Water/Hydrocarbon Co-Condensation and the Influence on Top-of-the-Line Corrosion

Top of the line corrosion (TLC) is a great concern in wet gas transportation where temperature gradient between internal pipeline and outside environment leads to the condensation of water vapor and a certain fraction of hydrocarbons. Liquid water from the condensation is greatly corrosive as it is saturated with the acid gases; e.g. CO₂, H₂S, HAc, etc. Extensive works have been focused primarily on the hydrocarbon-free system. In reality, the presence of condensable hydrocarbons affects the condensation process as two immiscible liquids with different wettability will form. As a result, less corrosion will be expected if hydrocarbons condense on the steel surface together with water. Therefore, the objective of this work is to investigate the influence of n-heptane on top of the line corrosion. The wettability of water and n-heptane on carbon steel (X65) was determined and corrosion tests under co-condensation were conducted. The results from the first experiment showed that water has higher affinity towards carbon steel than n-heptane in all cases. In a hydrocarbon-free system, corrosion rate increased with the water condensation rate whereas the presence of n-heptane provided some degree of protection in co-condensation scenario. In addition, iron carbonate scale was detected in a co-condensation system but not in a pure water system suggesting different chemistry in water condensate.

Keywords: Top of the line corrosion, Co-condensation, CO₂ corrosion