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Thermodynamics and adsorption study of the corrosion inhibition of mild steel by Euphorbia heterophylla L. extract in 1.5 M HCl
Journal Pre-proof Thermodynamics and Adsorption Study of the Corrosion Inhibition of Mild Steel by Euphorbia heterophylla L. Extract in 1.5 M HCl Olatunde Alaba Akinbulumo, Oludare Johnson Odejobi, Ebenezer Leke Odekanle PII:
S2590-048X(20)30016-9
DOI:
https://doi.org/10.1016/j.rinma.2020.100074
Reference:
RINMA 100074
To appear in:
Results in Materials
Revised Date:
27 October 2019
Accepted Date: 15 November 2019
Please cite this article as: O.A. Akinbulumo, O.J. Odejobi, E.L. Odekanle, Thermodynamics and Adsorption Study of the Corrosion Inhibition of Mild Steel by Euphorbia heterophylla L. Extract in 1.5 M HCl, Results in Materials, https://doi.org/10.1016/j.rinma.2020.100074. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
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Thermodynamics and Adsorption Study of the Corrosion Inhibition of Mild Steel by Euphorbia heterophylla L. Extract in 1.5 M HCl Olatunde Alaba Akinbulumo1, Oludare Johnson Odejobi1, Ebenezer Leke Odekanle2 1
Applied Thermodynamics and Process Design Laboratory Chemical Engineering Department Obafemi Awolowo University, Ile- Ife, Nigeria 2 Chemical Engineering Department Landmark University, Omu Aran, Nigeria E-mail: [email protected]; [email protected]; [email protected]
Abstract This work investigates the thermodynamics parameters and adsorption mechanism of Euphorbia heterophylla L. extract as a corrosion inhibitor for mild steel in 1.5M HCl. The gravimetric method was used to determine the inhibition efficiency and corrosion rate. Relevant thermodynamic equations were employed to determine the activation energy, enthalpy change and entropy change. The adsorption isotherms were used to evaluate the Gibbs free energy change. Observation from the results of the study showed that the activation energy of an inhibited process was higher when compared with the uninhibited process. Also, the enthalpy change was positive and less than 100 kJ/mol threshold. The adsorption study showed that the data fit into the Langmuir, Flory-Huggins, El-Awary's and Temkin isotherms but Flory-Hugins gave the best fit. The Gibb's free energy change of adsorption was negative and less than the -20 KJ/mol threshold. As concluded from the results, the adsorption of Euphorbia heterophylla L. on mild steel in 1.5M HCl medium is feasible, spontaneous and it occurred by physical adsorption according to Flory-Hugins isotherm model. Keywords: Corrosion Thermodynamics; Corrosion inhibition, Inhibitor Adsorption; Euphorbia heterophylla L.; Corrosion rate; Mild steel.
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1.0
Introduction Corrosion of mild steel is a challenge in industrial processes because of exposure of this
material to corrosive acids, alkalis and salt solutions. This problem has necessitated increasing research interest on mitigating the damaging effects of corrosion on metals and their alloys [1, 2]. Organic materials inhibit corrosion by adsorption; hence, inhibitors with the ability to adsorb on the metal surface will hinder the dissolution or corrosion reaction of such metal in the corrosive medium. Natural plant extracts are effective green corrosion inhibitors against mild steel [3] and were found to be the cheaper, biodegradable, renewable, more efficient, and environmentally friendly method of inhibiting corrosion rate in mild steel [4, 5]. The knowledge of thermodynamic parameters associated with such inhibitors is fundamental to an understanding of their efficiency at inhibiting corrosion rate, under corrosive prone medium at a given temperature of the process. Adsorption and thermodynamic behavior of organic inhibitors, especially a plant extract on mild steel in the acidic medium has been widely reported [6-15]. The spontaneity of the adsorption process depends on the sign of Gibbs free energy of adsorption, ∆Gads. The magnitude and sign of ∆Gads also depend on the value of adsorption equilibrium constant Kads. When the rate of adsorption is higher than desorption, the ∆Gads is negative, which indicates spontaneous adsorption. ∆Gads value less than -20KJ/mol indicates physical adsorption while -40KJ/mol and above is an indication for chemical adsorption [6-13]. Manimegalai and Manjula, [10] reported that the adsorption of Sargassum swartzii on mild steel from aqueous medium obeys Langmuir, Temkin and Freundlich adsorption isotherms. They also reported that the ∆Gads value obtained indicated dynamic and spontaneous adsorption of the Sargassum swartzii extract components on the metal. Ramananda et al., [11] reported
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adsorption of Musa Paradisiaca extract as a green inhibitor for corrosion of mild steel in 0.5 M sulphuric acid solution. Adsorption of the inhibitor molecules of the extract on mild steel surface was found to obey Langmuir adsorption isotherm. An increase in the activation energies of the corrosion process indicated that Musa paradisiaca extract could inhibit the corrosion rate of mild steel in 0.5 M H2SO4 solution. The negative values of ∆Gads and ∆H revealed that the inhibition of corrosion of mild steel through adsorption is spontaneous and exothermic. The researchers concluded that both physical and chemical adsorption are involved in the adsorption process. Umoren et al. and Alinnor and Ejikeme, [12, 13] also reported negative ∆Gads adsorption for some other green inhibitors systems. Avci, [14] reported that a ∆H higher than 100 KJ/mol indicates chemical adsorption while ∆H less than 100 KJ/mol indicates physical adsorption. Singh et al., [16] reported the corrosion inhibition study of a series of hydrazones derived from thiophene derivatives. According to the authors, the inhibitor effectively reduced the corrosion rate according to Langmuir adsorption isotherm. The work of Arshad et al. [17] revealed stable, spontaneous corrosion inhibition by synthetic anti-biotic derivatives inhibitors on mild steel in acidic media. The values evaluated for free energy change assured the involvement of chemisorption process, and the adsorption data was found well fitted in Langmuir isotherm. Olusegun et al., [18] reported the use of Jatropha curcas leaves extract as a corrosion inhibitor for mild steel in 1M hydrochloric acid. Their investigation showed that Jatropha curcas had good inhibition potentials. The chemisorption adsorption mechanism was proposed based on the results that revealed an increase in inhibition efficiency with increase in temperature. Alaneme et al., [7] reported corrosion inhibition and adsorption mechanism studies of Hunteria umbellate seed husk extracts on mild steel immersed in 1M HCl and 1M H2SO4. Hunteria umbellate corrosion inhibition efficiency was found to be higher in 1M HCl than in 1M H2SO4. Chauhan and Gunasekaran, [6] reported corrosion inhibition potentials of Zenthoxylum
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alatum extract on mild steel in dilute HCl medium. The adsorption of the extract on the mild steel surface was reported to obey the Langmuir adsorption isotherm. Nwabanne and Okafor, [19] reported their work on adsorption and thermodynamics study of the inhibition of corrosion of mild steel in H2SO4 medium using Vernonia amygdalina. Singh et. al [20] studied the effect of Aloe Vera gel on mild steel corrosion in 1 M HCl medium, and the activation parameters showed that the inhibitor is adsorbed by both physisorption and chemisorption. The thermodynamics and kinetic inhibition of aluminum in hydrochloric acid medium by date palm leaf extract have also been studied, and the result showed that hot-water extract of date palm leaves has inhibition efficiency (IE) of 40-88% at the tested conditions [21]. Odejobi and Akinbulumo [22] reported their work on the modeling and optimization of inhibition efficiency of Euphorbia heterophylla for mild steel in hydrochloric acid medium, and it was revealed that Euphorbia heterophylla extract is an efficient corrosion inhibitor at the investigated conditions. Falodun et al. [23] reported the secondary metabolites present in Euphorbia heterophylla as saponins, tannins, flavonoids and carbohydrates. These metabolites have been studied and discovered to be efficient for the enhancement of the performance of plant corrosion inhibitors [ 24, 25]. There is a shortage of information on the thermodynamic parameters and adsorption characteristics of Euphorbia heterophylla extract as a corrosion inhibitor for mild steel in 1.5M HCl medium. Hence, this study addresses the evaluation of the thermodynamics and adsorption parameters such as activation energy, enthalpy change, entropy change, adsorption equilibrium constant and free energy change of corrosion inhibition potentials of Euphorbia heterophylla L on mild steel in acid medium. 2.0
Methodology
2.1
Determination of Inhibition Effect of the Euphorbia heterophylla L Extract
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Corrosion inhibition effect of Euphorbia heterophylla L extract on mild steel in 1.5M HCl medium was investigated by determining the inhibition efficiency and corrosion rate. The inhibition efficiency and corrosion rate were obtained from the gravimetric method as reported by Odejobi and Akinbulumo [22] using Equations (1) and (2), respectively [22, 26].
I .E =
w0 − w × 100 w0
CR =
87.6w AtD
(1)
(2)
w0 is the weight loss of the uninhibited mild steel in mg, w is weight loss of the inhibited mild steel in mg, C R is corrosion rate in mmyr-1, I.E is inhibition efficiency in %, A is the area of the mild steel in cm2, t is immersion time in hr, D is density of mild steel ( 7.86g/cm3) [19, 27-29]. 2.2
Determination of Activation Energy (Ea)
The plot of log of corrosion rate, CR against 1/T in Eq. (3) gives a slope of
− Ea from which 2.303 RT
the activation energy, Ea was estimated. The Arrhenius equation described the relationship between the corrosion rate ( C R ) and temperature (T) as [12, 30]:
log C R =
− Ea + log γ 2.303RT
(3)
Ea is the activation energy, R is the gas constant, T is the temperature in Kelvin and γ is the exponential factor. 2.3
Determination of Enthalpy and Entropy Change
Enthalpy change and entropy change were evaluated through an alternative formula for the Arrhenius equation in the transition state, given as [31]:
6
CR =
RT ∆S ∆H exp exp − Nh R RT
(4)
Equation (4) is linearized, thus:
log
CR ∆S ∆H R = log + logexp + log exp − T Nh R RT
Since log(exp( x )) =
(4b)
x ln 10
Therefore, Equation (4b) becomes:
log
CR R ∆S ∆H = log + + − T Nh R ln 10 RT ln 10
(4c)
CR − ∆H 1 R ∆S = + + log T 2.303R T Nh 2.303R
(5)
log
where h is Planck’s constant, N is the Avogadro’s number, ∆S is the entropy change and ∆H the enthalpy change.
The enthalpy change, ∆H was evaluated from the slope
− ∆H C 1 of the plot of log R against 2.303 R T T
R ∆S and the entropy change, ∆S was evaluated from the intercept log + of the same Nh 2.303R plot. 2.4
Adsorption Isotherm and Adsorption Constant
Adsorption isotherm studies give the descriptive mechanism on how the organic inhibitors adsorb to the metal surface [32]. The adsorption isotherm model that best describes the adsorption of Euphorbia heterophylla extract on mild steel in 1.5M HCl medium was obtained
7
by fitting the corrosion rate, CR and the degree of surface coverage of the inhibitor, θ (i.e the reciprocal of inhibition efficiency) into the various adsorption isotherms models (Langmuir, Tempkin, Frendlich, Frumkim, El Awady, and Flory Huggins adsorption isotherm) expressed in linear form as: The Langmuir adsorption isotherm model [19, 33]:
CR
θ
=
1 + CR K ads
(6)
Frumkim adsorption isotherm model [19]: θ log C R 1−θ
= 2αθ + 2.303 log K ads
(7)
El-Awady’s thermodynamic/kinetic adsorption isotherm model [6, 19]: θ log = y log C R + log K 1 −θ
(8)
K ads = K 1 y
Temkin adsorption isotherm model [13,19, 34]:
θ = ln C R + K ads
(9)
Freundlish adsorption isotherm [34, 35]: log θ = log K ads + n log C R
(10)
Flory-Huggins adsorption isotherm [35]: log
θ CR
= b log(1 − θ ) + log K ads
(11)
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2.5.
Determination of Adsorption Thermodynamics Parameters The Gibb's free energy change of adsorption, ∆Gads presented in Equation 12 [35] was
used to investigate the feasibility and nature of the adsorption. ∆Gads = − RT ln (55.5 K ads )
(12)
K ads is the adsorption equilibrium constant obtained from the isotherm, and the number 55.5 is the molar concentration of water in solution. 3.0
Results and Discussion
3.1
Inhibition Efficiency and Corrosion Rate Inhibition efficiency and corrosion rate for 1.0, 1.5 and 2.0 g/l extract concentration are
presented in Figures 1, 2 and 3, respectively. For 1.0 g/l extract concentration shown in Figure 1, inhibition efficiency increased with immersion time. The reason for this observation is because, at such a relatively low extract concentration, considerable time is required for sufficient adsorption to take place. The is also the reason for the lowest inhibition efficiency observed at 3 hours immersion time and highest at 7 hours immersion time. For each immersion time, inhibition efficiency was found to increase with temperature initially but later declined. The decline was sharpest at 7 hours immersion time possibly because the inhibitor could not withstand high temperature for such a long period. Distortion of the extract molecular structure might occur at a higher temperature. According to Dauda et al. [36], an increase in temperature may also increase the solubility of the protective films of the metals, thus, increasing the susceptibility of the metal to corrosion. The corrosion rate decreased with immersion time and increased with temperature. The observation is accurate because reaction rates generally decrease with time and increase with temperature. The increase in the rate of corrosion of mild steel could be due to the rise in the average kinetic energy of the reacting molecules. The trend obtained for the corrosion rate is in
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agreement with the result reported by Dauda et al. [36] in which the rate of corrosion of iron and steel was observed to increase with temperature. The same trend was observed with increasing extract concentration as presented in Figure 2 for 1.5 g/l and Figure 3 for 2.0 g/l extract concentration. In Figures 1, 2 and 3, it is observed that the inhibition efficiency decreased more sharply as the temperature increased. This observation is common with most organic inhibitors [ 25] because, in inhibition reactions, at a high temperature many changes may occur on the metal surface, such as rapid etching, rupture, desorption of the inhibitor, decomposition and or rearrangement of the inhibitor [30]. Therefore, temperature variation affects the metal dissolution, inhibitor adsorption/desorption and inhibitor efficiency. 3.2
Activation Energy (Ea) Presented in Figures 4 to 6 are the plots of log CR against 1/T for 1.0, 1.5 and 2.0 g/l
extract concentrations at different immersion times. The perfect fit observed for the data points is an indication of the consistency of the results of the experimental data. The slopes obtained from the plots are therefore appropriate in estimating the activation energy of the process for different immersion time. The activation energy, Ea, got at different extract concentration for 3, 5 and 7 hours immersion time are presented in Tables 1, 2 and 3, respectively. As shown in the tables, the activation energy is higher for the inhibited process than for an uninhibited process. This observation shows that the adsorption process is by physisorption [37, 38]. The higher value of Ea in the presence of an inhibitor was due to the high-energy barrier. This incidence also confirms the formation of a complex compound between the inhibitor and of mild steel [ 38]. This result suggests that the corrosion inhibition by Euphorbia heterophylla is feasible because of the increase energy barrier for the metal dissolution. The formation of a thin film on the metal surface serves as a barrier to both energy and mass transfer, which increases the
10
activation energy. Therefore, the result shows that the adsorption of Euphorbia heterophylla on mild steel is by physical adsorption. As shown in the tables, the activation energy decreased as inhibitor concentration increased. However, the activation energy was still higher than that of the uninhibited solution. These results are in agreement with previous researchers [39 - 42]. For 1.0 and 1.5 g/l extract concentration, the activation energy at 5 hours of immersion time was the highest after which it began to decline. This result suggests that the activation energy increased with immersion time until 5 hours after which the Euphorbia heterophylla extract might become less potent in creating an energy barrier. Organic inhibitors can lose their inhibition potentials over time because they are sometimes biologically degraded, and they sometimes react with the environment and go into solution. For 2.0 g/l extract concentration, the activation energy does not decline after 5 hours probably because the concentration is high enough to overcome the effect of time. 3.2
Enthalpy and Entropy Change The values of enthalpy change, ∆H and entropy change, ∆S obtained at different
immersion time are also given in Tables 1 to 3. The ∆H value got was positive and observed to increase with immersion time but decreased with inhibitor concentration. However, the value of entropy change on the other hands was negative when the immersion time was 3 hours. This result indicates that a decrease in disorderliness took place on going from reactants to the activated complex. This result agrees with the work by Sudhish and Eno [37]. Further observation showed that the Ea values are more significant than the values of ∆H. This result indicates that the corrosion process must have involved a gaseous reaction [41]. The average value of the difference between activation energy and enthalpy change (Ea – ∆H) is 2.73 kJ mol-1 for the 3 hours immersion time, which is the exact average value of the product of gas constant (R=8.314 Jmol-1K-1) and the temperature of the experiment (T=328 K). That is, ܧ −
11
∆ܴܶ = ܪ. This result agrees with the work by Noor [41, 42]. He reported that such corrosion process is a unimolecular reaction with the evolution of hydrogen gas. 3.3
Adsorption Isotherms Figures 7 to 11 show the various adsorption isotherm models considered in this study.
The R2 values for each isotherm model presented in Table 4 were used to determine the most suitable model. The data fit into Langmuir, Flory-Huggins, El-Awary's and Temkin isotherms but Flory-Hugins gives the best fit. Flory-Huggins isotherm with R2 values of about 0.99, best describes the adsorption mechanism of Euphorbia heterophylla extract on mild steel in hydrochloric acid medium. Therefore, Flory-Hugins adsorption isotherm is appropriate for evaluating the adsorption equilibrium constant, K ads . 3.4
Adsorption Equilibrium Constant Table 5 shows the adsorption equilibrium constants obtained at various temperatures
from the intercept of the Flory-Huggins plot. The adsorption equilibrium constants, K ads are positive, indicating the feasibility of the adsorption of the inhibitor to the metal surface. The adsorption equilibrium constant increased with temperature up till 333K. However, at 343K, K ads declines, according to Hegazy [ 43], this indicates that at a higher temperature, the adsorbed inhibitor tends to desorb back from the mild steel surface. This result further confirms the trend obtained for activation energy. 3.5
Gibb’s Free Energy Change of Adsorption Presented in Table 5 is Gibb's free energy change of adsorption, ∆Gads got at different
temperatures. As shown in the table, the ∆Gads are negative and less than 20 kJ/mol. This result indicates that adsorption of the extract of Euphorbia heterophylla on mild steel surface is spontaneous, feasible and occurred according to the mechanism of physical adsorption. The
12
decrease in ∆Gads at 343K implies there is a reduction in the spontaneity and stability of the adsorption at a higher temperature. 3.6 Conclusion The study concluded that the adsorption of Euphorbia heterophylla L. on mild steel in 1.5M HCl medium is feasible, spontaneous and by physical adsorption according to FloryHugins isotherm model. Euphorbia heterophylla L extract is more effective in inhibiting mild steel corrosion in acidic medium at temperature below 343K.
References 1. C. A. Loto, A. I. Mohammed, The effect of anacardium occidentale (cashew) juice extract on the corrosion of mild steel in hydrochloric acid, J. Corros. Prevent. & Contr., 2, (2000), 50- 63. 2. Singh, A. K., Chugh B., Saha S. K., Banerjee P., Ebenso E. E., Thakur S., Pani B. Evaluation of anti-corrosion performance of an expired semi synthetic antibiotic cefdinir for mild steel in 1 M HCl medium: An experimental and theoretical study, Results in Physics, https://doi.org/10.1016/j.rinp.2019.1023 3. M. Chigondo M., F. Chigondo, Recent Natural Corrosion Inhibitors for Mild Steel: An Overview, J. Chem. (2016), Article ID 6208937, 1-7, http://dx.doi.org/10.1155/2016/6208937 4. N. Kavitha, P. Manjula, N. Anandha, Syneristic effect of C. papaya leaves extract-Zn2+ in corrosion inhibition of mild steel in aqueous medium, Resear. J. Chem. Sci., 8, (2014), 88-93. 5. C. N. Njoku, O. E. Onyelucheya, Response surface optimization of the inhibition efficiency of Gongronema latifolium as an inhibitor for aluminium corrosion in HCl Solutions, Int. J. Mater. Chem., 1, (2015), 4-13. 6. L. R. Chauhan, G. Gunasekaran, Corrosion inhibition of mild steel by plant extract in dilute HCl medium, Corros. Sci., 9, (2006), 1143-1161.
13
7. K. K. Alaneme, S. J. Olusegun, O. T. Adelowo, Corrosion inhibition and adsorption mechanism studies of Hunteria umbellata seed husk extract on mild steel immersed in acidic solutions, Alexandria Eng. J., 10, (2015), 45-51. 8. X. Li, S. Deng, H. Fu, Inhibition by tetra decylpyridinium bromide of the corrosion of aluminum in hydrochloric acid solution, Corros. Sci., 53, (2011), 1529-1536. 9. A. Y. Musa, A. A. H. Kadhum, A. R. Daud, A. B. Mohamad, M. S. Takriff, S.K. Kamarudin, A comparative study of the corrosion inhibition of mild steel in sulphuric acid by 4, 4-Dimethyloxazolidine-2-thione, Corros. Sci., 51, (2009), 2393–2399. 10. S. Manimegalai, P. Manjula, Thermodynamic and adsorption studies for corrosion inhibition of mild steel in aqueous media by Sargasam swartzii (Brown algae), J. Mater. Environ. Sci., 6, (2015), 1629-1637. 11. S. M. Ramananda, S. Vivek, S. Gurmeet, Musa paradisiaca extract as a green inhibitor for corrosion of mild steel in 0.5 M sulphuric acid Solution, Portugaliae Electrochimica Acta, 29, (2011), 405-417. 12. S.A. Umoren, I.B. Obot and E.E. Ebenso, Corrosion inhibition of aluminum using exudates gum from Pachylobus Edulis in the presence of halide ions in HCl, E. J. Chem., 5, (2008), 355-364. 13. I. J. Alinnor, and P. M. Ejikeme, Corrosion inhibition of aluminum in acidic medium by different extracts of Ocimum gratissimum, Ameri. Chemi. Sci. J., 2, (2012), 122-135. 14. G. Avci, Corrosion inhibition of indole-3-acetic acid on mild steel in 0.5 M HCl, Colloids and Surfaces A: Physicochem. Eng. Aspects 317, (2008), 730–736. 15. Singh A. K., Thakur S., Pani B. and Singh G. Green synthesis and corrosion inhibition study of 2-amino-N′-((thiophen-2-yl) methylene) benzohydrazide New J. Chem. 42, 2113-2124 16. A. K. Singh, S. Thakur, B. Pani, B. Chugh, J. Singh, Solvent-free microwave assisted synthesis and corrosion inhibition study of a series of hydrazones derived from thiophene derivatives: Experimental, surface and theoretical study, J. Molecular Liquids 283, (2019), 788-803 17. N. Arshad, A. K. Singh, B. Chugh, Experimental, theoretical, and surface study for corrosion inhibition of mild steel in 1 M HCl by using synthetic anti-biotic derivatives, Ionics, (2019) 1- 19, https://doi.org/10.1007/s11581-019-03028-y 18. S. J. Olusegun, B. A. Adeiza, K. I. Ikeke, M. O. Bodunrin, Jatropha curcas leaves as corrosion inhibitors for mild steel in 1M hydrochloric acid. J. Emerging Trends in Eng. Appl. Sci., 4, (2013), 138-143.
14
19. J. T. Nwabanne, V. N. Okafor, Adsorption and thermodynamics study of the inhibition of corrosion of mild steel in H2SO4 medium using Vernonia amygdalina, J. Minerals Mater. Characteri. Eng., 11, (2012), 885-890. 20. Singh A. K, Mohapatra S, Pani B. Corrosion inhibition effect of Aloe Vera gel: gravimetric and electrochemical study. J. Ind. Eng. Chem. 2016, 33, 288–297. 21. A. M. Al-Haj-Ali, N. A. Jarrah, N. D. Mu'azu, R. O. Rihan, Thermodynamics and kinetics of inhibition of aluminum in hydrochloric acid by date palm leaf extract J. Appl. Sci. Environ. Mgt. 18, (2014), 543 -551. 22. O. J. Odejobi, O. A. Akinbulumo, Modeling and optimization of the inhibition efficiency of Euphorbia heterophylla extracts based corrosion inhibitor of mild steel corrosion in HCl media using a response surface methodology, J. Chem. Technol. Metall., 54, 1, (2019), 217-232 23. A. Falodun, S. Ali, I. M. Quadir and M. I. C., Iqbal Phytochemical and biological investigation of chloroform and ethyl acetate fractions of Euphorbia heterophylla leaf (Euphorbiaceae), J. Medicinal Plants Research 2, 12, (2008) 365-369 24. H. Kaco, N. A. A. Talib, S, Zakaria, S. N. S. Jaafar, N. K. Othman, C. H.Chia and S. Gan, Enhanced Corrosion Inhibition Using Purified Tannin In HCl Medium Malaysian J. Analyt. Sci., 22, 6 (2018) 931-942 https://doi.org/10.17576/mjas-2018-2206-02 25. R. Amitha, B. Bharathi, J. Basu, Green Inhibitors for Corrosion Protection of Metals and Alloys: An Overview, Int. J. Corr. (2012), Article ID 380217,15pages doi:10.1155/2012/380217 26. M. Hegde, S. P. Nayak Aqueous extract of Dillenia PentagynaFruit as green inhibitor for mild steel corrosion in 0.5 M hydrochloric acid solution. J. Mater. Environ. Sci., Volume 10, Issue 1, (2019) Page 22-31 27. E. Osarolube, I.O. Owate, N.C. Oforka, The influence of acidic concentration on corrosion of copper and zinc, J. Corros. Sci. Tech. , 11, (2004), 66-69. 28. C.A. Loto, O. O. Joseph, R.T. Loto and A.P.I. Popoola, Inhibition effect of Vernonia amygdalina extract on the corrosion of mild steel reinforcement in concrete in 3.5M NaCl environment, Int. J. Electrochem. Sci., 8, (2013), 11087 – 11100. 29. O. O. Ige, Chrysophyllum albidium extracts as corrosion inhibition for aluminium in 0.5M H2SO4 solution, Afric. Corros. J., 2, (2015) 25-30. 30. M.M. Solomon, S.A. Umoren, I. I. Udosoro, A.P. Udoh, Inhibitive and adsorption behaviour of carboxymethyl cellulose on mild steel corrosion in sulphuric acid solution, Corro. Sci. 52, (2010), 1317–1325.
15
31. E. Ebenso, Effect of halide ions on the corrosion inhibition of mild steel in H2SO4 using methyl red. Bull. Electrochem., 19, (2003), 209-216. 32. M. G. Tsoeunyane, M. E. Makhatha, O. A. Arotiba Corrosion Inhibition of Mild Steel by Poly(butylene succinate)-L-histidine Extended with 1,6-diisocynatohexane Polymer Composite in 1 M HCl, Int. J. Corr., (2019)., Article ID 7406409, 12 pages, https://doi.org/10.1155/2019/7406409. 33. O. Dagdag, Z. Safi, R. Hsissou, H. Erramli, M. El Bouchti, N. Wazzan, L. Guo, C. Verma, E. E. Ebenso A. El Harfi, Epoxy pre-polymers as new and effective materials for corrosion inhibition of carbon steel in acidic medium: Computational and experimental studies, Scientific reports 9, 1, (2019). 1-14 34. K. K. Alaneme, S. J. Olusegun, Corrosion inhibition performance of lignin extract of sunflower (Tithonia Diversifolia) on medium carbon low alloy steel immersed in HCl solution, Leonardo J. Sci. 20, (2012), 59-70. 35. E.E. Ebenso, H. Alamu, S.A. Umoren, I.B. Obot, Inhibition of mild steel corrosion in sulphuric acid using alizarin yellow dye and synergistic iodide additive, Int. J. Electrochem. Sci., (2008), 1325-1339. 36. K. T. Dauda, V. N. Atasie, and A. A. Adetimilehin, Kinetics of the Inhibitory Effects of Ethanolic Extract of Vernonia amydgalina on the Corrosion of Aluminium in 1.0 M HCl. Chem. J., 3, (2013), 86-89. 37. K. S. Sudhish and E. E. Eno, Corrosion inhibition adsorption behavior and thermodynamic properties of streptomycin on mild steel in hydrochloric acid medium. Int. J. Electrochem. Sci., (2011), 3277-3291. 38. M. Erna , H. Herdini , D. Futra, Corrosion Inhibition Mechanism of Mild Steel by Amylose-Acetate/Carboxymethyl Chitosan Composites in Acidic Media int. J. Chem. Engg. Volume (2019), Article ID 8514132, 12 pages, https://doi.org/10.1155/2019/8514132 39. L.B. Tang, G.N. Mu, G.H. Liu, The effect of neutral red on the corrosion inhibition of cold-rolled steel in 1.0 M hydrochloric acid, Corros. Sci., 45, (2003), 2251-2262. 40. L. Tang, X. Li, L. Li, G. Mu, G. Liu, The effect of 1-(2-pyridylazo)-2-naphthol on the corrosion of cold-rolled steel in acid media: Part 2: inhibitive action in 0.5 M sulfuric acid, Mater. Chem. Phys., 97, (2006), 301-307. 41. E. A. Noor, Temperature effects on the corrosion inhibition of mild steel in acidic solutions by aqueous extract of fenugreek Leaves. Int. J. Electrochem. Sci., 3, (2007), 996-1017. 42. E. Noor, Potential of aqueous extract of hibiscus sabdariffa leaves for inhibiting the corrosion of aluminium in alkaline solutions. J. Appl. Electrochem., 39, (2009), 14651475. 43. M. A. Hegazy, H. M. Ahmed and A. S. El-Tabei, Investigation of the inhibitive effect of substituted 4-(N, N, N-d imet hyl dodecyl ammonium bromide) benzylidene-benzene-2-
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ylamine on corrosion of carbon steel pipelines in acidic medium, Corros. Sci., 53, (2011), 671-678.
Inhibition Efficiency (%)
70 200
60 50
150
40 100
30 20
50
10 0
Corrosion Rate (mm/yr)
250
80
0 310
320
330
340
60
250
50
200
40 150 30 100
20
50
10 0
0 320
330
340
Temperature (K)
350
Corrosion Rate (mm/yr)
Inhibition Efficiency (%)
300
Figure 2:
I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion
Inhibition efficiency and corrosion rate for 1.0 g/l extract concentration
70
310
I. E (%) at 5 hours immersion
C. Rate (mm/yr) at 7 hours immersion
350
Temperature (K) Figure 1:
I. E (%) at 3 hours immersion
I. E (%) at 3 hours immersion I. E (%) at 5 hours immersion I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion C. Rate (mm/yr) at 7 hours immersion
Inhibition efficiency and corrosion rate for 1.5 g/l extract concentration
17
400 350
50
300 40
250
30
200 150
20
100 10
50
0
0 310
320
330
340
350
Corrosion Rate (mm/yr)
Inhibition Efficiency (%)
60
I. E (%) at 3 hours immersion I. E (%) at 5 hours immersion I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion C. Rate (mm/yr) at 7 hours immersion
Temperature (K) Figure 3:
Inhibition efficiency and corrosion rate for 2.0 g/l extract concentration
at 3 hrs Immersion Time slope= -3214 R² = 0.998 3
at 5 hrs Immersion Time slope= -6459 R² = 0.928
at 7 hrs Immersion Time slope= -6307 R² = 0.919
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T (K-1)
Figure 4:
Plot of log CR versus 1/T for 1.0 g/l extract concentration
18 at 3 hrs Immersion Time slope= -3179 R² = 0.993 3
at 5 hrs Immersion Time slope= -5520 R² = 0.925
at 7 hrs Immersion Time slope= -4793 R² = 0.966
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 5:
Plot of log CR versus 1/T for 1.5g/l extract concentration
at 3 hrs Immersion Time slope = -2433.9 R² = 0.9975 3
at 5 hrs Immersion Time slope = -3150.3 R² = 0.9854
at 7 hrs Immersion Time slope = -4537.4 R² = 0.9418
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 6:
Plot of log CR versus 1/T for 2.0g/l extract concentration
19 12 10
C/Ө
8 313K 6
323K
4
333K 343K
2 0 0
0.5
1
1.5
2
2.5
C (g/l)
Figure 7:
Langmuir Adsorption Isotherms
0.6 0.4
log(Ө/1-Ө)
0.2 313K
0 -0.2
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
323K 333K
-0.4
343K
-0.6 -0.8
Figure 8:
log C
El-Awady’s Thermodynamic/Kinetic Adsorption Isotherm
20 0.8 0.7 0.6 Ө
0.5
313K
0.4
323K
0.3 0.2
333K
0.1
343K
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
ln CR
Figure 9:
Temkin Adsorption Isotherm
.
0 -0.1 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
log Ө
-0.2 -0.3
313K
-0.4
323K
-0.5
333K
-0.6
343K
-0.7 -0.8
Figure 10:
log C
Freundlich Isotherm
21 0 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0 -0.2
log(Ѳ/C)
-0.4 -0.6
40C/313K 50C/323K 60C/333K
-0.8
70C/343K
-1 -1.2
log(1-Ѳ) Figure 11:
Flory-Huggins Isotherms
Table 1:
Activation parameters at 3 hours immersion time
Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
27.51
-
-
1.0 g/l
61.54
58.78
-30.8
1.5 g/l
60.87
58.15
-43.75
2.0 g/l
46.59
43.89
-69.3
22
Table 2:
Activation parameters at 5 hours immersion time
Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
31.06
-
-
1.0 g/l
123.67
80.48
31.8
1.5 g/l
105.69
62.17
-14.72
2.0 g/l
60.31
57.59
-29.5
Ta ble 3:
Act iva
tion parameters at 7 hours immersion time Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
33.87
-
-
1.0 g/l
120.76
119.73
146.3
1.5 g/l
91.77
88.99
63.02
2.0 g/l
86.87
84.15
48.5
Table 4:
R2 Values for the Various Adsorption Isotherms Considered
23
Langmuir
Frumkin
FloryHuggins
ElAwary’s
Freundkich Temkin
R2
Temperature 313K
0.975
0.209
0.978
0.916
0.883
0.907
323K
0.967
0.780
0.994
0.942
0.901
0.931
333K
0.947
0.901
0.997
0.946
0.904
0.945
343K
0.872
0.920
0.992
0.922
0.879
0.954
Table 5:
Adsorption parameters
Tempt (K)
Kads (mol-1)
313
0.0526
-2.79
323
0.0687
-3.59
333
0.0689
-3.71
343
0.0515
-2.98
∆Gads (KJ /mol)
Table 1 Extract Conc.
Activation parameters at 3 hours immersion time Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
27.51
-
-
1.0g/l
61.54
58.78
-30.8
1.5g/l
60.87
58.15
-43.75
2.0g/l
46.59
43.89
-69.3
Table 2 Extract Conc.
Activation parameters at 5 hours immersion time Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
31.06
-
-
1.0g/l
123.67
80.48
31.8
1.5g/l
105.69
62.17
-14.72
2.0g/l
60.31
57.59
-29.5
Table 3
Activation parameters at 7 hours immersion time
Extract Conc.
Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
33.87
-
-
1.0g/l
120.76
119.73
146.3
1.5g/l
91.77
88.99
63.02
2.0g/l
86.87
84.15
48.5
Table 4
R2 Values for the Various Adsorption Isotherms Considered Langmuir
Frumkin
Flory-
El-
Huggins
Awary’s
Freundkich Temkin
R2
Temperature 313K
0.975
0.209
0.978
0.916
0.883
0.907
323K
0.967
0.780
0.994
0.942
0.901
0.931
333K
0.947
0.901
0.997
0.946
0.904
0.945
343K
0.872
0.920
0.992
0.922
0.879
0.954
Table 5 Adsorption parameters Tempt (K)
K ads (mol-1)
313
0.0526
-2.79
323
0.0687
-3.59
333
0.0689
-3.71
343
0.0515
-2.98
∆Gads (KJ /mol)
at 3 hrs Immersion Time slope= -3214 R² = 0.998 3
at 5 hrs Immersion Time slope= -6459 R² = 0.928
at 7 hrs Immersion Time slope= -6307 R² = 0.919
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 1
Plot of log CR versus 1/T for 1.0g/l extract concentration
at 3 hrs Immersion Time slope= -3179 R² = 0.993 3
at 5 hrs Immersion Time slope= -5520 R² = 0.925
at 7 hrs Immersion Time slope= -4793 R² = 0.966
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 2
Plot of log CR versus 1/T for 1.5g/l extract concentration
at 3 hrs Immersion Time y = -2433.9x + 9.6503 R² = 0.9975 3
at 5 hrs Immersion Time y = -3150.3x + 11.726 R² = 0.9854
at 7 hrs Immersion Time y = -4537.4x + 15.806 R² = 0.9418
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Plot of log CR versus 1/T for 2.0g/l extract concentration
Figure 3
12 10
C/Ө
8 313K 6
323K
4
333K 343K
2 0 0
0.5
1
1.5 C (g/l)
Figure 4 Langmuir Adsorption Isotherms
2
2.5
0.6 0.4
log(Ө/1-Ө)
0.2 313K
0 -0.2
0
0.05
0.1
0.15
0.2
0.25
0.3
323K
0.35
333K
-0.4
343K
-0.6 -0.8
log C
Figure 5
El-Awady’s Thermodynamic/Kinetic Adsorption Isotherm
0.8 0.7 0.6 Ө
0.5
313K
0.4
323K
0.3 0.2
333K
0.1
343K
0 0
0.1
0.2
0.3
0.4
ln C
Figure 6
Temkin Adsorption Isotherm
0.5
0.6
0.7
0.8
.
0 -0.1 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
log Ө
-0.2 -0.3
313K
-0.4
323K
-0.5
333K
-0.6
343K
-0.7 -0.8
Figure 7
log C
Freundlich Isotherm
0 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0 -0.2
log(Ѳ/C)
-0.4 -0.6
50C/323K 60C/333K
-0.8 -1
log(1-Ѳ) Figure 8
40C/313K
Flory-Huggins Isotherms
-1.2
70C/343K
Highlights The activation energy of inhibited process was found to be greater than that of the uninhibited. The enthalpy change is positive and less than 100 kJ/mol. The adsorption study showed that the data fit into Langmuir, Flory-Huggins, El-Awary’s and Temkin isotherms but Flory-Hugins gives the best fit. The Gibb’s free energy change of adsorption was negative and less than the -20 KJ/mol. It is concluded from the results that adsorption of Euphorbia heterophylla on mild steel in hydrochloric acid medium is feasible, spontaneous and by physical adsorption according to Flory-Hugins isotherm model.
S2590-048X(20)30016-9
DOI:
https://doi.org/10.1016/j.rinma.2020.100074
Reference:
RINMA 100074
To appear in:
Results in Materials
Revised Date:
27 October 2019
Accepted Date: 15 November 2019
Please cite this article as: O.A. Akinbulumo, O.J. Odejobi, E.L. Odekanle, Thermodynamics and Adsorption Study of the Corrosion Inhibition of Mild Steel by Euphorbia heterophylla L. Extract in 1.5 M HCl, Results in Materials, https://doi.org/10.1016/j.rinma.2020.100074. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
1
Thermodynamics and Adsorption Study of the Corrosion Inhibition of Mild Steel by Euphorbia heterophylla L. Extract in 1.5 M HCl Olatunde Alaba Akinbulumo1, Oludare Johnson Odejobi1, Ebenezer Leke Odekanle2 1
Applied Thermodynamics and Process Design Laboratory Chemical Engineering Department Obafemi Awolowo University, Ile- Ife, Nigeria 2 Chemical Engineering Department Landmark University, Omu Aran, Nigeria E-mail: [email protected]; [email protected]; [email protected]
Abstract This work investigates the thermodynamics parameters and adsorption mechanism of Euphorbia heterophylla L. extract as a corrosion inhibitor for mild steel in 1.5M HCl. The gravimetric method was used to determine the inhibition efficiency and corrosion rate. Relevant thermodynamic equations were employed to determine the activation energy, enthalpy change and entropy change. The adsorption isotherms were used to evaluate the Gibbs free energy change. Observation from the results of the study showed that the activation energy of an inhibited process was higher when compared with the uninhibited process. Also, the enthalpy change was positive and less than 100 kJ/mol threshold. The adsorption study showed that the data fit into the Langmuir, Flory-Huggins, El-Awary's and Temkin isotherms but Flory-Hugins gave the best fit. The Gibb's free energy change of adsorption was negative and less than the -20 KJ/mol threshold. As concluded from the results, the adsorption of Euphorbia heterophylla L. on mild steel in 1.5M HCl medium is feasible, spontaneous and it occurred by physical adsorption according to Flory-Hugins isotherm model. Keywords: Corrosion Thermodynamics; Corrosion inhibition, Inhibitor Adsorption; Euphorbia heterophylla L.; Corrosion rate; Mild steel.
2
1.0
Introduction Corrosion of mild steel is a challenge in industrial processes because of exposure of this
material to corrosive acids, alkalis and salt solutions. This problem has necessitated increasing research interest on mitigating the damaging effects of corrosion on metals and their alloys [1, 2]. Organic materials inhibit corrosion by adsorption; hence, inhibitors with the ability to adsorb on the metal surface will hinder the dissolution or corrosion reaction of such metal in the corrosive medium. Natural plant extracts are effective green corrosion inhibitors against mild steel [3] and were found to be the cheaper, biodegradable, renewable, more efficient, and environmentally friendly method of inhibiting corrosion rate in mild steel [4, 5]. The knowledge of thermodynamic parameters associated with such inhibitors is fundamental to an understanding of their efficiency at inhibiting corrosion rate, under corrosive prone medium at a given temperature of the process. Adsorption and thermodynamic behavior of organic inhibitors, especially a plant extract on mild steel in the acidic medium has been widely reported [6-15]. The spontaneity of the adsorption process depends on the sign of Gibbs free energy of adsorption, ∆Gads. The magnitude and sign of ∆Gads also depend on the value of adsorption equilibrium constant Kads. When the rate of adsorption is higher than desorption, the ∆Gads is negative, which indicates spontaneous adsorption. ∆Gads value less than -20KJ/mol indicates physical adsorption while -40KJ/mol and above is an indication for chemical adsorption [6-13]. Manimegalai and Manjula, [10] reported that the adsorption of Sargassum swartzii on mild steel from aqueous medium obeys Langmuir, Temkin and Freundlich adsorption isotherms. They also reported that the ∆Gads value obtained indicated dynamic and spontaneous adsorption of the Sargassum swartzii extract components on the metal. Ramananda et al., [11] reported
3
adsorption of Musa Paradisiaca extract as a green inhibitor for corrosion of mild steel in 0.5 M sulphuric acid solution. Adsorption of the inhibitor molecules of the extract on mild steel surface was found to obey Langmuir adsorption isotherm. An increase in the activation energies of the corrosion process indicated that Musa paradisiaca extract could inhibit the corrosion rate of mild steel in 0.5 M H2SO4 solution. The negative values of ∆Gads and ∆H revealed that the inhibition of corrosion of mild steel through adsorption is spontaneous and exothermic. The researchers concluded that both physical and chemical adsorption are involved in the adsorption process. Umoren et al. and Alinnor and Ejikeme, [12, 13] also reported negative ∆Gads adsorption for some other green inhibitors systems. Avci, [14] reported that a ∆H higher than 100 KJ/mol indicates chemical adsorption while ∆H less than 100 KJ/mol indicates physical adsorption. Singh et al., [16] reported the corrosion inhibition study of a series of hydrazones derived from thiophene derivatives. According to the authors, the inhibitor effectively reduced the corrosion rate according to Langmuir adsorption isotherm. The work of Arshad et al. [17] revealed stable, spontaneous corrosion inhibition by synthetic anti-biotic derivatives inhibitors on mild steel in acidic media. The values evaluated for free energy change assured the involvement of chemisorption process, and the adsorption data was found well fitted in Langmuir isotherm. Olusegun et al., [18] reported the use of Jatropha curcas leaves extract as a corrosion inhibitor for mild steel in 1M hydrochloric acid. Their investigation showed that Jatropha curcas had good inhibition potentials. The chemisorption adsorption mechanism was proposed based on the results that revealed an increase in inhibition efficiency with increase in temperature. Alaneme et al., [7] reported corrosion inhibition and adsorption mechanism studies of Hunteria umbellate seed husk extracts on mild steel immersed in 1M HCl and 1M H2SO4. Hunteria umbellate corrosion inhibition efficiency was found to be higher in 1M HCl than in 1M H2SO4. Chauhan and Gunasekaran, [6] reported corrosion inhibition potentials of Zenthoxylum
4
alatum extract on mild steel in dilute HCl medium. The adsorption of the extract on the mild steel surface was reported to obey the Langmuir adsorption isotherm. Nwabanne and Okafor, [19] reported their work on adsorption and thermodynamics study of the inhibition of corrosion of mild steel in H2SO4 medium using Vernonia amygdalina. Singh et. al [20] studied the effect of Aloe Vera gel on mild steel corrosion in 1 M HCl medium, and the activation parameters showed that the inhibitor is adsorbed by both physisorption and chemisorption. The thermodynamics and kinetic inhibition of aluminum in hydrochloric acid medium by date palm leaf extract have also been studied, and the result showed that hot-water extract of date palm leaves has inhibition efficiency (IE) of 40-88% at the tested conditions [21]. Odejobi and Akinbulumo [22] reported their work on the modeling and optimization of inhibition efficiency of Euphorbia heterophylla for mild steel in hydrochloric acid medium, and it was revealed that Euphorbia heterophylla extract is an efficient corrosion inhibitor at the investigated conditions. Falodun et al. [23] reported the secondary metabolites present in Euphorbia heterophylla as saponins, tannins, flavonoids and carbohydrates. These metabolites have been studied and discovered to be efficient for the enhancement of the performance of plant corrosion inhibitors [ 24, 25]. There is a shortage of information on the thermodynamic parameters and adsorption characteristics of Euphorbia heterophylla extract as a corrosion inhibitor for mild steel in 1.5M HCl medium. Hence, this study addresses the evaluation of the thermodynamics and adsorption parameters such as activation energy, enthalpy change, entropy change, adsorption equilibrium constant and free energy change of corrosion inhibition potentials of Euphorbia heterophylla L on mild steel in acid medium. 2.0
Methodology
2.1
Determination of Inhibition Effect of the Euphorbia heterophylla L Extract
5
Corrosion inhibition effect of Euphorbia heterophylla L extract on mild steel in 1.5M HCl medium was investigated by determining the inhibition efficiency and corrosion rate. The inhibition efficiency and corrosion rate were obtained from the gravimetric method as reported by Odejobi and Akinbulumo [22] using Equations (1) and (2), respectively [22, 26].
I .E =
w0 − w × 100 w0
CR =
87.6w AtD
(1)
(2)
w0 is the weight loss of the uninhibited mild steel in mg, w is weight loss of the inhibited mild steel in mg, C R is corrosion rate in mmyr-1, I.E is inhibition efficiency in %, A is the area of the mild steel in cm2, t is immersion time in hr, D is density of mild steel ( 7.86g/cm3) [19, 27-29]. 2.2
Determination of Activation Energy (Ea)
The plot of log of corrosion rate, CR against 1/T in Eq. (3) gives a slope of
− Ea from which 2.303 RT
the activation energy, Ea was estimated. The Arrhenius equation described the relationship between the corrosion rate ( C R ) and temperature (T) as [12, 30]:
log C R =
− Ea + log γ 2.303RT
(3)
Ea is the activation energy, R is the gas constant, T is the temperature in Kelvin and γ is the exponential factor. 2.3
Determination of Enthalpy and Entropy Change
Enthalpy change and entropy change were evaluated through an alternative formula for the Arrhenius equation in the transition state, given as [31]:
6
CR =
RT ∆S ∆H exp exp − Nh R RT
(4)
Equation (4) is linearized, thus:
log
CR ∆S ∆H R = log + logexp + log exp − T Nh R RT
Since log(exp( x )) =
(4b)
x ln 10
Therefore, Equation (4b) becomes:
log
CR R ∆S ∆H = log + + − T Nh R ln 10 RT ln 10
(4c)
CR − ∆H 1 R ∆S = + + log T 2.303R T Nh 2.303R
(5)
log
where h is Planck’s constant, N is the Avogadro’s number, ∆S is the entropy change and ∆H the enthalpy change.
The enthalpy change, ∆H was evaluated from the slope
− ∆H C 1 of the plot of log R against 2.303 R T T
R ∆S and the entropy change, ∆S was evaluated from the intercept log + of the same Nh 2.303R plot. 2.4
Adsorption Isotherm and Adsorption Constant
Adsorption isotherm studies give the descriptive mechanism on how the organic inhibitors adsorb to the metal surface [32]. The adsorption isotherm model that best describes the adsorption of Euphorbia heterophylla extract on mild steel in 1.5M HCl medium was obtained
7
by fitting the corrosion rate, CR and the degree of surface coverage of the inhibitor, θ (i.e the reciprocal of inhibition efficiency) into the various adsorption isotherms models (Langmuir, Tempkin, Frendlich, Frumkim, El Awady, and Flory Huggins adsorption isotherm) expressed in linear form as: The Langmuir adsorption isotherm model [19, 33]:
CR
θ
=
1 + CR K ads
(6)
Frumkim adsorption isotherm model [19]: θ log C R 1−θ
= 2αθ + 2.303 log K ads
(7)
El-Awady’s thermodynamic/kinetic adsorption isotherm model [6, 19]: θ log = y log C R + log K 1 −θ
(8)
K ads = K 1 y
Temkin adsorption isotherm model [13,19, 34]:
θ = ln C R + K ads
(9)
Freundlish adsorption isotherm [34, 35]: log θ = log K ads + n log C R
(10)
Flory-Huggins adsorption isotherm [35]: log
θ CR
= b log(1 − θ ) + log K ads
(11)
8
2.5.
Determination of Adsorption Thermodynamics Parameters The Gibb's free energy change of adsorption, ∆Gads presented in Equation 12 [35] was
used to investigate the feasibility and nature of the adsorption. ∆Gads = − RT ln (55.5 K ads )
(12)
K ads is the adsorption equilibrium constant obtained from the isotherm, and the number 55.5 is the molar concentration of water in solution. 3.0
Results and Discussion
3.1
Inhibition Efficiency and Corrosion Rate Inhibition efficiency and corrosion rate for 1.0, 1.5 and 2.0 g/l extract concentration are
presented in Figures 1, 2 and 3, respectively. For 1.0 g/l extract concentration shown in Figure 1, inhibition efficiency increased with immersion time. The reason for this observation is because, at such a relatively low extract concentration, considerable time is required for sufficient adsorption to take place. The is also the reason for the lowest inhibition efficiency observed at 3 hours immersion time and highest at 7 hours immersion time. For each immersion time, inhibition efficiency was found to increase with temperature initially but later declined. The decline was sharpest at 7 hours immersion time possibly because the inhibitor could not withstand high temperature for such a long period. Distortion of the extract molecular structure might occur at a higher temperature. According to Dauda et al. [36], an increase in temperature may also increase the solubility of the protective films of the metals, thus, increasing the susceptibility of the metal to corrosion. The corrosion rate decreased with immersion time and increased with temperature. The observation is accurate because reaction rates generally decrease with time and increase with temperature. The increase in the rate of corrosion of mild steel could be due to the rise in the average kinetic energy of the reacting molecules. The trend obtained for the corrosion rate is in
9
agreement with the result reported by Dauda et al. [36] in which the rate of corrosion of iron and steel was observed to increase with temperature. The same trend was observed with increasing extract concentration as presented in Figure 2 for 1.5 g/l and Figure 3 for 2.0 g/l extract concentration. In Figures 1, 2 and 3, it is observed that the inhibition efficiency decreased more sharply as the temperature increased. This observation is common with most organic inhibitors [ 25] because, in inhibition reactions, at a high temperature many changes may occur on the metal surface, such as rapid etching, rupture, desorption of the inhibitor, decomposition and or rearrangement of the inhibitor [30]. Therefore, temperature variation affects the metal dissolution, inhibitor adsorption/desorption and inhibitor efficiency. 3.2
Activation Energy (Ea) Presented in Figures 4 to 6 are the plots of log CR against 1/T for 1.0, 1.5 and 2.0 g/l
extract concentrations at different immersion times. The perfect fit observed for the data points is an indication of the consistency of the results of the experimental data. The slopes obtained from the plots are therefore appropriate in estimating the activation energy of the process for different immersion time. The activation energy, Ea, got at different extract concentration for 3, 5 and 7 hours immersion time are presented in Tables 1, 2 and 3, respectively. As shown in the tables, the activation energy is higher for the inhibited process than for an uninhibited process. This observation shows that the adsorption process is by physisorption [37, 38]. The higher value of Ea in the presence of an inhibitor was due to the high-energy barrier. This incidence also confirms the formation of a complex compound between the inhibitor and of mild steel [ 38]. This result suggests that the corrosion inhibition by Euphorbia heterophylla is feasible because of the increase energy barrier for the metal dissolution. The formation of a thin film on the metal surface serves as a barrier to both energy and mass transfer, which increases the
10
activation energy. Therefore, the result shows that the adsorption of Euphorbia heterophylla on mild steel is by physical adsorption. As shown in the tables, the activation energy decreased as inhibitor concentration increased. However, the activation energy was still higher than that of the uninhibited solution. These results are in agreement with previous researchers [39 - 42]. For 1.0 and 1.5 g/l extract concentration, the activation energy at 5 hours of immersion time was the highest after which it began to decline. This result suggests that the activation energy increased with immersion time until 5 hours after which the Euphorbia heterophylla extract might become less potent in creating an energy barrier. Organic inhibitors can lose their inhibition potentials over time because they are sometimes biologically degraded, and they sometimes react with the environment and go into solution. For 2.0 g/l extract concentration, the activation energy does not decline after 5 hours probably because the concentration is high enough to overcome the effect of time. 3.2
Enthalpy and Entropy Change The values of enthalpy change, ∆H and entropy change, ∆S obtained at different
immersion time are also given in Tables 1 to 3. The ∆H value got was positive and observed to increase with immersion time but decreased with inhibitor concentration. However, the value of entropy change on the other hands was negative when the immersion time was 3 hours. This result indicates that a decrease in disorderliness took place on going from reactants to the activated complex. This result agrees with the work by Sudhish and Eno [37]. Further observation showed that the Ea values are more significant than the values of ∆H. This result indicates that the corrosion process must have involved a gaseous reaction [41]. The average value of the difference between activation energy and enthalpy change (Ea – ∆H) is 2.73 kJ mol-1 for the 3 hours immersion time, which is the exact average value of the product of gas constant (R=8.314 Jmol-1K-1) and the temperature of the experiment (T=328 K). That is, ܧ −
11
∆ܴܶ = ܪ. This result agrees with the work by Noor [41, 42]. He reported that such corrosion process is a unimolecular reaction with the evolution of hydrogen gas. 3.3
Adsorption Isotherms Figures 7 to 11 show the various adsorption isotherm models considered in this study.
The R2 values for each isotherm model presented in Table 4 were used to determine the most suitable model. The data fit into Langmuir, Flory-Huggins, El-Awary's and Temkin isotherms but Flory-Hugins gives the best fit. Flory-Huggins isotherm with R2 values of about 0.99, best describes the adsorption mechanism of Euphorbia heterophylla extract on mild steel in hydrochloric acid medium. Therefore, Flory-Hugins adsorption isotherm is appropriate for evaluating the adsorption equilibrium constant, K ads . 3.4
Adsorption Equilibrium Constant Table 5 shows the adsorption equilibrium constants obtained at various temperatures
from the intercept of the Flory-Huggins plot. The adsorption equilibrium constants, K ads are positive, indicating the feasibility of the adsorption of the inhibitor to the metal surface. The adsorption equilibrium constant increased with temperature up till 333K. However, at 343K, K ads declines, according to Hegazy [ 43], this indicates that at a higher temperature, the adsorbed inhibitor tends to desorb back from the mild steel surface. This result further confirms the trend obtained for activation energy. 3.5
Gibb’s Free Energy Change of Adsorption Presented in Table 5 is Gibb's free energy change of adsorption, ∆Gads got at different
temperatures. As shown in the table, the ∆Gads are negative and less than 20 kJ/mol. This result indicates that adsorption of the extract of Euphorbia heterophylla on mild steel surface is spontaneous, feasible and occurred according to the mechanism of physical adsorption. The
12
decrease in ∆Gads at 343K implies there is a reduction in the spontaneity and stability of the adsorption at a higher temperature. 3.6 Conclusion The study concluded that the adsorption of Euphorbia heterophylla L. on mild steel in 1.5M HCl medium is feasible, spontaneous and by physical adsorption according to FloryHugins isotherm model. Euphorbia heterophylla L extract is more effective in inhibiting mild steel corrosion in acidic medium at temperature below 343K.
References 1. C. A. Loto, A. I. Mohammed, The effect of anacardium occidentale (cashew) juice extract on the corrosion of mild steel in hydrochloric acid, J. Corros. Prevent. & Contr., 2, (2000), 50- 63. 2. Singh, A. K., Chugh B., Saha S. K., Banerjee P., Ebenso E. E., Thakur S., Pani B. Evaluation of anti-corrosion performance of an expired semi synthetic antibiotic cefdinir for mild steel in 1 M HCl medium: An experimental and theoretical study, Results in Physics, https://doi.org/10.1016/j.rinp.2019.1023 3. M. Chigondo M., F. Chigondo, Recent Natural Corrosion Inhibitors for Mild Steel: An Overview, J. Chem. (2016), Article ID 6208937, 1-7, http://dx.doi.org/10.1155/2016/6208937 4. N. Kavitha, P. Manjula, N. Anandha, Syneristic effect of C. papaya leaves extract-Zn2+ in corrosion inhibition of mild steel in aqueous medium, Resear. J. Chem. Sci., 8, (2014), 88-93. 5. C. N. Njoku, O. E. Onyelucheya, Response surface optimization of the inhibition efficiency of Gongronema latifolium as an inhibitor for aluminium corrosion in HCl Solutions, Int. J. Mater. Chem., 1, (2015), 4-13. 6. L. R. Chauhan, G. Gunasekaran, Corrosion inhibition of mild steel by plant extract in dilute HCl medium, Corros. Sci., 9, (2006), 1143-1161.
13
7. K. K. Alaneme, S. J. Olusegun, O. T. Adelowo, Corrosion inhibition and adsorption mechanism studies of Hunteria umbellata seed husk extract on mild steel immersed in acidic solutions, Alexandria Eng. J., 10, (2015), 45-51. 8. X. Li, S. Deng, H. Fu, Inhibition by tetra decylpyridinium bromide of the corrosion of aluminum in hydrochloric acid solution, Corros. Sci., 53, (2011), 1529-1536. 9. A. Y. Musa, A. A. H. Kadhum, A. R. Daud, A. B. Mohamad, M. S. Takriff, S.K. Kamarudin, A comparative study of the corrosion inhibition of mild steel in sulphuric acid by 4, 4-Dimethyloxazolidine-2-thione, Corros. Sci., 51, (2009), 2393–2399. 10. S. Manimegalai, P. Manjula, Thermodynamic and adsorption studies for corrosion inhibition of mild steel in aqueous media by Sargasam swartzii (Brown algae), J. Mater. Environ. Sci., 6, (2015), 1629-1637. 11. S. M. Ramananda, S. Vivek, S. Gurmeet, Musa paradisiaca extract as a green inhibitor for corrosion of mild steel in 0.5 M sulphuric acid Solution, Portugaliae Electrochimica Acta, 29, (2011), 405-417. 12. S.A. Umoren, I.B. Obot and E.E. Ebenso, Corrosion inhibition of aluminum using exudates gum from Pachylobus Edulis in the presence of halide ions in HCl, E. J. Chem., 5, (2008), 355-364. 13. I. J. Alinnor, and P. M. Ejikeme, Corrosion inhibition of aluminum in acidic medium by different extracts of Ocimum gratissimum, Ameri. Chemi. Sci. J., 2, (2012), 122-135. 14. G. Avci, Corrosion inhibition of indole-3-acetic acid on mild steel in 0.5 M HCl, Colloids and Surfaces A: Physicochem. Eng. Aspects 317, (2008), 730–736. 15. Singh A. K., Thakur S., Pani B. and Singh G. Green synthesis and corrosion inhibition study of 2-amino-N′-((thiophen-2-yl) methylene) benzohydrazide New J. Chem. 42, 2113-2124 16. A. K. Singh, S. Thakur, B. Pani, B. Chugh, J. Singh, Solvent-free microwave assisted synthesis and corrosion inhibition study of a series of hydrazones derived from thiophene derivatives: Experimental, surface and theoretical study, J. Molecular Liquids 283, (2019), 788-803 17. N. Arshad, A. K. Singh, B. Chugh, Experimental, theoretical, and surface study for corrosion inhibition of mild steel in 1 M HCl by using synthetic anti-biotic derivatives, Ionics, (2019) 1- 19, https://doi.org/10.1007/s11581-019-03028-y 18. S. J. Olusegun, B. A. Adeiza, K. I. Ikeke, M. O. Bodunrin, Jatropha curcas leaves as corrosion inhibitors for mild steel in 1M hydrochloric acid. J. Emerging Trends in Eng. Appl. Sci., 4, (2013), 138-143.
14
19. J. T. Nwabanne, V. N. Okafor, Adsorption and thermodynamics study of the inhibition of corrosion of mild steel in H2SO4 medium using Vernonia amygdalina, J. Minerals Mater. Characteri. Eng., 11, (2012), 885-890. 20. Singh A. K, Mohapatra S, Pani B. Corrosion inhibition effect of Aloe Vera gel: gravimetric and electrochemical study. J. Ind. Eng. Chem. 2016, 33, 288–297. 21. A. M. Al-Haj-Ali, N. A. Jarrah, N. D. Mu'azu, R. O. Rihan, Thermodynamics and kinetics of inhibition of aluminum in hydrochloric acid by date palm leaf extract J. Appl. Sci. Environ. Mgt. 18, (2014), 543 -551. 22. O. J. Odejobi, O. A. Akinbulumo, Modeling and optimization of the inhibition efficiency of Euphorbia heterophylla extracts based corrosion inhibitor of mild steel corrosion in HCl media using a response surface methodology, J. Chem. Technol. Metall., 54, 1, (2019), 217-232 23. A. Falodun, S. Ali, I. M. Quadir and M. I. C., Iqbal Phytochemical and biological investigation of chloroform and ethyl acetate fractions of Euphorbia heterophylla leaf (Euphorbiaceae), J. Medicinal Plants Research 2, 12, (2008) 365-369 24. H. Kaco, N. A. A. Talib, S, Zakaria, S. N. S. Jaafar, N. K. Othman, C. H.Chia and S. Gan, Enhanced Corrosion Inhibition Using Purified Tannin In HCl Medium Malaysian J. Analyt. Sci., 22, 6 (2018) 931-942 https://doi.org/10.17576/mjas-2018-2206-02 25. R. Amitha, B. Bharathi, J. Basu, Green Inhibitors for Corrosion Protection of Metals and Alloys: An Overview, Int. J. Corr. (2012), Article ID 380217,15pages doi:10.1155/2012/380217 26. M. Hegde, S. P. Nayak Aqueous extract of Dillenia PentagynaFruit as green inhibitor for mild steel corrosion in 0.5 M hydrochloric acid solution. J. Mater. Environ. Sci., Volume 10, Issue 1, (2019) Page 22-31 27. E. Osarolube, I.O. Owate, N.C. Oforka, The influence of acidic concentration on corrosion of copper and zinc, J. Corros. Sci. Tech. , 11, (2004), 66-69. 28. C.A. Loto, O. O. Joseph, R.T. Loto and A.P.I. Popoola, Inhibition effect of Vernonia amygdalina extract on the corrosion of mild steel reinforcement in concrete in 3.5M NaCl environment, Int. J. Electrochem. Sci., 8, (2013), 11087 – 11100. 29. O. O. Ige, Chrysophyllum albidium extracts as corrosion inhibition for aluminium in 0.5M H2SO4 solution, Afric. Corros. J., 2, (2015) 25-30. 30. M.M. Solomon, S.A. Umoren, I. I. Udosoro, A.P. Udoh, Inhibitive and adsorption behaviour of carboxymethyl cellulose on mild steel corrosion in sulphuric acid solution, Corro. Sci. 52, (2010), 1317–1325.
15
31. E. Ebenso, Effect of halide ions on the corrosion inhibition of mild steel in H2SO4 using methyl red. Bull. Electrochem., 19, (2003), 209-216. 32. M. G. Tsoeunyane, M. E. Makhatha, O. A. Arotiba Corrosion Inhibition of Mild Steel by Poly(butylene succinate)-L-histidine Extended with 1,6-diisocynatohexane Polymer Composite in 1 M HCl, Int. J. Corr., (2019)., Article ID 7406409, 12 pages, https://doi.org/10.1155/2019/7406409. 33. O. Dagdag, Z. Safi, R. Hsissou, H. Erramli, M. El Bouchti, N. Wazzan, L. Guo, C. Verma, E. E. Ebenso A. El Harfi, Epoxy pre-polymers as new and effective materials for corrosion inhibition of carbon steel in acidic medium: Computational and experimental studies, Scientific reports 9, 1, (2019). 1-14 34. K. K. Alaneme, S. J. Olusegun, Corrosion inhibition performance of lignin extract of sunflower (Tithonia Diversifolia) on medium carbon low alloy steel immersed in HCl solution, Leonardo J. Sci. 20, (2012), 59-70. 35. E.E. Ebenso, H. Alamu, S.A. Umoren, I.B. Obot, Inhibition of mild steel corrosion in sulphuric acid using alizarin yellow dye and synergistic iodide additive, Int. J. Electrochem. Sci., (2008), 1325-1339. 36. K. T. Dauda, V. N. Atasie, and A. A. Adetimilehin, Kinetics of the Inhibitory Effects of Ethanolic Extract of Vernonia amydgalina on the Corrosion of Aluminium in 1.0 M HCl. Chem. J., 3, (2013), 86-89. 37. K. S. Sudhish and E. E. Eno, Corrosion inhibition adsorption behavior and thermodynamic properties of streptomycin on mild steel in hydrochloric acid medium. Int. J. Electrochem. Sci., (2011), 3277-3291. 38. M. Erna , H. Herdini , D. Futra, Corrosion Inhibition Mechanism of Mild Steel by Amylose-Acetate/Carboxymethyl Chitosan Composites in Acidic Media int. J. Chem. Engg. Volume (2019), Article ID 8514132, 12 pages, https://doi.org/10.1155/2019/8514132 39. L.B. Tang, G.N. Mu, G.H. Liu, The effect of neutral red on the corrosion inhibition of cold-rolled steel in 1.0 M hydrochloric acid, Corros. Sci., 45, (2003), 2251-2262. 40. L. Tang, X. Li, L. Li, G. Mu, G. Liu, The effect of 1-(2-pyridylazo)-2-naphthol on the corrosion of cold-rolled steel in acid media: Part 2: inhibitive action in 0.5 M sulfuric acid, Mater. Chem. Phys., 97, (2006), 301-307. 41. E. A. Noor, Temperature effects on the corrosion inhibition of mild steel in acidic solutions by aqueous extract of fenugreek Leaves. Int. J. Electrochem. Sci., 3, (2007), 996-1017. 42. E. Noor, Potential of aqueous extract of hibiscus sabdariffa leaves for inhibiting the corrosion of aluminium in alkaline solutions. J. Appl. Electrochem., 39, (2009), 14651475. 43. M. A. Hegazy, H. M. Ahmed and A. S. El-Tabei, Investigation of the inhibitive effect of substituted 4-(N, N, N-d imet hyl dodecyl ammonium bromide) benzylidene-benzene-2-
16
ylamine on corrosion of carbon steel pipelines in acidic medium, Corros. Sci., 53, (2011), 671-678.
Inhibition Efficiency (%)
70 200
60 50
150
40 100
30 20
50
10 0
Corrosion Rate (mm/yr)
250
80
0 310
320
330
340
60
250
50
200
40 150 30 100
20
50
10 0
0 320
330
340
Temperature (K)
350
Corrosion Rate (mm/yr)
Inhibition Efficiency (%)
300
Figure 2:
I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion
Inhibition efficiency and corrosion rate for 1.0 g/l extract concentration
70
310
I. E (%) at 5 hours immersion
C. Rate (mm/yr) at 7 hours immersion
350
Temperature (K) Figure 1:
I. E (%) at 3 hours immersion
I. E (%) at 3 hours immersion I. E (%) at 5 hours immersion I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion C. Rate (mm/yr) at 7 hours immersion
Inhibition efficiency and corrosion rate for 1.5 g/l extract concentration
17
400 350
50
300 40
250
30
200 150
20
100 10
50
0
0 310
320
330
340
350
Corrosion Rate (mm/yr)
Inhibition Efficiency (%)
60
I. E (%) at 3 hours immersion I. E (%) at 5 hours immersion I. E (%) at 7 hours immersion C. Rate (mm/yr) at 3 hours immersion C. Rate (mm/yr) at 5 hours immersion C. Rate (mm/yr) at 7 hours immersion
Temperature (K) Figure 3:
Inhibition efficiency and corrosion rate for 2.0 g/l extract concentration
at 3 hrs Immersion Time slope= -3214 R² = 0.998 3
at 5 hrs Immersion Time slope= -6459 R² = 0.928
at 7 hrs Immersion Time slope= -6307 R² = 0.919
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T (K-1)
Figure 4:
Plot of log CR versus 1/T for 1.0 g/l extract concentration
18 at 3 hrs Immersion Time slope= -3179 R² = 0.993 3
at 5 hrs Immersion Time slope= -5520 R² = 0.925
at 7 hrs Immersion Time slope= -4793 R² = 0.966
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 5:
Plot of log CR versus 1/T for 1.5g/l extract concentration
at 3 hrs Immersion Time slope = -2433.9 R² = 0.9975 3
at 5 hrs Immersion Time slope = -3150.3 R² = 0.9854
at 7 hrs Immersion Time slope = -4537.4 R² = 0.9418
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 6:
Plot of log CR versus 1/T for 2.0g/l extract concentration
19 12 10
C/Ө
8 313K 6
323K
4
333K 343K
2 0 0
0.5
1
1.5
2
2.5
C (g/l)
Figure 7:
Langmuir Adsorption Isotherms
0.6 0.4
log(Ө/1-Ө)
0.2 313K
0 -0.2
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
323K 333K
-0.4
343K
-0.6 -0.8
Figure 8:
log C
El-Awady’s Thermodynamic/Kinetic Adsorption Isotherm
20 0.8 0.7 0.6 Ө
0.5
313K
0.4
323K
0.3 0.2
333K
0.1
343K
0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
ln CR
Figure 9:
Temkin Adsorption Isotherm
.
0 -0.1 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
log Ө
-0.2 -0.3
313K
-0.4
323K
-0.5
333K
-0.6
343K
-0.7 -0.8
Figure 10:
log C
Freundlich Isotherm
21 0 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0 -0.2
log(Ѳ/C)
-0.4 -0.6
40C/313K 50C/323K 60C/333K
-0.8
70C/343K
-1 -1.2
log(1-Ѳ) Figure 11:
Flory-Huggins Isotherms
Table 1:
Activation parameters at 3 hours immersion time
Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
27.51
-
-
1.0 g/l
61.54
58.78
-30.8
1.5 g/l
60.87
58.15
-43.75
2.0 g/l
46.59
43.89
-69.3
22
Table 2:
Activation parameters at 5 hours immersion time
Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
31.06
-
-
1.0 g/l
123.67
80.48
31.8
1.5 g/l
105.69
62.17
-14.72
2.0 g/l
60.31
57.59
-29.5
Ta ble 3:
Act iva
tion parameters at 7 hours immersion time Extract Conc.
Ea (KJ/mol)
∆H (KJ/mol)
∆S (J/mol/K)
Blank
33.87
-
-
1.0 g/l
120.76
119.73
146.3
1.5 g/l
91.77
88.99
63.02
2.0 g/l
86.87
84.15
48.5
Table 4:
R2 Values for the Various Adsorption Isotherms Considered
23
Langmuir
Frumkin
FloryHuggins
ElAwary’s
Freundkich Temkin
R2
Temperature 313K
0.975
0.209
0.978
0.916
0.883
0.907
323K
0.967
0.780
0.994
0.942
0.901
0.931
333K
0.947
0.901
0.997
0.946
0.904
0.945
343K
0.872
0.920
0.992
0.922
0.879
0.954
Table 5:
Adsorption parameters
Tempt (K)
Kads (mol-1)
313
0.0526
-2.79
323
0.0687
-3.59
333
0.0689
-3.71
343
0.0515
-2.98
∆Gads (KJ /mol)
Table 1 Extract Conc.
Activation parameters at 3 hours immersion time Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
27.51
-
-
1.0g/l
61.54
58.78
-30.8
1.5g/l
60.87
58.15
-43.75
2.0g/l
46.59
43.89
-69.3
Table 2 Extract Conc.
Activation parameters at 5 hours immersion time Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
31.06
-
-
1.0g/l
123.67
80.48
31.8
1.5g/l
105.69
62.17
-14.72
2.0g/l
60.31
57.59
-29.5
Table 3
Activation parameters at 7 hours immersion time
Extract Conc.
Ea
∆H
∆S
KJ/mol
KJ/mol
J/mol/K
Blank
33.87
-
-
1.0g/l
120.76
119.73
146.3
1.5g/l
91.77
88.99
63.02
2.0g/l
86.87
84.15
48.5
Table 4
R2 Values for the Various Adsorption Isotherms Considered Langmuir
Frumkin
Flory-
El-
Huggins
Awary’s
Freundkich Temkin
R2
Temperature 313K
0.975
0.209
0.978
0.916
0.883
0.907
323K
0.967
0.780
0.994
0.942
0.901
0.931
333K
0.947
0.901
0.997
0.946
0.904
0.945
343K
0.872
0.920
0.992
0.922
0.879
0.954
Table 5 Adsorption parameters Tempt (K)
K ads (mol-1)
313
0.0526
-2.79
323
0.0687
-3.59
333
0.0689
-3.71
343
0.0515
-2.98
∆Gads (KJ /mol)
at 3 hrs Immersion Time slope= -3214 R² = 0.998 3
at 5 hrs Immersion Time slope= -6459 R² = 0.928
at 7 hrs Immersion Time slope= -6307 R² = 0.919
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 1
Plot of log CR versus 1/T for 1.0g/l extract concentration
at 3 hrs Immersion Time slope= -3179 R² = 0.993 3
at 5 hrs Immersion Time slope= -5520 R² = 0.925
at 7 hrs Immersion Time slope= -4793 R² = 0.966
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Figure 2
Plot of log CR versus 1/T for 1.5g/l extract concentration
at 3 hrs Immersion Time y = -2433.9x + 9.6503 R² = 0.9975 3
at 5 hrs Immersion Time y = -3150.3x + 11.726 R² = 0.9854
at 7 hrs Immersion Time y = -4537.4x + 15.806 R² = 0.9418
Log CR
2.5 2 1.5 1 0.5 0 0.0029 0.00295
0.003
0.00305 0.0031 0.00315 0.0032 0.00325
1/T
Plot of log CR versus 1/T for 2.0g/l extract concentration
Figure 3
12 10
C/Ө
8 313K 6
323K
4
333K 343K
2 0 0
0.5
1
1.5 C (g/l)
Figure 4 Langmuir Adsorption Isotherms
2
2.5
0.6 0.4
log(Ө/1-Ө)
0.2 313K
0 -0.2
0
0.05
0.1
0.15
0.2
0.25
0.3
323K
0.35
333K
-0.4
343K
-0.6 -0.8
log C
Figure 5
El-Awady’s Thermodynamic/Kinetic Adsorption Isotherm
0.8 0.7 0.6 Ө
0.5
313K
0.4
323K
0.3 0.2
333K
0.1
343K
0 0
0.1
0.2
0.3
0.4
ln C
Figure 6
Temkin Adsorption Isotherm
0.5
0.6
0.7
0.8
.
0 -0.1 0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
log Ө
-0.2 -0.3
313K
-0.4
323K
-0.5
333K
-0.6
343K
-0.7 -0.8
Figure 7
log C
Freundlich Isotherm
0 -0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0 -0.2
log(Ѳ/C)
-0.4 -0.6
50C/323K 60C/333K
-0.8 -1
log(1-Ѳ) Figure 8
40C/313K
Flory-Huggins Isotherms
-1.2
70C/343K
Highlights The activation energy of inhibited process was found to be greater than that of the uninhibited. The enthalpy change is positive and less than 100 kJ/mol. The adsorption study showed that the data fit into Langmuir, Flory-Huggins, El-Awary’s and Temkin isotherms but Flory-Hugins gives the best fit. The Gibb’s free energy change of adsorption was negative and less than the -20 KJ/mol. It is concluded from the results that adsorption of Euphorbia heterophylla on mild steel in hydrochloric acid medium is feasible, spontaneous and by physical adsorption according to Flory-Hugins isotherm model.