72
Fundamentals of Corrosion
are more severely embrittled than low-strength alloys, based on the
same metal. In iron, a nickel addition greater than 8% gives rise to
martensite with coarse slip lines. In precipitation-hardening aluminum and copper alloys, maximum susceptibility to LME coincides
with the peak strength of the alloys. All of these point to the generation of stress concentrations as a result of alloying.
3.11.1.3 Delayed Failure
Delayed failure refers to those failures taking place under a sustained load
after a period of time. In liquid metal environments, the embrittlement and
failure of some metals are time dependent.
Aluminum-copper and copper-beryllium in liquid mercury exhibit
delayed failure, as does AISI 4130 steel in molten lithium. Age-hardened
alloys exhibit the lowest time of fracture in the maximum hardened state.
The susceptibility increases with prior strain or cold-work.
3.11.1.4 Preventive Measures
Liquid metal embrittlement can be prevented or a reduction in occurrence
can be achieved by the following measures:
1. Introduction of impurity atoms in the solid metal. Examples are the
addition of phosphorus to monel to reduce embrittlement in liquid
mercury, or the addition of lanthanides to leaded steels.
2. In some cases the addition of a second metal to the embrittling liquid decreases the embrittlement.
3. An effective barrier between the solid metal and the liquid metal.
This may be a ceramic or covalent coating.
4. Cladding with a soft, high-purity metal such as zinc alloy clad with
pure zirconium to resist embrittlement in liquid cadmium.
5. Elimination of the embrittling metal.
6. Reduction in the level of applied or residual stress below the static
endurance limit.
3.11.2 Corrosion by Liquid Metals
Corrosion by liquid metals becomes a matter of concern when they have to
remain in contact with the solid metal over a long period of time. Because
of their excellent heat transfer properties, liquid metals are being used
extensively in nuclear power generation plants and in heat transfer systems
making use of heat pipes containing liquid metals. Examples include liquid
Fundamentals of Corrosion
are more severely embrittled than low-strength alloys, based on the
same metal. In iron, a nickel addition greater than 8% gives rise to
martensite with coarse slip lines. In precipitation-hardening aluminum and copper alloys, maximum susceptibility to LME coincides
with the peak strength of the alloys. All of these point to the generation of stress concentrations as a result of alloying.
3.11.1.3 Delayed Failure
Delayed failure refers to those failures taking place under a sustained load
after a period of time. In liquid metal environments, the embrittlement and
failure of some metals are time dependent.
Aluminum-copper and copper-beryllium in liquid mercury exhibit
delayed failure, as does AISI 4130 steel in molten lithium. Age-hardened
alloys exhibit the lowest time of fracture in the maximum hardened state.
The susceptibility increases with prior strain or cold-work.
3.11.1.4 Preventive Measures
Liquid metal embrittlement can be prevented or a reduction in occurrence
can be achieved by the following measures:
1. Introduction of impurity atoms in the solid metal. Examples are the
addition of phosphorus to monel to reduce embrittlement in liquid
mercury, or the addition of lanthanides to leaded steels.
2. In some cases the addition of a second metal to the embrittling liquid decreases the embrittlement.
3. An effective barrier between the solid metal and the liquid metal.
This may be a ceramic or covalent coating.
4. Cladding with a soft, high-purity metal such as zinc alloy clad with
pure zirconium to resist embrittlement in liquid cadmium.
5. Elimination of the embrittling metal.
6. Reduction in the level of applied or residual stress below the static
endurance limit.
3.11.2 Corrosion by Liquid Metals
Corrosion by liquid metals becomes a matter of concern when they have to
remain in contact with the solid metal over a long period of time. Because
of their excellent heat transfer properties, liquid metals are being used
extensively in nuclear power generation plants and in heat transfer systems
making use of heat pipes containing liquid metals. Examples include liquid