Corrosion resistance of nickel alloys in molten sodium hydroxide

H. B. Probst, C. E. May, Howard T. Mchenry
Jan 1958

The corrosion resistance of 11 nickel-based compositions to molten sodium hydroxide at 1500 degrees and 1700 degrees was studied in order to find a container material for the caustic at these temperatures. Although, in caustic, pure nickel is corrosion resistant (suffering only thermaal-gradient mass transfer), it is structurally weak. Therefore, 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 dispersions 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 1500 degrees F, only two nickel-base materials showed more than slight intergranular attack. This was a great 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 1700 degrees F was only slightly more severe than at 1500 degrees F. The only alloy that was as resistant to attack as pure nickel was the solid solution containing 30 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 investigation since corrosion was relatively slight. However, in the alloys containing columbium, manganese, silicon, and titanium, the corrosion was sufficiently severe so that these materials should be given no further consideration.

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