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Hydrogen damage

Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen. Hydrogen damage is the generic name given to a large number of metal degradation processes due to interaction with hydrogen. Hydrogen is present practically everywhere, 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 “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, such as steel, aluminium, titanium, and zirconium. 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. 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. Hydrogen damage may be of four types: solid solution hardening, creation of internal defects, hydride embrittlement, and hydrogen embrittlement. Each of these may further be classified into the various damaging processes. Metals like niobium and tantalum dissolve hydrogen and experience hardening and embrittlement at concentrations much below their solid solubility limit. 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. This latter kind of embrittlement is encouraged by increased strain-rates, decreased temperature and by the presence of notches in the material. Hydrogen present in metals can produce several kinds of internal defects like blisters, shatter fracture, flakes, fish-eyes and porosity. Carbon steels exposed to hydrogen at high temperatures experience hydrogen attack which leads to internal decarburization and weakening. 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. Metals with low hydrogen solubility (such as tungsten) are more susceptible to blister formation . While in metals with high hydrogen solubility like vanadium, hydrogen prefer to induce stable metal-hydrides instead of bubbles or blisters. 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 named for their appearance 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. In metals like iron, steel, aluminium, and magnesium, whose hydrogen solubilities increase with increasing temperature, liberation of excess hydrogen during cooling from the melt, (in ingots and castings) produces hydrogen gas porosity.

[ "Hydrogen embrittlement" ]
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