Corrosion of steels in liquid metals

1. MATGEN IV.2February 2- 8, 2009Stockholm – Kiruna, Sweden Corrosion of steels in liquid metals Concetta Fazio Program Nuclear Safety Research

2. Outline • Motivation • The role of Nuclear Energy in an Energy Mix • The Fast Reactor System and its fuel cycle • Transmutation objectives and Scenarios • Fast Reactor Systems and the role of liquid metals as coolant • Examples • Loop type Na cooled FR • Pool Type Pb cooled FR • ADS • Corrosion of steels in liquid metals • What is corrosion? • Parameters affecting corrosion • Corrosion mechanisms in HLM and Na • Experimental evaluation of corrosion mechanisms and rate • Models • Practical applications • Summary and Perspectives

3. 9 30 Other Renewable Biomass 8,5 25 Nuclear Gas 8 20 Oil Coal World Primary Energy Sources (Gtoe) World Population (Billions) 7,5 15 Population 7 10 6,5 5 6 0 1990 2000 2010 2020 2030 2040 2050 The role of Nuclear Energy in an Energy Mix Source IEA : Energy to 2050 - Scenarios for a Sustainable Future

4. Objectives of future implementation of FR in a power park (starting from ~ 2040)

5. Optimal resources utilisation …. G. Koch, Radiochimica Acta 37 (1984) 205

6. Generic objectives of P/T strategies: reduce the burden on a geological storage in terms of waste mass minimization, reduction of the heat load and of the source of potential radiotoxicity. Transmutation Objectives and Scenarios

7. More specific objectives can be defined according to the specific policy adopted towards nuclear energy and according to specific strategies of reactor development. Three categories of specific objectives: Waste minimization and sustainable development of nuclear energy and increased proliferation resistance of the fuel cycle. A transition from a LWR fleet to a FR fleet is foreseen. Reduction of MA inventory and use of Pu as a resource in LWRs, in the hypothesis of a delayed deployment of fast reactors. Use of dedicated burners (ADS or FR) Reduction of TRU inventory as unloaded from LWRs: Management of spent fuel inventories, as a legacy of previous operation of nuclear power plants in ADS. Transmutation Objectives and Scenarios It is a generally agreed conclusion that fast neutron spectrum systems are more appropriate for transmutation of TRU

8. Transmutation Objectives and Scenarios

9. Among the 6 preferred Gen IV systems, 3 are FRs Lead Fast Reactor Gas Fast Reactor Sodium Fast reactor Very High Temperature Reactor Supercritical Water Reactor Molten Salt Reactor

10. Examples - Loop type Na cooled FR: JSFR Ref. SMINS, 2007

11. Loop type Na cooled FR: JSFR Operational conditions Parameters to be considered for material assessment K. Mukai, Int. Seminar on coolants and Innovative Reactor Technologies, CEA Cadarache Nov. 2006

12. Examples - Pool type Pb cooled FR: ELSY Pump Impeller Alternative materials for pump impeller under investigation Maxthal, SiSiC, Noriloy HX - T91 or AISi 316L Cladding – T91 Vessel – AISI 316L L. Cinotti, Int. Seminar on coolants and Innovative Reactor Technologies, CEA Cadarache Nov. 2006

13. Pool type Pb cooled FR: ELSY Operational conditions Parameters to be considered for material assessment L. Cinotti, Int. Seminar on coolants and Innovative Reactor Technologies, CEA Cadarache Nov. 2006

14. ADS EFIT XT-ADS

15. ADS: Operational conditions EFIT Pump: T= 480 °C; dpa < 0.03; flow = 10 m/s (on impeller)

16. Liquid Metals • Fast reactors have: • Hard neutron spectrum (i.e. limited neutron thermalisation and as small neutron capture as possible) • High power density: need for effective coolant with high thermal exchange capability. • Therefore: liquid metals as coolant. Historically Na and, at a lesser extent, Heavy Liquid Metals (HLM) have been the preferred choices.

17. Liquid Metals Properties

18. Corrosion of steels in liquid metals What is Corrosion? Why it is important to study it?

19. „The word corrosion denotes the destruction of metal by chemical or electrochemical action; a familiar example is the rusting of iron” U. R. Evans Active Corrosion on Carbon Steel Manhole Pitting Corrosion: Corrosion Pits are the primary source of leaks in water handling systems Liquid metal corrosion Lecor impeller (presented at the ELSY Meeting by ENEA)

20. Why it is important to study it • Ensure Integrity of structures • Avoid Plugging of systems with corrosion products • Ensure thermal conductivity of fuel cladding and functional components • An example for HLM cooled FR: • Stringent safety requirement on the integrity of the cladding material has been put for design basis operating conditions and design extension conditions. • For the chosen temperature regime, the selected cladding material should withstand the combined effect of neutron irradiation, corrosion and mechanical stresses in order to comply with the safety requirements.

21. Temperature Temperature gradient (mass transfer) Cyclic temperature fluctuations Surface area to volume ratio Chemical purity of the liquid metal (wetting) Flow velocity (Reynolds number) Surface conditioning (surface films) Number of materials in contact with the same liquid metal (dissimilar mass transfer) Condition of the container material (carbides or nitrides at the grain boundary) Factors affecting liquid metal corrosion

22. Liquid metal corrosion mechanism Simple solution attack: Removal of the metal from the surface to saturate the liquid metal. Scheme of the simple dissolution mechanism Concentration gradient mass transfer dissimilar metals, e.g: Mo samples tested in Na contained in a Ni crucible at 1000°C: Ni had transferred through the Na and deposited on the Mo surface to produce Ni-Mo compounds Alloying between liquid and solid metals: for this type of mechanism there must be some solubility of the liquid metal in the solid metal. In some cases the liquid metal dissolves considerably in the solid metal with the formation of an intermetallic compound (e.g. V in Pb at 1000°C).

23. Liquid metal corrosion mechanism Temperature gradient mass transfer: the most damaging type of liquid metal corrosion is temperature gradient mass transfer. The driving force for temperature gradient mass transfer is the difference in solubility of the dissolved metal at the temperature extremes of the heat transfer system. By knowing the solubility limit of the solid in the liquid metal the driving force of these phenomena can be determined Pulg in an Inconel-Pb loop

24. Temperature Temperature gradient (mass transfer) Cyclic temperature fluctuations Surface area to volume ratio Chemical purity of the liquid metal (wetting) Flow velocity (Reynolds number) Surface conditioning (surface films) Number of materials in contact with the same liquid metal (dissimilar mass transfer) Condition of the container material (carbides or nitrides at the grain boundary) Part 2: Factors affecting liquid metal corrosion

25. At installation start up and in normal operating conditions From Cover gas and adsorbed gases on structures (O2, H2O) From neutron reaction and from spallation (e.g. Po, Hg, other activation products) Corrosion products from structural material (e.g. Fe, Cr, Ni, etc.) Intrinsic impurities (Ag, Cu, Sn, etc.) Off normal conditions From Fuel cladding failure (Pu, U, MA, etc.) Air entrance (N2, O2, H2O, ..) Steam entrance (H2O) Liquid metal quality: Sources and type of impurities

26. HLM quality control: the case of Oxygen Solubility of Oxygen in LBE Solubility of Oxygen in Pb Above solubility limit lead and bismuth oxide formation Oxides floating on the liquid metal

27. Na quality control: the case of Oxygen

28. Solubility of metallic elements In Na In HLM

29. Liquid metal corrosion depends from the solubility of the solid metal in the liquid metal and its solution rate. Solution rate and extent of solubility are affected by formation of surface intermetallic compounds (among the liquid and the solid) oxide or nitride films formation (due to the presence of oxygen / nitrogen in the liquid metal) Other impurities present in the liquid metal can increase the solution rate Temperature gradients and multimetallic systems From liquid metal quality to corrosion

30. 20mm ferrite layer PbBi Δ G (PbO) O 2 20mm PbBi Results from screening experiments: HLM Case 1) oxygen content in LBE < 10-9 wt.% T=400 °C Case 2) [O2]LBE > 10-8 wt.% and < 400 °C < T < 550 °C Dissolution of solid metal in the liquid metal Impurities in LBE forming simple or complex substances on the metal surface Leaching of Ni and ferritisation AISI 316L AISI 316L 2) T91 10000 h Uniform dissolution Transgranular and intergranular 1) Under controlled O2 content and T: the oxide can be considered as a corrosion protection layer Corrosion mechanism in HLM depends from: Temperature – oxygen content in the liquid metal – composition of steel

31. Results from experiments: Na Low Oxygen High Oxygen Similar mechanism is observed with Cr (Na-Cr-O are more stable than Na-Fe-O) Formation of ternary oxides increased corrosion rate In austenitic steels ferritisation can be observed (Ni solubility highest)

32. Experimental Programs to address Corrosion mechanism and rate

33. Materials selected Ferritic Martensitic steel T91 for the highly loaded parts (e.g. cladding, spallation target) Austentic steel AISI 316L for e.g. Vessel Fe, Al based corrosion protection barrier Choice and characterisation of reference structural materials: the case of HLM systems T91 AISI 316L

34. CEA FZK FZK/IPPE CIEMAT ENEA NRI Materials qualification program for HLM systems Corrosion studies Irradiation studies Corrosion / n-irradiation combined effect

35. Corrosion studies in HLM Ellingham Diagram For the operating condition of XT-ADS (300 – 400°C) and EFIT (480 – 530 °C) respectively an appropriate oxygen potential can be selected to avoid HLM oxides formation and to promote oxidation of the steel surface. HLM oxides precipitates causing hydraulics problems (e.g. plugging) Oxidation of the steel surface. The oxide layer can act as a protection barrier against a direct corrosive attack of the liquid metal These are thermo-chemical statements, which enables to identify the corrosion mechanism. However, no information are available on the corrosion rate and the hydraulics effect. Dissolution (uniform or transgranular) of the steel elements in the liquid metal

36. Corrosion studies: experimental results

37. CU2 IPPE/FZK Oxide: 39 mm CHEOPE /ENEA Oxide: ~ 20 mm Corrosion studies: experimental results Results after 2000h in LBE at 550°C and Pb at 500°C performed with a controlled oxygen potential • Oxide scale is formed by three layers: outer magnetite – intermediate Fe, Cr spinel oxide – inner oxygen diffusion zone • However, oxide scale tested in Corrida do not has the outer magnetite layer: Hydraulics effect (see next slide)? • After 2000h oxidation rate in Pb at 500°C is lower with respect to LBE at 550°C: temperature effect CORRIDA FZK Oxide: 25 mm 550°C , 10-6 wt.-%O flow= 1.3 m/s Oxide thickness: spinel+ internal oxidation = 22 µm Magnetite thickness: 17 µm 550°C; 10-6 wt.-%O flow= 2 m/s No magnetite detected (higher velocity) • 500°C , ~10-6 wt.-%O flow ~ 1 m/s • Lower temperature • Different oxygen potential • “low” flow velocity

38. А V=1 м/s V=2 м/s V=3 м/s 100mm 50mm 100mm 50mm 50mm 100mm А Corrosion studies: experimental results Confirmation of hydraulics effects on the oxide scale formation: Experiment performed with different flow velocities 550 °C, 2000 h, ~10-6 wt.% O V = 1 m/s V = 1,75 m/s V = 3,0 m/s Fe3O4 Fe3O4 (Fe, Cr)3O4 (Fe, Cr)3O4 (Fe, Cr)3O4 Internal oxidation Internal oxidation Internal oxidation At 1m/s outer magnetite scale, at 1,75 m/s small rests are visible, at 3m/s magnetite scale entirely eroded. FZK-IHM/IPPE collaboration

39. Corrosion studies experimental results Pressurised tube in HLM 10-6 wt.-%Oflow= 1 m/s t=2000h 550°C

40. Example of application of corrosion results to Design Axial profiles of clad inner temperature modified calculation with different additional oxide layers Oxide layer thickness should be limited to less than 20-30 mm in order to keep margin on the maximum allowable temperature for the T91 steel. Control of oxidation process in a reactor system might not be applicable GESA surface alloyed steel can be seen as a solution (D. Struwe, W. Pfrang, IRS/FZK)

41. Corrosion studies: Fe, Al corrosion protection barrier 1. LPPS of Fe, Al 2. GESA treatment on the LPPS coating • Enhance metallic bonding with substrate • Smoother surface • Reduced Al content 3. GESA treated samples tested for 10000 h in flowing LBE at three different temperatures, flow rate 1 m/s and oxygen ptential equivalent to 10-6 wt%  Up to 600°C and 10000 h no corrosion attack and no visible oxidation.  Thin alumina scales protect the surface alloyed steel. 500°C 550 °C 600°C

42. А V=1 м/s V=2 м/s V=3 м/s 100mm 50mm 100mm 50mm 50mm 100mm А Corrosion studies: Fe, Al corrosion protection barrier Confirmation of hydraulics effects on the Fe, Al GESA treated samples: Experiment performed with different flow velocities 550 °C, 2000 h, ~10-6 wt.% O V = 1 m/s V = 1,75 m/s V = 3,0 m/s Samples with proper LPPS coating and proper GESA treatment: no flow velocity effect on surface appearance, no dissolution attack, no severe oxidation, no erosion. FZK-IHM/IPPE collaboration

43. Parameters affecting corrosion of steels and modelling • Oxygen activity: oxidation/dissolution • Time, Temperature • Flow rate: high flow rate  erosion of Fe3O4 • Steel composition: high Cr content  increased oxidation resistance. • Stresses: hoop stress enhances Fe diffusion Effect of temperature, oxygen content and steel composition Effect of flow velocity

44. Oxygen content in Na and flow velocity have been identified as the two main variables affecting the corrosion rate. The corrosion mechanism is the dissolution. From experimental results two semi-empirical equations have been determined: Corrosion modelling: Example Na – steel system For v ≤ 4m/s - the corrosion rate depends from the velocity For v ≥ 4m/s – the corrosion rate depends only from the oxygen concentration

45. Corrosion modelling: Example Steel-HLM • Structural material corrosion in a closed an-isothermal system as a nuclear reactor, 2 kinds of model are needed: • Mechanistic model: give the structural material life timehox(t) or kp, hdiss(t), jox, jdiss, using the physical-chemical data characteristic of the mechanism (Dox, DLM, S…) • Mass transfer model: give the system life timeVcorr/prec(x) (prediction of plugging in the loop), using the output of the mechanistic model (kp, kpr, jox, jdiss…) • Data needed: • Mechanistic model is based on numerous specific experiments which can be partly performed in static conditions • Mass transfer model is based on long term experiments in LM closed loop • To develop these models, need of physical-chemical data as: Dox, DLM, SLM, kpr, kdiss which are very difficult to obtain and important lack of data to supply the corrosion model jox oxygen flux in steel; Jdiss Fe flux in steel, hox oxide thickness, hdiss dissolution thickness, D diffusion coefficient, S solubility limit, kox oxidation constant kpr precipitation rate

46. Example: Pb-Bi eutectic – steel system Corrosion modelling

47. Advantages of F/M steels with respect to austenitic steel: Better thermal properties: 1. higher thermal conductivity 2. lower thermal expansion (can have impact on the dimensioning, see e.g. Japanese Sodium Fast Reactor, JSFR) Lower Swelling However, experience on austenitic steels for the nuclear use is available Perspectives Data AISI316L from AAA handbook; Data T91 from RCC-MR

48. Advantages of ODS alloys, with respect to F/M steels 9% Cr ODS RAFM steel has been developed for future fusion reactors and has shown very promising mechanical properties at high temperature Perspectives R. Lindau et al., FZK …..but what about corrosion resistance of ODS steels?

49. For the development of nuclear energy, FRs provide solution to the key issues of sustainability and waste minimisation; Among the preferred systems of Gen IV three types of FRs and two among them are liquid metal cooled; In this respect corrosion issues have to be considered due to safety requirements; The corrosion control in HLM is more challenging when compared to Na; Current programs allow to consolidate and extend corrosion understanding and modelling; In future, new type of steels (e.g. ODS) can provide improved performances. These materials need to be characterised also for their corrosion resistance Summary

50. Na H.U. Borgstedt, C.K. Mathews, Applied Chemistry of Alkali Metals, Plenum, New York, 1987. K. Fink, L. Leibowitz, “Thermodynamic and Transport Properties of Sodium Liquid and Vapor” ANL/RE-95-2, January 1995 (www.insc.anl.gov) M. Konomura, M. Ichimiya „Design challenges for sodium cooled fast reactors“ J. Nucl. Mat. 371 (2007) 250–269 HLM “Handbook on Lead-bismuth Eutectic Alloy and Lead Properties, Materials Compatibility, Thermal-hydraulics and Technologies” issued by the OECD-NEA and available at the following link: http://www.nea.fr/html/science/reports/2007/nea6195-handbook.html Nuclear Technology September 2004 – Vol. 147, No3 Several Issues of J. Nucl. Mater (Vol. 296 (2001); Vol. 301 (2002);Vol. 318 (2003); Vol. 335 (2004), etc.) Comparative assessment HLM - Na „Comparative assessment of thermo-physical and thermohydraulic characteristics of Pb, LBE and Na coolants for fast reactors“, IAEA TECDOC – 1289, June 2002 F/M Steels High-Chromium Ferritic and Martensitic Steels for Nuclear applications, Ronald L. Klueh and Donald R. Harries, ASTM, MONO3 Selected References