Interfaces: Corrosion in Pb-alloy cooled nuclear reactors and advanced mitigation measures

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1 Interfaces: Corrosion in Pb-alloy cooled nuclear reactors and advanced mitigation measures and G. Müller KIT KIT Universität des Landes Baden-Württemberg und nationales Forschungszentrum in der Helmholtz-Gemeinschaft

2 Steel corrosion in liquid Pb-alloys Solubility of elements in Pb/PbBi Dissolution attack - corrosion steel exposed to LBE at 600 C Way Out: insoluble materials (W, Mo,.., ceramics) or like e.g. in atmospheric conditions protection by oxide scales growing in the media 2

3 Steel corrosion in liquid Pb-alloys Way out: Oxide scale on the steel surface prevent the dissolution Ellingham diagram steel liquid metal steel T 1 T 1 > T 2 T 2 a XT1 a XT1 > a LM > a XT2 a XT 2 oxide scale oxide scale diffusion barrier for Cations! f/m steel at 550 C in LBE At present only austenitic steels are considered in contact with liquid Pb-alloys due to LME of f/m steels 3

4 Steel corrosion in liquid Pb-alloys Oxygen content vs temperature 420 C / 4000 h 30µm Pb LBE 4 OECD Liquid metal handbook (2015) Austenitic steels can be used up to 450 C Oxygen content >10-7 wt% required Higher T or lower oxygen content Advanced mitigation measures

5 Steel corrosion in liquid Pb-alloys Incubation time for corrosion attack 5 Data from flowing LBE 2m/s Localized corrosion is one of the main concerns - origin still unknown Austenitic steels can be used up to 450 C? Oxygen content >10-7 wt% required? Higher T or lower oxygen content Advanced mitigation measures

6 Advanced mitigation measures (1) Insoluble ceramic coatings TiN, AlTiN, Al 2 O 3,.. (2) Formation of Alumina scale during operation Al- containing surface alloys FeCrAl Alumina forming steels FeCrAl, AFA Alumina forming HEA (3) MAXPHASES Some general requirements for advanced mitigation measures Corrosion resistant in HLM between 400 C and 650 C No delamination of the coating during operation or handling - coatings Self healing of mechanically damaged layers - surface alloys / bulk In-situ Formation Reservoir layer No negative influence on mechanical properties Irradiation stability under relevant fluxes The coating/alloying process should be of industrial relevance 6

7 Insoluble ceramic coatings TiN, AlTiN, Al 2 O 3 PLD of Al 2 O 3 scales on cladding tubes (IIT, Italy) γ-al 2 O 3 steel Al 2 O 3 substrate 500 nm 500 μm fully dense and compact microstructure Some droplet ejection 1% vol. γ-al 2 O 3 randomly oriented crystalline Al 2 O 3 nanodomains in an amorphous Al 2 O 3 matrix post-annealing (600 C,Ar) 10 nm γ-al 2 O 3 Exposure tests showed excellent corrosion resistance 1 st thermal cycling tests ( C 25X) no delamination no cracks Ceramic hardness combined with almost metallic E-Module Ion irradiation exhibit excellent irradiation stability Scalability?, No self-healing in service inspection, maintenance?? 7 F. García Ferré et al. Corros.Sci. 2017

8 In-service formation of Alumina as protective coating Al 2 O 3 layer Fe(Cr,Al)-phase Al- containing surface alloys FeCrAl Alumina forming steels FeCrAl, AFA Alumina forming HEA Steel Oxide map of FeCrAl - oxide at 900 C in gas Al 2 O 3 is very stable oxide Diffusion in Al 2 O 3 very low slowly growing perfect barrier No interaction with Pb-alloys No coating: either surface alloy or bulk Reservoir layer self healing Proven at other high temperature applications Al 2 O 3 8

9 Al - containing coatings Surface alloys The procedure consists in two steps: (i) pre-coating the surface steel with Al-containing alloy (Fe-Cr-Al system) (ii) (ii) melting the coating and the surface layer of the steel using intense pulsed electron beams (GESA process Karlsruhe Institute of Technology). LPPS E-beam -others Magnetic coils GESA target anode Electron beam parameter: electron energy: kev energy density : ~ 2 MW/ cm² pulse duration controllable: 1-200µs Beam diameter: 5-10 cm GESA treatment leads to: metallic bonding pore removal surface smoothening and reduced Al content 9

10 Pb/PbBi compatibility of Al surface alloyed steel at optimal oxygen concentration 10-6 wt% 500 C 600 C 550 C h h 20µm 20µm Up to 600 C and h no corrosion attack and no visible oxidation. Fe12Cr7Al Thin alumina scales protect the surface alloyed steel. Specimen at 550 C Al< 4wt% normal Fe-based scales?optimum Al/Cr content? Bulk FeCrAl to answer this question 10

11 Alumina forming steels - Bulk FeCrAl +RE Reactive elements influence on the corrosion behavior ( C) Exposure tests in Pb FeCrAl + RE manufactured and exposed to Pb alloy Fe-16Cr-8Al-0.5Y; Fe-16Cr-6Al-0.5Y Fe-16Cr-8Al-0.5Zr; Fe-16Cr-6Al-0.5Zr; Fe-16Cr-4Al-0.5Zr Fe-16Cr-8Al-0.5Hf; Fe-16Cr-6Al-0.5Hf; Fe-16Cr-4Al-0.5Hf Fe-16Cr-8Al-0.25Hf-0.25Zr; Fe-16Cr-6Al-0.25Hf-0.25Zr; Fe-16Cr- 4Al-0.25Hf-0.25Zr Fe-16Cr-8Al-0.5Mo; Fe-16Cr-6Al-0.5Mo; Fe-16Cr-4Al-0.5Mo Fe-16Cr-4Al-0.5Zr Fe-16Cr-4Al-0.5Hf 11

12 Protective oxides, balanced Zr/C (no surface carbides) C Oxygen~10-7 wt% TEM BF micrographs showing the alumina layer formed on Zr-0.2 at a. 450 C (1,000 h) and b. 550 C (8,760 h). 12

13 Oxide map of FeCrAl+RE in Pb-alloys between 450 and 600 C oxygen 10-6 wt% Al 2 O 3 formation at entire temperature range requires a certain Al, Cr ratio Reactive elements like Y, Zr, Hf foster the alumina formation Both Surface alloyed and bulk FeCrAl can protect the steels in Pb up to 750 C 13

14 Alumina forming steels AFA at KIT Metal powder Arc melter heat treatment (rapid cooling/quarz glas capsule) 1250 C 2h rapid cooling in water exposure in Pb at 600 C preliminary investigation Fe Cr Ni Nb Al Ti Mn Zr D ,2 0,32 41Z ,27 Alumina scales in Pb 8 different alloys: 10 Cr, Al (2-4), Ni (16-30) - dissolution attack Minimum Cr content required 14

15 15 Next steps with AFA- alloys selected and produced Based on 1 st results - start with simple model alloys to evaluate minimum Al and Cr content required, maximum Ni content allowed Phases are measured after heat treament Code Al(wt.%) Cr (wt.%) Ni (wt.%) Fe (wt.%) Phases Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC Sh FCC These AFA alloys are actually exposed to Pb at 600 C for 2000h Based on corrosion resistant small punch test to evaluate brittleness Additional elements to optimize mechanical properties

16 Alumina forming HEA -5 alloys AlCrFeNiX (X: Co Mn) Al 11.1 Cr 22.2 Fe 22.2 Ni 22.3 Mn 22.2 Al 12.7 Cr 21.6 Fe 21.8 Ni 21.6 Co 21.6 Y 0.7 HEA new class of materials Excellent mechanical properties also at high temperatures Indications to be stable under irradiation Adding Al compatible with Pb-alloys Corrosion resistance proved Mechanical and irradiation tests pending 16

17 Fretting depth [mm] Frettting depth / Fuel clad thickness Alumina forming surface allyos under combined load 0,20 0,4 0,15 T Ti GESA-T91 0,3 0,10 0,2 0,05 0,1 0, Temperature [ C] 0,0 Secondary creep rate in LBE and Airthreshold stress fretting damage as function of temperature Fig. 4b: fretting damage as function of temperatureure 17 In-situ formed Alumina-scales can mitigate influence of combined loads Surface alloyed f/m steels? Alumina forming f/m steels an option again?

18 Summary Outlook Corrosion is an issue for Pb-alloys foreseen as coolants f/m steels are not considered at present LME Austenitic steels (Ni-solubility) limit at ~450 C, but even there localized corrosion attack Open question regarding compatibility: What are the real limits of existing materials in Pb-alloys regarding temperature, oxygen content, flow rate, mechanical load? There are advanced mitigation strategies available > 600 C feasible Coatings Surface alloys and bulk alumina formers None of these is yet fully characterized None of these is ready now coatings and surface alloys more advanced All require more R&D EERA-JPNM PP ALCORE focus on Alumina forming - STAR-TREC focus on Al 2 O 3 coating - CERBERUS focus on corrosion all these PP are partially funded in the H2020 project GEMMA 18