Abstract: It is an object to establish a test method that enables a high-concentration ammonium bisulfide environment to be reproduced at a laboratory level, and enables the corrosivity of a material to be evaluated easily and highly accurately. According to the test method, when evaluating the corrosion resistance of a material under an ammonium bisulfide environment, the corrosion resistance of the material is evaluated by simulating the relationship between the ammonium bisulfide concentration of a test solution and the pressure in advance, and then determining the ammonium bisulfide concentration from the pressure.

Abstract: Corrosion of a combustion furnace is prevented by a coating of an alkali metal carbonate or an alkaline earth metal carbonate formed on metal surfaces of the furnace before it has started operation.

Abstract: An inert gas containing carbon dioxide and ammonia and separated from a urea synthesis effluent under urea synthesis pressures is brought into contact with an absorbent along with unreacted carbon dioxide and ammonia separated in high pressure distillation at a pressure of from 10 to 25 kg/cm.sup.2 G to absorb substantially all of the carbon dioxide and the major portion of the ammonia, and is cooled to be separated from the resulting liquid ammonia and then is discharged. The inert gas can be treated with little or no danger of explosion by the above method.