Polymorphous FeS corrosion products of pipeline steel under highly sour conditions

• 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.

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 (Fe1−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.