Electrochemical Behavior of Under-Deposit Corrosion of Oil Pipelines

Regarding the corrosion problem under CaCO₃scale in CO₂driven oil and gas fields, the influence of scale layer thickness on the corrosion behavior of Q235b steel used in oil pipelines was studied by electrochemical testing, image characterization, and corrosion product analysis methods. The results show that the corrosion effect of the scale layer on the sample could be mainly divided into three stages. (1) The scale layer promoted the diffusion of corrosive media, intensified substrate corrosion, and led to an increase in the size and maximum depth of substrate corrosion pits, manifested by a rapid negative shift in the open circuit potential. (2) The scale layer played a certain physical shielding role, reducing the size and maximum depth of corrosion pits, increasing the number of corrosion pits, manifested as the open circuit potential remaining basically unchanged. (3) The scale layer played an effective physical shielding role, and the oxygen concentration difference corrosion inside and outside the scale layer led to a significant increase in the number of corrosion pits, manifested as a slow negative shift in the open circuit potential. The scale layer mainly affected the cathodic reaction process of the sample. As the thickness of the scale layer increased, the corrosion of the sample intensified, manifested by an increase in corrosion current density.