Characterization of corrosion scale formed on 3Cr steel in CO2-saturated formation water

• No localized corrosion of 3Cr steel was observed in simulated CO2 solution.
• The inner layer is an amorphous-like structure containing some nanoscale crystalline grains.
• The amorphous-like inner layer is mainly composed of FeCO3 and Cr(OH)3.
• Transition of cathodic reactions on 3Cr steel occurred during the two stages.
• Growth of FeCO3 phase significantly decreased the scale porosity and caused passivation.

The evolution of the corrosion scale formed on 3Cr steel was investigated by scanning electron microscopy (SEM) and X-Ray Diffraction (XRD). The inner layer was characterized by Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Thermogravimetric analysis-differential scanning calorimetry (TG-DSC). The results show the inner layer is an amorphous-like structure containing some nano-quasicrystalline and nanocrystalline grains which is more uniform and less porous than the crystalline FeCO3 layer. At the 2nd stage, the transition of cathodic reactions was found, and the growth of the FeCO3 phase in the inner layer dramatically decreased the film porosity, resulting in the passivation.