尿素合成塔における316L鋼の腐食に対するプロセス条件の影響と耐食性の推定法

抄録

As corrosion failures of 316L in urea synthesis solutions are due to either the active region dissolution or the corroison at the transition zone from the passive to the transpassive region, effects of process variables of the urea reactor on these two types of corrosion were electrochemically examined in order to advance the reliability of corrosion preventive methods. For making simple methods to evaluate resistance of 316L to the corrosion at the transition zone from the passive to the transpassive region, seven 316L specimens were immersed in an urea synthesis solution and the results were discussed by comparison with the results of Huey test and chemical compositions of the test specimens. The conclusions obtained are as followws.
(1) 316L for the urea reactor required more amount of air to be injected for preventing active dissolution as the temperature of the reactor rose or the molar ratio of H₂O/CO₂ increased.
(2) Sulfide, an impurity in CO₂ gas, had a very detrimental effect on passivation of 316L. As it is not practical to compensate the harmfulness of sulfide by increase of the amount of air injection, the content of sulfide in CO₂ gas should be lowered before feeding to the urea plant.
(3) For reducing the corrosion at the transition zone from the passive to the transpassive region, higher molar ratio of NH₃/CO₂, lower ratio of H₂O/CO₂ and lower temperature were advantageous.
(4) Sulfide was also harmful to this corrosion.
(5) Annealed 316L was subject to weight loss owing to general corrosion and intergranular corrosion simultaneously at the transition zone from the passive to the transpassive region. Resistance of 316L to these two types of corrosion were correlated with Cr content in 316L and results of Huey test, respectively.