Raman Spectroscopy of Mackinawite FeS in Anodic Iron Sulfide Corrosion Products

Raman spectroscopy in a confocal microscope was used to study electrochemically synthesized corrosion products from sour gas experiments. When exposed to oxygen-containing atmosphere, the initial mackinawite FeS corrosion product transformed under laser irradiation to hematite, Fe2O3. Measurements with a thin water layer on top of the corrosion products prevented the transformation, as drying was prevented. In situ Raman measurements of mackinawite formation avoided the problem of transformation completely. In situ and operando, the initially formed mackinawite showed two Raman peaks in the wavenumber range > 180 cm− 1 centered around 200–215 cm− 1 and 285–300 cm− 1. On an empirical basis, these modes were assigned to a B1g mode of the iron sublattice and an A1g mode of the sulfur sublattice, respectively. A comparison with a literature assignment for aged mackinawite suggests that the aging observed involves significant changes in the sulfur sublattice.