Synthesis and application of a carboxylated benzimidazolium inhibitor for N80 steel in water-rich supercritical CO 2

The supercritical CO 2 (sc-CO 2 ) water-rich phase can severely corrode wellbore steels, requiring inhibitors effective at high temperature and pressure. In this study, a carboxyl benzimidazole quaternary ammonium salt (CBQ) was synthesized and evaluated for N80 steel in simulated downhole fluid under sc-CO 2 conditions. Its structure was characterized by Fourier-transform infrared spectroscopy (FT-IR) and 1 H NMR, and its weight-loss inhibition performance was evaluated after exposure at 120 °C and 20 MPa CO 2 for 168 h. Increasing CBQ concentration from 0 to 300 ppm decreased the corrosion rate from 0.754 to 0.014 mm a −1 , corresponding to a weight-loss inhibition efficiency of 98.07%. Subsequent ex situ electrochemical measurements were conducted at approximately 25 °C in a conventional three-electrode cell containing the same CBQ concentration used during autoclave exposure. The polarization inhibition efficiency reached 92.77% at 300 ppm, with a marked increase in charge-transfer resistance. Surface characterization showed that CBQ reduced corrosion-product accumulation and the representative line-profile peak-to-valley height difference from approximately 143.8 to 60.6 μm. XPS and XRD results indicate that adsorption through O- and N-containing groups, together with the FeCO 3 -containing corrosion-product layer, promotes formation of a protective interfacial film.