Top-of-the-line corrosion and benzylamine inhibition under water-hydrocarbon co-condensation

Wet gas pipelines transport unprocessed natural gas that contains water and carbon dioxide (CO₂), which combination is corrosive to carbon steel. Under specific conditions, severe CO₂ corrosion occurs at the top section of the pipeline, known as top-of-the-line corrosion (TLC), which can be mitigated through volatile corrosion inhibitor (VCI) injection. The focus of this study is to investigate TLC and VCI inhibition mechanisms in water-hydrocarbon environment, which effect is often ignored in TLC and VCI tests. This study conducted an improved simulation of the condensation occurring in a wet gas pipeline and showed the effect of hydrocarbon volume ratio and gas temperature towards TLC and VCI inhibition mechanism. This study is divided into three parts: first, VCI selection using molecular modelling, second, TLC glass cell experiments in uninhibited and inhibited conditions, which include condensed water analyses, uniform corrosion rate and pitting rate analyses, and corroded surface characterisation and finally, response surface methodology (RSM) analyses to evaluate the relationships between factors and TLC responses. Benzylamine was selected as the VCI through a score-based method that covered frontier molecular orbitals energy, total energy and adsorption energy calculated in BIOVIA Materials Studio. In TLC glass cell experiment, the pipeline condition was simulated using water and n-heptane at 0 vol%, 10 vol% and 25 vol% to create water-hydrocarbon co-condensation. The presence of n-heptane in TLC tests caused sharp decrease in water condensation rate (WCR), leading to lower uniform corrosion rates, downward trend of pitting rates and altered the condensed water chemistry. Possible iron carbonate containing precipitates were only observed in water-hydrocarbon co-condensation tests via scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX), which were absent in control tests. Benzylamine inhibition was affected by the test conditions with inhibition efficiency between 4.8 % - 58.3 % to mitigate uniform corrosion rate (UCR) and between 0 % - 88.9 % to mitigate pitting. Its inhibition efficiency to mitigate UCR is temperature dependent; it decreases with the increase in temperature, and it shows antagonism behaviour when combined with n-heptane. However, its inhibition efficiency to mitigate pitting did not show temperature dependence, and it showed potential synergism behaviour when combined with n-heptane. The proposed benzylamine TLC inhibition mechanism is a combined pH neutralisation and film formation. Finally, the RSM analyses were able to provide preliminary RSM models to predict the gas temperature for the TLC glass cell setup, and the pH of the condensed water within the boundary of the test conditions conducted in this study.