liquid metal embrittlement search for protective coatings

1. Modulo di “Tecniche di vuoto e films
sottili”
Study of hard coatings for
steel protection from
Liquid Metal Embrittlement
Prof. Enzo Palmieri
Laurea in Scienza dei Materiali
Università di Padova

2. 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

3. 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.

4. Part I
APPLICATION

5. 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.

6. PROPULSION
Submarines
The Soviet Alfa class submarine used a reactor cooled by a
lead-bismuth alloy. USS Sea wolf (SSN-575) was the
second nuclear submarine, and the only U.S. submarine to
have a sodium-cooled nuclear power plant.
Leaks in its superheaters made the submarine's Sodium-
cooled reactor replaced with pressurized water reactors.
Nuclear aircraft
Liquid metal cooled reactors were studied for use in
nuclear aircraft as part of the Aircraft Nuclear Propulsion
program up to 1979. From a few years there is a renewed
interest in France, India, USA, Italy

7. LEAD-BISMUTH COOLED
ACCELERATOR DRIVEN TRANSMUTATION SYSTEM
The reference target design assumes to
have a hemispherical beam window made of
Chromium-molybdenum steel cooled by
flowing Pb-Bi
One of the high priority
issues is
degradation of structural material in a
Pb-Bi coolant at high proton and neutron
fluxes and high temperatures

8. 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

9. ADS PREREQUISITES
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

10. Nuclear Spallation
A particle accelerator shoots on a cooled Hg, Ta or
other heavy metal target to produce a beam of
neutrons with 20 to 30 neutrons expelled after each
impact
European Spallation Source (ESS) should be in Lund,
Sweden and its construction is expected to be
completed around 2018–19
Either a liquid Pb-Bi alloy, liquid mercury or solid
tungsten will be used in quantities of around 20 tonnes

11. IV Generation
Research 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

12. 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.

13. Part II
Liquid Metal Cooling

14. 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

15. 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.

16. Mercury
At 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 by
Plutonium
The core was cooled by liquid Mercury since
it is liquid at room temperature
IT resulted that Hg was not an ideal cooling
medium due to its poor heat transfer characteristics,
high toxicity, high vapor pressure, low boiling point,
producing noxious fumes when heated, relatively low
thermal conductivity, high neutron cross section

17. 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

18. Lead
The 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

19. Lead
Pb 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.

20. Lead-Bismuth Eutectic
The 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.

21. LBE
an upper limit on the
The corrosivity of Pb-Bi
velocity of coolant flow through the reactor due
to safety considerations.
Furthermore, the higher melting points of Lead and
LBE may mean that solidification of the coolant
may be a greater problem when the reactor is operated
at lower temperatures.
Bi in LBE
Finally, upon neutron radiation the
coolant will undergo neutron capture and
subsequent beta decay, forming polonium, a potent
alpha emitter.

22. Part III
Corrosion due to liquid metal flow

23. Liquid Metal Embrittlement
For many systems in which a liquid metal is in
contact with a polycrystalline solid,
deep liquid grooves form where the grain
boundary meets the solid-liquid interface.

24. 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.

25. The liquid metal may invade grain &
interphase boundaries
Hg + Al = Hg(Al)
PLAY MOVIE
Hg(Al) + 6H2O = Al2O3.3H2O + H2 + Hg

26. 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

27. Skikda Algeria – January 19, 2004
Hg + Al = Hg(Al)
Hg(Al) + 6H2O = Al2O3.3H2O + H2 + Hg

29. Brittle intergranular fracture
Very deep grooves form at the intersections
of grain boundaries and at the surface of
systems where a liquid metal is in contact with a
polycrystalline solid.
In some systems, such as Al-Ga, Zn-Ga, Cu-Bi
and Ni-Bi, the liquid film quickly penetrates
deep into the solid along the grain boundary
and leads to brittle intergranular fracture under
the influence of even modest stresses.

30. LME and Grain Boundaries: Al/Ga
Ludwig et al. (2006)

31. Microradiographs showing liquid Ga penetration
along an Al bicrystal grain boundary

32. 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()]

33. 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.

34. Grain Boundaries (GBs)
Special type of interface in single phase materials. Play important role in properties of poly-
crystalline materials.

35. Anisotropy of Interfacial Properties

38. Interfacial Equilibrium
GB with surface
GB = (hkl)1 cos() + (hkl)2 cos()
or for isotropic surface: GB = 2s cos()

39. good wetting <90° bad wetting >90° Grain boundaries
SL
GB
/2
SL
2 SP cos/2 = GB
LV V (SV, LV, SL) S1L
 L GB
SV SL
mechanical equilibrium S2L
S chemical equilibrium
S1P + S2P = GB
SV - SL = LV cos (Young)

40. Example: AFM image of GB grooves at pure Cu surface

41. 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"

42. Factors influencing corrosion
• Solution pH
• Oxidizing agent
• Temperature
• Velocity
• Stresses
• Impurity content

43. Stresses

44. Stresses

45. STRESSES

46. 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

47. The effect of impurities
Polycrystalline Al

48. 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.

49. However, …
a quantitative prediction
of LME is still elusive

50. 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

51. 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

52. LME runs along Oxides fixtures
A perfectly and compact oxyde film is needed

53. Mercury readily “wets” most surfaces and
forms 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.

54. 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.

57. Thin films, coatings, cladding are
mandatory for steel protection

58. 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.

59. 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)

60. Legame Metallico Legame Covalente Legame Ionico
Boruri, Boruri, Carburi e Ossidi di
Carburi e Nitruri di Nitruri di Al, Si, B; Al, Zr, Ti, Be
Metalli di Transizione Diamante
Es.: TiB2, TiC, TiN, WC Es.: B4C, SiC, BN Es.: Al2O3, ZrO2, BeO

61. Schema delle proprieta’ di Boruri (b), Carburi (c) e Nitruri (n)
Durezza Fragilita’ Punto Stabilita’ Coeff.Esp. Aderenza Tendenza
fusione termica substrato interagire
In basso grado
n b n b b n n
i c c b c c c c
In alto grado
b n c n n b b

62. 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

63. Proprietà di differenti materiali metallici duri
Fase Densita' Punto di Durezza Young Resistiv. Coeff. espans. termica
(g/cm3) fusione ( C) (HV) Modulo (mW cm) (10-6/K)
kN/mm2
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

64. Fase Densita' Punto di Durezza Young Resistiv. Coeff. espans. termica
(g/cm3) fusione ( C) (HV) Modulo (mW cm) (10-6/K)
kN/mm2
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

65. Proprieta' di differenti materiali covalenti duri
Fase Densita' Punto di Durezza Modulo di Young Resistiv. Coeff. espans.
(g/cm3) fusione( C) (HV) kN/mm2 (mW cm) 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

66. Proprieta' di differenti materiali eteropolari duri
Fase Densita' Punto fusione Durezza Modulo Resistiv. Coeff. espans.
(g/cm3) ( C) (HV) Young (mW cm) termica (10-6/K)
kN/mm2
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

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

68. In sintesi, Cosa potrebbe funzionare?
Nitruri binari o ternari di Ti, Cr, Si
e ….. Ossidi?