Adriana Samide Patru and Mircea Preda
Keywords: carbon steel, corrosion, inhibition, NCBSA, ammonium chloride
Abstract:
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ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 The Study of the Process of Corrosion Inhibition of Carbon Steel in a Solution of Ammonium Chloride by using N-Cyclohexylbenzothiazolsulphen-amida Adriana Samide Patru*, Mircea Preda University of Craiova, Faculty of Chemistry, Calea Bucuresti no. 165BB, Dolj, Romania, Phone/Fax: 0251597048, E-Mail: samide_adriana@yahoo.com Abstract The inhibiting effect of N-Cyclohexylbenzothiazolsulphen-amida (NCBSA) on the corrosion of carbon steel in a solution of NH4Cl 10 -1M (pH = 5,5) at temperatures of 25º C, 35º C, 45º C and 55º C has been studied by using weight loss measurements and electrochemical measurements. The morphology of carbon-steel surface in the absence and in the presence of NCBSA was examined under the microscope. The activation energy (Ea ) was determined from Arrhenius equation. The activation enthalpy (ΔHº) and the activation entropy (ΔSº) were calculated from the diagram of the critical complex. The experimental data characterize an adsorption isotherm of type Langmuir. Key words: carbon steel, corrosion, inhibition, NCBSA, ammonium chloride © University of Manchester and the authors 2005. This is a preprint1 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 Introduction Treatments with organic compounds 1-9 have been proposed in order to improve anti-corrosion protection. The efficiency of the corrosion inhibition by organic compounds is closely related to the structure and properties of the film formed on the metal surface. For the carbon-steel corrosion inhibition in different mediums, there have been used organic compounds that contain sulphur and nitrogen, the results showing that several inhibitors act through adsorption on the metal surface 10-19 . It has been observed that the adsorption depends especially on the electronic structure of the molecule and that the inhibiting effect increases with the increase in the number of the aromatic nuclei. The adsorption process is also influenced by the metal nature and surface, by the electrolyte nature and the interactions between the organic molecules and the metal surface, and by temperature 20 -33. The action mechanisms of the organic inhibitors as well as the factors influencing the adsorption depend on the strength of the bond inhibitor-metal 34-39. The present study aims to determine the role of NCBSA in improving the protective film in case of generalized corrosion of carbon steel in a low acid medium (pH = 5,5) that contains NH4Cl 10-1M, by determining the parameters of activation of the dissolving reaction and the thermodynamic parameters of adsorption of the inhibitor on the metal surface. Experimental Weight loss measurements The experimental determinations follow the calculation of the corrosion rate of carbon steel in a solution of NH 4Cl 10-1M in the absence and presence of NCBSA by © University of Manchester and the authors 2005. This is a preprint2 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 using the gravimetric method. The utilized carbon steel has the following composition: C=0,1%, Si=0,035%, Mn=0,4%, Cr=0,3%, Ni=0,3% and the rest up to 100% Fe. For the gravimetrical determinations, the metal samples with a surface of 4 cm2 were mechanically polished with sand-paper of different sizes, decapped with a solution of 5% HCl, washed in double distilled water, degreased in ethylic alcohol and dried in warm air. To avoid the change of the solution concentration on through water evaporation, the experiments were effected in a closed system. The corrosion rates of carbon steel were determined in a solution of NH4Cl 10-1M in the absence and presence of some variable concentrations of NCBSA: 50 ppm, 100 ppm, 150 ppm, 200 ppm and respectively, 1,89.10-4M, 3,78. 10-4M, 5,68.10-4M, 7,56.10-4M. The immersion time of the plates in the respective media were 3 hours at temperatures of 25º C, 35º C, 45º C and 55º C. Before being introduced in corrosive media, the plates were exactely weighed on analytical scales. The corrosion products were removed once the samples were taken out of the corrosive mediums, by washing them in a warm solution of HCl 5%, after which they were degreased in ethilic alcohol, dried in warm air and weighed again on the analytical scales. The mass variation is conditioned by the composition; the nature of medium and work temperature Electrochemical measurements For the study of polarization a standard corrosion cell was used, with a working electrode made of carbon-steel with an active surface of 4 cm2 . The saturated calomel electrode (SCE) was used as a reference electrode. The auxiliary electrode was a carbon-steel plate identical to the one used as the working electrode. The carbon steel © University of Manchester and the authors 2005. This is a preprint3 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 electrodes were made of the same material as the plates used for weight loss measurements. The electrochemical measurements were carried out using a Keithley 2420 3A Source Meter, and the data were computerized. The morphology of steel’s surface before and after being treated with NCBSA was examined with a STEM LEO 911 OMEGA microscope Results and discussion Weight loss measurements For the carbon steel samples the weight losses were determined in a solution of NH4Cl 10-1M, in the absence and presence of several variable concentrations of NCBSA: 50 ppm, 100 ppm, 150 ppm, 200 ppm at different temperatures: 25º C, 35º C, 45º C, 55º C. The corrosion rate was expressed by the gravimetric indices (kg) determined according to relation 1. G kg S t (1) where: ΔG = mass loss due to corrosion (g), S = the corroded surface (m2), t = action time for corrosive processes. The variation of the corrosion rate with the concentration of NCBSA at different temperatures is presented in Figure 1. The increase in concentration of NCBSA leads to a decrease in the corrosion rate. This suggests that the inhibition of carbon steel corrosion in the presence of NCBSA is due to the adsorption of the latter on the metal surface. On the other hand, an increase in temperature from 25º C to 50º C leads to an increase in the corrosion rate, © University of Manchester and the authors 2005. This is a preprint4 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 the probable cause being the desorbtion of the inhibitor molecules from the metal surface. 0,7 0,6 kg (g/m2 h) 0,5 0,4 0,3 0,2 (1) (2) (3) (4) 0,1 0 0 50 100 150 200 250 C-NCBSA (ppm) Figure 1. The variation of the corrosion rate for carbon steel corroded in a solution of NH4Cl 10 -1M at different temperatures: (1) 550C; (2) 45 0C; (3) 350C; (4) 250C. The efficiency of inhibition expressed in percentage (P) for NCBSA was calculated using the relation 2. k k P g0 g 100 kg 0 (2) where: kg0 = the corrosion rate in the absence of inhibitor, kg = the corrosion rate in the presence of inhibitor. The inhibition efficiency determined under experimental conditions is presented in Figure 2. The inhibition efficiency depends on the inhibitor concentration and on temperature. It is observed that at a concentration of NCBSA of 200 ppm (7,56.10-4M) and a temperature of 25º C, the practically determined efficiency is of 81%. There is a possible formation of some complexes between NCBSA (as a binder) and different © University of Manchester and the authors 2005. This is a preprint5 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 metallic cations that are found in the steel composition. These complexes can form permanent stable films on the steel surface, thus decreasing the corrosion rate. The good adsorbability of NCBSA can be related to the basicity of the thiazolic nucleus and to the pair of non-participant electrons from the sulphur atom in the thiazolic cycle. 90 (1) (2) (3) (4) 80 70 P (%) 60 50 40 30 20 10 0 0 50 100 150 200 250 C-NCBSA (ppm) Figure 2. The variation of the inhibition efficiency expressed in percentage (P) with the concentration of NCBSA for carbon steel corroded in a solution of NH 4Cl 10-1M at different temperatures. (1) 250C;(2) 350C;(3) 45 0C;(4) 550C. It can be also presupposed that the adsorbability of the NCBSA molecules around the corrosion spots in incipient stages (initiation and propagation) is due to the non-participant electrons from the sulphur atom and nitrogen from the lateral chain of the thiazolic cycle, or that this chain contributes to the increase in the thickness of the adsorption layer. It can be said that a competitive adsorption takes place between the inhibitor molecules and the Cl- anion on the steel surface. © University of Manchester and the authors 2005. This is a preprint6 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 The activation energy that characterizes the corrosion inhibition of carbon steel in a solution of NH4Cl 10 -1M in the absence and presence of NCBSA was determined from the logarithmic equation of Arrhenius: E a log k g log A 2,303 RT (3) where: kg = corrosion rate, Ea = apparent activation energy, A = pre-exponential factor, R = gas constant, T = temperature By graphically representing logkg = f (1/T) straight lines can be obtained with slopes Ea /2,303R (Figure 3). 2,9 y = -1030x + 5,9385 R 2 = 0,9773 log kg (mg/m2 h) 2,7 2,5 (1) y = -1290x + 6,5005 R2 = 0,985 y = -1410x + 6,7145 R 2 = 0,9854 2,3 (2) 2,1 (3) 1,9 (4) y = -1460x + 6,752 R 2 = 0,985 y = -1470x + 6,6765 R 2 = 0,9623 (5) 1,7 0,003 0,0031 0,0031 0,0032 0,0032 0,0033 0,0033 0,0034 0,0034 1/T (K) Figure 3. The Arrhenius diagram for the corrosion rate of the carbon steel sample in NH 4Cl 10-1M in the absence and presence of different concentrations of NCBSA. (1) 0; (2) 1,89·10-4 M; (3) 3,78·10-4 M; (4) 5,68·10-4 M; (5) 7,56·10-4M. The enthalpy and entropy that characterize the processes of corrosion and inhibition were determined from the relation 4: RT S 0 0 kg exp exp R N ah RT (4) © University of Manchester and the authors 2005. This is a preprint7 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 where: h = the Planck constant, Na = Avogadro’s number, R = gas universal constant, ΔΗº = activation enthalpy, ΔSº = activation entropy. By graphically representing log(kg/T) function of (1/T) straight lines are obtained, as it can be observed in Figure 4. The slopes of these straight lines are equal to ΔΗº/2,303R, the intersection with the ordinate representing log R S 0 , out of Na h 2,303R which the values ΔΗº and ΔSº were calculated. 0,4 y = -887,2x + 2,9859 2 R = 0,9693 0,2 (1) log kg/T 0 -0,2 (2) -0,4 (3) -0,6 (4) (5) -0,8 0,003 0,0031 0,0032 0,0033 y = -1143x + 3,5363 R2 = 0,983 y = -1256x + 3,7292 2 R = 0,9807 y = -1307x + 3,7661 2 R = 0,9862 y = -1353x + 3,8073 2 R = 0,9498 0,0034 1/T (K) Figure 4. The diagram of the critical complex for the corrosion rate of carbon steel in a solution of NH4Cl 10-1M in the absence and presence of different concentrations of NCBSA. (1) 0;(2) 1,89·10-4M;(3) 3,78·10-4 M;(4) 5,68·10-4M;(5)7,56·10-4 M. The values obtained for Ea , ΔΗº and ΔSº are presented in Table 1. Ea and ΔΗº that characterize the inhibition process of carbon steel corrosion in a solution of NH4Cl 10-1M, in the presence of NCBSA as an inhibitor have values relatively close to those that characterize the corrosive process in the absence of the inhibitor. This proves that no energetic barrier is reached. The results show that the © University of Manchester and the authors 2005. This is a preprint8 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 reaction of corrosion inhibition is affected, but without any modification of the mechanism being produced. Table 1. The parameters of activation for the dissolving reaction of carbon steel in a solution of NH4Cl 10-1M in the absence and presence of NCBSA. NCBSA concentration E a (kJ/mol) ΔH 0 (kJ/mol) ΔS 0 (J/K·mol) (M) 0 19,7 16,77 -142,15 1,89· 10 - 4 24,67 21,86 -131,62 3,78· 10 - 4 26,97 24,03 -127,93 5,58· 10 - 4 27,92 25 -127,22 7,56· 10 - 4 28,12 25,88 -126,44 This implies that at the stage determining the rate, the activation complex represents rather an association than a dissociation. On studying the inhibitor we have reached the conclusion that the experimental data characterize an adsorption isotherm of Langmuir type expressed by the relation 5. Kc 1 (5) where: θ= degree of covering, K = the equilibrium constant of the adsorptiondesorption process, c = concentration of the inhibitor By graphically representing function of the NCBSA concentration, 1 straight lines are obtained with the slope K (Figure 5). © University of Manchester and the authors 2005. This is a preprint9 of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 5 4,5 (1) y = 5864,7x - 0,07 R2 = 1 4 (2) y = 4812,8x + 0,0147 2 teta/1-teta 3,5 R = 0,9964 (3) 3 (4) 2,5 y = 4035,3x + 0,0998 2 R = 0,9939 2 y = 3554,1x + 0,0498 2 R = 0,9952 1,5 1 0,5 0 0 0,0002 0,0004 0,0006 0,0008 C-NCBSA (M) Figure 5. The results of a Langmuir diagram in case of corrosion inhibition of carbon steel in a solution of NH4Cl 10 -1M with NCBSA at different temperatures. (1) 250C; (2) 350C; (3) 45 0C; (4) 550C. The thermodynamic parameters of adsorption ΔΗºads and ΔSºads were obtained from the dependence relation of the equilibrium constant (K) of temperature. 0 ads S 0 ads 1 log K log 2,303 RT 2,303R 55,5 (6) By graphically representing logK function of 1/T a straight line is obtained whose slope is equal to 0 ads , while the intersection with the ordinate axis is 2,303 RT S 0 ads 1 log (Figure 6). 2,303R 55,5 The free standard energy of adsorption (ΔGºads) was calculated with relation 7: ΔGºads = ΔΗºads - TΔSºads (7) © University of Manchester and the authors 2005. This is a preprint 10of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 3,8 y = -730x + 5,9875 R 2 = 0,9914 3,7 -1 log K (M ) 3,75 3,65 3,6 3,55 3,5 0,003 0,0031 0,0032 0,0033 0,0034 1/T (K) Figure 6. The graphical representation of logk function of 1/T for NCBSA derived from the experimental adsorption isotherm of carbon steel. The results are presented in Table 2. Table 2. The thermodynamic parameters for the NCBSA adsorption on the carbon steel surface in a solution of NH4Cl 10-1M. 25 0 C 35 0 C 45 0 C 55 0 C K (mol - 1 ) 5864,7 4812,8 4035,3 3554,1 ΔGa d s 0 (kJ/mol) -40,28 -42,1 -43,92 -45,73 Temperature The values of ΔGº are negative, which shows that the process of adsorption is spontaneous. The equilibrium constants K vary towards the same direction, in the sense that higher values of K imply a better adsorption, which leads to an increase in the inhibition efficiency. Electrochemical measurements The effect of the inhibitor has been studied by the anodic and cathodic galvanostatic polarization of the carbon-steel sample in a NH4Cl 10 -1M, both in the © University of Manchester and the authors 2005. This is a preprint 11of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 presence and absence of NCBSA. The anodic and cathodic polarization curves are presented in figure 7. -270 E (mV)/SCE -470 -670 -870 -1070 -1270 -1470 0,1 1,1 2,1 logi (mA/dm2) Figure 7. The polarization curves for carbon steel in a solution of NH4Cl 0,1M, in the presence of different concentrations of NCBSA: ( ( )150ppm; ( )0; ( )50ppm; ( ) 100ppm; )200ppm Five determinations were made for each solution, taking into consideration the most reproducible responses for the same current densities. In this study, the minimum recorded current was 10µA. Thus Ecor was defined as the potential from which a current larger than 10µA was observed. The anodic polarization of steel electrodes in the inhibitor containing NH4Cl 10-1M is shifted to less negative values, while the cathodic polarization shifts to more negative values with increasing current density. The presence of organic inhibitors in the corrosive medium increases the anodic and cathodic overpotentials and decreases the corrosion current (icor). These changes increase with increasing inhibitor concentration. This behaviour supports the inhibition function of these organic compounds. Decrease of the corrosion current (icor) was associated with an appreciable © University of Manchester and the authors 2005. This is a preprint 12of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 shift of corrosion potential (Ecor) to a less negative value. This suggests that although inhibition is of mixed type, it is predominantly anodic. The percentage inhibition efficiency (P) of these inhibitors was also determined from the polarization measurements according to the equation: i ' i P cor ' cor 100 i cor (8) where i’cor and i cor are the uninhibited and inhibited corrosion current density, respectively, obtained by extrapolation of the anodic and cathodic Tafel lines to the corrosion potential. In the presence of the inhibitor, the steel is active and dissolves by a Tafel slope of 85 ± 5 mV. The cathodic process is determined by the cathodic reduction of all the components, namely, O2, NH4+ . The reduction of water molecules is probably an additional cathodic process. Metal ionization is suppressed by the inhibitor more strongly (Figure 7) than the cathodic process is stimulated, and the free corrosion potential of steel becomes significantly higher with an increase in C-NCBSA The results are presented in table 3. Table 3. The electrochemical parameters for carbon steel in the presence and absence of NCBSA and the percentage P obtained by galvanostatic polarization. NCBSA E c o r (mV) b a (mV) i cor (mA/dm 2 ) P ( %) concentration (ppm) 0 -596 85 99,7 0 50 -545 83 52,8 47 100 -538 87 33,7 66,2 150 -520 84 22,4 77,5 200 -511 87 16,3 83,6 © University of Manchester and the authors 2005. This is a preprint 13of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 It can be observed that the inhibition efficiency calculated from the weight loss measurements is close to that calculated from electrochemical measurements, especially at high concentrations of inhibitor. The surface analysis of the corroded samples was effected under a microscope STEM LEO 912 omega. In the absence of the inhibitor (Figure 8.1) the experiments show that the surface was covered with a great number of corrosion spots. In the presence of NCBSA the corrosion spots decrease in intensity at the same time with the increase in inhibitor concentration 200 ppm (Figure 8.2). This demonstrates that the metal surface is covered by a superficial layer that offers it a good protection. Figure 8. The microscopic analysis of the metal surface after the corrosion of carbon steel in a solution of NH 4Cl 10-1M in the absence or presence of NCBSA (x1000). Conclusions NCBSA is an inhibitor for the corrosion of carbon steel in a solution of NH4Cl 10-1M at a concentration of 200 ppm (7,56.10-4M) in the corrosive medium it has an efficiency of 81% obtained by weight loss measurements respectively 83.6% by electrochemical measurements. NCBSA acts through adsorption on the metal surface. On studying the inhibitor it has been found that the experimental data observe the Langmuir adsorption isotherm. ΔGºads is negative, which shows that the adsorption process of NCBSA on the metal surface is spontaneous. © University of Manchester and the authors 2005. This is a preprint 14of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 31 January 2005 By the microscopic analysis of the carbon-steel surface it has been found that the corrosion spots decrease in intensity in the presence of the inhibitor . This demonstrates the formation of a superficial layer that ensures a good protection to the metal. References 1. R.D. Braun, E.E. Lopez, D.P. Vollmer, Corr. 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This is a preprint 16of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 7 Preprint 27 37. K.S. Rajagobaian, G. Venkata Chari, Corrosion, 36(1980)320 38. W.J. Lorenz, F. Eichhoen, J. Electrochem. Soc., 112(1965)1255 39. A. Hickling, Electrochem. Acta , 18(1973)635 31 January 2005 © University of Manchester and the authors 2005. This is a preprint 17of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers’ comments, be published online at http://www. corrosion.jcse in due course. Until such time as it has been fully published it should not normally be referenced in published work.