ABSTRACT

The overall objective of the present research is to investigate the effect of operating parameters on the inhibition efficacy of decanethiol in top of the line corrosion (TLC). The effect of water condensation rates, monoethylene glycol (MEG), H2S and hydrocarbon on inhibitor efficacy was evaluated. It was found that the presence of MEG, variation of gas temperatures and water condensation rates did not affect the inhibition efficacy of decanethiol. In sour environments, decanethiol was able to reduce localized corrosion of carbon steel and change the morphology of corrosion products on samples exposed to 30 ppm H2S. In the presence of condensable hydrocarbon (heptane), decanethiol lost its inhibition efficacy and showed very poor persistency. This was due to its low solubility in water.

INTRODUCTION

Top-of-the-line corrosion (TLC) is recognized as one of the most serious concerns encountered in the pipeline transmission of hydrocarbons, where carbon steel is the principal tubular alloy of choice.1TLC occurs in wet gas transportation where temperature gradients between the internal surfaces of the pipeline and the outside environment leads to condensation of water, as well as lighter hydrocarbons from the gas phase. Corrosive gases dissolve into the condensed water formed on the upper surface of the pipe resulting in TLC. Extensive prior research has primarily focused on the use of volatile corrosion inhibitors (VCIs) to mitigate TLC. 2,3,4 The importance and relevance of VCIs are evidenced by the existence of several patents, as well as recent publication of comprehensive reviews thereof.5,6 A few volatile organic compounds have been identified and tested as potential VCIs in the laboratory; their molecules have either filming or neutralizing properties.7,8 The two different classes of volatile corrosion inhibitors examined in this research possessed amine or thiol functionalities. Based on the previously reported results, morpholine and diethylamine show poor inhibition properties; these amines solely increase the pH of the condensed water and do not significantly decrease the corrosion rate.7 In contrast, thiols, especially decanethiol and 11-mercaptoundecanoic acid, show good persistency (in the absence of condensable hydrocarbons), filming behavior and superior mitigation of TLC.8 The good persistency of decanethiol and 11-mercaptoundecanoic acid could be due to their low association by hydrogen bonding and their lower solubility in water.

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