# Garcinia indica as an Environmentally Safe Corrosion Inhibitor for Aluminium in 0.5M Phosphoric Acid

Authors: Deepa Prabhu; Padmalatha Rao
Journal: International Journal of Corrosion
 (2013)

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## Abstract

The Inhibitive and adsorption properties of aqueous extract of seeds ofGarcinia indica extract (GIE) have been studied for corrosion control of aluminium in 0.5 M phosphoric acid solution using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques at 30∘C to 50∘C. The effects of inhibitor concentration on the inhibition action were investigated. Polarization measurements showed that the GIE acted as mixed inhibitor and the inhibitor molecules followed chemical adsorption on the surface of aluminium. The adsorption of GIE on metal surface obeyed Langmuir adsorption isotherm. The surface morphology of aluminium, in the absence and in the presence of GIE in 0.5 M phosphoric acid solution, was studied using scanning electron microscopy (SEM). Results obtained from both of the electrochemical techniques were in good agreement with each other.


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## Body

## 1. Introduction

Aluminium is the most commonly used metal in power transmission and the metallurgy of nonferrous metals due to its high-electrical conductivity, good working and forming properties, lightness, ease of recycling, low density, ductility, and high-mechanical strength [1, 2]. Aluminium and most of the aluminium alloys have good corrosion resistance towards natural atmosphere and other environments, because aluminium surfaces are covered with a natural oxide film. In the presence of aggressive environment the protective layer can be locally destroyed and corrosive attack takes place. Solutions of phosphoric acid are frequently employed for cleaning of aluminium [3] and in commercial preplating anodic oxidation and electropolishing of aluminium [4]. Phosphoric acid is most preferred for metal cleaning than hydrochloric [5] or sulphuric acid because of lower dissolution of metal in phosphoric acid medium. Even though dissolution rate of aluminium in phosphoric acid is lower, it is not at all negligible [6]. Therefore, it is necessary to use inhibitors for the corrosion of aluminium in phosphoric acid solution [7–10].Among the numerous organic compounds that have been tested and are applied industrially as corrosion inhibitors, those that are nontoxic are now far more strategic than in the recent past [7–11]. In the past two decades the research in the field of green inhibitors has been addressed toward the goal of using cheap, effective molecules at low or zero environmental impact [12–18]. In an attempt to find corrosion inhibitors that are environmentally safe and readily available, there has been a growing trend in the use of natural products such as plants extracts as corrosion  inhibitors for metals in acid-cleaning process [19–21].As a part of our studies on the corrosion inhibition of aluminium and 6063 aluminium alloy in acidic medium using plant extracts [22, 23] we report herein the results of using aqueous extract of Garcinia indica seed as a novel green inhibitor for corrosion control of aluminium in 0.5 M phosphoric acid solution.Garcinia indica, a plant in the mangosteen family (Clusiaceae), is a fruit-bearing tree that has culinary, pharmaceutical, and industrial uses. Seeds of Garcinia indica contain several organic compounds that are biodegradable and ecologically acceptable [24]; thus it is selected for the present investigation. The objective of the present work is extended to investigate the inhibition effect of Garcinia indica on the corrosion of aluminum in H3PO4 solutions for the first time by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) methods. Effects of inhibitor concentration and temperature on the corrosion inhibition were investigated. The adsorption and inhibition effect of GIE on aluminum in H3PO4 solutions are discussed.


## 2. Experimental

### 2.1. Material

The commercially available sample of aluminium was used for the study. The specimen contains 99.61% aluminium with 0.27% iron and 0.12% silicon.


### 2.2. Medium

Stock solution of 0.5 M phosphoric acid was prepared by diluting AR grade phosphoric acid (85%) with double distilled water and was standardized by potentiometric method.


### 2.3. Preparation of Test Coupons

Cylindrical test coupon was sealed with acrylic resin material in such a way that the area exposed to the medium was 0.7 cm2. The disc was polished with 180, 280, 400, 600, 800, 1000, 1500, and 2000 grade emery papers and further polished with disc polisher using levigated alumina till mirror surface was obtained. It was dried and stored in a desiccator to avoid moisture before being used for corrosion studies.


### 2.4. Preparation and Characterization ofGarcinia indica Extract (GIE)

A sample of the powder (25 g) was refluxed in 250 mL of water at 90°C for 3 h. The refluxed solution was kept overnight, filtered, and the filter liquor was evaporated to dryness. Then the deep brown-reddish solid residue was obtained and preserved in a desiccator. It was used to prepare the inhibitor solution of required strength in distilled water. The concentration range of GIE used was 100–500 ppm.FTIR spectra of the dried sample were recorded using spectrophotometer (Shimadz Model) in the frequency range of 4000 to 400 cm−1 using KBr pellet technique.


### 2.5. Electrochemical Measurements

Electrochemical measurements were carried out by using an electrochemical work station, (CH600D-series, U.S. Model with CH instrument beta software). The electrochemical cell used was a conventional three-electrode compartment having glass cell with a platinum counter electrode and a saturated calomel electrode (SCE) as reference. The working electrode was made up of aluminium. All the values of potential were with reference to the SCE. The polarization studies were carried out immediately after the EIS studies on the same electrode without any further surface treatment.


### 2.6. Tafel Polarization Studies

Finely polished specimens were exposed to the corrosion medium of 0.5 M phosphoric acid solution in the absence and in the presence of the inhibitor at different temperatures (30°C to 50°C) and allowed to establish a steady-state open circuit potential (OCP) for approximately 30 min. The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to the OCP at a scan rate of 1 mV s−1.


### 2.7. Electrochemical Impedance Spectroscopy (EIS) Studies

The impedance measurements were carried out in the frequency range from 100 kHz to 0.01 Hz, at the rest potential, by applying 10 mV sine wave AC voltage. The impedance data were analyzed using Nyquist plots. The charge transfer resistance,Rct, was extracted from the diameter of the semicircle in Nyquist plot.In all the above mentioned measurements, at least three similar results were considered and their average values are reported.


### 2.8. Scanning Electron Microscopy (SEM) Studies

The surface morphology of the aluminium specimen immersed in 0.5 M H3PO4 solution in the presence and the absence of inhibitor was compared by recording the SEM images of the samples using EVO MA18.


## 2.1. Material

The commercially available sample of aluminium was used for the study. The specimen contains 99.61% aluminium with 0.27% iron and 0.12% silicon.


## 2.2. Medium

Stock solution of 0.5 M phosphoric acid was prepared by diluting AR grade phosphoric acid (85%) with double distilled water and was standardized by potentiometric method.


## 2.3. Preparation of Test Coupons

Cylindrical test coupon was sealed with acrylic resin material in such a way that the area exposed to the medium was 0.7 cm2. The disc was polished with 180, 280, 400, 600, 800, 1000, 1500, and 2000 grade emery papers and further polished with disc polisher using levigated alumina till mirror surface was obtained. It was dried and stored in a desiccator to avoid moisture before being used for corrosion studies.


## 2.4. Preparation and Characterization ofGarcinia indica Extract (GIE)

A sample of the powder (25 g) was refluxed in 250 mL of water at 90°C for 3 h. The refluxed solution was kept overnight, filtered, and the filter liquor was evaporated to dryness. Then the deep brown-reddish solid residue was obtained and preserved in a desiccator. It was used to prepare the inhibitor solution of required strength in distilled water. The concentration range of GIE used was 100–500 ppm.FTIR spectra of the dried sample were recorded using spectrophotometer (Shimadz Model) in the frequency range of 4000 to 400 cm−1 using KBr pellet technique.


## 2.5. Electrochemical Measurements

Electrochemical measurements were carried out by using an electrochemical work station, (CH600D-series, U.S. Model with CH instrument beta software). The electrochemical cell used was a conventional three-electrode compartment having glass cell with a platinum counter electrode and a saturated calomel electrode (SCE) as reference. The working electrode was made up of aluminium. All the values of potential were with reference to the SCE. The polarization studies were carried out immediately after the EIS studies on the same electrode without any further surface treatment.


## 2.6. Tafel Polarization Studies

Finely polished specimens were exposed to the corrosion medium of 0.5 M phosphoric acid solution in the absence and in the presence of the inhibitor at different temperatures (30°C to 50°C) and allowed to establish a steady-state open circuit potential (OCP) for approximately 30 min. The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to the OCP at a scan rate of 1 mV s−1.


## 2.7. Electrochemical Impedance Spectroscopy (EIS) Studies

The impedance measurements were carried out in the frequency range from 100 kHz to 0.01 Hz, at the rest potential, by applying 10 mV sine wave AC voltage. The impedance data were analyzed using Nyquist plots. The charge transfer resistance,Rct, was extracted from the diameter of the semicircle in Nyquist plot.In all the above mentioned measurements, at least three similar results were considered and their average values are reported.


## 2.8. Scanning Electron Microscopy (SEM) Studies

The surface morphology of the aluminium specimen immersed in 0.5 M H3PO4 solution in the presence and the absence of inhibitor was compared by recording the SEM images of the samples using EVO MA18.


## 3. Results and Discussion

### 3.1. Fourier Transform Infrared (FTIR) Spectroscopy of GIE

Figure1 shows the FTIR spectroscopy of GIE. –OH stretching frequency appears at 3396.41 cm−1. The aromatic stretching frequency appears at 2923.88 cm−1. –C=O– stretching frequency is at 1785.96 cm−1. –C=C– stretching frequency is at 1608.52 cm−1. –CH2 bending frequency is at 1446.51 cm−1. –C–H bending frequency is at 1282.57 cm−1. –C–O bending frequency is at 1099.35 cm−1.Figure 1
FTIR spectrum of GIE.


### 3.2. Tafel Polarization Measurement

The effect of GIE on the corrosion reactions was determined by polarization techniques. The changes observed in the polarization curves after the addition of the inhibitor are usually used as the criteria to classify inhibitors as cathodic, anodic, or mixed [25, 26].Figure2 shows the Tafel polarization curves for aluminium in 0.5 M phosphoric acid solution at different concentrations of inhibitor at 30°C. Similar results were obtained at other temperatures as well.Figure 2
Tafel polarization curves for aluminum in 0.5 M phosphoric acid solution at different concentrations of inhibitor (CSE) at 30°C.Inhibitor efficiency was calculated using(1)η(%)=icorr-icorr(inh)icorr×100,
where icorr and icorr(inh) are the corrosion current densities obtained in uninhibited and inhibited solutions, respectively. The corrosion rate was calculated using
(2)υcorr(mmy-1)=3270×M×icorrρ×Z,
where 3270 is a constant that defines the unit of corrosion rate, icorr is the corrosion current density in A cm−2, ρ is the density of the corroding material (g cm−3), M is the atomic mass of the metal, and Z is the number of electrons transferred per atom [27, 28].The valuable potentiodynamic polarization parameters such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (bc and ba), corrosion current density (icorr) and corrosion rate (CR) were obtained from Tafel plots.  The inhibition efficiency values η(%) were calculated using (1). The results are tabulated in Table 1. It is obvious that corrosion current density decreases with increase in the concentration of the inhibitor.Table 1
Results of Tafel polarization studies on aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor GIE.
Temp.(°C)
Conc. of inhibitor(ppm)
E
corr (mV versus SCE)
i
corr (×m A cm−2)
-
b
c
(mV dec−1)
b
a
(mV dec−1)
CR(mm y−1)
η (%)
30
Blank
−797
0.17
637
354
1.89
—
100
−795
0.09
626
356
1.03
45.33
200
−805
0.08
634
346
0.91
51.96
300
−772
0.08
611
361
0.85
54.77
400
−801
0.06
686
333
0.70
62.99
500
−785
0.06
664
346
0.63
66.58
35
Blank
−793
0.22
610
353
2.39
—
100
−788
0.11
641
367
1.18
50.64
200
−779
0.10
674
342
1.06
55.66
300
−796
0.09
668
365
0.96
59.68
400
−796
0.07
685
353
0.80
66.70
500
−764
0.06
632
319
0.71
70.37
40
Blank
−797
0.28
596
364
3.00
—
100
−774
0.12
615
374
1.34
55.38
200
−789
0.11
621
373
1.16
61.30
300
−796
0.10
642
371
1.06
64.56
400
−777
0.08
670
321
0.84
72.15
500
−769
0.07
655
299
0.75
75.17
45
Blank
−804
0.40
595
367
4.38
—
100
−797
0.16
578
397
1.75
60.04
200
−799
0.14
616
380
1.49
66.01
300
−793
0.11
642
380
1.24
71.70
400
−780
0.10
716
312
1.14
74.03
500
−665
0.08
546
264
0.84
80.84
50
Blank
−800
0.48
579
379
5.20
—
100
−765
0.16
573
370
1.73
66.75
200
−806
0.14
586
396
1.56
70.09
300
−810
0.12
595
392
1.28
75.38
400
−738
0.09
609
366
1.02
80.44
500
−639
0.07
616
343
0.75
85.59From Figure2 and Table 1, it can be observed that the addition of GIE at all the studied concentrations resulted in the significant decrease in the corrosion current density (icorr) and decrease in the corrosion rate. It is also evident that the  percentage efficiency of the inhibitor increased with increase in the concentration of the inhibitor. Percentage efficiency of the inhibitor also increased with increase in temperature. There was no remarkable shift in the corrosion potential (Ecorr) value with respect to the blank. According to the literature report [28, 29], when corrosion potential is more than ±85 mV with respect to the corrosion potential of the blank, the inhibitor can be considered distinctively as either cathodic or anodic type. However, the maximum displacement in this study is less than ±85 mV. This suggests that GIE functions as mixed-type inhibitor.There is a slight change in the anodic and cathodic slopes after the addition of the inhibitor. The parallel cathodic Tafel curves suggest that the hydrogen evolution is activation controlled, and the reduction mechanism is not affected by the presence of the inhibitor. The shift in the anodic Tafel slope may be due to the surface/inhibitor molecules adsorbed on the metal surface.


### 3.3. Electrochemical Impedance Spectroscopy (EIS) Studies

The results of the Tafel polarization method were compared with the results of impedance measurements. EIS measurements were carried out at different temperatures at different inhibitor concentrations. The Nyquist plots of aluminium in 0.5 M phosphoric acid containing various concentrations of inhibitor are shown in Figure3.Figure 3
Nyquist plots and equivalent circuit for the corrosion of aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor (GIE) at 30°C.Impedance plots are semicircles both in the absence and in the presence of the inhibitor. The diameter of the capacitive loop increased with increase in the concentration of the inhibitor. This indicated that the impedance of the inhibited substrate increased with the inhibitor concentration. The impedance plots have a large capacitive loop at higher frequencies (HF), small inductive loop at intermediate frequencies (IF), followed by a second capacitive loop at low frequency (LF) value for uninhibited surface. Under inhibited conditions Nyquist plot consists of a large capacitive loop at high frequencies (HF) and an inductive loop at low frequencies (LF). Absence of additional inductive loop at intermediate frequency after the addition of inhibitor suggests the adsorption of inhibitor molecule, which changes the mechanism at the interface. The capacitive loop at HF is related to the charge transfer of the corrosion process and the double layer behavior [30]. It was also observed that addition of the inhibitor does not change the shape of the semicircle, indicating that there is absolutely no charge transfer taking place due to the addition of inhibitor.The high frequency capacitive loop could be assigned to the charge transfer of the corrosion process and to the formation of oxide layer [30]. The oxide film is considered to be a parallel circuit of a resistor due to the ionic conduction in the oxide film and a capacitor due to its dielectric properties. The capacitive loop is corresponding to the interfacial reactions, particularly the reaction of aluminium oxidation at the metal/oxide/electrolyte interface [31]. The process includes the formation of Al+ ions at the metal/oxide interface and their migration through the oxide/solution interface where they are oxidized to Al3+. At the oxide/solution interface, OH− or O2− ions are also formed. The fact that all the three processes are represented by only one loop could be attributed either to the overlapping of the loops of processes or to the assumption that one process only dominates, excluding the other processes. The other explanation offered to the high frequency capacitive loop is the oxide film itself. The origin of the inductive loop has often been attributed to surface or bulk relaxation of species in the oxide layer [31]. The LF inductive loop may be related to the relaxation process obtained by adsorption and incorporation of phosphate ions, oxide ions, and charged intermediates on and into the oxide film. The second capacitive loop observed at LF values could be assigned to the metal dissolution.The fact that all the three processes are represented by only one loop could be attributed either to the overlapping of the loops of processes or to the assumption that one process dominates and, therefore, excludes the other processes [32, 33]. The other explanation offered to the high frequency capacitive loop is the oxide film itself. The origin of the inductive loop has often been attributed to surface or bulk relaxation of species in the oxide layer [34]. The LF inductive loop may be related to the relaxation process obtained by adsorption and incorporation of phosphate ions on and into the oxide film [35].An equivalent circuit of nine elements depicted in Figure4(a) was used to simulate the measured impedance data of aluminium. The explanation given for the Nyquist plots is shown in Figure 4(b). In this equivalent circuit Rs is the solution resistance and Rct is the charge transfer resistance. RL and L represent the inductive elements. This also consists of constant phase element: CPE (Q) in parallel with the series capacitors C1, C2 and series resistors R1, R2, RL, and Rct. RL is parallel with the inductor L.(a) Equivalent circuit used to fit the experimental EIS data. (b) EIS data obtained for the corrosion of aluminum specimen in phosphoric acid at 30°C.
(a)
(b)The polarization resistanceRp and double layer capacitance Cdl can be calculated from
(3)Rp=RL+Rct+R1+R2,Cdl=C1+C2.
The circuit fitment was done by ZSimpWin software version 3.21. The results obtained from this are tabulated in Table 2. The CPE is composed of a component Qdl and a coefficient “a” which quantifies different physical phenomena like surface inhomogeneous resulting from surface roughness, inhibitor adsorption, porous layer formation, and so forth [18]. The parameter “a” quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness and so forth [32].Table 2
Electrochemical impedance values of aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor (GIE) at different temperatures.
T (°C)
Conc.(ppm)
R
p (Ω cm2)
CPE(μF cm−2)
η (%)
30
0
241.30
15
—
100
411.38
14
41.34
200
462.29
12
47.80
300
493.50
11
51.10
400
641.99
9
62.41
500
712.02
8
66.11
35
0
195.90
26
—
100
386.88
24
49.36
200
431.81
22
54.63
300
475.86
18
58.83
400
578.29
17
66.12
500
651.15
15
69.91
40
0
154.20
32
—
100
335.58
30
54.05
200
388.45
28
60.30
300
425.10
25
63.73
400
543.68
23
71.64
500
611.02
20
74.76
45
0
102.40
46
—
100
246.26
43
58.42
200
291.27
39
64.84
300
351.84
35
70.90
400
384.30
33
73.35
500
524.45
30
80.47
50
0
85.10
58
—
100
245.64
46
65.40
200
274.19
43
69.00
300
335.25
37
74.65
400
424.56
34
79.98
500
579.87
31
85.34The bode plots (bode phase plots and bode magnitude plots) obtained at OCP for aluminium in the presence and absence of inhibitor containing 0.5 M phosphoric acid obtained at 30°C are presented in Figures5 and 6, respectively. It is apparent that, the values of phase increase with increase in concentration of added GIE up to their optimal concentration. The bode magnitude plots indicate the presence of only one slope for the uninhibited and inhibited systems. The difference between the high frequency (HF) limit and low frequency (LF) limit in the bode plot is equal to Rp, the polarization resistance, which is associated with the dissolution and repassivation processes occurring at the interface as well as the electronic conductivity of the film. The difference between the HF and LF for the uninhibited and inhibited systems in the bode plot increases with increase in the concentration of GIE up to their critical concentration.Figure 5
Bode phase plots for the corrosion of aluminium in 0.5 M phosphoric acid at 30°C in the presence of different concentrations of GIE.Figure 6
Bode magnitude plots for the corrosion of aluminium in 0.5 M phosphoric acid at 30°C in the presence of different concentrations of GIE.The results of Table2 indicated that, the Rp values increased with the increase in the concentrations of the inhibitor. As the Rp value increased, there was a decrease in the Cdl values. Thus effective corrosion resistance was observed to be associated with high Rp and low Cdl values [33].Percentage efficiency of the inhibitor is calculated using the following relation:(4)η(%)=(Rp(inh)-RpRp(inh)),
where Rp(inh) and Rp are the polarization resistances in the presence and absence of inhibitor.The results of Tafel polarization method and electrochemical impedance method are in good agreement with one another. This proves that corrosion rate depends upon the nature of the inhibitor used and not on the technique employed for measuring the same.


### 3.4. Effect of Temperature

The study on the effect of temperature on the corrosion rate and inhibition efficiency facilitates the calculation of kinetic and thermodynamic parameters for the inhibition and the adsorption processes. These parameters are useful in interpreting the type of adsorption by the inhibitor.  In the present investigation, the corrosion potential (Ecorr), anodic and cathodic slopes (ba and bc) are not affected much after the addition of the inhibitor. This indicates that the increase in the temperature does not change the mechanism of the corrosion mechanism. In the present investigation there is an increase in the corrosion efficiency with increase in the temperature. The results reported in Tables 1 and 2 showed that the inhibition efficiency of GIE increases with increase in temperature.The apparent activation energy (Ea) for the corrosion process in the presence and absence of inhibitor can be calculated [36, 37] using the Arrhenius equation:
(5)ln(υcorr)=B-EaRT,
where B is a constant which depends on the metal type, R is the universal gas constant, and T is the absolute temperature. The plot of ln(υcorr) versus reciprocal of absolute temperature (1/T) gave a straight line with slope = -Ea/R, from which the activation energy values for the corrosion and the inhibition processes were calculated. The Arrhenius plots for the corrosion of aluminium in 0.5 M phosphoric acid solution in the absence and in the presence of different concentrations of inhibitor are shown in Figure 7.Figure 7
Arrhenius plots for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor (GIE).The enthalpy of activation (ΔH#) and entropy of activation (ΔS#) for the dissolution of alloy were calculated from the transition state equation  [37]:
(6)υcorr=RTNhexp(ΔS#R)exp(ΔH#RT),
where h is Plank’s constant and N is Avogadro’s number. A plot of ln(υcorr/T) versus 1/T gives a straight line with slope = -ΔH#/T and intercept = ln(R/Nh) + ΔS#/R.The plots ofln(υcorr/T) versus 1/T for the corrosion of aluminium in the presence of different concentrations of inhibitor are shown in Figure 8. The calculated values of activation parameters are tabulated in Table 3.Table 3
Activation parameters for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor CSE.
Conc. of inhibitor(ppm)
E
a (kJ mol−1)
Δ
H
a (kJ mol−1)
Δ
S
a (J mol−1 K−1)
0.0
45.70
43.15
−56.64
100
23.40
20.79
−176.29
200
23.23
20.62
−177.91
300
18.29
14.82
−197.46
400
17.43
15.68
−196.30
500
8.69
6.08
−228.52Figure 8
Plots ofln(υcorr/T) versus 1/T for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor (GIE).From Table3, it is obvious that the value of activation energy in 0.5 M phosphoric acid solution containing the inhibitor is lower than that without inhibitor. The extent of decrease is proportional to the inhibitor concentration. The decrease in apparent activation energy may be due to chemical adsorption [38, 39] of the inhibitor. Increase in the inhibition efficiency with increase in temperature may be attributed to increase in the adsorption of the inhibitor on the metal surface with increase in temperature. Large negative values of entropies show that the activated complex in the rate determining step is an association rather than dissociation step meaning that a decrease in disordering takes place on going from reactants to the activated complex [40].


### 3.5. Adsorption Behavior

In order to understand the mechanism of corrosion inhibition, the adsorption behavior of the adsorbate on the aluminium surface must be known. The information on the interaction between the inhibitor molecules and the metal surface can be provided by adsorption isotherm. The degree of surface coverage (θ) for different concentrations of inhibitor was evaluated from potentiodynamic polarization measurements. The data was applied to various isotherms including Langmuir, Temkin, Frumkin, and Flory-Huggins isotherms. It was found that the data best fit the Langmuir adsorption isotherm and can be represented using the following relation [33]:
(7)Cinhθ=1K+Cinh,
where K (L/mol) is the adsorption/desorption equilibrium constant, Cinh is the corrosion inhibitor concentration in the solution, and (θ) is the surface coverage, which is calculated using
(8)θ=η(%)100,
where η(%) is the percentage inhibition efficiency as calculated using (1). The plot of Cinh/θ versus Cinh gave a straight line with an intercept of 1/K.  The Langmuir adsorption isotherms for the adsorption of inhibitor on the aluminium surface are shown in Figure 9.Figure 9
Langmuir adsorption isotherms for the adsorption of GIE on aluminum in 0.5 M phosphoric acid solution at different temperatures.The slopes of the isotherms show deviation from the value of unity as would be expected for the ideal Langmuir adsorption isotherm equation. This deviation from unity may be due to the interaction among the adsorbed species on the metal surface. The Langmuir isotherm equation is based on the assumption that adsorbed molecules do not interact with one another, but this is not true in the case of organic molecules having polar atoms or groups from heterocyclic compounds which are adsorbed on the cathodic and anodic sites of the metal surface. The values of standard free energy (ΔGads0) of adsorption are related to K by the relation shown in the following equation.
(9)K=155.5exp(-ΔGads°RT),
where K (L/mol) is the adsorption/desorption equilibrium constant, value 55.5 is the concentration of water in solution in mol/L, R is the universal gas constant, and T is absolute temperature. The standard enthalpy of adsorption (ΔHads0) and standard entropy of adsorption (ΔSads0) were calculated from the plots of (ΔGads0) versus T. The thermodynamic parameters for the adsorption of GIE on the 6063 aluminium alloy are listed in Table 4.Table 4
Thermodynamic parameters for the adsorption of GIE on aluminium in 0.5 M phosphoric acid solution at different temperatures.
T(°C)
-
Δ
G
ads
°
Δ
H
ads
°
Δ
S
ads
°
(kJ mol−1)
(kJ mol−1)
(J mol−1 K−1)
303
21.08
34.25
−184.44
308
22.87
313
23.31
318
24.73
323
25.06Generally, value ofΔGads0 up to −20 kJ mol−1 is consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while the value more negative than −40 kJ mol−1 involves sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [37]. In the present study, the values of ΔGads0 are above −20 kJ mol−1. ΔGads0 and inhibition efficiency increase with the increase in temperature, indicating that the adsorption of GIE on the aluminium surface in phosphoric acid is favoured at high temperature and hence can be considered to be predominantly chemisorption [38]. The positive value of enthalpy of adsorption indicates that the adsorption of the inhibitor molecule is an endothermic process. Generally, an exothermic adsorption process signifies either physical adsorption or chemical adsorption, while the endothermic process is attributable unequivocally to chemical adsorption [39]. In the present investigation positive value of enthalpy of adsorption proves that adsorption of the inhibitor is by a chemical adsorption process. The chemical adsorption is also consistent with the increase in the efficiency of the inhibitor with increase in temperature. The ΔSads0 value is large and negative, indicating that the decrease in disordering takes place on going from the reactant to the adsorbed species. This can be attributed to the fact that adsorption is always accompanied by decrease in entropy [37].


### 3.6. Explanation for Inhibition

GIE is composed of numerous naturally occurring compounds. The principal active components of the aqueous extract are Garcinol, Isogarcinol, and Curcumin. The structure of Garcinol is shown in Figure10.Figure 10
Structure of Garcinol.Aluminium and aluminium alloys have air formed oxide film of amorphousγ alumina which initially thickens on exposure to neutral aqueous solution with the formation of crystalline hydrated alumina. In acid solution the mechanism of dissolution of aluminium is as follows [40]:
(10)Al+H2O⟶AlOH(ads)+H++e-(11)AlOH(ads)+5H2O+H+⟶Al3++6H2O+2e-(12)Al3++H2O⟶[AlOH]2++H+(13)[AlOH]2++X-⟶[AlOHX]+
Thus soluble complex ion formed leads to the dissolution of the metal. The major cathodic reaction is evolution of hydrogen gas according to the following steps:(14)H++e-⟶H(ads)(15)H(ads)+H(ads)⟶H2
The adsorption of the inhibitor molecule is often a displacement reaction involving removal of adsorbed water molecules from the metal surface [41]:
(16)Inhibitor(sol)+XH2O(ads)⟷Inhibitor(ads)+XH2O(sol)
Polar organic compounds containing sulphur, nitrogen, and oxygen are good corrosion inhibitors. Tannins, which are complex astringent aromatic acidic glycosides found in plants, are made up of polyphenols and their heterocyclic derivatives. The inhibition of GIE is mainly attributed to the presence of Garcinol and other organic compounds in the extract. Garcinol is a large molecule containing various hetero cyclic functional groups like –OH and –C=O. These are the centers of high electron density. According to the reported literature natural polyphenolic compounds containing vicinal hydroxyl groups readily form complexes with di-tri-valent metal ions [37, 38]. Thus Garcinol molecule gets chemically adsorbed over the surface of the metal in the anodic area and brings anodic process under control. Further, Garcinol molecules are chemically adsorbed due to interaction of π-orbitals with metal surface. The large Garcinol molecule may be responsible for the formation of an oriented film layer, which essentially blocks discharge of H+ and brings cathodic reaction under control.  Garcinol thus acts as a mixed type of inhibitor bringing both anodic and cathodic reactions under control. There will be a formation of a stable layer of the inhibitor molecule on the entire surface of the metal. This stable layer of the inhibitor forms the barrier between the metal and the corrosive environment and controls the rate of corrosion. The inhibitory action of the GIE is therefore explained by the formation of complexes in the form of chelates with aluminium ions in the solution and subsequent precipitation of complex formed at the surface of the alloy in the places where oxide film has been destroyed [42].


### 3.7. SEM Studies

The SEM image of corroded surface of pure aluminium is given in Figure11. It shows degradation of aluminium, with more or less uniform attack in phosphoric acid. Close observation of SEM images indicates the deposition of precipitates of aluminium phosphate on the surface of aluminium. Figure 12 represents the surface after being treated with GIE inhibitor. The surface of the aluminium has become more clear after the treatment with GIE inhibitor. This confirms the formation of the inhibitor layer on the surface of the metal, which is responsible for the decrease in the corrosion rate.Figure 11
SEM image of aluminum after immersion in 0.5 M H3PO4.Figure 12
SEM image of the surface of aluminum after 0.5 M H3PO4 at 30°C in presence of 500 ppm of GIE.


## 3.1. Fourier Transform Infrared (FTIR) Spectroscopy of GIE

Figure1 shows the FTIR spectroscopy of GIE. –OH stretching frequency appears at 3396.41 cm−1. The aromatic stretching frequency appears at 2923.88 cm−1. –C=O– stretching frequency is at 1785.96 cm−1. –C=C– stretching frequency is at 1608.52 cm−1. –CH2 bending frequency is at 1446.51 cm−1. –C–H bending frequency is at 1282.57 cm−1. –C–O bending frequency is at 1099.35 cm−1.Figure 1
FTIR spectrum of GIE.


## 3.2. Tafel Polarization Measurement

The effect of GIE on the corrosion reactions was determined by polarization techniques. The changes observed in the polarization curves after the addition of the inhibitor are usually used as the criteria to classify inhibitors as cathodic, anodic, or mixed [25, 26].Figure2 shows the Tafel polarization curves for aluminium in 0.5 M phosphoric acid solution at different concentrations of inhibitor at 30°C. Similar results were obtained at other temperatures as well.Figure 2
Tafel polarization curves for aluminum in 0.5 M phosphoric acid solution at different concentrations of inhibitor (CSE) at 30°C.Inhibitor efficiency was calculated using(1)η(%)=icorr-icorr(inh)icorr×100,
where icorr and icorr(inh) are the corrosion current densities obtained in uninhibited and inhibited solutions, respectively. The corrosion rate was calculated using
(2)υcorr(mmy-1)=3270×M×icorrρ×Z,
where 3270 is a constant that defines the unit of corrosion rate, icorr is the corrosion current density in A cm−2, ρ is the density of the corroding material (g cm−3), M is the atomic mass of the metal, and Z is the number of electrons transferred per atom [27, 28].The valuable potentiodynamic polarization parameters such as corrosion potential (Ecorr), cathodic and anodic Tafel slopes (bc and ba), corrosion current density (icorr) and corrosion rate (CR) were obtained from Tafel plots.  The inhibition efficiency values η(%) were calculated using (1). The results are tabulated in Table 1. It is obvious that corrosion current density decreases with increase in the concentration of the inhibitor.Table 1
Results of Tafel polarization studies on aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor GIE.
Temp.(°C)
Conc. of inhibitor(ppm)
E
corr (mV versus SCE)
i
corr (×m A cm−2)
-
b
c
(mV dec−1)
b
a
(mV dec−1)
CR(mm y−1)
η (%)
30
Blank
−797
0.17
637
354
1.89
—
100
−795
0.09
626
356
1.03
45.33
200
−805
0.08
634
346
0.91
51.96
300
−772
0.08
611
361
0.85
54.77
400
−801
0.06
686
333
0.70
62.99
500
−785
0.06
664
346
0.63
66.58
35
Blank
−793
0.22
610
353
2.39
—
100
−788
0.11
641
367
1.18
50.64
200
−779
0.10
674
342
1.06
55.66
300
−796
0.09
668
365
0.96
59.68
400
−796
0.07
685
353
0.80
66.70
500
−764
0.06
632
319
0.71
70.37
40
Blank
−797
0.28
596
364
3.00
—
100
−774
0.12
615
374
1.34
55.38
200
−789
0.11
621
373
1.16
61.30
300
−796
0.10
642
371
1.06
64.56
400
−777
0.08
670
321
0.84
72.15
500
−769
0.07
655
299
0.75
75.17
45
Blank
−804
0.40
595
367
4.38
—
100
−797
0.16
578
397
1.75
60.04
200
−799
0.14
616
380
1.49
66.01
300
−793
0.11
642
380
1.24
71.70
400
−780
0.10
716
312
1.14
74.03
500
−665
0.08
546
264
0.84
80.84
50
Blank
−800
0.48
579
379
5.20
—
100
−765
0.16
573
370
1.73
66.75
200
−806
0.14
586
396
1.56
70.09
300
−810
0.12
595
392
1.28
75.38
400
−738
0.09
609
366
1.02
80.44
500
−639
0.07
616
343
0.75
85.59From Figure2 and Table 1, it can be observed that the addition of GIE at all the studied concentrations resulted in the significant decrease in the corrosion current density (icorr) and decrease in the corrosion rate. It is also evident that the  percentage efficiency of the inhibitor increased with increase in the concentration of the inhibitor. Percentage efficiency of the inhibitor also increased with increase in temperature. There was no remarkable shift in the corrosion potential (Ecorr) value with respect to the blank. According to the literature report [28, 29], when corrosion potential is more than ±85 mV with respect to the corrosion potential of the blank, the inhibitor can be considered distinctively as either cathodic or anodic type. However, the maximum displacement in this study is less than ±85 mV. This suggests that GIE functions as mixed-type inhibitor.There is a slight change in the anodic and cathodic slopes after the addition of the inhibitor. The parallel cathodic Tafel curves suggest that the hydrogen evolution is activation controlled, and the reduction mechanism is not affected by the presence of the inhibitor. The shift in the anodic Tafel slope may be due to the surface/inhibitor molecules adsorbed on the metal surface.


## 3.3. Electrochemical Impedance Spectroscopy (EIS) Studies

The results of the Tafel polarization method were compared with the results of impedance measurements. EIS measurements were carried out at different temperatures at different inhibitor concentrations. The Nyquist plots of aluminium in 0.5 M phosphoric acid containing various concentrations of inhibitor are shown in Figure3.Figure 3
Nyquist plots and equivalent circuit for the corrosion of aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor (GIE) at 30°C.Impedance plots are semicircles both in the absence and in the presence of the inhibitor. The diameter of the capacitive loop increased with increase in the concentration of the inhibitor. This indicated that the impedance of the inhibited substrate increased with the inhibitor concentration. The impedance plots have a large capacitive loop at higher frequencies (HF), small inductive loop at intermediate frequencies (IF), followed by a second capacitive loop at low frequency (LF) value for uninhibited surface. Under inhibited conditions Nyquist plot consists of a large capacitive loop at high frequencies (HF) and an inductive loop at low frequencies (LF). Absence of additional inductive loop at intermediate frequency after the addition of inhibitor suggests the adsorption of inhibitor molecule, which changes the mechanism at the interface. The capacitive loop at HF is related to the charge transfer of the corrosion process and the double layer behavior [30]. It was also observed that addition of the inhibitor does not change the shape of the semicircle, indicating that there is absolutely no charge transfer taking place due to the addition of inhibitor.The high frequency capacitive loop could be assigned to the charge transfer of the corrosion process and to the formation of oxide layer [30]. The oxide film is considered to be a parallel circuit of a resistor due to the ionic conduction in the oxide film and a capacitor due to its dielectric properties. The capacitive loop is corresponding to the interfacial reactions, particularly the reaction of aluminium oxidation at the metal/oxide/electrolyte interface [31]. The process includes the formation of Al+ ions at the metal/oxide interface and their migration through the oxide/solution interface where they are oxidized to Al3+. At the oxide/solution interface, OH− or O2− ions are also formed. The fact that all the three processes are represented by only one loop could be attributed either to the overlapping of the loops of processes or to the assumption that one process only dominates, excluding the other processes. The other explanation offered to the high frequency capacitive loop is the oxide film itself. The origin of the inductive loop has often been attributed to surface or bulk relaxation of species in the oxide layer [31]. The LF inductive loop may be related to the relaxation process obtained by adsorption and incorporation of phosphate ions, oxide ions, and charged intermediates on and into the oxide film. The second capacitive loop observed at LF values could be assigned to the metal dissolution.The fact that all the three processes are represented by only one loop could be attributed either to the overlapping of the loops of processes or to the assumption that one process dominates and, therefore, excludes the other processes [32, 33]. The other explanation offered to the high frequency capacitive loop is the oxide film itself. The origin of the inductive loop has often been attributed to surface or bulk relaxation of species in the oxide layer [34]. The LF inductive loop may be related to the relaxation process obtained by adsorption and incorporation of phosphate ions on and into the oxide film [35].An equivalent circuit of nine elements depicted in Figure4(a) was used to simulate the measured impedance data of aluminium. The explanation given for the Nyquist plots is shown in Figure 4(b). In this equivalent circuit Rs is the solution resistance and Rct is the charge transfer resistance. RL and L represent the inductive elements. This also consists of constant phase element: CPE (Q) in parallel with the series capacitors C1, C2 and series resistors R1, R2, RL, and Rct. RL is parallel with the inductor L.(a) Equivalent circuit used to fit the experimental EIS data. (b) EIS data obtained for the corrosion of aluminum specimen in phosphoric acid at 30°C.
(a)
(b)The polarization resistanceRp and double layer capacitance Cdl can be calculated from
(3)Rp=RL+Rct+R1+R2,Cdl=C1+C2.
The circuit fitment was done by ZSimpWin software version 3.21. The results obtained from this are tabulated in Table 2. The CPE is composed of a component Qdl and a coefficient “a” which quantifies different physical phenomena like surface inhomogeneous resulting from surface roughness, inhibitor adsorption, porous layer formation, and so forth [18]. The parameter “a” quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness and so forth [32].Table 2
Electrochemical impedance values of aluminum in 0.5 M phosphoric acid solution containing different concentrations of inhibitor (GIE) at different temperatures.
T (°C)
Conc.(ppm)
R
p (Ω cm2)
CPE(μF cm−2)
η (%)
30
0
241.30
15
—
100
411.38
14
41.34
200
462.29
12
47.80
300
493.50
11
51.10
400
641.99
9
62.41
500
712.02
8
66.11
35
0
195.90
26
—
100
386.88
24
49.36
200
431.81
22
54.63
300
475.86
18
58.83
400
578.29
17
66.12
500
651.15
15
69.91
40
0
154.20
32
—
100
335.58
30
54.05
200
388.45
28
60.30
300
425.10
25
63.73
400
543.68
23
71.64
500
611.02
20
74.76
45
0
102.40
46
—
100
246.26
43
58.42
200
291.27
39
64.84
300
351.84
35
70.90
400
384.30
33
73.35
500
524.45
30
80.47
50
0
85.10
58
—
100
245.64
46
65.40
200
274.19
43
69.00
300
335.25
37
74.65
400
424.56
34
79.98
500
579.87
31
85.34The bode plots (bode phase plots and bode magnitude plots) obtained at OCP for aluminium in the presence and absence of inhibitor containing 0.5 M phosphoric acid obtained at 30°C are presented in Figures5 and 6, respectively. It is apparent that, the values of phase increase with increase in concentration of added GIE up to their optimal concentration. The bode magnitude plots indicate the presence of only one slope for the uninhibited and inhibited systems. The difference between the high frequency (HF) limit and low frequency (LF) limit in the bode plot is equal to Rp, the polarization resistance, which is associated with the dissolution and repassivation processes occurring at the interface as well as the electronic conductivity of the film. The difference between the HF and LF for the uninhibited and inhibited systems in the bode plot increases with increase in the concentration of GIE up to their critical concentration.Figure 5
Bode phase plots for the corrosion of aluminium in 0.5 M phosphoric acid at 30°C in the presence of different concentrations of GIE.Figure 6
Bode magnitude plots for the corrosion of aluminium in 0.5 M phosphoric acid at 30°C in the presence of different concentrations of GIE.The results of Table2 indicated that, the Rp values increased with the increase in the concentrations of the inhibitor. As the Rp value increased, there was a decrease in the Cdl values. Thus effective corrosion resistance was observed to be associated with high Rp and low Cdl values [33].Percentage efficiency of the inhibitor is calculated using the following relation:(4)η(%)=(Rp(inh)-RpRp(inh)),
where Rp(inh) and Rp are the polarization resistances in the presence and absence of inhibitor.The results of Tafel polarization method and electrochemical impedance method are in good agreement with one another. This proves that corrosion rate depends upon the nature of the inhibitor used and not on the technique employed for measuring the same.


## 3.4. Effect of Temperature

The study on the effect of temperature on the corrosion rate and inhibition efficiency facilitates the calculation of kinetic and thermodynamic parameters for the inhibition and the adsorption processes. These parameters are useful in interpreting the type of adsorption by the inhibitor.  In the present investigation, the corrosion potential (Ecorr), anodic and cathodic slopes (ba and bc) are not affected much after the addition of the inhibitor. This indicates that the increase in the temperature does not change the mechanism of the corrosion mechanism. In the present investigation there is an increase in the corrosion efficiency with increase in the temperature. The results reported in Tables 1 and 2 showed that the inhibition efficiency of GIE increases with increase in temperature.The apparent activation energy (Ea) for the corrosion process in the presence and absence of inhibitor can be calculated [36, 37] using the Arrhenius equation:
(5)ln(υcorr)=B-EaRT,
where B is a constant which depends on the metal type, R is the universal gas constant, and T is the absolute temperature. The plot of ln(υcorr) versus reciprocal of absolute temperature (1/T) gave a straight line with slope = -Ea/R, from which the activation energy values for the corrosion and the inhibition processes were calculated. The Arrhenius plots for the corrosion of aluminium in 0.5 M phosphoric acid solution in the absence and in the presence of different concentrations of inhibitor are shown in Figure 7.Figure 7
Arrhenius plots for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor (GIE).The enthalpy of activation (ΔH#) and entropy of activation (ΔS#) for the dissolution of alloy were calculated from the transition state equation  [37]:
(6)υcorr=RTNhexp(ΔS#R)exp(ΔH#RT),
where h is Plank’s constant and N is Avogadro’s number. A plot of ln(υcorr/T) versus 1/T gives a straight line with slope = -ΔH#/T and intercept = ln(R/Nh) + ΔS#/R.The plots ofln(υcorr/T) versus 1/T for the corrosion of aluminium in the presence of different concentrations of inhibitor are shown in Figure 8. The calculated values of activation parameters are tabulated in Table 3.Table 3
Activation parameters for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor CSE.
Conc. of inhibitor(ppm)
E
a (kJ mol−1)
Δ
H
a (kJ mol−1)
Δ
S
a (J mol−1 K−1)
0.0
45.70
43.15
−56.64
100
23.40
20.79
−176.29
200
23.23
20.62
−177.91
300
18.29
14.82
−197.46
400
17.43
15.68
−196.30
500
8.69
6.08
−228.52Figure 8
Plots ofln(υcorr/T) versus 1/T for the corrosion of aluminum in 0.5 M phosphoric acid containing different concentrations of inhibitor (GIE).From Table3, it is obvious that the value of activation energy in 0.5 M phosphoric acid solution containing the inhibitor is lower than that without inhibitor. The extent of decrease is proportional to the inhibitor concentration. The decrease in apparent activation energy may be due to chemical adsorption [38, 39] of the inhibitor. Increase in the inhibition efficiency with increase in temperature may be attributed to increase in the adsorption of the inhibitor on the metal surface with increase in temperature. Large negative values of entropies show that the activated complex in the rate determining step is an association rather than dissociation step meaning that a decrease in disordering takes place on going from reactants to the activated complex [40].


## 3.5. Adsorption Behavior

In order to understand the mechanism of corrosion inhibition, the adsorption behavior of the adsorbate on the aluminium surface must be known. The information on the interaction between the inhibitor molecules and the metal surface can be provided by adsorption isotherm. The degree of surface coverage (θ) for different concentrations of inhibitor was evaluated from potentiodynamic polarization measurements. The data was applied to various isotherms including Langmuir, Temkin, Frumkin, and Flory-Huggins isotherms. It was found that the data best fit the Langmuir adsorption isotherm and can be represented using the following relation [33]:
(7)Cinhθ=1K+Cinh,
where K (L/mol) is the adsorption/desorption equilibrium constant, Cinh is the corrosion inhibitor concentration in the solution, and (θ) is the surface coverage, which is calculated using
(8)θ=η(%)100,
where η(%) is the percentage inhibition efficiency as calculated using (1). The plot of Cinh/θ versus Cinh gave a straight line with an intercept of 1/K.  The Langmuir adsorption isotherms for the adsorption of inhibitor on the aluminium surface are shown in Figure 9.Figure 9
Langmuir adsorption isotherms for the adsorption of GIE on aluminum in 0.5 M phosphoric acid solution at different temperatures.The slopes of the isotherms show deviation from the value of unity as would be expected for the ideal Langmuir adsorption isotherm equation. This deviation from unity may be due to the interaction among the adsorbed species on the metal surface. The Langmuir isotherm equation is based on the assumption that adsorbed molecules do not interact with one another, but this is not true in the case of organic molecules having polar atoms or groups from heterocyclic compounds which are adsorbed on the cathodic and anodic sites of the metal surface. The values of standard free energy (ΔGads0) of adsorption are related to K by the relation shown in the following equation.
(9)K=155.5exp(-ΔGads°RT),
where K (L/mol) is the adsorption/desorption equilibrium constant, value 55.5 is the concentration of water in solution in mol/L, R is the universal gas constant, and T is absolute temperature. The standard enthalpy of adsorption (ΔHads0) and standard entropy of adsorption (ΔSads0) were calculated from the plots of (ΔGads0) versus T. The thermodynamic parameters for the adsorption of GIE on the 6063 aluminium alloy are listed in Table 4.Table 4
Thermodynamic parameters for the adsorption of GIE on aluminium in 0.5 M phosphoric acid solution at different temperatures.
T(°C)
-
Δ
G
ads
°
Δ
H
ads
°
Δ
S
ads
°
(kJ mol−1)
(kJ mol−1)
(J mol−1 K−1)
303
21.08
34.25
−184.44
308
22.87
313
23.31
318
24.73
323
25.06Generally, value ofΔGads0 up to −20 kJ mol−1 is consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while the value more negative than −40 kJ mol−1 involves sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorption) [37]. In the present study, the values of ΔGads0 are above −20 kJ mol−1. ΔGads0 and inhibition efficiency increase with the increase in temperature, indicating that the adsorption of GIE on the aluminium surface in phosphoric acid is favoured at high temperature and hence can be considered to be predominantly chemisorption [38]. The positive value of enthalpy of adsorption indicates that the adsorption of the inhibitor molecule is an endothermic process. Generally, an exothermic adsorption process signifies either physical adsorption or chemical adsorption, while the endothermic process is attributable unequivocally to chemical adsorption [39]. In the present investigation positive value of enthalpy of adsorption proves that adsorption of the inhibitor is by a chemical adsorption process. The chemical adsorption is also consistent with the increase in the efficiency of the inhibitor with increase in temperature. The ΔSads0 value is large and negative, indicating that the decrease in disordering takes place on going from the reactant to the adsorbed species. This can be attributed to the fact that adsorption is always accompanied by decrease in entropy [37].


## 3.6. Explanation for Inhibition

GIE is composed of numerous naturally occurring compounds. The principal active components of the aqueous extract are Garcinol, Isogarcinol, and Curcumin. The structure of Garcinol is shown in Figure10.Figure 10
Structure of Garcinol.Aluminium and aluminium alloys have air formed oxide film of amorphousγ alumina which initially thickens on exposure to neutral aqueous solution with the formation of crystalline hydrated alumina. In acid solution the mechanism of dissolution of aluminium is as follows [40]:
(10)Al+H2O⟶AlOH(ads)+H++e-(11)AlOH(ads)+5H2O+H+⟶Al3++6H2O+2e-(12)Al3++H2O⟶[AlOH]2++H+(13)[AlOH]2++X-⟶[AlOHX]+
Thus soluble complex ion formed leads to the dissolution of the metal. The major cathodic reaction is evolution of hydrogen gas according to the following steps:(14)H++e-⟶H(ads)(15)H(ads)+H(ads)⟶H2
The adsorption of the inhibitor molecule is often a displacement reaction involving removal of adsorbed water molecules from the metal surface [41]:
(16)Inhibitor(sol)+XH2O(ads)⟷Inhibitor(ads)+XH2O(sol)
Polar organic compounds containing sulphur, nitrogen, and oxygen are good corrosion inhibitors. Tannins, which are complex astringent aromatic acidic glycosides found in plants, are made up of polyphenols and their heterocyclic derivatives. The inhibition of GIE is mainly attributed to the presence of Garcinol and other organic compounds in the extract. Garcinol is a large molecule containing various hetero cyclic functional groups like –OH and –C=O. These are the centers of high electron density. According to the reported literature natural polyphenolic compounds containing vicinal hydroxyl groups readily form complexes with di-tri-valent metal ions [37, 38]. Thus Garcinol molecule gets chemically adsorbed over the surface of the metal in the anodic area and brings anodic process under control. Further, Garcinol molecules are chemically adsorbed due to interaction of π-orbitals with metal surface. The large Garcinol molecule may be responsible for the formation of an oriented film layer, which essentially blocks discharge of H+ and brings cathodic reaction under control.  Garcinol thus acts as a mixed type of inhibitor bringing both anodic and cathodic reactions under control. There will be a formation of a stable layer of the inhibitor molecule on the entire surface of the metal. This stable layer of the inhibitor forms the barrier between the metal and the corrosive environment and controls the rate of corrosion. The inhibitory action of the GIE is therefore explained by the formation of complexes in the form of chelates with aluminium ions in the solution and subsequent precipitation of complex formed at the surface of the alloy in the places where oxide film has been destroyed [42].


## 3.7. SEM Studies

The SEM image of corroded surface of pure aluminium is given in Figure11. It shows degradation of aluminium, with more or less uniform attack in phosphoric acid. Close observation of SEM images indicates the deposition of precipitates of aluminium phosphate on the surface of aluminium. Figure 12 represents the surface after being treated with GIE inhibitor. The surface of the aluminium has become more clear after the treatment with GIE inhibitor. This confirms the formation of the inhibitor layer on the surface of the metal, which is responsible for the decrease in the corrosion rate.Figure 11
SEM image of aluminum after immersion in 0.5 M H3PO4.Figure 12
SEM image of the surface of aluminum after 0.5 M H3PO4 at 30°C in presence of 500 ppm of GIE.


## 4. Conclusions

Based on the results of investigation, the following conclusions are drawn.(1)
GIE is a good eco-friendly green inhibitor for the corrosion control of aluminium in 0.5 M phosphoric acid solution.(2)
Inhibition efficiency of the GIE increases with increase in the concentration of the inhibitor and also with temperature.(3)
GIE acts as a mixed inhibitor on the surface of the metal.(4)
GIE gets chemically adsorbed on the surface of aluminium and obeys Langmuir adsorption isotherm.(5)
GIE is a renewable, easily available, cost effective, and environmentally benign inhibitor.



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*Source: 945143-2013-12-19.xml*