Corrosion Behavior of N80 Oil Casing Steel in Simulated Underground Supercritical CO₂ Environment

The corrosion rate, corrosion morphology and electrochemical behavior of tubing and casing N80 oil casing steel in underground supercritical CO₂ environment (temperature 81.7 ℃, pressure 52.3 MPa) were analyzed by simulated corrosion test and electrochemical test. The results showed that the corrosion rate of the steel increased with the increase of test time. When the time increased from 800 h to 1 000 h, the pitting corrosion rate increased from 1.470 mm/a to 1.782 mm/a. Compared with the change trend of the corrosion current density of the sample without corrosion product layer was slowing increasing with the increase of time. The corrosion current density of the sample with corrosion product was smaller than that of the sample without corrosion product at 0-800 h. However, when the time was extended from 800 h to 1 000 h, the corrosion current density of the sample with corrosion product increased instantaneously from 36.99 μA/cm² to 67.53 μA/cm², and the Tafel constant ratio also showed the same change characteristics. With the increase of the simulated corrosion test time, the charge transfer resistance decreased, the protective effect of the corrosion product layer gradually disappearred, the time constant changed from two to one, and the peak value of the phase angle of the samples with and without corrosion products was basically equal. The characteristics of the low frequency phase angle indicated that the surface corrosion products of the samples with corrosion product layer were formed and ruptured.