The influence of oxygen on the morphology and protectiveness of iron carbonate scales formed on carbon steel in CO2 environments at near-neutral pH was investigated. Specimens were tensioned in four-point bending jigs for stress corrosion cracking tests, and unloaded specimens were used for electrochemical tests. All specimens were exposed to CO2 aqueous solutions with and without dissolved oxygen, and corrosion rates were monitored by linear polarization resistance technique. Corrosion scales were characterized by scanning electron microscopy and X-ray diffraction, while the analysis of the corroded surface of the specimens after scale removal was carried out using scanning electron microscopy and optical profilometry. The iron carbonate scales were destabilized and became less protective in the presence of oxygen, which resulted in localized attacks, but no cracks were observed.


High strength carbon steel tensile wires confined in the annulus of flexible pipes might experience corrosion when the annulus is flooded with water, either due to outer sheath breaches or to condensation of water molecules permeating from the bore through the inner sheath.1-3 Carbon dioxide (CO2) molecules may also permeate from the bore and reach the annulus,1,2,4,5 where it dissolves into water to form carbonic acid (H2CO3).

Carbon steel corrodes in carbonic acid according to Equation 1, producing ferrous ions (Fe2+), an equivalent amount of bicarbonate (HCO3−) and hydrogen gas (H2). In deaerated aqueous CO2 environments, iron carbonate (siderite, FeCO3) is the typical corrosion product formed.4,6,9 It precipitates on the steel surface according to Equation 2, forming a scale which gives a varying degree of protection, depending on the conditions in which it is formed, and may reduce corrosion rates.6,7



Protective FeCO3 scales cannot be formed, however, when oxygen is dissolved in the solution. Oxygen reacts with Fe2+ to form Fe3+, from which iron oxides and oxy-hydroxides are formed.8,9 In aerated aqueous CO2 solutions, the Fe2+ concentration is therefore reduced and the formation of FeCO3 is hindered, limited to occur in regions of the scale which are depleted of oxygen,8-10 while previously formed FeCO3 scales are destabilized and dissolve where the Fe2+ concentration is lower than the solubility limit of FeCO3.8,11,12

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