Abstract: In a process for manufacturing food-grade caustic such as sodium hydroxide wherein hydrazine or a derivative or salt thereof is used as a corrosion inhibitor, residual hydrazine is removed from the concentrated caustic product solution by oxidation. The oxidation of residual hydrazine is accomplished by adding to the hydrazine-containing concentrated caustic solution, which is characterized by a pH of 14 or higher, a 5- to 10-fold stoichiometric excess of hydrogen peroxide or an equivalent peroxy compound or hypochlorite.

Abstract: The addition of a small amount of hydrazine or a derivative or salt thereof serves to inhibit the corrosive effect of caustic such as sodium hydroxide on metal surfaces during the manufacture of the caustic or in processes using same. For instance, aqueous sodium hydroxide solutions having hydrazine or a derivative or salt thereof added thereto in an effective concentration in the range of from as little as about 2 ppm or less and up to about 1000 ppm, preferably of from about 2 to about 200 ppm, and most preferably from about 3 to about 40 ppm, can be concentrated by evaporation in nickel or nickel alloy equipment at temperatures as high as 150.degree.-175.degree. C. without causing undue corrosion.