Kinetics of Iron Sulfide and Mixed Iron Sulfide/Carbonate Scale Precipitation in CO2/H2S Corrosion

Abstract Glass cell experiments were conducted to investigate kinetics of iron sulfide and mixed iron sulfide/carbonate scale precipitation in CO2/H2S corrosion. Weight gain/loss (WGL) method was used to investigate the scale formation using X65 carbon steel as substrates. Scanning Electron Microscopy (SEM/EDS), X-ray Diffraction methodology (XRD), X-ray Photoelectron Spectroscopy (XPS), and Electron Probe Micro-analyzer (EPMA) were used to analyse the scale. The experimental results show that the corrosion products formed in CO2/H2S system depend on the competitiveness of iron carbonate and mackinawite. At high H2S concentration and low Fe2+ concentration, mackinawite was the predominant scale formed on the steel surface. At low H2S concentration and high Fe2+ concentration, both iron carbonate and mackinawite form. It was also found that ferrous ions forming mackinawite scale mainly come from Fe2+ released from the steel surface.