Corrosion of N80 tubing steel in brine at 1·2 MPa CO ₂ containing trace amounts of H ₂ S

The corrosion of N80 tubing steel was measured electrochemically in an autoclave at an overall carbon dioxide pressure of 1·2 MPa. Both potentiodynamic polarisation curves and electrochemical impedance data were analysed to obtain the relevant kinetic parameters. The influences of H2S pressure and immersion time on the corrosion of N80 were analysed in detail. The results indicate that protective sulphide scales may form at higher partial pressures of H2S and greatly decreased the corrosion rate of N80 steel as the immersion time increased. Thus, the capacitive loop shrinks gradually in the low frequency region and eventually shifts to a single capacitive loop at low H2S concentration, whereas at higher H2S concentrations, the capacitive loop gradually disappeared, and diffusion controlled processes then predominated owing to the formation of a protective corrosion scale at longer immersion times.