added is no more than 200 ppm and most preferably no more than 100 ppm the actual concentration of sodium borohydride in the caustic after the addition is likely lower than the aforementioned values and possibly even zero due to reactions that consume sodium borohydride typically liquid sodium borohydride is added using a caustic resistant dosage pump dilution of liquid sodium borohydride with aqueous sodium hydroxide e g 20 40 sodium hydroxide may increase efficiency by improving mass transport of the sodium borohydride sodium borohydride containing solution may be injected into the caustic process stream at any point where the equipment allows introduction of an additive one preferred addition point is prior to the steam heat exchanger preferably addition is done after the preliminary concentration of caustic to approximately 75 the method of the present invention is applicable to any evaporator having a contact surface i e a surface in contact with liquid sodium hydroxide and having a temperature of 300 u00b0 c to 450 u00b0 c that is subject to corrosion by sodium hydroxide materials of construction for evaporator contact surfaces include for example nickel alloys such as low carbon nickel nickel and stainless steels such as e brite u2122 in a preferred embodiment of the invention the contact surfaces are constructed of low carbon nickel a typical evaporator used in production of solid sodium hydroxide is a forced draft evaporator the method of the present invention is applicable to an evaporator operating at a contact surface temperature from 300 u00b0 c to 450 u00b0 c preferably the temperature is at least 350 u00b0 c preferably the temperature is.