Chapter 5 Temperature Kinetics Studiesshodhganga.inflibnet.ac.in/bitstream/10603/17993/11/11_chapter...

Chapter 5 Temperature Kinetics Studies

Temperature Kinetics Studies

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Temperature Kinetics Studies This chapter explains the role of adsorption in corrosion inhibition. It deals with the various adsorption isotherms, their theoretical basis and the calculations of various thermodynamic and kinetic parameters. It entails the studies conducted on mild steel in acidic medium and provides an insight into the probable mechanism of inhibition followed by the selected inhibitors. Galvanostatic polarization parameters were used for temperature kinetics studies and also related calculations of various parameters. 5.1. Adsorption of inhibitor at the metal surface It is generally agreed that corrosion inhibition is due to the adsorption of the inhibitor molecule at the metal–solution interface, which is accompanied by a change in potential difference between the metal electrode and the solution due to the non-uniform distribution of electric charges at the interface. The metal–electrolyte interface is characterized by an electrical double layer, sometimes by a triple layer. A schematic representation of the electrical double layer is given in Fig. 5.1. The first layer is a sheet of charges at the metal surface caused by an excess or deficiency of electrons. The second layer (region A) is formed on the solution side of the interface by specially adsorbed ions. The loci of the centers of these charges form the inner Helmholtz plane of the double layer. These anions lose their coordinated water molecules or water sheaths, displace adsorbed water molecules from the metal surface, and in turn are adsorbed on portions of the bare metal surface. These ions are known as potential-determining ions. The charges are balanced in part by hydrated ions of opposite charge in the outer Helmholtz plane in region B called counter ions. Outside this area (i.e., region C in the figure) is known as the Gouy–Chapman diffuse layer, where the concentrations of the counter ions decrease toward that of bulk electrolyte and balance the net charge close to the metal surface.


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Fig. 5.1: Schematic representation of the electric double layer (Source: Green Corrosion

Inhibitors: Theory and Practice, V. S. Sastri, Copy right 2011 John Wiley & Sons)

When an inhibitor I approaches and adsorbs at the metal–solution interface, it may be written as:

M (nH2O)ads + I(sol) = MIads + nH2O(sol)

In the process of adsorption of the inhibitor, the inhibitor displaces n water molecules initially adsorbed on the metal. The adsorption of the inhibitor on the metal occurs because the interaction energy between the metal and the inhibitor is more favorable than the interaction energy between the metal and the water molecules.

The adsorption of inhibitors on the metal surface is governed by the residual charge on the metal and the chemical structure of the inhibitor. The two types of adsorption of an organic inhibitor on a metal surface are physical or electrostatic and/or chemisorption.

Physical adsorption is due to electrostatic attraction between inhibiting ions or dipoles and the electrically charged metal surface. The forces involved in electrostatic


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adsorption are generally weak. The inhibitor adsorbed on the metal due to electrostatic force can be desorbed easily. The main feature of electrostatic adsorption is that the adsorbed ions are not in direct physical contact with the metal. A layer of solvent molecules separates the metal from the ions. The physical adsorption process has low activation energy and is relatively independent of temperature.

Chemisorption involves charge sharing or charge transfer from the inhibitor molecules to the metal surface to form a coordinate type bond. The chemisorption process is slower than electrostatic sorption and has higher activation energy. The temperature dependence shows higher inhibition efficiency at higher temperatures. Unlike electrostatic adsorption, chemisorption is specific for certain metals and is not completely reversible. The nature of the metal and the organic inhibitor has a decisive effect on the bond between the metal and the inhibitor, and charge transfer from the inhibitor molecule to the metal is facilitated when the inhibitor molecule has a functional group with an atom containing a lone pair of electrons. Availability of π electrons due to the presence of multiple bonds and/or aromatic rings in the inhibitor molecule is thought to facilitate charge (electron) transfer from the inhibitor to the metal. The organic inhibitors used have reactive functional groups, which are the sites for the chemisorption process. The strength of the adsorption bond depends upon the electron density on the donor atom present in the functional group and its polarizability.

In order to understand the adsorption behavior of an inhibitor, thermodynamic adsorption parameters and kinetic corrosion parameters are required. In an earlier work, Riggs and Hurd [1] reported that the latent heat of adsorption could be obtained from a comparison of the activation energies of uninhibited and inhibited corrosion reaction. It was found that the positive heat of adsorption, ΔHads. > 0 (endothermic process), is attributed unequivocally to chemisorptions [2], and a negative heat of adsorption, ΔHads. < 0 (exothermic process), may involve either physisorption [3] or chemisorptions [4] or a mixture of both processes (comprehensive adsorption) [5].

It is well established that the effect of temperature on the inhibition of an acid-metal reaction is highly complex, due to the many changes that take place on the metal surface, such as rapid etching and desorption of the inhibitor, and due to the fact that


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the inhibitor itself may undergo decomposition and/or rearrangement [6]. The temperature dependence of the inhibiting effect and a comparison of the values of the apparent activation energy Ea, of the corrosion process in the absence and presence of the inhibitor of interest could provide further evidence concerning its mechanism of inhibition [7].

The negative values of ΔGads° suggest that the adsorption of inhibitor molecule onto metal surface is a spontaneous process [8] and having strong interaction between the inhibitor molecules and the metal surface [9]. Generally, values of ∆Gads° upto -20 kJ/mol are consistent with physisorption, while those around -40 kJ/mol or higher are associated with chemisorptions [10]. 5.2. Adsorption isotherms The relationships between corrosion rate and concentration of inhibitor on one hand and corrosion inhibition and concentration of inhibitor on the other have been shown in Figs. 5.2 and 5.3. These were earlier investigated by Sieverts and Leug [11]. The figures resemble adsorption isotherms.

Fig. 5.2 Fig. 5.3

Adsorption isotherms are often used to demonstrate the performance of organic adsorbent-type inhibitors. The first isotherm was proposed by Freundlich and Kuster (1894) and it is a purely empirical formula which is generally valid for gaseous substances.


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There are several isotherms that are commonly used to characterize inhibitor performance. The following adsorption isotherms have been considered in the present study.

1. Langmuir Adsorption Isotherm 2. Freundlich Adsorption Isotherm 3. Temkin Adsorption Isotherm 4. El-Awady Adsorption Isotherm 5. Flory-Huggins Adsorption Isotherm 6. Frumkin Adsorption Isotherm 5.2.1. Langmuir adsorption isotherm In 1916, Irving Langmuir developed an isotherm to describe the dependence of the surface coverage of an adsorbed gas on the pressure of the gas above the surface at a fixed temperature. Following are the assumptions while plotting this type of adsorption isotherm:

� The surface of the adsorbent is uniform, that is, all the adsorption sites are equal. � Adsorbed molecules do not interact i.e. molecule adsorption is independent of

occupation of neighboring sites. � All adsorption occur through the same mechanism. � Only monolayer is formed i.e. adsorption takes place only on the free surface of

the adsorbent.

If θ is the fraction of surface of adsorbent covered, (1 - θ) is fraction of uncovered sites and C is the concentration, and then the Langmuir adsorption isotherm equation can be written as:

�� = �� + �

where K is equilibrium constant which depends on temperature and strength of adsorption.


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This adsorption isotherm is followed if �� vs. � graph gives straight line with

correlation constant (R2) close to one [12 - 14]. 5.2.2. Freundlich adsorption isotherm Herbert Max Finley Freundlich gave an empirical isotherm which relates the concentration of a solute on the surface of an adsorbent, to the concentration of the solute in the liquid with which it is in contact. It can be expressed as:

� � � ��

where K is an approximate indicator of adsorption capacity and C is the concentration. n is an adsorption intensity, which is a function of the strength of adsorption or the mechanism of adsorption. The value of n between 2 and 10 shows good adsorption.

Taking log on both the sides, we can write as:

log � = log K + �� log C

log � vs. log C graph gives a straight line with slope equal to 1/n and intercept is equal to log K. If the correlation coefficient (R2) is nearly equal to one, it indicates that Freundlich adsorption isotherm is followed [15 - 17]. 5.2.3 Temkin Adsorption Isotherm Temkin suggested that the deviations from the Langmuir isotherm at high coverage’s could be accounted by regarding the surface of the metal as being composed of small patches of equal size, at each of which the Langmuir isotherm holds independently with a characteristics local standard free energy of adsorption that depends on patch distribution. The standard free energy for each patch was assumed to decrease by equal small decrements over successively covered patches with increase in coverage. If the value of free energy parameter (a) is small, θ varies linearly with the coverage in the intermediate region, whereas if (a) is sufficiently large, θ varies logarithmically with C in the intermediate region.


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Temkin’s equation can be written as:

�� = K C

where ‘a’ is energetic inhomogeneity factor. A low value signifies a weak dependence of the free energy of adsorption on the surface coverage.

Taking log on both side and on rearranging it, we can write as:

� = ��.�� - �.��

θ vs. log C graph gives a straight line with slope equal to �.�� and intercept is equal to �.��

�� . If the correlation coefficient (R2) is nearly equal to one, it means Temkin adsorption isotherm is being followed [18 - 20]. 5.2.4. El-Awady adsorption isotherm If the slope and Karl Pearson’s coefficient (correlation coefficient) obtained from Langmuir’s adsorption isotherm is one, then it can be said that ideal Langmuir adsorption isotherm model is followed. But if R2 = 1 and slope ≠ 1, then it is El-Awady adsorption isotherm [21 - 23].

El-Awady adsorption isotherm equation is: �� = K’Cy

Taking log on both the sides, log �� = log K’ + y log C

log �� vs. log C will give a straight line with slope y = �� where x is the number of

inhibitor molecules occupying one active site (or the number of water molecules replaced by one molecule of the inhibitor) and intercept is log K’ which is related with equilibrium constant K as K = K’(1/y). 5.2.5. Flory-Huggins adsorption isotherm The Flory-Huggins adsorption isotherm is used for polymer systems and is given by the equation:

�� = K (1 - �)x


86

where x, represents the number of desorbed water molecules from the surface replaced by the adsorption of one organic molecule.

Taking log on both the sides

log (��) = log K + x log (1 – �) The graph between log (��) and log (1 – �) gives a straight line with slope equal to x and intercept is equal to log K. If Karl Pearson’s coefficient for this straight line is nearly one, then Flory-Huggins adsorption isotherm is followed [24 - 27]. 5.2.6. Frumkin adsorption isotherm Frumkin isotherm is an extension of Langmuir isotherm. It states that adsorbed molecules do interact and affect further adsorption by either repulsion or attraction of molecules.

Mathematically, it can be represented as: �� = K C

Taking natural log on both sides:

ln �� = ln K + 2a�

where ‘a’ is an interaction parameter.

The graph between ln �� and � gives a straight line with slope equal to 2a and

intercept of ln K. If the correlation coefficient (R2) is equal to one then Frumkin adsorption isotherm is followed [28 - 30]. 5.3. Thermodynamic parameters Gibb’s free energy, enthalpy of adsorption and entropy of adsorption can be calculated from the adsorption isotherm [31 - 33].


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Standard Gibb’s free energy (ΔG°ads) can be calculated by using the relation:

ΔG°ads = -RT ln (55.5 Keq) = -2.303 RT log (55.5 Keq)

where Keq is an equilibrium adsorption constant which is calculated from the adsorption isotherm to be followed and it has been calculated with reference to its molarity of water, under standard condition, the value is 55.5 M.

From standard free energy, enthalpy and entropy of adsorption can be calculated by using the following relation:

ΔG°ads = ΔH°ads - T ΔS°ads = -2.303 RT log (55.5 Keq)

A negative ΔG°ads indicates the spontaneity of the adsorption of inhibitor on the surface of metal and vice versa.

Taking log on both the sides and on rearranging the equation:

log Keq = ��°

�. � �� + ��°

�. � �� - log 55.5

When log Keq vs. 1/T is plotted, a straight line is obtained where ΔH°ads is calculated from the slope and ΔS°ads is calculated from the intercept. 5.4. Adsorption kinetics Energy of activation is calculated by using the Arrhenius equation i.e.,

k = A' exp �� where Ea is the activation energy of adsorption A is the pre-exponential (frequency) factor or Arrhenius constant. k is the rate constant which is proportional to Icorr

Taking log on both sides, we can write as:

log Icorr = log A - � ��. � ��

Clearly, this is a straight line equation for log Icorr vs. 1/T graph whose slope gives Ea and intercept gives the value of A.


88

5.5. Adsorption behavior of 1H-1,2,3-triazole (Trz) on the mild steel surface in 0.5 M H2SO4

Corrosion and the surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of various concentrations of (Trz) at different temperatures was obtained from Galvanostatic polarization measurements and is listed in Table 5.1 [34].

Table 5.1: Corrosion and surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of (Trz) at different temperatures.

Temperature (K)

Concentration (M)

Icorr (µA cm-2)

I.E. (%)

θ

298

10-2 464 73.33 0.7333 5 x 10-3 625 64.08 0.6408 10-3 689 60.40 0.6040

5 x 10-4 752 56.80 0.5680 0 1740 - -

308

10-2 798 81.61 0.8161 5 x 10-3 981 77.40 0.7740 10-3 1060 75.58 0.7558

5 x 10-4 1120 74.19 0.7419 0 4340 - -

318

10-2 1350 86.76 0.8676 5 x 10-3 1420 86.08 0.8608 10-3 1810 82.25 0.8225

5 x 10-4 2510 75.39 0.7539 0 10200 - -

328

10-2 3560 75.94 0.7594 5 x 10-3 6250 57.77 0.5777 10-3 6420 56.62 0.5662

5 x 10-4 10200 31.08 0.3108 0 14800 - -

Fig. 5.4 illustrates the variation of inhibition efficiency (IE) with the concentration of (Trz) at different temperatures. It is evident from the Table 5.1 and Fig. 5.4 that the inhibition efficiency increases with increase in concentration of (Trz) and subsequently the extent of surface coverage by the inhibitor on the metal surface increases. Fig. 5.4 and Table 5.1 shows that IE increases with increase in experimental temperature up to 318 K, which indicates that this range of temperature


89

favors adsorption of (Trz) on the mild steel surface. Further, IE decreases at 328 K at all inhibitor concentrations which indicates that higher temperature might cause desorption of (Trz) from the mild steel surface. The adsorption-desorption dynamic equilibrium might have got shifted to the desorption process at higher temperature which in other words leads to shorter time lag between adsorption-desorption processes.

Fig. 5.4: Inhibition Efficiency (IE%) against concentration for corrosion of mild steel in 0.5 M

H2SO4 in the presence of (Trz) at different temperature. Various adsorption isotherms like Langmuir, Freundlich, Temkin, El-Awady, Flory-Huggins, Frumkin were plotted and analyzed, and the best fitted adsorption isotherm for adsorption of (Trz) on mild steel surface was found to be Temkin adsorption isotherm as is clear from Table 5.2 (whose R2 ≈ 1). (Note: Langmuir adsorption isotherm cannot be considered as the slope > 1 and intercept = 0 at all range of temperature considered)

Table 5.2: The Karl Pearson’s coefficient of different adsorption isotherm of (Trz) in 0.5 M H2SO4.

Adsorption Isotherm R2 Freundlich 0.843 Langmuir 0.992 Frumkin 0.759 El-Awady 0.842

Flory Huggins 0.792 Temkin 0.850

020406080

100120140

-4 -3 -2 -1 0 1

IE %

log C

At 298 K

At 308 K

At 318 K

At 328 K


90

Fig. 5.5: Temkin adsorption isotherm of (Trz) on the mild steel surface in 0.5 M H2SO4.

Fig. 5.5 gives a plot of Temkin adsorption isotherm and the corresponding values of linear regression parameters are given in Table 5.3.

Table 5.3: The Linear Regression parameters of Temkin adsorption isotherm of (Trz) in 0.5 M H2SO4.

Temperature (K)

Slope Intercept R2

298 0.110 0.929 0.877 308 0.050 0.904 0.877 318 0.080 1.038 0.858 328 0.271 1.272 0.786

The intercept of the plot was used to calculate the adsorption equilibrium constant which was further used to calculate the Gibbs’s free energy change for the adsorption process (Table 5.4). The negative values of ΔG°ads suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. The calculated value of ΔG°ads was found to be more than -40 kJ/mol at the studied temperature(s), which suggests that the adsorption mechanism involved here in the present case is chemisorption.

Table 5.4: Thermodynamic parameters of adsorption of (Trz) on the mild steel surface.

Temperature (K)

log Keq - ΔG°ads (kJ mol-1)

298 8.4470 58.142 308 18.0799 116.910 318 12.9750 89.622 328 4.6940 40.434

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-4 -3 -2 -1 0 1

θ

log C

AT 298 K

AT 308 K

AT 318 K

AT 328 K


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Fig. 5.6: Plot of variation of log Keq vs 1/T.

Plot of log Keq vs. 1/T (Fig. 5.6) was required to calculate other thermodynamic parameters i.e. ΔH°ads and ΔS°ads which are listed in Table 5.5.

Table 5.5: Enthalpy and Entropy value for the adsorption of (Trz) on the mild steel surface in 0.5 M H2SO4.

ΔH°ads (kJ mol-1)

ΔS°ads (J K-1mol-1)

-284.53 -665.34

Since the ΔH°ads value is negative, the adsorption of inhibitor molecules onto the mild steel surface is an exothermic process. Further, the high negative value (> 100 kJ/mol) proves that this was a chemisorption process [35]. It is obvious that the values of ΔS°ads are negative, as inhibitor molecules adsorbed onto the mild steel surface become more orderly, resulting in a decrease in entropy [9].

Values of apparent activation energy of corrosion (Ea) as calculated for mild steel in 0.5 M H2SO4 without and with various concentration of (Trz) determined from the slope of log Icorr vs. 1/T plot (Fig. 5.7) are shown in Table 5.6. The results show that the apparent activation energies at relatively lower concentration (5 x 10-4 M) increases, while in the range (10-2 M - 10-3 M), it decreases as compared to the one where no inhibitor was used.

02468

101214161820

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log K

eq

1/T (K-1)


92

Fig. 5.7: Plot of variation of log Icorr vs 1/T.

Table 5.6: Parameters of Linear Regression between log Icorr and 1/T.

Concentration (M)

log A R2 Ea (kJ mol-1)

10-2 6.025 0.966 53.63 5 x 10-3 6.971 0.870 58.57 10-3 6.982 0.916 58.34

5 x 10-4 8.965 0.921 69.60 0 7.686 0.978 59.37

According to Riggs and Hurd [1], the decrease in apparent activation energy at higher levels of inhibition arises from a shift of the net corrosion reaction, from one on the uncovered surface to one directly involving the adsorbed sites. Increase in apparent activation energy indicates that the energy barrier of the corrosion reaction increased in the presence of inhibitor without changing the mechanism of dissolution [36]. This also reveals that the entire process is surface-reaction controlled, since the energy of activation for the corrosion process, both in the absence and presence of inhibitor, was > 20 kJ/mol [37]. 5.6. Adsorption behavior of 1-hydroxybenzotriazole (HOBT) on the mild

steel surface in 0.5 M H2SO4 Corrosion and the surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of various concentrations of (HOBT) at different temperatures were obtained from Galvanostatic polarization measurements and are listed in Table 5.7 [34].

-4-3.5

-3-2.5

-2-1.5

-1-0.5

0

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log IC

ORR

1/T (K-1)

0.01 M0.005 M0.001 M0.0005 MACID


93

Table 5.7: Corrosion and surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of HOBT at different temperatures.

Temp. (K)

Conc. (M)

-Ecorr (mV vs. SCE)

Icorr (µA cm-2)

I.E. (%)

θ

298

10-2 424 31.7 98.18 0.9818 5 x 10-3 428 46.5 97.32 0.9732 10-3 468 157 90.98 0.9098

5 x 10-4 465 249 85.69 0.8569 0 475 1740 - -

308

10-2 482 263 93.94 0.9394 5 x 10-3 461 286 93.41 0.9341 10-3 439 500 88.48 0.8848

5 x 10-4 471 691 84.08 0.8408 0 484 4340 - -

318

10-2 463 615 93.97 0.9397 5 x 10-3 478 1100 89.22 0.8922 10-3 473 1540 84.90 0.8490

5 x 10-4 472 1660 83.73 0.8373 0 459 10200 - -

328

10-2 458 1230 91.69 0.9169 5 x 10-3 480 2140 85.54 0.8554 10-3 460 2230 84.93 0.8493

5 x 10-4 467 5200 64.86 0.6486 0 458 14800 - -

Fig. 5.8 illustrates the variation of inhibition efficiency (IE) with the concentration of (HOBT) at different temperatures. It is evident from the Table 5.7 and Fig. 5.8 that the inhibition efficiency increases with increase in concentration of (HOBT) and subsequently the extent of surface coverage by the inhibitor on the metal surface increases. Fig. 5.8 and Table 5.7 show that IE decreases with increase in experimental temperature. IE decreases at all inhibitor concentration which indicates that the higher temperature might cause desorption of (HOBT) from the mild steel surface. The adsorption-desorption dynamic equilibrium might have got shifted to the desorption process with the increase in temperature. This also decreases the time lag of adsorption-desorption process.


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Fig. 5.8: Inhibition Efficiency (IE %) against concentration for corrosion of mild steel in 0.5 M

H2SO4 in the presence of HOBT at different temperature. Various adsorption isotherms like Langmuir, Freundlich, Temkin, El-Awady, Flory-Huggins, Frumkin were plotted and analyzed, and the best fitted adsorption isotherm for adsorption of (HOBT) on mild steel surface was found to be El-Awady adsorption isotherm as clear from Table 5.8 (whose R2 ≈ 1). (Note: Langmuir adsorption isotherm cannot be considered as the slope > 1 and intercept = 0 at all range of temperature considered).

Table 5.8: The Karl Pearson’s coefficient of different adsorption isotherm of HOBT in 0.5 M H2SO4



Fig. 5.9 gives the plot of El-Awady adsorption isotherm and the corresponding values of linear regression parameters are given in Table 5.9.

0

20

40

60

80

100

120

140

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5

IE % At 298 K

At 308 K

At 318 KAt 328 K

log C


95

Fig. 5.9: El-Awady adsorption isotherm of HOBT on the mild steel surface in 0.5 M H2SO4.

Table 5.9: The Linear Regression parameters of El-Awady adsorption isotherm of HOBT in

0.5 M H2SO4.

Temperature (K)

Slope Intercept R2

298 0.747 3.250 0.997 308 0.334 1.886 0.983 318 0.340 1.796 0.880 328 0.469 1.953 0.769

The intercept of the plot was used to calculate the adsorption equilibrium constant which was further used to calculate the Gibbs’s free energy change for the adsorption process (Table 5.10). The negative values of ΔG°ads suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. The calculated values of ΔG°ads are around – 40 kJ/mol, indicates that the adsorption mechanism of HOBT on mild steel in 0.5 M H2SO4 at the studied temperatures is comprehensive adsorption involving both electrostatic adsorption and chemisorption.

0

0.5

1

1.5

2

2.5

3

3.5

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5

log {θ

/(1-θ)

}

log C

AT 298 K

AT 308 K

AT 318 K

AT 328 K


96

Table 5.10: Thermodynamic parameters of adsorption of HOBT on mild steel surface.

Temperature (K)


298 4.3541 34.801 308 5.6464 43.586 318 5.2810 42.782 328 4.1644 37.107



Table 5.11: Enthalpy and Entropy value for the adsorption of HOBT on the mild steel

surface in 0.5 M H2SO4.

ΔH°ads (kJ mol-1)


-14.60 79.77 Since the ΔH°ads value is negative, the adsorption of inhibitor molecules onto the mild steel surface is an exothermic process. Further, the negative values (< 40 kJ/mol) suggest that the process is physical adsorption process [35]. The positive value of ΔS°ads implies that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of inhibitor onto the mild steel surface [38].

0123456

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log K

eq

1/T (K-1)


97



Concentration (M)


10-2 12.62 0.938 96.79 5 x 10-3 14.14 0.971 104.83 10-3 9.25 0.967 74.18 5 x 10-4 10.59 0.995 81.11 0 7.68 0.978 59.37

Values of apparent activation energy of corrosion (Ea) calculated for mild steel in 0.5 M H2SO4 without and with various concentration of (HOBT) determined from the slope of log Icorr vs. 1/T plot (Fig. 5.11) are shown in Table 5.12. Clearly, from the Table 5.12, the values of Ea are higher in the presence of inhibitor than in a blank solution which indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution [36]. 5.7. Adsorption behavior of 1-hydroxy-7-azabenzotriazole (HOAT) on the

mild steel surface in 0.5 M H2SO4 Corrosion and the surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of various concentrations of (HOAT) at different temperatures were obtained from Galvanostatic polarization measurements and are listed in Table 5.13 [34].

-5-4.5

-4-3.5

-3-2.5

-2-1.5

-1-0.5

0

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log ICO

RR

1/T (K-1)

0.01 M0.005 M0.001 M0.0005 MACID


98

Table 5.13: Corrosion and surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of HOAT at different temperatures.

Temp. (K)

Conc. (M)

-Ecorr (mV vs. SCE)

Icorr (µA cm-2)

I.E. (%)

θ

298

10-2 424 44.8 97.42 0.9742 5 x 10-3 439 50 97.12 0.9712 10-3 442 121 93.05 0.9305

5 x 10-4 457 211 87.87 0.8787 0 475 1740 - -

308

10-2 429 145 96.64 0.9664 5 x 10-3 460 192 95.57 0.9557 10-3 479 373 91.40 0.9140

5 x 10-4 480 666 84.65 0.8465 0 484 4340 - -

318

10-2 479 373 96.34 0.9634 5 x 10-3 470 558 94.53 0.9453 10-3 476 895 91.22 0.9122

5 x 10-4 480 2830 72.25 0.7225 0 459 10200 - -

328

10-2 501 1210 91.82 0.9182 5 x 10-3 479 1600 89.19 0.8919 10-3 493 5660 61.76 0.6176

5 x 10-4 491 8040 45.67 0.4567 0 458 14800 - -

Fig. 5.12 illustrates the variation of inhibition efficiency (IE) with the concentration of (HOAT) at different temperatures. It is evident from the Table 5.13 and Fig. 5.12 that the inhibition efficiency increases with increase in concentration of (HOAT) and subsequently the extent of surface coverage by the inhibitor on the metal surface increases. Fig. 5.12 and Table 5.13 show that IE decreases with increase in experimental temperature. IE decreases at all inhibitor concentrations with increase in temperature which indicates that higher temperature might cause desorption of (HOAT) from the mild steel surface. The adsorption-desorption dynamic equilibrium might have got shifted to the desorption process with the increase in temperature.


99

Fig. 5.12: Inhibition Efficiency (IE %) against concentration for corrosion of mild steel in

0.5 M H2SO4 in the presence of HOAT at different temperature. Various adsorption isotherms like Langmuir, Freundlich, Temkin, El-Awady, Flory-Huggins, Frumkin were plotted and analyzed, and the best fitted adsorption isotherm for the adsorption of (HOAT) on mild steel surface was found to be El-Awady adsorption isotherm as it is clear from Table 5.14 (whose R2 ≈ 1). (Note: Langmuir adsorption isotherm cannot be considered as the slope > 1 and intercept = 0 at all range of temperatures considered).

Table 5.14: The Karl Pearson’s coefficient of different adsorption isotherm of HOAT in 0.5 M H2SO4



Fig. 5.13 gives the plot of El-Awady adsorption isotherm and the corresponding values of linear regression parameters are given in Table 5.15.

020406080

100120140160180

-4 -3 -2 -1 0 1

IE %

log C

At 298 K

At 308 K

At 318 K

At 328 K


100

Fig. 5.13: El-Awady adsorption isotherm of HOAT on the mild steel surface in 0.5 M H2SO4.

Table 5.15: The Linear Regression parameters of El-Awady adsorption isotherm of HOAT in 0.5 M H2SO4.

Temperature (K)

Slope Intercept R2

298 0.556 2.747 0.965 308 0.526 2.535 0.971 318 0.670 2.798 0.854 328 0.898 2.906 0.989

The intercept of the plot was used to calculate the adsorption equilibrium constant which was further used to calculate the Gibbs’s free energy change for the adsorption process (Table 5.16). The negative values of ΔG°ads suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. The calculated values of ΔG°ads are between – 20 kJ/mol and – 40 kJ/mol which indicates that the adsorption mechanism of HOAT on mild steel in 0.5 M H2SO4 at the studied temperatures is comprehensive adsorption involving both electrostatic and chemisorption.

Table 5.16: Thermodynamic parameters of adsorption of HOAT on mild steel surface.

Temperature (K)


298 4.9390 38.134 308 4.8189 38.706 318 4.1761 36.056 328 4.2380 31.285

-0.50

0.51

1.52

2.53

3.5

-3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5

log {θ

/(1-θ)

}

log C

AT 298 KAT 308 KAT 318 KAT 328 K


101



Table 5.17: Enthalpy and Entropy value for the adsorption of HOAT on the mild steel

surface in 0.5 M H2SO4

ΔH°ads (kJ mol-1)


-51.54 -44.52

Since the ΔH°ads value is negative, the adsorption of inhibitor molecules onto the mild steel surface is an exothermic process. Further, the negative value (> 40 kJ/mol) suggest that this is a comprehensive adsorption process [39-41]. It is obvious that the values of ΔS°ads are negative, as inhibitor molecules adsorbed onto the mild steel surface become more orderly, resulting in a decrease in entropy of this process [9].

Values of apparent activation energy of corrosion (Ea) calculated for mild steel in 0.5 M H2SO4 with and without various concentration of (HOAT) determined from the slope of log Icorr vs. 1/T plot (Fig. 5.15) are shown in Table 5.18. Clearly, from the Table 5.18, the values of Ea are higher in the presence of inhibitor than in a blank solution which indicates that the energy barrier of the corrosion reaction increased in the presence of the inhibitor without changing the mechanism of dissolution [36].

44.14.24.34.44.54.64.74.84.9

55.1

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log K

eq

1/T (K-1)


102



Concentration (M)


10-2 11.07 0.997 88.02 5 x 10-3 12.06 0.998 93.27 10-3 13.62 0.966 100.43

5 x 10-4 13.92 0.996 100.50 0 7.68 0.978 59.37

5.8. Adsorption behavior of 5-methyl-1H-benzotriazole (MBTA) on the

mild steel surface in 0.5 M H2SO4 Corrosion and the surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of various concentrations of (MBTA) at different temperatures were obtained from Galvanostatic polarization measurements and are listed in Table 5.19 [34].

-5-4.5-4

-3.5-3

-2.5-2

-1.5-1

-0.50

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log ICO

RR

1/T (K-1)

0.01 M0.005 M0.001 M0.0005 MACID


103

Table 5.19: Corrosion and surface coverage parameters of mild steel in 0.5 M H2SO4 in the presence of MBTA at different temperatures.

Temp. (K)

Conc. (M)

-Ecorr (mV vs. SCE)

Icorr (µA cm-2)

I.E. (%)

θ

298

10-2 436 13.1 99.25 0.9925 5 x 10-3 483 39.1 97.75 0.9775 10-3 486 80.2 95.39 0.9539

5 x 10-4 474 106 93.91 0.9391 0 475 1740 - -

308

10-2 479 35.3 99.19 0.9919 5 x 10-3 466 71.2 98.36 0.9836 10-3 478 158 96.36 0.9636

5 x 10-4 502 309 92.88 0.9288 0 484 4340 - -

318

10-2 485 65.4 99.36 0.9936 5 x 10-3 505 152 98.51 0.9851 10-3 485 351 96.56 0.9656

5 x 10-4 464 455 95.54 0.9554 0 459 10200 - -

328

10-2 458 149 98.99 0.9899 5 x 10-3 480 223 98.49 0.9849 10-3 460 483 96.74 0.9674

5 x 10-4 467 1840 87.57 0.8757 0 458 14800 - -

Fig. 5.16 illustrates the variation of inhibition efficiency (IE) with the concentration of (MBTA) at different temperatures. It is evident from the Table 5.19 and Fig. 5.16 that the inhibition efficiency increases with increase in concentration of (MBTA) and subsequently the extent of surface coverage by the inhibitor on the metal surface increases. Fig. 5.16 and Table 5.19 shows that IE remains almost unaffected with increase in experimental temperature up to 318 K, which indicates that this range of temperature favors adsorption of (MBTA) on the mild steel surface. Further, IE decreases at 328 K at all inhibitor concentration which indicates that the higher


104

temperature might cause desorption of (MBTA) from the mild steel surface. The adsorption-desorption dynamic equilibrium might have got shifted to the desorption process at higher temperature.

Fig. 5.16: Inhibition Efficiency (IE %) against concentration for corrosion of mild steel in

0.5 M H2SO4 in the presence of MBTA at different temperature. Various adsorption isotherms like Langmuir, Freundlich, Temkin, El-Awady, Flory-Huggins, Frumkin were plotted and analyzed, and the best fitted adsorption isotherm for adsorption of (MBTA) on mild steel surface was found to be Langmuir adsorption isotherm as is clear from Table 5.20 (whose R2 = 1).

Table 5.20: The Karl Pearson’s coefficient of different adsorption isotherm of MBTA in 0.5 M H2SO4.



Fig. 5.17 gives the plot of Langmuir adsorption isotherm and the corresponding values of linear regression parameters are given in Table 5.21.

0

20

40

60

80

100

120

140

-4 -3 -2 -1 0 1

IE %

log C

At 298 K

At 308 K

At 318 K

At 328 K


105

Fig. 5.17: Langmuir adsorption isotherm of MBTA on the mild steel surface in 0.5 M H2SO4. Table 5.21: The Linear Regression parameters of Langmuir adsorption isotherm of MBTA

in 0.5 M H2SO4.

Temperature (K)

Slope Intercept R2

298 1.004 0.00005 1.000 308 1.005 0.00004 1.000 318 1.004 0.00003 1.000 328 1.005 0.00005 1.000

The intercept of the plot was used to calculate the adsorption equilibrium constant which was further used to calculate the Gibbs’s free energy change for the adsorption process (Table 5.22). The negative values of ΔG°ads suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. The calculated values of ΔG°ads are between – 20 kJ/mol and – 40 kJ/mol which indicates that the adsorption mechanism of MBTA on mild steel in 0.5 M H2SO4 at the studied temperatures is a comprehensive are involving both electrostatic and chemisorption.

Table 5.22: Thermodynamic parameters of adsorption of MBTA on mild steel surface.

Temperature (K)


298 4.3010 34.49 308 4.3979 36.22 318 4.5185 38.16 328 4.3010 37.97

0

0.002

0.004

0.006

0.008

0.01

0.012

0 0.002 0.004 0.006 0.008 0.01 0.012

C / θ

C

AT 298 KAT 308 KAT 318 KAT 328 K


106



Table 5.23: Enthalpy and Entropy value for the adsorption of MBTA on the mild steel

surface in 0.5 M H2SO4.

ΔH°ads (kJ mol-1)


2.62 125.64 Since the ΔH°ads value is positive, the adsorption of inhibitor molecules onto the mild steel surface is an endothermic process. Further, the positive value suggest for the chemisorption process [2]. The positive value of ΔS°ads implies that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of inhibitor onto the mild steel surface [38].

Values of apparent activation energy of corrosion (Ea) calculated for mild steel in 0.5 M H2SO4 with and without various concentration of (MBTA) as determined from the slope of log Icorr vs. 1/T plot (Fig. 5.19) are shown in Table 5.24. According to Riggs and Hurd [1], the decrease in apparent activation energy at higher levels of inhibition arises from a shift of the net corrosion reaction, from one on the uncovered surface to one directly involving the adsorbed sites. Increase in apparent activation energy indicates that the energy barrier of the corrosion reaction increased in the presence of

4.25

4.3

4.35

4.4

4.45

4.5

4.55

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log K

eq

1/T (K-1)


107

inhibitor without changing the mechanism of dissolution [36]. This also reveals that the entire process is surface-reaction controlled, since the energy of activation for the corrosion process, both in the absence and presence of inhibitor, was > 20 kJ/mol [37].



Concentration (M)


10-2 6.414 0.994 64.33 5 x 10-3 4.128 0.989 48.67 10-3 4.755 0.980 50.40

5 x 10-4 8.728 0.954 72.51 0 7.686 0.978 59.37

5.9. Summary � The extent of surface coverage by the inhibitor molecules on the mild steel surface

increases with the increase in concentration. � The Karl Pearson’s coefficient (R2) values suggest that 1H-1,2,3-triazole (Trz)

follows Temkin adsorption isotherm, 1-hydroxybenzotriazole (HOBT) and 1-hydroxy-7-azabenzotriazole (HOAT) follows El-Awady adsorption isotherm, and Langmuir adsorption isotherm is followed by 5-methyl-1H-benzotriazole (MBTA).

-6

-5

-4

-3

-2

-1

0

0.003 0.00305 0.0031 0.00315 0.0032 0.00325 0.0033 0.00335 0.0034

log IC

ORR

1/T (K-1)

0.01 M0.005 M0.001 M0.0005 MACID


108

� The value of ΔH°ads suggests that 1H-1,2,3-triazole (Trz) and 5-methyl-1H-benzotriazole (MBTA) shows chemisorptions, 1-hydroxybenzotriazole (HOBT) shows physisorptions, and 1-hydroxy-7-azabenzotriazole (HOAT) indicates comprehensive adsorptions. � The negative values of ∆G°ads ensure the spontaneity of the adsorption process

and stability of the adsorbed layer on the mild steel surface by the inhibitor molecules.


109

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