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Influence of Hydrogen on Materials

All materials deform under load. The stress which a structural material is able to withstand is conditioned by its ductility that is the ability to deform permanently prior to fracture. A material is elastic if, after being elongated under stress, it returns to its original shape as soon as the stress is removed. Elastic deformation is recoverable and involves both a change of shape and volume. At a certain strain, when the load exceeds the yield stress, the stress strain behaviour becomes non-linear and the material will retain a permanent deformation. A further increase of the strain eventually reaches the ultimate load called ‘tensile stress’ beyond which the stress decreases finally leading to rupture. Ductile materials can accommodate local stress concentrations, they can be greatly bent and reshaped without breaking. In contrast, brittle materials have only a small amount of elongation at fracture. The strength of ductile material is approximately the same in tension and compression, whereas that of brittle material is much higher in compression than it is in tension. Brittle materials do not show the phase of permanent elongation. They fail suddenly and catastrophically when they are exposed to tensile stress

Hydrogen can have two main damaging effects on materials:

- Low temperature effect. At low temperature for example, when it is stored in liquid form, it can have an indirect effect called “cold embrittlement”. This effect is not specific to hydrogen and can occur with all the cryogenic gases if the operating temperature is below the ductile-brittle transition temperature. Cryogenic temperatures can affect structural materials. With decreasing temperature, there is a decrease in toughness that is very slight in face centred cubic materials, but can be very marked in body centred cubic ones such as ferritic steels. Metals that work successfully at low temperatures include aluminium and its alloys, copper and its alloys, nickel and some of its alloys, as well as stable austenitic stainless steels.

– Hydrogen embrittlement. Hydrogen can have a direct effect on the material by degrading its mechanical properties; this effect is called “hydrogen embrittlement” and is specific to the action of hydrogen and some other hydrogenated gases. Hydrogen has to breakdown from its molecular into the atomic form in order to be able to enter the material and to produce the deleterious effect on its properties.

The effect of hydrogen on material behaviour, on its physical properties, is a fact. Hydrogen may degrade the mechanical behaviour of metallic materials and lead them to failure. Hydrogen embrittlement affects the three basic systems of any industry that uses hydrogen: Production, Transport/ Storage and Use.

When tensile stresses are applied to a hydrogen embrittled component, it may fail prematurely in an unexpected and sometimes catastrophic way. An externally applied load is not required as the tensile stresses may be due to residual stresses in the material. The threshold stresses to cause cracking are commonly below the yield stress of the material. Thus, catastrophic failure can occur without significant deformation or obvious deterioration of the component.

It can take place in two different ways:

– Internal Hydrogen Embrittlement. Takes place when hydrogen enters the metal during its processing. It is a phenomenon that may lead to the structural failure of material that never has been exposed to hydrogen before. Internal cracks are initiated showing a discontinuous growth. Not more than 0.1 - 10 ppm hydrogen in the average are involved. The effect is observed in the temperature range between 173 and 373 K and is most severe at near room temperature.

– External Hydrogen Embrittlement. Occurs when the material is subjected to a hydrogen atmosphere, e.g. storage tanks. Absorbed and/or adsorbed hydrogen modifies the mechanical response of the material without necessarily forming a second phase. The effect strongly depends on the stress imposed on the metal. It also maximizes at around room temperature. The hydrogen that has entered the material can be either in interstitial solid solution state or in a combined state forming H2, CH4 or hydrides. When hydrogen is dissolved in the material as interstitial solid solution promoting lattice decohesion, the phenomenon is called «gaseous hydrogen embrittlement». It takes place generally at temperatures close to ambient and the transport of the hydrogen occurs mainly by the net dislocations when the material is undergoing deformation. When hydrogen is present in a combined state, it is a matter of «hydrogen attack». It is a phenomenon in which the hydrogen chemically reacts with a constituent of the metal to form a new microstructural element or phase. The hydrogen can react with the carbon of the alloy to form molecules of methane; this leads to the formation of micro-cavities and to a lack of carbon in the alloy. Atomic hydrogen can also recombine forming hydrogen molecules. Hydrogen can react with a specific element forming hydrides precipitating in a new phase. In all these cases, hydrogen is transported mainly by diffusion and the phenomenon is increased with the temperature. The case of hydride formation in titanium alloys is a typical one. The microstructure of these alloys consists usually of two phases with different hydrogen solubility and diffusivity. Hydrogen enters the alloy via grain boundaries or other easy paths as beta phase, forming hydrides that precipitate in the alfa phase.

Materials

Material suitability for hydrogen service should be evaluated carefully before it is used. A material should not be used unless data are available to prove that it is suitable for the planned service conditions. In case of any doubt the material can be subjected to hydrogen embrittlement susceptibility testing (e.g. ISO 11114-4).

According to the information included in the ISO/TR 15916:2004 Basic considerations for the safety of hydrogen systems /Technical Report most of the metallic materials present a certain degree of sensitivity to hydrogen embrittlement. However, there are some that can be used without any specific precautions as for example brass and most of the copper alloys, aluminium and its alloys and austenitic stainless steel. On the other hand, nickel and high nickel alloys or titanium and its alloys are known to be sensitive to hydrogen embrittlement. For steels the sensitivity may depend on several factors as the exact chemical composition, heat or mechanical treatment, microstructure, impurities and strength. Concerning non-metallic materials, ISO/TR 15916:2004 also provides information as far as the suitability of some selected materials.

Fortunately many materials can be safely used under controlled conditions ( e.g. limited stress , absence of stress raisers such as surface defects….).

And thanks to Sandia National Laboratories and especially to Mr. Moen and Drs. Somerday and San Marchi for their contribution to this chapter.

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