Michael Emmanuel, Bethrand T. Nwufo and Mbanefo M. Ekwenchi
Keywords: Mild steel, Dioctyl Phthalate, Weight loss, Corrosion inhibition.
Abstract:
The corrosion inhibition properties of dioctyl phthalate on mild steel in 0.5 M H2SO4 medium was carried out using the weight loss method at temperature range of 303 K to 313 K for three hours. The concentration of the inhibitor was set at a range of 0.01 M to 0.07 M and a maximum reaction time of three (3) hours. The adsorption of the inhibitor on the surface of the mild steel was studied using Langmuir, Freundlich and the newly proposed Adejoh – Ekwenchi Isotherm. The maximum inhibition efficiency obtained for the corrosion of mild steel in H2SO4 was 85.11% at a temperature of 303 K. All values of ∆Hads were found to be positive indicating the endothermic nature of the dissolution of the mild steel. The values of entropy of adsorption ∆Sads was found to increase with increase in concentration of the inhibitor indicating that the rate determining combination step is a more orderly arrangement relative to the initial state. The values of Eads and those of ∆Hads varied in a similar manner, and the average values of Eads - ∆Hads equals 2.41 kJmol-1, which is very close to the value of RT (2.51 kJmol-1) indicating that the adsorption process is unimolecular in nature. The values of ∆Gads and Kads for the corrosion inhibition studies of mild steel in 0.5 M H2SO4 and dioctyl phthalate as the inhibitor were found to be suggestive of a physisorption mechanism. The adsorption values were in agreement with the three isotherms employed, with Freundlich isotherm as best fit for the modelling of the adsorption of dioctyl phthalate on mild steel in acid medium.
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Corrosion Inhibition Properties of Dioctyl Phthalate on Mild Steel in 0.5 M Sulphuric Acid Medium Michael Emmanuel1*, Bethrand T. Nwufo2 and Mbanefo M. Ekwenchi2 Department of Chemistry, Gombe State University, P.M.B. 127, Gombe, Gombe State, Nigeria) 2(Department of Pure and Industrial Chemistry, University of Jos, Jos, Plateau State, Nigeria) *E-mail: mikolalaem143@gmail.com 1( Abstract The corrosion inhibition properties of dioctyl phthalate on mild steel in 0.5 M H2SO4 medium was carried out using the weight loss method at temperature range of 303 K to 313 K for three hours. The concentration of the inhibitor was set at a range of 0.01 M to 0.07 M and a maximum reaction time of three (3) hours. The adsorption of the inhibitor on the surface of the mild steel was studied using Langmuir, Freundlich and the newly proposed Adejoh – Ekwenchi Isotherm. The maximum inhibition efficiency obtained for the corrosion of mild steel in H2SO4 was 85.11% at a temperature of 303 K. All values of ∆Hads were found to be positive indicating the endothermic nature of the dissolution of the mild steel. The values of entropy of adsorption ∆Sads was found to increase with increase in concentration of the inhibitor indicating that the rate determining combination step is a more orderly arrangement relative to the initial state. The values of E ads and those of ∆Hads varied in a similar manner, and the average values of Eads - ∆Hads equals 2.41 kJmol-1, which is very close to the value of RT (2.51 kJmol-1) indicating that the adsorption process is unimolecular in nature. The values of ∆Gads and Kads for the corrosion inhibition studies of mild steel in 0.5 M H2SO4 and dioctyl phthalate as the inhibitor were found to be suggestive of a physisorption mechanism. The adsorption values were in agreement with the three isotherms employed, with Freundlich isotherm as best fit for the modelling of the adsorption of dioctyl phthalate on mild steel in acid medium. Keyword: Mild steel, Dioctyl Phthalate, Weight loss, Corrosion inhibition. 1 Introduction Corrosion is a major industrial problem all over the world and a nightmare to most manufacturing and construction companies. According to the American Galvanizers Association, steel corrosion is an expensive problem costing about $2.2 trillion annually [1]. Although corrosion could be said to be nature’s method of recycling, or of returning of a metal to its lowest energy form, it is an insidious enemy that destroys our cars, plumbing systems, buildings, bridges, engines, factories etc [2]. In Nigeria, especially in the oil producing states where there is a large amount of emitted acid gases, zinc roofs and metallic parts have been corroded and appear as a brownish colour on the surfaces. These tends to be washed down by rain leading to a considerable reduction in metal thickness and loss of strength, which may as a result leads to perforation of roofs and leaking [3]. Corrosion inhibitors are classified into inorganic and organic corrosion inhibitors, the mechanism of corrosion can be described as; chemical adsorption or chemisorption of the inhibiting material to the surface of the steel or by combination between inhibitor ions and metallic surface [4, 5]. Phthalates are esters of 1,2-benzenedicarboxylic acid. The most widely used phthalates are the dibutyl phthalates and the dioctyl phthalate [6]. Dioctyl phthalate (DOP) with IUPAC nomenclature dioctyl benzene-1,2-dicarboxylate, is a clear, colourless, and viscous liquid with a slight, characteristic odor, soluble in ethanol, ether, mineral oil, and the majority of organic solvents, immiscible with water, resistant to hydrolysis and air oxygen activity. Its high plasticizing efficiency, fusion rate, viscosity, low volatility, UV-resisting property, water-extracting proof, cold-resisting property, and also good softness, found a lot of applications in many offshoots of the industry [7]. Many researchers reported that the inhibition effect mainly depends on some physicochemical and electronic properties of the organic inhibitor which relate to its functional groups, steric effects, electronic density of donor atoms, and orbital character of donating electrons, and so on [8,9]. Most of the research conducted on corrosion inhibition are Zenthoxylum alatum plant extract [11], extract of Ricinus communis var minor [12], Newbouldia leavis leaf extract [13], leaf extract of Euphorbia hirta [14], e.t.c. Most of these extracts studied do not give a clear through the use of plant extracts; from the juice of grapes [10], view of the compound in the extract responsible for the inhibition process, and as such makes the application ambiguous. This particular study is centred on one compound – dioctyl phthalate, and described it corrosion inhibition properties, hence its mechanism of adsorption on the surface of the mild steel. 2 Materials and Method All the materials used for this research are of analytical grade. The method used for the study of the corrosion inhibition properties of Dioctyl phthalate on mild steel is the weight loss method [15], with coupons of size 5 cm by 4 cm, and thickness of 0.5 mm obtained from the mechanical workshop of the University of Jos, Nigeria. Determinations The evaluation of the inhibition efficiency (%I) was determined from the weight loss measurement at temperatures ranging from 303K to 318K [16]. The kinetic and thermodynamic parameters were determined as described below. The adsorption parameters and surface coverage were also determined as outlined below. Determination of Parameters The data obtained from the experiment above was used to determine the following parameters; Inhibition Efficiency (%IE) and Surface Coverage (θ) The inhibition efficiency (%IE) and the surface coverage (θ) were determined from the following equations %I = [1 – ( 𝑊0−𝑊𝑡 )] 𝑋 𝑊 100 𝑊0−𝑊𝑡 ) 𝑊 θ=1–( 1 Where W0 = initial weight of the mild steel before suspending in the solutions, W= weight of the mild steel without the inhibitor, that is, weight in blank solution, and; Wt = weight of the mild steel with the inhibitor The inhibitor efficiency gives important information about the performance of the inhibitor in various medium [17] Corrosion Rate This is a method of monitoring corrosion. The rate of corrosion C r, is the weight loss per unit area per unit time of a metallic material [17]. 3 This is given by the equation Cr (mg cm-2 hr-1) = 𝑊0−𝑊𝑡 𝐴𝑇 2 Where A = surface area of the coupon (cm2) T = time of exposure in hrs W0 – Wt = weight loss Thermodynamic Parameters These parameters tell more of the adsorption properties of the inhibitor. This includes the Activation energy of adsorption Eads, the heat of adsorption Qads, enthalpy of activation ∆Hads, entropy ∆Sads and free energy change ∆Gads. 1. Activation Energy Eads: this is the energy that adsorbing species must acquire in order to be adsorbed on the surface of the mild steel to form a protective monolayer. This is obtained from a modified form of the Arrhenius equation [18]; Cr = A 𝑒 −𝐸𝑎𝑑𝑠/𝑅𝑇 This equation can be linearized to give ln Cr = ln A – (Eads/RT) 3 Where Cr = rate of corrosion A = a frequency factor R = Gas constant (Atm. L. mol-1 K-1) T = temperature (K) A graphical plot of ln Cr against 1/T, gives a straight line graph with slope equals to E ads/R and an intercept of ln A [18]. The values of Ea greater than 20kJ indicate that the adsorption process is controlled by the surface reactions. [19] Heat of Adsorption Qads This parameter is used to tell whether an adsorption process is endothermic or exothermic. A positive value of Qads indicates an endothermic process while a negative value indicates exothermic process of adsorption. This can be evaluated from equation 3.4 reported by 4 [20] Where 𝜃 1 = is the initial surface coverage at temperature T1 𝜃 2 = is the final surface coverage at temperature T2 R = the gas constant Equation 3.4 can be written as; Qads = 𝑅𝑇 𝜃 4 ln( 1− 𝜃) A graphical plot of ln( 𝜃 ) 1− 𝜃 against 1/T gives the value of the Qads obtained from the slope of the graph which is Qads/R. Negative Qads values indicate an exothermic adsorption process. This exothermic reaction can either be physisorption or chemisorption. This can be distinguished by considering the values of the heat of adsorption. Value of energy less than 40kJ is indicative of physical adsorption, otherwise, chemical adsorption. [20] Enthalpy of Adsorption ∆Hads and Entropy of Adsorption ∆Sads The enthalpy of adsorption and the entropy of adsorption (Hads and Sads) can be determined from the Eyring transition state equation; Cr = 𝑅𝑇 ∆𝑆𝑎𝑑𝑠 ∆𝐻𝑎𝑑𝑠 exp ( 𝑅 ) exp(− 𝑅𝑇 ) 𝑁ℎ Rearranging the equation gives; 𝑙𝑛 𝐶𝑟 𝑇 = 𝑙𝑛 𝑅 ∆𝑆𝑎𝑑𝑠 ∆𝐻𝑎𝑑𝑠 +( )−( ) 𝑁ℎ 𝑅 𝑅𝑇 5 A plot of ln Cr/T against 1/T is a straight line with a slope equal to the graph is equal to 𝑅 ∆𝑆𝑎𝑑𝑠 𝑙𝑛 𝑁ℎ + ( 𝑅 ). ∆𝐻𝑎𝑑𝑠 and 𝑅 the intercept of Where Cr is the corrosion rate, T is the temperature in Kelvin, N is the Avogadro’s number 6.03 x 1023 mol-1, h is the Planck’s constant 6.62 x 1034 Js, and R is the gas constant. The values of ∆Hads indicates whether the adsorption is endothermic (+∆H) and exothermic (-∆H). [21] Free Energy of Adsorption (∆Goads) This value also indicates the type of adsorption mechanism, whether by physical or chemical adsorption mechanism. Values of ∆Goads from -20kJ or more indicates physical adsorption mechanism, while values of -40kJ or less is an indicative of chemical adsorption. This value was calculated from the equation below; 1 ∆𝐺𝑎𝑑𝑠 ) exp(− ) 55.5 𝑅𝑇 1 ∆𝐺𝑎𝑑𝑠 Kads = ln(55.5) − ( 𝑅𝑇 ) Kads = ( ln 5 ∆Gads = - RT(ln 55.5Kads) 6 Where Kads is the equilibrium constant for the adsorption of the inhibitor on mild steel surface, which is obtained from the adsorption isotherm given by equation 3.9 – 3.11, T is the Absolute temperature of the system, R is the gas constant, and C is the concentration of the inhibitor. [22] Kinetic Parameters Corrosion reactions are usually first order and are expected to obey the equation of a first order reaction as given below ln wf = ln wi – kt 7 ln wf/wi = -kt ln wi/wf = kt Where wi and wf are respectively the weights of the mild steel in blank acid solution and after immersion in the inhibitor solution. This value tells how fast the adsorption process is taking place. A plot of ln(wi/wf) against t, gives a straight line with a slope which is equal to k [12]. Also the half life of this process was determined from the value of the rate constant that was obtained from above using t1/2 = ln 2 𝑘 8 Adsorption Studies The variation at constant temperature of the surface coverage of adsorbing species with the concentration or pressure of the adsorbing solutes is known as the adsorption isotherm. Adsorption parameters obtained from adsorption studies give important information that will help in understanding the type of adsorption taking place during the inhibition. Adsorption isotherms are important in the optimization of the adsorption mechanism and aiding the understanding of the surface properties of the inhibitor. [23]. Most frequently used adsorption isotherms are Langmuir, Freundlich, Frumkin, Temkin, Forry – Huggins, BET, e.t.c. The newly proposed Adejo – Ekwenchi isotherm was also used to confirm the claims about its effectiveness in deciphering the mechanism of adsorption. The isotherms used to test the linearity of the adsorption results are; 6 𝐶 1 Langmuir (𝜃) = 𝑘 + 𝐶 9 Freundlich log 𝜃 = log 𝑘 + 𝑛𝑙𝑜𝑔 𝐶 Adejoh – Ekwenchi log( 1− 𝜃) = log 𝐾AE + blog C 1 10 11 k is the equilibrium constant for adsorption in all the equations. The values of k, gives information about how favourable the inhibition process is, and higher values of k indicates favourable adsorption process. C is the concentration of the solution of the inhibitor. 7 [24, 25] Results and Discussion The corrosion inhibition properties of Dioctyl phthalate was determined at temperature ranges of 303 – 313 K, in 0.5 M H2SO4 solution. The average initial weight of the mild steel coupon before immersion into the inhibitor solution is 2.512 g. Table 1 represents the weight loss of mild steel at different concentration of the inhibitor in 0.5 M H 2SO4 solution as the inhibitor. Table 1: weight loss of mild steel in inhibitor and 0.5 M H 2SO4 solution at different temperatures Conc. of inhibitor (M) in 0.5M H2SO4 Blank 0.01 0.03 0.05 0.0362 0.0257 0.0128 0.0023 0.0424 0.0365 0.0212 0.0083 0.07 0.0011 0.0026 Weight Loss (g) (303 K) (w0 – wt) 1 hr 2hr 3hr Weight Loss (g) (308 K) (w0 – wt) 1 hr 2hr 3hr Weight Loss (g) (313 K) (w0 – wt) 1 hr 2hr 3hr 0.0512 0.0401 0.0386 0.0158 0.0471 0.0366 0.0261 0.0155 0.0537 0.0426 0.0317 0.0216 0.0611 0.0523 0.0478 0.0321 0.0563 0.0459 0.0351 0.0248 0.0623 0.0549 0.0442 0.0324 0.0713 0.0645 0.0522 0.0411 0.0071 0.0103 0.0149 0.0218 0.0137 0.0209 0.0340 The percentage inhibition efficiency (%IE) was calculated according to equation (1a) and the values obtained at different concentration of the inhibitor and different temperatures in 0.5 M H2SO4 solution are recorded in Table 2. Table 2: Percentage Inhibition Efficiency (%IE) of the inhibitor on mild steel in 0.5M H 2SO4 Conc. Of inhibitor in 0.5M H2SO4 Blank 0.01 0.03 0.05 0.07 % IE at 303K % IE at 308K % IE at 313K 1 Hr 2Hrs 3Hrs 1 Hr 2Hrs 3Hrs 1 Hr 2Hrs 3Hrs 0 29.0055 64.6409 93.6464 96.9613 0 13.9151 50.0000 80.4245 93.8679 0 21.6797 24.6094 69.1406 86.1328 0 22.2930 44.5860 67.0913 78.1316 0 20.6704 40.9683 59.7765 72.2533 0 14.4026 21.7676 47.4632 64.3208 0 18.4725 37.6554 55.9503 75.6661 0 11.8780 29.0530 47.9936 66.4526 0 9.53717 26.7882 42.3562 52.3142 Also, the surface coverage of the inhibitor solution on mild steel was calculated according to equation (1b). Table 3, represents the surface coverage (θ) of the inhibitor on mild steel in 0.5 M H2SO4 solution 8 Table 3: Surface coverage (θ) of the inhibitor on mild steel in 0.5M H2SO4 Conc. Of inhibitor in 0.5M H2SO4 1 Hr 2 Hrs 3 Hrs 1 Hr 2 Hrs 3 Hrs 1 Hr 2 Hrs 3 Hrs Blank 0.01 0.03 0.05 0.0000 0.2901 0.6464 0.9365 0.0000 0.1392 0.5000 0.8043 0.0000 0.2168 0.2461 0.6914 0.0000 0.2229 0.4459 0.6709 0.0000 0.2067 0.4097 0.5978 0.0000 0.1440 0.2177 0.4746 0.0000 0.1847 0.3766 0.5595 0.0000 0.1188 0.2905 0.4799 0.0000 0.0954 0.2679 0.4236 0.07 0.9696 0.9387 0.8613 0.7813 0.7225 0.6432 0.7567 0.6645 0.5231 θ at 303K θ at 308K θ at 313K The rate of corrosion of mild steel in solution of the inhibitor was also calculated according to equation 2. These values are represented in Table 4 for the inhibitor in 0.5 M H2SO4. Corrosion rate (Cr) increases with increasing temperature from 303 K to 313 K in H 2SO4 medium, and the corrosion rate decreases with increase in the concentration of the inhibitor. This is because as the concentration of the inhibitor increases, more molecules of the inhibitor become available to cover the surface of the mild steel, preventing it from the grip of the corrodant. Table 4: Corrosion rate (Cr) of the mild steel in inhibitor/0.5M H2SO4 Conc. Of inhibitor in 0.5M H2SO4 1 Hr 2 Hrs 3 Hrs 1 Hr 2 Hrs 3 Hrs 1 Hr 2 Hrs 3 Hrs Blank 0.01 0.03 0.05 0.07 0.0018 0.0013 0.0006 0.0001 0.0001 0.0011 0.0009 0.0005 0.0002 0.0001 0.0009 0.0007 0.0006 0.0003 0.0001 0.0024 0.0018 0.0013 0.0008 0.0005 0.0013 0.0011 0.0008 0.0005 0.0004 0.0010 0.0009 0.0008 0.0005 0.0004 0.0028 0.0023 0.0018 0.0012 0.0007 0.0016 0.0014 0.0011 0.0008 0.0005 0.0012 0.0011 0.0009 0.0007 0.0006 Cr at 303K Cr at 308K Cr at 313K As presented in table 2, there is a significant increase in inhibition efficiency as the concentration of the inhibitor increases and it could be observed that the efficiency decreases with increase in temperature. This is suggestive of a physisorption mechanism of adsorption of the inhibitor onto the surface of the metal 9 [16]. Thermodynamic Considerations/parameters Activation energy of adsorption Eads, the quantity of energy Qads involved in the adsorption process, the enthalpy of adsorption ∆Hads, and the entropy of the system ∆Sads, where determined according to equations, 3b, 4b and 5b respectively. The results obtained for these parameters are shown in Table 5. Table 5: Thermodynamic parameters of the dioctyl phthalate inhibitor in 0.5 M H 2SO4 solution at different concentrations of the inhibitor Concentration of inhibtor (moldm-3) in 0.5M H2SO4 Blank 0.01 0.03 0.05 0.07 Activation energy of Adsorption Eads (kJ) R2 38.67 50.84 89.15 211.81 226.20 0.998 0.997 0.974 0.932 0.883 Heat of Adsorption Qads (kJ) -46.53 -87.55 -193.92 -184.45 Enthalpy of adsorption and Entropy of Adsorption Free Energy of Adsorption R2 ∆Hads (kJ) ∆Sads (JK-1) R2 ∆Gads (kJ) R2 0.989 0.932 0.894 0.802 32.29 43.23 77.1 185.54 197.12 -2.76 -2.28 -0.74 4.25 4.72 0.988 0.984 0.947 0.894 0.837 -46.5 -87.55 -194 -184.5 0.989 0.932 0.894 0.802 A change in the value of Eads in the presence of the inhibitor may be due to the modification of the mechanism of the corrosion process in presence of adsorbed inhibitor molecules. Bouyanzer et’al in Ejikeme et’al [26] indicated that generally, higher values of Eads in presence of additives support physical adsorption mechanism whereas an unchanged or lower value of Eads for inhibited systems compared to the blank is indicative of chemisorption mechanism. From table 5, it can be seen that the values of the activation energies for the inhibited systems are higher than that of the uninhibited or blank system. This is an indication of a physisorption mechanism of adsorption. The values of Enthalpy of adsorption and Entropy of Adsorption; ∆H ads and ∆Sads were obtained from the graphical plot of the Eyring transition state equation (equation 6). The values of ∆Hads and Eads are nearly the same in 0.5 M H2SO4, and are higher in the presence of the inhibitor. This indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution [27]. The positive values of ∆Hads for both corrosion processes with and without the inhibitor reveal the endothermic nature of the mild steel dissolution process and indicate that the dissolution of steel is difficult [27]. The values of entropy of adsorption ∆Sads listed in Table 5 for 0.5M H2SO4, clearly showed that entropy of activation increases in the presence of the 10 studied inhibitor compared to free acid solution. The negative Qads values indicate that adsorption and inhibition efficiency (Table 2) decreases with rise in temperature which is in line with the findings of Onen et al, [15] and indicates that the adsorption of the inhibitor to the surface of the mild steel is spontaneous, and also, this is indicative of a strong interaction between the inhibitor and the mild steel [15]. Adsorption Studies Three isotherms were selected for the adsorption study; these are Langmuir, the Freundlich and the newly proposed Adejoh – Ekwenchi isotherms. Table 6: Values for adsorption studies of the inhibitor on mild steel in 0.5 M H 2SO4 solutions at different temperatures. Temperature Langmuir (K) Isotherm Freundlich Isotherm Adejoh-Ekwenchi Isotherm KL R2 KF nF R2 KAE B R2 303 37 0.965 5.25 0.732 0.782 41.4 0.814 0.705 308 25 0.961 4.47 0.772 0.907 6.97 0.413 0.765 313 18.5 0.857 5.86 0.889 0.996 4.56 0.317 0.934 “b” (from the Adejoh – Ekwenchi Isotherm) is a parameter called “variation factor” and it relates the surface coverage by the adsorbate to the change of adsorbate bulk concentration. A decrease in the value of b with rise in temperature is an indication of physical adsorption mechanism, while increase or fairly constant in its value with rise in temperature shows the mechanism of adsorption to be chemical adsorption [24]. KAE is the isotherm adsorption equilibrium constant known as Adejo-Ekwenchi constant. Just like any isotherm it is a temperature dependent parameter, which gives the adsorption strength. High value of it is indicative of favourable adsorption process. It can be seen that K AE, followed the required trend similar to those of Langmuir and Freundlich, indicating a favourable adsorption process. It can also be seen that the values of “b” decreases with increase in temperature. This supports the claims by Adejoh et’ al, [24] that the mechanism for this type of trend is physisorption. This is in line with all the experimental data analysed previously in this research. The positive adsorption equilibrium constant KF (Freundlich Isotherm) values are indication of favourable adsorption. The parameter nF in the Freundlich isotherm relates to intensity of adsorption and it varies with heterogeneity of the material positive, but not an integer, with typical value is 0.6 11 [29]. [28], and the value is always The average value of nF is 0.798 for the inhibitor in 0.5 M H2SO4 indicating that this adsorption process can reasonably be modelled by Freundlich isotherm and is heterogeneous [16]. The adsorption equilibrium constant Kads (for all the Isotherms tested) decreases with increase in experimental temperature in the corrosion media, indicating that the interactions between the adsorbed molecules and the metal surface are weakened and consequently, the adsorbed molecules could become easily removable. Such data explains the decrease in the inhibition efficiency with increasing temperature [30]. Free Energy of Adsorption ∆Goads The standard free energy of adsorption, ΔG°ads, which can characterize the interaction of adsorbed molecules and metal surface, was calculated using Equation 10. The values of K ads were determined from the different isotherm plots, and are represented in table 7 for 0.5 M H2SO4 solution. The negative values of ΔG°ads ensure the spontaneity of adsorption process and stability of the adsorbed layer on the metal surface. Generally, the values of ΔG°ads below 40kJ/mol are consistent with physisorption, while those above 40 kJ/mol, involve chemisorption [31]. From table 7 below, the values of the standard free energy of adsorption are negative. This indicates that the adsorption process is spontaneous. The values also, were found to be below 40 kJ/mol; which concur with the aforementioned physisorption mechanism for the adsorption process in 0.5 M H2SO4 solution. Table 7: values for the standard free energy of adsorption ∆Goads of the solution of the inhibitor in 0.5 M H2SO4 at different temperatures Ekwenchi Adejoh - Freundlich Langmuir Isotherm Temperature (K) Kads R2 ∆Goads (kJ/mol) 303 37 0.965 -19.21 308 25 0.961 -18.53 313 18.5 0.857 -18.04 303 5.25 0.782 -14.30 308 4.47 0.907 -14.12 313 5.86 0.996 -15.05 303 41.4 0.705 -19.50 308 6.97 0.765 -15.26 313 4.56 0.934 -14.40 12 Kinetic Parameters The corrosion of mild steel follows a first order rate law with respect to the concentration of the inhibitor [32]. A decrease in the rate constant k, with increase in temperature indicates that the rate of the mild steel corrosion is greatly reduced in the presence of the inhibitor (dioctyl phthalate) in H2SO4 Solution. The half – life presented in table 8, indicates that the life span of the mild steel is increased as the temperature increased[32]. Table 8: rate constant k, and half – life t1/2 of mild steel in the inhibitor/0.5 M H2SO4 solution at different temperatures Temperature (K) Rate Constant k (hr-1) k (hr-1) R2 t1/2 (hr) 303 0.79 0.943 0.877 308 0.233 0.993 2.975 313 0.134 0.982 5.173 Summary Dioctyl phthalate can be said to be a good corrosion inhibitor, and gave a maximum percentage inhibition efficiency of 85.11% at a concentration of 0.07 M of the inhibitor in 0.5 M H2SO4 solution at a temperature of 303 K. The Freundlich adsorption isotherm was found to be the best fit for the adsorption studies of the corrosion inhibition of dioctyl phthalate in H2SO4 medium with an average R2 value of 0.9. It was also discovered that the Adejoh – Ekwenchi Isotherm can be used to model adsorption process for corrosion inhibition studies. All values of ∆Hads were found to be positive indicating an endothermic nature of the dissolution of the mild steel. The values of entropy of adsorption ∆S ads was found to increase with increase in concentration of the inhibitor indicating that the rate determining combination step is a more orderly arrangement relative to the initial state. It was also discovered that the values of Eads increases with increase in the concentration of the inhibitor. The values of Eads and those of ∆Hads varies in a similar manner, and the average values of Eads - ∆Hads equal to 2.41 kJmol-1, which is very close to the value of RT (2.51 kJmol-1) indicating that the adsorption process is unimolecular in nature[16]. 13 All values of Qads were found to be negative, indicating that the process of adsorption of the inhibitor on mild steel is exothermic and spontaneous. It also indicates strong interaction between the inhibitor and the mild steel surface. The corrosion kinetics showed an adherence to first order kinetics and the half- life values showed that the presence of the inhibitor extends the lifespan of the mild steel. The values of ∆Gads and Kads for the corrosion inhibition studies of mild steel in 0.5 M H 2SO4 and dioctyl phthalate as the inhibitor were found to be suggestive of a physisorption mechanism. Conclusion The corrosion inhibition properties of dioctyl phthalate were determined in 0.5 M H 2SO4 solution and at a temperature range of 303 to 313 K using the weight loss method. The results obtained, showed that dioctyl phthalate is a good corrosion inhibitor of mild steel. This efficiency was found to decrease with increasing temperature and increases with increase in the concentration of the inhibitor. The most suitable isotherm to be used for modelling the corrosion inhibition studies of the dioctyl phthalate is the Freundlich adsorption isotherm. 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