Use of Sapindus (reetha) as corrosion inhibitor of aluminium in acidic medium

The specimens of 1 cm² dimensions were used for weight loss, potentiodynamic polarization measurements and electrochemical impedance studies. These specimens were abraded with different grades of emery papers, washed with acetone and then rinsed with distilled water before inserting them into the test solution. 1 M HCl was used as a corrosive medium for Aluminium. Raw Sapindus fruits (reetha) were purchased from market and were shade dried for 48 h and were grinded to powder [16]. 100 grams of this powder was used in a soxhlet apparatus in order to make aqueous extract of Sapindus. This aqueous extract found was then filtered and was tray dried for 48 h at 318.15 K. The dried powder obtained after drying was preserved in desiccator. The test solutions of different concentrations were made using this dried extract in 1 M HCl as a medium.

Weight loss method was employed to evaluate corrosion rate, inhibition efficiency and surface coverage of Sapindus because of its simplicity and accuracy. This technique involves the specimen to be tested for its weight loss by using weighing balance of sensitivity of ±0.01 mg. Aluminum coupons having the following composition: 0.35% Fe, 0.25% Si, 0.05% Cu, 0.05% Zn, 0.03% Mg, 0.03% Mn, 0.03% Ti and 99.21% being Al were used to perform the tests. The specimen (metal) is treated with acid and variety of concentrations of the inhibitors and the weight loss is seen in the specimen before it is immersed in the acid solution and after immersion for some specific time. The concentration of inhibitor in this technique is taken in mgL^-1. By using this technique the corrosion rate, inhibition efficiency as well as surface coverage were calculated by using equations (1)–(3) respectively.

$$\begin{eqnarray}&&CR=\displaystyle \frac{87.6\times w}{AtD}\end{eqnarray} \tag{ 1 }$$

Here

CR = Corrosion rate in mpy

W (mg) = the Aluminium weight loss

A = the coupon area (cm²)

t = the time of exposure

D = the Aluminium density (g/cm³)

$$\begin{eqnarray}&&\eta \% =\displaystyle \frac{{W}_{(0)}-{W}_{(i)}}{{W}_{(0)}}\times 100\end{eqnarray} \tag{ 2 }$$

Here

η = Inhibition efficiency

W_(i) = average weight loss with the inhibitor

W₍₀₎ = average weight loss without the inhibitor

$$\begin{eqnarray}&&\theta =\displaystyle \frac{{W}_{(0)}-{W}_{(i)}}{{W}_{(0)}}\end{eqnarray} \tag{ 3 }$$

Electrochemical experiments were done using three electrode cells. The three electrodes are saturated calomel electrode, platinum counter electrode, which was attached with the luggin capillary. Luggin capillary acted as the reference electrode. The Luggin capillary tip was kept near to working electrode, to minimize the ohmic contribution. To embed the working electrode Teflon holder was used having the epoxy resin area of 0.785 cm².

The potential of potentiodynamic polarization curves started from potential of −250 mV to potential of +250 mV with respect to open circuit potential at a scan rate of started rate of 0.5 mV s⁻¹. Inhibition efficiency η_p (%) is written as:

$$\begin{eqnarray}&&{\eta }_{p}( \% )=\displaystyle \frac{{I}_{\,corr}^{0}-{I}_{corr}}{{I}_{corr}^{0}}\times 100\end{eqnarray} \tag{ 4 }$$

Where

I°_corr = Standard corrosion current density of blank.

I_corr = Corrosion current density of inhibitor.

This technique was used in order to know the values of corrosion potential (E_corr), current densities which are denoted by (I_corr), anodic tafel slopes and cathodic tafel slopes (βa and βc), and surface coverage (θ) and inhibition efficiency as functions of inhibitor concentration was measured from the curves. The equation (4) mentioned above was used for calculation of the corrosion current [17].

The measurement of EIS was done by the help of corrosion potential (E _corr) with frequency from 100 000 to 0.1 Hz. The result of EIS were obtained by utilizing electrical equivalent circuit system (figure 2) obtained along with the charge transfer resistance which is donated by R_ct value. In this method the IE % is calculated by the formula:

$$\begin{eqnarray}&&IE \% =\displaystyle \frac{{R}_{ct(I)}-{R}_{ct(0)}}{{R}_{ct(I)}}\times 100\end{eqnarray} \tag{ 5 }$$

Here R_{ct (0)} = charge resistance transfer of Aluminium without inhibitor

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R_{ct (I)} = charge resistance transfer of Aluminium having the inhibitor.

The optimum concentration of extracts was calculated by using Gamry instrument 600 AC. Voltametry was used to understand the nature of Aluminium surface adsorption. Bode plot method (figures 3(d) and 4(c)) used to identified the maximum frequency of sample. The final and initial range of potential was set at 1 V to −1 V with voltage at 0.01. The evaluation of surface morphology of Aluminium specimen is done by scanning electron microscope [18].

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2.6.1. Scanning electron microscopy (SEM) and AFM

Gaussian software was used to determine HOMO, LUMO and dipole moment of the inhibitors on the basis density function theory method. The computational methodology of quantum chemistry was being used in order to obtain the structure and electronic parameters. The tendency of an inhibitor to donate electrons to acceptor molecule is shown by the value of E_HOMO and to accept electrons is given by E_LUMO. The reactivity of an inhibitor towards metal is determined by the energy gap [21].

These studies were performed by using reliable DMol³ module implemented in the high-performance software (Materials Studio version 6.0). For the whole simulation procedure the compass force field was used to optimize the structure of all component of the system of interest. The simulations were carried out in the simulation boxes (42 Å × 42 Å × 55 Å) having α = 90.00; β = 90.00 and γ = 90.00 with periodic boundary condition in order to simulate a representative part of an interface devoid of any arbitrary boundary effects. The Al (111) planes were next enlarged to a (10*10) super cell. After that, a vacuum slab was built above the surface to convert the system to 3D periodicity. The optimized inhibitor using the Forcite code was then added near the surface of Al (111) and a Monte Carlo simulation annealing procedure was carried out [22].

Values of corrosion rate as reported in table 1 signified that corrosion rate was inversely proportional to Sapindus concentration inacidic medium. Such behavior was observed due to the higher adsorption of inhibitor on the surface of metal. The concentration of inhibitor was varied from 400 to 4000 mg L⁻¹ and highest inhibition was attained at 2000 mg L⁻¹ after which inhibition efficiency did not considerably vary. Therefore, the other variations like temperature variation and electrochemical studies were performed at an inhibitor concentration of 2000 mg L⁻¹.

Weight loss measurement of Aluminium was carried out at temperature ranging from 293.15 to 308.15 K in the presence and absence of standard concentration (2000 mg L⁻¹) of inhibitor (Sapindus). The results of temperature variation have been represented in table 2.

Adsorption isotherm signified the mechanism of adsorption of inhibitor molecule on the surface of metal. The surface coverage of different concentration of inhibitor at different temperatures enabled to get the isotherm plot. Equation (7) was utilized to calculate K_ads using Langmuir isotherm, similarly equation was for Freundlich isotherm

$$\begin{eqnarray}&&\displaystyle \frac{\theta }{1-\theta }={K}_{ads}C\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&\theta ={K}_{ads}C\,\end{eqnarray} \tag{ 7 }$$

Here C is the concentration and θ is the surface coverage and K_ads is the equilibrium constant.

Langmuir absorption was the method used to check the adsorption behavior of Sapindus molecule on the surface of metal of Aluminium. Figure 3(a) represents the (Arrhenius Plot)graph between the log C/θ versus log C. The slope of this graph was found to be 0.999 25 (=1). This signified that the Langmuir desorption isotherm was obeyed for the adsorption of Sapindus on the surface of Aluminium.

The activation energy was calculated using Arrhenius equation (9) from the slope of the plot between Log Cr and 1000/T (figure 3(b)):

$$\begin{eqnarray}&&{C}_{R}={\rm{Aexp}}\left(\displaystyle \frac{-Ea}{RT}\right)\end{eqnarray} \tag{ 8 }$$

Where, A is the pre-exponential factor, T is the temperature, R is the gas constant, and Ea is the activation energy.

Table 3 represents the value of activation energy for inhibitor as well as for blank and due to the physical barrier created by the adsorbed inhibitor molecules for charge and mass transfer, the value of the activation energy is high for inhibitor as compared to the value of activation energy for blank. To calculate the enthalpy of activation Erying equation (10) was used. Figure 3(c) represents the plot between ln Cr/T versus 1/T and the values of slope (−ΔH/R) and intercept $(\,\displaystyle \frac{{\rm{lnR}}}{Nh}+\displaystyle \frac{{\rm{\Delta }}S}{R})$ of this plot was used to calculate the values of ΔH and ΔS.

$$\begin{eqnarray}&&\mathrm{ln}\,\displaystyle \frac{Cr}{T}=\left(\displaystyle \frac{-{\rm{\Delta }}{\rm{H}}}{R}\right).\displaystyle \frac{1}{T}+\left(\displaystyle \frac{\mathrm{ln}\,{\rm{R}}}{Nh}+\displaystyle \frac{{\rm{\Delta }}S}{R}\right)\end{eqnarray} \tag{ 9 }$$

Here,

ΔH = enthalpy of activation.

ΔS = entropy of activation.

R = gas constant.

N = Avogadro's number.

h = plank's constant.

Table 4 gives the values of derived activation parameters. The relation between the standard free energy of adsorption ΔG⁰_ads and K_ads is given by the equation:

$$\begin{eqnarray}&&{\rm{\Delta }}{G}_{ads}^{0}=-2.303RT\,\mathrm{log}(55.5Kads)\end{eqnarray} \tag{ 10 }$$

Here the 55.5 value is the molar concentration of water in the solution and from the values of ΔG⁰_ads and ΔH⁰_ads value of ΔS⁰_ads was calculated using equation (12):

$$\begin{eqnarray}&&{\rm{\Delta }}{G}_{ads}^{0}={\rm{\Delta }}{H}_{ads}^{0}-T{\rm{\Delta }}{S}_{ads}^{0}\end{eqnarray} \tag{ 11 }$$

The value of ΔG⁰_ads for Sapindus is −10.181 KJ mol⁻¹. The negative value tells the spontaneity of the adsorption as well as the stability of adsorbed layer which is made by the Sapindus on the metal surface. The ΔG⁰_ads value confirmed the Physisorption mechanism followed by Sapindus for inhibition of acidic corrosion of Aluminium in 1 M HCl. The calculated values of ΔG⁰_ads suggest a strong interaction between the inhibitor and the surface of Aluminium [23]. The dissolution of Aluminium is endothermic in nature due to the positive value of ΔH⁰_ads and the higher positive value in presence of inhibitor than in blank is an evidence of slower dissolution of metal in presence of inhibitor [24]. The disordering increases which is the driving force for inhibitor adsorption on Aluminium surface due to the positive value of ΔS⁰_ads. The calculated values for standard enthalpy of adsorption ΔH⁰_ads and standard entropyof adsorption ΔS⁰_ads has been reported in table 5.

Polarization curves of the Aluminium in 1 M HCl solution in the presence and absence of Sapindus with different concentrations were used to determine various electrochemical parameter like corrosion potential (E_corr), corrosion current densities (I_corr), and tafel slopes (figure 4(a)). I_corr or corrosion current densities are used to calculate the inhibition efficiency. The tafel plot demonstrated that the increase in inhibitor concentration leads to the decrease in the corrosion rate densities. This suggested that reduction of electrochemical rate due to the formation of barrier layer over the surface of Aluminium. Cathodic and anodic curves are too observed to fluctuate towards lesser current densities in the presence of inhibitor. This can be presumed that the inhibition here occurs by blocking of active sites on surface of metal. The surface coverage (θ) was calculated by:

$$\begin{eqnarray}&&\theta =\displaystyle \frac{Icorr(blank)}{Icorr(blank)}-\displaystyle \frac{{\rm{Icorr}}({\rm{inhibitor}})}{Icorr(blank)}\end{eqnarray} \tag{ 12 }$$

Here I_corr(blank) is the corrosion current density of without inhibitor and I_corr (inhibitor) is the corrosion current density with inhibitor. If anodic slope (ba) > cathodic slope (bc), it suggests that anodic disintegration process is more preferred and inhibitor will go about as an anodic type of inhibitor and If cathodic slope (bc) > anodic slope (ba), then cathodic disintegration process is more preferred and inhibitor will go about as a cathodic type of inhibitor [25, 26]. In the present studies, the values of Ecorr remain almost constant, indicating the mixed type of inhibitor. It is clear from the table 6 that both cathodic and anodic slopes are affected in presence of inhibitor, it can therefore be assumed that Sapindus gets adsorbed by inhibiting the corrosion of Al in HCl solution by precipitation of chloride salt on both cathodic and anodic sites of metal oxide surfaces.

The Aluminium metal corrosion from the HCl is explored by the EIS at 298.15 K after immersion of 1 h. By EIS we can calculate the double fold layer capacitance (cdl) and charge transfer resistance value (Rct) depicted in figures 4(b)–(d). By measuring the width of semicircle and the two fold layer capacitance, calculation of charge transfer resistance. The equation (14)was used to calculate that:

$$\begin{eqnarray}&&{\boldsymbol{Cdl}}=\displaystyle \frac{1}{2\pi {f}_{\max }{R}_{ct}}\end{eqnarray} \tag{ 13 }$$

Here, f_max is the frequency where imaginary part of impedance i.e., Z' has maximum magnitude.

Also the corrosion inhibition efficiency was calculated by the below equation:

$$\begin{eqnarray}&&\displaystyle \frac{{R}_{ct(inh)}-{R}_{ct(acid)}}{{R}_{ct(inh)}}\times 100\end{eqnarray} \tag{ 14 }$$

Here

R_ct(inh) = charge transfer resistance exhibited by Aluminium in the presence of Sapindus in 1 M HCl,

R_ct(acid) = charge transfer resistance exhibited by Aluminium in the absence of Sapindus in 1 M HCl, From table 7 it is clearly seen that as the concentration is increasing charge transfer resistance is also increasing and this will help to decrease the corrosion current for aluminium in 1 M HCl [27].