Properties of MetalsDOE-HDBK-1017/1-93CORROSIONHYDROGEN EMBRITTLEMENTPersonnel need to be aware of the conditions for hydrogen embrittlement and itsformation process when selecting materials for a reactor plant. This chapterdiscusses the sources of hydrogen and the characteristics for the formation ofhydrogen embrittlement.EO 1.21DESCRIBE hydrogen embrittlement, including the tworequired conditions and the formation process.EO 1.22IDENTIFY why zircaloy-4 is less susceptible to hydrogenembrittlement than zircaloy-2.Another form of stress-corrosion cracking is hydrogen embrittlement. Although embrittlementof materials takes many forms, hydrogen embrittlement in high strength steels has the mostdevastating effect because of the catastrophic nature of the fractures when they occur. Hydrogenembrittlement is the process by which steel loses its ductility and strength due to tiny cracks thatresult from the internal pressure of hydrogen (H2) or methane gas (CH4), which forms at the grainboundaries. In zirconium alloys, hydrogen embrittlement is caused by zirconium hydriding. Atnuclear reactor facilities, the term "hydrogen embrittlement" generally refers to the embrittlementof zirconium alloys caused by zirconium hydriding.Sources of hydrogen causing embrittlement have been encountered in the making of steel, inprocessing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogenas a contaminant in the environment that is often a by-product of general corrosion. It is thelatter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions suchas rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactorcoolant to remove oxygen from reactor coolant systems.As shown in Figure 10, hydrogen diffuses along the grain boundaries and combines with thecarbon (C), which is alloyed with the iron, to form methane gas. The methane gas is not mobileand collects in small voids along the grain boundaries where it builds up enormous pressures thatinitiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintainedat a neutral or basic pH in plants without aluminum components.Rev. 0Page 37MS-02
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