The Effect of Elevated H2S on Corrosion Behaviour of API 5L X65 Carbon Steel in High Partial Pressure CO2 Environments

Abstract

The corrosion behaviour of API 5L X65 carbon steel was investigated under high pressure carbon dioxide environments, containing elevated hydrogen sulfide, to simulate the condition of high carbon dioxide-containing natural gas subsea pipelines. It was systematically studied under high pressure carbon dioxide (120 bars) with a variation in other key parameters (hydrogen sulfide concentration and temperature). The corrosion rates were tested using High Pressure and High Temperature (HPHT) autoclave and measured using the techniques such as linear polarisation resistance (LPR), potentiodynamic sweep measurements, iron count, weight loss (WL) and electrochemical impedance spectroscopy (EIS). The surface morphology and the composition of the corrosion product layers were analysed using Infinite Focus Microscope (IFM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). The results showed that in the CO2- saturated water phase, the addition of 2000 ppm hydrogen sulfide (H2S) instantaneously decreased the corrosion rate of carbon steel API 5L X65 at both 25°C and 80°C. The surface morphology and the composition of the corrosion product layers reveal the formation of mackinawite. The inhibitive effect of hydrogen sulfide at elevated concentration was observed and contributed to the significant reduction in corrosion. The effect on corrosion was despite the fact that water chemistry equilibrium was unchanged with the presence of elevated hydrogen sulfide.