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Influence of Liquid Sodium on Mechanical Properties of Steels, Refractory Alloys and Ceramics Hans Ulrich Borgstedt Formerly Section Head Inst. of Materials.

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Presentation on theme: "Influence of Liquid Sodium on Mechanical Properties of Steels, Refractory Alloys and Ceramics Hans Ulrich Borgstedt Formerly Section Head Inst. of Materials."— Presentation transcript:

1 Influence of Liquid Sodium on Mechanical Properties of Steels, Refractory Alloys and Ceramics Hans Ulrich Borgstedt Formerly Section Head Inst. of Materials Research of Inst. of Materials Research of FZK Head of Liquid Metal Laboratory

2 Part 1: Corrosion and Reactions with Impurities Dissolved in Sodium 1.Corrosion mechanisms 2.Influence of impurities dissolved in liquid sodium 3.Sodium corrosion of reactor materials 4.Corrosion of some ceramic materials 5.Methods of corrosion studies

3 Corrosion and Reactions with Impurities Dissolved in Sodium 1.Corrosion mechanisms 1.1 Corrosion in stagnant liquid sodium 1.2 Corrosion in turbulent liquid sodium flow 1.3 Selective leaching of alloy components

4 Corrosion Mechanisms Na corrosion is based on dissolution ! The dissolution rate  S/  t is ruled by the difference of the chemical activities in the solid phase, a (s), minus that in the liquid phase, a (l)  S/  t = ß [a (s) - a (l) ] (1) Based on the work of Epstein and of Weeks

5 Corrosion Mechanisms Corrosion in stagnant liquid sodium is characterized by the fact, that the difference of the activities, a (s) - a (l), is dependent on time. The activity of the solute, a (l), increases from very low values at the beginning of the process to values close to that in the solid phase, a (s ),, thus, corrosion rate approaches zero

6 Corrosion Mechanisms Corrosion in turbulent liquid sodium flow is determined by equation (1) Flow and temperature gradients keep the difference of activities a s and a d on its level Dissolution is influenced by the apparent solubility that can be different from the physical one

7 Corrosion Mechanisms Chemical activity in the bulk stream in non- isothermal flowing systems is orders of magnitude lower than saturation. Its value can be neglected, when the temperature gradient is in the order of  100 K. The corrosion rate becomes proportional to the activity at saturation :  S/  t = ß a sat (2) ß is the corrosion constant a sat can be influenced by side reactions

8 Corrosion Mechanisms ß depends on the hydraulic parameters of the flow. Epstein and Weeks published equation (3): ß = Sh  D  d H -1 (3) Sh = Sherwood number, D = diffusion coefficient and d H = hydraulic diameter of the test tube. The Sherwood number is related to the flow velocity via the Reynolds & Schmitt number: Sh =  Re 0.75  Sc 0.42 (4) Re is the Reynolds number Re = v  d H  -1 (v is the flow velocity and the viscosity) and Sc the Schmitt number Sc =  D -1

9 Corrosion Mechanisms ß depends on temperature through viscosity and diffusion coefficient D. Corrosion rates are further related to temperature via the solubility as a function of temperature. With the knowledge of ß and a sat one can estimate the Na corrosion rate (using equs. 1-4) Confirmed by corrosion experiments with Ni in Na circuit and solubility data

10 Saturation concentrations of steel components in liquid sodium at 773 and 973 K Steel component C sat [mol%] at 773 KC sat [mol%] at 973 K Fe Ni Cr Mo

11 Selective leaching of alloy components Different apparent solubilities cause selective leaching of alloy components Stainless steels form surface layers depleted in Cr, Ni and Mn Fe and Mo are enriched in surface layers Austenitic structure is changed to ferritic one Grain boundaries are depleted to larger depth

12 Surface layer depletion zone

13 Ferritic layer and grain boundaries

14 Corrosion and Reactions with Impurities Dissolved in Sodium 2.Influence of impurities dissolved in liquid sodium 2.1 Sodium oxide 2.2 Carbon 2.3 Analytical procedures to determine Na 2 O and C

15 Influence of impurities in liquid sodium Thorley and Tyzack exhibited the influence of oxygen on the Na corrosion of a group of steels in a systematic study log S = log c O –18000/2.3RT (with mass loss S in mils/y, c O in ppm and T in K Their results are shown in the following figure Na corrosion of stainless steels can be significally reduced by decreasing the oxygen content of sodium

16 Corrosion results of Thorley and Tyzack (1967)

17 Stability of sodium chromite in Na Influence of temperature and O concentration Na chromite stable at moderately high temperature and high oxygen concentration in Na

18 Influence of carbon on Na corrosion of stainless steel The formation of chromium carbides acts as barrier against depletion of Cr in the surface layers However, the formation of chromium carbides reduces the stability of the austenitic phase Since C reacts with Cr to form carbides, C decreases the chemical activity of Cr in steel

19 Analytical procedures to determine Na 2 O and C Oxygen Online with electrochemical cells using solid electrolytes (ThO 2 /Y 2 O 3 ) Vacuum distillation technique, alkalimetric measurement Carbon 18/8 steel foil equilibration technique Electrochemical cell using molten salt electrolyte Diffusion cell, C diffuses through Fe membran into gas phase, subsequent gas analysis

20 Corrosion and Reactions with Impurities Dissolved in Sodium 3.Sodium corrosion of reactor materials 3.1 Corrosion of iron base alloys 3.2 Corrosion of nickel base alloys 3.3 Corrosion of some refractory alloys

21 Na Corrosion of Reactor Materials Core materials have to withstand sodium at highest temperature and flow velocity Life time of these materials is limited to about 2 years Cladding materials are stabilized or unstabilized austenitic stainless steels Structural materials are exposed to sodium of lower temperature and flow velocity Life time is in the order of more than 30 years Austenitic steels 304 and 316 are used

22 Schematic view on corrosion effects of cladding tubes

23 Corroded surface of steel after 5000 h in Na at 700 °C Bright spots are Mo rich particles on ferritic surface grains

24 Downstream effect of mass loss Corrosion is influenced by the position in the tubes The geometric factor L/D (tube length of specimen position over hydraulic diameter) was introduced by Zebroski: R = v c O exp [ –13300/T – L/D /(t+1)] (R = rate of metal removal in mg/dm 2 /month, V = sodium velocity in ft/sec, c O = concentration of oxygen in ppm, t = time in months, T = sample temperature in K)

25 Downstream effect reported by Suzuki and Mutoh (steel 316 at 700°C, 2 ppm O & 4 m/s, cw and st)

26 Na Corrosion of nickel base alloys Inconel 600 and Nimonic were already studied by Thorley and Tyzack (1967) Corrosion rates at 650°C were 3 times higher than with steel AISI 316 There was no influence of the oxygen activity on corrosion rates Increasing flow velocity of sodium increased corrosion rates significantly Surface layers were not changed in their composition

27 Corrosion of refractory metals Mo, W, and Re have extremely low solubility in Na Alloys based on these metals are well compatible with Na Mo alloys are sensitive to oxidation in Na with oxygen concentrations close to saturation Such alloys may be used for particular high temperature applications

28 Corrosion of vanadium alloys Solubility of vanadium in sodium is very low V alloys react with non-metals (C,N,O) present in sodium They are oxidized in cold trapped sodium V-Ti-Cr alloys are corroded by internal oxidation V alloys have been considered as alternative fuel element clads Hardened surface layers are formed Depths of such layers depends on the composition

29 Corrosion of vanadium alloys formation of hardened layers through internal oxidation V-1Ti-15Cr (1), V-2Ti-15Cr (2), V-3Ti-15Cr (3), 1000 h at 600 °C

30 Corrosion and Reactions with Impurities Dissolved in Sodium 4. Corrosion of some ceramic materials (structural and functional ceramics, graphite core catcher minerals and concrete)

31 Corrosion of some ceramic materials Materials satisfying the thermodynamic stability criterion for sodium compatibility: Alumina, beryllia, magnesia, thoria, zirconia are compatible oxides Compatibility depends on the purity of the ceramic materials Unsufficient purity causes intercrystalline corrosion and desintegration of samples

32 Corrosion of some ceramic materials Basalt and concrete that are proposed as core catcher material are poorly compatible Their humidity causes violent reactions Functional ceramics as ThO 2 -Y 2 O 3 are compatible up to 400 °C Graphite as getter material for radioactive cesium is compatible to high temperatures

33 Corrosion and Reactions with Impurities Dissolved in Sodium 5.Na Corrosion studies 5.1Corrosion studies in stagnant and isothermal liquid metal 5.2Corrosion studies in flowing liquid Na and thermal gradients

34 Na Corrosion studies - tests in stagnant sodium Corrosion tests in stagnant liquid sodium are performed in sealed capsules Capsule material should not be different from specimen material Capsules to be filled in inert atmosphere glove boxes Closure by vacuum tight flanges or welding Purity of sodium is not always well defined in capsules Oxygen meters allow studies of chemical reactions between Na, materials and oxygen in capsules

35 Na Corrosion studies - testing in sodium loops Simple figure-of-eight corrosion loop (Klueh) Hot part separated from cold one by heat exchanger Heat exchanger acts as economizer Test sections in hot part, pump & flow meter in cold one cold trap in by-pass Expansion tank is an important component of a Na loop Auxiliary inert gas circuit

36 Conclusions concerning Na corrosion Corrosion of clad and duct materials under present core conditions is within the allowable range Corrosion in the core must be minimized to reduce activation of the cooling systems Mass loss and formation of corrosion layers does not lead to problems in piping and vessels Selection of steels should avoid mass transfer due to different chemical activities Minimized corrosion demands for low oxygen activities Downstream effect restricts effects in components

37 Conclusions concerning Na corrosion Materials tested in the work referred are limited to 700 °C in the core and 600 °C in the pipes and vessels Advanced materials for higher temperature have to be studied in respect to their behaviour in contact with Na The possibility of mass transfer has to be considered

38 Thanks to the Pioneers of Na corrosion Pioneer work in 50th and 60th mainly in USA, UK, France, Russia, Japan, Germany DeVan, Epstein, Weeks, Zebroski (USA) Champeix (France) Thorley (UK) Furukawa, Suzuki (Japan) Ivanovski, Ioltukhovsky (Russia)

39 Thanks to colleagues of the SNR & EFR projects Casteels, Tas, Grosser, Menken, Hissink and Kolster in the co-operation for the SNR 300 project Debergh, Horton, Huthmann, Wood in the EFR co-operation Drechsler, Frees, Peric, Schneider, Wittig in the sodium work at KfK The former Fast Breeder Project of KfK as generous supporter of our work


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