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670 Liquid Alkali Metals

For alkali metals dissolved in each other, the coefficients in lithium; reaction occurred, giving a residue of lithium
are relatively small when the metals adjoin one another cyanamide, Li 2 NCN. The same product was formed when
in the group (e.g., lithium in sodium) but increase as the the reactants were added in the reverse order. The remark-
solute and solvent metals are further separated in atomic able feature here is that the simple cyanide LiCN is not
number (e.g., sodium in cesium). produced, and all attempts to isolate it from liquid lithium
have failed. Once again, the chemical behavior of common
elements in the electronic environment of a liquid metal is
VIII. REACTIONS BETWEEN quite unusual; in this case, the cyanamide ion NCN 2− has
−
DISSOLVED ELEMENTS highest stability, though the cyanide ion CN is stable in
liquid sodium.
Many species dissolved in the liquid alkali metals do in-
teract when present together, but often in ways that cannot
be anticipated from existing knowledge of the chemistry IX. CORROSION BY THE LIQUID
of these species in other liquid media. Two examples will ALKALI METALS
illustrate this.
Wheneveraliquidmetalhastobestored,andmanipulated,
on a large scale, the problem of corrosion of containers is
A. The Nitrogen Group II Metal Reaction
of paramount importance. The general principles govern-
in Liquid Sodium
ing corrosion by liquid metals bear very little resemblance
The technical significance of this reaction arises from the to those involved in corrosion of metals by aqueous me-
observation that dissolved calcium (the main metallic im- dia; they have been defined largely as a result of research
purity in sodium) permits some nitrogen to dissolve in on liquid sodium, but they apply to the other alkali metals
the sodium, and this in turn promotes the nitriding of steel also. Essentially, corrosion results from the solution of the
containers. However, when excess nitrogen is present, this container metal (or one of the constituents in the case of an
corrosion is diminished. Because of the small solubility alloy such as steel) in the liquid alkali metal, and this can
of calcium in sodium, the solutions are difficult to study, be augmented by intergranular attack of the solid metal.
but barium has a higher solubility, and the chemistry of A phenomenon known as thermal gradient mass transfer
barium solutions is similar to that of the calcium solu- is one of the most important and most troublesome conse-
tions. In a typical experiment a solution of barium (about quences of this solubility. When used as a coolant, sodium
4 at.%) in sodium was exposed to progressive aliquots of contained in (say) steel is in constant circulation; one part
◦
nitrogen gas at 300 C. The nitrogen continued to dissolve of the loop (the hot leg) is receiving heat, and at another
until the nitrogen:barium ratio reached 1:4. The N 3− ion region (the cold leg) heat is being withdrawn. If the tem-
is held in solution as a result of the high solvation energy perature difference amounts to several hundred degrees,
3−
arising from the solvation of each N ion by four barium structural metal will dissolve in the hot leg and deposit in
atoms. When further nitrogen is added, precipitation of the cold leg; this is thermal gradient mass transfer.
barium nitride commences, some of the barium atoms in Corrosion also results from chemical reaction with non-
the Ba 4 N unit have to be replaced by sodium atoms, and metals dissolved in the liquid metal. Oxides, nitrides, or
the solvation shell becomes weaker and eventually col- carbides are then formed on the container surface as sur-
lapses. With excess nitrogen, pure liquid sodium remains, face layers, which can thicken or be swept away in the
and the precipitate is a barium nitride of composition stream of flowing liquid metal. The formation of such
Ba 2 N. Such behavior has been found in many other liquid films, however, is a highly selective process and depends
metal solutions and is quite different from the chemistry of on the free energies of formation of the compounds in-
these species that is normally observed in molecular liquid volved. Thus, the high value of − G for lithium oxide
◦
f
media. (Table IV) ensures that oxygen dissolved in liquid lithium
will stay in the liquid rather than react with most of the
transition metals used as containers. This suggests that
B. The Nitrogen–Carbon Reaction
oxygen in lithium should not act directly as a corrosive
in Liquid Lithium
impurity, and this is found to be the case in practice. How-
Nitrogen and carbon are the main impurities in liquid ever, because of the lower free energies of formation of
lithium, and for the competent handling of liquid lithium sodium oxide and potassium oxide, solutions of oxygen in
on an industrial scale it is necessary to determine the these two liquids are more reactive toward transition metal
species actually present in the liquid. To this end, elemen- surfaces. Some of the corrosion products formed on the
tal carbon was added to an unsaturated solution of nitrogen surface of various metals are shown in Table VIII. While