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Highlights • The amorphous corrosion scales formed on the surface of HP-13Cr stainless steel in extremely aggressive environment. • Cr(OH) 3 precipitation contributed to the increase of scales thickness and inhibited the precipitation of FeCO 3. • As the temperature and pressure increased, the corrosion scales changed from monolayer to double layer, and then transformed into single layer. • The corrosion scales exhibited p-type semiconductivity, and density and diffusion coefficient of acceptor increase with the increasing of temperature and pressure. Abstract The nature of corrosion scales formed on HP-13Cr stainless steel (HP-13Cr SS) in the extremely aggressive environment was investigated by means of microstructure characterization and high-temperature-high-pressure electrochemical measurements. The results of these studies indicated that the precipitation of Cr(OH) 3 is the dominating factor affecting on the formation of corrosion scales, and its effect can be categorized based on two compromising aspects. On the one hand, Cr(OH) 3 precipitation contributed to the increase of scales thickness. On the other hand, it inhibited the precipitation of FeCO 3 due to the hydrolysis of Cr3+. Because of these reasons, the corrosion scales undergo significant microstructural changes, i.e., from monolayer (95 °C/2.8 MPa) to bilayer (120 °C/3.2 MPa and 150 °C/3.6 MPa), then to single layer (180 °C/3.8 MPa). Therefore, the corrosion-resistance performance of corrosion scales decreased with increasing temperature and CO 2 pressure, wherein the decreasing pitting potential and repassivation potential accompanied with the increasing density and diffusivity of acceptor in the scales. [ABSTRACT FROM AUTHOR]