Abstract:
The nature of the passive film formed and the extent of corrosion within the Ohaaki high and intermediate pressure steamlines was investigated. A recirculating autoclave system and a Fluid Flow Test Rig were used successfully to simulate the corrosion of low carbon steel exposed to Ohaaki condensate. The corrosion test rigs used various techniques to monitor the corrosion: corrosion potentials, linear polarization resistance, electrochemical impedance spectroscopy, Thin Layer Activation, electrical resistance, the hydrogen probe and weight loss coupons. Surface analysis by optical and scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy and Auger electron spectroscopy was used to characterise the corrosion product in terms of composition and morphology. Pourbaix diagrams accurately predicted the formation of magnetite in the Fe-H2O system, and a duplex iron sulphide-magnetite film in the Fe-S-H2O system, provided sufficient time was allowed for the system to come to equilibrium. The iron sulphides formed ranged from mackinawite to pyrite. The iron sulphide and magnetite layers were of roughly equal thickness, with a surface layer of iron sulphide crystals, grown by recrystallization from solution, also present on the test coupons. The addition of SiO2 to the Fe-S-H2O system caused more Fe2+ to be retained within the film. Average corrosion rates decreased with increasing condensate pH. Steady-state corrosion rates were achieved after two or three days, and were independent of the film thickness. The final film thickness was a linear function of the average corrosion rate. The addition of SiO2 to the test solution inhibited corrosion in the Fe-H2O system. In the Fe-S-H2O system the iron sulphide prevented the SiO2 from reaching the magnetite layer and no inhibition was observed. A model for the corrosion in the presence of magnetite and duplex iron sulphide-magnetite films was developed. The rate of corrosion was limited by ionic migration across a barrier layer at the base of pores in the film. Diffusion fluxes of Fe2+ and H2S within the pores were of the same order of magnitude. SiO2 inhibits corrosion at high concentrations by blocking the pores in magnetite films.