Forms of Metallic Corrosion
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3.11.1 Liquid Metal Embrittlement (LME)
The failure of a solid metal under stress in contact with a liquid metal is known
as liquid metal embrittlement (LME). It is also known as liquid metal cracking.
The loss of ductility of a normally ductile metal is manifested as a reduction in
fracture stress, or strain, or both. Normally there is a change in fracture mode
from ductile to brittle intergranular or brittle transgranular (cleavage).
The failure resulting from LME may be instantaneous or it may take place
after a lapse of time following the exposure of the stressed metal to a liquid environment. The former is the classical LME while the latter is often
referred to as “delayed failure” or “static fatigue.” In either case, the presence
of stress is necessary. The stress may be shear, tensile, or torsional in nature
— but not compressive. LME and SCC are similar in that stress must be present; however, the propagation of fracture is much faster in LME than in SCC.
If sufficient time is allowed, intergranular penetration of liquid metal may
render a solid metal brittle, even if stress is absent.
The elongation and reduction in area of the metal or alloy are lowered as the
result of LME. The fracture stress is also reduced and, in the cases of severe
embrittlement, may be less than the yield stress of the material. However,
there is no change in the yield strength and strain hardening behavior of the
solid metal. The liquid metal acts only to limit the total ductility before fracture or the stress at fracture if failure occurs before the normal yield point.
The failure of mild steel in lithium occurs at only 2 to 3% elongation, but the
lower yield point, upper yield point, and the yield point elongation remain
unaffected.
As with SSC, all liquid metals do not embrittle all solid metals. For
example, liquid mercury embrittles zinc but not cadmium; liquid gallium
embrittles aluminum but not magnesium. Table 3.6 lists the known embrittlement combinations.
3.11.1.1 Requirements for Embrittlement
The general requirements for LME to occur in a ductile metal are as follows:
1. There must be a wetting or intimate contact of the solid metal by the
liquid metal.
2. The solid metal must be stressed to the point of producing plastic
deformation.
3. There must be an adequate supply of liquid metal.
The most critical condition for LME is intimate contact between the solid
metal and the liquid metal. This is required in order to initiate embrittlement
and guarantee the presence of liquid metal at the tip of the propagating crack
to cause brittle failure. An adequate supply of liquid metal is necessary to
absorb at the propagating crack tip. The total amount need not be large; a
69
3.11.1 Liquid Metal Embrittlement (LME)
The failure of a solid metal under stress in contact with a liquid metal is known
as liquid metal embrittlement (LME). It is also known as liquid metal cracking.
The loss of ductility of a normally ductile metal is manifested as a reduction in
fracture stress, or strain, or both. Normally there is a change in fracture mode
from ductile to brittle intergranular or brittle transgranular (cleavage).
The failure resulting from LME may be instantaneous or it may take place
after a lapse of time following the exposure of the stressed metal to a liquid environment. The former is the classical LME while the latter is often
referred to as “delayed failure” or “static fatigue.” In either case, the presence
of stress is necessary. The stress may be shear, tensile, or torsional in nature
— but not compressive. LME and SCC are similar in that stress must be present; however, the propagation of fracture is much faster in LME than in SCC.
If sufficient time is allowed, intergranular penetration of liquid metal may
render a solid metal brittle, even if stress is absent.
The elongation and reduction in area of the metal or alloy are lowered as the
result of LME. The fracture stress is also reduced and, in the cases of severe
embrittlement, may be less than the yield stress of the material. However,
there is no change in the yield strength and strain hardening behavior of the
solid metal. The liquid metal acts only to limit the total ductility before fracture or the stress at fracture if failure occurs before the normal yield point.
The failure of mild steel in lithium occurs at only 2 to 3% elongation, but the
lower yield point, upper yield point, and the yield point elongation remain
unaffected.
As with SSC, all liquid metals do not embrittle all solid metals. For
example, liquid mercury embrittles zinc but not cadmium; liquid gallium
embrittles aluminum but not magnesium. Table 3.6 lists the known embrittlement combinations.
3.11.1.1 Requirements for Embrittlement
The general requirements for LME to occur in a ductile metal are as follows:
1. There must be a wetting or intimate contact of the solid metal by the
liquid metal.
2. The solid metal must be stressed to the point of producing plastic
deformation.
3. There must be an adequate supply of liquid metal.
The most critical condition for LME is intimate contact between the solid
metal and the liquid metal. This is required in order to initiate embrittlement
and guarantee the presence of liquid metal at the tip of the propagating crack
to cause brittle failure. An adequate supply of liquid metal is necessary to
absorb at the propagating crack tip. The total amount need not be large; a