Magnetite Formation and Localized Corrosion of Carbon Steel in a Simulated Geothermal Environment at Elevated Temperature and Pressure

This study investigates the formation of magnetite (Fe₃O₄) corrosion product layers on carbon steel surfaces in carbon dioxide (CO₂)-containing conditions representative of produced fluids in geothermal energy systems. X65 carbon steel coupons were immersed in CO₂-containing, 0.3 wt.% sodium chloride solution at pH ~7.5. The test solution was heated to a temperature of 250 °C and pressurized to 45 bar using an autoclave system, with X65 carbon steel coupons positioned in the liquid phase, vapor phase and at the liquid-vapor interface for a period of 48 h and 96 h. Gravimetric measurements showed that corrosion rates were similar between the three different phases and uniform corrosion rates were low (<0.5 mm/year). Ex-situ analysis confirmed that Fe₃O₄ crystals formed widely across the carbon steel surface, acting to minimize uniform corrosion rates in all three conditions. However, white light interferometry analysis showed that significant localized corrosion was observed, with pit-like features observed on the carbon steel surface in regions uncovered by the Fe₃O_4. Iron carbonate (FeCO₃) crystals were also detected in the mixed and vapor phase conditions but were detected in significantly lower quantities than Fe₃O₄.