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. MotivationHydrogen is a promising medium for energy transfer Advantages: Plentiful Inexpensive Flexible Renewable Sustainable Safe Drawbacks:ConversionefficienciesGenerators 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. HYDROGENEFFECTS 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. Embrittlement 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. The effects of HE Effect of hydrogenation time on yield strength of steel.[2]

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

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

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