Erosion-Corrosion of a Carbon Steel Elbow in a Natural Gas Gathering Pipeline

A failed carbon steel elbow from a natural gas gathering pipeline in a gas field in Northeast China was investigated by macroscopic and microscopic examinations, chemical composition analysis, metallographic examination, and numerical simulation methods. The investigation results show that the intrados of the elbow was subject to slight general corrosion, while the extrados suffered from severe localized corrosion. The damage of the elbow resulted from an erosion-corrosion in the natural gas containing a few amount of corrosive impurities, liquid water, and solid particles. The impurities in the natural gas, specifically CO₂ and chlorides, would be dissolved into water droplets in the natural gas. These corrosive droplets reacted with the pipe metal, resulting in typical CO₂ corrosion of carbon steel pipe. Furthermore, the droplets and solid particles in the gas would destroy the protectiveness of the corrosion product film on the intrados by mechanical erosion, finally leading to the deterioration of the local environment and then the acceleration of corrosion failure. For controlling corrosion, some measures should be given. However, considering the difficulty of the increase in the curvature radius or the internal diameter of the pipeline, increasing wall thickness of the elbow pipe was a relatively feasible measure to mitigate the erosion-corrosion of the pipe.