51312-01075-Degradation of Cement-steel Composite at Bonded Steel-cement Interfaces in Supercritical CO2 Saturat

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Product Number: 51312-01075-SG

ISBN: 01075 2012 CP

Author: Jiabin Han

Publication Date: 2012

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In wellbore systems Portland cement can passivate and protect steel against corrosion by forming an iron oxide film. However exposure to CO2 changes the cement properties and may damage the pre-formed passive film and allow enhanced corrosion rates of steel. In order to investigate this effect steel-cement composites with varying thicknesses of cement were exposed to CO2-brine mixtures. Electrochemical techniques were employed to probe the corrosion behavior of steel as the cement was progressively carbonated by CO2. In low-pressure experiments (1-bar CO2 pressure) the steel was initially pre-passivated by cement and the corrosion rates were < 0.0005 mm/year. The corrosion rates then increased to 0.005 mm/year as cement carbonation resulted in depassivation of steel. These rates were independent of the cement thickness. However the carbonation depth of cement increased linearly with the square root of time demonstrating diffusive mass transfer control of CO2. In high-pressure experiments (100-bar CO2 pressure) the initial corrosion rates on pre-passivated steel were negligibly small (0.002-0.006 mm/year) compared with rates of unprotected bare steel (10 mm/year). The rates increased three orders of magnitude to 1.0 mm/yr as the pre-formed iron oxide passive film was damaged by infiltrating CO2 brine. With time an iron carbonate scale developed eventually resulting in final steady corrosion rates that were almost equal to those of the initial passivated steel surface. This indicates that the corrosion surface was re-healed by iron carbonate formation after the pre-formed iron oxide was damaged by CO2 brine. 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 iron-oxide passivated film compared with the iron carbonate scale. We also observed that corrosion rate was a function of the thickness of cement coating with a 1 mm coating having a corrosion rate two times higher than that with a 5 mm thick coating indicating improved protection with greater cement thickness. In addition we observed that the steady state corrosion rate was two orders of magnitude higher when the cement was completely leached (decarbonated and decalcified) compared with that covered by carbonated cement.