Polymorphous FeS corrosion products of pipeline steel under highly sour conditions

In this work, the crystal structures and micromorphology of H 2 S 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− x S) is a hexagonal crystal. Mackinawite mainly forms at low temperatures and low H 2 S partial pressures while the formation of pyrrhotite takes place at the higher temperatures and higher H 2 S partial pressures because of the difference between their thermodynamic properties. Due to the protection of pyrrhotite, corrosion rate of X60 steel was reduced.