Iraqi Journal of Chemical and Petroleum Engineering Vol.13 No.2 (June 2012) 1- 9 ISSN: 1997-4884 Phenyl Thiourea as Corrosion Inhibitor for Mild Steel in Strong Hydrochloric Acid Aprael S. Yaro and Dhuha A. Abdulaaima Department of Chemical Engineering, College of Engineering, University of Baghdad, Iraq Abstract The inhibitive action of Phenyl Thiourea (PTU) on the corrosion of mild steel in strong Hydrochloric acid, HCl, has been investigated by weight loss and potentiostatic polarization. The effect of PTU concentration, HCl concentration, and temperature on corrosion rate of mild steel were verified using 2 levels factorial design and surface response analysis through weight loss approach, while the electrochemical measurements were used to study the behavior of mild steel in 5-7N HCl at temperatures 30, 40 and 50 °C, in absence and presence of PTU. It was verified that all variables and their interaction were statistically significant. The adsorption of (PTU) is found to obey the Langmuir adsorption isotherm. The effect of temperature on the adsorption process showed that the adsorption process is exothermic, spontaneous and represents mixed chemical and physical adsorption for PTU on the metal surface. Key Words Corrosion, Phenyl Thiourea, Electrochemical Measurements, dimensionless separation factor, Factorial Design, Adsorption, Activation parameters Introduction The corrosion of metals remains a world -wide scientific problem as it affects the metallurgical, chemical, and oil industries [1]. Hydrochloric acid is widely used for the removal of the rust and scale in several industrial operations [2]. When mild steel is used in these operations it suffers sever corrosion [3]. Using inhibitors is one of the most practical methods for protection against corrosion, especially in acid solutions, to prevent metal dissolution and acid consumption [4]. Organic compounds containing N and S have proved to be good inhibitors for the preventation of corrosion under acidic conditions [5]; therefore, thiourea and its derivatives have been extensively investigated as corrosion inhibitors in acidic media [6]. The main objective of this investigation is to study the inhibitive effect of Phenyl Thiourea (PTU) for the mild steel corrosion in strong hydrochloric acid using weight loss and potentiostatic methods. Experimental Method A mild steel sheet was used as working electrode of 20 mm (width), 30 mm (length) and 1 mm thickness in weight loss method. Area of (0.8 cm²) was used for polarization method. Its composition is :( C=0.17-0.2, Mn<1.4, S<0.045, P< 0.045, Fe reminder). (5 Iraqi Journal of Chemical and Petroleum Engineering University of Baghdad College of Engineering Phenyl Thiourea as Corrosion Inhibitor for Mild Steel in Strong Hydrochloric Acid 2 IJCPE Vol.13 No.2 (June 2012) -Available online at: www.iasj.net and 7 N) Hydrochloric acid solutions were prepared using distilled water. Weight Loss Method The specimens were polished with emery papers and then cleaned with tap water, distilled water, benzene and acetone. After that they were dried and weighed on a digital scale. Each of the specimens is designated and its initial weight is noted. After each test, the specimen was washed with running tap water, scrubbed with a brush to remove corrosion products, then washed with tap water followed by distilled water and dried on clean tissue, immersed in benzene, dried, immersed in acetone, dried and left in a desiccators over silica gel for 1 hour before weighting. The time of immersion in HCl solutions was two hours. Polarization Techneque By using a Wenking M Lab potentiostat and a three electrode cell, electrochemical studies were performed. Platinum over Titanium (Pt/Ti) electrode was used as the auxiliary electrode and a saturated silver electrode Ag/AgCl as the reference electrode. The corrosion rates are determined by Tafel extrapolation technique. The experiments were conducted at 30, 40, and 50 °C. Results and Discussions Weight Loss Method Table (1) represents the low and high levels factor, the matrix of the factorial design. Table 1, Factors and levels used in 2³ factorial design variables Low level (-1) High level (+1) Inhibitor concentration (ppm) 100 1000 Acid concentration (N) 5 7 Temperature (˚C) 30 70 Table (2) shows the experimental results. Table 2, Effect of HCl concentration, PTU concentration, and temperature on corrosion rate of mild steel Table (3) shows the main effects of the factors under study and their interaction on the corrosion rate of mild steel in HCl acid media in presence of phenyl thiourea (PTU). Table 3, The variables effect and their interaction using PTU Factor Main effect or interaction X₁ -7282.25 X₂ 3443.25 X₃ 10478.75 X₁ X₂ -998.75 X₁ X₃ -6818.25 X₂ X₃ 2860.25 It is clear from Table (3) that the acid concentration and the temperature accelerate corrosion, and the effect of temperature is about 4 times larger than acid concentration. The inhibitor, on the other hand, decreases the corrosion rate sharply under the operating conditions. Yates method [7] was followed on the data which are given in Table (3). A mathematical expression to describe the design matrix combination mentioned in Table (2) as low and high level of each factor and its corresponding corrosion rates mentioned in Table (3) in code values were obtained as follows: No. PTU (ppm) X₁ HCl (N) X₂ Temperatur e (˚C) X₃ Corrosion rate g/m².day 1 100 5 30 135 2 100 5 70 13970 3 100 7 30 1115 4 100 7 70 21874 5 1000 5 30 68 6 1000 5 70 1470 7 1000 7 30 254 8 1000 7 70 6173 Aprael S. Yaro and Dhuha A. Abdulaaima -Available online at: www.iasj.net IJCPE Vol.13 No.2 (June 2012) 3 Y=5632.375- 3641.125X₁+1721.625X₂+5239.375X₃ +1430.125X₂X₃-499.375X₁X₂- 3409.125X₁X₃ …(1) Where: Y is the corrosion rate at each variable combination. X₁, X₂, X₃ are inhibitor concentration, acid concentration, and temperature, respectively. The results were analyzed using the analysis of variance (ANOVA) as appropriate to experimental design used. From ANOVA, the variables and their interaction effect on the corrosion rate were significant. Polarization Technique The polarization curves in the absence and the presence of PTU in (7 N HCl solutions) at different temperatures are presented in Figures 1 through 12, respectively. Table (4) shows the values of corrosion parameters obtained using Tafel extrapolation method. Fig.1, Polarization behavior of mild steel in 7N HCl in absence of PTU at temperature =30˚C Fig.2, Polarization behavior of mild steel in 7N HCl in presence of 100 ppm of PTU at temperature =30˚C Fig. 3, Polarization behavior of mild steel in 7N HCl in presence of 550 ppm of PTU at temperature =30˚C Fig. 4, Polarization behavior of mild steel in 7N HCl in presence of 1000 ppm of PTU at temperature =30˚ C Fig. 5, Polarization behavior of mild steel in 7N HCl in absence of PTU at temperature =40˚C Fig. 6, Polarization behavior of mild steel in 7N HCl in presence of 100 ppm of PTU at temperature =40˚C Phenyl Thiourea as Corrosion Inhibitor for Mild Steel in Strong Hydrochloric Acid 4 IJCPE Vol.13 No.2 (June 2012) -Available online at: www.iasj.net Fig. 7, Polarization behavior of mild steel in 7N HCl in presence of 550 ppm of PTU at temperature =40˚ Fig. 8, Polarization behavior of mild steel in 7N HCl in presence of 1000 ppm of PTU at temperature =40˚ C Fig. 9, Polarization behavior of mild steel in7N HCl in absence of PTU at temperature =50˚C Fig. 10, Polarization behavior of mild steel in 7N HCl in presence of 100 ppm of PTU at temperature =50˚C Fig. 11, Polarization behavior of mild steel in 7N HCl in presence of 550 ppm of PTU at temperature =50˚C Fig. 12, Polarization behavior of mild steel in 7N HCl in presence of 1000 ppm of PTU at temperature = 50°C Table 4, Corrosion parameters obtained for mild steel in 7N HCl at different temperatures and concentrations of PTU Effect of Temperature and Activation Studies Activation energy, Eact., activation entropy, ΔSact., and enthalpy of activation, ΔHact. were calculated using Arrhenius equation: PTU ppm Temp K Ecorr mV icorr μA/cm² ba mV/dec -bc mV/dec IE (%) Nil 303 -365 2380 76 124 ...... 313 -364 7190 69 139 ...... 323 -392 9840 83 79 ...... 100 303 -437 372 54 183 84.4 313 -420 1440 74 106 80 323 -453 935 55 147 90.5 550 303 -404 222 56 78 90.7 313 -438 1240 60 129 82.8 323 -441 2180 67 144 77.8 1000 303 -440 305 53 137 87.2 313 -424 440 61 125 94 323 -445 1940 72 173 80.3 Aprael S. Yaro and Dhuha A. Abdulaaima -Available online at: www.iasj.net IJCPE Vol.13 No.2 (June 2012) 5 log icorr = log A - Eact/2.303 RT …(2) and its alternative formulation called transition state equation: icorr = (RT/Nh) exp(ΔSact./R) exp(- ΔHact./RT) …(3) where, T is the absolute temperature, R, the universal gas constant, h is Planck´s constant, and N is Avogadro´s number. From the corrosion current densities obtained from polarization curves at different temperatures in the absence and the presence of PTU as corrosion inhibitor, Arrhenius plots are shown in Figure 13 for a temperature range of (303-323 K) in 7N acid concentration. The activation energies calculated from Arrhenius plots, and the values of ΔSact. and the ΔHact. obtained from transition state plots with accepted regression coefficient are listed in table (5). Figure (14) shows transition state plots for temperature range of (303- 323 K) in 7N acid concentration. Table 5, Activation parameters for adsorption of PTU on mild steel at different conditions From Table (5), it is observed that the activation energy, Eact., and activation enthalpy, ∆Hact., for uninhibited acid were lower than in inhibited acid. The higher values in the presence of PTU inhibitor indicate physical adsorption of the inhibitor on the metal surface. The results showed the positive sign for both Eact. and ∆Hact., reflecting the endothermic nature of corrosion process [4]. The endothermic process is attributed to chemisorptions. All values of Eact. are larger than the analogous values of ∆Hact indicating that the corrosion process must involve a gaseous reaction, simply the hydrogen evolution reaction, associated with a decrease in total reaction volume [8]. Fig.13, Arrhenius plot of mild steel in 7 N HCl contains different concentrations of PTU at different temperatures Fig.14, Transition state plot of mild steel in 7 N HCl contains different concentrations of PTU at different temperatures The negative values of ∆Sact. pointed to a greater order produced during the process of activation. This can be achieved by the formation of activated complex and represents association or fixation with consequent loss in the degrees of freedom of the system PTU ppm Eact. kJ/mol R² ΔHact. kJ/mol ΔSact. J/mol. K R² Nil 56.52 0.9 17 54.15 -0.919 0.914 100 56.59 0.9 38 54.26 -16.05 0.901 550 90.91 0.9 36 88.17 92.14 0.932 1000 72.63 0.8 73 70.08 32.21 o.867 Phenyl Thiourea as Corrosion Inhibitor for Mild Steel in Strong Hydrochloric Acid 6 IJCPE Vol.13 No.2 (June 2012) -Available online at: www.iasj.net during the process [9]. It means that a decrease in disordering take place on going from reactants to the activated complex [10, 11]. The increase of ∆Sact. reveals that an increase in disordering take place from reactant to activated complex [12]. Adsorption Isotherm Studies Figure (15) shows the linear plots for C/Ѳ versus C, suggesting that the adsorption obeys the Langmuire´s isotherm: C/Ѳ = 1/Kads + C …(4) Where C is the inhibitor concentration, and Kads the adsorptive equilibrium constant, representing the degree of adsorption (i.e., the higher value of Kads indicates that the inhibitor is strongly adsorped on the metal surface); the value of Kads obtained from the reciprocal of intercept of Langmuir´s plot lines and the slop of these lines is near unity, which mean that each inhibitor molecule occupies one active site on the metal surface. Moreover, the essential characteristic Langmuir isotherm can be expressed in terms of a dimensionless separation factor, RL [4], which describes the type of isotherm and is defined by: R = ads C …(5) The smaller RL value indicates a highly favorable adsorption. If RL >1, unfavorable; RL=1, linear; 0< RL<1, favorable; and if RL=0, irreversible. Table (6) gives the estimated values of RL for PTU in 7N HCl at different temperature. It was found that all RL values are less than unity, confirming that the adsorption is favorable. Fig.15, Langmuir adsorption isotherm of PTU on mild steel in7N HCl at different temperatures The standard adsorption free energy (∆Gads°) was calculated using the following equation [13]: Kads = (1/55.5) exp (-∆G ° ads/RT) …(6) Where, 55.5 is the concentration of water in solution expressed in molar, R is the gas constant, and T is the absolute temperature. The average value of standard adsorption free energy is ∆Gads=-25.769 kJ/mol. The negative values of ∆Gads ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the metal surface. Generally, values of ∆Gads up to -20 KJ/mol are consistent with electrostatic interaction between the charged molecule and the charged metal (physisorption), while those around -40 KJ/mol or higher are associated with chemisorptions as a result of sharing or transfer of electrons from the organic molecules to the metal surface to form a coordinate type of bond [14]. While other researchers suggested that the range of ∆Gads of chemical adsorption processes for organic inhibitor in aqueous media lies between -21 to -42 KJ/ mol [15]. Therefore, for the present work the value of ∆Gads is larger than the common physical adsorption values, but smaller than the common chemical Aprael S. Yaro and Dhuha A. Abdulaaima -Available online at: www.iasj.net IJCPE Vol.13 No.2 (June 2012) 7 adsorption values [16], probably meaning that both physical and chemical adsorption take place (i.e. comprehensive adsorption). The dependence of ∆Gads on temperature can be explained by two cases as follows [17]: 1. ∆Gads may increase (becomes less negative) with the increase in temperature which indicates the occurrence of exothermic process. 2. ∆Gads may decrease (becomes more negative) with the increase in temperature which indicates the occurrence of endothermic process. Table 6, Dimensionless separation factor RL for PTU at various temperatures Temp (K) PTU (g/l) RL 303 0.1 0.218 0.55 0.04 1 0.027 313 0.1 0.253 0.55 0.058 1 0.0328 323 0.1 0.555 0.55 0.185 1 0.111 Values of other thermodynamic parameters such as enthalpy (∆Hads) and entropy (∆Sads) can provide supplementary information about the mechanism of corrosion inhibition. The enthalpy (∆Hads) and entropy (∆Sads) of adsorption on mild steel in hydrochloric acid in the presence of inhibitor can be calculated by using the following equation [18]: ln Kads = ln( /55.5) ∆Sads/R-∆Hads/RT …(7) Using equation (7), the values of enthalpy (∆Hads) and entropy (∆Sads) of adsorption were evaluated from the slope and intercept of the plot of ln Kads versus 1/T as shown in Figure (16). The thermodynamic data of adsorption are depicted in Table (7). Fig.16, Plot of ln Kads against 1/T for PTU on mild steel in 7N HCl at different temperatures Table 7, Thermodynamic parameters for adsorption of PTU on mild steel surface in 7N HCl at different temperatures Temp (K) Kads l/g slope ∆Gads KJ/mol ∆Hads KJ/mol ∆Sads J/mol.K 303 35.71 1.100 -26.406 -58.74 -105.03 313 29.41 1.056 -26.773 323 8 1.096 -24.13 The values obtained confirm the exothermic behavior of the adsorption process of PTU on mild steel surface in hydrochloric acid. While an endothermic adsorption process (∆Hads>0) is attributed unequivocally to chemisorptions, an exothermic adsorption process (∆Hads <0) may involve either physisorption or chemisorption or a mixture of both processes [19, 20]. In the present work, the value obtained may introduce physisorption and chemisorption processes which are confirmed by previous discussion. Also, the negative values of ∆Hads show that the adsorption is exothermal with an ordered phenomenon ascribed by the negative values of ∆Sads. This order may more probably be explained by the possibility of formation of iron complex on the metal surface [21, 22], or inhibitor molecules may freely move in the bulk of solution before the adsorption process, while with progress in adsorption the inhibitor molecules were orderly adsorbed on the metal surface, which resulted in the decrease in entropy[16]. 1 1.5 2 2.5 3 3.5 4 0.003050.00310.003150.00320.003250.00330.00335 ln k a d s( l/ g ) 1⁄T (K¯¹) Phenyl Thiourea as Corrosion Inhibitor for Mild Steel in Strong Hydrochloric Acid 8 IJCPE Vol.13 No.2 (June 2012) -Available online at: www.iasj.net Conclusions 1. Phenyl thiourea (PTU) represents effective inhibitor in 7N HCl at temperature range of 30 -50°C. The maximum inhibition efficiency was found to be 94% at 40°C and 1000 ppm of PTU. 2. The endothermic nature and chemisorption of corrosion process. 3. The adsorption of PTU is spontaneous and exothermic and follows Langmuir adsorption isotherm. 4. The adsorption of PTU is comprehensive (physical and chemical adsorption) for the inhibition process. Acknowledgments The authors would like to express all their thanks to ALLAH, Who enabled them to overcome all the difficulties associated with this study till producing this project in its final form. They would also like to thank PRDC- Petroleum Research and Development Center (Contract monitor: D. Shehab) for the financial support of this project. References 1. A. O. James, N. C. Oforka, Olusegum . Abiola, “Inhibition of acid corrosion of mild steel by Pyridoxal and Pyridoxal Hydrochloride”, Electrochem. Sci., 2(2007) 278-284. 2. G.Y. Elewady, “Pyrimidine Derivatives as Corrosion Inhibitors for Carbon-Steel in 2M Hydrochloric Acid Solution”, Electrochem. Sci., 3 (2008) 1149 – 1161. 3. P. Bothi Raja and M.G. Sethuraman, “Studies on the Inhibition of Mild Steel Corrosion by Rauvolfia serpentina in Acid Media” ,Journal of Materials Engineering and Performance, 19(5) 2010 761-766. 4. A. 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