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National Advisory Committee for Aeronautics, Technical Notes - Corrosion Resistance of Nickel Alloys in Molten Sodium Hydroxide

The corrosion resistance of 11 nickel—base compositions to molten
sodium hydroxide at 15000 and 17000 F was studied in order to find a con-
tainer material for the caustic at these temperatures. Although, in
caustic, pure nickel is corrosion resistant (suffering only thermal-
gradient mass transfer), it is structurally weak. ltherefore , ways of
strengthening nickel were sought that would not decrease its corrosion
resistance. The materials selected for testing were as follows:

(1) Solid solutions: (nickel plus copper, nickel plus molybdenum,
nickel plus zirconium, nickel plus tin, nickel plus columbium,
nickel plus manganese, and nickel plus silicon)

(2) Two-phase materials: (a) materials containing mechanical disper—
sions of refractory particles (nickel plus titanium carbide, nickel
plus magnesium oxide, and nickel plus aluminum oxide); and (b)
precipitation—hardened alloy (nickel plus titanium)

At 15000 F only two nickel—base materials showed more than slight
intergranular attack. This was a mat improvement over commercial nickel-
base alloys. However, other types of corrosion were still prevalent in
some of the materials. The most common of these were the leaching of the
solute, or second phase, and the formation of foreign nonmetallic phases
within the alloy. In general, corrosion at 17000 F was only slightly
more severe than at 15000 F. The only alloy'that was as resistant to
attack as pure nickel was the soud solution containing 50 percent copper.
(This material still exhibited thermal-gradient mass transfer, however.)
The materials containing molybdenum, zirconium, tin, titanium carbide,
magnesium oxide, and aluminum oxide might be worthy of further investiga—
tion since corrosion was relatively slight. However, in the alloys con-
taining columbim, manganese, silicon, and titanium the corrosion was
sufficiently severe so that these materials should be given no further
consideration.

Two essential components of a thermal nuclear powerplant are a coolant
and a moderator. If both functions, cooling and moderating, could be per—
formed by a single material, reactor design might be considerably simpli—
fied (ref. 1). Molten sodium hydroxide is such a material. It also has
the desirable properties of good resistanceto radiation damage (ref. 2)
and a wide temperature range between melting point (608° F) and boiling
point (25540 F).