Comparative study of morpholine-based ligands on corrosion inhibition of C-steel in 0.5 M H2SO4 solution: Chemical and electrochemical exploration

Abstract

The inhibition of carbon-steel (C-steel) corrosion in 0.5 M H2SO4 media using three morpholine-based compounds, N1-(2-morpholinoethyl)-N1-([pyridine-2-yl]methyl)propane-1,3-diamine (H1), 2-((3-(2-morpholinoethylamino)-N3-((pyridine-2-yl)methyl) propylimino) methyl)pyridine (H2), and N1-(2-morpholinoethyl)-N1,N3-bis(pyridine-2-ylmethyl)propane-1,3-diamine (H3), at room temperature (RT) and higher temperatures, was investigated using electrochemical and chemical methods. The Tafel results showed that the highest corrosion current density (Icorr) is related to the sample without an inhibitor, with a value of 6.3 × 10−5 A.cm−2, and adding inhibitors reduced the values. The lowest Icorr is associated with the sample containing H3 with a concentration of 2.0 mM, which is 0.3 × 10−5 A.cm−2. According to the electrochemical impedance spectroscopy (EIS) results, with an increase in the concentrations of inhibitors H1, H2, and H3 from 0.2 to 2.0 mM, the value of charge transfer resistance (Rct) increases. In addition, Tafel and EIS results confirm that the inhibitor H3 shows better results in corrosion protection. The immersion results of C-steel in a 0.5 M H2SO4 solution without the inhibitor also showed that the highest corrosion rate occurs at 30 and 50 °C. At 30 °C, inhibitors H1, H2, and H3 offer stability for one, two, and three days, respectively. However, at 50 °C, inhibitors H1 and H2 decompose on the first day and do not provide good protective conditions, while sample H3 shows relatively good stability in a 0.5 M H2SO4 environment at 50 °C. In general, inhibitor H3 offers better corrosion protection performance at 30 and 50 °C compared with H1 and H2. According to the R2 values, the adsorption isotherm model of inhibitor H1 follows the Freundlich model, while inhibitors H2 and H3 follow the Langmuir model. The scanning electron microscopy (SEM) results also suggest that the surface of the C-steel sample after corrosion in the inhibitor-free solution has the highest corrosion and surface roughness. Adding inhibitors H1, H2, and H3 significantly reduces the corrosion, and sample H3 shows better surface conditions than the other samples.