{"id":28010,"date":"2020-11-23T20:52:22","date_gmt":"2020-11-23T20:52:22","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=28010"},"modified":"2023-08-10T07:39:27","modified_gmt":"2023-08-10T07:39:27","slug":"hydrogen-embrittlement","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/materials-science\/material-properties\/toughness\/hydrogen-embrittlement\/","title":{"rendered":"Hydrogen Embrittlement"},"content":{"rendered":"
Hydrogen embrittlement<\/strong> is one of many forms of stress-corrosion cracking. Hydrogen embrittlement<\/strong> results from the combined action of applied tensile stress and a corrosive hydrogen environment. Both influences are necessary. In this case, the corrosive agent is hydrogen<\/a> in its atomic form (H as opposed to the molecular form, H2<\/sub>), which diffuses interstitially through the crystal lattice<\/a>, and concentrations as low as several parts per million can lead to cracking. Although embrittlement of materials takes many forms, hydrogen embrittlement in high-strength steels has the most devastating effect because of the catastrophic nature of the fractures. Hydrogen embrittlement is the process by which steel loses its ductility<\/a> and strength<\/a> due to tiny cracks that result from the internal pressure of hydrogen (H2<\/sub>), which forms at the grain boundaries. In the case of steel, hydrogen diffuses along the grain boundaries and combines with the carbon to form methane gas. The methane gas collects in small voids along the grain boundaries, where it builds up enormous pressures that initiate cracks and decrease the ductility of the steel. Brittle fracture<\/strong> can occur if the metal is under high tensile stress.<\/p>\n See also: Fracture Toughness<\/a><\/p>\n It is a complex process that is not completely understood because of the variety and complexity of mechanisms that can lead to embrittlement. Many mechanisms have been proposed to explain hydrogen embrittlement. Mechanisms proposed to explain embrittlement include the formation of brittle hydrides, the creation of voids that can lead to bubbles, and pressure build-up within a material. Hydrogen is introduced to the surface of a metal, and individual hydrogen atoms diffuse through the metal structure. Because the solubility of hydrogen increases at higher temperatures, raising the temperature can increase the diffusion of hydrogen.<\/p>\n For hydrogen embrittlement to occur, a combination of three conditions is required:<\/p>\n In zirconium alloys<\/strong>, hydrogen embrittlement is caused by zirconium hydriding. At nuclear reactor facilities, the term \u201chydrogen embrittlement\u201d generally refers to the embrittlement of zirconium alloys caused by zirconium hydriding.<\/p>\n Special Reference: U.S. Department of Energy, Material Science.\u00a0DOE Fundamentals Handbook,\u00a0Volume 1 and 2.\u00a0January\u00a01993.<\/p>\n Cladding<\/strong><\/a> prevents radioactive fission products<\/a> from escaping the fuel matrix into the reactor coolant and contaminating it. Various fuel failure root <\/a>causes<\/a> have\u00a0been identified in the past. IThesecauses were predominantly fabrication defects or fretting in the early days of PWR and BWR operations. One possible cause is also:<\/p>\n The aggressive agent in this respect is primary circuit water, at a temperature of some 300\u00b0 C. This oxidizes zirconium according to the reaction:<\/p>\n Zr + 2H2<\/sub>O\u2192ZrO2<\/sub> + 2H2<\/sub><\/strong><\/p>\n resulting in the formation of solid oxide on the metal\u2019s surface.<\/p>\n Part of the hydrogen produced by the corrosion of zirconium<\/a> in water combines with the zirconium<\/a> to form a separate phase of zirconium hydride (ZrH1.5) platelets. Hydrogen migrates under the effect of the thermal gradient to accumulate in the less hot regions, forming hydrides<\/strong> that are liable to cause brittleness<\/strong> in the cladding as the fuel cools down. The metal then becomes embrittled (ductility decreases) and fractures easily. Cracks begin to form in the zirconium hydride platelets and are propagated through the metal.<\/p>\n Hydrogen embrittlement is also very important for high-temperature steam oxidation of zirconium alloys<\/a>.<\/p>\n<\/div><\/div>\n\n
Hydrogen Embrittlement of Zirconium Alloys<\/h3>\n
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