One-dimensional Array Electrode Method Was Used To Study The Crevice Corrosion Mechanism Of Oil Well Pipe Joints

Oil well tubing joint is connected to each other through screw thread. There might be a slight gaps between the coupling and tubing, in which corrosion occurs more easily than the other parts of the tubing. Once the corrosion happens, the tubing joint will become the weakest part and causes a failure of the complete oil well tubing. Therefore, it is necessary to study the mechanism of crevice corrosion and find a reasonable protective method.In this research, the commonly used P110 oil casing steel was adopted as a study sample, in which one-dimensional array electrode was applied to study the process of crevice corrosion. Additionally, potentiodynamic polarization (PDP) technique, electrochemical impedance spectroscopy (EIS), scanning electrochemical microscopy (SECM) and scanning electron microscope (SEM) were used to measure the sensitivity and electrochemical behavior of samples as well as the corresponding morphologies after corrosion in the environment with CO2. Also, the effect of inhibitor on P110 steel crevice corrosion will be revealed.The effects of gap structure and temperature on the susceptibility of crevice corrosion for P110 steel were studied through weight-loss and the observation of surface morphology. The results indicated that the corrosion rate of P110 steel was accelerated by the gap between the coupling and tubing. The susceptibility of crevice corrosion was gradually strengthened by the increasing temperature, and thus the corrosion occurred. The corrosion product film formed at the gap was weak and displayed no protection to the substrate. Moreover, severe pitting corrosion occurred on the surface of the substrate and the inner region of the pit was even worse than crevice opening.In order to understand the corrosion damage to the inner surface of screw thread joint between the tubing and coupling, artificial crevice was built by one-dimensional array electrode. The results showed that the crevice corrosion was found on the surface of P110 steel in brine solution with CO2. The size of most incidental gap was between 0.1 to 0.15 mm. As the crevice corrosion is often accompanied by pitting, and thus even more serious damage was caused to the screwed connection. Based on the field and lab investigations, a multi-staged model for the crevice corrosion at tubing joint was proposed, which can explicitly explain the development of the crevice corrosion.The effects of mannich bases and imidazoline corrosion inhibitor on the crevice corrosion of P110 steel in acid environment were tested. The results indicated that both inhibitors were able to reduce the tendency of the crevice corrosion for P110 steel, i.e., the decreased corrosion current density and low the corrosion rate of P110 steel caused by the addition of inhibitor. The corrosion inhibition of these two types of inhibitors was produced by the protective film on electrode surface ascribed to the adsorption mechanism. This protective film mainly inhibited the reaction at the anode electrode. It was clear that the two types of inhibitors inhibited both cathodic and anodic processes during corrosion. Also, the inhibiting effect of imidazoline inhibitor on the crevice corrosion of P110 steel was higher than that of mannich bases.