Forms of Metallic Corrosion
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sodium in fast-breeding reactors, and lithium, sodium, or sodium-potassium
liquid metals as the working fluid in heat transfer systems.
Liquid metal corrosion can take place through any one or a combination of
the following processes:
1. Direct dissolution. Direct dissolution is the release of atoms of the
containment material into the molten metal. As the liquid metal
becomes saturated with the dissolving metal, the dissolution reaction decreases or stops altogether. However, in a nonisothermal liquid metal system, this may not occur because of the convection from
hotter to colder regions. Under this condition, the dissolved metal
from the “hot leg” is carried to the “cold leg” where it gets deposited.
Plugging of the coolant pipes results. Then dissolution results. The
dissolution may be uniform or selective. The selective leaching may
proceed to such an extent that voids are left in the steel.
2. Corrosion product formation. At times, the corrosion or reaction products form protective layers on the containment metal surface, thereby
reducing further attack. For example, the addition of aluminum or
silicon to steel helps in forming such a protective layer. The addition
of zirconium to liquid bismuth or mercury has an inhibiting effect
on the corrosion of steel in these liquid metals. The nitrogen present
in steel forms a surface layer of ZnN, a very stable compound and an
effective diffusion barrier.
3. Elemental transfer. Elemental transfer refers to the net transfer of
impurities to or from a liquid metal. In such a case, the liquid metal
atoms do not react with the atoms of the containment metal atoms.
Carburization of refractory metals and of austenitic stainless steels
has been observed in liquid sodium contaminated with carbon.
Decarburization of iron-chromium-molybdenum steels in liquid
sodium or lithium is another example of elemental transfer.
4. Alloying. An alloying action can be observed between the atoms of
the liquid metals and the constituents of the material. Systems that
form alloys or stable intermetallic compounds (nickel in molten aluminum) should be avoided.
3.12 Exfoliation
When intergranular corrosion takes place in a metal with a highly directional grain structure, it propagates internally, parallel to the surface of the
metal. The corrosion product formed is about five times as voluminous as
the metal consumed, and it is trapped beneath the surface. As a result, an
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sodium in fast-breeding reactors, and lithium, sodium, or sodium-potassium
liquid metals as the working fluid in heat transfer systems.
Liquid metal corrosion can take place through any one or a combination of
the following processes:
1. Direct dissolution. Direct dissolution is the release of atoms of the
containment material into the molten metal. As the liquid metal
becomes saturated with the dissolving metal, the dissolution reaction decreases or stops altogether. However, in a nonisothermal liquid metal system, this may not occur because of the convection from
hotter to colder regions. Under this condition, the dissolved metal
from the “hot leg” is carried to the “cold leg” where it gets deposited.
Plugging of the coolant pipes results. Then dissolution results. The
dissolution may be uniform or selective. The selective leaching may
proceed to such an extent that voids are left in the steel.
2. Corrosion product formation. At times, the corrosion or reaction products form protective layers on the containment metal surface, thereby
reducing further attack. For example, the addition of aluminum or
silicon to steel helps in forming such a protective layer. The addition
of zirconium to liquid bismuth or mercury has an inhibiting effect
on the corrosion of steel in these liquid metals. The nitrogen present
in steel forms a surface layer of ZnN, a very stable compound and an
effective diffusion barrier.
3. Elemental transfer. Elemental transfer refers to the net transfer of
impurities to or from a liquid metal. In such a case, the liquid metal
atoms do not react with the atoms of the containment metal atoms.
Carburization of refractory metals and of austenitic stainless steels
has been observed in liquid sodium contaminated with carbon.
Decarburization of iron-chromium-molybdenum steels in liquid
sodium or lithium is another example of elemental transfer.
4. Alloying. An alloying action can be observed between the atoms of
the liquid metals and the constituents of the material. Systems that
form alloys or stable intermetallic compounds (nickel in molten aluminum) should be avoided.
3.12 Exfoliation
When intergranular corrosion takes place in a metal with a highly directional grain structure, it propagates internally, parallel to the surface of the
metal. The corrosion product formed is about five times as voluminous as
the metal consumed, and it is trapped beneath the surface. As a result, an