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National Advisory Committee for Aeronautics, Technical Notes - The Mechanism of Thermal-Gradient Mass Transfer in the Sodium Hydroxide Nickel System

Sodium hydroxide is in many ways an attractive choice as a high-
temperature heat-transfer fluid. Unfortunately, only a few materials are
suitable for containing it in the molten condition at the temperatures
desired, 15000 to 17000 F. Among these are nickel (refs. 1 to 5), copper,
silver, gold (ref. 4), and some nickelébase alloys recently produced at
the Lewis laboratory. Even these materials exhibit corrosion termed
"thermal-gradient mass transfer“, although they do not show intergranular
attack or rapid solution. Thermal—gradient mass transfer is the phenome-
non by which the metal is removed (either chemically or physically) from
the hotter regions of a system and deposited in colder regions. With the r
previously listed metals the deposit is in the form of needle—like crystals,
the size of which depends upon the experimental conditions.

In order to find ways of inhibiting this type of corrosion, an in-
vestigation was conducted concerning the possible mechanisms for mass
transfer. Although mass transfer also occurs in liquid metal systems
(ref. 5; e.g., molten sodium in copper), the mechanism in such cases is
undoubtedly purely physical in nature, being due to solution of the con-
tainer material in the molten metal. In systems using sodium hydroxide,
transfer is prdbably chemical in nature, and the process is believed to
be similar for all the container materials previously mentioned. The
purpose of this report is to establish the most prdbable mechanism for
mass transfer in this system. This report discusses methods of measure-
ment, choice of mechanism, kinetics, effect of additives, and alternate
mechanisms. Many of the proofs require the use of previously unpublished A
experimental data, the details of which are included in the appendixes.

At the Iewis laboratory two methods hays been used to study the
phenomenon of mass transfer; they are the static capsule test and the
dynamic toroid test. Previous reports (refs. l to 5) describe these in
detail. In the static test a vertical temperature gradient exists in
nickel capsules (crucibles) with the hot zone at the bottom. This type
of gradient was intended to produce thermal convection. The nickel dis—
solves from the bottom of the capsule and precipitates as a narrow ring
at the liquid level of the sodium hydroxide. From radiographs of such
capsules the relative amounts of transfer can be estimated qualitatively.