Iron Sulfide Precipitation Kinetics, Solubility, Phase Transformation, and Corrosion versus Temperature and Ionic Strengt

A reliable anoxic plug flow reactor has been developed to study iron sulfide (FeS) precipitation kinetics, solubility, phase transformation, and corrosion simultaneously. The effects of temperature (23 -- 125 °C), ionic strength (0.00886 -- 5.03 mol/kg), and ferrous iron (Fe(II)) to sulfide (S(-II)) concentration ratio (1:20 to 1:5) were studied. The kinetics of FeS precipitation was found to be a pseudo first order reaction with respect to Fe(II) concentration, when Fe(II) concentration is significantly lower than S(-II) concentration. FeS precipitation kinetics can be accelerated by high temperature and high ionic strength, but not be affected by Fe(II) to S(-II) concentration ratio at the tested ratio range. A model for predicting FeS solubility and precipitation was developed by using FeS solubility data measured in this study and data from literature. At temperature ≤ 100 °C, freshly precipitated FeS was found to be mackinawite. Mackinawite can transform to troilite at temperature ≥ 50 °C, and low ionic strength favors the phase transformation. Also, mackinawite formed from steel corrosion seems to be easier to transform to troilite than the mackinawite formed from precipitation. H2S corrosion and FeS scale retention on carbon steel can be significantly accelerated by high temperature and impeded by extra high ionic strength (IS ≥ 4 mol/kg). This study presented a new approach for iron sulfide study and contributed valuable FeS thermodynamics and kinetics data for FeS prediction and control in industry.