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 FeCO3 due to the hydrolysis of Cr3+. Because of these reasons, the corrosion scales undergo significant microstructural changes, i. e., from monolayer (95 degrees C/2.8 MPa) to bilayer (120 degrees C/3.2 MPa and 150 degrees C/3.6 MPa), then to single layer (180 degrees C/3.8 MPa). Therefore, the corrosion-resistance performance of corrosion scales decreased with increasing temperature and CO2 pressure, wherein the decreasing pitting potential and repassivation potential accompanied with the increasing density and diffusivity of acceptor in the scales.