The corrosion behaviour of low-alloy steel 3Cr was investigated under high pressure carbon dioxide environments, containing small amount of hydrogen sulfide, to simulate the condition of high carbon dioxide containing natural gas transporting offshore pipeline. It was systematically studied under high pressure carbon dioxide (range from 80 and 120 bars) with variation in other key parameters (temperature and hydrogen sulfide concentration). The corrosion rates were tested using High Pressure and High Temperature (HPHT) Autoclave and measured using the techniques such as linear polarisation resistance (LPR), electrochemical impedance spectroscopy (EIS), iron count and weight loss (WL). The surface morphology and the composition of the corrosion product layers were analysed by using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Raman spectroscopy and Infinite Focus Microscope (IFM). The results showed that in the CO2- saturated water phase, the addition of 200 ppm hydrogen sulfide (H2S) instantaneously decreased the corrosion rate of low-alloy steel 3Cr at both 25°C and 80°C. © 2018 Author(s).