Environmental Engineering Reference
In-Depth Information
excellent heat transfer properties liquid metals are being extensively used in nu-
clear power generation plants, e.g., molten sodium in liquid metal fast-breeder
reactors (LMFBR) or molten sodium-potassium as static heat sinks in automotive
and aircraft valves. Heat transfer systems utilizing heat pipes use a liquid metal,
e.g., lithium, sodium, or sodium-potassium, as the working fluid. These and the
other applications of liquid metal make the containment material vulnerable to
corrosive attack, which sometimes reduces its life considerably.
Liquid metal corrosion can take place through any one or combination of the
following process:
1.
Direct dissolution
2.
Corrosion product formation
3.
Elemental transfer
4.
Alloying
Direct dissolution is the release of atoms of the containment material into the
melt. As the adjacent melt becomes saturated with the dissolving metal, the disso-
lution reaction decreases or ceases altogether. However, in a nonisothermal liquid
metal system such a situation may not be attained because of the convection from
hotter to colder regions. As a consequence, the dissolved metal from the ''hot
leg'' is carried to the ''cold leg'' where it gets deposited. Plugging of coolant
pipes as a result of such depositions of dissolved species in the colder zones has
been encountered. The dissolution may be uniform or selective. For example,
preferential leaching of silicon from stainless steels in molten sodium and prefer-
ential dissolution of nickel from a type 316 stainless steel in molten lithium so-
dium or bismuth have been reported [11,12]. The selective leaching sometimes
proceeds to such an extent that voids are left in the steel.
The solubilities of refractory metals, as well as iron and nickel base alloys,
are very low (of the order of a fraction of a ppm) in alkali metals. However, the
interstitials in the metals such as carbon, oxygen, and nitrogen, and impurities
in the liquid metal, particularly oxygen, dominate in the corrosion process either
by the formation of corrosion products or by elemental transfer.
The solubility of oxygen as Na
2
O in sodium is 3 ppm O
2
at 150
°
C, which
enhances to 1000 ppm O
2
at 500
C. Na
2
O reacts with iron to form (Na
2
O)
2
FeO,
which increases the apparent solubility of iron in molten sodium. In chromium-
containing steels, Na
2
O reacts with chromium to form NaCrO
2
as corrosion prod-
uct. Niobium forms double oxides with Na
2
O, sometimes accompanied by a
change in valency state, e.g.,
°
O
2
In flowing sodium, the double oxides are removed from the metal surface. The
attack is aggravated under a thermal gradient. Reduction of the oxygen concentra-
tion of the sodium to less than 3 ppm provides protection. This is achieved by
Nb
2
O
5
Na
2
O
→
Na
2
O, Nb
2
O
3
