Assessment of carbon steel and corrosion resistant alloy corrosion in geothermal environments containing sulphuric acid

This work focusses on understanding corrosion severity and pipeline material performance within model geothermal sulphuric acid (H 2 SO 4 )-containing environments. A tantalum-lined high pressure/high temperature autoclave is used to assess the corrosion behaviour of N80 carbon steel compared to 13 Cr and 22 Cr duplex stainless steel when exposed to H 2 SO 4 solutions between a pH of 2-4, and at a temperature of 150 °C. Significant increases in solution pH were observed during 20-hour exposure experiments, resulting from acid consumption due to ongoing corrosion reactions. The resulting pH shift led to a notable reduction in corrosion rates over time, creating uncertainty when assessing long-term material degradation under variable environmental conditions. To address this uncertainty, corrosion rates were analysed in the early stages of exposures, enabling a reliable correlation between temperature, pH and metal composition. Linear polarisation resistance (LPR) measurements revealed that corrosion of carbon steel was primarily influenced by solution acidity, with temperature playing a secondary role. A comparative analysis of corrosion performance revealed that uniform and pitting corrosion rates decreased in the following order: carbon steel > 13 Cr stainless steel > 22 Cr duplex stainless steel. These results offer valuable insights into material selection and corrosion mechanisms relevant to geothermal energy applications. • N80 carbon steel, 13Cr and 22Cr stainless steels assessed at high temperatures • In-situ electrochemical measurements performed in pH 2, 3 and 4 H 2 SO 4 environments • No formation of FeSO 4 layers on N80 and aggressive localised corrosion observed • At pH 2, 150 °C corrosion rates of 10 (13Cr) and 0.1 (22Cr) mm/year measured • Formed layer on 13Cr was thicker, porous, and less protective than 22Cr's