Effect of ionic liquid as corrosion inhibitor for 6061 aluminium alloy-(electrochemical and quantum chemical approaches)

ABSTRACT

Ionic liquid 1,3-dimethylimidazolium dimethyl phosphate (DIDP) is used as a possible green inhibitor for the corrosion control of 6061 aluminium alloy in 0.25 mol/L HCl is described in the study. Study involved electrochemical methods carried out at various temperatures by changing the concentrations of DIDP. Kinetic and thermodynamic parameters were determined using the Arrhenius rate law and transition state equations, respectively. Physisorption of the inhibitor takes place and the adsorption follows Freundlich isotherm. Surface morphology was studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy-dispersive X-ray analysis (EDAX) techniques. Quantum chemical studies were done by the density functional theory (DFT). The maximum inhibition efficiency of DIDP on 6061 aluminium alloy was about 78% for the concentration of 1000 ppm at 303 K. The mechanistic aspects of DIDP adsorption onto the metal surface were supported by quantum chemical studies. HOMO and LUMO of the optimized structure and quantum chemical descriptors confirmed the adsorption of the inhibitor on the metal surface. Mulliken charge population was used to identify the DIDP molecule’s high electron density region, and Fukui indices confirmed the interaction between metal and inhibitor.

GRAPHICAL ABSTRACT

KEYWORDS:

• 6061 aluminium alloy
• electrochemical studies
• adsorption
• quantum chemical calculations
• ionic liquid
• surface morphology

Introduction

Corrosion is an unintentional natural process having interaction between material and its environment, the interaction being either chemical or electrochemical reaction resulting in material loss and useful properties. Corrosion studies are important for various reasons such as economic, health hazards, material loss, loss of useful properties and failure of corroded parts in machine components and structures. The study of corrosion is equally important for the conservation of materials, particularly metals and also from aesthetic and academic points of view. Corrosion can lead to loss of products, plant shutdown, repair and replacement costs, loss of life, etc., A moderate estimate regarding the cost of corrosion is about 4% of a country’s GDP, and it could be more in the case of industrially advanced countries, matching almost with the amount spent on education. Corrosion at best can be combated but not prevented [1,2].

Aluminium alloys are commonly used in engineering structures and components that require lightweight and corrosion resistance. Aluminium has unique properties like low density, durability, high strength, good conductivity, corrosion resistivity, etc., making it a choice for many applications. In aerospace industry and transportation aluminium and its alloys are extensively used [3]. Aluminium, on exposure to air, forms an oxidized layer on the metal surface which can control its further corrosion. Corrosion of aluminium and its alloys in many of the industrial environments is less compared to ferrous materials. Aluminium alloys exhibit good service life in air or in neutral aqueous chloride-free solutions at room temperature. However, it can corrode when exposed to an acid medium, or caustic environment. In industries, aluminium alloys undergo corrosion during the pickling or descaling process, and the thickness of the aluminium alloy will decrease. To prevent this, inhibitors are added to the corrosive medium. Nevertheless, aluminium alloys can offer a long service life by carefully choosing the proper alloy, good design, and use of corrosion inhibitors [4,5].

Corrosion inhibitors are either inorganic or organic heterocyclic compounds that decrease the corrosion rate without undergoing any chemical modification when added to a corrosive environment. The presence of heteroatom, unsaturation, low cost, non-toxic nature, readily available, miscible with the medium are the desirable features of a good inhibitor. The lone pair of electrons present in some of the hetero atoms are the reason for the inhibitor’s adsorption on the metal surface. Certain chemical inhibitors show excellent inhibition efficiency towards the corrosion of materials. A few examples of chemical inhibitors used in acid corrosion inhibition of aluminium are 6-diallylamino-1,3,5-triazine-2,4-dithiol monosodium (DAN) and 6-dibutylamino-1,3,5-triazine-2,4-dithiol monosodium (DBN) [6], 2-bromo-2-nitro-1,3-propanediol (bronopol) [7], and Sodium [(1,5-dimethyl-3-oxo-2-phenylpyrazol-4-yl)-methylamino] methanesulfonate (analgin) [8]. Bronopol is a biocide, and analgin is a pain-killer and antipyretic.

However, some of the chemical inhibitors are toxic and costly. Some of the inhibitors may affect the health of humans and the environment. Corrosion inhibitors have always raised safety and environmental concerns in the industrial world because of the toxic effects. Eco-friendly and harmless green inhibitors are now widely recommended. Green inhibitors are non-toxic, biodegradable, do not contain heavy metals, and do not cause any harmful effects on the environment. Plant extracts, biopolymers, surfactants, and ionic liquids are some of the major green inhibitors reported in the literature.

Nowadays, ionic liquids are widely used. They contain an organic cation and an inorganic anion. Few ionic liquids have unique properties like biodegradability, non-inflammability, low toxicity, low volatility, and low vapour pressure, because of which it is considered a ‘green corrosion inhibitor’. There are protic and aprotic ionic liquids. Both protic and aprotic ionic liquids can be used for corrosion inhibition studies. Protic ionic liquids are more reactive towards metals than aprotic ionic liquids and hence suitable for corrosion inhibition studies [9]. By proper selection of anion and cation, efficient tailor made ionic liquids can be designed as corrosion inhibitors [10–12]. The present study uses ionic liquids as green corrosion inhibitors.

A few examples for ionic liquids which are successfully used as corrosion inhibitors for various metals are 1-allyl-3-ethylimidazolium bromide ([AEIM]Br), 1-allyl-3-butylimidazolium bromide ([ABIM]Br), 1-allyl-3-hexylimidazolium bromide ([AHIM]Br), and 1-allyl-3-octylimidazolium bromide ([AOIM]Br) are used for copper [13], 1-ethyl-3- methylimidazolium bis(trifluoromethylsulfonyl)amide [EMIm] [NTf2],, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide [HMIm][NTf2], and 1-decyl-3-methylimidazolium bis(tri fluoromethylsulfonyl)amide [DMIm][NTf2] used for magnesium [14], 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄⁻] and 1-decyl-3-methylimidazolium tetrafluoroborate [DMIM][BF₄⁻] used for zinc [15]. Some of the ionic liquids used to control the corrosion of aluminium and its alloys are 1-octyl-3-methylimidazolium bromide ([OMIM]Br) and 1-allyl-3-octylimidazolium bromide ([AOIM]Br) [16]. 1-butyl-3- methylimidazolium chlorides (BMIC), 1-hexyl-3-methylimidazolium chlorides (HMIC), and 1-octyl-3-methylimidazolium chlorides (OMIC) [17], (poly(1-vinyl-3- dodecylimidazolium hexafluorophosphate) PImC₁₂, poly(1-vinyl-3-octylimidazolium hexafluorophosphate) PImC₈, and poly(1-vinyl-3-butylimidazolium hexafluorophosphate) PImC₄ [18]. tetradecylpyridinium bromide (TDPB) [19], 1,3-bis(2-oxo-2-phenylethyl)-1H-imidazol-3-iumbromide (OPEIB) [20]. From the above reports of ionic liquids used for aluminium and its alloys, it is evident that the inhibition efficiency is more in the imidazolium derivative than the pyridinium derivative, because of the presence of two nitrogen atoms. The nitrogen atoms are responsible for easy adsorption of inhibitors onto the metal surface. It is also observed that the increase in the length of the carbon side chain in the imidazolium derivative increases the inhibition efficiency. In the present work, only the methyl group is attached to both the nitrogen; still, the inhibitor showed very good inhibition efficiency.

In this paper, we present the results of ‘1,3-dimethylimidazolium dimethyl phosphate’ (DIDP) as an effective inhibitor. The corrosion inhibition study using DIDP has not been done so for aluminium alloy in 0.25 mol/L HCl. The molecular structure of DIDP is depicted in Figure 1. The large number of heteroatoms like nitrogen, oxygen, and phosphorous makes 1,3-dimethylimidazolium dimethyl phosphate a good candidate as a corrosion inhibitor.

Figure 1. DIDP structure.

DFT studies help to identify the electron-rich and deficit site in DIDP. It also identifies which part of the molecule is adsorbed on the metal surface. Mulliken charge population helps to know the charge on each atom. Fukui indices analysis helps to identify the reactivity of atoms present in the molecule.

Materials and methods

Sample

The test coupon of 6061 aluminium alloy was cast with dental resin exposing a 1 cm² surface area. The exposed surface area was polished with different grit emery papers and later disc polished with levigated Al₂O₃ suspension. Table 1 shows the wt. (%) of the 6061 aluminium alloy.

Table 1. Composition of test specimen.

Element	Ti	Zn	Cu	Cr	Si	Fe	Mg	Al
Wt. (%)	0.15	0.20	0.24	0.25	0.52	0.55	0.95	Balance

Medium

A stock solution of 1 mol/L HCl solution was prepared from procured HCl (Sigma-Aldrich, 35%) using double-distilled water. The stock solution was standardized using standard sodium carbonate solution with indicator methyl orange. 0.25 mol/L HCl was prepared as and when required from stock solution by diluting it with double distilled water.

Inhibitor

Commercially available (Sigma Aldrich make) DIDP was procured. Various known concentrations of DIDP in 0.25 mol/L HCl were prepared.

Electrochemical studies

The electrochemical workstation (CH600 D-series with beta software) was made use for conducting experiments of potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) studies. Pyrex glass beaker, along with three different electrodes such as platinum electrode (auxiliary electrode), saturated calomel (reference electrode), and the working electrode (6061 aluminium alloy), was used as an electrochemical cell.

The polished 6061 aluminium alloy was dipped in 0.25 mol/L HCl medium without and with the various DIDP concentrations at 303 K to 323 K temperature range. The electrochemical cell was placed in a thermostat set to a particular temperature of study. Without stirring, the measurements were carried out.

Before performing electrochemical measurements, open circuit potential (OCP) was noted after 1800 s.

PDP

To perform PDP measurements, 6061 aluminium alloy was polarised from OCP by applying − 250 mV cathodically to +250 mV anodically at a 1 mVs⁻¹ scan rate. Corrosion current density and electrochemical parameters were obtained from the PDP plot. Corrosion current density was used to calculate corrosion rate by the equation (1)(1) $CR=\frac{K\times i_{corr}\times M}{\rho \times Z}$(1)

(1) $CR=\frac{K\times i_{corr}\times M}{\rho \times Z}$(1)

K = proportionality constant which is equal to 0.00327 mm g μ A⁻¹ cm⁻¹ y⁻¹, M (molar mass = 27), i_corr = corrosion current density in μA cm⁻², z (metal valence electrons = 3), and ρ (density of metal = 2.7 g cm⁻³)

EIS

At the OCP, an AC signal of amplitude 10 mV and 100 kHz to 0.01 Hz frequency range was applied. The resultant Nyquist plot were fit into the appropriate circuit using ZSimpwin software version 3.21. The double-layer capacitance (C_dl) and polarization resistance (R_P) were determined.

EIS was conducted first followed by PDP without disturbing the experimental setup. For the addition of each inhibitor concentration at each temperature, both PDP and EIS techniques were done 3–4 times. The inhibition efficiency reported is averaged out separately for each technique.

Surface analysis

The surface morphology was examined by SEM (JEOL JSM-6380 L) at 500 X magnification. Three small coupons of 1 cm² area each were polished using different grades of emery paper followed by a disc polisher. After polishing, the coupons were washed using distilled water and cleaned. The first coupon was used as such after polishing (blank). The second metal coupon was immersed in 0.25 mol/L HCl for 12 h. The third coupon was immersed in 0.25 mol/L HCl and 1000 ppm of DIDP inhibitor and kept for 12 h. All three coupons were used for SEM analysis. Elemental mapping was carried out by EDAX (Energy-dispersive X-ray) analysis at 200 µm magnification. The roughness in the surface was obtained by AFM (1B342 Innova model) technique by contact mode.

Computational studies

Schrodinger suite was used for theoretical calculations. The basis set for the analyses was (B3LYP) and 6-31 G. The highest occupied molecular orbital (E_HOMO), lowest unoccupied molecular orbital (E_LUMO), energy gap (ΔE), hardness ($\eta$), softness (σ), electrophilicity (ω), nucleophilicity (ϵ), and so on was calculated by DFT technique. The electron density of different atoms of the DIDP molecule was known by the Mulliken charge population. Electron donating and accepting properties of DIDP molecule were evaluated by Fukui indices analysis.

Results and discussion

Potentiodynamic polarisation (PDP) technique

Figure 2 depicts the potentiodynamic polarization plot for 6061 aluminium alloy in 0.25 mol/L HCl for various DIDP concentrations at 313 K. At different temperatures, similar plots were obtained.

Figure 2. Potentiodynamic polarization plot.

The anodic curve of the potentiodynamic polarisation plot depicts metal dissolution and cathodic curve hydrogen evolution in an acidic environment. As the anodic curve has an inflection point, corrosion current density was difficult to obtain. Hence, extending the linear part of the cathodic curve provided corrosion current density. The inhibition efficiency was derived using the corrosion current density from equation (2)(2) $IE\left(\mathrm{\%}\right)=\frac{i_{corr}-i_{corr\left(inhi\right)}}{i_{corr}}\times 100$(2) .

(2) $IE\left(\mathrm{\%}\right)=\frac{i_{corr}-i_{corr\left(inhi\right)}}{i_{corr}}\times 100$(2)

i_corr = corrosion current density (blank) and

i_corr(inhi) = corrosion current density (inhibitor).

PDP data are given in Table 2. Tafel slopes reported in Table 2 indicate no significant variation. This suggested that the added inhibitor did not affect the corrosion mechanism [21,22]. The inhibitor simply blocks the available sites thereby minimizing corrosion.

Table 2. PDP results of 6061 aluminium alloy corrosion in 0.25 mol/L HCl for different DIDP concentrations.

Temp (K)	[DIDP] (ppm)	Ecorr vs. SCE (mV)	icorr (μA cm^−2)	−βc (V dec^−1)	CR (mmy^−1)	IE (%)
303	Blank	─695	114.92	7.33	1.24	─
	200	─702	78.03	6.91	0.85	31.57 ±0.9
	400	─690	66.57	6.92	0.72	41.60 ±1.5
	600	─695	52.63	6.60	0.57	53.83 ±1.2
	800	─683	43.22	6.48	0.47	62.08 ±1.5
	1000	─683	24.64	5.78	0.26	78.38 ±0.9
308	Blank	─702	141.97	7.38	1.53	─
	200	─694	105.39	6.98	1.14	25.25 ±1.3
	400	─705	87.49	6.58	0.95	37.95 ±1.2
	600	─691	78.03	6.77	0.85	44.65 ±1.3
	800	─704	63.85	6.31	0.69	54.71 ±1.2
	1000	─697	49.86	6.18	0.54	64.63 ±1.4
313	Blank	─717	175.38	7.53	1.90	─
	200	─690	136.96	7.13	1.49	21.73 ±1.2
	400	─691	118.95	6.96	1.29	32.02 ±1.3
	600	─692	99.81	6.96	1.08	42.96 ±0.9
	800	─702	84.19	6.54	0.91	52.00 ±1.3
	1000	─695	66.39	6.66	0.72	62.14 ±1.2
318	Blank	─714	216.67	7.50	2.35	─
	200	─702	181.55	7.63	1.97	15.94 ±0.9
	400	─708	157.68	7.39	1.71	27.00 ±1.1
	600	─661	136.96	7.07	1.49	36.59 ±1.2
	800	─724	118.95	6.92	1.29	44.93 ±1.3
	1000	─715	103.32	6.75	1.12	52.31 ±1.2
323	Blank	─711	267.67	7.59	2.91	─
	200	─711	235.50	7.50	2.56	11.79 ±1.2
	400	─705	209.02	7.38	2.36	21.71 ±0.9
	600	─706	181.55	7.24	1.97	32.00 ±1.1
	800	─706	157.68	7.33	1.71	40.94 ±1.5
	1000	─704	141.97	7.38	1.54	46.82 ±1.0

According to the PDP results, i_corr and CR increased with increasing temperature. The corrosion rate decreased after the addition of DIDP, resulting in increased inhibition efficiency. The inhibition efficiency decreased as the temperature increased, implying that the DIDP molecule may have physically adhered to the metal’s surface.

According to the literature [23,24], when the change in corrosion potential (E_corr) is greater than ±85 mV when inhibitor was added, then the inhibitor is considered anodic or cathodic. In the present study, the corrosion potential difference when the DIDP inhibitor was added was found to be below −85 mV, indicating that DIDP acted as a mixed inhibitor.

The maximum efficiency of DIDP is 78.38% for 1000 ppm of DIDP immersed in 0.25 mol/L HCl at 303 K.

Electrochemical impedance spectroscopic (EIS) method

Nyquist plot for 6061 aluminium alloy for various concentrations of DIDP in 0.25 mol/L HCl at 313 K is shown in Figure 3.

Figure 3. Nyquist plot for varying DIDP concentrations in 0.25 mol/L HCl at 313 K.

The shape of the plot agrees completely with those previously reported for aluminium corrosion with HCl and other mediums [25]. The Nyquist plot has two loops. The high-frequency capacitive loop is the alloy’s resistance towards corrosion and is also called charge transfer resistance. Initially, when no inhibitor was added, charge transfer resistance was low. That means corrosion takes place faster. After the addition of an inhibitor, resistance to charge transfer increases. An increase in the semicircle diameter indicates the same. Further, with a concentration increase in the inhibitor, resistance to the charge transfer increased. In other words, resistance to corrosion increases. Hence, there is an increase in the diameter of the Nyquist semicircle. A depressed Nyquist semicircle represents surface inhomogeneity [26].

The inductive loop represents the adsorption and penetration of Cl⁻ ions into the oxide film layer [27,28]. The aluminium oxide layer is regarded as a parallel circuit because of the conduction of oxide film and a capacitor due to its dielectric properties.

Polarization resistance increases when the concentration of the inhibitor increases. An increase in polarization resistance could be due to the inhibitor forming a protective film on the surface of alloy. An increase in double-layer thickness decreases the double-layer capacitance. % inhibition efficiency increases with an increase in the concentration of inhibitor [29,30]

ZSimpwin software was used for equivalent circuit fitment. It fits best into the five-element circuit. Of these, the resistances are solution resistance (R_S), charge transfer resistance (R_ct), and inductive resistance (R_L). There is a constant phase element (Q) and an inductive element (L). Figure 4 shows the circuit fitment graph.

Figure 4. Equivalent circuit fitment.

Charge transfer and inductive resistances were parallel. Therefore, polarization resistance was calculated by taking the sum of the reciprocal of R_CT and R_L, which is given in equation (3)(3) $\frac{1}{R_P}=\frac{1}{R_{CT}}+\frac{1}{R_L}$(3) .

(3) $\frac{1}{R_P}=\frac{1}{R_{CT}}+\frac{1}{R_L}$(3)

Using equation (4)(4) $C_{dl}=Q_{dl}({\omega }_{max}{)}^{n-1}$(4) , C_dl was calculated.

(4) $C_{dl}=Q_{dl}({\omega }_{max}{)}^{n-1}$(4)

Q_dl denotes the CPE constant, ω_max denotes the frequency at which the imaginary impedance (Z^//) is greatest, and n denotes the CPE exponent, which explains the electrode surface unevenness. When n = 1, CPE acts as an ideal capacitor.

The inhibition efficiency of the DIDP was calculated by the formula given in equation (5)(5) $IE\left(\mathrm{\%}\right)=\frac{R_{P\left(inhi\right)}-R_{P\left(blank\right)}}{R_{P\left(inhi\right)}}$(5) [31]:

(5) $IE\left(\mathrm{\%}\right)=\frac{R_{P\left(inhi\right)}-R_{P\left(blank\right)}}{R_{P\left(inhi\right)}}$(5)

Polarization resistance in the presence and absence of a DIDP is represented by R_{P (inh)} and R_{P (blank)}. EIS results are given in Table 3 for 6061 aluminium alloy.

Table 3. EIS results of 6061 aluminium alloy corrosion in 0.25 mol/L HCl containing different DIDP concentrations.

Temp (K)	[DIDP] (ppm)	Rp (Ω cm^2)	Cdl (µF cm^−2)	χ^2 (X 10^−3)	IE (%)
303	Blank	173.0	10.13	4.90	─
	200	242.1	3.29	1.83	28.54 ±1.0
	400	305.7	2.12	3.27	43.40 ±1.6
	600	367.5	1.22	5.17	52.92 ±1.3
	800	444.8	0.69	2.65	61.10 ±1.4
	1000	628.2	0.26	2.33	72.46 ±1.3
308	Blank	79.2	22.39	5.89	─
	200	104.0	11.35	3.66	23.84 ±1.3
	400	123.0	8.83	1.63	35.60 ±1.2
	600	145.1	5.26	3.77	45.41 ±1.2
	800	176.9	3.13	1.39	55.22 ±1.5
	1000	218.2	2.00	9.76	63.70 ±1.3
313	Blank	52.6	40.56	9.77	─
	200	65.7	29.73	5.02	19.93 ±1.6
	400	77.5	19.58	4.32	32.12 ±1.5
	600	94.3	12.75	4.44	44.22 ±1.4
	800	104.6	10.65	2.44	49.71 ±1.4
	1000	137.6	5.98	2.98	61.77 ±1.5
318	Blank	39.5	69.26	5.96	─
	200	46.2	46.22	6.12	16.96 ±1.2
	400	54.4	34.29	5.89	27.38 ±1.3
	600	60.1	23.95	3.18	34.27 ±1.6
	800	67.8	16.13	7.68	41.74 ±1.6
	1000	78.5	9.20	1.17	49.68 ±1.5
323	Blank	29.2	115.8	4.39	─
	200	32.1	96.22	5.05	9.03 ±1.4
	400	38.6	64.18	6.35	24.35 ±1.5
	600	44.2	43.76	7.90	33.93 ±1.5
	800	47.8	37.97	4.29	38.91 ±1.6
	1000	52.9	29.51	2.37	44.80 ±1.6

Figure 5 shows the Bode plot of 6061 aluminium alloy at various inhibitor concentrations.

Figure 5. Bode plot.

The phase angle increases as the DIDP concentration increases. This represents the DIDP molecule that covers the 6061 aluminium alloy surface [32]. The impedance modulus is greatest at 1000 ppm DIDP concentration, implying the maximum efficiency of DIDP at 1000 ppm [33]. EIS studies show the highest efficiency of 72.46% for 1000 ppm of DIDP concentration in 0.25 mol/L HCl at 303 K.

Effect of temperature

Arrhenius rate law equation gives the activation energy from equation (6)(6) $ln\left(CR\right)=B-\frac{E_a}{RT}$(6) .

(6) $ln\left(CR\right)=B-\frac{E_a}{RT}$(6)

The transition state equation entropy and enthalpy of activation from equation (7)(7) $ln\left(\frac{CR}{T}\right)=\left[ln\left(\frac{R}{Nh}\right)+\left(\frac{\mathrm{\Delta }{S}_a}{R}\right)-\left(\frac{\mathrm{\Delta }{H}_a}{RT}\right)\right]$(7) .

(7) $ln\left(\frac{CR}{T}\right)=\left[ln\left(\frac{R}{Nh}\right)+\left(\frac{\mathrm{\Delta }{S}_a}{R}\right)-\left(\frac{\mathrm{\Delta }{H}_a}{RT}\right)\right]$(7)

CR = corrosion rate,

B = Arrhenius constant,

Ea = activation energy (obtained from the slope),

R = universal gas constant (8.314 JK⁻¹ mol⁻¹), and

T = temperature (K) [34,35].

N is Avogadro’s number (6.022×10²³ mol⁻¹), and

h is Planck’s constant (6.626×10⁻³⁴ Js).

Figure 6 shows the lnCR vs. 1/T plot and lnCR/T vs. 1/T plot for 6061 aluminium alloy at varying DIDP concentrations in 0.25 mol/L HCl medium.

Figure 6. (a) Arrhenius plot and (b) Transition state plot of 6061 aluminum alloy corrosion in 0.25 mol/L HCl with different concentrations of DIDP.

Table 4 shows activation/kinetic parameters obtained from the Arrhenius rate law and transition state equations.

Table 4. Activation parameters.

[DIDP] (ppm)	Ea (kJ mol^−1)	∆Ha (kJ mol^−1)	∆Sa (J mol^−1 K^−1)
Blank	34.74	32.13	−137.25
200	44.79	42.18	−107.15
400	48.17	45.57	−97.51
600	49.52	46.92	−94.60
800	52.24	49.64	−87.37
1000	61.59	58.84	−59.68

With the increase in DIDP concentration, activation energy increased, resulting in decreased corrosion rate. This is due to the DIDP molecule, which is adsorbed, forming a barrier between the medium and metal. The enthalpy of activation (∆H_a) is similar to that of E_a, which explains the exothermic process of adsorption [36]. The entropy of activation (∆S_a) has a negative value moving towards positive because of increased disorderliness while forming the activated complex and implies physical adsorption [34].

Adsorption study

Adsorption isotherms like Temkin, Freundlich, Frumkin, and Langmuir have been tried to fit into the results obtained. None have fitted into the following adsorption isotherms except Freundlich. The best fitted is the one with an R-square value equal to unity [37,38]. Corrosion of 6061 aluminium alloy gave a linear plot in the presence of DIDP, which fits into the Freundlich adsorption isotherm and has a regression coefficient (R²) nearly equal to unity. The formula of Freundlich adsorption isotherm is in equation (8)(8) $log\theta =logK+\frac{1}{n}logC$(8) .

(8) $log\theta =logK+\frac{1}{n}logC$(8)

K = adsorption/desorption constant [39],

‘n’ = Freundlich constant, and

C = concentration of DIDP.

Different adsorption isotherm plots are shown in Figure 7.

Figure 7. Plots for (a) Langmuir adsorption isotherm, (b) Frumkin adsorption isotherm, (c) Temkin adsorption isotherm, and (d) Freundlich adsorption isotherm.

Figure 7 (d) depicts a Freundlich adsorption isotherm plot. Table 5 lists the parameters of the Freundlich adsorption isotherm.

Table 5. Freundlich adsorption isotherm parameters.

Temp [K]	K	Slope	n	R^2
303	0.71	.52	1.92	0.98
308	0.62	.55	1.81	0.99
313	0.58	.64	1.56	0.99
318	0.52	.73	1.36	0.99
323	0.48	.86	1.16	0.99

As the temperature increased, the value of K decreased, which implies physical adsorption. In Table 5, the slope value was below one, and the value of n was above 1. If the value of the slope is in the range of 0 and 1 and n in the range of 0 and 10, it favours the adsorption. The regression coefficient (R²) was nearly equal to one. As a result, the results best fit into the Freundlich adsorption isotherm.

The intercept of the Freundlich plot was used to calculate K_ads. Standard free energy can be calculated by incorporating the value of K into equation (9)(9) $K_{ads}=\frac{1}{55.55}exp\left[\frac{{\left(-\mathrm{\Delta }{G}^{\circ }\right)}_{ads}}{RT}\right]$(9) .

(9) $K_{ads}=\frac{1}{55.55}exp\left[\frac{{\left(-\mathrm{\Delta }{G}^{\circ }\right)}_{ads}}{RT}\right]$(9)

T = temperature in Kelvin,

R = universal gas constant [40], and

55= molar concentration of water in the solution in mol dm⁻³.

Plotting the graph ΔG⁰_ads Vs. Temperature, standard enthalpy of adsorption (ΔH⁰_ads), and standard entropy of adsorption (ΔS⁰_ads) can be determined. Equation (10)(10) $\left(\mathrm{\Delta }G\right){ }_{ads}^{\circ }=\hspace{0.17em}(\mathrm{\Delta }H){ }_{ads}^{\circ }-T\left(\mathrm{\Delta }S\right){ }_{ads}^{\circ }$(10) gives Gibb’s-Helmholtz equation.

(10) $\left(\mathrm{\Delta }G\right){ }_{ads}^{\circ }=\hspace{0.17em}(\mathrm{\Delta }H){ }_{ads}^{\circ }-T\left(\mathrm{\Delta }S\right){ }_{ads}^{\circ }$(10)

Figure 8 depicts ΔG⁰_ads Vs. T graph, for DIDP adsorbing on 6061 aluminium alloy in 0.25 mol/L HCl.

Figure 8. The plot of ΔG⁰_ads vs. T.

The thermodynamic parameters are given in Table 6.

Table 6. Thermodynamic parameters of 6061 aluminium alloy in 0.25 mol/L HCl on DIDP adsorption.

Temp (K)	ΔG ^0ads (kJ mol^−1)	△H ^0ads (kJ mol^−1)	△S ^0ads (kJ mol^−1K^−1)
303	−9.25	−15.89	−0.021
308	−9.13
313	−9.03
318	−8.91
323	−8.81

The standard free energy of adsorption (ΔG⁰_ads) for DIDP adsorption onto the metal surface ranges from −8.81 to −9.25 kJ mol⁻¹, referring to the physical adsorption of DIDP [41]. For physical adsorption, the value of ΔG⁰_ads must be below −20 kJ mol⁻¹; chemisorption must be between −40 and −100 kJ mol⁻¹ [42]. The negative standard enthalpy of adsorption (△H⁰_ads) refers to physical adsorption, whereas the negative standard entropy of adsorption (△S⁰_ads) is due to a decrease in the system’s randomness [43].

Mechanism of corrosion inhibition

Aluminium corrosion in the HCl medium is spontaneous. Equations (11)(11) $Al\to A{l}^{3+}+3e^{-}$(11) and (12(12) $A{l}^{3+}+C{l}^{-}\to AlC{l}^{2+}$(12) ) show anodic reactions.

Anodic reaction:

(11) $Al\to A{l}^{3+}+3e^{-}$(11)

(12) $A{l}^{3+}+C{l}^{-}\to AlC{l}^{2+}$(12)

Aluminium loses its electrons during oxidation and converts to Al³⁺. From the HCl medium, chloride ions adhere to the surface of the aluminium to satisfy the positive charge on the alloy surface [44]. The Al³⁺ attracts more chloride ions to form [Al(Cl)₆]^3-. At the cathode, hydrogen evolution occurs, as shown in the equations (13)(13) $H^{+}+e^{-}\to H_{ads}$(13) and (14(14) $H_{ads}+H_{ads}\to H_2$(14) ).

Cathodic reactions:

(13) $H^{+}+e^{-}\to H_{ads}$(13)

(14) $H_{ads}+H_{ads}\to H_2$(14)

The H⁺ ions in the medium take up electrons released at the anode and liberate the H⁺ ions as hydrogen gas [45]. Equation (15)(15) ${[Al{\left(Cl\right)}_6{]}^{3-}}_{ads}+3{\left[DIDP\right]}^{+}\to [AlC{l}_6{]}^{3-}.3{\left[DIDP\right]}^{+}$(15) depicts the adsorption of the positively charged organic part of the DIDP molecule forming complex on the surface of 6061 aluminium.

(15) ${[Al{\left(Cl\right)}_6{]}^{3-}}_{ads}+3{\left[DIDP\right]}^{+}\to [AlC{l}_6{]}^{3-}.3{\left[DIDP\right]}^{+}$(15)

Figure 9 depicts the mechanism of DIDP molecule physical adsorption on 6061 aluminium surface.

Figure 9. Mechanism of DIDP on 6061 aluminum alloy.

Surface analysis

SEM analysis

Figure 10 shows SEM pictures of finely polished 6061 aluminium alloy and in 0.25 mol/L HCl in the presence and absence of DIDP for 12 h at normal room temperature.

Figure 10. SEM pictures of (a) Finely polished 6061 aluminum alloy, (b) 6061 aluminum alloy + 0.25 mol/L HCl (c) 6061 aluminum alloy + 0.25 mol/L HCl + 1000 ppm of DIDP.

The metal surface looks smooth in freshly polished 6061 aluminium alloy. When the metal was immersed in 0.25 mol/L HCl, it appeared rough comparatively as more corrosion occurred [46]. In the presence of a DIDP, the surface appeared smooth because the DIDP forms a protective layer and prevents corrosion.

EDX analysis

EDX data of 6061 aluminium alloy in 0.25 mol/L HCl in the presence and absence of DIDP is shown in Table 7.

Table 7. EDX data of 6061 aluminium alloy in 0.25 mol/L HCl.

Samples	Wt % Composition
	Al	Si	C	Mg	Fe	Cr	O	Cl	N
Freshly polished 6061 aluminum alloy	95.43	3.44	─	0.76	0.25	0.12	─	─	─
6061 aluminum alloy + 0.25 mol/L HCl	51.40	10.85	10.71	─	─	─	25.89	1.16	─
6061 aluminum alloy + 0.25 mol/L HCl + DIDP	85.66	0.22	4.94	1.10	0.93	─	4.19	0.16	1.13

The wt % composition of aluminium was more in the freshly polished 6061 aluminium alloy. When dipped in 0.25 mol/L hydrochloric acid, wt % aluminium composition decreased as corrosion occurred. Wt % composition of aluminium increased when DIDP is present, as DIDP formed a protective barrier and prevented aluminium from corroding. Chlorine present is because of the medium and metal interaction. Nitrogen is because of the interaction between metal and DIDP [47,48].

AFM studies

Figure 11 shows AFM images of finely polished 6061 aluminium alloy and when the alloy dipped in 0.25 mol/L HCl with and without DIDP.

Figure 11. AFM pictures of a) Freshly polished 6061 aluminum alloy, b) 6061 aluminum alloy + 0.25 mol/L HCl, c) 6061 aluminum alloy + 0.25 mol/L HCl + DIDP.

AFM pictures of 6061 aluminium alloy dipped in 0.25 mol/L HCl has comparatively more roughness than finely polished 6061 aluminium alloy and the alloy along with DIDP. The roughness values are given in Table 8.

Table 8. AFM data.

Samples	Ra (nm)	Rq (nm)	Rmax (nm)
Freshly polished 6061 aluminium alloy	18.4	23.9	351
6061 aluminium alloy + .25 mol/L HCl	221	299	2608
6061 aluminium alloy + .25 mol/L HCl + DIDP	39.7	60.1	678

Table 8 displays the values for average roughness (R_a), Root means square roughness (R_q), and maximum roughness (R_max). As more corrosion occurs in 0.25 mol/L HCl medium, the roughness values for aluminium dipped in 0.25 mol/L HCl were higher. The roughness values for 6061 aluminium alloy immersed in 0.25 mol/L HCl medium and 1000 ppm of DIDP have decreased.

The mean height calculated over the measured length/area is the average roughness (Ra). The term (Ra) is commonly used to describe the roughness of machined surfaces. Roughness (Rq) is defined as the square root of the surface height distribution. Maximum peak-to-valley height (Rmax) describes the overall roughness of the surface.

This is because DIDP adsorbed on the alloy surface, by forming a barrier and stopping the attack from acid [49,50].

Computational studies

DFT technique

Quantum calculations were conducted using (B3LYP) and 6-31 G as the basis set. The structure of 1,3-dimethylimidazolium dimethyl phosphate (DIDP) is given in Figure 12.

Figure 12. DIDP optimized structure.

The aliphatic chain of the DIDP structure is planar. The presence of a high molecular weight anionic part of DIDP expands the size of the ionic liquid. The increased molecular size of the counter ion and the planarity property increases the DIDP’s surface coverage on the metal [51]. The nitrogen in the heterocyclic ring structure enhanced the adsorption of DIDP on the metal surface. Therefore, it reduces the rate of corrosion [52,53]. The optimized structures of HOMO and LUMO are depicted in Figure 13. The HOMO frontier molecular orbital helps in the donation of electrons towards the metal surface. The LUMO frontier molecular orbital helps in accepting the electrons from the metal surface [54].

Figure 13. (a) HOMO and (b) LUMO structures.

The electron distribution in HOMO and LUMO helps us identify the DIDP and metal interaction. The HOMO and LUMO structures refer to DIDP’s ability to donate and accept electrons [4]. HOMO of DIDP is present in the counter ion, and LUMO is in the organic cationic part. Therefore, electron donation occurs through the anionic part and electron acceptance through the cationic part. DIDP’s adsorption occurs by sharing electrons from both anionic and cationic parts to the metal surface [55]. Table 9 shows the electronic parameters.

Table 9. Electronic parameters of DIDP molecule.

	EHOMO	ELUMO	ΔE	X	$\eta$	σ	ω	ϵ	ΔN	μ
DIDP	─6.950	─0.6324	6.312	3.791	3.158	0.316	1.137	0.879	0.045	13.127

All units are in eV, except σ in eV⁻¹ and dipole moment in Debye

The E_HOMO and E_LUMO values of the DIDP molecule are −6.950 eV and −6.324 eV. The energy gap (ΔE) predicts the reactivity of the DIDP and can be obtained by the formula given in equation (16)(16) $\mathrm{\Delta }E=E_{LUMO}-E_{HOMO}$(16) .

(16) $\mathrm{\Delta }E=E_{LUMO}-E_{HOMO}$(16)

E_LUMO is the lowest unoccupied molecular orbital, and E_HOMO is the highest occupied molecular orbital. The energy gap of the DIDP molecule is 6.312 eV. Electronegativity (χ) refers to the ability to hold electrons and can be calculated using the equation (17)(17) $\chi =\frac{1}{2}\left(I+A\right)$(17)

(17) $\chi =\frac{1}{2}\left(I+A\right)$(17)

DIDP has a χ value of 3.791 eV. Hardness ($\eta$) was calculated by the formula given in equation (18)(18) $\eta =\frac{1}{2}\left(I-A\right)$(18)

(18) $\eta =\frac{1}{2}\left(I-A\right)$(18)

I (Ionization potential) is negative of E_HOMO value, and A (Electron affinity) is negative of E_LUMO value. The hardness of DIDP was found to be 3.158 eV. Softness (σ) was calculated by taking the reciprocal of the hardness value and was found to be 0.316 eV⁻¹. The electrophilicity index (ω) shows the DIDP’s ability to accept electrons. It also implies the DIDP molecule’s participation in a back donation. Electrophilicity was calculated by the formula given in equation (19)(19) $\omega =\frac{{\chi }^2}{4\eta }$(19) and was found to be 1.137 eV.

(19) $\omega =\frac{{\chi }^2}{4\eta }$(19)

Where χ is the electronegativity and $\eta$ is the hardness value. The reciprocal of electrophilicity gives nucleophilicity (ϵ) which is 0.879 eV. $N$ represents the fraction of electron transferred, calculated by the formula given in equation (20)(20) $\mathrm{\Delta }N=\frac{{\mathrm{\Phi }}_{Al}-{\chi }_{DIDP}}{2{\eta }_{DIDP}}$(20) .

(20) $\mathrm{\Delta }N=\frac{{\mathrm{\Phi }}_{Al}-{\chi }_{DIDP}}{2{\eta }_{DIDP}}$(20)

Where ${\mathrm{\Phi }}_{Al}$ is the work function of aluminium which is 4.2 eV, χ represents the electronegativity of DIDP and $\eta$ represents the hardness of DIDP. △N is a fraction of an electron transferred from an inhibitor molecule to a metal. Any value greater than zero indicates that electron transfer is taking place. Effective electron transfer means it easily forms a complex with the metal ion at the interface and forms a protective film. As per the reported literature [56,57], if the value of △N is more than zero and below 3.6 eV, then the electron-donating property is more. According to the present study, the value of △N is positive, which is 0.045 eV, indicating easy adsorption of DIDP on the metal surface and showing inhibition action. DIDP has a greater dipole moment of 13.127 Debye than a water molecule of 1.88 Debye. As a result, DIDP molecules easily displace water molecules, and the DIDP and aluminium alloy has a strong dipole-dipole interaction.

Mulliken charge density identifies the high electron density region of DIDP. The electron density was more around hetero atoms as hetero atoms containing lone pair of electrons, and the bond formed by heteroatoms will be strong [28]. The accumulation of negative charges in oxygen and nitrogen is shown in Figure 14. In the present study, the positively charged nitrogen adsorbs on the alloy surface and forms bond [58,59]. DIDP has more number of active sites. From the mechanism, it is confirmed that nitrogen with positive charge adsorbs on alloy surface. The total electron density value for organic cation part is more positive compared to inorganic anion part. Therefore, the positively charged organic cation will adhere on to the metal surface. In Figure 14 the overall charge on both the nitrogen is shown as negative probably due to resonance, although one of the nitrogen in organic cation part has positive charge.

Figure 14. Mulliken change density of DIDP.

Fukui indices analysis

Table 10 shows the results of the Fukui indices analysis. Fukui indices analysis helps identify the reactivity of atoms present in the molecule. The results obtained $f_k^{+}$values of DIDP are located in C1, C2, N3, C4, N5, H19, H22, and H23. This suggested that these atoms accept electrons readily and act as electrophiles [60]. The $f_k^{-}$ values of DIDP is located in P8, O9, O10, O11, and O12, indicating these atoms donate electrons and act as nucleophiles [61,62]. The atoms in the DIDP molecule contribute electrons to the metal and accept electrons from the metal [57].

Table 10. Data of Fukui indices analysis.

Atoms			Atoms
C1	0.1032	0.0022	C2	0.0128	0.0094
N3	0.1596	0.0002	C4	0.4779	0.0009
N5	0.1817	0.0006	C6	0.0042	0.0023
C7	0.0036	0.0001	P8	0.0001	0.0257
O9	0.0002	0.3422	O10	0.0001	0.4687
O11	0.0000	0.1100	O12	0.0000	0.0126
C13	0.0000	0.0008	C14	0.0000	0.0019
H15	0.0007	0.0001	H16	0.0000	0.0018
H17	0.0040	0.0000	H18	0.0082	0.0003
H19	0.0126	0.0008	H20	0.0011	0.0004
H21	0.0000	0.0000	H22	0.0140	0.0000
H23	0.0159	0.0000	H24	0.0000	0.0005
H25	0.0000	0.0006	H26	0.0000	0.0019
H27	0.0000	0.0004	H28	0.0000	0.0073
H29	0.0000	0.0081	─	─	─

The comparison table of previously studied ionic liquids and the present work are listed in Table 11.

Table 11. Comparison between the previous studies and the present study.

Material	Medium	Inhibitor	IE%	Ref.
Copper	0.5 mol/L H2SO4	1-allyl-3-ethylimidazolium bromide ([AEIM]Br) 1-allyl-3-butylimidazolium bromide([ABIM]Br) 1-allyl-3-hexylimidazolium bromide ([AHIM]Br) 1-allyl-3-octylimidazolium bromide ([AOIM]Br)	92.5 96.4 97.2 97.5	[13]
AZ31B Mg alloy	0.5 wt % NaCl	1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [EMIm][NTf2], 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [HMIm][NTf2], 1- decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [DMIm][NTf2]	82.22 84.83 88.37	[14]
Zinc	1.0 mol/L HCl	1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4-] 1-decyl-3-methylimidazolium tetrafluoroborate [DMIM][BF4-]	20.65 70.57	[15]
mild steel	0.5 mol/L H2SO4	1-octyl-3-methylimidazolium bromide ([OMIM]Br) 1-allyl-3-octylimidazolium bromide ([AOIM]Br)	92.9 93.3	[16]
Aluminium	1.0 mol/L HCl	1-butyl-3-methylimidazolium chlorides (BMIC) 1-hexyl-3-methylimidazolium chlorides (HMIC) 1-octyl-3-methylimidazolium chlorides (OMIC)	82.4 90.4 94.0	[17]
Aluminium alloy AA6061	0.1 mol/L H2SO4	(poly(1-vinyl-3-dodecyl-imidazolium) (PImC12), poly(1-vinyl-3-octylimidazolium) (PImC8) poly(1-vinyl-3-butylimidazolium) (PImC4) hexafluorophosphate)	61 53 50	[18]
Aluminium	1.0 mol/L HCl	tetradecylpyridinium bromide (TDPB)	95.2	[19]
6061 Al-15 Vol. Pct. SiC(p) Composite	0.1 M H2SO4	1,3-bis(2-oxo-2-phenylethyl)-1H-imidazol-3-ium bromide (OPEIB)	96.7	[20]
6061 Al-alloy	0.25 mol/L HCl	1,3 dimethyl imidazolium dimethyl phosphate (DIDP)	72.46	Present work

It is observed from the table that ionic liquid in the present study shows inhibition efficiency in the same order as those of reported molecules. The inhibition efficiency was observed to be 72.46% for 6061 aluminium alloy, which is considered an effective eco-friendly green inhibitor. The inhibition efficiency increases with the increase in the carbon side chains and the number of hetero atoms.

Conclusion

• PDP study showed decreased corrosion current density in the presence of DIDP.

• The percentage inhibition efficiency of DIDP decreased with an increase in temperature. The inhibition efficiency increased with the increase in the concentration of DIDP.

• The maximum inhibition efficiency of DIDP on 6061 aluminium alloy was 78.38% for the concentration of 1000 ppm at 303 K.

• The results of thermodynamic parameters were found to obey the Freundlich adsorption model and suggested physisorption.

• SEM analysis showed a smooth surface after the addition of DIDP. EDAX analysis results also confirmed the interaction of DIDP on the 6061 aluminium alloy surface. AFM study showed a decrease in surface roughness after the addition of DIDP.

• The computational study also helped to arrive at a suitable mechanism for adsorption of DIDP. DIDP acted as a potential green inhibitor in controlling the corrosion of 6061 aluminium alloy in 0.25 M HCl.

Acknowledgments

Mrs. Namitha Kedimar is grateful to MAHE Manipal for the Dr. TMA Pai scholarship. The authors are thankful to the Chemistry Department, MIT MAHE Manipal for the laboratory facilities. They also acknowledge Central Instrumentation Facilities, MIT MAHE Manipal.

Disclosure statement

No potential conflict of interest was reported by the authors.