Purpose - In this paper, the main aim is to study the synergistic effects of alkanethiols upon inhibition of Schiff bases, N,N'-ethylen-bis and N,N'-ortho-phenylen-bis in 0.5 M sulphuric acid solutions. Design/methodology/approach - Measurements were carried out in a three-electrode cell assembly connected to the corrosion measurement system using software for Tafel polarization. Impedance measurements were carried out at open circuit potential using an electrochemical interface and frequency response analyzer at frequencies from 100 kHz to 10 MHz. Findings - Results obtained revealed that at high concentrations, each of the investigated Schiff bases and 1 -dodecanethiol (DT) were good inhibitors when added separately to the corrosive media. The decrease in corrosion rate was associated with an increase in their concentrations. It was found that the presence of DT together with Schiff bases in corrosive medium increased inhibition effect, especially at low concentrations. Synergistic effect is discussed from viewpoint of their co-adsorption. The adsorption of combined inhibitors was found to follow Langmuir isotherm. Thermodynamic calculations revealed that adsorption of combined inhibitors had a physical nature. Research limitations/implications - In order to improve further the performance of Schiff bases and identifying their synergistic effects with other additives it is suggested that some more researches on other Schiff bases with other alkanethiol molecules must be done. Practical implications - The results of this paper can be used for acid cleaning of carbon steel in which suitable inhibitor systems with the highest efficiency is required. Originality/value - This paper reveals that the presence of DT molecules in solution encourage the adsorption of Schiff bases by facilitating the electrostatic co-adsorption of Schiff base molecules through a synergistic effect.
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