Abstract

Volatile corrosion inhibitors (VCIs), such as thiol- and amine- based formulations, can be used to mitigate top of the line corrosion (TLC) that occurs during transportation of wet gas via transmission pipeline. However, the presence of condensable hydrocarbons can compromise VCI inhibition efficiency. This phenomenon can be related to the higher solubility of VCIs in the hydrocarbon phase, compared to water, which leads to a deleterious partitioning effect. This partitioning can happen both in the bulk fluid at the bottom of the line, hindering the evaporation of VCIs, and in any condensed hydrocarbon phase at the top of the line, leading to a decrease in the availability of VCI molecules. The presence of condensable hydrocarbon can even lead to the VCI desorption from the steel surface. The inhibition efficiency of three VCI mixture combinations for TLC scenarios was studied in the presence and absence of n-heptane, as a representative liquid hydrocarbon phase. A mixture of thiols (decanethiol and hexanethiol) and two mixtures of thiol and amines (decanethiol and diethylamine/t-butylamine) were considered. In the presence of n-heptane, only the thiol mixture, where the molecules involved had different tail lengths, conferred high inhibition efficiency. This behavior was attributed to the superior inhibition efficiency provided by thiol-based molecules with a shorter carbon tail (hexanethiol) in the presence of n-heptane. It was further demonstrated that the decanethiol and amine mixtures did not provide any improved corrosion inhibition with n-heptane in the system.

Introduction

Top of the line corrosion (TLC) is a phenomenon encountered in the transportation of wet gas, where temperature differences between the pipelines and the surroundings lead to condensation of water and subsequent metal degradation. This kind of corrosion occurs due to the condensation of saturated vapors present in the unprocessed gas stream which collects on the internal surface of the cold pipe wall. The condensed liquid contains hydrocarbons and water. It forms a thin film and/or droplets of liquid on the pipeline. The condensed water phase can be, at least initially, very corrosive to typical pipeline (made of carbon steel), because it contains dissolved acid gases (such as carbon dioxide [CO2] and hydrogen sulfide [H2S]) and organic acids (such as acetic acid [CH3COOH]).1

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