Corrosion And Anti-corrosion Of Carbon Steel Of The Aqueous Phase Heat-Exchanger In Refineries

At present, carbon steel is used as the major materials of the aqueous phase heat exchanger in oil refineries. But the corrosion of carbon steel is a serious problem for aqueous phase heat exchanger. Therefore, the corrosion mechanism and corrosion protection methods need to be investigated to extend the equipment life. The combined protection of coatings and sacrificial anodes and electroless nickel plating are useful corrosion protection methods for the aqueous phase heat exchanger. The corrosion medium of the aqueous phase heat exchanger is general alkaline medium, sulfur-bearing solution, so it is necessary to study of the corrosion electrochemical behavior of electroless nickel plating in alkaline medium and sulfur-bearing solution.In present paper, aiming at serious corrosion phenomena of electric-desalting heat exchanger and reforming regeneration alkali-washing cooler in China Sinopec Changling Refinery, the compositions of corrosion medium were analyzed and the corrosion process of carbon steel in corrosion medium was studied. The synthetic protection of coatings and sacrificial anodes was taken as a corrosion protection measure for heat exchanger and the corrosion electrochemical behavior of electroless nickel plating in alkaline medium and sulfur-bearing solution was investigated. The main results obtained in this paper are shown as follows:1. The compositions of corrosion medium for electric-desalting heat exchanger or reforming regeneration alkali-washing cooler in China Sinopec Changling Refinery were analyzed, and the results showed that main composition of corrosive medium for electric-desalting heat exchanger corrosive medium composition were Cl-, H2S, NH3, and etc., and main composition of corrosive medium for reforming regeneration alkali-washing cooler were Cl-, Ca2+, CO32-, SO42-, and etc. The blade-hanging experiment and electrochemical test were used to study the corrosion rate of carbon steel in corrosive medium. Metallographic analysis and electrochemical impedance spectroscopy(EIS) were used to analyze the corrosion process of carbon steel, and the results showed that pit corrosion was the main corrosion forms for electric-desalting heat exchanger, uniform corrosion was the main corrosion forms for carbon steel in circulating water regeneration alkali-washing cooler, and Cl- could destroy the passive film in alkali solution. Corrosion product was analyzed by EDS and XRD, and the results showed that pitting caused by H2S and Cl- was the main reason in electric-desalting heat exchanger leaks, and under-deposit corrosion caused by scaling of the cycle water was the main reason that resulted in corrosion perforation of the reforming regeneration alkali-washing cooler bundle.2. The synthetic protection of coatings and sacrificial anodes was taked as a corrosion protection measure for the aqueous phase heat exchanger, and the related research and design had been done. The optimum cathode protection potential and current density for electric-desalting heat exchanger or reforming regeneration alkali-washing cooler had been investigated by open circuit potential, cathodic polarization curves and coupon weight loss, and the result was -0.87V, -0.86V, 25mA/m2, 36mA/m2, respectively. The suitable sacrificial anode material for electric-desalting heat exchanger or reforming regeneration alkali-washing cooler was Al-Zn-In-Mg-Ti or Zn-Al-Cd, respectively. The study for electrochemical performance of the combied protection was also processed, and the results showed that the combied protection could satisfy the engineering design requirements.3. The surface morphology and elemental composition of Ni-P coating was determined by environmental scanning electron-microscopy. The corrosion electrochemical behavior of electroless Ni-P coating in alkaline medium and sulfur-bearing solution was studied by polarization curves and EIS. The results showed that Na2S could decrease corrosion resistant of electroless Ni-P coating in alkaline medium. Furthermore, corrosion resistant of electroless Ni-P coating decreased with the temperature increasing and increased with the soaking time prolonging.