Electrochemical study of 2-mercaptoimidazole as a novel corrosion inhibitor for steels

From electrochemical methods, polarization resistance and electrochemical impedance spectroscopy, the corrosion susceptibility of pipeline steel samples immersed in 1 M H2SO4 was determined using nil and different concentrations of the molecule 2-mercaptoimidazole (2MI). It was found that a corrosion inhibiting efficiency, IE, value of 98.5% was reached when the 2MI concentration in the system increased to 25 ppm. Moreover, at this concentration, the 2MI IE was measured as a function of time finding that the IE kinetics follows the relationship: %IE = 98.5 ? 0.03t after 800 h of evaluation. During the first 200 h 2MI IE was higher that 90% then, it decreased to 70% and it remained constant up to 1200 h. It is shown that this compound can affect both the anodic and cathodic processes, thus it can be classified as a mixed-type inhibitor however, from variation of both corrosion potential and polarization resistance with [2MI] it was possible to state that the anodic reaction rate, of the corrosion process, decreases at a greater proportion than the cathodic one. 2MI follows an adsorption mechanism, which can be adequately described by the Langmuir isotherm with an adsorption standard free energy difference (?Gads) of ?26.8 kJ mol?1. In order to analyze the influence of substituting groups, both electron-donating and electron-attracting and the number of ?-electrons on the corrosion inhibiting properties of organic molecules, an electrochemical study was carried out on four different molecules having similar chemical framework structure: 2-mercaptoimidazole (2MI), 2-mercaptobenzoimidazole (2MBI) 2-mercapto-5-methylbenzimidazole (2M5MBI) and 2-mercapto-5-nitrobenzimidazole (2M5NBI). It was found that the IE order followed by the molecules tested was 2MI > 2MBI > 2M5MBI > 2M5NBI. Thus 2MI turned out to be the best inhibitor, even superior to the 2MBI. This fact strongly suggests that, contrary to a hitherto generally suggested notion, an efficient corrosion inhibiting molecule does not require to be a large one, also bearing an extensive number of ?-electrons.