Corrosion characteristics of X65 steel in CO₂/oil/water environment of gathering pipeline

The corrosion behavior of X65 steel in the CO₂/oil/water environment of gathering pipeline was investigated by mass loss, X-ray powder diffraction (XRD), scanning electron microscopy with X-ray microanalysis (SEM-EDS), and electrochemical analysis through dynamic reactor tests. The results indicate that the corrosion rate increases and the corrosion morphology changes with an increase in the water cut. When the water cut of crude oil is within the range 40%-50%, uniform corrosion occurs along with a lower corrosion rate due to the protection of crude oil, but pitting corrosion appears due to the inhomogeneous adsorption of crude oil. When the water cut is between 70%-80%, the barrier effect of crude oil on the steel surface is weakened, the corrosion scales become thick and loose, and the partial loss of corrosion scales cause mesa corrosion. When the water cut is 90%, the damage area of platform corrosion is enlarged. Consequently, the corrosion rate dramatically increases. Crude oil can hinder the corrosion scales from being dissolved in a corrosive medium and change the dimension and accumulation pattern of the crystal grain, thickness and structure of corrosion scales, thus influencing the corrosion rate. Under the corrosion inhibition effect of crude oil, the temperature-sensitive point of X65 steel corrosion moves to a low temperature at -50℃, lower corrosion rate interval is broadened, and the corrosion resistance of X65 steel is enhanced.