A furnace, a reaction column, a heat exchanger, and so on of, for example, a hydrodesulfurization apparatus are exposed to fluids containing high-temperature sulfides during the operation, whereby iron sulfide is formed on the surface thereof. This iron sulfide, when exposed to the air, is hydrolyzed by the action of oxygen and moisture and is converted into polythionic acid, causing the occurrence of stress-corrosion cracking of an austenitic stainless steel used in the equipment.
In order to eliminate this problem, a method in which in stopping the operation, the fluids are withdrawn from the equipment and the inside of the equipment is washed and neutralized with an aqueous solution of an inorganic alkali such as sodium carbonate, caustic soda, or ammonia has heretofore been employed (see NACE Standard, RP01-70, titled "Protection of Austenitic Stainless Steel in Refineries Against Stress Corrosion Cracking by Use of Neutralizing Solutions During Shut Down").
In accordance with the above method comprising washing and neutralizing with an aqueous alkali solution, however, because the surface of the equipment is wet with fluids containing sulfides and repels the aqueous alkali solution, contact of the aqueous alkali solution with iron sulfide formed on the surface of the equipment is achieved insufficiently such that protection from the occurrence of stress-corrosion cracking cannot be ensured. Furthermore, the aqueous alkali solution for washing and neutralization sometimes remains in dead portions of the equipment and pipes to cause corrosion. Moreover, the above method involves such a problem that it is necessary to once withdraw the fluid remaining in the equipment and then introduce the aqueous alkali solution, which makes the operation complicated.