Properties of MetalsDOE-HDBK-1017/1-93HYDROGEN EMBRITTLEMENTIf the metal is under a high tensile stress, brittleFigure 10 Hydrogen Embrittlementfailure can occur. At normal room temperatures, thehydrogen atoms are absorbed into the metal latticeand diffused through the grains, tending to gather atinclusions or other lattice defects. If stress inducescracking under these conditions, the path istransgranular. At high temperatures, the absorbedhydrogen tends to gather in the grain boundaries andstress-induced cracking is then intergranular. Thecracking of martensitic and precipitation hardenedsteel alloys is believed to be a form of hydrogenstress corrosion cracking that results from the entryinto the metal of a portion of the atomic hydrogen that is produced in the following corrosionreaction.3 Fe + 4 H2O Fe3O4 + 4 H2Hydrogen embrittlement is not a permanent condition. If cracking does not occur and theenvironmental conditions are changed so that no hydrogen is generated on the surface of themetal, the hydrogen can rediffuse from the steel, so that ductility is restored.To address the problem of hydrogen embrittlement, emphasis is placed on controlling the amountof residual hydrogen in steel, controlling the amount of hydrogen pickup in processing,developing alloys with improved resistance to hydrogen embrittlement, developing low or noembrittlement plating or coating processes, and restricting the amount of in-situ (in position)hydrogen introduced during the service life of a part.Hydrogen embrittlement is a problem with zirconium and zirconium alloys, which often are usedas cladding materials for nuclear reactors. Zirconium reacts with water as follows.Zr + 2 H2O ZrO2 + 2H2Part of the hydrogen produced by the corrosion of zirconium in water combines with thezirconium to form a separate phase of zirconium hydride (ZrH1.5) platelets. The metal thenbecomes embrittled (ductility decreases) and it fractures easily. Cracks begin to form in thezirconium hydride platelets and are propagated through the metal. Zircaloy-2 (a zirconium alloy),which has been used as a fuel rod cladding, may absorb as much as 50% of the corrosion-produced hydrogen and is subject to hydrogen embrittlement, especially in the vicinity of thesurface. Studies at Westinghouse, Batelle, and elsewhere have revealed that the nickel in thezircaloy-2 was responsible for the hydrogen pickup. This has led to the development of zircaloy-4, which has significantly less nickel than zircaloy-2 and is less susceptible to embrittlement. Inaddition, the introduction of niobium into zircaloy-4 further reduces the amount of hydrogenabsorption.MS-02Page 38Rev. 0
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