Electrochemical and quantum chemical studies of some indole derivatives as corrosion inhibitors for C38 steel in molar hydrochloric acid
Introduction
Acid solutions are generally used for the removal of undesirable scale and rust in several industrial processes. Hydrochloric and sulphuric acid are widely used in the pickling processes of metals. Inhibitors are used to prevent metal dissolution as well as acid consumption [1]. Several researchers [2], [3], [4], [5], [6], [7], [8], [9] studied the effect of some organic inhibitors on the corrosion of steel in hydrochloric and sulphuric acid solutions. Numerous attempts are made to link the corrosion inhibitor efficiency with a number of parameters of the molecules [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. In many cases the parameters connected with the electronic and the chemical structure of the molecule act simultaneously on the inhibitor efficiency and it is difficult to decide which parameter plays the most important role in increasing the inhibitor efficiency [22], [23]. For this reason organic compounds, which have planar molecules of similar area were chosen in this investigation. It is assumed that in this way the influence of the chemical structure will be very similar and the influence of the electronic structure will be better expressed. Indole is a heterocyclic compound with a fused structure of the benzene ring with a pyrrole ring. Previous studies have shown that indole and some of its derivatives display good inhibiting properties on corrosion of steel and copper in acidic solutions [24], [25], [26], [27], [28].
In this work, some indole derivatives, namely, 9H-pyrido[3,4-b]indole (norharmane) and 1-methyl-9H-pyrido[3,4-b]indole (harmane) have been studied as possible corrosion inhibitors for C38 steel in molar hydrochloric acid (1 M HCl), in order to determine the effect of alkyl group in pyridine. Measuring changes in open-circuit potential (OCP) with immersion time, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were carried out to study the mechanism of corrosion inhibition. Adsorption mechanisms of studied compounds were described by finding their PZC values. Raman spectroscopy was carried out to establish the mechanism of corrosion inhibition of indole derivatives of C38 steel in acid solution. Also the relationship between DFT quantum chemical calculations and experimental inhibition efficiencies of the inhibitors was discussed using QSAR approach.
Section snippets
Electrode and solution
Corrosion tests have been carried out on electrodes cut from sheets of C38 steel. Steel strips containing 0.36% C, 0.66% Mn, 0.27% Si, 0.02% S, 0.015% P, 0.21% Cr, 0.02% Mo, 0.22% Cu, 0.06% Al and the remainder iron. The specimens were embedded in epoxy resin leaving a working area of 0.78 cm2. The working surface was subsequently polished with 180 grit and 1200 grit abrading papers, cleaned by distilled water and ethanol. The solutions (1 M HCl) were prepared by dilution of an analytical reagent
Open-circuit potential versus time measurements
As mentioned in the experimental part, special care was taken in the stability of the OCP before each polarization or impedance run. Fig. 2, Fig. 3 represent the variation of the OCP of the steel electrode with time in 1 M HCl solution in absence and presence of various concentrations of harmane and norharmane at 25 °C. In absence of either harmane or norharmane (see curve blank in Fig. 2, Fig. 3), the steady-state Ecorr value, that achieved after about some minutes of immersion, corresponds to
Conclusion
The following results can be drawn from this study:
- (1)
The examined molecules were found effective corrosion inhibitors for C38 steel in 1 M HCl solution at low concentration. The harmane gives higher inhibition efficiency than norharmane and efficiencies increases with increasing its concentrations.
- (2)
The potentiodynamic polarization curves indicated that harmane and norharmane inhibit both anodic metal dissolution and also cathodic hydrogen evolution reactions.
- (3)
Data obtained from ac impedance
Acknowledgement
This work was supported by European Union through DEGRAD framework (FEDER funds, PRESAGE 30070).
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