The Influence of Surface State and Weldment on the Corrosion Behavior of X65 Steel in Seawater and Production Water Environments

In this study, the service behavior of an X65 oil and gas pipeline in seawater and production water environments was simulated by a corrosion experiment, and the influence of surface treatment (polishing and scratching) on its corrosion behavior was systematically analyzed. The corrosion resistance of the material was evaluated by means of scanning electron microscopy (SEM), an electrochemical test, and uniform corrosion rate calculations. The results show that the corrosion degree of X65 steel in an oilfield production water environment is significantly higher than that in a seawater environment. The uniform corrosion rate of the welding area is as high as 1.05 mm/y, which is more sensitive than that of the matrix material. The surface treatment has a significant effect on the corrosion behavior. The polishing treatment reduces the corrosion current density of the matrix material from 472.44 μA/cm2 to 313.10 μA/cm2, and the polarization resistance increases to 14.07 kΩ·cm2, which effectively improves its corrosion resistance. The scratch treatment significantly reduces the corrosion resistance of the material, and the corrosion current density of the welding area at the scratch site is as high as 313.00 μA/cm2, even more than that of the untreated matrix material. The study further points out that the scratches and welding areas generated during the pipeline cleaning process will significantly aggravate the tendency of local corrosion and pitting corrosion due to their microstructure heterogeneity. This study provides a clear theoretical basis and engineering guidance for the anti-corrosion design and maintenance of offshore oil and gas pipelines in complex water quality environments.