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Hydrogen is present practically everywhere, in the atmosphere, several kilometres above the earth and inside the earth. Engineering materials are exposed to hydrogen and they may interact with it resulting in various kinds of structural damage. Damaging effects of hydrogen in metallic materials have been known since 1875 when W. H. Johnson reported (1) “some remarkable changes produced in iron by the action of hydrogen and acids”. During the intervening years many similar effects have been observed in different structural materials like steels, aluminium, 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.
Additional recommended knowledge
Importance of hydrogen damage
With advancing technology, use of high strength structural materials becomes a necessity. Depletion of fossil fuels and the search for other sources of energy is a current activity of mankind. Hydrogen is believed to be a possible future source of energy (Engineering note: Hydrogen could not be used as a "source" of energy but only as a means to transport energy from one place to another) 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 limit 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
A classification of the various hydrogen damage processes is shown in Fig.1. The various processes are briefly explained below; 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 Fig.1. These processes are briefly explained 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.
In hydride forming metals like titanium, zirconium and vanadium, 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 encouraged by increased strain-rates, decreased temperature and by the presence of notches in the material.
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).
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, aluminium 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 produces 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)
In metals like iron and steel, aluminium 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.
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.
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.
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.
Degradation of other mechanical properties
Hydrogen may also affect the plastic flow behaviour 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 is to control contact between the metal and hydrogen. Many steps can be taken to reduce the entry of hydrogen into metals during critical operations like melting, casting, working (rolling, forging etc), fabrication (welding), surface preparation, like chemical cleaning, electro plating 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.
1) W. H. Johnson, Proc.Royal Soc. (London), 23 (1875), 168
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Hydrogen_damage". A list of authors is available in Wikipedia.|