Molecular simulation, quantum chemical calculations and electrochemical studies for inhibition of mild steel by triazoles

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1 Available online at Electrochimica Acta 53 (2008) Molecular simulation, quantum chemical calculations and electrochemical studies for inhibition of mild steel by triazoles K.F. Khaled Electrochemistry Laboratory, Chemistry Department, Faculty of Education, Ain Shams University, Roxy, Cairo, Egypt Received 19 October 2007; received in revised form 5 December 2007; accepted 6 December 2007 Available online 15 December 2007 Abstract The inhibition performance of three triazole derivatives on mild steel in 1 M HCl were tested by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The adsorption behavior of these molecules at the Fe surface was studied by the molecular dynamics simulation method and the quantum chemical calculations. Results showed that these compounds inhibit the corrosion of mild steel in 1 M HCl solution significantly. Molecular simulation studies were applied to optimize the adsorption structures of triazole derivatives. The iron/inhibitor/solvent interfaces were simulated and the charges on the inhibitor molecules as well as their structural parameters were calculated in presence of solvent effects. Aminotriazole was the best inhibitor among the three triazole derivatives (triazole, aminotriazole and benzotriazole). The adsorption of the inhibitors on the mild steel surface in the acid solution was found to obey Langmuir s adsorption isotherm Elsevier Ltd. All rights reserved. Keywords: Molecular simulation; Quantum chemical calculation; Mild steel; Corrosion inhibition; Triazoles; EIS 1. Introduction Iron and its alloys are widely used in many applications, which have resulted in research into the corrosion resistance in various aggressive environments. In efforts to mitigate electrochemical corrosion, the primary strategy is to isolate the metal from corrosive agents. Among the different methods available, the use of corrosion inhibitors is usually the most appropriate way to achieve this objective [1]. Corrosion inhibition occurs via adsorption of the organic molecule on the corroding metal surface following some known adsorption isotherms with the polar groups acting as the reactive centers in the molecules. The resulting adsorption film acts as a barrier that isolates the metal from the corroding environment and efficiency of inhibition depends on the mechanical, structural, and chemical characteristics of the adsorption layers formed under particular conditions. Although various experimental and theoretical techniques [2] have been developed to study the structural properties of inhibitor molecules, little is known about the interactions that Present address: Chemistry Department, Faculty of Science, Taif University, 888 Hayia, Saudi Arabia. Tel.: ; fax: address: khaledrice2003@yahoo.com. occurred between the adsorbed molecules and metal surfaces. A practical route to study these complex processes are computer simulations of suitable models. In recent years, the quantum chemistry computing method has become an effective way to study the correlation of the molecular structure and its inhibition properties and much achievement was reached [3 8]. The quantum chemistry computing method is often used to study the simple systems. For more complex systems, such as systems involving a relatively large number of molecules, the quantum chemistry computing method is not suitable anymore. Khaled et al. [9] concluded that the quantum mechanical approach may well be able to foretell molecule structures that are better for corrosion inhibition purposes if it is taken into account that (i) the effect depends only on the inhibitor molecule properties and (ii) everything else in its vicinity is uninvolved either with respect to competition for the surface or with respect to itself. Also, it is clear that there is no general way for predicting compound usefulness to be good corrosion inhibitor or find some universal type of correlation. A number of excluded parameters that should be involved as effect of solvent molecules, surface nature, adsorption sites of the metal atoms or oxide sites or vacancies, competitive adsorption with other chemical species in the fluid phase and solubility give at least simplified inspection. In this circumstance, a molecular simulation method is the best choice in /$ see front matter 2007 Elsevier Ltd. All rights reserved. doi: /j.electacta

2 K.F. Khaled / Electrochimica Acta 53 (2008) an attempt to take into account the effect some of these excluded parameters. Molecular simulation has been used to investigate the formation of adsorption layers on metals and provide some valuable information about the microstructures of the surfaces [10,11]. It maybe has a potential application towards the design and development of organic corrosion inhibitors in corrosion field. Molecular modeling has not been used widely for inhibition studies. Edwards et al. [12] investigated the adsorption of the oil field pipeline inhibitor oleic imidazoline using both molecular orbital and molecular mechanics methods whilst Fitzwater [13] employed molecular mechanics and molecular dynamics to simulate the adsorption of both poly(acrylic acid) and poly(aspartic acid) on various CaCO 3 surfaces. Ramachandran et al. [14] studied the oleic imidazolines and reported on the use of molecular orbital calculations and subsequent molecular dynamics simulations to investigate the adsorption of 1,2-dimethylimidazoline on an Fe(OH) 3 (H 2 O) 2 surface. These methods were also used to simulate the adsorption of imidazolines to the Fe site of Fe 2 O 3 [15], whilst in another article [16] molecular mechanics was employed to model the interactions between imidazolines and Fe 3 O 4 surface. Further work on the adsorption of imidazolines has been done by Wang et al. [17]. These workers designed three imidazoline derivatives and their MO predictions as to inhibition efficiency were in good agreement with experimental corrosion studies. Triazole derivatives have been studied extensively in the literature as corrosion inhibitors for iron and copper in acidic media [18 22]. Relation between inhibition performance and quantum chemical calculations have been reported by several authors [23 26]. In this study, molecular simulation studies were performed to simulate the adsorption of some triazole derivatives on iron surface and advance the understanding of interactions between these molecules and iron surface. In essence molecular mechanics has been coupled with molecular dynamics to simulate adsorption of triazoles with a clean (1 1 0) iron surface. In each case interaction of a single, triazole derivatives molecule only has been considered. The aim of this work is to investigate the inhibition mechanism of these molecules on mild steel in an aerated solution of 1 M HCl, using chemical (weight loss) electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization as well as molecular modeling of the corrosion system which allow determining the possible anchoring site suitable for the inhibitor to bond with steel surface. 2. Experimental 2.1. Structure of triazole derivatives The structure of the studied compounds are listed below: All compounds investigated were obtained from Aldrich Chemical Co. They were put in the 1 M HCl (Fisher Scientific) without pretreatment at concentrations of 10 4,10 3, and 10 2 M Modeling calculations The molecular simulation product called MS Modeling 4.0 was used to perform MS Modeling using the amorphous cell module which provides a comprehensive set of tools to perform atomistic simulations on complex systems containing dense amorphous polymers, liquids and other non-crystalline materials. Using these tools, one can construct systems containing bulk solvent and phases, perform structural analysis and property calculation to obtain estimates of experimentally measurable quantities and parameters, including estimates of properties that are sometimes difficult or even impossible to obtain in the laboratory. Also, the study was carried out using Dewar s linear combinations of atomic orbitals selfconsistent field molecular orbital (LCAO SCF MO) [27,28]. Semi-empirical PM3 method [29,30] with commercially available quantum chemical software HyperChem, Release 7.5 [31]. A full optimization of all geometrical variables without any symmetry constraint was performed at the restricted Hartree Fock (RHF) level [32,33]. It develops the molecular orbitals on a valence basis set and also calculates electronic properties, optimized geometries and total energy of the triazoles molecules. Molecular structures were optimized to a gradient <0.01 in the solvent phase. As an optimization procedure, the built-in Polak Ribiere algorithm was used [34] Weight loss measurements The experiments were carried out using mild steel (99.14%) specimens. The steel coupons of 3.0 cm 1.0 cm 0.20 cm with an exposed total area of 7.6 cm 2 were used for weight loss measurement studies. A mild steel rod of the same composition was mounted in Teflon with an exposed area of 0.28 cm 2 used for polarization and electrochemical impedance EIS measurements the coupons were polished, dried and weighted and then suspended in a 100-cm 3 aerated solution of 1 M HCl with and without the different concentrations of triazoles for exposure period (8 h). After the designated exposure to the test solution, the specimens were rinsed with distilled water, washed with acetone to remove a film possibly formed due to the inhibitor, dried between two tissue papers, and weighted again. Weight loss measurements were made in triplicate and the loss of weight was calculated by taking an average of these values. Prior to all measurements, the steel samples are abraded with a series of emery paper up to 0000 grit size. The specimens are washed thoroughly with bidistilled water, degreased and dried with acetone Electrochemical measurements Electrochemical experiments were carried out using a conventional electrolytic cell with three-electrode arrangement:

3 3486 K.F. Khaled / Electrochimica Acta 53 (2008) saturated calomel reference electrode (SCE), platinum mesh as a counter electrode, and the working electrode (WE) had the form of rod. The counter electrode was separated from the working electrode compartment by fritted glass. The reference electrode was connected to a Luggin capillary to minimize IR drop. Solutions were prepared from bidistilled water of resistivity 13 M cm, Prior to each experiment, the specimen was polished with a series of emery papers of different grit sizes up to 0000 grit size, polished with Al 2 O 3 (0.5 mm particle size), washed several times with bidistilled water then with acetone and dried using a stream of air. The electrode potential was allowed to stabilize 30 min before starting the measurements. The aggressive environment used was 1 M HCl solution with different concentrations of triazole derivatives. All experiments were conducted at 300 K. The exposed electrode area to the corrosive solution is 0.28 cm 2. Potentiodynamic polarization curves were obtained by changing the electrode potential automatically from ( 1100 to +400 mv vs. SCE) at open circuit potential with scan rate of 3mVs 1. EIS measurements were carried out in a frequency range of 100 khz to 50 mhz with amplitude of 5 mv peak-to-peak using ac signals at open circuit potential. Measurements were performed with a Gamry Instrument Potentiostat/Galvanostat/ZRA. This includes a Gamry Framework System based on the ESA400, Gamry applications that include DC105 for dc corrosion measurements, EIS300 for electrochemical impedance spectroscopy measurements to calculate the corrosion current and the Tafel constants along with a computer for collecting the data. Echem Analyst 4.0 Software was used for plotting, graphing and fitting data. 3. Results and discussions 3.1. Molecular modeling Here, molecular simulation studies were performed to simulate the adsorption structure of the triazole derivatives and press on the understanding of interactions between these triazoles and iron surface. Molecular structure of triazoles shows that it is likely for these molecules to adsorb on iron surface by sharing the electrons of nitrogen atoms with iron to form coordinated bonds with nitrogen and -electron interactions of the aromatic rings [35]. Both interactions can make it possible for the triazoles to form coordination bond with iron. The adsorption progress of triazoles on iron surface is investigated by performing molecular mechanics (MM) using MS Modeling Software. As the three kinds of Fe surfaces (1 1 0, 1 0 0, 1 1 1), Fe (1 1 1) and Fe (1 0 0) surfaces have relatively open structures while Fe (1 1 0) is a density packed surface and has the most stabilization, so we choose Fe (1 1 0) surface to simulate the adsorption process [36]. The periodic boundary conditions (PBC) are applied to the simulation cell. The size of simulation box is 23.0 Å 23.0 Å Å. The force field used in the current MM is COMPASS (condensed phase optimized molecular potentials for atomistic simulation studies) force field. All molecules are energy optimized, iron surface and solvent layers Fig. 1. The amorphous cell containing the iron substrate, the solvent molecule and the triazole derivatives. was constructed using the amorphous cell module, the whole system was energy optimized and the possibility of triazoles adsorption on the iron surface were simulated as in Fig. 1. Fig. 1 shows that the close contact between aminotriazole is more efficient than triazole and bensotriazole, respectively. The charges on nitrogen atoms and other quantum chemical parameters for the three triazoles are presented in Fig. 2 and Table 1, respectively. Fig. 2 shows the molecular orbital plots (Fig. 2c) as Table 1 The calculated quantum chemical properties for triazole derivatives Compound Aminotriazole Triazole Benzotriazole E HOMO (ev) E LUMO (ev) E (total energy) (kcal mol 1 ) Maximum charge on N atoms Dipole moment (D) Molecular weight

4 K.F. Khaled / Electrochimica Acta 53 (2008) Fig. 2. Structure of triazole derivatives, molecular orbital plots and the charge density distribution. well as the charge density distribution (Fig. 2d) on the three triazoles. It is worth noting that the charge density distribution on aminotriazole are more intense than triazole and benzotriazole which enhance the possibility of aminotriazole to adsorb more strongly on iron surface than triazole and benzotriazole. It is confirmed that the more negative the atomic charges (Fig. 2b) of the adsorbed centre, the more easily the atom donates its electrons to the unoccupied orbital of metal [37]. So these negative atomic charges indicated that nitrogen atoms are the active adsorption sites. The electronegativity of the nitrogen atoms and the conjugated -electrons of the triazoles make it possible to form coordination bond with iron surface as can be seen in Fig. 1. The structure and electronic parameters can be obtained by means of theoretical calculations using the computational methodologies of quantum chemistry. The geometry of the inhibitor in its ground state, as well as the nature of their molecular orbitals, HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) are involved in the properties of activity of inhibitors. Table 1 shows the calculated quantum chemical properties for triazole compounds, E HOMO (ev), E LUMO (ev), dipole moment, μ (D) total energy. In fact the adsorption power (efficiency of the inhibitor) increases with lower dipole moments, with decreasing molecular size and with increasing nitrogen charge, respectively [38,39]. E HOMO is often associated with the electron donating ability of a molecule, whereas E LUMO indicates its ability to accept electrons. As E HOMO is often associated with the electron donating ability of the molecule, high values of E HOMO are likely to indicate a tendency of the molecule to donate electrons to appropriate acceptor molecules with low energy and empty molecular orbital. From Table 1, it is clear that E HOMO in case of aminotriazole is higher than both triazole and benzotriazole which enhance the assumption that aminotriazole will adsorb more strongly on iron surface than both triazole and benzotriazole. Fig. 2 shows the molecular orbital distribution of triazole compounds. The negative sign of the E HOMO indicates that the adsorption is physisorption [39] Weight loss measurements The corrosion parameters such as inhibition efficiency, E I (%) and corrosion rate (mg cm 2 h 1 ) at different concentration of triazoles in 1 M HCl at 300 K are presented in Table 2. Ascan

5 3488 K.F. Khaled / Electrochimica Acta 53 (2008) Table 2 Corrosion rate and efficiency data obtained from weight loss measurements for mild steel in 1 M HCl solutions in absence and presence of different concentrations from triazole derivatives Concentration (M) CR (mg cm 2 h 1 ) Coverage ( ) E w (%) Benzotriazole Triazole Aminotriazole Fig. 3. Potentiodynamic polarization curves of the mild steel electrode in 1 M HCl without and with various concentrations from benzotriazole. be seen in Table 2, triazoles inhibit the corrosion of mild steel at all concentrations in 1 M HCl. From the determined weight loss values, the inhibition efficiencies, E w (%) and coverage θ, were calculated using the following equations [40]: ) E w (%) = (1 ww0 100 (1) ) θ = (1 ww0 (2) where w 0 and w are the weight loss in absence and in the presence of triazoles, respectively. Inspection of these data in Table 2, reveal that the inhibition efficiency increases with increasing the concentration of triazoles, and aminotriazole is better inhibitor than both triazole and benzotriazole. The corrosion inhibition can be attributed to the adsorption of triazoles molecules at mild steel acid solution interface. Adsorption of triazole derivatives can be explained on the basis that adsorption of these compound are mainly via the nitrogen atoms in the triazole ring, in addition to the availability of electrons (by resonance structures) in the aromatic system. In case of aminotriazole, the presence of an amino group enhances its adsorption more than triazole itself, while the presence of benzene ring in benzotriazole lower the efficiency of benzotriazole than triazole compound due to the electron withdrawing ability of benzene ring. Fig. 4. Potentiodynamic polarization curves of the mild steel electrode in 1 M HCl without and with various concentrations from triazole Polarization measurements Figs. 3 5 are showing typical polarization curves for the inhibition characteristics of triazole derivatives. These curves show anodic and cathodic polarization plots recorded on mild steel electrode in 1 M HCl at various concentrations in the presence and absence of triazole derivatives. As would be expected both anodic and cathodic reactions of mild steel electrode corrosion were inhibited with the increase of triazole derivatives concentration. This result suggests that the addition of triazole derivatives reduces anodic dissolution and also retards the Fig. 5. Potentiodynamic polarization curves of the mild steel electrode in 1 M HCl without and with various concentrations from aminotriazole.

6 K.F. Khaled / Electrochimica Acta 53 (2008) Table 3 Electrochemical kinetic parameters obtained form potentiodynamic polarization curves shown in Figs. 3 5 for the mild steel electrode in 1 M HCl in different concentrations of triazole derivatives Concentration (M) i corr ( Acm 2 ) E corr (mv) β c (mv dec 1 ) β a (mv dec 1 ) E p (%) Benzotriazole Triazole Aminotriazole hydrogen evolution reaction. Table 3 shows the electrochemical corrosion kinetic parameters, i.e., corrosion potential (E corr ), cathodic and anodic Tafel slopes (β c, β a ) and corrosion current density i corr obtained by extrapolation of the Tafel lines. The calculated inhibition efficiency, E p (%) are also reported from the following equation: E p (%) = ( 1 i corr i 0 corr ) 100 (3) where i 0 corr and i corr correspond to uninhibited and inhibited current densities, respectively. The best inhibition efficiency was about 90.2% at concentration 10 2 M. It can be seen that by increasing inhibitor concentration, the corrosion rate decreased and inhibition efficiency E p (%), increased. No definite trend was observed in the shift of E corr values, in the presence of various concentrations of triazole derivatives, suggesting that these compounds behave as mixed-type inhibitors. Moreover, these inhibitors cause no change in the anodic and cathodic Tafel slopes, indicating that the inhibitors are first adsorbed onto steel surface and therefore impedes by merely blocking the reaction sites of iron surface without affecting the anodic and cathodic reaction mechanism [40]. the impedance behavior of steel in 1 M HCl solutions are given in Figs These curves show a typical set of Nyquist plots for mild steel in 1 M HCl in the absence and presence of various concentrations of triazole derivatives. It is clear from these plots that the impedance response of mild steel has significantly changed after the addition of triazole derivatives in the corrosive media. This indicates that the impedance of an inhibited substrate increases with increasing concentration of inhibitor in 1 MHCl. It is worth noting that the change in concentration of triazole derivatives did not alter the profile of the impedance behavior, suggesting similar mechanism for the corrosion inhibition of mild steel by triazole derivatives, Figs The impedance parameters derived from Figs. 7 9 are given in Table 4. The charge transfer resistance R ct, double layer capacitance C dl and inhibition efficiency E I (%) are calculated from the following equations: E I (%) = ( ) 1 R0 ct 100 (4) R ct 3.4. Electrochemical impedance measurements Results obtained from EIS can be interpreted in terms of the equivalent circuit of the electrical double layer shown in Fig. 6, which was used previously to model the iron/acid interface [41]. The effects of triazole derivatives concentrations on Fig. 6. Suggested equivalent circuit model for the studied system. Fig. 7. Nyquist plots for mild steel in 1 M HCl in absence and presence of different concentrations of benzotriazole.

7 3490 K.F. Khaled / Electrochimica Acta 53 (2008) Table 4 Electrochemical kinetic parameters obtained by EIS technique for mild steel in 1 M HCl in absence and presence of various concentrations of triazole derivatives Concentration (M) R s ( cm) R ct ( cm) C dl ( Fcm 2 ) E I (%) Benzotriazole Triazole Aminotriazole Fig. 8. Nyquist plots for mild steel in 1 M HCl in absence and presence of different concentrations of triazole. f ( Z img ) = 1 (5) 2πC dl R ct where R 0 ct and R ct are the charge transfer resistances in 1 M HCl solution without and with different concentrations of tria- zole derivatives, respectively, Z img is the maximum imaginary component of the impedance. The Nyquist plots obtained in the real system represent a general behavior where the double layer on the interface of metal/solution does not behave as a real capacitor. On the metal side electrons control the charge distribution whereas on the solution side it is controlled by ions. As ions are much larger than the electrons, the equivalent ions to the charge on the metal will occupy quite a large volume on the solution side of the double layer [42]. From Table 4, it was clear that charge transfer resistance R ct values were increased and the capacitance values C dl decreased with increasing inhibitors concentration. Decrease in the capacitance, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the inhibitor molecules act by adsorption at the metal/solution interface [43]. The addition of triazole derivatives provides lower C dl values, probably as a consequence of replacement of water molecules by triazoles at the electrode surface. Also the inhibitor molecules may reduce the capacitance by increasing the double layer thickness according to the Helmholtz model [44]: C dl = εε 0A (6) δ where ε is the dielectric constant of the medium, ε 0 is the vacuum permittivity, A is the electrode surface area and δ is the thickness of the protective layer. The value of C dl is always smaller in the presence of the inhibitor than in its absence, as a result of the effective adsorption of the triazole derivatives. The results obtained from EIS measurements are in good agreement with that obtained from potentiodynamic polarization and weight loss measurements Adsorption isotherm Fig. 9. Nyquist plots for mild steel in 1 M HCl in absence and presence of different concentrations of aminotriazole. It has been assumed that organic inhibitor molecules establish their inhibition action via the adsorption of the inhibitor onto the metal surface. The adsorption processes of inhibitors are influenced by the chemical structures of organic compounds, the

8 K.F. Khaled / Electrochimica Acta 53 (2008) nature and surface charge of metal, the distribution of charge in molecule and the type of aggressive media. In general, two modes of adsorption can be considered. The proceeding of physical adsorption requires the presence of electrically charged metal surface and charged species in the bulk of the solution. Chemisorption process involves charge sharing or charge transfer from the inhibitor molecules to the metal surface. The presence, with a transition metal, having vacant, low-energy electron orbital, of an inhibitor molecule having relatively loosely bound electrons or heteroatoms with lone-pair electrons facilitates this adsorption [45,46]. Assuming the corrosion inhibition was caused by the adsorption of triazole derivatives, and the values of surface coverage (θ) for different concentrations of inhibitors in 1 M HCl were evaluated from weight loss measurements from Eq. (2). Adsorption isotherms are very important in determining the mechanism of organic electrochemical reactions. The most frequently used adsorption isotherms are Langmuir, Temkin and Frumkin. So several adsorption isotherms were tested for the description of adsorption behaviour of studied compounds and it is found that adsorption of triazoles on mild steel surface in HCl solution obeys the Langmuir adsorption isotherm given by the following equations [47]: C inh θ = 1 b + C inh (7) b = exp ( G ) ads RT C inh is the inhibitor concentration, θ is the fraction of the surface covered, b is the adsorption coefficient and G ads is the standard free energy of adsorption. Fig. 10 shows the dependence of the fraction of the surface covered C/θ as a function of the concentration (C) of triazole derivatives. It should be explained that other adsorption isotherms (Frumkin and Temkin) were checked and Langmuir adsorption isotherm is the best approximate between them. This is why the assumption is true for Langmuir adsorption isotherm. (8) The obtained plots of the inhibitors is linear with correlation coefficient higher than The intercept permits the calculation of the equilibrium constant b which are , and M 1 for aminotriazole, triazole and benzotriazole, respectively. The values of b which indicate the binding power of the inhibitor to the steel surface can lead to calculate the adsorption energy. Values of G ads = , , kj mol 1. The negative value of G ads means that the adsorption of triazoles on mild steel surface is a spontaneous process, and furthermore the negative values of G ads also show the strong interaction of the inhibitor molecule onto the mild steel surface [48,49]. Generally, values of G ads around 20 kj mol 1 or lower are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption). Those more negative than 40 kj mol 1 involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [50,51]. For investigated aminotriazole inhibitor, one can see that the calculated G ads values, equals kj mol 1, indicating, that the adsorption mechanism of the aminotriazole on mild steel in 1 M HCl solution was typical of physisorption. 4. Conclusions The following results can be drawn from this study: This study is unique in that the simulation included the presence of solvent molecules; traditionally the solvent has been ignored to save computational time. The work is a good example of how computational chemistry can not only be used as a screening tool to test several different molecules, but more importantly to develop an understanding on the behavior of different systems as a function of their molecular characteristics. This reduces the number of experiments required and allows one to do more intelligent experiments. Molecular modeling techniques incorporating molecular mechanics and molecular dynamics can be used to simulate the adsorption from 1 M HCl solution of a single target molecule from triazole derivatives on iron (1 1 0) surface. The quantum mechanical approach may well be able to foretell molecule structures that are better for corrosion inhibition. Double layer capacitances decreases with respect to the blank solution when these inhibitors are added. This fact may be explained on the basis of adsorption of these inhibitors on the steel surface. In determining the corrosion rates, electrochemical studies and weight loss measurements give similar results. Triazole derivatives can be used as corrosion inhibitors for mild steel in 1 M HCl. References Fig. 10. Langmuir s adsorption plots for mild steel in 1 M HCl containing various concentrations from triazole derivatives. [1] E.S. Ferreira, C. Giacomelli, F.C. Gicomelli, A. Spinelli, Mater. Chem. Phys. 83 (2004) 129. [2] A. Halperin, M. Tirell, T.P. Lodge, Adv. Polym. Sci. 100 (1992) 31.

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