Hydrocarbon chain, hydrophobic in nature, plays an important role in the efficiency of corrosion inhibitors. Increasing hydrocarbon chain length in a corrosion inhibitor tends to increase its inhibition efficiency (IE) but decrease its solubility in aqueous solutions. In this study, quaternary amine corrosion inhibitors with variable hydrocarbon chain lengths ranging from 10 to 16 carbons were synthesized and evaluated as acid corrosion inhibitors for coiled tubing steel. The substituents containing double and triple bonds were incorporated into the corrosion inhibitors to enhance their performance in HCl acid solutions. In addition to the quaternary nitrogen, three additional adsorption sites including propargyl and two allyl groups are present in this corrosion inhibitor.

Behavior of these compounds in concentrated HCl acids was examined using gravimetric and electrochemical methods (polarization and Electrochemical Impedance Spectroscopy (EIS)). Weight loss tests were conducted in 28 wt% HCl acid solutions at different temperatures and a soaking time of two hours was applied. The temperatures examined ranged from 30 to 90 °C. The results showed that IE increased in the order of C10 < C12 < C16 at 30 °C and while a reverse order was observed at higher examined temperatures (90 °C). At a temperature of 30 °C, an inhibitor concentration of 50 ppm gave the corrosion IE of 80.4, 78.3 and 67.2% for C16, C12 and C10, respectively, while at 90 °C, the corresponding inhibition efficiencies were 25.7, 21.1 and 15.4%. Polarization measurements showed that the inhibition mechanisms for C12 and C16 was via suppressing/retarding anodic reaction while a mixed- type mechanism was obtained for C10. The IE obtained by the polarization technique at 30 °C showed the same order obtained by weight loss method (C16>C12>C10) while at a temperature of 70 °C, the order was C16>C10>C12.

This paper will present in detail, 1) the synthesis and characterization of the corrosion inhibitors, 2) the variation in their performance, and 3) subsequently, the potential mechanisms for obtained variations as a function of temperature are discussed.

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