Delinkage of Metal Surface Saturation Concentration and Micellization in Corrosion Inhibition

Abstract

Long-distance transmission of oil is usually carried out in large-diameter steel pipelines; water present therein may cause severe internal corrosion. An effective method of mitigating such corrosion is to inject organic corrosion inhibitors (CIs). Their surface adsorption, via heteroatom functionalities, can markedly enhance the corrosion resistance of metals. In this study, three CI model compounds with different head groups but the same alkyl tail length (-C14H29), specifically tetradecyltetrahydropyrimidinium (THP-C14), tetradecylphosphate ester (PE-C14) and tetradecylimidazolinium (IMID-C14) were synthesized, their purities being determined using nuclear magnetic resonance spectroscopy (NMR). The critical micelle concentrations (CMCs) of each compound were measured using surface tensiometry (Du Noüy ring) and fluorescence spectroscopy techniques, differences being found between these indirect and direct methods. In addition, linear polarization resistance was used to determine inhibition efficiencies (IEs) for carbon steel immersed in a 5 wt.% NaCl electrolyte saturated with CO2. CI surface saturation concentrations, with maximum IEs, were compared with the determined CMCs. Excellent IEs were observed at concentrations of THP-C14, PE-C14 and IMID-C14 which do not correspond with their CMCs, the differences involved being significantly greater than what has previously been reported for tetradecylbenzyldimethylammonium (BDA-C14). These results demonstrate that there is no direct link between CMC and metal surface saturation/corrosion inhibition efficiency that can be made on a generalized basis for the different head groups but with the same tail length corrosion inhibitors, indicating that the selection of the appropriate CI concentration for an industrial application should not be based on CMC alone.

Electrochemical Study of Corrosion Inhibition of Carbon Steel During Oil/Water Intermittent Wetting

“…For experiments without inhibitor, the final corrosion rate did not change significantly after two oil-wetting stages, which is fully expected. However, in the presence of 20 ppm of THP-C14 inhibitor (above the surface saturation concentration 19 ) in EP condition (Fig. 4a), the corrosion rate decreased significantly by one order of magnitude after the #1 oil wetting stage.…”

“…4b). However, with experiments conducted in 50 g l −1 NaCl with THP-C14 present above its surface saturation concentration, 19 the final corrosion rate was much lower than in the absence of oil (Fig. 4a).…”

“…The physical properties of the LVT 200 model oil are shown in Table II. The inhibitor model compound employed in this work, tetradecyltetrahydropyrimidinium (THP-C14), was synthesized in-house as described in a prior paper, 19 with a chemical structure where the positive charge of the head group delocalizes between two nitrogen atoms as shown in Fig. 1.…”

“…Electrochemical measurements.-Baseline pure water wetting.-For pure water wetting experiments, a three-electrode setup was used to perform electrochemical experiments, equipped with a C1018 rotating cylinder electrode (RCE) as a working electrode (exposed area 2.89 cm 2 ), a platinum-coated titanium mesh counter electrode, and an Ag/AgCl (KCl saturated) reference electrode; a similar configuration was introduced in previous work 19 and is also shown as a part of the setup shown in Fig. 2.…”

“…For experiments without THP-C14, i.e., blank (intermittent wetting) and blank (pure water wetting), the final corrosion rate did not change significantly after two oil-wetting stages and their final corrosion rates were very close to each other. When the THP-C14 inhibitor concentration was 20 ppm (above surface saturation concentration 19 ), CR gradually decreased after the #1 and #2 oil-wetting stage for the intermittent wetting experiment. Compared with blank tests without oil (pure water wetting), the final corrosion rate was close to that determined in the experiment conducted without oil.…”