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Inhibition Effect of Hydantoin Compounds on the Corrosion of Iron in Nitric and Sulfuric Acid Solutions Loutfy H. Madkour
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  • 1. Monatshefte fuÈr Chemie 132, 245±258 (2001) Inhibition Effect of Hydantoin Compounds on the Corrosion of Iron in Nitric and Sulfuric Acid Solutions Loutfy H. Madkour, Amera M. Hassanein, Mohamed M. GhoneimÃ, and Safwat A. Eid Chemistry Department, Faculty of Science, Tanta University, 31527 Tanta, Egypt Summary. The inhibition of corrosion of iron in 2 M nitric acid and 2 M sulfuric acid solutions by substituted phenylhydantoin, thiohydantoin, and dithiohydantoin compounds was measured using thermometric, weight loss, and polarization methods. The three methods gave consistent results. The polarization curves indicated that the hydantoin compounds act as mixed-type inhibitors. The adsorption of the inhibitors were found to obey the Temkin adsorption isotherm. The higher inhibition ef®ciency of the additives in nitric with respect to sulfuric acid solution may be attributed to the reduced formation of soluble quaternary nitrogen salts in nitric acid medium, favouring adsorption of the parent additive on the metal surface. The obtained results indicate that the corrosion rate of iron in both acids increases with increasing temperature, both in absence and presence of the tested inhibitors. Kinetic-thermodynamic model functions and Temkin isotherm data are compared and discussed. The synergistic effect of halide anions on the inhibition ef®ciency of the hydantoin compounds was also investigated. Keywords. Acid corrosion; Inhibition; Iron; Hydantoins; Synergistic effect. Introduction One of today's most important considerations in industry is the reduction of overall costs by protection and maintenance of materials used. Because iron is the back- bone of industrial constructions, the inhibition of iron corrosion in acidic solutions has been studied in considerable detail. Many N-heterocyclic compounds with polar groups and/or %-electrons are ef®cient inhibitors for iron corrosion in acidic media [1]. These organic molecules can adsorb on the metal surface, forming a bond between the electron pair of the nitrogen and/or the %-electron cloud and the metal, thereby reducing the corrosive attack [1, 2]. It has also been reported that the inhibition ef®ciency of sulfur-containing compounds is superior to that of nitrogen- containing ones [3]. In nearly all cases there was evidence of chemisorption of the inhibitor, and the inhibitors were of a mixed type, i.e. both the anodic and cathodic polarization processes were affected. No research work is available in the literature à Corresponding author
  • 2. to date about the application of hydantoin compounds as inhibitors for surface metal corrosion. Accordingly, the objective of this work is to study the hydantoin com- pounds effects toward corrosion process of iron in nitric and sulfuric acid solutions and also to determine the adsorption isotherms and to compare them with kinetic- thermodynamic models of corrosion inhibition. Results and Discussion Thermometric measurements The temperature change of the system involving iron in 2 M HNO3 or 2 M H2SO4 was followed in the absence as well as in the presence of different concentrations of compound 3 as an example (Fig. 1). Upon increasing the concentration of the additive, the time required to reach Tmax increases. This indicates that the inhibitor retards the dissolution of iron in both corrosive acidic media, presumably by Fig. 1. Temperature vs. time curves of iron corrosion in 2 M HNO3 in the absence and in the presence of different concentrations of 3 246 L. H. Madkour et al.
  • 3. adsorption onto the surface of the metal. The extent of retardation depends on the degree of coverage of the metal surface with the adsorbate. The temperature vs. time curves provide a means of differentiating between weak and strong adsorption [4]. Strong adsorption is noted in both acidic solutions, since a simultaneous increase in t and a diminution in Tmax takes place, and both factors cause a large decrease in RN (reduction in reaction number) of the system. The results reported in Table 1 reveal that the inhibition ef®ciency of the additive, as determined from the percentage reduction in RN, increases with increasing concentration of additives. Figure 2 shows the relation between % RN and the molar concentration of different additives. The curves obtained are invariably sigmoid in nature, substantiating the idea that the present inhibitors retard the corrosion rate by adsorption according to the Temkin isotherm [5] (Eq. (1)). ˆ c1 Á ln…c2 Á c† …1† Fig. 2. Effect of additives concentration on % reduction in reaction number (% RN) of iron corrosion in 2 M HNO3 Hydantoin Derivatives as Corrosion Inhibitors 247
  • 4. In Eq. (1), c is the concentration of the additive in the bulk of the solution, is the degree of coverage of the investigated metal surface by the adsorbed molecules, and c1 and c2 are constants. The order of increasing the inhibition ef®ciency of the hydantion compounds as determined by % RN is 3 2 4 1 6 5 7. Weight loss measurements Figure 3 shows the effect of the time of immersion on the corrosion of iron in 2 M H2SO4 solution in the absence as well as in the presence of different amounts of 3. The curves obtained in the presence of additives fall below that of the free acid. The Table 1. Effect of concentration 3 on the thermometric parameters of Fe in 2 M HNO3 c/mol Á ia C maxa C t/min Át/min log RNa CÁ % Red Á dmÀ3 …Átamin† minÀ1 in RN 0 35.0 59.0 60 0.400 5  10À7 35.0 45.9 75 15 1.18 0.686 0.150 62.5 1  10À6 35.0 44.0 75 15 1.18 0.749 0.120 70.0 5  10À6 34.8 44.0 90 30 1.48 0.791 0.100 75.0 1  10À5 35.1 42.2 90 30 1.48 0.835 0.078 80.5 5  10À5 34.9 39.8 105 45 1.65 0.899 0.048 88.0 1  10À4 35.0 40.1 120 60 1.78 0.921 0.041 89.8 Fig. 3. Weight loss vs. time curves of iron corrosion in 2 M H2SO4 in the absence and in the presence of different concentrations of 3 at 303 K 248 L. H. Madkour et al.
  • 5. weight loss of iron depends on type and concentration of the additive in a similar way as thermometric measurements do. The ef®ciency of the inhibitors under investigation increases in the order 3 2 4 1 6 5 7. The inhibition effect may be explained by considering the adsorption of the hydantoin molecules (with high negative charge density at the hetero atom) on the metal surface [6, 7] consisting of iron atoms with incomplete d shells [8]. Also, formation of a metal-inhibitor com- plex on the corroding iron surface (surface chelation) may play a role [9]. Polarization measurements Anodic and cathodic polarization of iron was carried out under potentiostatic conditions in 2 M HNO3 and 2 M H2SO4 in the absence as well as in the presence of different concentrations of inhibitor at 303 K. Figure 4 shows the polarization curves of iron in 2 M nitric acid solution at different concentrations of 3; the results obtained for the other compounds were quite similar. The inhibition ef®ciency depends on many factors including number of adsorption sites or functional groups, basicity, and molecular size. In the present case, the N-atom and the O-atom probably act as the centers of adsorption, their basicity being affected by the character of the substituents in -position. The results show that the inhibitive power increases with increasing chain length: 3-carbethoxy-1-phenylhydantoin (3) is found to be the most ef®cient inhibitor. This may be attributed to the presence of the carbethoxy group which increases the electron density on the molecule and provides an active adsorption center (oxygen atom) in addition to the two nitrogen centers already present. Fig. 4. Potentiostatic polarization curves of iron in 2 M HNO3 in the absence and in the presence of different concentrations of 3 Hydantoin Derivatives as Corrosion Inhibitors 249
  • 6. On the other hand, 1,3-dimethyl-5-phenylazo-2-thiohydantoin (6) has the lowest inhibition ef®ciency owing to the formation of an iron complex (see formula) which is obviously less adsorbed in acidic solutions. This interpretation is supported experimentally by spectroscopic analysis (mainly ultraviolet spectra). The Temkin adsorption isotherm is found to be ideally obeyed in acidic solutions (Fig. 5), indicating that the main inhibition process takes place through adsorption [10, 11]. The degree of surface coverage () by the adsorped molecules was calculated from Eq. (2), where u0 and u are the dissolution rates of iron in the absence and in the presence of hydantoins, respectively. ˆ …1 À uau0† …2† From Table 2 it can be seen that the three different techniques afford the same results for the inhibition of corrosion of iron in both acidic media. Fig. 5. Variation of iron surface coverage () with the logarithmic concentration of different additives in 2 M HNO3 at 303 K 250 L. H. Madkour et al.
  • 7. Effect of temperature The effect of temperature on the rate of corrosion of iron in 2 M HNO3 and 2 M H2SO4 containing 1  10À5 M hydantoin was studied in the temperature range of 303±323 K. The corrosion rate increases with increasing temperature in the absence as well as in the presence of the inhibitors. The increase of icorr is due to the absence of a protective layer at the iron surface. Thus, the increase of temperature enhances both the iron dissolution and the additive desorption processes without leading to Fe(II)-hydantoin complex formation. The protective layer decreases as the temperature increases. The Arrhenius parameters as purely empirical quantities enable us to discuss the variation of rate constants with temperature. It was found experimentally that a plot of lnk against 1aT gives a straight line according to Eq. (3) [5]. lnk ˆ lnA À EaaRT …3† The activation energies calculated from the slopes of lnIcorr vs. 1aT plots (Fig. 6) are reported in Table 3. The enthalpy change of activation (ÁHz ) can be calculated from Eq. (4), the free energy change of activation (ÁGz ) is obtained from the Eyring equation [12]: (Eq. (5)). Another convenient form of Eq. (5) is Eq. (6). ÁHz ˆ Ea À RT …4† k ˆ kBT h eÀÁGz aRT …5† ÁGz ˆ RT Á lnkBT h À lnk …6† From ÁHz and ÁGz , the entropy change of activation (ÁSz ) can be obtained according to Eq. (7). ÁSz ˆ …ÁHz À ÁGz †aT …7† Table 2. Comparison between the inhibition ef®ciency of 1±7 in 2 M acid solutions as determined by thermometric, weight loss, and polarization methods (1  10À4 M inhibitor, 303 K) % Inhibition Inhibitor Thermometric Weight loss Polarization HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4 1 71.4 74.4 78 73 75.7 88.2 2 85.5 80.9 87 82 90.9 90 3 92.1 88.3 90 85 93.8 91 4 77.3 77.1 85 76 84.8 90 5 62.5 60.1 74 66 69.6 87.3 6 64.9 64.9 76 63 74.1 88.1 7 53.8 53.1 69 59 68.3 81.4 Hydantoin Derivatives as Corrosion Inhibitors 251
  • 8. Fig. 6. Arrhenius plot of the current corrosion rate constant (icorr) vs. 1aT of iron in (a) 2 M HNO3 and (b) 2 M H2SO4 in the absence and in the presence of 1 Â 10À5 M hydantoin inhibitors 252 L. H. Madkour et al.
  • 9. From the thermodynamic parameters in Table 3 it can be seen that Ea increases as the inhibition ef®ciency of the additives increases. This suggests that the process is controlled by a surface reaction, since the energy of activation for the corrosion process is above 20 kJ Á molÀ1 [13]. The Ea value for iron dissolution in 5 M H2SO4 has been reported as 20X2 kJ Á molÀ1 [12] and 51.4 kJ molÀ1 [14±16]. For iron in 3 M HCl and 1 M HNO3, an Ea values of 11.8 kJ Á molÀ1 has been reported [17], whereas in 2 M HNO3 20.06 kJ Á molÀ1 have been found [18]. Generally, one can say that the nature and the concentration of the electrolyte greatly affect the activation energy of the corrosion process. The presence of the inhibitor causes a change in the value of the apparent activation energy. Thus, it indicates no change in the rate-determining step brought about by the presence of the hydantoin inhibitor. Kinetic-thermodynamic model of corrosion inhibition To evaluate the kinetic parameters and correlate them to the corrosion inhibition mechanism, it is of value to analyze the kinetic data obtained in the presence of hydantoin inhibitors from the standpoint of the generalized mechanistic scheme proposed by El-Awady et al. [19, 20]. Table 4 comprises the values of 1ay which give the number of active sites occupied by a single organic molecule; K is the binding constant [21]. The values of B (equilibrium constant) and f (lateral interaction parameter, 1/c1) are also reported in Table 4. The large negative values of ÁG indicate that the reaction proceeds spontaneously and is accompanied by a high ef®cient adsorption. Large values of K and B (c2) point to better inhibition ef®ciency of the tested hydantoin compounds, i.e. stronger electrical interaction between the double layer at the phase boundary and the adsorbing molecules. In general, the equilibrium constant of the adsorption process was found to rise with increasing inhibition ef®ciency. Table 3. Activation energy (Ea), enthalpy change (ÁHz ), free energy change (ÁGz ), and entropy change (ÁSz ) for the dissolution of Fe in 2 M acid in the presence of 1 Â 10À5 M inhibitor at 313 K Inhibitor Ea kJ Á molÀ1 ÁHz kJ Á molÀ1 ÁGz kJ Á molÀ1 ÀÁSz J Á KÀ1 Á molÀ1 HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4 HNO3 H2SO4 No 8.98 7.27 6.38 4.67 71.54 71.68 208.18 214.09 1 40.15 23.84 37.55 21.24 75.55 72.82 121.40 164.79 2 38.97 30.15 36.37 27.55 76.35 73.38 127.73 146.42 3 38.40 32.07 35.80 29.47 76.65 73.63 130.51 141.09 4 43.29 27.56 40.69 24.96 75.81 73.14 112.20 153.93 5 30.71 20.90 28.11 18.30 74.77 72.50 149.07 173.16 6 29.47 22.35 26.87 19.75 75.26 72.56 154.60 168.72 7 23.92 20.31 21.32 17.71 74.45 72.36 169.74 174.60 Hydantoin Derivatives as Corrosion Inhibitors 253
  • 10. Synergistic effects of halide anions The effects of IÀ , BrÀ , and ClÀ on the polarization curves of iron at 298 K in the absence and presence of hydantoin inhibitors in 0X5 M H2SO4 solution were studied. The in¯uence on the inhibition ef®ciency was observed in presence of each anion alone or in the presence of any of the different additives together with the anion. The extent of the effect follows the order IÀ BrÀ ClÀ . Since the iodide ions have a higher inductive effect than BrÀ and ClÀ [22], they are less attached to the metal surface and, consequently, easily displaced by the inhibitor molecules. The net increment of the inhibition ef®ciency is de®ned in Eq. (8), where Px and Pinh are the protection ef®ciency of the anion and inhibitor, respectively, and Ptot is the total protection ef®ciency of the corrosive medium containing the anion and inhibitor together. ÁP ˆ Ptot À …Px ‡ Pinh† …8† The correlation of ÁP vs. logc for the applied synergistic anions in acidic solution was studied. Since the halide anions interact strongly with the iron surface due to chemisorption [23, 24], the inhibitory effect is strengthened due to the coadsorption of the anions. As a result, the surface coverage area …ÁP† and, consequently, the inhibition ef®ciency both increase. KI is the most effective among the investigated salts. Addition of 10À3 M KI in the presence of a very low concentration …1  10À5 M† of 3 rises the inhibition ef®ciency from 48 to 93% as shown in Table 5. Table 4. Curve ®tting of data to the kinetic-thermodynamic model (r ˆ 0X94) and the Temkin isotherm for hydantoin inhibitors in 2 M acid at 303 K Kinetic model Temkin isotherm Medium 1ay K ÀÁG kJ Á molÀ1 1 C1 C2  104 ÀÁG kJ Á molÀ1 1 HNO3 12.50 1X30  109 41.96 7X20  10À4 133.10 24.46 H2SO4 6.67 5X47  108 39.82 1X51  10À3 16.72 19.44 2 HNO3 10.77 1X63  1010 53.93 1X32  10À2 254.61 26.03 H2SO4 5.88 9X63  1010 52.62 2X56  10À3 141.25 24.60 3 HNO3 8.43 1X01  1012 58.44 6X43  10À2 258.67 26.09 H2SO4 8.24 5X02  1011 56.71 3X35  10À2 243.60 25.93 4 HNO3 9.10 1X90  109 42.90 1X12  10À3 167.49 25.02 H2SO4 7.14 5X57  109 45.57 1X85  10À3 32.69 21.06 5 HNO3 4.79 9X50  108 41.19 2X16  10À4 87.70 23.45 H2SO4 10.75 1X15  108 35.96 6X56  10À4 2.72 15.04 6 HNO3 8.33 1X20  109 41.77 4X90  10À4 118.85 24.19 H2SO4 5.88 3X59  108 38.78 1X26  10À3 13.55 18.93 7 HNO3 3.96 2X30  108 37.64 9X36  10À5 68.07 22.84 H2SO4 5.56 1X69  107 31.21 1X76  10À4 2.30 14.63 254 L. H. Madkour et al.
  • 11. Experimental Iron specimens and electrolytes Iron specimens (0.16% C, 0.05% Si, 0.37% Mn, 0.015% S) were used in the present study. Prior to each experiment, the electrodes were mechanically polished with successive grades of emery paper, degreased in pure acetone, washed in running bidistilled water, dried, and weighed before being inserted in the cell to remove any oxide layer or corrosion product from the surface [25, 26]. 2 M nitric and 2 M sulfuric solutions were prepared by diluting Analar reagents by bidistilled water. Additives The structural formulae of the investigated hydantoin derivatives 1±7 are given below. The compounds were prepared according to methods reported in the literature [27±29]. Their purity was checked by melting point determinations and spectroscopy. The hydantoin solutions were prepared by dissolving the appropriate amount of compound in 25 cm3 Analar EtOH. The desired volume of the free inhibitor was added to the electrolyte solution. The solvent effect must be considered by mixing a de®nite volume of EtOH to the free acid to reach a constant ratio of EtOH in each test in the absence and presence of different concentrations of inhibitor. Thermometric measurements The reaction vessel used was basically the same as that described by Mylius [30]. An iron piece …1  10  0X1 cm† was immersed in 30 cm3 of either 2 M HNO3 or 2 M H2SO4 in the absence and presence of additives, and the temperature of the system was followed as a function of time. The procedure for the determination of the metal dissolution rate by the thermometric method has been described previously [4, 30]. The reaction number (RN) is de®ned as given in Eq. (9) [31]. RN ˆ …Tmax À Ti†at …9† Tmax and Ti are the maximum and initial temperatures, respectively, and t is the time (in minutes) required to reach the maximum temperature. The percent reduction in RN [32] is then given as ……RNfree À RNinh†aRNfree†  100. Table 5. Electrochemical parameters of Fe in the presence of 10À3 M KI and different concentrations of 3 in 0X5 M H2SO4 at 298 K logC mol Á dmÀ3 Ecorr mV vsX SCE icorr mAacm2 Rcorr mpy c V Á decadeÀ1 c V Á decadeÀ1 % lnh Free acid À506 10.72 4.92 0.123 0.049 ± 1  10À3 M KI À471 9.57 4.39 0.113 0.042 10.7 1  10À5 M 3 À482 5.60 2.57 0.113 0.046 47.8 1  10À3 M KI ‡ 1  10À3 M 3 À438 0.77 0.35 0.111 0.041 92.8 1  10À4 M 3 À438 0.77 0.35 0.111 0.041 92.8 1  10À3 M KI ‡ 1  10À4 M 3 À431 0.75 0.34 0.115 0.032 93.0 1  10À3 M 3 À445 0.73 0.34 0.107 0.025 93.2 1  10À3 M KI ‡ 1  10À3 M 3 À420 0.72 0.33 0.109 0.035 93.3 1  10À2 M 3 À395 0.63 0.28 0.108 0.026 94.2 1  10À3 M KI ‡ 1  10À2 M 3 À440 0.41 0.19 0.102 0.022 96.2 Hydantoin Derivatives as Corrosion Inhibitors 255
  • 12. Weight loss measurements The reaction basin used in this method was a graduated glass vessel of 6 cm inner diameter and a total volume of 250 cm3 . 100 cm3 of the test solution at 303X0 Æ 1X0 K were employed in each experiment. The iron pieces (2 Â 2 Â 0X1 cm) were prepared as described before, weighed, and suspended under the surface of the test solution by about 1 cm by suitable glass hooks. After speci®ed periods of time, three pieces of iron were taken out of the test solution, rinsed with doubly distilled water, dried, and re-weighed. The average weight loss at a certain time for each test of three samples was taken. The percentage of inhibition (% In) of different concentrations of the inhibitors was calculated accordin
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