The complex chemical environment present in the well tubulars used for the oil and gas production as well as during CO₂ capture and storage process could highly influence the initiation, precipitation, and growth of the corrosion products/scales in their surface region due to the CO₂-electrochemical reactions. In order to systematically elucidate the corrosion mechanisms involved when steels are electrochemically exposed to the CO₂-saturated environments at moderately high temperatures, the detailed microstructural characterization of the corroded steel during and after subjecting to the CO₂-containing environments is essential. For this purpose, in addition to the several (electron) microscopical and X-ray computed tomography investigations, ex-situ and in-situ synchrotron X-ray diffraction (XRD) in the grazing incidence geometry were employed. The ex-situ depth-resolved phase identification of the corrosion products precipitated on the steels’ surface with different initial microstructure was conducted using synchrotron XRD at different grazing incidence angles. The results reveal the presence of various crystalline phases at different information depths of the applied X-ray radiation where the changes in the diffracted intensity of (oxy-) hydroxide (chloride) and carbonate reflections acquired at different grazing incidence angles are distinct. This suggests that the corrosion products have different extensions in depth, which is perfectly in line with the observed microstructure. Furthermore, in-situ synchrotron XRD combined with the electrochemical measurements of steels in CO₂-saturated Ca²⁺-containing brine at 80°C using a flow cell was carried out for directly examining the initiation and the formation of (oxy-) hydroxide (chloride) phases as well as precipitation of Fe(and Ca)CO₃ over the thickness of the scale in real-time.