Mitigation of Elemental Sulfur and Iron Sulfide Under-Deposit Corrosion

Elemental sulfur deposition commonly occurs in sour gas pipelines, especially from high H₂S reservoirs. In aqueous environments, contact of solid sulfur with mild steel can result in aggressive corrosion due to sulfur's oxidizing capability. Iron sulfide is formed on the internal pipeline surface as a corrosion product of carbon steel in an environment containing H₂S. Corrosion caused by deposited elemental sulfur can be further accelerated in the presence of iron sulfide due to its semiconductive nature. Currently, there is minimal prior literature on the corrosion caused by the deposition of both iron sulfide and elemental sulfur in combination. This study was performed to further investigate the effect of iron sulfide and elemental sulfur corrosion of mild steel using an autoclave design to simulate an under-deposit corrosion mechanism. The development of a new effective inhibitor for corrosion and pitting protection in the presence of sulfur and iron sulfide under sour conditions will also be described.

Production of highly sour oil and gas fields has increased recently, which introduces new challenges for corrosion control. In recent years, some unexpected and unexplained pipeline failures have occurred, resulting in complex investigations and laboratory corrosion performance testing studies to assess the inhibitors that are applied in these environments. While some of these failures are due to cracking in sour environments which are prevented by alternative material selection or different operation pipeline maintenance, we were specifically looking at cases where chemical inhibitors could be used to mitigate corrosion. Through this process, it was identified that some of these systems are experiencing under-deposit corrosion (UDC) in the presence of a combination of sand, iron sulfide and elemental sulfur¹. UDC is often experienced in stagnant regions of a pipeline and where pigging has been ineffective or impractical. It is often viewed as one of the most aggressive forms of corrosion in the oil and gas industry and commonly results in severe localized corrosion.

Most published work addressing UDC has focused on sand^2-3, iron sulfide^4-6 or elemental sulfur^7-10 independently, and corrosion inhibitors that effectively mitigate these corrosion accelerants on their own ¹⁰. Discussion regarding corrosion under the combination of multiple types of deposit is sparse in the literature^8-10. Blais et al.⁸ evaluated various testing conditions with polysulfide, elemental sulfur, and iron sulfide. They observed that sulfur, sulfur and iron sulfide, or sulfur and polysulfide accelerate the pitting corrosion of carbon steel. Work carried out by Gregg et al.¹⁰ showed that the combination of a batch corrosion inhibitor and high concentration of continuous corrosion inhibitor are required to mitigate elemental sulfur and iron sulfide UDC. Kvarekval et. al.¹¹ investigated the effect of different sand deposit loadings with iron sulfide. When the sand loading was > 50%, both general and localized corrosion were significantly reduced. SEM images and EDS mapping showed that a continuous network of conductive iron sulfide is required to generate maximum cathodic activity on the deposit. The presence of sand acted as a resistive blockage.