ppm, more concentrated waters were ppm and ppm. ph levels 4, 6 and 8. Temperature 40, 60 and 80 o C. Air or nitrogen purging during the 16 hours test. Pure copper and oxidized copper (7 days at 100 o C). Quartz Crystal Microbalance. 43

44 What-if scenarios water composition Corrosion rates in room temperature OL-SR water under nitrogen purging were 1-2 mm/a for copper and oxidized copper, several hundreds mm/a in hot, oxic solution. Decreasing ph, increasing salinity and temperature and air purging vs. nitrogen purging increased corrosion rate. Corrosion rate of oxidized copper was 6 times higher than pure copper. Models based on factorial test (air = 1, N 2 = 0): Copper mm/a = [Cl] C gas [Cl] gas C gas Oxidized mm/a = C gas- 22,80 C gas 44

45 What-if scenarios anoxic and H 2 Corrosion by water and hydrogen evolution is based on the idea that copper is not thermodynamically stable in oxygenfree water. Literature review SKB TR by King concludes that this unproven mechanism does not have an adverse impact on canister lifetime. Test reports SKB R-13-34, SKB TR and SKB TR did not detect copper corrosion even hydrogen evolution was noticed. 45

46 What-if scenarios anoxic and H 2 Immersion test in vacuum chamber, corrosion monitored by pressure increase caused by hydrogen. Immersion test using real-time weight loss by Quartz Crystal Microbalance. Ultrapure water and OL- SR anoxic ground water 46

47 What-if scenarios anoxic and H 2 Pure water, 23 ºC, oxic. Corrosion products Saline water, 23 ºC, anoxic. Clean surface Saline water, 65 ºC, anoxic. Salt deposits, copper deposits 47

48 What-if scenarios anoxic and H 2 Corrosion rate measured by hydrogen pressure increase is less than one nm/year in anoxic pure water and approximately 10 nm/year in anoxic saline groundwater. The surface of the test chamber becomes green-black when in contact in air, but remains clear in oxic conditions. Salt and copper depsoits were seen after test in anoxic groundwater at high temperature. Consumption of remaining oxygen can take up to month based on potential measurements, but no corresponding changes seen in hydrogen pressure increase. Dissolved copper analysis indicated 1000 times higher corrosion rates than hydrogen pressure increase. 48

49 Summary In the analyses the estimated corrosion rates have decreased 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. A scenario analysis of the whole lifetime starting from canister manufacture to years could be useful. 49