Sour corrosion performance and sensitivity to hydrogen induced cracking in the X70 pipeline steel: Effect of microstructural variation and pearlite percentage

Abstract

In this study, the effect of the amount of pearlite and microstructural evolution on sour corrosion performance as well as sensitivity to hydrogen induced cracking (HIC) of an X70 steel was assessed. Heat treatment was performed at 875, 950, 1025 and 1100 °C on an API X70 steel that contained various amounts of pearlite (4.61%, 8.94%, 14.46%, and 17.18%). Optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and ImageJ software were employed to assess the effect of pearlite percentage and microstructural development on the mentioned properties. For the sour corrosion performance, open circuit potential (OCP), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) in the H2S containing environment were employed. Also, HIC sensitivity of the X70 steel was measured using hydrogen charging of the cathode followed by tensile testing. Eventually, the damage caused due to the sensitivity to HIC was assessed using FE-SEM technique. Microstructural observations and phase analysis showed that a combination of ferrite, pearlite and (Ti, Nb) carbonitrides were developed in this steel, in which ferrite was considered as the main phase. Increasing the heat treatment temperature increased ferrite grain size as well as its stability, and reduced the amount of pearlite in the structure. Sour corrosion results showed that no active layer was formed on the surface of this pipeline steel. These latest results also showed that increasing the amount of pearlite reduced resistance against corrosion and increased sensitivity to corrosion of microgalvanic localization. As well, increasing the pearlite percentage and reduction of ferrite considerably raised the sensitivity to HIC of the API X70 steel. Finally, the increased sensitivity to HIC was observed in the form of surface initiated cracks in the charged samples.