Corrosion Study of 80S Steel under the Coexistence of CO2 and H2S

Abstract

The continuous rise in the energy demand has shifted the extraction environment in oil and gas fields towards a more hostile environment, and has ultimately increased the corrosion of extraction and transmission facilities. One of the most effective solutions for mitigating the corrosion problem is the use of corrosion-resistant metals. In this paper, we investigated the corrosion behavior of 80S steel that was being employed in an oilfield underground gathering pipeline at different temperatures and partial pressures of H2S and CO2 using an autoclave. Moreover, the loss-in-weight method was used to simulate the corrosive environment in the oilfield. Electrochemical studies were then carried out to investigate the corrosion mechanism. The results show that: (1) In the corrosive environment of CO2 and H2S coexistence, temperature is a major factor affecting the corrosion rate of 80S steel, and increase in temperature accelerates the corrosion process. (2) Corrosion rate is also affected by the CO2 and H2S partial pressure ratio; high S content at high temperatures will inhibit the corrosion process, and vice versa for low temperature. (3) With an increase in the temperature, the corrosion potential decreases, corrosion current density increases, and polarization curve gradually moves to the right. (4) The shape of the cathodic branch moves in the X-negative direction by increasing S content, and the cathodic reaction is jointly controlled by activation and diffusion processes, when the temperature is 100 °C, whereas the anodic branch of the polarization curve at a 3% concentration of Na2S.9H2O changes significantly and a passivation zone appears. (5) The results of the impedance spectra showed that the impedance radius of the metal decreases significantly at increasing temperatures. In addition, the Warburg impedance showed a more pronounced diffusion phenomenon with the increasement of H2S concentration.