Properties of Metals
Personnel need to be aware of the conditions for hydrogen embrittlement and its
formation process when selecting materials for a reactor plant. This chapter
discusses the sources of hydrogen and the characteristics for the formation of
DESCRIBE hydrogen embrittlement, including the two
required conditions and the formation process.
IDENTIFY why zircaloy-4 is less susceptible to hydrogen
embrittlement than zircaloy-2.
Another form of stress-corrosion cracking is hydrogen embrittlement. 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 when they occur. Hydrogen
embrittlement is the process by which steel loses its ductility and strength due to tiny cracks that
result from the internal pressure of hydrogen (H2) or methane gas (CH4), which forms at the grain
boundaries. In zirconium alloys, hydrogen embrittlement is caused by zirconium hydriding. At
nuclear reactor facilities, the term "hydrogen embrittlement" generally refers to the embrittlement
of zirconium alloys caused by zirconium hydriding.
Sources of hydrogen causing embrittlement have been encountered in the making of steel, in
processing parts, in welding, in storage or containment of hydrogen gas, and related to hydrogen
as a contaminant in the environment that is often a by-product of general corrosion. It is the
latter that concerns the nuclear industry. Hydrogen may be produced by corrosion reactions such
as rusting, cathodic protection, and electroplating. Hydrogen may also be added to reactor
coolant to remove oxygen from reactor coolant systems.
As shown in Figure 10, hydrogen diffuses along the grain boundaries and combines with the
carbon (C), which is alloyed with the iron, to form methane gas. The methane gas is not mobile
and collects in small voids along the grain boundaries where it builds up enormous pressures that
initiate cracks. Hydrogen embrittlement is a primary reason that the reactor coolant is maintained
at a neutral or basic pH in plants without aluminum components.