Hydrogen Damage of Metallic Materials

Hydrogen Damage of Metallic Materials

Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen. Damaging effects of hydrogen in metallic materials are known since 1875 when W.H.Johnson reported (1) “some remarkable changes produced in iron by the action of hydrogen in acids”. During the intervening years many similar effects have been observed in different structural materials like steels, aluminum, titanium, zirconium etc. Because of the technological importance of hydrogen damage, many people explored the nature, causes and control measures of hydrogen related degradation of metals. Hardening, embrittlement and internal damage are the main hydrogen damage processes in metals. This article consists of a classification of hydrogen damage, brief description of the various processes and their mechanisms, and some guidelines for the control of hydrogen damage.

 

IMPORTANCE OF HYDROGEN DAMAGE

 

With advancing technology, use of high strength structural materials becomes a necessity. Fast depletion of fossil fuels and the global warming arising from the burning of fossil fuels have lead to the search for alternate sources of energy.  Hydrogen is believed to be the future source of energy and a “hydrogen economy” is a strong possibility within the next 50 years. In such a scenario, large scale production, storage, transportation and use of hydrogen become necessary (2). Materials’ problems caused by hydrogen damage could be limiting the progress of such an economy.

Hydrogen may be picked up by metals during melting, casting, shaping and fabrication. They are also exposed to hydrogen during their service life. Materials susceptible to hydrogen damage have ample opportunities to be degraded during all these stages.

 

CLASSIFICATION OF HYDROGEN DAMAGE

Hydrogen damage may be of four types; (i) solid solution hardening, (ii) creation of internal defects, (iii) Hydride embrittlement, and (iv) hydrogen embrittlement(3).Each of these may further be classified into the various damaging processes shown in the figure below.

 

Solid solution hardening

Metals like niobium and tantalum dissolve hydrogen and experience hardening and embrittlement at concentrations much below their solid solubility limit (4). The hardening and embrittlement are enhanced by increased rate of straining

Hydride embrittlement

In hydride forming metals like Ti, Zr and V hydrogen absorption causes severe embrittlement. At low concentrations of hydrogen, below the solid-solubility limit, stress-assisted hydride formation causes the embrittlement which is enhanced by slow straining. At hydrogen concentrations above the solubility limit, brittle hydrides are precipitated on slip planes and cause severe embrittlement (5). This latter kind of embrittlement is enhanced by increased strain-rates, decreased temperature and by the presence of notches in the material. A ductile-to- brittle transition is produced when the test temperature is lowered.

Creation of internal defects

Hydrogen present in metals can produce several kinds of internal defects like blisters, shatter cracks, flakes, fish-eyes and porosity. Carbon steels exposed to hydrogen at high temperatures experience hydrogen attack which leads to internal decarburization and weakening (6).

Blistering

Atomic hydrogen diffusing through metals may collect at internal defects like inclusions and laminations and form molecular hydrogen. High pressures may be built up at such locations due to continued absorption of hydrogen leading to blister formation, growth and eventual bursting of the blister. Such hydrogen induced blister cracking has been observed in steels, aluminum alloys, titanium alloys and nuclear structural materials (3).

 Shatter cracks, flakes, fish-eyes and micro perforations

Flakes and shatter cracks are internal fissures seen in large forgings. Hydrogen picked up during melting and casting segregates at internal voids and discontinuities and produces these defects during forging. Fish-eyes are bright patches resembling eyes of fish seen on fracture surfaces, generally of weldments. Hydrogen enters the metal during fusion-welding and produce this defect during subsequent stressing. Steel containment vessels exposed to extremely high hydrogen pressures develop small fissures or micro perforations through which fluids may leak. (3)

 Porosity

In metals like iron & steel, aluminum and magnesium whose hydrogen solubilities decrease with decreasing temperature, liberation of excess hydrogen during cooling from the melt, (in ingots and castings) produces gas porosity

By far, the most damaging effect of hydrogen in structural materials is hydrogen embrittlement. Materials susceptible to this process exhibit a marked decrease in their energy absorption ability before fracture in the presence of hydrogen. This phenomenon is also known as hydrogen-assisted cracking, hydrogen-induced blister cracking . The embrittlement is enhanced by slow strain rates and low temperatures, near room temperature.As shown in Fig.1, hydrogen embrittlement may be of four types.

1.  Hydrogen stress cracking  Brittle delayed failure of normally ductile materials when hydrogen is present within is called hydrogen stress cracking or internal hydrogen embrittlement. This effect is seen in high strength structural steels, titanium alloys and aluminum alloys.
2. Hydrogen environment embrittlement Embrittlement of materials when tensile loaded in contact with gaseous hydrogen is known as hydrogen environment embrittlement or external hydrogen embrittlement. It has been observed in alloy steels and alloys of nickel, titanium, uranium and niobium.
3. Loss in tensile ductility  Hydrogen lowers tensile ductility in many materials. In ductile materials like austenitic stainless steels, aluminum alloys etc no marked embrittlement may not occur, but may exhibit significant lowering in tensile ductility (% elongation or % reduction in area) in tensile tests.
4. Degradation of other mechanical properties  Hydrogen may also affect the plastic flow behavior of metals. Increased or decreased yield strengths, serrated yielding, altered work hardening rates as well as lowered fatigue and creep properties have been reported.(3).

 

CONTROL OF HYDROGEN DAMAGE

 

The best method of controlling hydrogen damage would be to control the contact between the metal and hydrogen. Many steps can be taken to reduce the entry of hydrogen in to metals during critical operations like melting, casting, working (rolling, forging etc), fabrication (welding), surface preparation, like chemical cleaning, electroplating etc, and corrosion during their service life. Control of the environment and metallurgical control of the material to decrease its susceptibility to hydrogen are the two major approaches to reduce hydrogen damage.

References

1)      W. H. Johnson, Proc.Royal Soc. (London), 23 (1875), 168

2)      J.  O’M. Bockris, Int. J. Hydrogen Energy, 6 (1981), 223

3)      T. K. G. Namboodhiri, Trans. Indian Inst. Metals,  37(1984), 764

4)      B. A. Kolachev, Hydrogen embrittlement of non-ferrous metals, Translated from Russian, Israel Program for scientific translations, (1968)

5)      W. J. Pardee and N. E. Paton, Metall. Trans. 11A (1980), 1391

6)      G. A. Nelson, in Hydrogen Damage, C. D. Beachem (Ed.), American Society for Metals, Metals Park, Ohio, (1977), p. 377