| Hydrocracking is the core technology in oil refining which make the heavy crude oil to light oil. Failures of the reaction effluent air cooler (REAC) and the connected pipelines have been the major obstacle to safe operation of the hydrocracking unit. NH3, HS- were mainly corrosive medium of the REAC export pipeline system, the pH of the aqueous solution was 8.5 to 10. This study mainly focus on 20# carbon steel in aqueous solution of REAC system, and the corrosion rate and the factors of corrosion behaviors of carbon steel in aqueous solution of sulfur-containing were also studied. The morphology, elements and chemical composition of corrsion products and corrosion processes of carbon steel in REAC system aqueous solution were analysised.The factors of corrosion behaviors of carbon steel in aqueous sodium sulfide were investgated using steady state polarization curves and EIS spectra. The results showed that with increasing concentration of sodium sulfide, the corrosion rate first increases and then decreases; when mass fraction of sodium sulfide was 2%, the maximum corrosion rate was reached; when mass fraction of sodium sulfide was 1% and carbon steel experience anodic polarization, passive film can be generated on the surface of carbon steel electrode, which will provent the further anodic dissolution of carbon steel. Add ammonia and SO42-, the basic corrosion behavioral characteristics of carbon steel in sodium sulfide solution have no effect; but the addition of ammonia in aqueous sodium sulfide, corrosion of carbon steel was significantly accelerated. When adding low concentration of Cl~-, corrosion of carbon steel in sodium sulfide solution could be mitigated, and while its concentration exceeds a certain value, the carbon steel corrosion might be exacerbated; in a sum, Cl~- will weaken the passivation capability of carbon steel electrode. Passivation potential at 100 and 200 mV, diffusion impedance barrier layer of EIS can be observed, indicating that carbon steel in aqueous sodium sulfide, the carbon steel electrode surface will generate a certain thickness of the corrosion products while carbon steel through passivation potential.The corrosion rates of carbon steel in REAC system aqueous solution were caculated with methods of weight loss, and the corrosion behaviors of carbon steel in REAC system aqueous solution were studied by steady state polarization curves and electrochemical impedance spectroscopy (EIS). The results showed that with the immersion time goes on, the corrosion rates decreased first and then increased, after 50 hours immersion the critical points of pitting was arrived; with the temperature increases, the corrosion rates increased. The carbon steel had mainly been experienced active dissolution, activation-passivation, passivation and trans-passivation processes while the anodic potentials through the carbon steel. The active dissolution process took two steps, and the first step was experienced the pre-passivation process. The current platform was absevred when carbon steel experiencing catholic polarization. The EIS behaviors were different which obtained at various polarization potentials. The capacitance semi-circles with negative differential resistance transfer from the first quadrant to the second quadrant had been observed which marked the nature of passivation. The morphology, elements and chemical compositions of corrosion products in REAC system aqueous solution were investgated using scanning electron microscopy (SEM), Energy Dispersive X-ray Detector (EDS) and X-ray diffraction analysis (XRD) after constant potential electrolysis by 30 min. The results showed that under the passivation potential of half an hour after constant potential electrolysis, a layer of black corrosion products could be generated on surface of carbon steel electrode. In electrolytic corrosion process, rupture and regeneration of membrane have been emerged. Some non-conducting material of corrosion products could be observed by SEM, and most of the corrosion products joined together. Corrosion products were mainly including S, Fe, and O by EDS. The mainly corrosion products of carbon steel in REAC aqueous solution were composited by elemental S and sulfur-iron compounds (FexSy) and Fe2O3 investigated by XRD. And formation of Fe2O3 was mainly by spontaneous combustion of sulfur-iron compounds in the air. |
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