A comprehensive study on the inhibition behaviour of four carboxylate-based corrosion inhibitors focusing on efficiency drop after the optimum concentration for carbon steel in the simulated concrete pore solution

Abstract

In this research, the inhibition action of four carboxylate derivatives with different numbers of functional carboxylate groups on carbon steel in 0.5 M NaCl simulating concrete pore solution was investigated using electrochemical measurements in conjunction with the XPS technique. Quantum chemical calculations were also applied to discuss the EIS data and the inhibitor's adsorption behaviour. This paper focuses on the inhibitive action of carboxylate derivates after optimal concentration. The results show that after a critical concentration, the corrosion inhibition efficiency drops to lower values, which the reasons for this matter are still uncertain. We suggest that the adsorption process is directly affected by the number of attractive and repulsive intermolecular interactions. At higher concentrations, electrostatic forces cause the local molecular clusters on the surface, which results in an imperfect layer of molecules that is vulnerable to aggressive agents such as chloride ions. Although with increasing the number of carboxylate groups, the corrosion inhibition efficiency decreases by 35%. However, quantitative XPS results reveal no noticeable reduction in the oxide layer thickness, which indicates that localized attacks lead the corrosion.