Volume 18 Preprint 83


Dipron: An Eco-Friendly Corrosion Inhibitor for Iron in HCl Media in Both Micro and Nano Scale Particle Size - Comparative Study

Enas Mohamed Attia

Keywords: Corrosion, Inhibition, Adsorption, Potentiodynamic, Potentiostatic, micro- scale, nano- scale

Abstract:
This study was performed in order to investigate the ability of using sulfa drug (Dipron) with heterogeneous particle size in both micro and nano scale forms as eco-friendly corrosion inhibitor for iron (Fe) in hydrochloric acid solutions, in addition to decide which form would be more effective to inhibit Fe corrosion. The particle size analysis for micro- and nano- scale Dipron was determined using BT-2001(Liquid) laser particle size analyzer and dynamic light scattering (DLS) respectively. Open circuit potential measurements, potentiodynamic and potentiostatic polarization techniques were performed at temperature range 20–60°C. The outcomes show that inhibition takes place by adsorption of the Dipron on Fe surface without altering the mechanism of corrosion process. The adsorption of Dipron on Fe surface is consistent with Langmuir’s adsorption isotherm. Physical adsorption mechanism is proposed from the calculated activation energy and thermodynamic parameters for the two forms of Dipron. The negative values of free energy of adsorption (〖∆G〗_ads^°) indicate that adsorption of Dipron follows a spontaneous process. The obtained results indicate that nano- scale Dipron (NSD) has higher efficiency in corrosion inhibition than micro- scale Dipron (MSD). The inhibition efficiency increases with a corresponding increase in the inhibitor’s concentration and decreases with rise of temperature.

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Dipron: An Eco-Friendly Corrosion Inhibitor for Iron in HCl Media in Both Micro and Nano Scale Particle Size - Comparative Study Enas Mohamed Attia Chemistry Department, Faculty of Science (Girls), AL-Azhar University, Nasr City, Cairo, Egypt enasmattia@yahoo.com Abstract This study was performed in order to investigate the ability of using sulfa drug (Dipron) with heterogeneous particle size in both micro and nano scale forms as eco-friendly corrosion inhibitor for iron (Fe) in hydrochloric acid solutions, in addition to decide which form would be more effective to inhibit Fe corrosion. The particle size analysis for micro- and nano- scale Dipron was determined using BT-2001(Liquid) laser particle size analyzer and dynamic light scattering (DLS) respectively. Open circuit potential measurements, potentiodynamic and potentiostatic polarization techniques were performed at temperature range 20–60°C. The outcomes show that inhibition takes place by adsorption of the Dipron on Fe surface without altering the mechanism of corrosion process. The adsorption of Dipron on Fe surface is consistent with Langmuir’s adsorption isotherm. Physical adsorption mechanism is proposed from the calculated activation energy and thermodynamic parameters for the two forms of ° ) indicate that adsorption of Dipron. The negative values of free energy of adsorption (∆ ��� Dipron follows a spontaneous process. The obtained results indicate that nano- scale Dipron (NSD) has higher efficiency in corrosion inhibition than micro- scale Dipron (MSD). The inhibition efficiency increases with a corresponding increase in the inhibitor’s concentration and decreases with rise of temperature. Keywords: Corrosion, Inhibition, Adsorption, Potentiodynamic, Potentiostatic, micro- scale, nano- scale. Introduction Fe has a significant impact on our day – to – day life, directly or indirectly. The main reason behind large scale usage of this metal is that it is not very expensive as compared to other metals, and is also abundantly available on the surface of the earth. This metal which is the most commonly used corrodes in many media especially acidic one. The application of acid corrosion inhibitors in the industry is widely used to 1 prevent or minimize material loss during contact with acid [1]. Most of organic compounds which are largely used as corrosion inhibitors are toxic in nature. This revealed the need of environmentally friendly inhibitors. Recently, there is a growing interest on the development of drugs as inhibitors for metallic corrosion due to their non-toxic nature [2]. The choice of drugs used as corrosion inhibitors is based on the following facts: (a) the molecules have oxygen, nitrogen and sulphur as active centers, (b) they are healthy i.e. not hazardous and environmentally friendly and (c) they can be easily produced and purified [3]. Sulphonamides are drugs extensively used for the treatment of infections caused by gram-positive microorganisms, some fungi, and certain protozoa. Other therapeutic uses of sulfonamides are as diuretic and hypoglycemic agents. Dipron (one of the simplest sulphonamides) has numbers of functional adsorption centers (e.g. –NH2 group, –SO2–NH– group and aromatic ring). It is strongly basic hence it can be readily soluble in the acid medium [4]. In view of the above, the aims of the present work are: (1) to make a comparative study on the inhibition of Fe corrosion in HCl solution by Dipron in both micro and nano scale forms using electrochemical methods. (2) to study the effect of temperature and concentration on the inhibitive properties of the drug in its two particle size scales. (3) to study the adsorption characteristics of the drug in its two scales by fitting the adsorption data into different adsorption models. Experimental Specimen’s preparation The specimen of Fe (99.99% purity, Alfa Ventron) with 1.643 cm2 exposed area was made of massive cylindrical rod mounted into glass tube by epoxy resin. A copper wire was employed for an electrical contact. Before each experiment, the Fe surface was polished mechanically using emery papers of Grade Nos. 220, 400, 600, 800 and 1200, until its surface became smooth and mirror-like bright, then it was cleaned by washing thoroughly with distilled water and finally degreased with acetone. Solutions and inhibitor The aggressive solution was prepared by dilution of AR grade 37% HCl in bidistilled water. The acid concentrations (0.05, 0.10, 0.25, 0.50, 0.75, 1.00 and 2.00 M) were prepared at 25°C. 2 The sulfa drug (Dipron), 4-(Aminosulfonyl)aniline, is of AR grade purchased from Nile Company for Pharmaceuticals and Chemical Industries and used as inhibitor without further purification. Its molecular weight is 172.2 g/mol with melting point and density 165 °C and 1.08 g/cm³, respectively. The chemical structure of Dipron is represented in Figure 1. The particle size of the purchased Dipron was determined using a high performance BT-2001(Liquid) laser particle size analyzer. Figure 2 represents the relation between diffraction and accumulation of Dipron molecules with its size. O S H2N O NH2 Figure 1: Chemical structure of Dipron: (4-(Aminosulfonyl)aniline) 5 120 (a) (b) 100 4 Cumu% Diff % 80 3 2 60 40 1 20 0 0 -1 0 1 2 3 -1 log Size, (μm) 0 1 log Size, (μm) 2 3 Figure 2: Relation between particle size with (a) diffraction and (b) accumulation of molecules. The particle size analysis illustrate that the different percentages of particles with its different micro size are as follows: D3: 0.607 μm, D6: 0.754 μm, D10: 0.931 μm, D16: 1.182 μm, D25: 1.550 μm, D50: 2.811 μm, D75: 5.607 μm, D84: 8.160 μm, D90: 11.40 μm, D97: 19.55 μm and D98: 21.87 μm with medium index =1.00. On the other hand the percent of each present specified size in the system medium is illustrated in Table 1. Figure 2 and Table 1 illustrate the heterogeneity of the system 3 which all falls in the micro scale level. Thus purchased micro scale Dipron can be referred as MSD. Table 1: Relation between particle size and its percentage in the medium Size(μm) 0.323 0.548 0.929 1.576 2.674 4.537 7.698 13.060 22.160 37.670 Percent 0.00 1.94 9.94 25.62 47.88 68.38 82.74 92.06 98.10 100.00 The nano scale Dipron (NSD) was prepared by grinding MSD using ball mill at 200 rpm for 10 hours. The particle size of NSD was determined using dynamic light scattering (DLS). As can be seen from Figure (3), in number distribution technique, there is only one peak of 100% particle sizes equal to 159.4 nm. The polydispersity index value (PDI = 0.364) reflects the slightly mono-disperse in size and indicates that the sample has a relatively narrow size distribution with width equal to 89.68 nm. Different concentrations of Dipron (0.19, 0.95, 1.90, 3.80, 9.50, 10.00 and 13.31 mM) were prepared at 25 °C by dissolving a calculated amount of the inhibitor in 0.5 M HCl. 16 Number (%) 12 8 4 0 0,1 1 10 100 Size (d. nm) Figure 3: Size distributions of NSD by number. 4 1000 10000 Electrochemical techniques Open circuit potential measurements Open circuit potential (OCP) measurements were carried out in a double- walled glass cell filled with 25 ml of the test solution. The potentials were measured with the aid of digital multimeter (KEITHLEY, Model 175, USA) using saturated calomel electrode (SCE) as a reference electrode. Potentiodynamic polarization In potentiodynamic polarization technique, measurements were performed by changing the electrode potential from negative to positive direction at scan rate 3mV/s. The corrosion rate (CR ) in mpy was calculated using Eq.1 [5]: where . CR = . × Ico × ⁄� (1) is the metric and time conversion factor, Ico , is the corrosion current density in μA/cm2, is the equivalent weight of metal in geq/mol and �, is the density in g/cm3. Values of Ico and corrosion potential Eco were determined from the extrapolation of linear Tafel segments of anodic and cathodic curves. The inhibition efficiency (IE% was evaluated from the measured Ico ° where Ico IE% = [ − and Ico Icorr I°corr ]× values using Eq.2: (2) are the corrosion current densities in absence and presence of inhibitor, respectively. Potentiostatic polarization In potentiostatic polarization, the experiments were undertaken by applying a constant voltage values (-150, -100, +10, +50, +100 and +200 mV(SCE)) to the working Fe electrode in 0.5 M HCl in absence and presence of Dipron in each of MSD and NSD forms and the corresponding current densities were recorded as a function of time. The quantity of electricity ( ) passed through the metal surface was calculated from the relation Q = It, where I is the current density recoded corresponding to the applied voltage at time �. The surface charging capacity (�) is calculated from Eq. 3: 5 ⁄� (3) = �� ° � − (4) �= where � is the constant voltage applied across the metal surface. The reciprocal capacitance (� − ) was in proportion to the thickness of the oxide film ( ) according to Eq. 4 [6]: is the electrode surface area, � is the dielectric constant of oxide and � ° is the ⁄ permittivity of free space 8.8 × − . Potentiodynamic and potentiostatic where polarization measurements were generated using a Wenking Electronic Potentioscan (Mode l73). The indicated results are the mean of four experiments. Results and Discussion Open circuit potential measurements Effect of different concentrations of Dipron on OCP of Fe electrode Figure 4 illustrates the OCP of Fe immersed in 0.5 M HCl in absence and presence of different concentrations (0.19 -13.31 mM) of Dipron. The electrode potential shifts to more passive values in presence of Dipron in MSD and NSD forms compared with that of free HCl solution, indicating the inhibiting action of Dipron in its two forms. Considering the lowest (0.19 mM) and the highest (13.31mM) Dipron concentrations, the steady state potential ( �� ) is ranging from -353 to -337 mV(SCE) in presence of MSD and from -353 to -308 mV(SCE) in presence of NSD after 120 minute of immersion ( �� of Fe in free 0.5 M HCl = -360 mV(SCE)). This means that at the studied concentrations, NSD shifts the electrode potentials to positive values more than MSD, indicating better effect on corrosion inhibition process in 0.5 M HCl. These results indicate that in OCP measurements, Dipron in both MSD and NSD forms stimulate the corrosion inhibition process in 0.5 M HCl and the effect of NSD is more pronounced than MSD. On the other hand, 10 mM Dipron concentration represents an odd behavior where its �� values are - 290 and -302 mV(SCE) for MSD and NSD respectively. These values exceed that corresponding to the highest Dipron concentration (13.31 mM). Thus, 10 mM represents an optimum concentration for corrosion inhibition process in OCP measurements. 6 (a) (b) -280 -300 -300 E, mV(SCE) E,mV(SCE) -280 -320 -340 -320 -340 -360 -360 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Time, min Time, min Figure 4: Effect of Dipron concentrations on OCP of Fe in 0.5 M HCl: (a) MSD, (b) NSD. Effect of different concentrations of HCl in absence and presence of 10 mM Dipron on Fe electrode The potentials of Fe electrode immersed in different concentrations (0.05-2.00 M) of HCl in absence and presence of 10 mM Dipron were measured as a function of immersion time as shown in Figure 5. The potentials of Fe electrode in free HCl (Fig. 5(a)) tends towards more negative potential represent the breakdown of the preimmersion air formed oxide film present on the surface according to Eq.5 [7]: + � → � + (5) The general shift, to noble direction, points to oxidation of to according to Figure 5(b and c) indicates that addition of 10 mM Dipron in MSD and NSD Eq.6 [8]. + → (6) forms to the medium produces a slight positive shift in the open circuit potential due to the retardation of the anodic reaction. 7 (a) (b) -260 -280 -300 -300 -300 E,mV(SCE) -280 -320 -320 -340 -340 0 20 40 -360 0 60 -320 -340 -360 -360 (c) -260 -280 E, mV(SCE) E, mV(SCE) -260 20 40 60 0 20 40 60 Time, min Time, min Time, min Figure 5: Potential – time curves of Fe in different concentrations of HCl in absence and presence of 10 mM Dipron: (a) Free HCl (b) MSD (c) NSD. The variation of �� with the logarithm of the molar concentration, �, of the acid solutions, is represented in Figure 6. The results reveal that according to Eq. 7: where, �� = + log � (7) �� varies linearly with log � represents the steady state potential in solution of 1 M concentration, and is the slope of the straight line. The constants composition of solution. Values of and and are depending on the for Fe in free HCl are equal to -0.316 V(SCE) and 20 mV per decade concentration respectively. Addition of 10 mM Dipron in MSD and NSD forms shifts the steady state potentials positively with extrapolated values of are -0.284 and -0.283 V(SCE) and values of are 41 and 38 mV per decade concentration, respectively. These values indicate that 10 mM Dipron in MSD and NSD forms have similar inhibiting effect on open circuit potential of Fe in different concentrations of HCl solutions with preferential action of NSD form. 8 -260 Free HCl MSD Ess, mV (SCE) -280 NSD -300 -320 -340 -360 -1,5 -1 -0,5 0 0,5 log� Figure 6: �� vs. log � for Fe in different concentrations of HCl solutions in absence and presence of 10 mM Dipron in MSD and NSD forms. Potentiodynamic polarization measurements Effect of HCl concentrations on corrosion behavior of Fe electrode Polarization plots of Fe in different concentrations of HCl (0.05 - 2.00 M) at 25°C are shown in Figure 7. The respective potentiodynamic parameters including corrosion potential (Eco , corrosion current density (Ico cathodic (bc) Tafel slopes are listed in Table 2. , corrosion rate (CR , anodic (ba) and The analysis of the data in Table 2 revealed that Ico and CR increased with increasing acid concentration. This indicates that the aggressive Cl¯ ions participate in the dissolution process. The Fe metal was oxidized at the anode where corrosion occurs according to Eq. 8: = + + − (8) Simultaneously, reduction occurs at cathodic sites according to Eq. 9: + + − = (9) Chloride ions typically migrate into Fe from the environment and, upon reaching embedded Fe and exceeding a certain critical concentration, destroys the passivating oxide layer on Fe forming complexes with ferrous ion. This led to formation of expansive corrosion products. Some of these products are soluble and assist transport 9 of corrosion products away from sites where they might otherwise give physical protection [9]. This corrosion process can be suppressed by using an appropriate dose of inhibitor, which reacts with and complex ferrous ions, or adsorbs on its surface thus interrupting the corrosion process. 2 log I, mA/cm2 1 0 0.05M -1 0.10M 0.25M -2 0.50M 0.75M -3 1.00M 2.00M -4 -1000 -500 0 500 E, mV(SCE) Figure 7: Potentiodynamic polarization plots of Fe in different concentrations of free HCl solutions. Table 2: Potentiodynamic parameters of Fe in different concentrations of free HCl Ico CR CHCl Ecorr ba - bc M mV(SCE) μA/cm2 mpy mV/dec mV/dec 0.05 -320 100 46 100 200 0.10 -320 355 164 214 167 0.25 -320 950 439 94 36 0.50 -300 970 448 111 143 0.75 -300 1400 647 91 143 1.00 -340 1995 921 100 200 2.00 -280 2512 1160 91 100 10 Effect of HCl concentrations on the inhibition efficiency of 10 mM Dipron using Fe electrode The effect of changing HCl concentrations on the inhibition efficiency of 10 mM Dipron was investigated in Figure 8 and the corresponding potentiodynamic parameters are represented in Table 3. It can be seen that, with increasing acid concentration and in the presence of 10 mM Dipron, the anodic and cathodic Tafel slopes changed greatly from that of free acid solutions (Table 2). This shows that addition of 10 mM Dipron to HCl solutions reduces the anodic dissolution of Fe and delays the hydrogen evolution reaction. The presence of Dipron in MSD and NSD forms lowered both of Ico and CR of Fe electrode compared with obtained values of free HCl. Moreover NSD achieved lower Ico and CR values than MSD. Also, increasing acid concentration resulted in decreasing IE% in the presence of the two forms of Dipron. Thus, at high acid concentration, the adsorbed inhibitor molecules could be expected to be desorbed by the retardation action of Cl¯ ions [10]. The results illustrated that adsorbed NSD was stronger than MSD form. The inhibition efficiency in presence of 10 mM NSD reached its maximum value, 98.22 %, at 0.05 M HCl, and it is higher than that of MSD which reached to 91.60 % at the same concentration. This may be due to the presence of smaller nano particles of inhibitor in case of NSD form. However, as the size of a particle decreases, its surface area increases and also allows a greater proportion of its atoms or molecules to be displayed on the surface rather than the bigger molecules [11]. 2 (a) (b) 0 0 0.05M 0.10M 0.25M 0.50M 0.75M 1.00M 2.00M -1 -2 -3 -1000 -500 0 E, mV(SCE) 500 logI, mA/cm2 logI, mA/cm2 1 -2 0.05M 0.10M 0.25M 0.50M 0.75M 1.00M 2.00M -4 -6 -8 -600 1000 -500 -400 -300 -200 E, mV(SCE) -100 0 Figure 8: Potentiodynamic polarization plots of Fe in presence of 10 mM Dipron soluble in different concentrations of HCl solutions: (a) MSD, (b) NSD 11 Table 3: Potentiodynamic parameters of Fe in different concentrations of HCl solutions in presence of 10 mM Dipron in MSD and NSD forms Ico CR CHCl Ecorr ba Medium 2 mpy M mV(SCE) μA/cm mV/dec - bc mV/dec IE% 0.05 -350 84 4 167 154 91.60 0.10 -350 40 18 222 318 88.72 0.25 -150 120 55 250 214 87.37 0.50 -300 138 64 222 400 85.77 0.75 -300 200 92 222 375 85.71 1.00 -300 300 138 167 250 84.96 2.00 -300 400 185 200 250 84.07 0.05 -350 2 0.82 75 50 98.22 0.10 -300 8 3.69 50 50 97.74 0.25 -300 30 13.8 40 31 96.84 10 mM 0.50 -300 40 18.4 182 29 95.88 NSD 0.75 -300 68 31.4 20 31 95.14 1.00 -300 100 46.1 21 17 94.98 2.00 -300 250 115.4 11 18 90.05 10 mM MSD The natural logarithms of CR of Fe in absence and presence of 10 mM MSD and NSD versus the molar concentration of HCl ���� illustrate a straight line broken at approximately 0.25 M HCl for all solutions. The following rate equation was set up to describe the straight line relationship [12]. ln �� = ln � + ���� (10) where � is the specific corrosion reaction rate constant and is a constant for the reaction. Equation 10 is shown graphically in Figure 9 and the calculated kinetic parameters are listed in Table 4. It is clear from Figure 9 and Table 4 that in the presence of MSD and NSD, the magnitude of CR is suppressed as its tendency to increase with increasing acid concentrations. This may be due to suppression of Fe chloride by the adsorption of Dipron molecules [10]. The decrease of slope of the second part of broken lines may be arise from the formation of a tightly adsorbed, more protective Fe chloride compound on the metal at higher acid concentrations (> 12 0.25 M) [12]. On the other hand, the presence of 10 mM NSD soluble in different concentrations of HCl considerably decrease the magnitude of CR compared with that of MSD. β1 and � in Table 4 represent the defined constants of equation 10 at acid concentrations of 0.05, 0.10 and 0.25 M HCl. While β2 and � represent the same constants at acid concentrations of 0.50, 0.75, 1.00 and 2.00 M HCl. Data in Table 4 illustrates that, the constants β1 and β2 for inhibited solutions are higher than that for free HCl solutions. The rate constants of corrosion reaction, � and � , decrease in the following order: Free HCl > MSD > NSD. This indicates the inhibiting action of Dipron and the better action of NSD form. 8 6 ln �� 4 2 Free HCl MSD 0 NSD -2 0 0,5 1 1,5 2 2,5 CHCl , M Figure 9: Variation of ln�� of Fe with different concentrations of HCl in absence and presence of 10 mM Dipron in MSD and NSD forms at 25 °C. Table 4: kinetic parameters for the corrosion of Fe in HCl solutions in absence and presence of 10 mM Dipron in MSD and NSD forms Medium β2 β1 mpy M k2 k1 mpy Free HCl 25.33 0.59 1.30E-2 394 MSD 31.21 0.70 8.14E-4 51 NSD 30.06 1.21 1.83E-4 11 13 Effect of different concentrations of Dipron on corrosion behavior of Fe electrode The effect of different concentrations of MSD and NSD on the potentiodynamic polarization of Fe is represented in Figure 10. From the figure it can be observed that the addition of different concentrations of NSD to 0.5 M HCl shifts the anodic and cathodic branches of polarization curves of free acid solution towards lower current densities than MSD does. The potentiodynamic parameters of Dipron in the two forms are illustrated in Table 5. The values of Ico and CR decrease with increasing Dipron concentration. This decrement indicates that the addition of Dipron molecules inhibits the corrosion process by decreasing the surface area for corrosion. This demonstrates that Dipron acts as inhibitor by adsorption onto Fe surface, and the inhibition degree depends on the nature of inhibitor and its concentration. Both ba and bc values changes noticeably in presence of the two forms of inhibitor than that observed in free acid solution. The inhibition efficiencies increase with increasing Dipron concentration. The highest IE% of 97.9 occurred at 13.31 mM NSD compared with 89.7 for MSD. This is most likely due to the adsorption and formation of a good protective film by Dipron in MSD and NSD forms. The formation of a protective film is a result of the interaction of Dipron molecules with atoms of the Fe surface. The smaller NSD particles present in HCl solution have high surface area that provides high reaction activities leading to an increase in the formation of a protective film [13]. Values of corrosion potential are not affected by addition of any concentration of MSD indicating that this form could act as pickling inhibitor which in spite of leaving the corrosion potential virtually unaffected, cause a significant decrease in the corrosion rate [10]. The same behavior was recorded for NSD at concentrations 3.80, 9.50, 10.00 and 13.31 mM. On the other hand, a positive displacement of the Ecorr values exhibited by 0.19 mM NSD was 200 mV(SCE) and by 0.95 and 1.90 mM was 100 mV(SCE). This means that, at these concentrations, the dissolution of the anode can be controlled and the hydrogen evolution controlling process is reduced. An inhibitor can be classified as anodic or cathodic type when the change in Ecorr value is higher than 85 mV in relation to that measured for the free solution [14]. Hence, it can be concluded that NSD form could act as pickling inhibitor at high concentrations and behaves as mixed type but predominantly anodic inhibitor at low concentrations. 14 2 2 (a) 0 0.00mM 0.19mM 0.95mM 1.90mM 3.80mM 9.50mM 10.00mM 13.31mM -2 -4 -6 -8 -1000 -700 Figure 10: -400 -100 200 E, mV(SCE) Potentiodynamic 500 log I, mA/cm2 logI, mA/cm2 0 (b) 0.00mM 0.19mM 0.95mM 1.90mM 3.80mM 9.50mM 10.00mM 13.31mM -2 -4 -6 -8 -1000 -700 -400 -100 800 200 500 800 electrode in E, mV(SCE) polarization plots of Fe different concentrations of Dipron in its two form: (a) MSD, (b) NSD. Table 5: Potentiodynamic parameters of Fe in 0.5M HCl in absence and presence of different concentrations of MSD and NSD at 25 °C Ico CR Cinh Ecorr ba Medium 2 μA/cm mpy mM mV(SCE) mV/dec Free HCl MSD NSD -bc mV/dec IE% 0.00 -300 970 448 111 143 - 0.19 -300 316 146 185 200 67.4 0.95 -300 200 92 100 160 79.4 1.90 -300 180 83 100 200 81.4 3.80 -300 160 74 100 100 83.5 9.50 -300 145 67 133 171 85.0 10.00 -300 140 64 222 400 85.6 13.31 -300 100 46 142 250 89.7 0.19 -100 100 46 360 560 89.7 0.95 -200 60 28 228 133 93.8 1.90 -200 49 22 333 160 94.9 3.80 -300 45 21 314 280 95.3 9.50 -300 42 19 300 366 95.7 10.00 -300 40 18 182 29 95.9 13.31 -300 20 9 267 533 97.9 15 Adsorption isotherms Freundlich isotherm The Freundlich isotherm model was chosen to explain the adsorption of Dipron on the iron surface with uniform energy according to Eq. 11 [10, 15]: 1 � � = ���� ���ℎ (11) The linearized form is given in Eq. 12: log � = log ���� + � log ���ℎ (12) where, ���� (the equilibrium constant) and � are indicators of the adsorption capacity and adsorption intensity respectively. ���ℎ is the inhibitor concentration (mM) and � is the surface coverage degree. Figure (11-a) represents the adsorption of MSD and NSD on Fe surface according to this model. The adsorption parameters are listed in Table 6 and compared with that of Langmuir model. It has been stated that the magnitude of � gives an indication of the favorability and capacity of the adsorbent/adsorbate system. A value for � < 1 as in the present study indicates a normal Freundlich isotherm in which a significant adsorption takes place at low concentration but the increase in the amount adsorbed with concentration becomes less significant at higher concentrations and vice versa, while � > 1 is indicative of cooperative adsorption [10, 16]. Also, the higher ���� value for NSD indicates the greater the adsorption capacity. ���� is related to the free energy of adsorption, ∆ ° ��� ���� = , by Eq. 13: ⁄ . exp −(∆ ° ��� ⁄� ) (13) . is the molar concentration of water, � is the universal gas constant and where the absolute temperature. The obtained negative value of ∆ ° ��� is indicates that the adsorption process of Dipron in MSD and NSD forms on the Fe surface is spontaneous. Langmuir isotherm A mathematical representation of the Langmuir model is illustrated in Eq. 14 [10, 17]: ���ℎ ⁄� = ⁄���� � + ���ℎ ⁄ (14) � 16 where �, is the adsorbate binding capacity, that is, the maximum adsorption upon complete saturation of adsorbent surface. (a) 15 (b) 0,00 12 -0,05 Cinh/ Ө log Ө 9 -0,10 6 MSD -0,15 MSD 3 NSD NSD -0,20 0 -1,0 -0,5 0,0 0,5 1,0 1,5 0 3 log Cinh 6 9 12 15 Cinh , mM Figure 11: Adsorption isotherms of Dipron in MSD and NSD form on Fe surface: (a) Freundlich, (b) Langmuir. Figure (11-b) and Table 6 show that for Langmiure adsorption isotherm, the linear correlation coefficients (R2) and the slopes of the two forms of Dipron are almost very close to unity, which indicates that the adsorption of Dipron in the micro and nano scales follows Langmuir adsorption isotherm. This means that the solid surface contains a fixed number of adsorption sites and each site holds one adsorbed species [18]. The higher values of ���� and � suggested higher capacity of the adsorption process and stability of the adsorbed layer on the Fe surface in the presence of NSD than that in presence of MSD. Values of ∆ ° ��� between 0 and -20 kJ/mol, as in the present study, are consistent with spontaneous physical adsorption [19]. 17 Table 6: Freundlich and Langmuir isotherms constants for adsorption of Dipron in MSD and NSD forms on Fe surface at 25 °C Isotherm MSD NSD R2 0.913 0.890 ���� , mol-1 0.766 0.931 17.513 59.523 0.057 0.017 -9.293 -9.773 R2 0.998 0.999 slope 1.128 1.027 0.886 0.973 4.660 16.860 -13.765 -16.949 parameters Freundlich isotherm ⁄ ∆ ° ��� , kJ/mol Langmuire isotherm � ���� , ∆ mol-1 ° ��� , kJ/mol Effect of temperature on the corrosion behavior of Fe in 0.5 M HCl in absence and presence of 10 mM Dipron Temperature is an important parameter in the metal dissolution studies. The effect of temperature on the inhibition of metal corrosion reactions is very complex because different changes may occur on the metal surface such as rapid etching, desorption of inhibitor and the inhibitor may undergo decomposition [20]. In this study, the effect of changing the temperature from 20 to 60 °C on the corrosion behavior of Fe in free 0.5 M HCl and in presence of 10 mM of Dipron in both of MSD and NSD forms was studied. It was observed that variation of temperature has almost no effect on the general shape of the polarization curves as shown in Figure 12. Table 7 illustrates the effect of temperature on the corrosion potentials and corrosion rates of Fe in 0.5 M HCl in absence and presence of 10 mM Dipron in MSD and NSD forms and its corresponding inhibition efficiencies. By increasing the temperature, the corrosion rates increases and the free acid solution showed maximum corrosion rates than that in the presence of Dipron. This signified that the dissolution of the metal increased at higher temperatures. This is because the reactant molecules gain more 18 energy and are able to overcome the energy barrier more rapidly. An increase in temperature may also increase the solubility of the adsorbed films on the metals, thus increasing the susceptibility of the metal to corrosion [21]. The decrease in IE% with the increase in temperature might be attributed to the weakening of the physical adsorption process of inhibitor on the metal surface. For a physical adsorption mechanism, IE% of an inhibitor decreases with increasing temperature while for a chemical adsorption mechanism, values of inhibition efficiency increases with temperature [21]. It is noted from Table 7 that MSD has the best IE% value at 20 °C whereas the inhibition efficiency decreased at higher temperatures. On the other hand NSD keeps its tendency for inhibition efficiency constant even at higher temperatures owing to its greater activity which resulted from the presence of smaller nano particle size in the corrosive HCl solution. (a) 0 2 -2 -4 -6 -2 -4 0 500 1000 -1200 -4 -8 -8 -1500 -1000 -500 -2 -6 -6 -8 (c) 0 0 logI, mA/cm2 log I, mA/cm2 (b) 2 log I, mA/cm2 2 -900 -600 -300 E, mV(SCE) 0 300 -1000 -500 0 500 E, mV(SCE) E, mV(SCE) Figure 12: Effect of temperature on potentiodynamic polarization of Fe electrode in 0.5M HCl in absence and presence of 10 mM Dipron: (a) free HCl (b) MSD (c) NSD. 19 Table 7: Corrosion potential and corrosion rates of Fe electrode in 0.5M HCl in absence and presence of 10 mM of Dipron in MSD and NSD forms and the corresponding inhibition efficiencies at different temperatures Free HCl Ecorr MSD �� Ecorr NSD �� IE% mV(SCE) �� mpy IE% Ecorr °C mV(SCE) mpy mV(SCE) mpy 20 -250 369 -300 23.1 93.75 -350 6.9 98.13 30 -300 453 -350 64.7 85.71 -300 18.9 95.82 40 -300 508 -350 73.9 85.45 -300 22.2 95.64 50 -300 647 -300 97.9 84.85 -350 28.6 95.57 60 -400 785 -350 161.6 79.41 -350 35.1 95.53 The dependence of corrosion rate, CR , on temperature can be expressed by the Arrhenius equation [20]: where log �� = − �⁄ . � (15) is a constant representing the frequency factor, � is the apparent activation energy of the Fe dissolution reaction, � is the universal gas constant and T is the absolute temperature. The values of � can be calculated from the slopes of the straight line obtained by plotting log CR vs 1/ as illustrated in Figure 13(a). The thermodynamic functions for dissolution of Fe in 0.5M HCl in absence and presence of 10 mM of Dipron in MSD and NSD forms were obtained by applying the Eyring transition-state equation (Eq. 16) [18, 20]: where log CR ⁄ = log � ⁄ ℎ + ∆ ° ⁄ . � − ∆ °⁄ . � is Avogadro’s number, ℎ is Planck’s constant, ∆ (16) ° and ∆ ° are the entropy and enthalpy of activation, respectively. A plot of log CR ⁄ vs. / gave straight line with slope of [─∆ ° ⁄ . �] and an intercept of [log � ⁄ ℎ + ∆ ° ⁄ . � ] as illustrated in Figure 13 (b). The obtained values were tabulated in Table 8. 20 3,0 (a) log CR/T, mpyK-1 2,5 log CR, mpy (b) 0,5 2,0 1,5 0,0 -0,5 -1,0 -1,5 1,0 Free HCl MSD Free HCl NSD MSD NSD -2,0 0,5 2,9 3,0 3,1 3,2 3,3 3,4 2,9 3,5 3,0 3,1 1000/T 3,2 3,3 3,4 3,5 1000/T Figure 13: (a) Arrhenius plot, (b) Transition state plot. Table 8: Activation parameters of Fe in 0.5M HCl in absence and presence of 10 mM Dipron Medium The increase in � kJ/mol ∆ ° kJ/mol ∆ ° J/mol K Free HCl 15 12 -153 MSD 35 32 -106 NSD 30 27 -133 � and ∆ ° in the presence of MSD and NSD forms implies that addition of Dipron to the acid solution increases the height of the energy barrier of the corrosion reaction and indicating that as the temperature is raised a decrease in protection efficiency is obtained [22]. It is noted from Table 8 that � for the corrosion process, in presence of Dipron, is greater than 20 kJ/mol and hence the entire process is controlled by surface reaction [23]. Based on the temperature effects, the relationships between the temperature dependence of IE% of an inhibitor and the can be classified into three groups [24]: 21 � (1) IE% decreases with the increase in temperature: (inhibited solution) > � (inhibited solution) < � (uninhibited solution); (2) IE% increases with the increase in temperature: (uninhibited solution); (3) IE% does not change with temperature: (inhibited solution) = solution). (uninhibited � According to Tables 7 and 8, group (1) is the applied case in which the values of � suggests the physisorption mechanism. The positive signs of ∆ ° reflect the endothermic nature of Fe dissolution process which suggests its difficult and slow dissolution in presence of Dipron in MSD and NSD forms [18]. Values of entropy of activation ∆ ° illustrated less negativity for inhibited solutions than that for the uninhibited one. This implied that the activated complex represents an association rather than a disordering going from reactants to the activated complex [18]. This reflects the formation of an ordered stable layer of inhibitor on the Fe surface [25]. Thus, one can say the nature of inhibitor and the temperature of solution affect greatly the activation parameters of the corrosion process. Potentiostatic polarization Effect of applied potential Figure 14(a, b and c) shows the variation of current density with time at different applied potentials (EA) for Fe electrode in 0.5M HCl in absence and presence of 10 mM Dipron in MSD and NSD forms, respectively. The corresponding values of initial current density (ii), stabilized current density (is), quantity of electricity ( ) and surface charging capacity (�) are summarized in Table 9. It is clear that, for each type of solution, as a general trend, the ii, is and increases by increasing the applied potentials. Comparing the three types of solutions, values of ii, is, and C for Fe electrode generally decreases in the following order: Free HCl > MSD > NSD This illustrates the effective action of Dipron in MSD and NSD forms on the corrosion inhibition of Fe in 0.5M HCl and ensured the better action of NSD form. 22 12 12 6 4 8 6 4 2 2 0 0 0 2 4 6 8 10 Current density, mA/ cm2 Current density, mA /cm2 8 8 6 4 2 0 0 2 4 6 Time, min Time, min (c) 10 10 10 Current density, mA /cm2 12 (b) (a) 8 10 0 2 4 6 8 10 Time, min Figure 14: Potentiostatic polarization plots of Fe electrode at different applied potentials in 0.5M HCl in absence and presence of 10 mM Dipron: (a) free HCl (b) MSD (c) NSD. These results are in good agreement with those obtained in OCP and potentiodynamic polarization measurements. The highest values observed for free 0.5M HCl were due to the preferential adsorption of Cl¯ ions on the oxide surface acting as a depolarizer for the main anodic process of oxygen discharge [26]. The results showed that the surface charging capacity reached its maximum value at 10 mV(SCE) in the three types of solutions. Effect of inhibitor concentration Figure 15 and Table 10 illustrate the effect of Dipron concentration at constant applied voltage of 10 mV(SCE) on the potentiostatic behavior of Fe in 0.5 M HCl at 25 °C. It is clearly observed that the presence of different concentrations of Dipron in MSD and NSD forms was accompanied by a reduction in stabilized current density compared with that recorded for free acid solution. The current densities decreased with increasing inhibitor concentrations. This was an indicative of passivation of the electrode in the presence of the two forms of Dipron. The decrease in � values in the presence of inhibitor is attributed to a decrease in active sites because of the adsorbed Dipron in micro and nano scale particle size [27]. The values of film thickness ( ) did not change regularly with the Dipron concentration. But in all cases, it is higher in the 23 Table 9: Potentiostatic parameters of Fe in 0.5M HCl in absence and presence of 10 mM Dipron in MSD and NSD forms at different applied potentials at 25 °C EA ii � is Medium mV(SCE) mA/cm2 mA/cm2 mC/cm2 mF/cm2 Free HCl MSD NSD -150 0.40 0.95 85 0.57 -100 0.80 1.45 130 1.31 10 4.91 5.20 1092 109.20 50 6.30 6.45 290 5.81 100 7.50 6.75 1012 10.12 200 11.25 8.40 1008 5.04 -150 0.30 0.85 76 0.51 -100 0.75 1.40 84 0.84 10 3.20 1.65 198 19.80 50 4.00 4.10 184 3.69 100 5.00 4.30 645 6.45 200 7.00 5.40 972 4.86 -150 0.04 0.26 24 0.16 -100 0.41 0.71 42 0.42 10 0.39 0.87 157 15.68 50 4.00 2.60 156 3.12 100 5.00 4.10 615 6.15 200 5.80 4.20 630 3.15 presence of Dipron than that of free acid solution, and in most cases it is higher in the presence of NSD compared with MSD indicating its better protective properties. It is worthy to mention that 10 mM Dipron concentration show better protective properties than other concentrations and can be considered as an optimum concentration for this study. This result is in good agreement with that obtained in OCP measurements. 24 (MSD) (NSD) 6 5 current density, mA/cm2 current density, mA/cm2 6 4 3 2 1 5 4 3 2 1 0 0 0 10 20 30 0 10 Time, min 20 30 Time, min Figure 15: Potentiostatic polarization plots of Fe in 0.5 M HCl in absence and presence of different concentrations of Dipron at constant applied voltage of 10 mV(SCE). Table 10: Potentiostatic parameters of Fe in 0.5 M HCl in absence and presence of different concentrations of Dipron at constant applied voltage of 10 mV(SCE) at 25°C MSD Cinh mM ii is NSD � mA/cm2 mA/cm2 mF/cm2 ii Ǻ*10-6 mA/cm2 mA/cm2 � mF/cm2 Ǻ*10-6 is 0.00 4.91 5.20 109.20 1.89 4.91 5.20 109.20 1.89 0.19 5.00 5.00 60.00 3.45 5.50 4.22 25.32 8.17 0.95 4.80 4.75 57.00 3.63 5.20 4.04 24.24 8.54 1.90 4.50 3.93 82.53 2.51 4.80 3.91 19.54 10.59 3.80 3.40 3.84 80.64 2.56 4.80 3.81 45.72 4.53 9.50 2.70 3.72 78.12 2.65 3.60 3.66 76.86 2.69 10.00 3.20 1.80 86.40 2.40 0.39 1.20 21.60 9.58 13.31 4.70 3.45 103.50 2.00 1.90 2.10 37.80 5.47 25 Effect of temperature Figure 16 represents the temperature effect on corrosion behavior of Fe electrode immersed in 0.5 M HCl in absence and presence of 10 mM Dipron in MSD and NSD forms at constant applied voltage of 10 mV(SCE). The potentiostatic parameters are tabulated in Table 11. It is clear that, for the three types of solutions, ii, i s, and �, all generally increase with increasing temperature obeying the following order: Free HCl > MSD > NSD. This can be attributed to an appreciable decrease in the adsorption of Dipron on the Fe surface with increase in temperature and a corresponding increase in corrosion rates expected due to the fact that greater area of metal is exposed to acid environment. The reciprocal capacitances which represent the film stability are always having higher values for acid solutions containing NSD than that containing MSD at all temperatures. From Figure 16 and Table 11 it can be concluded that the presence of Dipron in its two forms inhibit the corrosion of Fe in 0.5 M HCl. Also, the presence of NSD in 0.5 M HCl solution resists the increasing Fe corrosion accompanied by increasing temperature more effectively than MSD does. (a) (b) 14 10 8 6 4 2 12 10 8 6 4 2 0 2 4 6 Time, min 8 10 12 10 8 6 4 2 0 0 0 (c) 14 Current density, mA/cm2 12 Current density, mA/cm2 Current density, mA /cm2 14 0 2 4 6 Time, min 8 10 0 2 4 6 8 10 Time, min Figure 16: Effect of temperature on Fe electrode at constant applied voltage of 10 mV(SCE) in 0.5M HCl in absence and presence of 10 mM Dipron: (a) free HCl (b) MSD (c) NSD. 26 Table 11: Potentiostatic parameters of Fe electrode in 0.5 M HCl in absence and presence of 10 mM Dipron in MSD and NSD forms at constant applied voltage of 10 mV(SCE) at different temperatures ii i Temp. Medium °C mA/cm2 mA/cm2 mC/cm2 � � -1 mF/cm2 cm2/mF 20 3.00 4.30 258 25.80 3.88E-02 Free 30 5.00 5.30 636 63.60 1.57E-02 HCl 40 9.00 7.60 912 91.20 1.10E-02 50 10.40 9.00 270 27.00 3.70E-02 60 11.25 13.25 795 79.50 1.26E-02 20 0.45 1.30 156.00 15.60 6.40E-02 30 1.80 2.27 204.30 20.43 4.89E-02 40 5.00 4.00 480.00 48.00 2.08E-02 50 7.30 7.40 1332.00 133.20 7.51E-03 60 10.00 9.45 1984.50 198.45 5.04E-03 20 0.12 0.30 36.00 3.60 2.78E-01 30 0.27 0.88 158.00 15.80 6.31E-02 40 1.30 1.65 346.50 34.65 2.89E-02 50 2.00 2.50 375.00 37.50 2.67E-02 60 4.40 4.40 924.00 92.40 1.08E-02 MSD NSD 3-4- The inhibition mechanism In hydrochloric acid medium, the Fe surface is negatively charged due to the specifically adsorbed chloride ions on the surface. Owing to the acidity of the medium, the –NH2 group in Dipron could not remain in solution as free base. It exists as a neutral species or in the cationic form as indicated below. Also, the oxygen and sulfure atoms in the sulfamidic group can be protonated easily, due to high electron density on it, leading to positively charged inhibitor species. Thus, Dipron inhibitor may adsorb through the electrostatic interactions between the positively charged Dipron molecules and the negatively charged Fe surface. Moreover, the adsorption of Dipron molecules could also be occurred due to the formation of links between the 27 d-orbital of Fe atoms, involving the displacement of water molecules from the metal surface, and the lonely sp2 electron pairs present on the N and O atoms of the sulfamidic group [28]. O S H2 N O O O S H3N + H NH2 NH2 Molecular form Cationic form Conclusions 1. From OCP, potentiodynamic and potentiostatic polarization measurements, the corrosion of Fe in HCl solutions is retarded in the presence of Dipron in MSD and NSD forms and the effect of NSD is more pronounced than MSD. 2. 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