Corrosion of Nickel tubes in the steam chest of Caustic soda evaporators

Abstract

The investigation on corrosion of nickel tubes in steam chest of caustic soda evaporator was taken up at the instance of M/s D.C.M.(Chem.) Ltd. to study the cause of the attack on the steam side of the nickel tube of evaporators. The party supplied the pieces of attacked and unattached nickel tubes and also relevant operating data of this process. Caustic solution from the electrolysis of brine by diaphragm process is received at a concentration of about 10%. chloride and other components e.g. Na2SO4, Na2CO3 etc. This solution of caustic soda is generally concentrated to strength of 50% by evaporation of the 10% solution, carried out in evaporators in different stages of concentration. The concentration from 10% to 50% is not brought out in a single evaporator but is carried out in steps of different concentrations. The various steps are known as ‘effect’ and depending on the number of steps; the evaporators are known as single effect, triple effect etc. The process consists of arranging a series of evaporators in an order of descending pressure (and therefore of descending boiling points) so that the vapors from one unit can heat the lower pressure unit. The solution is heated by passing the steam through the steam chests which consists of several expanded tubes of nickel inside a closed chamber. The solution is circulated through the tubes. (The circulation is either natural or forced concentration of caustic soda.)