Kinetics of corrosion layer formation. Part 2 - Iron sulfide and mixed iron sulfide/carbonate layers in carbon dioxide/hydrogen sulfide corrosion

Glass cell experiments were conducted to investigate kinetics of iron sulfide and mixed iron sulfide/carbonate layer formation in carbon dioxide/hydrogen sulfide (CO2/H2S) corrosion of mild steel using the weight change method. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS), x-ray diffraction methodology (XRD), and x-ray photoelectron spectroscopy (XPS) were used to analyze the layer. The experimental results show that mackinawite is the predominant type of iron sulfide layer formed in short exposures in pure H2S solutions. Me type of layer formed in a CO2/H2S solution depends on the competitive mechanism of iron carbonate and mackinawite formation. At high H2S concentration and low dissolved iron carbonate supersaturations, mackinawite was the predominant component in the layer; at low H2S concentration and iron carbonate supersaturations, both iron carbonate and mackinawite may form on the steel surface. It was also found that the corrosion rate of mild steel in H2S corrosion is affected by H2S concentration, temperature, velocity, and the protectiveness of the layer.