Effect of hydrocarbon volume ratio in sweet top-of-the-line corrosion under water-hydrocarbon co-condensation

Abstract

Wet gas pipelines transport unprocessed natural gas that contains water and carbon dioxide (CO₂), which combination can lead to severe wall loss caused by CO₂ or sweet corrosion in carbon steel. Under specific conditions, CO₂ corrosion occurs at the top section of the pipeline, which is known as top-of-the-line corrosion (TLC). TLC tests were conducted in the water-hydrocarbon co-condensation environment using 10 vol% n-heptane and 25 vol% n-heptane to simulate the internal condition of a wet gas pipeline to study the effect of hydrocarbon volume ratio towards TLC. The presence of n-heptane showed minimal effect on the gas temperature profile; however, n-heptane suppressed the water condensation, resulting in a tremendous decrease in the water condensation rate (WCR). The TLC rates were found to be lower in the presence of n-heptane, which can be attributed to the reduced WCR and water-wetted areas. The presence of n-heptane had no significant effect on the pitting rates but showed an increasing pit ratio as the n-heptane volume increased. TLC tests were conducted at three durations: one day, two days and three days showing that n-heptane has no significant effect on corrosion kinetics.