Degradation of Cement-steel Composite at Bonded Steel-cement Interfaces in Supercritical CO₂ Saturated Brines Simulating Wellbore Systems Available to Purchase

Geologic sequestration of CO₂ is a proposed method to dispose of greenhouse gas emissions in the earth’s subsurface. Steel used in sequestration wells is protected by Portland cement, and this paper considers whether CO₂ can destroy this protection, resulting in corrosion. Steel-cement composites were exposed to supercritical CO₂ saturated brine mixtures at 50 °C, pCO₂ 1 MPa simulating a typical wellbore environment. Electrochemical techniques were employed to probe corrosion behavior as the cement was progressively carbonated by CO₂. The initial corrosion rates on pre-passivated steel were negligibly small (0.005 mm/year) compared with rates of unprotected bare steel (10 mm/year). The rates increased three orders of magnitude as the iron oxide passive film was damaged by infiltrating CO₂ brine. With time, a calcium/iron carbonate scale developed resulting in corrosion rates that were almost equal to those of the initial passivated surface. Although the corrosion rates of the initial and final states were almost the same, the corrosion potentials were significantly different (-0.3 vs. -0.6 V) reflecting the different properties of the scales. When CO₂ was allowed to fully carbonate and leach the cement, the corrosion rate increased ca. 100 times and stabilized at the same level as steel without any cement coating.