In this study, CO 2 -and Fe 3 O 4 -assisted corrosion of the cement-casing interface for the magnetite (Fe 3 O 4 ) high-density cement-casing steel system in simulated formation water is investigated under the normal and high CO 2 pressure conditions. Results show that the degradation of the cement-casing interface is affected by the competition of cement hydration and CO 2 corrosion. The migration of CO 2 and corrosive species to the cement-casing interface is facilitated under the high pressure condition, resulting in enhanced corrosion rate as compared to the normal pressure condition. The casing steel is in active corrosion state while the corrosion rate gradually increases with time. After long term of immersion, a layer of thick corrosion products with multiple cracks/ microgaps forms at the cement-casing interface, which can hardly protect the casing steel from further corrosion. The corrosion products on the steel surface are mainly FeCO 3 accompanied by a small amount of CaCO 3 and Fe 3 O 4 , suggesting the prevalence of CO 2 corrosion. The mechanism of the CO 2 -and Fe 3 O 4 -facilitated corrosion of the cement-casing interface for the magnetite high-density cement-casing system is proposed.
A high-temperature autoclave was used to grow CO2 corrosion-product films on P110SS steel specimens while the surface of the specimens was continuously subjected to tensile stress in a four-point bending jig; the autoclaving times were 6, 18, 36, and 72 h. A scanning electron microscope was used to observe the surface topography of the corrosion-product films formed on the P110SS steels. An X-ray diffraction was used to analyze the phase compositions of the corrosion products. The electrochemical performance of the films was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization curves. The results showed that tensile stress could hinder the formation of corrosion-product films; the integrity and compactness of the films worsened, but the phase compositions of the films did not change. The applied tensile stress resulted in a smaller grain size of the corrosion-product films, and the grain boundaries increased. In addition, owing to the induced tensile stress, the charge transfer resistances decreased, and the corrosion current densities increased for the P110SS steels with corrosion-product films in a 3.5 wt.% NaCl solution saturated with CO2.
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