Study of hard coatings forsteel protection from

Liquid Metal Embrittlement

Modulo di “Tecniche di vuoto e filmssottili”

Prof. Enzo PalmieriLaurea in Scienza dei Materiali

Università di Padova

Liquid metal cooling

• Leno Facility LNL

• Liquid metal cooled reactors

Nucleare IV generazione

Nuclear Propulsion

• Spallation Target cooling for AcceleratorDriven 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.

Part I

APPLICATION

Liquid Metal cooled nuclear Reactors

An advanced type of fast neutron reactor

where the primary coolant is a liquid metal.

Liquid metal cooled reactors were first

adapted for nuclear submarine use, but have

also been extensively studied for power generation 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 submarine's 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 COOLEDACCELERATOR 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 the

thermal conductive materials (1Cr-0.5Mo steel) used under the

condition of natural convection with temperature around 400°C

• Extensive experience in the use as Russian submarines and in

R&D during about 50 years are available. As a result, it will be

able to lead approximately zero corrosion for Cr-Si materials by

adjusting oxygen film with oxygen concentration control between

10-7 to 10-5% mass

Polonium forms PbPo in Pb-Bi, and the evaporation rate

become less three factor than that of Po, and

furthermore, the rate decreases in the atmosphere. The

effects of Po on employee and environment will not be

dominant in comparison with those of fission products

In Bi-resource, a confirmed amount will be 260 000

tonnes and an estimated amount will become ten times of

the confirmed ones by including resources in Russia. This

shows there are enough amounts for ADS developments

ADS PREREQUISITES

Nuclear Spallation

A particle accelerator shoots on a cooled Hg, Ta or

other 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 solid

tungsten will be used in quantities of around 20 tonnes

IV GenerationResearch into these reactor types was officially started by the Generation

IV 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-spectrum

liquid-metal-cooled reactor with a closed fuel cycle and a large

monolithic plant option at 1,200 MW.

The fuel is metal or nitride-based containing fertile uranium and

transuranics. The LFR is cooled by natural convection with a reactor

outlet coolant temperature of 550 °C, possibly ranging up to

800 °C with advanced materials.

Part II

Liquid Metal Cooling

Liquid metal coolants

Coolant Melting point Boiling point

Mercury -38.8°C 356.7°C

NaK -11Cº 785ºC

Sodium 97.7°C 883°C

Lead-bismuth eutectic 123.5°C 1670°C

Lead 327.5 °C 1749 °C

Cooling Criteria

Water's boiling point is also much lower than most

metals demanding that the cooling system be kept at

high pressure to effectively cool the core.

Pressurized water could theoretically be used for a fast

reactor, but it tends to slowdown neutrons and absorb

them.

This limits the amount of water that can be allowed to

flow through the reactor core, and since fast reactors have a

high power density most designs instead use molten metals.

MercuryAt LANL, Clementine was the code name for the world's

first fast neutron nuclear experimental scale reactor

The maximum output was 25kW and was fueled byPlutonium

The core was cooled by liquid Mercury since

it 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 don't corrode steel to any

significant degree and are compatible with many nuclear

fuels

They do however ignite spontaneously on contact

with air and react violently with water, producing

hydrogen gas

Neutron activation of sodium also causes these

liquids to become intensely radioactive during operation,

though the half-life is short

LeadThe advantage of a high boiling point,

compared to water, makes not needed the

pressurization of the reactor at high temperatures.

This improves safety as it reduces the

probability of a dramatic loss of coolant accident,

and allows for safer designs

LeadPb has excellent neutron properties

(reflection, low absorption) and is a very potent

radiation shield against gamma rays. However,

because lead has a high melting point and a high

vapor pressure, it is tricky to refuel and service

a lead cooled reactor.

Lead-Bismuth EutecticThe Lead melting point can be lowered for lead-bismuth eutectic

that is unfotunately highly corrosive to most metals used

for structural materials.

The eutectic alloy of lead (44.5%) and bismuth (55.5%) is a

proposed coolant for the lead-cooled fast reactor, part of the

Generation IV reactor initiative.

It has a melting point of 123.5°C (pure lead melts at

327°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

The corrosivity of Pb-Bi an upper limit on thevelocity 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.

Finally, upon neutron radiation the Bi in LBEcoolant will undergo neutron capture andsubsequent beta decay, forming polonium, a potentalpha emitter.

Part III

Corrosion due to liquid metal flow

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 from the combined action of a tensile stress and a liquid metal in contact with the alloy surface. Metals with low melting temperatures, such as mercury, cadmium and zinc, can cause liquid metal cracking.”

For example, liquid Ga quickly penetrates deep into grain boundaries in Al, leading to intergranular fracture under very small stresses.

PLAY MOVIE

The liquid metal may invade grain &

interphase boundaries

Hg + Al = Hg(Al)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

Skikda Algeria – January 19, 2004

Hg + Al = Hg(Al)Hg(Al) + 6H2O = Al2O3.3H2O + H2 + Hg

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 penetration along an Al bicrystal grain boundary

Energies of GBs for simplified orientation space (a: symmetric tilt GB, b: symmetric twist GB

e.g. energy of symmetric tilt GB (Read and Shockley):

GB = B[A – ln()]

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.

Schematic of GB with solute segregation

Grain Boundaries (GBs)

Special type of interface in single phase materials. Play important role in properties of poly-crystalline materials.

Anisotropy of Interfacial Properties

Interfacial Equilibrium

GB with surface

GB = (hkl)1 cos() + (hkl)2 cos()

or for isotropic surface: GB = 2s cos()

good wetting <90° bad wetting >90°

LV

SV SL L

V

S

GB

/2

SL

SL

GB

S1L

S2L

S1P + S2P = GB

2 SP cos/2 = GB

(SV, LV, SL)

mechanical equilibrium

chemical equilibrium

SV - SL = LV cos (Young)

Grain boundaries

Example: AFM image of GB grooves at pure Cu surface

Example of GB wetting

Since GB is more anisotropic than SL, there can be conditions where some high energy GBs are completely wet while low energy GBs are still dry.

Wet GBs will lead to "liquid metal embrittlement"

Factors influencing corrosion

• Solution pH

• Oxidizing agent

• Temperature

• Velocity

• Stresses

• Impurity content

Stresses

Stresses

STRESSES

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

The effect of impurities

Polycrystalline Al

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 fixtures

A 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 protected from attack by air and water by an oxide layer.

If the protective oxide layer is and liquid mercury is present then an amalgam is formed and this will allow rapid reaction with air or water.

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 are mandatory 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.

NITRIDING

Beneficial 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 Ionico

Boruri,

Carburi e Nitruri di

Metalli di Transizione

Boruri, Carburi e

Nitruri di Al, Si, B;

Diamante

Ossidi di

Al, Zr, Ti, Be

Es.: TiB2, TiC, TiN, WC Es.: B4C, SiC, BN Es.: Al2O3, ZrO2, BeO

Durezza Fragilita’ Punto

fusione

Stabilita’ Coeff.Esp.

termica

Aderenza

substrato

Tendenza

interagire

In basso gradon b n b b n n

ic c b c c c c

In alto grado b n c n n b b

Schema delle proprieta’ di Boruri (b), Carburi (c) e Nitruri (n)

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

Fase Densita'

(g/cm3)

Punto di

fusione ( C)

Durezza

(HV)

Young

Modulo

kN/mm2

Resistiv.

(mW cm)

Coeff. espans. termica

(10-6/K)

TiB2 4.50 3225 3000 560 7 7.8

TiC 4.93 3067 2800 470 52 8.0 - 8.6

TiN 5.40 2950 2100 590 25 9.4

ZrB2 6.11 3245 2300 540 6 5.9

ZrC 6.63 3445 2560 400 42 7.0 - 7.4

ZrN 7.32 2982 1600 510 21 7.2

VB2 5.05 2747 2150 510 13 7.6

VC 5.41 2648 2900 430 59 7.3

VN 6.11 2177 1560 460 85 9.2

NbB2 6.98 3036 2600 630 12 8.0

NbC 7.78 3613 1800 580 19 7.2

NbN 8.43 2204 1400 480 58 10.1

Proprietà di differenti materiali metallici duri

TaB2 12.58 3037 2100 680 14 8.2

TaC 14.48 3985 1550 560 15 7.1

CrB2 5.58 2188 2250 540 18 10.5

Cr3C2 6.68 1810 2150 400 75 11.7

CrN 6.12 1050 1100 400 640 (2.3)

Mo2B5 7.45 2140 2350 670 18 8.6

Mo2C 9.18 2517 1660 540 57 7.8 - 9.3

W2B5 13.03 2365 2700 770 19 7.8

WC 15.72 2776 2350 720 17 3.8 - 3.9

LaB6 4.73 2770 2530 (400) 15 6.4

Fase Densita'

(g/cm3)

Punto di

fusione ( C)

Durezza

(HV)

Young

Modulo

kN/mm2

Resistiv.

(mW cm)

Coeff. espans. termica

(10-6/K)

Fase Densita'

(g/cm3)

Punto di

fusione( C)

Durezza

(HV)

Modulo di Young

kN/mm2

Resistiv.

(mW cm)

Coeff. espans.

termica (10-6/K)

B4C 2.52 2450 3-4000 441 0.5e+6 4.5 (5.6)

BN cub.) 3.48 2730 ~ 5000 660 1e+18 -

C (diam.) 3.52 3800 ~ 8000 910 1e+20 1.0

B 2.34 2100 2700 490 1e+12 8.3

AlB12 2.58 2150 (dec) 2600 430 2e+12 -

SiC 3.22 2760 (dec) 2600 480 1e+5 5.3

SiB6 2.43 1900 2300 330 1e+7 5.4

Si3N4 3.19 1900 1720 210 1e+18 2.5

AlN 3.26 2250 (dec) 1230 350 1e+15 5.7

Proprieta' di differenti materiali covalenti duri

Fase Densita'

(g/cm3)

Punto fusione

( C)

Durezza

(HV)

Modulo

Young

kN/mm2

Resistiv.

(mW cm)

Coeff. espans.

termica (10-6/K)

Al2O3 3.98 2047 2100 400 1e+20 8.4

Al2TiO5 3.68 1894 - 13 1e+16 0.8

TiO2 4.25 1867 1100 205 - 9.0

ZrO2 5.76 2677 1200 190 1e+16 11 (7.6)

HfO2 10.2 2900 780 - - 6.5

ThO2 10.2 3300 950 240 1e+16 9.3

BeO 3.03 2550 1500 390 1e+23 9.0

MgO 3.77 2827 750 320 1e+12 13.0

Proprieta' di differenti materiali eteropolari duri

Cella unitaria del TiN con inclusioni di Alluminio

Guardando il rapporto d’impacchettamento si capisce immediatamenteperché introduzione dell’ Al rende il materiale più duro

Il Ti-Al-N è alquanto simile al TiN. Ha la stessa struttura fcc, con la differenza che gli atomi di Alsostituiscono quelli di Ti

Parametro reticolare a: aTi-Al-N < aTiN in funzione del contenuto di Al

In sintesi, Cosa potrebbe funzionare?

Nitruri binari o ternari di Ti, Cr, Si

e ….. Ossidi?