ORNL-TM-7207 - the Molten Salt Energy Technologies Web Site

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57
Corrosion of Hastelloy-N by MSRE and MSBR flue1 mixtures without ir-
radiation and without the consequences of fission has been studied in
sophisticated equipment for many years. It has been thoroughly de-
Table 26 clearly indicates that chromium is the most easily oxidized
of the major Hastelloy-N components. C~rrosion of the alloy, therefure,
is essentially by selective leaching of chromium from the alloy. A rapid
initial attack can result from reactions such as
and
if the fuel salt is impure or if the metal system is poorly cleaned.
*
These reactions proceed to completion at all temperatures within the re-
actor circuit and do not afford a basis for continued attack.
The most oxidizing of the major constituents sf the fuel is UF4, and
the reaction
has an equilibrium constant with a small temperature dependence. When
the salt is forced to circulate very rapidly through a large (140°C) tem-
perature gradient, as is the case within the reactor circuit, a mechanism
exists for mass transfer uf chromium and for continued attack. The re-
sult is that chromium is selectively removed from the alloy in higk-
temperature regions and deposited on the alloy in low-temperature regions
* Molybdenum fluorides are somewhat less stable than NdF2. M[SPS(~)
at 906 K has a standard free energy of formatfon25 of about 21% kJ (-51.4
kcal) per gram-atom of F--