Study On Inhibition Mechanism Of Cleaning And Passivation For Pyrophoric Ferrous Sulfide

Most of Chinese imported crude oil were sour crude or high-sulfur crude, and the problem of sulfur corrosion got worse in practical production. Ferrous sulfide resulted from sulfur corrosion had high spontaneous combustion tendency, so they often caused the fire and explosive accidents. Then studying on preventive measures, especially chemical cleaning and passivating inhibition, for spontaneous combustion of ferrous sulfide was very important in safe production.According to the principle of complex cleaning, cleaning formula was made up of complexing agents, surface active agents, corrosion inhibitors and additives. The effects of key components on cleaning ferrous sulfide were studied in this paper. We found that EDTA which cleaned pyrophoric ferrous sulfide had peak efficiency. Surface active agents inhibited complexing cleaning of EDTA, but had no effect on complexing cleaning of citric acid. Adding excessive corrosion inhibitors and strong oxidizing passivators to the cleaning agent reduced the cleaning efficiency. The optimal formulations which were obtained by orthogonal experiments were c(LAS)=0.5g/L, c(EDTA)=30g/L, c(THPS)=70mJ/L, c(NH4Cl)=9g/L, c(thiourea)=lg/L. The optimal technological parameters which were obtained by single factor analysis were pH=5, cleaning temperature90°C, mixing cleaning time5hours, soak cleaning time18hours. The best corrosion inhibitor for complexing cleaning which was chosen by immersion experiments and electrochemical analysis was thiourea. When c(thiourea)=2g/L the highest inhibiting rate for carbon steel was95.55%. The result satisfied the needs of cleaning industry standards of the state.Spontaneous combustion tendency of ferrous sulfide and the inhibiting effects of passivators were studied at the low and high temperature. We got that ferrous sulfide generated from iron oxide had high spontaneous combustion tendency at the low temperature. The inhibiting effects of passivators on spontaneous combustion were so obvious, and the highest inhibition ratio of temperature was94.35%. Inhibition mechanism of passivators was the aggregation effect. When the oxidability of the passivator was weaker, the inhibiting effect was more obvious. Ferrous sulfide generated from iron oxidehydroxide had low spontaneous combustion tendency at the low temperature, and the passivators had no effect of inhibition. Ferrous sulfide generated at the high temperature had lower spontaneous combustion tendency than the ones generated at the low temperature, and the passivators also had the inhibiting effects. Ferrous sulfide which had low spontaneous combustion tendency, produced more sulfur after the passivating reaction. It meant that it would planted hidden dangers in the future.According to the features of chemical cleaning and passivating inhibition, the way of passivating inhibition should be carried on at the low-temperature services, because ferrous sulfide had high spontaneous combustion tendency at the low temperature. The way of chemical cleaning should be carried on at the high-temperature services, because the high-temperature services almost all had the dynamical systems.