Modulo di “Tecniche di vuoto e films sottili” Study of hard coatings for steel protection fromLiquid Metal EmbrittlementProf. Enzo PalmieriLaurea in Scienza dei MaterialiUniversità di Padova
Liquid metal cooling• Leno Facility LNL• Liquid metal cooled reactors Nucleare IV generazione Nuclear Propulsion• Spallation Target cooling for Accelerator Driven System for Nuclear Wast Trasmutations
EXTRAORDINARILY HIGH HEAT EXCHANGING POWER• Fast neutron reactor cores tend to generate a lot of heat in a small space when compared to reactors of other classes.• The liquid metals used typically have extremely good heat transfer characteristics• Ideally the coolant should never boil as that would make it more likely to leak out of the system, resulting in a loss of coolant accident.
Liquid Metal cooled nuclear Reactors An advanced type of fast neutron reactorwhere the primary coolant is a liquid metal. Liquid metal cooled reactors were firstadapted for nuclear submarine use, but havealso been extensively studied for powergeneration applications.
PROPULSIONSubmarinesThe Soviet Alfa class submarine used a reactor cooled by alead-bismuth alloy. USS Sea wolf (SSN-575) was thesecond nuclear submarine, and the only U.S. submarine tohave a sodium-cooled nuclear power plant.Leaks in its superheaters made the submarines Sodium-cooled reactor replaced with pressurized water reactors.Nuclear aircraftLiquid metal cooled reactors were studied for use innuclear aircraft as part of the Aircraft Nuclear Propulsionprogram up to 1979. From a few years there is a renewedinterest in France, India, USA, Italy
LEAD-BISMUTH COOLED ACCELERATOR DRIVEN TRANSMUTATION SYSTEM The reference target design assumes tohave a hemispherical beam window made ofChromium-molybdenum steel cooled byflowing Pb-Bi One of the high priority issues isdegradation of structural material in aPb-Bi coolant at high proton and neutronfluxes and high temperatures
ADS PREREQUISITES• In Japan, there is enough employment experience for liquid Pb-Bi in period of about 17 years and absence of corrosion for thethermal conductive materials (1Cr-0.5Mo steel) used under thecondition of natural convection with temperature around 400°C• Extensive experience in the use as Russian submarines and inR&D during about 50 years are available. As a result, it will beable to lead approximately zero corrosion for Cr-Si materials byadjusting oxygen film with oxygen concentration control between10-7 to 10-5% mass
ADS PREREQUISITES Polonium forms PbPo in Pb-Bi, and the evaporation ratebecome less three factor than that of Po, andfurthermore, the rate decreases in the atmosphere. Theeffects of Po on employee and environment will not bedominant in comparison with those of fission products In Bi-resource, a confirmed amount will be 260 000tonnes and an estimated amount will become ten times ofthe confirmed ones by including resources in Russia. Thisshows there are enough amounts for ADS developments
Nuclear Spallation A particle accelerator shoots on a cooled Hg, Ta orother heavy metal target to produce a beam ofneutrons with 20 to 30 neutrons expelled after eachimpact European Spallation Source (ESS) should be in Lund,Sweden and its construction is expected to becompleted around 2018–19 Either a liquid Pb-Bi alloy, liquid mercury or solidtungsten will be used in quantities of around 20 tonnes
IV GenerationResearch into these reactor types was officially started by the GenerationIV International Forum (GIF) based on eight technology goals.• improve nuclear safety,• improve proliferation resistance• minimize waste and natural resource utilization• decrease the cost to build and run such plants. The claimed benefits include: • Nuclear waste that lasts decades instead of millennia. • 100-300 times more energy yield from the same amount of nuclear fuel.• The ability to consume existing nuclear waste for production of electricity
IV Generation The lead-cooled fast reactor features a fast-neutron-spectrumliquid-metal-cooled reactor with a closed fuel cycle and a largemonolithic plant option at 1,200 MW. The fuel is metal or nitride-based containing fertile uranium andtransuranics. The LFR is cooled by natural convection with a reactoroutlet coolant temperature of 550 °C, possibly ranging up to800 °C with advanced materials.
Liquid metal coolantsCoolant Melting point Boiling pointMercury -38.8°C 356.7°CNaK -11Cº 785ºCSodium 97.7°C 883°CLead-bismuth eutectic 123.5°C 1670°CLead 327.5 °C 1749 °C
Cooling Criteria Waters boiling point is also much lower than mostmetals demanding that the cooling system be kept athigh pressure to effectively cool the core. Pressurized water could theoretically be used for a fastreactor, but it tends to slowdown neutrons and absorbthem. This limits the amount of water that can be allowed toflow through the reactor core, and since fast reactors have ahigh power density most designs instead use molten metals.
Mercury At LANL, Clementine was the code name for the worldsfirst fast neutron nuclear experimental scale reactor The maximum output was 25kW and was fueled byPlutonium The core was cooled by liquid Mercury sinceit is liquid at room temperature IT resulted that Hg was not an ideal coolingmedium due to its poor heat transfer characteristics,high toxicity, high vapor pressure, low boiling point,producing noxious fumes when heated, relatively lowthermal conductivity, high neutron cross section
Sodium and NaK Sodium and NaK dont corrode steel to anysignificant degree and are compatible with many nuclearfuels They do however ignite spontaneously on contactwith air and react violently with water, producinghydrogen gas Neutron activation of sodium also causes theseliquids to become intensely radioactive during operation,though the half-life is short
Lead The advantage of a high boiling point,compared to water, makes not needed thepressurization of the reactor at high temperatures.This improves safety as it reduces theprobability of a dramatic loss of coolant accident,and allows for safer designs
Lead Pb has excellent neutron properties(reflection, low absorption) and is a very potentradiation shield against gamma rays. However,because lead has a high melting point and a highvapor pressure, it is tricky to refuel and servicea lead cooled reactor.
Lead-Bismuth Eutectic The Lead melting point can be lowered for lead-bismuth eutecticthat is unfotunately highly corrosive to most metals usedfor structural materials. The eutectic alloy of lead (44.5%) and bismuth (55.5%) is aproposed coolant for the lead-cooled fast reactor, part of theGeneration IV reactor initiative. It has a melting point of 123.5°C (pure lead melts at327°C) and a boiling point of 1670°C. Alloys with between 30% and 75% bismuth all have melting points below 200°C. . While lead expands slightly on melting and bismuth contracts slightly on melting, LBE has negligible change in volume on melting.
LBE an upper limit on the The corrosivity of Pb-Bivelocity of coolant flow through the reactor dueto safety considerations. Furthermore, the higher melting points of Lead andLBE may mean that solidification of the coolantmay be a greater problem when the reactor is operatedat lower temperatures. Bi in LBE Finally, upon neutron radiation thecoolant will undergo neutron capture andsubsequent beta decay, forming polonium, a potentalpha emitter.
Liquid Metal Embrittlement For many systems in which a liquid metal is incontact with a polycrystalline solid, deep liquid grooves form where the grainboundary meets the solid-liquid interface.
Liquid Metal Cracking “A form of embrittlement that results fromthe combined action of a tensile stress and aliquid metal in contact with the alloy surface.Metals with low melting temperatures, such asmercury, cadmium and zinc, can cause liquidmetal cracking.” For example, liquid Ga quickly penetratesdeep into grain boundaries in Al, leading tointergranular fracture under very small stresses.
The liquid metal may invade grain & interphase boundaries Hg + Al = Hg(Al) PLAY MOVIE Hg(Al) + 6H2O = Al2O3.3H2O + H2 + Hg
Skikda Algeria – January 19, 2004(Liquid Metal Embrittlement, LNG Plant, 27 killed 72 injured, USD 30,000,000)The report concluded that the escaped gas was from the cryogenic heat exchanger due to LME
Brittle intergranular fracture Very deep grooves form at the intersectionsof grain boundaries and at the surface ofsystems where a liquid metal is in contact with apolycrystalline solid. In some systems, such as Al-Ga, Zn-Ga, Cu-Biand Ni-Bi, the liquid film quickly penetratesdeep into the solid along the grain boundaryand leads to brittle intergranular fracture underthe influence of even modest stresses.
LME and Grain Boundaries: Al/Ga Ludwig et al. (2006)
Microradiographs showing liquid Ga penetrationalong an Al bicrystal grain boundary
Energies of GBs for simplified orientation space (a: symmetric tilt GB, b: symmetric twistGB e.g. energy of symmetric tilt GB (Read and Shockley): GB = B[A – ln()]
Schematic of GB with solute segregation Orientation space of GBs is 5-d (compared with surfaces that have 2-d orientation space). 5-d space often described by 3 Euler angles + vector perpendicular to GB plane.
Grain Boundaries (GBs)Special type of interface in single phase materials. Play important role in properties of poly-crystalline materials.
Interfacial EquilibriumGB with surface GB = (hkl)1 cos() + (hkl)2 cos() or for isotropic surface: GB = 2s cos()
good wetting <90° bad wetting >90° Grain boundaries SL GB /2 SL 2 SP cos/2 = GB LV V (SV, LV, SL) S1L L GBSV SL mechanical equilibrium S2L S chemical equilibrium S1P + S2P = GB SV - SL = LV cos (Young)
Example: AFM image of GB grooves at pure Cu surface
Example of GB wettingSince GB is more anisotropic than SL, there can be conditions where some highenergy GBs are completely wet while low energy GBs are still dry. Wet GBs will lead to "liquid metal embrittlement"
Velocity• High velocity of corrosive medium increases corrosion.• Corrosion pdts are formed rapidly, mainly because chemicals are brought to the surface at a high rate.• The accumulation of insoluble film on the metallic surface is prevented. So corrosion resistance of these films decreases.• The corrosion products s are easily stifled and carried away, thereby exposing the new surfaces for corrosion
Many theories have been proposed for LME• The dissolution-diffusion model of Robertson and Glickman says that adsorption of the liquid metal on the solid metal induces dissolution and inward diffusion. Under stress these processes lead to crack nucleation and propagation.• The brittle fracture theory of Stoloff and Johnson, Westwood and Kamdar proposed that the adsorption of the liquid metal atoms at the crack tip weakens inter-atomic bonds and propagates the crack.• Gordon postulated a model based on diffusion-penetration of liquid metal atoms to nucleate cracks which under stress grow to cause failure.• The ductile failure model of Lynch and Popovich predicted that adsorption of the liquid metal leads to weakening of atomic bonds and nucleation of dislocations which move under stress, pile- up and work harden the solid. Also dissolution helps in the nucleation of voids which grow under stress and cause ductile failure.
However, …a quantitative prediction of LME is still elusive
Galvanic corrosion• It is associated with the flow of current to a less active metal from a more active metal in the same environment.• Coupling of two metals, which are widely separated in the electrochemical series, generally produces an accelerated attack on the more active metal
Oxygen conc cell• due to the presence of oxygen electrolytic cell• i.e. diff in the amt of oxygen in solution at one point exists when compared to another
LME runs along Oxides fixturesA perfectly and compact oxyde film is needed
Mercury readily “wets” most surfaces andforms amalgams with a number of metals.This is a potentially reactive metal protectedfrom attack by air and water by an oxide layer.If the protective oxide layer is and liquidmercury is present then an amalgam is formedand this will allow rapid reaction with air orwater.
Hydrogen embrittlement• hydrogen can penetrate carbon steel and react with carbon to form methane.• The removal of carbon result in decreased strength.• Corrosion is possible at high temp as significant hydrogen partial pressure is generated.• This cause a loss of ductility, and failure by cracking of the steel.• Resistance to this type of attack is improved by allowing with chromium / molybdenum.
Thin films, coatings, cladding aremandatory for steel protection
DLC does not work mainly because of Graphite corrosion• When carbon steel is heated for prolonged periods at temp greater than 455 C, carbon may segregated, which is then transformed in to graphite. So the structural strength of the steel is affected.• Employing killed steels of Cr and Molybdenum or Cr and Ni can prevent this type of corrosion.
NITRIDINGBeneficial Effect of Nitriding:• Obtain high surface hardness• Increase wear resistance• Improve fatigue life• Improve corrosion resistance (except for stainless steels)• Obtain a surface that is resistant to the softening effect of heat (at temperatures up to the nitriding temperature)
Legame Metallico Legame Covalente Legame IonicoBoruri, Boruri, Carburi e Ossidi diCarburi e Nitruri di Nitruri di Al, Si, B; Al, Zr, Ti, BeMetalli di Transizione DiamanteEs.: TiB2, TiC, TiN, WC Es.: B4C, SiC, BN Es.: Al2O3, ZrO2, BeO
Schema delle proprieta’ di Boruri (b), Carburi (c) e Nitruri (n) Durezza Fragilita’ Punto Stabilita’ Coeff.Esp. Aderenza Tendenza fusione termica substrato interagireIn basso grado n b n b b n n i c c b c c c cIn alto grado b n c n n b b
Tipi di matching all’interfaccia Film/Substrato (a) (b) (c)(a) Interfaccia fra sistemi coerenti fra materiali duri a legame metallico(b) Interfaccia a fasi miste fra materiali duri metallici e materiali ionici(c) Interfaccia a fasi non interagenti fra materiali a legame covalente
Il Ti-Al-N è alquanto simile al TiN. Ha la stessa struttura fcc, con la differenza che gli atomi di Alsostituiscono quelli di TiParametro reticolare a: aTi-Al-N < aTiN in funzione del contenuto di Al Cella unitaria del TiN con inclusioni di Alluminio Guardando il rapporto d’impacchettamento si capisce immediatamente perché introduzione dell’ Al rende il materiale più duro
In sintesi, Cosa potrebbe funzionare?Nitruri binari o ternari di Ti, Cr, Sie ….. Ossidi?