Effect of Gas Temperature for Sweet Top-of-the-Line Corrosion in Water-Hydrocarbon Co-condensation Environment

When carbon dioxide (CO2) dissolves in water that condenses at the top of the pipeline, carbonic acid is generated, which is corrosive to carbon steel; consequently, sweet top-of-the-line corrosion (TLC) occurs. Gas temperature (Tgas) and water condensation rate (WCR) are important factors that drive the severity of TLC, which effect has yet to be established for water-hydrocarbon co-condensation environment. TLC tests were conducted in control (without n-heptane) and water-hydrocarbon co-condensation environment at 10 vol% n-heptane and 25 vol% n-heptane with average Tgas of 30.0°C and 41.6 °C. At higher Tgas, WCR for control test increased by 250%, from 0.215 ml/(m2s) to 0.754 ml/(m2s); however, only marginal increase between 70% - 79% were recorded for the co-condensation tests. The WCR of 10 vol% n-heptane co-condensation test increased from 0.012 ml/(m2s) to 0.021 ml/(m2s) and the WCR of 25 vol% n-heptane test increased from 0.012 ml/(m2s) to 0.020 ml/(m2s). A similar trend observed for uniform corrosion, whereby the average uniform corrosion rate for control test increased by 43%, from 0.396 mm/y to 0.564 mm/y and co-condensation tests recorded marginal increase between 28% - 32%. The uniform corrosion rate of 10 vol% n-heptane co-condensation test increased from 0.169 mm/y to 0.216 mm/y and the uniform corrosion rate of 25 vol% n-heptane test increased from 0.173 mm/y to 0.229 mm/y. Meanwhile, average pitting rates are similar for all test conditions due to the overlapping error bar. Scanning electron microscope (SEM) of the corrosion layer showed iron carbonate crystals formed in water-hydrocarbon co-condensation even at low temperatures due to localized supersaturation condition, which were not present in the control environment.