Corrosion inhibition properties of pyrazolylindolenine compounds on copper surface in acidic media
© Ebadi et al.; licensee Chemistry Central Ltd. 2012
Received: 26 August 2012
Accepted: 18 December 2012
Published: 31 December 2012
The corrosion inhibition performance of pyrazolylindolenine compounds, namely 4-(3,3-dimethyl-3H-indol-2-yl)-pyrazole-1-carbothioamide (InPzTAm), 4-(3,3-dimethyl-3H-indol-2-yl)-1H-pyrazole-1-carbothiohydrazide (InPzTH) and 3,3-dimethyl-2-(1-phenyl-1H-pyrazol-4-yl)-3H-indole (InPzPh),) on copper in 1M HCl solution is investigated by electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and linear scan voltammetry (LSV) techniques.
The results show that the corrosion rate of copper is diminished by the compounds with the inhibition strength in the order of: InPzTAm> InPzTH > InPzPh. The corrosion inhibition efficiencies for the three inhibitors are 94.0, 91.4 and 79.3, for InPzTAm, InPzTH and InPzPh respectively with the same inhibitor concentration (2 mM).
From the EIS, OCP and LSV results it was concluded that pyrazolylindolenine compounds with S-atom (with an amine group) have illustrated better corrosion inhibition performance compared to hydrazine and phenyl group.
KeywordsCorrosion behaviour Copper Electro-corrosion FESEM Acid inhibition
Corrosion reactions are thermodynamically favourable on less noble metals and alloys when exposed to a corrosive environment such as chloridric acid. The inhibition of these reactions can be controlled by many types of organic and inorganic compounds [1–4], but organic compounds are the more common type of corrosion inhibitors. Most organic compounds which are efficient corrosion inhibitors contain functional groups which incorporate phosphorus, oxygen, nitrogen, sulfur atoms and multiple bonds [5, 6]. The action of these inhibitors are closely related to factors such as: the types of functional groups, the number and type of adsorption sites, the charge distribution in the molecules and the type of interaction between the inhibitors and the metal surface . A large number of organic compounds have been investigated as corrosion inhibitors for different types of metals [8–10]. With increased awareness towards environmental pollution and control, the search for less toxic and environment friendly corrosion inhibitors are becoming increasingly important.
Preparation of the pyrazolylindolenine compounds
The three pyrazolylindolenine compounds were synthesised through the reaction of 2-(diformylmethylidene)-3,3-dimethylindole with thiosemicarbazide (for InPzTAm), with thiocarbohydrazide (for InPzTH) and with phenylhydrazide (for InPzPh). The details of the synthetic and characterization methods are described elsewhere .
Corrosion characterization techniques
All experiments were done using Cu plates of 99.9% purity (Goodfellow Cambridge) in 1 M HCl aqueous media containing various amount of pyrazolylindolenine compounds: InPzTAm (0–2.5 mM l-1), InPzTH (0–2.5 mM l-1) and InPzPh (0–4 mM l-1). The Cu plates (0.3× 1× 1 cm) were polished with emery paper (2000 grit), rinsed in distilled water and ultrasound in acetone to remove oily stains. A three-compartment cell was used in the corrosion measurements with a saturated calomel electrode (SCE) as the reference electrode and Cu plate and platinum wire with the same surface area as the working and counter electrodes respectively. Frequency response analysis (FRA) software was used in the EIS experimental and simulation process, while general purpose electrochemical software (GPES) was used in the linear scan voltammetry (LSV, Tafel) and open circuit potential (OCP) techniques. The software was installed in a computer interfaced with an Autolab (302 N) potentiostat/galvanostat instrument. The scan rate for LSV was 10 mV s-1, while the potential rage was between 0.3 V to −0.4 V). The EIS measurements were carried out at the OCP value with a frequency domain of 100 kHz-10 mHz with an amplitude of 5 mV. Prior to all analysis (EIS and LSV) the Cu plates were immersed in the corrosive solution (1 M HCl) containing different amount of inhibitors for 1 hr to obtain the OCP value. A JEOL JSM-840A field emission scanning electron microscopy (FESEM) instrument was used to capture the images of the copper surface after immersion in the corrosive solution for 1 week.
Results and discussions
Electrochemical impedance spectroscopy
where ωmax is the angular frequency when Z imaginary (Zi) is maximum and n is the phase shift and is the degree of surface in-homogeneity. The value of “n” is also related to the slope of the log ℜ Z ℜ vs. log f in the Bode plot, when n=1, then Q = Cdl.
EIS simulation results of copper surface in 1 M HCl with the presence of different inhibitor concentration
Inhi. Conc. (mM l-1)
Potentiodynamic measurements and thermodynamic calculations
Electrochemical impedance parameters for copper plates in 1 M HCl solution in the absence and presence of different concentrations of the pyrazolylindolenine compounds
Conc. Inhib. (mM)
In this work the displacement range of Ecorr. for InPzTAm (Figure 3a) is 15.9-62.9 mV towards the anodic region, whereas InPzTH shows a 32.8-7.3 mV displacement to the anodic region (Figure 3c). Figure 3e shows that the Tafel curve has a small shift to the cathodic zone (< 85 mV) with the increase of InPzPh (54.4-5.2 mV). Therefore it can be concluded that the pyrazolylindolenine inhibitors (InPzTAm, InPzTH and InPzPh) show mixed type behaviour. From Figure 3a, the InPzTAm concentration of 2.5 mM shows maximum inhibition efficiency. The corrosion rate of the copper plate decreases from 4.69 mm y-1 to 1.9×10-1 mm y-1 with the addition of 0 to 2.5 mM l-1 for InPzTAm; from 4.69 mm y-1 to 2.1×10-1 mm y-1 with the addition of 0 to 2.5 mM l-1 for InPzTH; and from 4.69 mm y-1 to 7.2×10-1 mm y-1 with the addition of 0 to 4.0 mM l-1 for InPzPh. These results are consistent with the EIS results. The inhibition efficiency (η %) is calculated using the data from the corrosion potential (Ecorr.), cathodic and anodic Tafel slopes (βc and βa, respectively) and corrosion current density (Icorr.), and are tabulated in Table 2. From Table 2, larger differences in the cathodic slopes are shown by InPzTAm and InPzTH. This can be attributed to the thickening of the electrical double layer due to the adsorbed inhibitor molecules. According to Bockris and Srinivason , this behaviour is associated with the changes in the mechanism of the cathodic reaction, e.g. hydrogen evolution reaction (HER). On the other hand, there are no significant changes for the anodic and cathodic Tafel slopes (βa, βc, respectively) for InPzPh. It can be suggested that there are also no significant changes in the inhibition mechanism of cathodic and anodic reactions with the increase of the InPzPh concentration.
The change of the corrosion rate with temperature (298 to 318 K) is also examined on the copper plates. Figure 3 (b and d) shows that the activity of the inhibitors decreases with the increase of temperature from 298 to 318 K. This is due to the decrease of adsorption strength of the inhibitor on the metal surface with the rise in temperature. The adsorption of the inhibitor is dependent on many factors such as temperature, adsorption of solvent, type of ions, thickness of the electrical double layer and charge density. The surface coverage (θ) is an important factor which can be used to describe the type of adsorption mechanism of the inhibitors. The common isotherms which are used to calculate the surface coverage (θ) are :
Frumkin isotherm: (θ/1-θ)exp(−2Fθ) = Kads. C
Freundlich isotherm: θ = Kads. C
Temkin isotherm: exp(f.θ) = Kads. C
Computed molecular parameters for the pyrazolylindolenine compounds
∆Sads. (kJ mol-1)
∆Hads. (kJ mol-1)
λ (mg cm-2 h-1)
Ea (kJ mol-1)
∆E= ELUMO - EHOMO
Gaussian software 9.0 is used to study the quantum chemical behaviour of the inhibitor. The quantum chemical properties of the inhibitors are studied by calculating the energy of the highest occupied molecular orbital (EHOMO) and the energy of lowest unoccupied molecular orbital (ELUMO). Prior to the calculation using the Gaussian 9.0, optimizations of the inhibitor molecules are done by the same software.
The hydrazine group in InPzTH with two N-atoms and the amine group in InPzTAm with one N-atom have slight difference in energies. Figure 5a and c show higher electron density in the vicinity of the N-atom in InPzTAm compared to N-atoms in the hydrazine group in InPzTH. Thus InPzTAm can be absorbed on copper activated sites via the donation of the unshared electron pair from the N and S-atoms to the copper metal. It was found that more electron density transfer from inhibitors to metal surface results in increased adsorption of the inhibitor molecules on the surface, thus better corrosion protection properties. Table 3 shows that the ΔE for compounds is in the order of InPzTAm > InPzTH > InPzPh. It can be shown that the electron transfer process in InPzTAm is faster compared to the other compounds.
From Figure 7, the lowest shift towards positive potential is for InPzPh due to the low surface coverage while the largest shift of towards positive potential is for InPzTAm. This phenomenon can be ascribed to the decrease of the charge transfer and subsequently mass transport of the Cl- decreases with the formation of the inhibitor adsorption on the copper surface. From the measurements of surface coverage θ and Kads. the strength of the inhibitor adsorption on the copper surface is in the order of InPzTAm > InPzTH >InPzPh.
The EIS, OCP and LSV measurements show that all three pyrazolylindolenine compounds (InPzTAm, InPzTH and InPzPh) give good inhibition performance against copper corrosion in 1 M HCl solution. OCP measurements show that the OCP moves towards more noble potentials with the increase in the inhibitor concentration. Tafel polarization experiments show that the Rp also increases with the increase of inhibitors in acidic media. EIS simulation of the experimental data shows that InPzTAm and InPzTH have the same equivalent circuit but the Warburg element is present in the equivalent circuit for the InPzPh compound. The corrosion inhibition performance of these compounds for the same concentration (2 mM) is η = 94.0%, η = 91.4% and η = 79.3% for InPzTAm, InPzTH and InPzPh respectively. From the EIS, OCP and LSV results, the higher corrosion efficiency for both InPzTAm and InPzTH compared to InPzPh is due to the presence of sulfur atoms in those compounds. From quantum chemical calculations, the presence of the sulfur atoms promote better electron donor ability for both compounds which give higher inhibition efficiency.
Electrochemical impedance spectroscopy
LSV: Open circuit potential, linear scan voltammetry
Field emission scanning electron microscopy
Saturated calomel electrode
Frequency response analysis
General purpose electrochemical software
Outer Helmholtz plane
Constant phase element
Hydrogen evolution reaction
- E HOMO :
Occupied molecular orbital
- E LUMO :
Unoccupied molecular orbital.
The authors would like to thank the University of Malaya for financial support provided by research grants FP039 2010B and RG181-12SUS.
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