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Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract
Accepted Manuscript Original article Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract N. Raghavendra, J. Ishwara Bhat PII: DOI: Reference:
S1018-3639(17)30078-8 http://dx.doi.org/10.1016/j.jksues.2017.06.003 JKSUES 250
To appear in:
Journal of King Saud University - Engineering Sciences
Received Date: Accepted Date:
13 March 2017 6 June 2017
Please cite this article as: Raghavendra, N., Bhat, J.I., Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract, Journal of King Saud University - Engineering Sciences (2017), doi: http://dx.doi.org/10.1016/ j.jksues.2017.06.003
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Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract Graphical abstract
and aluminum immersion (contact) time was also evaluated. Parameters such as Ea, ∆H*, ∆S*, Kads and ∆Go ads were calculated by the gravimetric (weight loss) technique. Results of mass loss technique indicated that, the Areca flower extract molecules are strongly adsorbed at Al- 0.5 M HCl solution interface and follows the law of the Langmuir adsorption model. Tafel curves show that, the Areca flower extract molecules effectively adsorbed on the active Al sites by replacing the H2O molecules and strongly interact with cathodic and anodic sites impeding the dissolution reaction by mixed mode. Hence, Al metal is protected. Electrochemical impedance results indicate that, area of a semicircle (Nyquist plots) in the presence of the Areca flower extract is high compared to the bare system. The variation in the surface topography of aluminum was analyzed through scanning electron microscopy and atomic force microscopy techniques. Further, AFM results show that, surface roughness values in the protected Al system are low compared to the unprotected Al system. Keywords Aluminum; Areca flower; Hydrochloric acid; Langmuir adsorption model; Surface morphology
1. Introduction
Abstract The corrosion inhibition property of Areca flower extract species on the Al surface in acid (0.5 M HCl) medium was tested by weight loss, Tafel plot, impedance, scanning electron microscopy and atomic force microscopy techniques. The influence of different concentrations of Areca flower extract, 0.5 M HCl solution temperature
Acid solutions, especially HCl are greatly employed in the metallurgical industries for the removal of rust and dust. It is widely used in the de-scaling, etching, oil well acidification, ore fabrication, cleaning of boilers and other materials in several chemical, manufacturing and engineering industries. Hydrochloric acid solutions are highly corrosive and cause degradation of aluminum metal by dissolving the natural protective film (which is amphoteric in nature) (Nnanna et al., 2010; Oguzie et al., 2007). The Al corrosion in acid solution generally is inevitable, and leads to personal injury and economic loss. Among the corrosion protection methods, inhibitors are the best option to defend the metals from the corrosion owing to the low cost and high efficiency.
The corrosion inhibitors are the chemical substances contain sulfur (S), phosphor (P), oxygen (O) and nitrogen (N) elements in their moieties which plays a very important role in the Al corrosion inhibition process in hydrochloric acid solutions (Raghavendra and Ishwara Bhat, 2017; El-Etre, 2003; Li., et al., 2005; Raghavendra and Ishwara Bhat, 2016; Abdallah, 2004; Gunasekaran and Chauhan, 2004; Zucchi, Omar, 1985: Fouda et al., 2016). The electron rich species in chemical molecules form a protective film on aluminum surface and effectively isolates the electrode surface from the hydrochloric acid solution. Many synthetic compounds effectively block the corrosion reactions at aluminum sites by adsorption mode. But, majority synthetic compounds are expensive, noxious and not environmental friendly. Hence, nowadays, the use of synthetic organic species as corrosion inhibitors has been discouraged due to strict environmental rules. Hence, there is growing interest in non-poisonous and low cost corrosion inhibitors to replace the poisonous and expensive corrosion inhibitors. The study of corrosion of Al in hydrochloric acid solution by plant products is of immense interest from ecological viewpoint and is attracting a noteworthy level of attention. Because, natural products are non-toxic, low cost and eco-friendly substances. Adding, they are easily available and biodegradable (Bulstein, et al., 2005; AbdelGaber, et al., 2010; Khaled, 2010; Oguzie, 2008; Ebenso, et al., 2009; Mejeha, et al., 2010; Ebenso, et al., 2004; Abiola, et al., 2007; Martinez, 2002). Up to now, many plant extract species have been reported as inexpensive, less polluting and effective corrosion inhibitors for Al in 0.5 M HCl medium. The basic components in the plant or natural extracts are polyphenols, alkaloids, tannins, fats and flavanoids. These groups increase the thickness of the protective film on the surface of the aluminum, and hinder the aluminum corrosion rate in acid solution. Areca flower is one of the greenest products possessing special elements (heteroatoms) in their
molecules. Table 1 confirms the rich sources of natural molecules in Areca flower extract (EnShyh Lin and Chia-Ching Li, 2010; Wei-Min Zhang et al., 2009; Wei Peng, et al., 2015). The antioxidant property of Areca flower extract has been previously reported. But, no research paper on the corrosion inhibition efficiency (protection efficiency) of the Areca flower extract for any metal in acid (0.5 M hydrochloric acid) solution. Therefore, in this, we tested the corrosion inhibiting action of the Areca flower extract on the aluminum surface in 0.5 M HCl solution by chemical (weight loss), electrochemical (Tafel plot, AC impedance spectroscopy ) scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods. Table 1 The main constituents of Areca flower extract
Polyphenols
Flavanoids
Alkaloids
Fatty acids
Rutin
Sorhamnetin
Arecoline
Lauric acid
Catechin,
Chrysoeriol
Arecaidine
myristic acid
Epicatechin
Uteolin
Guavacoline
palmitic acid
Procyanidin B1
Quercetin
Guavacine
stearic acid
Gallic acid,
Iquiritigenin
Arecolidine
oleic acid
Leucocyanidin,
acareubin
isoguvacine
Arachidonic acid
2. Experimental Part 2.1 Materials The important materials used for the present study are Al, 0.5 M hydrochloric acid solution, distilled water, acetone, ethanol, weighing balance, distilled water, water bath, Soxhlet apparatus and heating mantle. The corrosion inhibition test was carried out on the aluminum surfaces having the following chemical composition (in weight percentage): 0.1% Cu, 0.3–0.7 Si %, 0.3% Mn, 0.6% Fe, 0.4-
0.9% Mg, 0.2% Cr, 0.1%Tl or other grainrefining elements, 0.2% Zn and plus Al. Al pieces of 2.4 × 1.1 × 0.2 cm were used for the gravimetric studies. For electrochemical, SEM and AFM studies, 1 cm2 of the Al surface was exposed to acid solution and the remaining portion was enclosed with epoxy resin. The Al electrode employed for the examination was carefully polished with emery papers (grade: 320-1200) in order to remove the corrosion products and dust on the electrode surface, rinsed with distilled water, degreased with polar solvent (acetone/ethanol) and finally dried at laboratory temperature. 2.2 Methods The different amounts (3, 6, 12 and 18 (g/L) of inhibitor (Areca flower extract) were prepared by placing the 230 grams of Areca flower in the Soxhlet chamber. The extraction was run by using distilled water as a solvent. Extraction was repeated in many cycles. After that, the resulting solution was cooled, Whatman filter paper was used to remove the impurities present in the solution and purified sample was stored in the refrigerator in order to avoid the side reactions.
The Al surface exposed to acid (0.5 M HCl) solution without and with the Areca flower extract for 2 hours duration was examined via scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The attendance of functional groups in the Areca flower extract molecules was confirmed by the Fourier transform infrared spectroscopy (FT-IR) method.
3. Results and discussion 3.1. Mass loss (gravimetric) technique The Al weight loss in the 0.5 M hydrochloric acid solution without and with the Areca flower extract of four different concentrations was examined at 1, 2, 3, 4, 5 and 10 hours duration at laboratory temperature 303 K. The weight loss experiment was also performed in 308, 313, 318 and 323 K in order to study the effect of solution temperature on protection efficiency of the inhibitor. Corrosion rate values in mils penetration per year were evaluated by applying following relation, Corrosion
rate=
(Fontana,
1987) Prior to exposure of metal to an acid solution, the wiped and dried Al metals were weighed using an analytical balance. Tests (both chemical and electrochemical) were conducted with four different concentrations of Areca flower extract. At the end, the Al surface was carefully rinsed with acetone and wiped with tissue paper and a loss in the weight of Al was recorded. To support the truth of the results, the experiment was repeated. For electrochemical test, CHI 660C instrument was used. In which, Al pieces as working electrode, the counter electrode is platinum cell and reference is calomel cell. The Tafel plots (plot of current density versus applied potential) are recorded with a potential range from -200 mV to +200 mV at speed rate 0.01 V/s. Whereas, Nyquist plot obtained from the frequency range 105 to 1 Hz with an amplitude of 0.01V.
et
al.,
(1)
where, W is the Al weight loss in milligrams , A is the Al immersed surface area in square inches, T is the immersion time of Al in 0.5 M HCl solution in hours , and D is the density of the Al in gram per cubic centimeter. The protection efficiency (in percentage) of the green inhibitor (Areca flower extract) was obtained by the following equation, Protection ( )
efficiency
×100
(in
percentage)= (2)
where, W2 is the Al weight loss when exposed to protected solution and W1 is the Al weight loss when exposed to unprotected solution.
From the Tables 2 and 3, it is observed that, the degree of Al dissolution rate decreases with an increase in the Areca flower extract concentrations is due to the adsorption of natural extract molecules on the surface of the Al which creates a smooth film. The formed protective film effectively blocks the attack of H + ions on the surface of the electrode in 0.5 M HCl system and controls the Al corrosion rate. Lowest protection efficiency value at lower inhibitor concentration is due to less coverage of inhibitor molecules on the Al surface. The lowest Al dissolution rate was noticed at 18 g/L of Areca flower extract concentration, showing that Areca flower extract molecules cover more Al active sites where a direct attack of H + ions occurs and greatly protects Al against dissolution. The loss in the weight of Al decreases with an increase in the amount of plant products. As a result, high protection efficiency (low Al corrosion rate) values are obtained. The effect of temperature and time on corrosion inhibition efficiency indicates that, protection of Al metal from 0.5 M HCl solution is generally reduced with enhancement in both temperature and time. The increase of both time and temperature directly impact on the stability of protective film formed on the electrode surface. The effect of test solution temperature on the acid-metal interface is complex, because lots of changes occur on the surface of the substrate such as desorption of Areca flower extract molecules, etching and adsorbed Areca flower extract molecules undergo decomposition. Degradation of Areca flower extract molecules on the Al surface weakness the metal-inhibitor interaction. Hence, protection efficiency decreases with a rise in test solution temperature from 303 to 323 K. With the rise in time, the insoluble protective film formed on the Al surface loses its stability and dissolves in the corrosive solution. As a result, the free Al surface (uncovered by Areca flower extract molecules) is direct contact with 0.5 M hydrochloric acid solution and undergo dissolution. Hence, protection to Al slightly
weaker with increasing time. Therefore, the rate of corrosion of Al is enhanced with time. Activation parameters are required to understand the mechanism of the inhibition of the Al corrosion process. The activation energy was obtained from the slope of an Arrhenius plot (ln υcorr vs. 1000/T, Fig. 1). Activation enthalpy and entropy were calculated from the slope and intercept of transition state plot (ln(υcorr/T) vs. 1000/T, Fig. 2) respectively. The calculated parameters are placed in the Table 4. From the Table 4, it is clear that, the values of the activation energy (Ea*) in protected environment were higher than the unprotected environment and increases with Areca flower extract concentration, which clearly showing that, difficulty in the movement of corrosive ions towards active Al sites was enhanced with inhibitor concentration. The attack of H+ ions on the Al surface was hindered by the Areca flower extract molecules. Hence, the more energy required in a protected environment for the oxidation process. The endothermic nature of the Al dissolution process in the acid medium was confirmed by positive activation enthalpy (∆H*) values, indicating the difficulty of the Al dissolution process. Both activation energy and activation enthalpy values are varied in the same fashion and are good agreement with each other. A similar observation was reported by a previous author (Adriana Patru Samidea, et al., 2008). The obtained activation entropy (∆S*) values in both unprotected and protected systems are high, negative and moves in the direction of positive side represents the decrease in the disorderness of the system. The gravimetric results are best fitted to the Langmuir adsorption model ( plot of C inh vs cinh/θ, where, Cinh is Areca flower extract concentration and θ is surface coverage , Fig.3) and which provides the nature of interaction between the Areca flower extract molecules and electrode (Al) surface. The parameters such as the equilibrium constant of the inhibitor adsorption (Kads) and free energy of adsorption (∆Goads) obtained from Fig.3 and are shown in the Table 5.
The strong adsorption of Areca flower extract molecules on the Al surface was confirmed by high Kads values. Strong interaction of plant extract molecules with the Al surface leads to the formation of a thick insoluble layer on electrode surfaces.
Table 2 Weight loss parameters for different contact times without and with Areca flower extract of different concentrations at laboratory temperature 303 K
Time (hrs)
1
2
3
4
5
10
Concentration (g/L)
Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18
Corrosion rate (× 10-4 (mpy) 8.699 1.933 1.449 0.966 0.483 11.116 2.658 1.933 1.449 1.208 16.110 4.188 3.383 2.416 2.094 18.124 4.833 4.229 3.141 2.779 24.166 7.346 6.089 4.833 5.558 42.291 14.983 12.083 9.666 8.699
Stronger adsorption of Areca flower extract molecules is an indication of high protection to the Al surface by inhibitory molecules in the 0.5 M hydrochloric acid system. Table 5 also shows that, the addition of Areca flower extracts to the 0.5 M HCl solution shifts the ∆Goads values towards the negative side, which confirms that Areca flower extract molecules adsorb spontaneously on the Al surface in acid medium. The resulted ∆Goads values in the range -34.759 to -36.443 verified that adsorption of Areca flower extract molecules on the surface of the Al are comprehensive adsorption type (includes both physical and chemical adsorption type).
Protection efficiency (%)
77.777 83.333 88.888 94.444 76.086 82.608 86.956 89.130 74.000 79.000 85.000 87.000 73.333 76.666 82.666 84.666 69.600 74.800 80.000 81.666 64.571 71.428 77.142 79.428
Figure 1 Arrhenius plots without and with inhibitor (Areca flower extract) of four different concentrations
Table 3 Effect of solution temperature on the protection efficiency of the inhibitor at different concentrations of Areca flower extract with contact period of one hour
Temperature (K)
Concentration (g/L)
Protection efficiency (%)
303
3 6 12 18 3 6 12 18 3 6 12 18 3 6 12 18 3 6 12 18
77.777 83.333 88.888 94.444 75.000 80.000 85.000 90.000 72.727 77.272 81.818 86.363 72.000 72.000 80.000 84.000 66.666 70.370 77.777 81.481
308
313
318
323
Figure 2 Transition state plots in the absence and presence of Areca flower extract The image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again.
Figure 3 Langmuir plots
Table 4 Activation parameters
Concentration (g/L)
Ea* ( kJ mol-1)
∆H* (kJ mol-1)
∆S* (J mol-1 K-1)
Blank 3 6 12 18
16.830 31.856 41.071 44.194 63.926
14.230 29.255 38.471 41.593 61.325
-313.901 -276.782 -248.672 -241.253 -181.159
Table 5 Thermodynamic parameters Temperature (K) 303 308 313 318 323
Kads (L g-1)
∆Go ads (kJ mol-1)
980.411 998.502 1019.794 885.002 780.944
-34.759 -35.380 -36.009 -36.209 -36.443
3.2. Electrochemical measurements 3.2.1. Tafel plots Fig.4 shows the polarization curves ( Roland T. Loto, Olukeye Tobilola, 2016) of Al in 0.5 M hydrochloric acid system in the absence and presence of the Areca flower extract with four different concentrations. Equation 3 was
employed for the determination of protection efficiency of Areca flower extract, , !""
( %) = [ 1 −
] ×100
!""
(3) where, i’corr = protected Al corrosion current density value and icorr = unprotected Al corrosion current density value. The parameters obtained from the Tafel curves are detailed in the Table 6. From the Table 6, it is noticed that, the value of corrosion current density is inversely proportional to the Areca flower extract concentrations, indicates that inhibitory species obstruct the both anodic and cathodic reactions on the surface of the Al via adsorption mechanism. The protective film formed on the active Al sites prevents corrosive ions attack on the electrode surface. Hence, corrosion current density values decreased and protection efficiency of the inhibitor increases with an increase in the amounts of Areca flower extract. The obtained corrosion potential (E corr), cathodic (βc) and anodic (βa) Tafel constant values did not exhibit monotonic trend and maximum displacement in the corrosion potential values with respect to bare solution is 27 mv, which showing that, Areca flower extract act as a mixed type inhibitor for Al metal in 0.5 M HCl solution by controlling the both active anodic and cathodic reactions.
the Areca flower extract is an indication of the protective role of plant extracts on the Al electrode. Appearance of depressed semicircular shows that, the Al corrosion in 0.5 M HCl system is mainly controlled by the single charge transfer process. The shapes of Nyquist curves in the absence and presence of the Areca flower extract of different concentrations are similar, which confirms that, the presence of the Areca flower extract of four different concentrations does not alter the Al corrosion dissolution mechanism in the studied environment. The deviation from the perfect semicircles is often referred to as frequency dispersion, which ascribed to inhomogeneities and the roughness of the surface.
Figure 4 Tafel curves (without and with addition of Areca flower extract) The image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again.
3.2.2. AC impedance spectroscopy The effect of four different concentrations of the Areca flower extract on the surface of Al in 0.5 M hydrochloric acid system is shown in Fig. 5 (Nyquist plots). Appraisal of Fig. 5 shows that, all the Nyquist plots exhibit the depressed semi circle. The area of the semicircle increases with the increment in the Areca flower extract concentrations, which shows that corrosion inhibition property of the Areca flower extract is mainly depends on its concentration. The area of the semicircle is low on a bare system (inhibited free system) compared to the inhibited system. The larger area of the semicircle in the presence of
Figure 5 Nyquist curves
(formed from Areca flower extract molecules) on the surface of Al, which is accountable for the inhibition of Al corrosion.
Table 6 Tafel plot results Concentration
Ecorr
(g /L)
(mV)
Blank 3 6 12 18
-770 -750 -760 -743 -767
icorr×(10-)3 (A)
βc
βa (V dec-1)
(V dec-1) 8.916 1.824 1.815 1.189 1.008
6.679 3.398 5.338 5.694 6.580
6.031 6.694 6.107 6.153 6.255
Protection efficiency (%)
79.542 79.643 86.664 88.694
3.3. FT-IR spectroscopy FT-IR spectroscopy technique was used to confirm the presence of electron rich elements in the Areca flower extract. Fig. 6 represents the different functional moieties in Areca flower extract molecules. The detailed results are shown in Table 7. Table 7 shows that, Areca flower extract species possessing free hydroxyl (in alcohols or phenols), carbonyl (in phenols), C-O, O-C=O and other electron rich elements in their moieties. It is well-known that, the Al corrosion can be inhibited by species possessing heteroatoms. Hence, it was concluded that, Areca flower extract species strongly adsorb on the active Al sites and inhibit the Al corrosion process by adsorption mechanism.
3.4. Scanning technique
electron
microscopy
3.5. Atomic force microscopy (AFM) technique AFM tool was also applied to study the truth of SEM results. The obtained AFM results (Fig. 8 (a, b) and Table 8) shows that, surface roughness values (both average roughness and root mean square roughness) in the inhibited Al system are low compared to the uninhibited Al system, which fully supports the SEM results.
Figure 6 IR spectra of Areca flower extract
(SEM) a)
The SEM technique was used to examine the Al surface condition in unprotected and protected systems. Figure 7 a represents the morphology of polished Al surface prior to the experiment. The polished Al metal surface was badly attacked when polished Al exposed to 0.5 M HCl solution, as a result of many cracks and scratches on the Al surface (Fig. 7 b). In another case, the surface of Al significantly developed (Fig. 7 c) when Al metal exposed to inhibited system (0.5 M HCl plus Areca flower extract) showing a remarkable decrease in the Al corrosion rate. The upgrading the surface of Al morphology is due to the existence of a tenacious green inhibitor layer
b)
(C Figure 7 (a-c) SEM images, a) polished Al surface, b) uninhibited Al surface and c) inhibited Al surface
Table 7 FT-IR spectrum results a) groups
Bands
3294 cm-1 1602 cm-1 1090 cm-1 2351 cm-1 2905 cm-1 1387 cm-1
b)
Figure 8 (a-b) AFM images, a) without inhibitor b) with an inhibitor
monomeric hydroxyl carbonyl C-O O-C=O C-H stretching vibrations
Table 8 AFM results Concentration (g/L)
Average roughness (Sa)
Root mean square roughness (Sq)
0
475. 84 nm
627.56 nm
18
179. 98 nm
253. 17 nm
4. Conclusions The present study demonstrated a good corrosion inhibition property of the Areca flower extract on the Al surface in 0.5 M HCl medium. Weight loss results show that, the inhibition property of the Areca flower extract was depends on its concentration, immersion period and test solution temperature. The highest protection efficiency (94.444 %) was observed at 303 K with 18 g/L of Areca flower extract for one hour immersion time. Langmuir isotherm affords the best depiction of the adsorption character of the Areca flower extract on the Al surface at all studied temperatures. Tafel curves show that, Areca flower extract act as a mixed type of controlling the both active anodic and cathodic reactions on the Al surface in 0.5 M HCl medium. Nyquist plots confirm that, protective action of inhibiting is directly proportional to the amounts of Areca flower extract. Further, SEM and AFM figures complement the weight loss, Tafel plot and impedance results.
Acknowledgment The authors thank Dr. B. E Kumaraswamy, Kuvempu University, for electrochemical facility.
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Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract N. Raghavendra, J. Ishwara Bhat* Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India. Email:[email protected]
Author: 1. N. Raghavendra (first author) Ph.D student, Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India.
2. J. Ishwara Bhat (corresponding author) Professor, Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India. Email:[email protected]
S1018-3639(17)30078-8 http://dx.doi.org/10.1016/j.jksues.2017.06.003 JKSUES 250
To appear in:
Journal of King Saud University - Engineering Sciences
Received Date: Accepted Date:
13 March 2017 6 June 2017
Please cite this article as: Raghavendra, N., Bhat, J.I., Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract, Journal of King Saud University - Engineering Sciences (2017), doi: http://dx.doi.org/10.1016/ j.jksues.2017.06.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract Graphical abstract
and aluminum immersion (contact) time was also evaluated. Parameters such as Ea, ∆H*, ∆S*, Kads and ∆Go ads were calculated by the gravimetric (weight loss) technique. Results of mass loss technique indicated that, the Areca flower extract molecules are strongly adsorbed at Al- 0.5 M HCl solution interface and follows the law of the Langmuir adsorption model. Tafel curves show that, the Areca flower extract molecules effectively adsorbed on the active Al sites by replacing the H2O molecules and strongly interact with cathodic and anodic sites impeding the dissolution reaction by mixed mode. Hence, Al metal is protected. Electrochemical impedance results indicate that, area of a semicircle (Nyquist plots) in the presence of the Areca flower extract is high compared to the bare system. The variation in the surface topography of aluminum was analyzed through scanning electron microscopy and atomic force microscopy techniques. Further, AFM results show that, surface roughness values in the protected Al system are low compared to the unprotected Al system. Keywords Aluminum; Areca flower; Hydrochloric acid; Langmuir adsorption model; Surface morphology
1. Introduction
Abstract The corrosion inhibition property of Areca flower extract species on the Al surface in acid (0.5 M HCl) medium was tested by weight loss, Tafel plot, impedance, scanning electron microscopy and atomic force microscopy techniques. The influence of different concentrations of Areca flower extract, 0.5 M HCl solution temperature
Acid solutions, especially HCl are greatly employed in the metallurgical industries for the removal of rust and dust. It is widely used in the de-scaling, etching, oil well acidification, ore fabrication, cleaning of boilers and other materials in several chemical, manufacturing and engineering industries. Hydrochloric acid solutions are highly corrosive and cause degradation of aluminum metal by dissolving the natural protective film (which is amphoteric in nature) (Nnanna et al., 2010; Oguzie et al., 2007). The Al corrosion in acid solution generally is inevitable, and leads to personal injury and economic loss. Among the corrosion protection methods, inhibitors are the best option to defend the metals from the corrosion owing to the low cost and high efficiency.
The corrosion inhibitors are the chemical substances contain sulfur (S), phosphor (P), oxygen (O) and nitrogen (N) elements in their moieties which plays a very important role in the Al corrosion inhibition process in hydrochloric acid solutions (Raghavendra and Ishwara Bhat, 2017; El-Etre, 2003; Li., et al., 2005; Raghavendra and Ishwara Bhat, 2016; Abdallah, 2004; Gunasekaran and Chauhan, 2004; Zucchi, Omar, 1985: Fouda et al., 2016). The electron rich species in chemical molecules form a protective film on aluminum surface and effectively isolates the electrode surface from the hydrochloric acid solution. Many synthetic compounds effectively block the corrosion reactions at aluminum sites by adsorption mode. But, majority synthetic compounds are expensive, noxious and not environmental friendly. Hence, nowadays, the use of synthetic organic species as corrosion inhibitors has been discouraged due to strict environmental rules. Hence, there is growing interest in non-poisonous and low cost corrosion inhibitors to replace the poisonous and expensive corrosion inhibitors. The study of corrosion of Al in hydrochloric acid solution by plant products is of immense interest from ecological viewpoint and is attracting a noteworthy level of attention. Because, natural products are non-toxic, low cost and eco-friendly substances. Adding, they are easily available and biodegradable (Bulstein, et al., 2005; AbdelGaber, et al., 2010; Khaled, 2010; Oguzie, 2008; Ebenso, et al., 2009; Mejeha, et al., 2010; Ebenso, et al., 2004; Abiola, et al., 2007; Martinez, 2002). Up to now, many plant extract species have been reported as inexpensive, less polluting and effective corrosion inhibitors for Al in 0.5 M HCl medium. The basic components in the plant or natural extracts are polyphenols, alkaloids, tannins, fats and flavanoids. These groups increase the thickness of the protective film on the surface of the aluminum, and hinder the aluminum corrosion rate in acid solution. Areca flower is one of the greenest products possessing special elements (heteroatoms) in their
molecules. Table 1 confirms the rich sources of natural molecules in Areca flower extract (EnShyh Lin and Chia-Ching Li, 2010; Wei-Min Zhang et al., 2009; Wei Peng, et al., 2015). The antioxidant property of Areca flower extract has been previously reported. But, no research paper on the corrosion inhibition efficiency (protection efficiency) of the Areca flower extract for any metal in acid (0.5 M hydrochloric acid) solution. Therefore, in this, we tested the corrosion inhibiting action of the Areca flower extract on the aluminum surface in 0.5 M HCl solution by chemical (weight loss), electrochemical (Tafel plot, AC impedance spectroscopy ) scanning electron microscopy (SEM) and atomic force microscopy (AFM) methods. Table 1 The main constituents of Areca flower extract
Polyphenols
Flavanoids
Alkaloids
Fatty acids
Rutin
Sorhamnetin
Arecoline
Lauric acid
Catechin,
Chrysoeriol
Arecaidine
myristic acid
Epicatechin
Uteolin
Guavacoline
palmitic acid
Procyanidin B1
Quercetin
Guavacine
stearic acid
Gallic acid,
Iquiritigenin
Arecolidine
oleic acid
Leucocyanidin,
acareubin
isoguvacine
Arachidonic acid
2. Experimental Part 2.1 Materials The important materials used for the present study are Al, 0.5 M hydrochloric acid solution, distilled water, acetone, ethanol, weighing balance, distilled water, water bath, Soxhlet apparatus and heating mantle. The corrosion inhibition test was carried out on the aluminum surfaces having the following chemical composition (in weight percentage): 0.1% Cu, 0.3–0.7 Si %, 0.3% Mn, 0.6% Fe, 0.4-
0.9% Mg, 0.2% Cr, 0.1%Tl or other grainrefining elements, 0.2% Zn and plus Al. Al pieces of 2.4 × 1.1 × 0.2 cm were used for the gravimetric studies. For electrochemical, SEM and AFM studies, 1 cm2 of the Al surface was exposed to acid solution and the remaining portion was enclosed with epoxy resin. The Al electrode employed for the examination was carefully polished with emery papers (grade: 320-1200) in order to remove the corrosion products and dust on the electrode surface, rinsed with distilled water, degreased with polar solvent (acetone/ethanol) and finally dried at laboratory temperature. 2.2 Methods The different amounts (3, 6, 12 and 18 (g/L) of inhibitor (Areca flower extract) were prepared by placing the 230 grams of Areca flower in the Soxhlet chamber. The extraction was run by using distilled water as a solvent. Extraction was repeated in many cycles. After that, the resulting solution was cooled, Whatman filter paper was used to remove the impurities present in the solution and purified sample was stored in the refrigerator in order to avoid the side reactions.
The Al surface exposed to acid (0.5 M HCl) solution without and with the Areca flower extract for 2 hours duration was examined via scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. The attendance of functional groups in the Areca flower extract molecules was confirmed by the Fourier transform infrared spectroscopy (FT-IR) method.
3. Results and discussion 3.1. Mass loss (gravimetric) technique The Al weight loss in the 0.5 M hydrochloric acid solution without and with the Areca flower extract of four different concentrations was examined at 1, 2, 3, 4, 5 and 10 hours duration at laboratory temperature 303 K. The weight loss experiment was also performed in 308, 313, 318 and 323 K in order to study the effect of solution temperature on protection efficiency of the inhibitor. Corrosion rate values in mils penetration per year were evaluated by applying following relation, Corrosion
rate=
(Fontana,
1987) Prior to exposure of metal to an acid solution, the wiped and dried Al metals were weighed using an analytical balance. Tests (both chemical and electrochemical) were conducted with four different concentrations of Areca flower extract. At the end, the Al surface was carefully rinsed with acetone and wiped with tissue paper and a loss in the weight of Al was recorded. To support the truth of the results, the experiment was repeated. For electrochemical test, CHI 660C instrument was used. In which, Al pieces as working electrode, the counter electrode is platinum cell and reference is calomel cell. The Tafel plots (plot of current density versus applied potential) are recorded with a potential range from -200 mV to +200 mV at speed rate 0.01 V/s. Whereas, Nyquist plot obtained from the frequency range 105 to 1 Hz with an amplitude of 0.01V.
et
al.,
(1)
where, W is the Al weight loss in milligrams , A is the Al immersed surface area in square inches, T is the immersion time of Al in 0.5 M HCl solution in hours , and D is the density of the Al in gram per cubic centimeter. The protection efficiency (in percentage) of the green inhibitor (Areca flower extract) was obtained by the following equation, Protection ( )
efficiency
×100
(in
percentage)= (2)
where, W2 is the Al weight loss when exposed to protected solution and W1 is the Al weight loss when exposed to unprotected solution.
From the Tables 2 and 3, it is observed that, the degree of Al dissolution rate decreases with an increase in the Areca flower extract concentrations is due to the adsorption of natural extract molecules on the surface of the Al which creates a smooth film. The formed protective film effectively blocks the attack of H + ions on the surface of the electrode in 0.5 M HCl system and controls the Al corrosion rate. Lowest protection efficiency value at lower inhibitor concentration is due to less coverage of inhibitor molecules on the Al surface. The lowest Al dissolution rate was noticed at 18 g/L of Areca flower extract concentration, showing that Areca flower extract molecules cover more Al active sites where a direct attack of H + ions occurs and greatly protects Al against dissolution. The loss in the weight of Al decreases with an increase in the amount of plant products. As a result, high protection efficiency (low Al corrosion rate) values are obtained. The effect of temperature and time on corrosion inhibition efficiency indicates that, protection of Al metal from 0.5 M HCl solution is generally reduced with enhancement in both temperature and time. The increase of both time and temperature directly impact on the stability of protective film formed on the electrode surface. The effect of test solution temperature on the acid-metal interface is complex, because lots of changes occur on the surface of the substrate such as desorption of Areca flower extract molecules, etching and adsorbed Areca flower extract molecules undergo decomposition. Degradation of Areca flower extract molecules on the Al surface weakness the metal-inhibitor interaction. Hence, protection efficiency decreases with a rise in test solution temperature from 303 to 323 K. With the rise in time, the insoluble protective film formed on the Al surface loses its stability and dissolves in the corrosive solution. As a result, the free Al surface (uncovered by Areca flower extract molecules) is direct contact with 0.5 M hydrochloric acid solution and undergo dissolution. Hence, protection to Al slightly
weaker with increasing time. Therefore, the rate of corrosion of Al is enhanced with time. Activation parameters are required to understand the mechanism of the inhibition of the Al corrosion process. The activation energy was obtained from the slope of an Arrhenius plot (ln υcorr vs. 1000/T, Fig. 1). Activation enthalpy and entropy were calculated from the slope and intercept of transition state plot (ln(υcorr/T) vs. 1000/T, Fig. 2) respectively. The calculated parameters are placed in the Table 4. From the Table 4, it is clear that, the values of the activation energy (Ea*) in protected environment were higher than the unprotected environment and increases with Areca flower extract concentration, which clearly showing that, difficulty in the movement of corrosive ions towards active Al sites was enhanced with inhibitor concentration. The attack of H+ ions on the Al surface was hindered by the Areca flower extract molecules. Hence, the more energy required in a protected environment for the oxidation process. The endothermic nature of the Al dissolution process in the acid medium was confirmed by positive activation enthalpy (∆H*) values, indicating the difficulty of the Al dissolution process. Both activation energy and activation enthalpy values are varied in the same fashion and are good agreement with each other. A similar observation was reported by a previous author (Adriana Patru Samidea, et al., 2008). The obtained activation entropy (∆S*) values in both unprotected and protected systems are high, negative and moves in the direction of positive side represents the decrease in the disorderness of the system. The gravimetric results are best fitted to the Langmuir adsorption model ( plot of C inh vs cinh/θ, where, Cinh is Areca flower extract concentration and θ is surface coverage , Fig.3) and which provides the nature of interaction between the Areca flower extract molecules and electrode (Al) surface. The parameters such as the equilibrium constant of the inhibitor adsorption (Kads) and free energy of adsorption (∆Goads) obtained from Fig.3 and are shown in the Table 5.
The strong adsorption of Areca flower extract molecules on the Al surface was confirmed by high Kads values. Strong interaction of plant extract molecules with the Al surface leads to the formation of a thick insoluble layer on electrode surfaces.
Table 2 Weight loss parameters for different contact times without and with Areca flower extract of different concentrations at laboratory temperature 303 K
Time (hrs)
1
2
3
4
5
10
Concentration (g/L)
Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18 Blank 3 6 12 18
Corrosion rate (× 10-4 (mpy) 8.699 1.933 1.449 0.966 0.483 11.116 2.658 1.933 1.449 1.208 16.110 4.188 3.383 2.416 2.094 18.124 4.833 4.229 3.141 2.779 24.166 7.346 6.089 4.833 5.558 42.291 14.983 12.083 9.666 8.699
Stronger adsorption of Areca flower extract molecules is an indication of high protection to the Al surface by inhibitory molecules in the 0.5 M hydrochloric acid system. Table 5 also shows that, the addition of Areca flower extracts to the 0.5 M HCl solution shifts the ∆Goads values towards the negative side, which confirms that Areca flower extract molecules adsorb spontaneously on the Al surface in acid medium. The resulted ∆Goads values in the range -34.759 to -36.443 verified that adsorption of Areca flower extract molecules on the surface of the Al are comprehensive adsorption type (includes both physical and chemical adsorption type).
Protection efficiency (%)
77.777 83.333 88.888 94.444 76.086 82.608 86.956 89.130 74.000 79.000 85.000 87.000 73.333 76.666 82.666 84.666 69.600 74.800 80.000 81.666 64.571 71.428 77.142 79.428
Figure 1 Arrhenius plots without and with inhibitor (Areca flower extract) of four different concentrations
Table 3 Effect of solution temperature on the protection efficiency of the inhibitor at different concentrations of Areca flower extract with contact period of one hour
Temperature (K)
Concentration (g/L)
Protection efficiency (%)
303
3 6 12 18 3 6 12 18 3 6 12 18 3 6 12 18 3 6 12 18
77.777 83.333 88.888 94.444 75.000 80.000 85.000 90.000 72.727 77.272 81.818 86.363 72.000 72.000 80.000 84.000 66.666 70.370 77.777 81.481
308
313
318
323
Figure 2 Transition state plots in the absence and presence of Areca flower extract The image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again.
Figure 3 Langmuir plots
Table 4 Activation parameters
Concentration (g/L)
Ea* ( kJ mol-1)
∆H* (kJ mol-1)
∆S* (J mol-1 K-1)
Blank 3 6 12 18
16.830 31.856 41.071 44.194 63.926
14.230 29.255 38.471 41.593 61.325
-313.901 -276.782 -248.672 -241.253 -181.159
Table 5 Thermodynamic parameters Temperature (K) 303 308 313 318 323
Kads (L g-1)
∆Go ads (kJ mol-1)
980.411 998.502 1019.794 885.002 780.944
-34.759 -35.380 -36.009 -36.209 -36.443
3.2. Electrochemical measurements 3.2.1. Tafel plots Fig.4 shows the polarization curves ( Roland T. Loto, Olukeye Tobilola, 2016) of Al in 0.5 M hydrochloric acid system in the absence and presence of the Areca flower extract with four different concentrations. Equation 3 was
employed for the determination of protection efficiency of Areca flower extract, , !""
( %) = [ 1 −
] ×100
!""
(3) where, i’corr = protected Al corrosion current density value and icorr = unprotected Al corrosion current density value. The parameters obtained from the Tafel curves are detailed in the Table 6. From the Table 6, it is noticed that, the value of corrosion current density is inversely proportional to the Areca flower extract concentrations, indicates that inhibitory species obstruct the both anodic and cathodic reactions on the surface of the Al via adsorption mechanism. The protective film formed on the active Al sites prevents corrosive ions attack on the electrode surface. Hence, corrosion current density values decreased and protection efficiency of the inhibitor increases with an increase in the amounts of Areca flower extract. The obtained corrosion potential (E corr), cathodic (βc) and anodic (βa) Tafel constant values did not exhibit monotonic trend and maximum displacement in the corrosion potential values with respect to bare solution is 27 mv, which showing that, Areca flower extract act as a mixed type inhibitor for Al metal in 0.5 M HCl solution by controlling the both active anodic and cathodic reactions.
the Areca flower extract is an indication of the protective role of plant extracts on the Al electrode. Appearance of depressed semicircular shows that, the Al corrosion in 0.5 M HCl system is mainly controlled by the single charge transfer process. The shapes of Nyquist curves in the absence and presence of the Areca flower extract of different concentrations are similar, which confirms that, the presence of the Areca flower extract of four different concentrations does not alter the Al corrosion dissolution mechanism in the studied environment. The deviation from the perfect semicircles is often referred to as frequency dispersion, which ascribed to inhomogeneities and the roughness of the surface.
Figure 4 Tafel curves (without and with addition of Areca flower extract) The image cannot be display ed. Your computer may not hav e enough memory to open the image, or the image may hav e been corrupted. Restart y our computer, and then open the file again. If the red x still appears, y ou may hav e to delete the image and then insert it again.
3.2.2. AC impedance spectroscopy The effect of four different concentrations of the Areca flower extract on the surface of Al in 0.5 M hydrochloric acid system is shown in Fig. 5 (Nyquist plots). Appraisal of Fig. 5 shows that, all the Nyquist plots exhibit the depressed semi circle. The area of the semicircle increases with the increment in the Areca flower extract concentrations, which shows that corrosion inhibition property of the Areca flower extract is mainly depends on its concentration. The area of the semicircle is low on a bare system (inhibited free system) compared to the inhibited system. The larger area of the semicircle in the presence of
Figure 5 Nyquist curves
(formed from Areca flower extract molecules) on the surface of Al, which is accountable for the inhibition of Al corrosion.
Table 6 Tafel plot results Concentration
Ecorr
(g /L)
(mV)
Blank 3 6 12 18
-770 -750 -760 -743 -767
icorr×(10-)3 (A)
βc
βa (V dec-1)
(V dec-1) 8.916 1.824 1.815 1.189 1.008
6.679 3.398 5.338 5.694 6.580
6.031 6.694 6.107 6.153 6.255
Protection efficiency (%)
79.542 79.643 86.664 88.694
3.3. FT-IR spectroscopy FT-IR spectroscopy technique was used to confirm the presence of electron rich elements in the Areca flower extract. Fig. 6 represents the different functional moieties in Areca flower extract molecules. The detailed results are shown in Table 7. Table 7 shows that, Areca flower extract species possessing free hydroxyl (in alcohols or phenols), carbonyl (in phenols), C-O, O-C=O and other electron rich elements in their moieties. It is well-known that, the Al corrosion can be inhibited by species possessing heteroatoms. Hence, it was concluded that, Areca flower extract species strongly adsorb on the active Al sites and inhibit the Al corrosion process by adsorption mechanism.
3.4. Scanning technique
electron
microscopy
3.5. Atomic force microscopy (AFM) technique AFM tool was also applied to study the truth of SEM results. The obtained AFM results (Fig. 8 (a, b) and Table 8) shows that, surface roughness values (both average roughness and root mean square roughness) in the inhibited Al system are low compared to the uninhibited Al system, which fully supports the SEM results.
Figure 6 IR spectra of Areca flower extract
(SEM) a)
The SEM technique was used to examine the Al surface condition in unprotected and protected systems. Figure 7 a represents the morphology of polished Al surface prior to the experiment. The polished Al metal surface was badly attacked when polished Al exposed to 0.5 M HCl solution, as a result of many cracks and scratches on the Al surface (Fig. 7 b). In another case, the surface of Al significantly developed (Fig. 7 c) when Al metal exposed to inhibited system (0.5 M HCl plus Areca flower extract) showing a remarkable decrease in the Al corrosion rate. The upgrading the surface of Al morphology is due to the existence of a tenacious green inhibitor layer
b)
(C Figure 7 (a-c) SEM images, a) polished Al surface, b) uninhibited Al surface and c) inhibited Al surface
Table 7 FT-IR spectrum results a) groups
Bands
3294 cm-1 1602 cm-1 1090 cm-1 2351 cm-1 2905 cm-1 1387 cm-1
b)
Figure 8 (a-b) AFM images, a) without inhibitor b) with an inhibitor
monomeric hydroxyl carbonyl C-O O-C=O C-H stretching vibrations
Table 8 AFM results Concentration (g/L)
Average roughness (Sa)
Root mean square roughness (Sq)
0
475. 84 nm
627.56 nm
18
179. 98 nm
253. 17 nm
4. Conclusions The present study demonstrated a good corrosion inhibition property of the Areca flower extract on the Al surface in 0.5 M HCl medium. Weight loss results show that, the inhibition property of the Areca flower extract was depends on its concentration, immersion period and test solution temperature. The highest protection efficiency (94.444 %) was observed at 303 K with 18 g/L of Areca flower extract for one hour immersion time. Langmuir isotherm affords the best depiction of the adsorption character of the Areca flower extract on the Al surface at all studied temperatures. Tafel curves show that, Areca flower extract act as a mixed type of controlling the both active anodic and cathodic reactions on the Al surface in 0.5 M HCl medium. Nyquist plots confirm that, protective action of inhibiting is directly proportional to the amounts of Areca flower extract. Further, SEM and AFM figures complement the weight loss, Tafel plot and impedance results.
Acknowledgment The authors thank Dr. B. E Kumaraswamy, Kuvempu University, for electrochemical facility.
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Inhibition of Al corrosion in 0.5 M HCl solution by Areca flower extract N. Raghavendra, J. Ishwara Bhat* Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India. Email:[email protected]
Author: 1. N. Raghavendra (first author) Ph.D student, Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India.
2. J. Ishwara Bhat (corresponding author) Professor, Department of Chemistry, Mangalore University, Mangalagangotri, Karnataka 574199, India. Email:[email protected]