Technical Note: Monitoring X65 Steel Internal Corrosion in Humidified N₂-Containing H₂S Using Membrane-Based Electrochemical Sensors

This paper will report on a new membrane-based electrochemical sensor that may provide an important utility in monitoring and characterizing internal corrosion of natural gas pipelines. Using this sensor, the corrosion rate of X65 steel exposed to H₂S in humidified environments up to 60°C has been measured. Consistent with the authors’ earlier CO₂ study, the membrane’s conductivity did not change when exposed to H₂S-containing acidic gas. Introducing H₂S consistently increased the measured corrosion rate between testing conditions, though corrosion rates were typically less than 2 μm/y. At 30°C, the corrosion rate doubled from 7.3 nm/y to 14 nm/y below a relative humidity of 30%, and it increased by an order of magnitude (0.19 μm/y to 1.9 μm/y) at 55% relative humidity, showing that the influence of H₂S on corrosion increases dramatically with greater humidity. Trends with relative humidity match industry expectations: the corrosion rate is low (<0.25 μm/y) without the presence of a condensed aqueous phase, but it increases as the water content of the system increases. The membrane-based electrochemical sensor (MBES) was therefore able to capture relevant corrosion trends, even while the corrosion rates would not have presented a serious threat to any natural gas pipeline. As such, the MBES can be used to detect the onset of emerging corrosion threats before they occur. Field emission scanning electron microscopy and energy-dispersive x-ray spectroscopy confirmed that H₂S reacted with the metal covered by the membrane phase, showing evidence of sulfur-rich sites on the X65 surface. In addition, finite element analysis (FEA) confirmed that electrochemical measurements and data analysis techniques could be successfully used for this membrane-based sensor, despite its unconventional cell geometry.