Influence of the Co-Condensation of Water and n-Heptane on Top of the Line Corrosion

Abstract

Top of the line corrosion (TLC) is a serious concern in wet gas transportation pipelines operating in a stratified flow regime. Extensive research on TLC has been conducted in hydrocarbon-free environments while, in production environments, condensable hydrocarbons are always present. When water and hydrocarbons co-condense, the condensate comprises two immiscible liquids which have different wettability and corrosivity. In this study, the co-condensation process was investigated using a borescope for visual observation and a set of conductivity probes for quantification of water-wetted area. The corrosion behavior of carbon steel under water-only condensation and water/hydrocarbon co-condensation scenarios was evaluated with weight loss measurements and surface analytical techniques (scanning electron microscopy, energy dispersive spectroscopy, and 3D profilometry). The results showed that water and hydrocarbon condensation processes interacted with each other. Even though the rate of hydrocarbon condensation was much greater than that of water, the hydrophilicity of the steel surface always ensured that it would be mainly wetted by water. However, the presence of n-heptane interfered with the water droplet coalescence and segregated water into smaller droplets which affected the water chemistry. This resulted in a potentially higher pH and iron ion concentration in the water droplet as compared to a water-only scenario. The corrosion rate consequently decreased and was found to be less dependent on the water condensation rate in the co-condensation scenario, as compared to a pure water system.