This document discusses hydrogen embrittlement of metals. It provides background on hydrogen properties and issues with metals. There are three main mechanisms of hydrogen embrittlement: hydride-induced embrittlement, hydrogen-enhanced decohesion, and hydrogen-enhanced localized plasticity. Prevention methods include reducing corrosion, heat treating to remove hydrogen, and using less susceptible materials. The effects of hydrogen embrittlement include increased strength and decreased ductility as hydrogen concentration increases.

1. Hydrogen Embrittlement
of Metals
M.Vishnukumar
B.E. Mechanical department
Gojan School of Business and Technolgy
Chennai

2. SYNOPSIS
 HYDROGEN PROPERTIES
 TECHNICAL ISSUES
 TYPES OF HYDROGEN EFFECTS
 HYROGEN EMBRITTLEMENT
 TYPES OF HE MECHANISMS
 VARIOUS EFFECTS
 PREVENTION
 CONCLUSION

3. Motivation
Hydrogen is a promising medium for
energy transfer
 Advantages: Plentiful
 Inexpensive
 Flexible
 Renewable
 Sustainable
 Safe
 Drawbacks:Conversion efficienciesGenerators
 Fuel cells
 Compressors
 Energy content per unit volumeElectrons v. Gas
 Tube trucks
 Pipelines
 Safety (infrastructure)Flammability Range (≈ 4-75 v% in air)
 Hydrogen alters properties (embrittlement)
 Codes and standards for safe high pressure containers
 Reliable, evaluated, data on properties in hydrogen
 Standardized test methods for measuring properties in hydrogen

4. Outline
 What is hydrogen embrittlement (HE)?
 The effect of HE in metals.
 The mechanisms of HE.
 The reduction of HE.
 Conclusion

5. Technical Issues(1) Materials
 All types of structural materials will be exposed to
hydrogen at elevated pressures and expected to
perform their function. For example:
 Steels-transmission pipelines, compressors, storage
containers, dispensers
 Al Alloys-containers, tube trucks, on-board storage
 Polymers and PMCs-distribution pipelines, storage
containers
 Stainless Steels-fuel cell containers and components,
on-board tubing and storage
 Ni Based Alloys-compressors, valves, bearing surfaces
 Others-Cu alloys, Ti Alloys, …

6. HYDROGEN EFFECTS
Hydrogen can degrade metals by:
 hydrogen blistering;
 hydrogen embrittlement;
 decarburization;
 hydrogen attack.

7. TYPES OF HE MECHANISMS
No. of Mechanism
1 Hydride-induced embrittlement
(Second-phase mechanism)
2 Hydrogen-enhanced decohesion
mechanism, HEDE (brittle fracture)
3Hydrogen-enhanced localized
plasticity mechanism HELP (ductile
fracture)

8. Hydride-induced embrittlement
 The stress-induced hydride formation and cleavage mechanism is one
of the well established hydrogen embrittlement mechanisms with
extensive experimental and theoretical support.
 The nucleation and growth of an extensive hydride field ahead of a
crack has been observed dynamically by Robertson et al. [26] in α-Ti
charged from the gas phase in situ in a controlled environment
transmission electron microscope [27]. In their observations the
hydrides first nucleated in the stress-field of the crack and grew to
large sizes not by the growth of individual hydrides but by the
nucleation and growth of new hydrides in the stress 2 field of the
others. They showed that these small hydrides grew together to form
the larger hydrides. This auto-catalytic process of hydride nucleation
and growth together with brittle nature of them seems to be the main
cause of embrittlement of typical hydride former element, i.e. the
element of the group Vb; e.g., V, Nb, Ti and Zr.

9. Hydrogen-enhanced decohesion
 The decohesion model is one of the oldest models used to
represent the change of properties as a result of atomar
hydrogen. It was described first in 1941 by Zapffe and Sims
 It is based on the increased solubility of hydrogen in a
tensile strength field, for instance on the tip of a crack or in
areas with internal tensile strength or in the tension field
of edge dislocations .The increased solubility of hydrogen
in this tension field results in a decrease in the atom
binding forces of the metal lattice. The influence of stress
results in a premature brittle-material fracture along the
grain boundaries (intergranular cleavage) or network levels
(transgranular cleavage) owing to the decrease of the
binding forces.

10. Hydrogen-enhanced localized
plasticity
 The most recent process model by far is the so-called HELP (Hydrogen Enhanced Local
 plasticity) process [1].
 A prerequisite for the HELP process is, as is the case with the decohesion model, the
accumulation of hydrogen in the field of stress, for instance, in the vicinity of the tips of
cracks or in the stress areas of dislocations (carriers of plasticdeformation in a metal
grid).
 During the initiation of a dislocation movement by
 introducing external stresses, the existing active hydrogen considerably eases the
 dislocation movement through shielding the fields of stress of the dislocations against
 each other as well as against other grid defects (see Fig. 2).
 Therefore, a local dislocation movement will already occur at low levels of shearing
stress, which is caused by a local drop of yield stress due to hydrogen. A sliding
localization occurs, leading to a micro crack caused by the formation of micro pores and
shearing action.
 As soon as the crack leaves the area of reduced yield stress, it will not propagate any
further.

11. Embrittlement
Similar to blistering . . . hydrogen enters metal lattice . .
.BUT . . .interaction with metal lattice different. High-
strength (and more brittle) steels are susceptible.
H-embrittlement different from SCC in nature of cracks .
. . stress-corrosion cracks usually propagate anodically;

12. Hydrogen embrittlement is a type of failure that
affects almost all metals and alloys. When
hydrogen is present, materials fail at load
levels that are very low compared with those
that a hydrogen-free material can sustain. And
a lot of scientists have studied on this field for
many years.

13. Embrittlement (?)
 Reverse strain rate sensitivity from normal
embrittlement mechanisms
 •Hydrogen must be present during loading (exception
blistering)
 •Effect fully reversed on removal of H unless damage
occurred
 •Temperature minimum
 •Slow, sub-critical, crack propagation

14. Issues(2) Hydrogen effects
properties (embrittlement)
 Hydrogen is one of the most influential alloying elements
 2.Many metals and alloys readily absorb hydrogen from
H2(g) or other hydrogen bearing environments
 3.Hydrogen dissolves as an interstitial and diffuses rapidly
 4.Hydrogen alters physical properties
 5.Hydrogen can dramatically alter the deformation and
fracture behavior of metals and alloys by
 Modifying normal modes of deformation and fracture
 Precipitating as brittle metal hydrides or H2(g) blisters
 Induce new brittle "cleavage-like" fracture modes
 Lowering the cohesive strength of interfaces

16.  In-situ charging is requiredHydrogen must be present
in the microstructure at the concentrations expected
in service for an experiment to accurately represent the
effects
 Solubility varies with cold work
 Cold work varies in the plastic zone at the crack tip (K,
x,y,z)
 Rapid diffusion in and out (pre-charging)

18. Susceptibility
 Susceptibility tends to increase with increasing strengthHigh
strength steels are very susceptible, but not the only alloys
susceptibleLit. show H effects pure Fe and other elements
 Other alloy classes are influenced (Ni, Ti, Al)
 Existing H pipelines use low yield strength steels (<60ksi)
 Existing H pipelines require special practices to keep yield
strengths low
 Existing natural gas and oil pipelines regularly fail by
hydrogen embrittlement typically at "hard spots" at welds,
bends, gouges, dings, and dents

24. a) Effect of H on the ductility of
ultra high purity Nb

25. c) Effect of increasing H fugacity on
the fracture mode of a steel

26. d) MVC-IG Transitionin a Ni alloy

27. b) Effect of H on the fracture mode
of a titanium alloy

28.  Existing susceptibility measures tend to vary with the testing conditionsStrain rate
dependence
 Surface treatment and finish dependent
 Gas purity dependent
 Solubility dependence
 Diffusivity (rate) dependence
 5.Fracture (and deformation) properties do not scale well with sample sizeSample
thickness constraint (plane strain v. plane stress)
 Diffusion and sample size effects
 6.Fatigue (cyclic) loading is relatively unexplored and provides bare surface and time for
solid state diffusion during each load cycle
 7.The high pressure hydrogen gas "in-situ" testing chamber and the time required for
hydrogen saturation of large samples becomes a "bottle-neck" limiting high throughput
testing

29. Effect of hydrogenation time on yield strength of steel.[2]
The effects of HE

30. The effects of HE
Plasticity loss induced by atomic hydrogen I(H) vs hydrogen
concentration C0 for type 308, 347L and 304L.I(H) reach a
saturation value after C0 >100ppm[1]

31. The mechanisms of HE
 Diffusion of hydrogen in steels will result in interactions of hydrogen
with dislocations. Interference with dislocations will cause a change in
mechanical properties of the steels.
 The increasing in yield strength on hydrogenated steel is based on
interpretation that hydrogen drag upon moving dislocation or
impeding cross slip.
 The later reduction of fracture stress of the material is due to the
diffusion of hydrogen atoms into the lattice precipitate. The pressure
exerted by the gas adds to the external applied tensile load on the
specimen.
 Three specific mechanism of HE is viable: induced hydride formation
and cleavage; hydrogen-enhanced localized plasticity; hydrogen-
induced decohesion.[3]

32. Prevention of Embrittlement
 reduce corrosion rate (inhibitors, coatings, etc.);
 change electroplating process to minimize H effects
(voltage, current density, bath composition, etc.);
 bake material to remove H;
 minimize residual stresses;
 use less susceptible material;
 maintain clean conditions during welding.

33. C: corroded metal CH: heating corroded metal CR: removal of the
corrosion layer CRH: removal of corrosion layer and heating[4]
The reduction of HE.

34. The reduction of HE.
The results are interpreted
as follows: Corrosion
creates damage, resulting
a depth of attack. Below
the corrosion layer, a
hydrogen zone is
established due to the
diffusion of hydrogen
released by the corrosion
reactions.(a) Removal of
corrosion removes all the
above defects and
thus restores strength. (b)
Heating the alloy liberates
all hydrogen and restore
ductility.(c)[4]

35. The reduction of HE.
Tensile properties of steel specimens tested in air and a sulfureted
hydrogen solutions shows redution of HE susceptibility.[5]

36. Conclusion
 The HE affect the tensile properties of metals. When exposure to hydrogen the
strength properties is increasing primarily due to the presence of a large
concentration of hydrogen at various trap sites and the interaction of hydrogen
with these traps.
 The strength properties is decreasing later because of the initiation and growth
of cracks at the pre-existing stress defects.
 The plasticity loss caused by atomic hydrogen increased with increasing in
concentration of hydrogen.
 Removal of the corrosion layer and heating to activate all hydrogen traps leads
complete restoration of strength and ductility.
 Other method such as laser surface annealing can reduce the HE susceptibility,
however may cause the decreasing in tension strength in general condition.

37. References[1] C. Pan, Y.J. Su, W.Y. Chu, et al, “Hydrogen embrittlement of weld metal
of austenitic stainless steels”, Corrosion Science, 44 (2002), p1983–1993
[2] R.A. Siddiqui, H.A. Abdullah, “Hydrogen embrittlement in 0.31% carbon
steel used for petrochemical applications”, Journal of Materials Processing
Technology, 170 (2005) ,p430–435
[3] P. Sofronis, I.M. Robertson, “Viable Mechanisms of Hydrogen
Embrittlement-A Review”, American Institute of Physics, 2006, p837
[4] H. Kamoutsi, G.N. Haidemenopoulos, V. Bontozoglou, S. Pantelakis,
“Corrosion-induced hydrogen embrittlement
in aluminum alloy 2024”, Corrosion Science, 48 (2006), p1209–1224
[5] L . W. TSAY, T. Y. YANG, “Reduction of hydrogen embrittlement in an
ultra-high-strength steel by laser surface annealing”, Fatigue Fract Engng
Mater Struct, 23, p325–333