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.