The study of corrosion inhibition of pyridine and its derivatives on steel using electrochemical methods

JOURNAL OF SCIENCE OF HNUE Chemical and Biological Sci., 2012, Vol. 57, No. 8, pp. 28-35 This paper is available online at THE STUDY OF CORROSION INHIBITION OF PYRIDINE AND ITS DERIVATIVES ON STEEL USING ELECTROCHEMICAL METHODS Hoang Van Hung, Dang Thi Tuoi and Nguyen Ngoc Ha Faculty of Chemistry, Hanoi National University of Education Abstract. Two advanced methods in electrochemistry have been employed to study corrosion inhibition of pyridine (PY) and its methyl derivatives: 2-methyl pyridine (2MPY), 3-methyl pyridine (3MPY) and 4-methyl pyridine (4MPY) on steel surface in acidic media of a 1 M HCl solution. The obtained results show that all methyl derivatives of pyridine are more efficient than pyridine in inhibiting corrosion. The obtained results are also in good agreement with our theoretical results which were previously published. The relationship between the electronic structure of pyridine and its derivatives and corrosion inhibition efficiency has been explained referring to theoretical and experimental results. Keywords: Pyridine, impedance spectroscopy, polarization measurements, corrosion inhibition. 1. Introduction Acid solutions are generally used in several processes: industrial cleaning, acid descaling and in petrochemical processes. Hydrochloric acid is widely used in cleaning and pickling processing of metals where the use of inhibitors is one of the most practical methods used to protect metals from corrosion [1]. Organic substances containing heteroatoms such as nitrogen, sulfur and oxygen have frequently been used as corrosion inhibitors of metals in acidic media for the last few decades [1-3]. These substances are adsorbed onto the metallic surface and prevent the degradation of the metal in a corrosive media [4]. Among heteroatom-containing compounds, nitrogen-containing compounds function more effectively in hydrochloric solutions, whereas compounds containing sulfur are preferred for sulfuric acid solutions. Many experimental studies have been carried out which have revealed the corrosion Received May 28, 2012. Accepted October 1, 2012. Contact Hoang Van Hung, e-mail address: hunghv@hnue.edu.vn 28 The study of corrosion inhibition of pyridine and its derivatives on steel... efficiency of chemical compounds containing heteroatoms in acidic media. It was revealed that pyridine compounds are effective corrosion inhibitors for steel in acidic solutions. Replacing hydrogen atoms in pyridine rings with several substituted groups has shown improved corrosion inhibition efficiency [5, 6]. In order to examine the systematic relationship between the electronic properties of heteroatoms and atoms in the molecules of inhibitors, the present work made use of pyridine and its derivatives to study corrosion-inhibition efficiency on steel surfaces in a 1 M HCl medium using a combination of advanced theoretical and electrochemical methods. Theoretical calculations were carried out using the density functional theory (DFT) method and experiments were done using two advanced electrochemical methods: polarization measurements and electrochemical impedance spectroscopy. Our theoretical calculations have been published in a previous article [7]. In this article, we present the results of an experiment investigating corrosion inhibition effectiveness of pyridine and its methyl derivatives in a 1 M HCl medium on steel. 2. Content 2.1. Materials and methods *Materials Pyridine, 1-methyl pyridine, 2-methyl pyridine, 3-methyl pyridine and 4-methyl pyridine were purchased from the MERK chemical company. An aggressive 1 M HCl solution was prepared using analytical grade 37% HCl and bi-distilled water. All acidic solutions were used directly with no deaerating procedure. For electrochemical measurements, steel samples (C = 0.08%, Mn = 2.00%, P = 0.045%, S = 0.030%, Si = 0.75%, Cr = 18.00%, Ni = 8.00%, N = 0.10% and remainder iron) were made in the form of disc electrodes, sealed by epoxy resin, with an exposed surface diameter of 2.0 mm. *Methods Cathodic and anodic polarization curves were potentiodynamically recorded with the scan rate of 5mVs−1 using a conventional three-electrode cell connected to an Autolab potentiostat/galvanostat instrument supplied by Metrohm AUTOLAB B. V, Netherland. The steel disc electrode with a diameter of 2.0 mm was used as a working electrode (WE), a platinum wire was the counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. All experiments were performed at (25◦C). The working electrode (WE) was polished mechanically with different emery papers up to 1000 grade. After polishing, the electrode was washed several times with bi-distilled water, degreased with ethanol and dried. The electrochemical impedance spectroscopy (EIS) measurements were carried out using an electrochemical system and instruments similar to that used in polarization 29 Hoang Van Hung, Dang Thi Tuoi and Nguyen Ngoc Ha measurements with modulation amplitude of 10 mV in the frequency range between 1 Hz to 100 kHz. Evaluation of the impedance data was performed assuming equivalent circuit with NOVA software version 1.5. 2.2. Results and discussion 2.2.1. Concentration dependence of the inhibition efficiency of pyridine Polarization curves in the Tafel form of steel in a 1.0 M HCl solution with and without the addition of PY at different concentrations are shown in Figure 2. The electrochemical parameters associated with polarization measurements, for instance polarization resistances Rp and corrosion current densities icorr are listed in Table 1. The values of corrosion current density and polarization resistance Rp were obtained by extrapolating Tafel plots and using the Stern-Geary equation, respectively, [8, 9] with the help of NOVA 1.5 software. Rp = ba · bc 2.303(ba + bc) · 1 icorr · A (Stern-Geary equation) The corrosion inhibition efficiencies were calculated making use of the following equation [10]: η(%) = icorr − icorr(inhibitor) icorr where ba and bc are Tafel slopes and A is the surface area of the steel electrode. icorr and icorr(inhibitor) are the corrosion current densities for uninhibited and inhibited solutions respectively. Figure 1. Structures of pyridine (a), 2M-Py (b), 3M-Py (c) and 4M-Py (d) From electrochemical polarization measurements, it is clear that the addition of the inhibitor caused a decrease in current density. The values of corrosion current density of steel in inhibited solutions were smaller than those for the inhibitor free solution. The Tafel plots remain almost unchanged indicating that the presence of these inhibitors 30 The study of corrosion inhibition of pyridine and its derivatives on steel... has no effect on the mechanism of the dissolution process of steel and the adsorbed molecules mechanically screen the coated part of electrode and therefore protect the steel surface from corrosive media [11]. In general, the presence of inhibitors will prevent the degradation of a metal surface that is caused by the attack of oxidants present in the electrolyte. These inhibitors are adsorbed onto the metallic surfaces to form a barrier isolating the metal surface from attacking species [12]. The inhibition efficiency of inhibitors depends on their concentration in solution (see Table 1). In the case of PY, we found that inhibition efficiency reaches a maximum value at the concentration of 10−5 M (the highest value of polarization resistance, 181 ω). In order to compare the inhibition efficiencies of inhibitors conveniently, the concentration of 10−5 M was employed for all inhibitors in this study. Figure 2. Tafel plot for the steel electrode in 1.0 M HCl with and without pyridine at different concentrations Table 1. Electrochemical parameters of steel in various concentrations of PY in 1.0 M HCl CM /mol L −1 icorr/µA cm −2 RP 5.10−3 M 26.1 175 10−3 M 29.3 156 5.10−4 M 31.1 148 10−4 M 28.9 158 5.10−5 M 27.3 165 10−5 M 25.1 181 5.10−6 M 29.8 153 blank 33.4 136 31 Hoang Van Hung, Dang Thi Tuoi and Nguyen Ngoc Ha The results obtained from electrochemical impedance measurements are similar to those obtained using polarization measurements. Figure 3 shows Nyquist plots of steel electrode in 1.0 M HCl with and without the addition of pyridine. The impedance data were analyzed using an equivalent circuit composed of a double layer capacitance, CDL, charge transfer resistance, RCT , and solution resistance, RS , as in Figure 4. In general, the number of elements in an employed equivalent circuit depends on each particular system. The number of time constants and other elements needed to fully describe the impedance data were based on the condition of a fit with minimal deviation between measured and calculated results. The corrosion rate is inversely proportional to the value of RCT and a high RCT value corresponds to a low corrosion rate [13]. Figure 3. Nyquist plots of steel electrode in 1.0 M HCl with and without the addition of inhibitors Figure 4. Equivalence circuit for steel electrode in 1.0 M HCl with and without the addition of inhibitors 2.2.2. Inhibition efficiency of pyridine and its methyl derivatives Figure 5 shows a Tafel plot for steel electrode in 1.0 M HCl with the addition of different inhibitors at a concentration of 10−5 M. It is clear that when a hydrogen atom 32 The study of corrosion inhibition of pyridine and its derivatives on steel... at any position in a pyridine ring is replaced by a methyl group, inhibition efficiency will increase (see Table 2). However, the increase in inhibition efficiency depends on the position of the substituted group. The inhibition efficiency of 2Me-PY (60.5%) and 4Me-PY (63.2%) is higher than that of 3Me-PY (55.7%) and PY (24.8%). There are some parameters which can affect the inhibition efficiency of the inhibitors. In the case of PY and its methyl-derivatives, efficiency depends much on their electronic and geometric structures. When a methyl group is at position number 2 or 4, the donating effect (or inductive effect) of the methyl group and the conjugation effect creates an increase in electron density at the N atom which is much more greater in comparison to the electron density of the N atom in pyridine and 3Me-PY. This increases the bond strength between the steel surface and the inhibitor molecules which leads to an increase in inhibition efficiency of the inhibitors. In 3Me-PY the donating effect of the methyl group does not have as much affect as it does in 2 and 4Me-PY and therefore inhibition efficiency is less in 3Me-PY than it is in 2 and 4Me-PY. Figure 5. Tafel plots for the steel electrode in 1.0 M HCl with different inhibitors at a concentration of 10−5 M Table 2. Electrochemical parameters of steel at concentration of 10−5 M of PY and its derivatives in 1.0 M HCl Parameter HCl HCl + PY HCl + 2Me-PY HCl + 3Me-PY HCl + 4Me-PY icorr/µA cm−2 33.4 25.1 13.2 14.8 12.3 RP /ω 136 181 348 299 368 RCT //ω 127 156 336 253 299 η (%) – 24.8 60.5 55.7 63.2 33 Hoang Van Hung, Dang Thi Tuoi and Nguyen Ngoc Ha Figure 6. Nyquist plots for the steel electrode in 1.0 M HCl with different inhibitors at concentration of 10−5 M The results obtained from experimental work are in good agreement with the theoretical results published elsewhere [7] and with experimental study on an aluminum surface of PY and its derivatives. The polarization resistances calculated from Tafel plots (RP ) are almost in agreement with the values of charge transfer resistance (RCT ) calculated from impedance data. The relatively small difference between these two values at the same electrode is expected because at frequency zero the sum of all ohmic components of the impedance is equal to polarization resistance. The relatively small contribution of the solution resistance, RS , and the double layer results in the present case in fairly good agreement between RP and RCT [14]. 3. Conclusion Corrosion inhibition efficiencies of pyridine and its methyl derivatives were studied experimentally using electrochemical methods. 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