Effect of Environmental Factor and Microstructure on Morphology of Corrosion Products in CO₂ Environments Available to Purchase

The effect of environmental factors such as $HCO3+$ ion, CH₃COO^- ion, and Ca²⁺ ion, and microstructure on morphology of corrosion products was investigated in CO₂ environments with 0.3MPa CO₂ at 80°C. Steel J55 with ferritic-pearlitic microstructure and Steel N80 with martensitic microstructure were used as testing materials. Steel J55 showed good localized-corrosion resistance compared with Steel N80. A linear relationship between pH and corrosion rate was obtained in Steel J55 with homogeneous corrosion, but the relationship was not so good in Steel N80 with heterogeneous corrosion. The good localized-corrosion of Steel J55 can be explained from the morphology of corrosion products. Namely, as Steel J55 corrodes away, there is lamellar cementite left behind. An increase in the local concentration of Fe²⁺ ions arises in the cavities between lamellar cementites and leads to FeCO₃ formation between those. Then, the lamellar cementite helps to anchor the corrosion product. In Steel N80 with the homogeneous dispersed-cementite to act as cathodic site, the corrosion product peels off partially because not having the anchor effect in Steel J55. Therefore, Steel N80 suffered from severe corrosion on locations without the corrosion product. In CaCl₂ solution, FeCO₃ and CaCO₃ were identified as corrosion product of Steel J55 by X-ray analysis. In this case, FeCO₃ forms in the cavities between lamellar cementites due to a local flow stagnation and higher local Fe²⁺ ion concentration, but in former ferrite phase, the higher local Fe²⁺ concentration does not arise and CaCO₃ crystallizes.