Corrosion inhibition exploration of synthesized carboxylic acid and amino esters on selected metals in acid medium

Abstract

In this study, carboxylic acid and two ionic liquids-based amino esters were used as corrosion inhibitors for aluminium and mild steel. These compounds were selected because they are said to be non-toxic and environmentally friendly. The non-toxicity of these compounds is essential due to the increasing implementation of strict environmental regulations. The investigated compounds as corrosion inhibitors in this study include a carboxylic acid, namely, N-benzoyl2-aminobutyric acid (2-NBABA), and two amino esters which are 1-(benzyloxy)-1-oxopropan2-aminium 4-methylbenzenesulfonate (1-BOPAMS) and 4-(benzyloxy)-4-oxobutan-1aminium 4-methylbenzenesulfonate (4-BOBAMS) for aluminium and mild steel corrosion in 1.0 M hydrochloric solution (HCl) at 303-333 K. The three compounds were synthesized and characterized by spectroscopic methods (FT-IR, 13C-NMR and 1H-NMR). Their anti-corrosive properties, inhibition mechanism, inhibitor-metal adsorption behaviour and corrosion inhibition efficiency were investigated by employing several techniques such as the gravimetric, potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Fourier transform infrared spectroscopy (FT-IR) was used to investigate the functional group responsible for the adsorption/desorption process of the inhibitor molecules and those that disappeared or formed on the surfaces of mild steel and aluminium. Gravimetric analysis exhibited an increase in the inhibition efficiency as the concentrations of the inhibitors were increased for all the three inhibitors studied. Amongst the several isotherms plotted, Langmuir adsorption isotherm was found to be the best-fit isotherm for the three compounds on the metal surfaces. The isotherm provided a clear indication on the mechanism of adsorption which was mixed-type adsorption for both aluminium and mild steel. The impendence result showed an increase in the Rct values as the concentration of the inhibitors was raised, resulting in an increase in the surface coverage by the inhibitor molecules. The adsorption of the inhibitor molecules on the aluminium and mild steel surface led to the decrease in the double-layer capacitance (Cdl) as the molecules of the inhibitors replaced the water molecules at the metal/solution interface reducing the local dielectric constant while increasing the double layer thickness. The data gathered showed that the adsorption of the three inhibitors prevented the dissolution of the mild steel and aluminium in 1.0 M HCl without altering the corrosion mechanism of the metals. The PDP results obtained indicated that the three inhibitors affected both the anodic and cathodic half-reactions to a similar extent. The anodic and cathodic Tafel curves were both affected by the introduction of the inhibitors which revealed that the investigated inhibitors act as mixed-type corrosion inhibitors. Fourier transform infrared spectroscopy studies showed that the corrosion process was reduced due to the interaction of the three inhibitors with mild steel and aluminium resulting in the formation of a Fe-inhibitor and Al-inhibitor complexes.