1 Introduction

Aluminum metal possesses exceptional thermal, electrical, and mechanical properties. Because of high mechanical and corrosion resistance, specific weight (2.7 g/cm3), low cost, and high stiffness, aluminum metal was widely applied in building material, transportation, aerospace, electrical engineering, household, and technological applications [1,2,3,4]. Aluminum metal oxidizes easily due to its high electronegative property. Hence, aluminum metal reacts with oxygen and produce aluminum oxide layer (Al2O3) [thickness 0.01 mm]. This impenetrable layer further ceases the reaction on aluminum metal.

$$2{\text{Al}}+3{\text{/}}2{{\text{O}}_2} \to {\text{A}}{{\text{l}}_2}{{\text{O}}_3}\quad {{\text{G}}^ \circ }= - 1675{\text{ kJmo}}{{\text{l}}^{ - 1}}$$

The natural oxide layer formed on the surface of the aluminum is stable in between the pH range 3–8 and hence protects the metal from corrosion in such existing conditions. The hydrochloric acid and sodium hydroxide solutions are needed for several industrial applications. The natural oxide layer loses its strength when it is exposed to high concentration of acid and base solutions [5, 6]. Therefore, acid and base solutions attack on the free aluminum surface and deteriorate the metal structure.

The aluminum surface layer undergoes a subsequent series of reactions when in contact with an acid solution,

$${\text{A}}{{\text{l}}_2}{{\text{O}}_3}+3{{\text{H}}_2}{\text{O}} \to 2{\text{Al}}{\left( {{\text{OH}}} \right)_3}+3{{\text{H}}^+}$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_3}+{{\text{H}}^+} \to {\text{Al}}{\left( {{\text{OH}}} \right)_2}^{+}+{{\text{H}}_2}{\text{O}}$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_2}^{+}+2{{\text{H}}^+} \to {\text{A}}{{\text{l}}^{3+}}+2{{\text{H}}_2}{\text{O}}$$

Overall reaction:

$${\text{A}}{{\text{l}}_2}{{\text{O}}_3}+6{{\text{H}}^+} \to 2{\text{A}}{{\text{l}}^{3+}}+3{{\text{H}}_2}{\text{O}}$$
$${\text{A}}{{\text{l}}^{3+}}{\text{exists as Al}}{\left[ {{{\left( {{{\text{H}}_2}{\text{O}}} \right)}_6}} \right]^{3+}}{\text{in acid solution}}.$$

Aluminum dissolution in alkaline systems involves following reactions:

$${\text{A}}{{\text{l}}_2}{{\text{O}}_3}+3{{\text{H}}_2}{\text{O}} \to 2{\text{Al}}{\left( {{\text{OH}}} \right)_3}$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_3}+{\text{O}}{{\text{H}}^ - } \to {\text{Al}}{\left( {{\text{OH}}} \right)_4}^{ - }\left( {{\text{aq}}} \right)$$
$${\text{A}}{{\text{l}}_{({\text{S}})}} \to {\text{A}}{{\text{l}}^+}+{e^ - }$$
$${{\text{H}}_2}{\text{O}}+{e^ - } \to {\text{O}}{{\text{H}}^ - }+{{\text{H}}^+}$$
$${\text{H}}+{\text{H}} \to {{\text{H}}_2}$$
$${\text{Al}}+{\text{O}}{{\text{H}}^ - } \to {\text{Al}}{\left( {{\text{OH}}} \right)_{{\text{ads}}}}$$
$${\text{A}}{{\text{l}}_{\left( {\text{S}} \right)}}+{{\text{H}}_2}{\text{O}} \to {\text{Al}}{\left( {{\text{OH}}} \right)_{{\text{ads}}}}+\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} {{\text{H}}_2}$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_{{\text{ads}}}}+{\text{O}}{{\text{H}}^ - } \to {\text{Al}}{\left( {{\text{OH}}} \right)_{2,}}_{{{\text{ads}}}}+{e^ - }$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_{{\text{ads}}}}^{2}+{\text{O}}{{\text{H}}^ - } \to {\text{Al}}{\left( {{\text{OH}}} \right)_{3,}}_{{{\text{ads}}}}+{e^ - }$$
$${\text{Al}}{\left( {{\text{OH}}} \right)_{3,}}_{{{\text{ads}}}}+{\text{O}}{{\text{H}}^ - } \to {\text{Al}}{\left( {{\text{OH}}} \right)_4}^{-}$$

H2 gas is produced due to reduction of H2O molecules in the main cathodic process,

$${{\text{H}}_2}{\text{O}}\left( {{\text{surface}}} \right)+{e^ - } \to {\text{H}}+{\text{O}}{{\text{H}}^ - }$$
$${\text{H}}+{{\text{H}}_2}{\text{O}}\left( {{\text{surface}}} \right)+{e^ - } \to {{\text{H}}_2}+{\text{O}}{{\text{H}}^ - }$$

Due to aluminum corrosion, huge losses occur in each year in all countries. The losses due to corrosion are categorized into two types: one is direct losses (includes corroded metal cost and labor charge of replacement) and the other one is indirect losses (includes losses in efficiency losses, product loss due to leakage, product contamination).

The indirect losses cannot be anticipated in terms of currency. So it is so particularly vital to recognize the mechanism of aluminum corrosion [7].

Corrosion inhibitors are the one of the best methods for aluminum metal degradation in both acid and base systems due to its low cost and high adsorption efficiency. Because of strict environmental regulations, industries consider the following points for the selection of inhibitors for various applications:

  1. (i)

    Cost and availability of the inhibitor,

  2. (ii)

    Stability of the inhibitor,

  3. (iii)

    Affinity with corroding solution,

  4. (iv)

    Eco-friendly nature,

  5. (v)

    Biodegradable, and

  6. (vi)

    High adsorption ability.

Majority organic compounds possessing sulfur, oxygen, nitrogen, and phosphorous atoms were greatly employed as effective corrosion inhibitors. But, great number of organic species are pricey and harmful to the environment. Therefore, more attention is paid on non-toxic and cheap corrosion inhibitors. Plant products are the richest sources of natural organic species which can be employed as ecological corrosion inhibitors [8,9,10,11,12,13].

Green arecanut husk is one of the waste natural products and employed as a fuel during arecanut processing. In epoxy composites, it is used as a reinforcing material. Arecanut husk possessing special elements in their lignin (amorphous in nature), hemicelluloses (amorphous in nature), cellulose (crystalline in nature), furfuraldehyde, propectin, and pectin groups [14,15,16,17,18]. Results of literature analysis revealed that special elements (nitrogen, phosphorus, oxygen, and sulfur) in organic species participate in the corrosion inhibition process through the adsorption mechanism (Fig. 1).

Fig. 1
figure 1

Schematic demonstration of plant extract adsorption on the surface of the metal molecules on metal surface

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No specific information exists on the arecanut husk extract as a potential corrosion inhibitor for aluminum in both 0.5 M HCl and 0.1 M NaOH solution. Therefore, the present investigation is focused on finding the inhibiting property of arecanut husk extract at the electrode (aluminum)–electrolyte (0.5 M HCl/0.1 M NaOH solution) interface by weight loss (both stirred and unstirred conditions), Tafel plot, impedance, scanning electron microscopy (SEM) and atomic force microscopy (AFM) techniques. Further, crystalline or amorphous nature of the arecanut husk extract and its protective film on the surface of the metal was confirmed by powder X-ray diffraction technique.

2 Experimental Sections

2.1 Metal and Corrosive Media Under Study

The chemical composition of aluminum metal in percentage employed in the studies of corrosion is given in Table 1 [19]. 0.5 M HCl and 0.1 M NaOH solution are the media employed in the aluminum corrosion inhibition study. Standard procedure is followed for the preparation of test solutions.

Table 1 Al specimen composition
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2.2 Inhibitor Used

The 100 g of small pieces of arecanut husk (submerged in 450 ml of 1% HCl solution for 1 day) was refluxed in a Soxhlet apparatus with 450 ml of 1% HCl solution for effective 5-h duration. Different concentrations (3, 6, 12, and 18 g/L) were prepared from the filtered solution.

2.3 Chemical Method

The polished (with different grades of sand papers) and cleaned aluminum metal was submerged in 100 ml of 0.5 M HCl/0.1 M NaOH solutions without and with the arecanut husk extract for predetermined temperature and time.

The removed aluminum metal was washed with running tap water and acetone to eliminate the corrosion products and differences in the weight of the aluminum metal were noted.

The protection efficiency of the arecanut husk extract was calculated by using the following formula:

$${\text{Protection efficiency of arecanut husk extract}}=\frac{{\left( {{W_1} - {W_2}} \right)}}{{{W_1}}} \times 100$$
(1)

where W2 is the aluminum weight loss by inhibiting corroding system (0.5 M HCl/0.1 M NaOH) and W1 is the aluminum weight loss in uninhibited corroding system (0.5 M HCl/0.1 M NaOH).

Equation 2 was used to calculate the aluminum corrosion rate in protected and unprotected systems [20],

$${\text{Corrosion rate in mils penetration per year = }}\frac{{534W}}{{ATD}},$$
(2)

where W is weight loss (milligrams); A is the surface area (square inches); T is the immersion time (hours), and D is the density (gram per cubic centimeter).

2.4 Electrochemical Method

The electrochemical results (both potentiodynamic polarization and AC impedance spectroscopy) were obtained by CHI660C workstation, which consists three electrode systems, namely aluminum (working) electrode, platinum (counter) electrode, and calomel (reference) electrode. The 1 cm2 area of aluminum electrode was immersed in 0.5 M HCl/0.1 M NaOH solution without and with different concentrations of the arecanut husk extract. Tafel curves are recorded potentiodynamically with a scan rate of 0.01 V/s in between the potential range ± 200 mV. Nyquist plots were obtained in the frequency range of 100,000 to 1 Hz with an amplitude of 0.01 V.

2.5 Powder X-ray Diffraction Technique

Powder X-ray diffraction technique (Rigaku miniflex 600 W) was used to investigate the amorphous/crystalline nature of the arecanut husk extract and its protective film on the surface of the aluminum in 0.5 M HCl/0.1 NaOH solution after the 2-h immersion time.

2.6 Surface Examination

The aluminum surface condition in inhibiting and uninhibited systems after 2-h duration (simple immersion) was analyzed through scanning electron microscopy and atomic force microscopy techniques at 303 K.

3 Results and Discussion

3.1 Gravimetric Experiment

Mass loss (weight loss) experiment was carried out with four different concentrations of arecanut husk extract, to study the effect of the arecanut husk extract on aluminum surface in both 0.5 M HCl and 0.1 NaOH environments, gravimetric study was performed with different immersion times and temperatures. The parameters obtained from this method are listed in Tables 2, 3, 4. It is observed that the rise in the arecanut husk extract concentration from 3 to 18 g/L decreases the aluminum corrosion rate from higher to lower value. This indicated that increased concentration of arecanut husk extract increases the adsorption of plant extract constituents on aluminum surface, which strongly defend the aluminum metal surface in 0.5 M HCl and 0.1 M NaOH environments by creating a barrier for charge and mass transfer. Hence, the aluminum metal surface was strongly protected in 0.5 M HCl and 0.1 M NaOH solution by arecanut husk extract constituents.

Table 2 Weight loss results at laboratory temperature (303 K)
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Table 3 Effect of 0.5 M HCl/0.1 M NaOH solution temperature on corrosion inhibition efficiency
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Table 4 Effect of 0.5 M HCl/0.1 M NaOH stirring solution on corrosion inhibition efficiency with an immersion period of one hour at 303 K
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The effect of contact time on protection efficiency of the arecanut husk extract was also studied by submerging aluminum metal in 0.5 M HCl/0.1 M NaOH solution from 1 to 72 h. It is noticed that protection efficiency of arecanut husk extract decreases in both 0.5 M HCl and 0.1 M NaOH environments by increasing the exposure time from 1 to 72 h. This is due to the fact that solubility of protective film increases with immersion time. Hence, the more free area is available for corrosion process. As a result, protection efficiency decreases with increasing immersion time from 1 to 72 h. Similar observation has been reported by previous authors [21,22,23]. The weight loss experiment was also carried out in stirred condition without and with inhibitor (with an expose time of 1 h). It is concluded that protection efficiency obtained in stirred condition is higher than the unstirred condition, which indicates that stable, compact, uniform, and persistence protective film formed on the surface of the aluminum in stirred condition.

3.2 Reason for Lower Protection Efficiency at Low Inhibitor Concentrations

Results in the table have also shown that lower aluminum protection is achieved at lower arecanut husk extract constituents; this is due to soluble protective film on aluminum surface in both corrosive conditions, as the amount of arecanut husk extract increases the soluble protective film changes to the insoluble complex which responsible for high protection efficiency values.

3.3 Effect of Inhibitor Storage Time on Protection Efficiency

The protection efficiency of the inhibitor was also investigated after its storage in room temperature for 2 years. During the storage time, it is found that the arecanut husk extract is free from bacterial and fungal attack. The protection efficiency of the inhibitor after its storage time 2 years was well agreement with an inhibition efficiency of freshly prepared inhibitor. Thus, arecanut husk extract molecules inhibit the aluminum corrosion in both 0.5 M HCl and 0.1 M HCl solutions effectively with a shelf life of at least 2 years. Therefore, arecanut husk extract has significant shelf period.

It is also observed that protection efficiency of arecanut husk extract decreases with enhancing the solution temperature from 303 to 323 K. This is due to the enhanced aluminum dissolution rate and partial desorption of plant extract constituents from the electrode surface. By increasing the 0.5 M HCl/0.1 M NaOH solution temperature from 303 to 323 K, the equilibrium shifts to higher desorption side than adsorption side and hence the stability of protective film weakness.

As a result lower protection efficiency at higher solution temperature, which indicates the physical adsorption nature of arecanut husk extract molecules on aluminum surface in 0.5 M HCl/0.1 M NaOH environments.

The activation parameters of the aluminum corrosion process in free and arecanut husk extract containing 0.5 M HCl/0.1 M NaOH solution were determined by Arrhenius and transition state equations (Figs. 2a, b, 3a, b; Table 5). The activation energy (Ea) values in free 0.5 M HCl/0.1 M NaOH solution were lower than inhibited systems, which confirms that electrode (aluminum) dissolution rate is very slow with the introduction of arecanut husk extract constituents and also supports the physical nature of protective film. The endothermic nature of the aluminum corrosion process in both 0.5 M HCl/0.1 M NaOH solutions was confirmed by positive activation enthalpy (ΔH) values, which indicates the adsorption plant extract constituents on the surface of aluminum. The decrease in the entropy of the system was confirmed by negative entropy of activation (ΔS) values.

Fig. 2
figure 2

Arrhenius plot, a aluminum in 0.5 M HCl solution without and with inhibitor, b aluminum in 0.1 M NaOH solution without and with inhibitor

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Fig. 3
figure 3

Transition state plot, a aluminum in 0.5 M HCl without and with inhibitor, b aluminum in 0.1 M NaOH without and with inhibitor

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Table 5 Activation parameters
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It is generally assumed that, during aluminum corrosion process, plant extract molecules adsorbed on the surface of the metal (aluminum). The adsorbed plant species strongly interact with active aluminum sites and inactive the metal (aluminum) corrosion process. Basic information regarding the interaction between plant extract molecules and aluminum surface was obtained from adsorption isotherm models. The values of inhibitor concentration and surface coverage (Ɵ) were applied to different adsorption isotherm models and best R2 value was obtained in the Langmuir isotherm model. The thermodynamic parameters obtained from the Langmuir adsorption model are listed in Table 6.

Table 6 Thermodynamic parameters
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The large equilibrium constant of adsorption (Kads) values supports the high protection efficiency of arecanut husk extract molecules and strong interaction between the arecanut husk extract molecules and electrode (aluminum) surface. Hence, aluminum metal is protected in both 0.5 M HCl/0.1 M NaOH solutions in the presence of four different concentrations of plant extract. The free energy of adsorption \((\Delta G_{{{\text{ads}}}}^{ \circ })\) values was in negative side, which indicates that adsorption of arecanut husk extract molecules on the surface of the aluminum obeys spontaneous process. On present investigation, the obtained ΔG° values are in between the literature values which confirm that the aluminum corrosion inhibition process occurs through comprehensive adsorption (both physical and chemical adsorption) mode.

Heat of adsorption (Q) (obtained from plot of log (Ɵ/1 − Ɵ) vs 1/T, Fig. 4a, b and Table 7) is an evidence of release of more quantity of heat.

Fig. 4
figure 4

Plot of log (Ɵ/1 − Ɵ) vs 1/T, a aluminum metal in 0.5 M HCl solution, b aluminum metal in 0.1 M NaOH solution

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Table 7 Heat of aluminum corrosion reaction in both 0.5 M HCl and 0.1 M NaOH solutions
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4 Electrochemical Studies

4.1 Potentiodynamic Polarization Studies (Tafel Plot Studies)

The Tafel curve parameters for aluminum metal in both 0.5 M HCl and 0.1 M NaOH solution containing four different concentrations of the arecanut husk extract are listed in Table 8. The Tafel curve (potentiodynamic polarization plot) is shown in Fig. 5a, b.

Table 8 Tafel parameters
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Fig. 5
figure 5

Tafel plot, a aluminum in 0.5 M HCl without and with inhibitor, b aluminum in 0.1 M NaOH without and with inhibitor

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The protection efficiency (corrosion inhibition efficiency) of the arecanut husk extract for the aluminum metal in 0.5 M HCl/0.1 M NaOH solution was calculated by using below mathematical relation,

$${\text{Protection~efficiency}}=\left[ {1 - \frac{{{{i^{\prime}}_{{\text{corr}}}}}}{{{i_{{\text{corr}}}}}}} \right] \times 100$$

where i′corr is the value of electrode (aluminum) corrosion current density in protected 0.5 M HCl/0.1 M NaOH solution and icorr the value of electrode (aluminum) corrosion current density value in free 0.5 M HCl/0.1 M NaOH solution.

It is observed that the values of corrosion potential (Ecorr) and Tafel slopes (βa and βc) did not show any regular trend in the presence of arecanut husk extract molecules indicating that arecanut husk extract molecule blocks the both anodic and cathodic reactions of aluminum surface in both 0.5 M HCl and 0.1 M NaOH solution through mixed mechanism. The aluminum corrosion inhibition process occurs through the adsorption mechanism. The corrosion current (icorr) values decrease with increase in arecanut husk extract concentrations from 3 to 18 g/L, which shows that the aluminum corrosion reaction was controlled by arecanut husk extract constituents. The enhancement in the protection efficiency values with the rise in arecanut husk extract concentration is due to the formation of a thick barrier layer which effectively isolates the electrode surface from 0.5 M HCl/0.1 M NaOH solution [24,25,26].

4.2 Impedance Spectroscopy Technique

Impedance studies were carried out in order to support the weight loss and Tafel plot results. The increased depressed semi-circle with natural extract concentration is clearly is an indication of corrosion inhibition property of plant extract on metal surface in corrosive media (Nyquist plots, Fig. 6a, b).

Fig. 6
figure 6

Nyquist plots, a aluminum in 0.5 M HCl without and with inhibitor, b aluminum in 0.1 M NaOH without and with inhibitor

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4.3 Powder X-ray Diffraction Technique

Figure 7 represents that, sharp, intense peak in arecanut husk extract. The protective film formed (after 2-h immersion period) on the aluminum surface in 0.5 M HCl solution also contains sharp peak with less intensity (compared to arecanut husk extract). The less intensity of peak is may be due to the adsorption of extract constituents on the aluminum metal surface in 0.5 M HCl solution. But, during aluminum corrosion inhibition process in 0.1 M NaOH medium, the protective film formed by arecanut husk extract constituents (after 2-h immersion period) on aluminum surface in 0.1 M NaOH solution lost its crystalline nature that was evidenced by the absence of sharp peaks as shown in the figure.

Fig. 7
figure 7

Powder XRD pattern of arecanut husk extract and its protective film on aluminum surface in both 0.5 M HCl and 0.1 NaOH media

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4.4 Surface Inspection

4.4.1 Scanning Electron Microscopy (SEM) Technique

The changes in the aluminum surface topography due to corrosion reaction without and with the arecanut husk extract were analyzed using scanning electron microscopy technique. The resulted image are shown in Fig. 8a, b.

Fig. 8
figure 8

a Aluminum metal in 0.5 M HCl solution. b Aluminum metal in 0.1 M NaOH solution

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When the aluminum metal immersed in 0.5 M HCl/0.1 M NaOH solution, the much variation in the surface morphology was observed with many numbers of lines and cracks, which is an indication of higher dissolution rate of aluminum metal in 0.5 M HCl/0.1 M NaOH solution. When arecanut husk extract was introduced into the 0.5 M HCl/0.1 M NaOH solution, the surfaces are in good condition with the least number of lines and cracks. This change is due to the adsorption of arecanut husk extract molecules on the surface of the electrode (aluminum) in 0.5 M HCl/0.1 M NaOH solution.

4.4.2 Atomic Force Microscopy Technique (AFM) Technique

The decrease in average roughness (Sa) values (Fig. 9a, b; Table 9) with the introduction of arecanut husk extract to the 0.5 M HCl/0.1 M NaOH solution highly favors the SEM results.

Fig. 9
figure 9

a Aluminum surface in 0.5 M HCl solution without and with inhibitor, respectively. b Aluminum surface in 0.1 M NaOH solution without and with inhibitor, respectively

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Table 9 Average roughness values obtained by AFM technique
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5 Conclusions

Arecanut husk extract showed the good inhibitory action over the electrode (aluminum) surface in both acid (0.5 M HCl) and alkali (0.1 M NaOH) media. Arecanut husk extract controls the aluminum corrosion inhibition process by mixed mechanism. The endothermic aluminum corrosion process in both 0.5 M HCl and 0.1 NaOH solutions was confirmed by weight loss parameters. The adsorption process follows the Langmuir model in the studied temperature range 303–323 K. Both Tafel and impedance studies showed the adsorption of arecanut husk extract constituents on aluminum surface in 0.5 M HCl and 0.1 M NaOH environments. The SEM and AFM images confirmed the inhibitory action of arecanut husk extract molecules in both 0.5 M HCl and 0.1 M NaOH solutions.