Polymorphous FeS corrosion products of pipeline steel under highly sour conditions

Highlights: • As H₂S corrosion products, mackinawite is tetragonal crystal and pyrrhotite is hexagonal crystal. • Mackinawite mainly forms at low temperature and low H₂S partial pressure. • Pyrrhotite forms at the high temperature and high H₂S partial pressure. • The corrosion rate decreases gradually when pyrrhotite forms. - Abstract: In this work, the crystal structures and micromorphology of H₂S corrosion products were investigated by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The corrosion behavior of X60 steel under different conditions was analyzed. The results show that mackinawite (FeS) is a tetragonal crystal and pyrrhotite (Fe_1−xS) is a hexagonal crystal. Mackinawite mainly forms at low temperatures and low H₂S partial pressures while the formation of pyrrhotite takes place at the higher temperatures and higher H₂S partial pressures because of the difference between their thermodynamic properties. Due to the protection of pyrrhotite, corrosion rate of X60 steel was reduced.