Vanillin as eco-friendly corrosion inhibitor for aluminum immersed in 0.5 M H₂SO₄

Abstract

The possibility of using Vanillin as an eco-friendly corrosion inhibitor for Al in 0.5 M H₂SO₄ has been evaluated, examining concentrations ranging from 0 to 10 mM at 25, 40, and 60 °C. Results indicate that Vanillin is a mixed corrosion inhibitor. Its inhibitory effect can be attributed to aldehyde functional groups forming a protective layer by reacting with Al³⁺ ions, which are chemisorbed onto the metal’s surface, as was demonstrated via SEM analysis of the metal surface. The I_corr and R_p values estimated from the electrochemical characterization tend to improve in the presence of Vanillin. However, inhibition efficiency is affected by temperature, resulting in lower values. Also, the weight loss analyses demonstrated that Vanillin is desorbed over time and temperature. The SEM micrographs taken after weight loss show an aluminum surface with several pits in the absence of inhibitor. Otherwise, EDS analyses show the presence of carbon, confirming the Vanillin adsorption on the metal surface.

Introduction

Metals and their alloys are exposed to the action of acids in industry, processes in which acids play a very important role, amongst others, including pickling. To avoid the corrosion effect generated by these working conditions several authors are proposed the use of corrosion inhibitors to prevent or reduce the corrosion effect on metal surfaces exposed to acidic mediums¹. Aluminum and its alloys are important materials, due to their multiple applications in industries such as the automotive and aircraft, and, in the manufacture of appliances, shipbuilding, military equipment, etc^2–4. Their usefulness derives from their physical and mechanical properties such as high strength, weight ratio and outstanding corrosion resistance which lead to the formation of a compact film on the metal surface.

This surface film is amphoteric and is substantially dissolved when the metal is exposed to high concentrations of acids or bases^5–7. Corrosion processes develop rapidly after the alteration of the film, changing the composition and properties of the metallic surface. In this sense the corrosion is an issue that has received considerable interest to mitigate its effect through the use of films and inhibitors which are being one of the most popular methods used to control and prevent it⁸. Green inhibitors receive attention since they can replace synthesized inorganic inhibitors which are often toxic, expensive, and environmentally unfriendly¹.

Based on the latter, Vanillin (4-hydroxy-3-methoxybenzaldehyde) is the main component of natural vanilla and is one of the most used flavorings around the world, its chemical structure is shown in Fig. 1. The natural source of Vanillin is the bean or pod Vanilla (Vanilla planifolia mainly). It is native to Mexico but has spread though the tropics being Madagascar one of its largest producers. Vanillin is widely used as a flavoring agent in food, especially in baking. It is also used in the pharmaceutical industry as a gastric stimulant, in perfumery, creams, ice creams, cakes, and other culinary preparations since digestive, calming, aphrodisiac, and antipyretic properties are attributed to it^9,10. Like many other low molecular weight substances such as phenolic compounds, Vanillin has antioxidant and antimicrobial properties that make it suitable for its use as a preservative¹¹. It is an organic compound with the formula and its functional groups include aldehyde, ether, and phenol. The strength of the adsorption bond depends on the electron density in the donor atom of the functional group as well as its polarizability. When an H atom attached to the C in the ring is replaced by a substituent group (, , , or ) it improves its inhibitory power¹².

Fig. 1.

Chemical structure of Vanillin (4-hydroxy-3-methoxybenzaldehyde).

Several studies have been tested the Vanillin as corrosion inhibitor, e.g., Fernandes et al., evaluated three Vanillin derivatives as corrosion inhibitors for mild steel at room temperature, finding the best inhibition efficiency with Vanillin cyanoguanidine imine (VCNG) suggesting based on SEM and AFM analyses the formation of a protective film onto the metal surface¹³. Furtado et al., investigated the temperature effect of Schiff bases synthesized via the reaction of Vanillin with aromatic amines using them as corrosion inhibitors for a carbon steel immersed in HCl. The weight loss (WL) conducted at 313, 333, and 353 K over 24 h demonstrated that these shift bases decrease the corrosion rate achieving efficiencies of nearly 90%. The potentiodynamic analysis revealed that these compounds act as mixed inhibitors¹⁴. Other Schiff bases, such as chitosan derivative of Vanillin, have been tested as corrosion inhibitors for mild steel, showing improved corrosion resistance^15,16. Other authors report that chitosan derivatives from Vanillin can reach efficiencies above 90% in acid media¹⁷. However, most research has been focused on analyzing Vanillin’s effect on steel corrosion; few analyses have been conducted in other metals such as Cu and Al¹⁸. Then, in the present research, the analysis of Vanillin as eco-friendly corrosion inhibitor for aluminum is proposed, using gravimetric tests and electrochemical approaches. Also, DFT analysis is conducted to observe the electronic properties of Vanillin and its correlation with the inhibition performance. The analyses are conducted at 25, 40, and 60 °C using 0.5 M H₂SO₄ as the corrosion environment.

Experimental procedure

Analytical grade Vanillin was used to prepare a stock solution. Aliquots were taken from this stock to prepare desired concentrations by dilution, i.e., 0, 2, 4, 6, 8, and 10 mM in deionized water. Each concentration was dissolved in the corrosive media, eliminating the need for organic solvents. Prior to dissolution, Vanillin was characterized using FT-IR.

For the weight loss experiments, aluminum rods of 0.6 cm diameter were used as specimens with a length of 2.5 cm. They were rubbed with fine emery paper 120, 300, 400, 600, 800, and 1200 grids to achieve a less heterogeneous surface, in accordance with ASTM D 1384-05. After rubbed, the samples were rinsed with acetone to remove any impurities and stored. Then, the samples were immersed in an aggressive solution of 0.5 M H₂SO₄ for 24 and 72 h. Then specimens were taken out, washed with distilled water, degreased with acetone, dried, and weighed accurately. To eliminate corrosion products formed on the metal surface, chemical cleaning procedure was conducted in accordance with ASTM-G1-03. WL test was performed at 25, 40, and 60 °C. The corrosion rate was calculated in terms of weight loss measurements, , as follows:

1

where is the mass of the specimen before corrosion, is the mass of the specimen after corrosion, and is the exposed area of the specimen.

Electrochemical techniques, such as Potentiodynamic polarization (PDP) curves and electrochemical impedance spectroscopy (EIS), were used to characterize aluminum’s corrosion behavior. To conduct the electrochemical analyses, aluminum samples were encapsulated in epoxy resin, previously welded to a copper wire, to ensure conductivity and communication with the equipment to obtain the corrosion data. Also, an area of 1 cm² was left exposed to contact the electrolyte. Then, the surface was rubbed with emery paper120, 300, 400, 600, 800, and 1200 grid and degrease with acetone. All experiments allowed the aluminum electrode to reach a stable open circuit potential value, E_corr. Polarization curves were recorded at a constant sweep rate of 1 mV/s from − 500 to + 500 mV vs. E_corr. Measurements were obtained using a conventional glass cell of three electrodes using the aluminum sample encapsulated as working electrodes, a graphite rod as auxiliary electrode, and an Ag/AgCl electrode as reference electrode. Corrosion current density values, I_corr, were obtained using Tafel extrapolation. EIS analyses were carried out versus E_corr using a sinusoidal signal with an amplitude of 10 mV in a frequency interval of 100 mHz–20 kHz. An ACM Potentiostat controlled by a desktop computer was used to carry out the polarization curves, whereas a model PC4 300 Gamry Potentiostat was used to carry out the EIS measurements. After that, EIS results were fitted using Zview software 3.0. Each plot was fitted using instant fit, first from high to middle frequencies and then from middle to low frequencies. An equivalent circuit was fitted with the values readied from the instant fit circuit. A chi-square (χ²) of 10^− 3 was used to warrant a suitable fitting. All tests were carried out at 25, 40, and 60 °C. Figure 2 illustrates the schematic procedure that was followed.

Fig. 2.

Experimental schematic procedure for the analysis of Vanillin as corrosion inhibitor for the aluminum.

The surface characterization to observe and identify the corrosion products and the inhibitor presence on the metal surface was carried out using X-ray diffraction (XRD) and Scan Electron Microscopy techniques. The XRD patterns of the as-prepared materials were recorded with a Bruker D8 Advanced diffractometer in the range of 20–80°(2θ) with a Cu-Kα radiation (λ = 1.541 Å). SEM images and elemental mapping of the samples subjected to electrochemical corrosion conditions were recorded using a JEOL JSM-IT 500 scanning electron microscope (SEM) coupled with a Bruker energy dispersive X-ray spectroscope (EDS).

Results and discussion

FT-IR

The FT-IR analysis done on the Vanillin is shown in Fig. 3, where the significant peaks appear at 1500 and 3500 cm^− 1 wavenumbers. The peak observed at 3410 cm^− 1 corresponds to the O–H bond or hydroxyl group. The peak observed at 2927 cm^− 1 corresponds with the hydrogen bond adding to an aromatic ring. At 2855 cm^− 1, another peak was observed, corresponding with the C–H bond from the –CH₃ group. A C–O bond appears at 1060 cm^− 1. Meanwhile, at 1463 cm^− 1, the aromatic ring can be visualized through the conjugate C=C double bonds. Aldehyde group appears at 2855 and 1744 cm^− 1 through the HC=O bond.

Fig. 3.

FT-IR for Vanillin extract.

Weight loss test

The weight loss (WL) analysis for pure aluminum immersed in 0.5 M H₂SO₄ without and with Vanillin after 24 and 72 h is displayed in Table 1. After 24 h, the inhibitory efficiency (IE) was highest at 2 mM of Vanillin, showing values of 97, 74, and 72% at temperatures of 25, 40, and 60 °C, respectively. Except for 10 mM concentration, the remaining showed efficiencies above 80%. However, there is a trend to decrease as the temperature increases. After 72 h, the WL for the blank sample was 33.23 mg/cm² at 25 °C. When Vanillin is added at 2 and 4 mM, the mass loss of aluminum decreased to 15.63 and 12.23 mg/cm², respectively. A similar trend was observed at 10 mM, resulting in a WL of 15.22 mg/cm². However, at 6 and 8 mM, the WL increased. At 40 and 60 °C, the inhibitory efficiency of Vanillin is decreased across all concentrations, indicating that the immersion time reduces the effectiveness of Vanillin as corrosion inhibitor.

Table 1.

Weight loss for the aluminum without and with Vanillin immersed by 24 and 72 h.

C inh	24 h						72 h
	25 °C	%IE	40 °	%IE	60 °C	%IE	25 °C	%IE	40 °	%IE	60 °C	%IE
0	38.47		17.73		15.97		33.23		14.659		47.33
2	1.33	97	4.59	74	4.53	72	15.63	53	13.141	10	43.141	9
4	3.60	91	5.90	67	12.97	19	12.23	63	15.131	-	43.56	8
6	1.77	95	4.67	74	6.40	60	23.20	30	13.403	9	42.931	9
8	6.33	84	6.23	65	4.97	69	22.30	33	10.367	29	47.12	–
10	11.87	69	5.67	68	4.33	73	15.22	54	10.786	26	49.216	–

Electrochemical analyses

Potentiodynamic polarization (PDP)

The PDP curves conducted for the Al without and with Vanillin at different concentrations (2, 4, 6, 8, and 10 mM) and temperatures (25, 40, and 60 °C) immersed in 0.5 M H₂SO₄ are shown in Fig. 4. The parameters calculated via Tafel extrapolation are detailed in Table 2.

Fig. 4.

Polarization curves for the aluminum immersed in 0.5 M H₂SO₄ in the absence and presence of Vanillin at (a) 25, (b) 40, and (c) 60 °C.

Table 2.

Electrochemical parameters estimating from PDP curves for aluminum immersed in 0.5 M H₂SO₄ in the absence and presence of Vanillin at different concentrations.

Cinh (mM)	Icorr (µA/cm2)	Ecorr (mV vs. Ag/AgCl)	βc mV/dec	βa mV/dec	%IE
25 °C
0	4.1	− 572	− 101.0	99.8
2	0.7	− 587	− 130.4	72.5	82
4	1.6	− 582	− 93.5	81.0	60
6	2.9	− 613	− 194.0	108.1	29
8	2.8	− 685	− 198.8	119.6	33
10	1.4	− 569	− 191.3	92.0	65
40 °C
0	79	− 660	− 66.5	100.8
2	23	− 683	− 50.8	83.4	71
4	17	− 691	− 67.1	109.7	77
6	19	− 657	− 70.8	107.6	76
8	20	− 682	− 60.0	70.8	74
10	27	− 695	− 63.5	72.1	65
60 °C
0	54	− 886	− 93.2	122.4
2	27	− 781	− 89.3	97.8	65
4	31	− 733	− 96.3	105.1	61
6	37	− 785	− 80.3	124.3	53
8	34	− 842	− 81.9	130.8	57
10	19	− 695	− 86.0	125.3	75

The PDP for the blank solution at room temperature (Fig. 4a) shows the highest corrosion current (I_corr) with 4.1 µA/cm². Also, its E_corr shifted in the positive direction. When the Vanillin is introduced into the solution, the corrosion potential (E_corr) shifts toward negative values, reaching at 2 mM the lowest I_corr with 0.7 µA/cm² with an efficiency of 82%. However, at a Vanillin concentration of 10 mM, the E_corr is shifted in the positive direction, and its I_corr increases, reaching an efficiency of 65%. The other concentrations yield lower I_corr values than the test without inhibitor where at 4, 6. And 8 mM Vanillin, the aluminum reached efficiencies of 60, 29 and 33%, respectively.

The current density values in the anodic branch are relatively similar, indicating the presence of a limiting current (vertical straight line with a staggering increase), suggesting the formation of a passive layer formed by Al₂O₃ (Eqs. 2 and 3), which is dissolved slowly due to the and anions from the acid media (Eqs. 4 and 5) delaying the initiating of the corrosion process leaving active sites to be continued attack by them¹⁹.

2

3

4

5

As these active sites are formed, the heteroatoms in the functional groups of the Vanillin are adsorbed, forming a protective layer on the aluminum surface²⁰.

On the other hand, the cathodic branch is shifted toward low current values. Both anodic and cathodic behaviors suggest that this inhibitor acts as a mixed corrosion inhibitor, as is reported by^21,22, who tested the Vanillin as inhibitor for the AA6061 alloy in neutral media. Moreover, the decrease in the slope suggests reduced rates of cathodic corrosion reactions due to the barrier oxide layer formed on the surface, which prevents the hydrogen ions diffusion from the bulk solution into the metal surface. However, the protective film on the aluminum surface is dissolved in the presence of H₂SO₄, leading to hydrogen evolution (Eq. 6). Consequently, the protective layer is broken down.

6

At 40 °C, the E_corr shifts toward positive values (Fig. 4b) except at 6 mM, which exhibits a more negative E_corr (Table 2). Meanwhile, its I_corr decreases compared to the blank solution, with the lowest current density at 6 mM of Vanillin with 19 µA/cm². The 8 mM concentration follows closely behind with an I_corr of 20 µA/cm²; their inhibition efficiencies are of 76% and 74%, respectively. The other concentrations showed low efficiency around 70%, as detailed in Table 2.

At 60 °C (Fig. 4c), the E_corr for the blank is shifted toward more negative values (Table 2), suggesting an increase in the mass transport phenomena from the surface to the bulk solution with an I_corr of 54.3 µA/cm². When Vanillin is added at 8 and 10 mM, the I_corr is diminished to 34 and 19 µA/cm², respectively. The 6 mM concentration showed an I_corr of 37 µA/cm². Also, there is not significant change in the anodic branch, while the cathodic branch shifted toward high current values, indicating an increase in the hydrogen evolution reaction.

Electrochemical impedance spectroscopy (EIS)

Nyquist and Bode plots for aluminum immersed in 0.5 M H₂SO₄ without and with Vanillin at 25. 40 and 60 °C are shown in Fig. 5. The Nyquist plots carried out at 25 °C (Fig. 5a) show a depressed, capacitive loop from high to middle frequencies (between 10⁴ and 10⁰ Hz) and an inductive loop at low frequencies (< 10⁰ Hz). For the test without Vanillin, the semicircle has an amplitude of 700 Ω cm² with a time constant from 10³ to 10² Hz, with a phase angle of 73° (Fig. 5b). In the presence of Vanillin, the semicircle amplitude increased to the maximum impedance at an concentration of 2 mM with a value around 10³ Ω cm², rising at least one order of magnitude against the test without inhibitor, observing that its time constant is extended from 10³ to 10⁰ Hz with an increased in the phase angle of 80° which suggesting that Vanillin forming a barrier that reduces the mass transport phenomena between the metal surface and the electrolyte²³. At 4, 6, and 8 mM, the semicircle diameter declines, which is observed in its phase angle, decreasing 78°. At this point, it should be mentioned that the capacitive loop at high-frequency range is related to adsorption processes, including charge transfer reaction, time constant of electrical double layer, and structural or interfacial heterogeneity²⁴. In contrast, the inductive loop relates to the relaxation of the materials adsorbed on the electrode surface^25,26. Also, at low frequencies a second capacitive loop is observed, which could be due to metal dissolution and/or inhibitor adsorption²⁷.

Fig. 5.

Nyquist and Bode plots for aluminum immersed in 0.5 M H₂SO₄ without and with Vanillin as corrosion inhibitor at temperatures of (a) and (b) 25 °C, (c) and (d) 40 °C, and (e) and (f) 60 °C.

At 40 °C (Fig. 5c and d), the semicircle diameter for the blank decreased around 200 Ω cm², observing a capacitive behavior in its phase angle, with the formation of one constant time from high to middle frequencies (10⁴ to 10⁰), which suggests that at this stage the process is controlled via charge transfer resistance with the formation of a depressive semicircle (Fig. 5c). At 2 mM of Vanillin, the semicircle diameter increased by at least 1 order against the blank with a total impedance of around 10³ Ω cm² with an extended phase angle at 80°. At 4, 6, and 8 mM, the semicircle diameter decreased again around of 10² Ω cm², showing a contraction in its phase angle underneath 80° (Fig. 5d). As in the test at 25 °C, an inductive loop is observed in the presence of Vanillin, suggesting that the corrosion process is controlled by species adsorbed such as H, O, and C, which form complexes in the active sites of the aluminum.

With the solution temperature at 60 °C, the diameter of the semicircle in the Nyquist plot (Fig. 5e) decreases for the analyses without and with Vanillin. As is known, an increase in the temperature accelerates the corrosion process over the metal surfaces due to electron exciting, allowing the free transit of the current, as consequence the mass transport phenomena is accelerated, as is observed in the test without Vanillin with a lowest semicircle diameter with a total impedance (Fig. 5f) around 10² Ω cm². However, all impedances, even with the inhibitor, are in the same order of magnitude with a slight increase, confirming that the Vanillin helps to mitigate some anodic processes on the metal surface. Also, the time constant with the inhibitor addition is displacement toward high frequencies with an increase in the phase angle (Fig. 5f) with its maximum at 2 and 10 mM of Vanillin.

Electrical equivalent circuit (EEC)

To explain the electrical behavior of the layer formed on the metal surface without and with the Vanillin in the corrosion media, the electrical equivalent circuit (EEC) shown in Fig. 6 was used to fit the experimental data from EIS, where a resistance element (R_s) was configurated to simulate the resistance solution. Afterward, it was connected in series with a constant phase element and placed in parallel with a charge transfer resistance (R_ct) to simulate the effect of the double layer formed from high to middle frequencies. Finally, an inductor (L) connected in series with a resistance to simulate the repulsion of the inhibitor to the electron flow (R_L) was added. Also, an inhibitor resistance (R_inh) in parallel with a CPE_inh were added, connecting them in parallel, with the R_ct and CPE_dl of the double layer²⁸.

Fig. 6.

EEC to Fit the EIS data of the aluminum immersed in 0.5 M H₂SO₄.

The fitting results obtained for the aluminum without and with Vanillin at 25, 40 and 60 °C are shown in Table 3. For all data, the chi-square (χ²) was in the order of 10^− 5, suggesting a good fitting with the experimental data. At 25 °C is observed that the n_dl values, which are an indicative factor about the roughness of the metal surface, are keeping stable around 0.9 near 1 without and with Vanillin, suggesting a non-capacitive ideal behavior, explaining the depressive behavior observed in the Nyquist plots. The inductor values (L) shown an increment with the inhibitor addition, helping to avoid the current and voltage fluctuation through the metal surface; as consequence, the R_inh is increased and reached its maximum value at 2 mM with a polarization resistance (R_p) of 3165.58 Ω cm² followed by the concentration at 10 mM with an R_p of 2543.25  Ω cm² with an efficiency of 75 and 68%, respectively. Then, it is possible to assume that the corrosion reaction is strictly controlled by electron transfer because an increase in the R_p is related to corrosion inhibition. In contrast, the decrease in the CPE_dl value is attributed to both the decrease in the local dielectric constant and the increase in the thickness of the electric double-layer, indicating, that Vanillin displaces the water molecules and other ions on the metallic surface and adsorb at the metal/solution interface⁶.

Table 3.

Parameters obtained via EEC for the EIS data for al immersed in 0.5 M H₂SO₄ without and with Vanillin as corrosion inhibitor.

Cinh (mM)	Rs (Ω cm2)	CPEdl (×10− 5, F cm− 2)	n dl	Rct (Ω cm2)	L (H cm2)	RL (Ω cm2)	CPEinh (×10− 5, F cm− 2)	n inh	Rinh (Ω cm2)	Rp (Ω cm2)	%IE
25 °C
0	9.17	1.94	0.9	702	53.51	73.94	2.29 × 10− 8	0.4	17.93	803.05
2	4.78	9.20	0.9	1200	229.5	334.82	1.06 × 10− 5	0.8	1626.00	3165.58	75
4	4.86	1.57	0.9	713.8	66.71	106.91	1.06 × 10− 4	0.8	228.00	1053.56	24
6	4.75	1.10	0.9	970.4	102.6	173.60	9.01 × 10− 5	0.8	224.40	1373.15	42
8	8.16	1.14	0.9	751.9	78.47	130.30	1.06 × 10− 4	0.6	401.70	1292.06	38
10	62.15	1.46	0.9	323	266.1	291.13	5.87 × 10− 7	0.8	1867.00	2543.25	68
40 °C
0	2.70	1.49	0.9	25.93	1.446	11.24	1.87 × 10− 6	0.9	131.50	171.374
2	3.64	5.58	0.9	1307	42.77	353.8	2.72 × 10− 7	0.4	788.10	2452.55	93
4	2.55	1.30	0.9	11.69	4.25	27.77	3.99 × 10− 8	0.4	249.70	291.75	41
6	2.47	1.36	0.9	23	13.9	169.5	1.34 × 10− 7	0.4	745.50	940.47	82
8	2.95	1.33	0.9	23	3.927	18.14	1.74 × 10− 7	0.3	288.50	332.60	48
10	2.51	1.36	0.9	40.8	12.82	54.04	6.55 × 10− 9	0.3	444.40	541.75	68
60 °C
0	7.47	1.47	0.9	102.60	1.31	15.34	1.46 × 10− 2	0.4	39.74	165.15
2	3.03	1.02	0.9	60.22	2.81	17.93	2.37 × 10− 5	0.7	207.6	288.78	43
4	10.37	1.77	0.9	198.02	14.3	27.58	2.45 × 10− 2	0.4	23.45	259.42	36
6	2.36	1.93	0.9	188.70	1.60	16.57	5.27 × 10− 2	0.4	52.33	259.96	36
8	2.97	1.85	0.9	89.41	3.32	8.02	9.58 × 10− 2	0.4	22.76	123.17	–
10	2.14	7.92	0.9	16.00	3.25	43.74	2.67 × 10− 7	0.9	228.3	290.18	43

At 45 °C, the n_dl values keep near 1, suggesting a less heterogeneous surface and non-ideal capacitive system behavior. An inductor was added to explain the species adsorbed on the surface, which is usually evidenced by the inductive loop at low frequencies. The L values increased in the presence of the Vanillin, with its maximum values at 2 and 6 mM. Consequently, the R_inh increased at this concentration with a total R_p of 2452.55  Ω cm² and 940.47  Ω cm², respectively. All concentrations got values above the blank, bringing inhibit efficiencies over 90% at 2 mM. This could be due to the temperature at which the oxide film formed by Al₂O₃ is dissolved, allowing the adsorption of the Vanillin in the active sites at the correct angle. At 60 °C, the R_p decreased even in the presence of inhibitor, suggesting that the temperature increased the mass transport phenomenon from the metal surface to the electrolyte. Also, as at the other temperatures, the higher efficiency was again at 2mM of Vanillin followed by 10 mM, indicating that 2 mM is the optimal concentration to adequately perform the inhibitor adsorption on the active sites covering the metal surface.

Adsorption isotherm

To evaluate the adsorption of Vanillin on the aluminum surface, several adsorption isotherms were tested between them: Flory-hugging, Freundlich, Hill de Boer, Langmuir, and Temkin²⁹. The best linear correlation was with Frumkin isotherm, which got a R² above 0.97 (Fig. 7). Meanwhile, the remains got R² values lower than 0.6 for all temperatures. The equation used to fit the data was in its linear form, as described in Eq. (7)³⁰.

7

where is the surface coverage, k_ads the adsorption-desorption equilibrium constant, C is the inhibitor concentration, and α is the lateral interaction parameter, which, based on its values can describe the molecular interaction on the metal surface, as is mentioned negative values mean repulsive forces between the molecules adsorbed on the metal surface that came from the corrosion inhibitor under test which in this case correspond with the Vanillin³¹. Meanwhile, positive values mean an attractive behavior of the inhibitor on the aluminum surface³². In this sense, Vanillin presents positive values, suggesting an attractive behavior with the metal surface (Table 4). The k_ads values shown in Table 4 correspond with the equilibrium constant of adsorption estimated from the intercept of the linear fitting of the Frumkin isotherm used to calculate the adsorption energy with Eq. (8).

8

where corresponds with free energy of adsorption, R is the molar gas constant in kJ mol^− 1 K^− 1, and T is the absolute temperature in K. The values can be negative, suggesting spontaneous adsorption of the inhibitor on the metal surface; otherwise, it means nonspontaneous. Based on the literature values lower that − 20 kJ mol^− 1 suggest an electrostatic interaction between organic molecules and charged metallic surfaces knowing as physisorption mechanism, and values higher that − 40 kJ mol^− 1 are associated with charge sharing or transfer from the organic molecules to the metallic surface to form a coordinate type of bond knowing as chemisorption mechanism. Values between 20 and 40 kJ mol^− 1 suggesting a mix type mechanism adsorption. Based on the latter the free energy calculated (Table 4) suggested a spontaneous chemisorption of the vanillin. However, based on Kokalj, it is not deterministic, making it necessary conduct other analysis to distinguish between chemisorption or physisorption such as computational modeling studies which can give a better overview of the Energies between the inhibitor and the metallic surface³³.

Fig. 7.

Frumkin isotherm for adsorption of Vanillin on aluminum surface.

Table 4.

Adsorption parameters for Vanillin as corrosion inhibitor in aluminum immersed in 0.5 M H₂SO₄.

log kads (M− 1)	Temperature (K)	Isotherm propierty (α)	(kJ/mol)
− 0.35	298	0.46	− 48.13
− 0.28	313	0.34	− 40.44
− 0.41	333	0.68	− 63

Density functional theory (DFT) analyses

Vanillin’s electronic structure behavior was optimized using a DFT Technique with the functional basis set B3LYP 631G ⁺⁺ with solvation model IEFPCM using water as solvent. Parameters such as highest and lower occupied molecular orbital (HOMO and LUMO, respectively), energy gap (ΔE_gap), electron affinity (A), ionization potential (I), electronegativity (χ), chemical harness (η) and softness (σ), global electrophilicity index (ω), and nucleophilicity (ε) were calculated with the Eqs. (9) to (16) respectively. Then, with χ and η calculated from the inhibitor and using hardness and electronegativity theoretical values for Al which are 5.6 and 0, respectively, it is possible to determine the number of electrons transferred (ΔN, Eq. (17)). Fukui functions , , and were calculated with the Eqs. (18) to (20), respectively, where , , and are the anionic, neutral, and cationic Mulliken atomic charges³⁴. Some authors mentioned that large values of or make an atom more susceptible to nucleophilic or electrophilic attacks, respectively. Also, they mentioned that the Fukui dual Function is a better descriptor for the local reactivities in a molecule where atoms with values above 0 are considered electrophiles and values lower than 0 are considered nucleophiles; this means that the first one is more susceptible to nucleophilic attacks and the second one is more susceptible to electrophilic attacks^35–37.

9

10

11

12

13

14

15

16

17

18

19

20

Table 5 lists the results obtained from the Vanillin molecule optimization (Fig. 8), where the orbital with higher energy (E_HOMO) got a value of -6.089 eV. In comparison, the orbital with the lower energy (E_LUMO) got a value of -1.685 eV. Based on the E_LUMO value, Vanillin tends to accept electrons, which increases the binding ability of the inhibitor since more negative E_LUMO values decrease the accepted electron process; therefore, its electron affinity (A = 1.685 eV) is larger. On the other hand, its E_HOMO is more negative than other compounds reported, suggesting a decrease in the donation electron process to the metal surface³⁸. Also, The ΔE_gap calculated got a value of 7.775 eV, which is higher compared with other organic inhibitors previously reported, such as Artichoke, Chamomile flower, and Thymus vulgaris, which got ΔE_gap between 3 and 5 eV³⁹, meaning that Vanillin´s molecule needs more energy to release an electron from the HOMO to the LUMO orbitals which based on Kokalj et al.⁴⁰, it does not mean a reduction in the inhibitor efficiency of Vanillin. Moreover, as was demonstrated by other research, the model used to carry out the DFT analyses can influence the values calculated as was demonstrated by Guo et al., who reported ΔE_gap higher or lower based on the model to the DFT analysis done³⁷. The ΔN value was under 0 with − 0.389 eV, suggesting that the electron donation of the inhibitor increased due to the low softness and hardness calculated⁴¹.

Table 5.

DFT quantum parameters estimated for Vanillin molecule as corrosion inhibitor.

ELUMO (eV)	EHOMO (eV)		ΔEgap (eV)	A (eV)	I (eV)	µ (eV)	χ (eV)	η (eV)	σ (eV− 1)	ω (eV)	ε (eV− 1)	Δ N
− 1.685	− 6.089		7.775	1.685	6.089	3.888	− 3.888	− 2.202	− 0.454	− 3.431	− 0.291	− 0.389

Fig. 8.

HOMO and LUMO electronic densities for Vanillin.

Fukui index

Fukui indexes and Mulliken atomic charges are shown in Table 6. Figure 9 shows the position of atoms in the Vanillin. The highest and values were for the atom in the first position (1O), suggesting that it is susceptible to nucleophilic and electrophilic attacks from the atoms on the metal Surface. However, its Fukui dual function () got a value less than 0 being more susceptible to an electrophilic attack (accepting electrons) than a nucleophilic attack, since values < 0 are nucleophiles and > 0 are electrophiles. Conversely, the 2 C atom got the highest , making this site susceptible to nucleophilic attack (donor electron). Based on the latter, as Vanillin is an organic compound, this can be adsorbed on the metal surface from heteroatoms contained and those with conjugated double bonds, as is the case of the oxygen and carbon in the first and second position previously described as aldehyde group in the FT-IR analysis, which can react with the aluminum oxide products forming a barrier that reduced the mass transport phenomena from the metal surface to the bulk solution^42,43.

Table 6.

Fukui functions and mulliken atomic charges for Vanillin as corrosion inhibitor.

Atom
1O	0.6494	0.13368	0.4893	0.1602	0.3556	− 0.1954
2 C	− 0.1734	− 0.3515	− 0.3068	0.1335	0.0447	0.0888
3 C	0.2325	0.19623	0.1794	0.0531	− 0.0168	0.0700
4 C	0.2456	0.12465	0.1704	0.0751	0.0458	0.0293
5 C	0.3341	0.28337	0.3087	0.0255	0.0253	0.0002
6 C	− 0.1893	− 0.4005	− 0.3290	0.1397	0.0715	0.0682
7O	0.6966	0.58338	0.6430	0.0535	0.0597	− 0.0061
8 C	− 0.2430	− 0.29179	− 0.2616	0.0186	0.0302	− 0.0117
9O	0.5333	0.47162	0.5068	0.0265	0.0352	− 0.0087
10 C	0.3328	0.34762	0.3409	− 0.0081	− 0.0067	− 0.0014
11 C	0.3534	0.2677	0.2681	0.0853	0.0004	0.0849

Fig. 9.

Distribution electron densities mapping for Vanillin.

Inhibition mechanism

Vanillin contains functional groups in its structure such as O–H, C–H, –CH₃, C–O, C=C, and HC=O, which are desirable for the good performance of a corrosion inhibitor. Based on the theorical calculation the Vanillin´s chemisorption can be occurred via coordinate bonding between the Al₂O₃ and the oxygen and carbon found in the first and second position in the aldehyde group of the Vanillin, forming a barrier that reduced the mass transport phenomena from the metal surface to the bulk solution, which can be occur based on the mentioned by Errili et al., who suggests that a flat-adsorbed structure in the inhibitor molecule can optimize the interaction of the O with the metal surface. Meanwhile, based on Ennafaa et al., the C–O polar bond enhances solubility and facilitates uniform surface coverage^44,45.

To investigate the role of Vanillin in the corrosion products formed on the aluminum surface, XRD patterns were conducted on the aluminum surface before (Fig. 10a) and after exposure to the corrosive environment at 25 °C, the latter without and with Vanillin (0 and 2 mM) (Fig. 10b and c). After exposure to H₂SO₄, the metal surface formed corrosion products such as Al₂O₃, Al(OH)₃, and AlO(OH) for regardless of whether Vanillin was present. While the Al₂O₃ is produced as described in Eq. (3); the Al(OH)₃ is formed via the reaction of the hydroxyl ion (OH⁻) from Vanillin with the Al³⁺ from Eq. (2) as follows:

21

Fig. 10.

XRD patterns for the aluminum surface: (a) before immersed in 0.5 M H₂SO₄, (b) after being immersed in 0.5 M H₂SO₄ without vanillin and (c) immerse in 0.5 M H₂SO₄ with 2 mM of Vanillin by 24 h at 25 °C.

Then, the Al(OH)₃ undergoes dehydration, resulting in the formation of aluminum oxyhydroxide (AlO(OH)) as represented by the following equation:

22

Based on the above, the presence of hydroxyl groups can enhance the formation of stable compounds such as Al₂O₃, AlO(OH), and Al(OH)₃, which are recognized as highly protective corrosion products, thereby decreasing the corrosion rate of the aluminum surface⁴⁶. This effect can be correlated with the inductive loop observed at low frequencies in the Nyquist plots from the EIS analyses, which confirms the formation and adsorption of these complexes at the active sites of aluminum. Additionally, there is an increase in the inductive values (L) in the presence of Vanillin, suggesting that its -CHO functional group provides a stronger anchoring for the inhibitor attachment on the aluminum surface⁴⁷.

SEM micrographs and EDS analyses

To ensure the presence of Vanillin, some micrographs were taken from weigh lost analysis carried out after 24 h. For the sample immersed in 0.5 M H₂SO₄ in the absence of Vanillin, the EDS showed the presence of O and Al over the metal surface. Also, the micrograph (Fig. 11a) shows the presence of pits and honeycomb corrosion products on the metallic surface. When the corrosion products are removed (Fig. 11b) from the surface several cracks and pits are reveal. On the other side, EDS analysis for the test with Vanillin at 2mM shows the presence of carbon on the metal surface (Fig. 11c), which demonstrated the adsorption of the inhibitor. It shows the presence of a layer of corrosion products more compact and with less porosities. When the corrosion products are removed from the surface (Fig. 11d), less cracks are observed confirming the adsorption of the vanillin on the aluminum surface. However, due to exposition to the acid media, the metal surface presented pits still.

Fig. 11.

SEM micrographs and EDS analyses for the Aluminum immersed in 0.5 M H₂SO₄ by 24 h at 25 °C, (a) and (b) before and after cleaning without vanillin, respectively; and (c) and (d) before and after cleaning with vanillin at a concentration of 2 mM, respectively.

Conclusions

The effect of Vanillin as corrosion inhibitor for aluminum immersed in H₂SO₄ was evaluated at different concentrations and temperatures. Weight loss analyses conducted at 25, 40, and 60 °C showed that temperature decreased Vanillin efficiency. The impact of the immersion time was also explored, showing that at 24 h, Vanillin provides over 60% efficiency in protecting the surface. However, its effectiveness is diminished after 72 h. In the electrochemical characterization, it was observed that Vanillin could be adsorbed on the metal surface since PDP curves showed that at 25 °C, the I_corr reached 82% efficiency at 2 mM. At elevated temperatures of 40 and 60 °C, efficiencies of 77% and 75% were reached at 4 mM and 10 mM, respectively. This behavior suggests that corrosion products may react with Vanillin, forming a protective layer with the temperature. However, weight loss analyses indicate that this protective layer tends to degrade over time. In the case of EIS, the maximum R_p at all temperatures was observed at 2 mM of Vanillin, suggesting that this is the optimal concentration for it use. The FT-IR characterization showed some functional groups such as hydroxyl, methyl, aldehyde and conjugated C = C double bonds. So, based on the Fukui indexes, the Aldehyde group in the molecule corresponds with the sites more susceptible to electrophilic and nucleophilic attacks (1O and 2 C atoms) got the highest - and values, meaning that this group is susceptible to electrophilic and nucleophilic attacks, and could react with the Aluminum oxide, forming a barrier that reduced the mass transport phenomena from the metal surface to the bulk solution.

Declarations.

Author contributions

A.M.R.A. and M.P.H.V. conducted the experiments described in the manuscript and carried out the electrochemical analyses with the Vanillin as corrosion inhibitor. A.K.L.G. carried out the chemical characterization of the Vanillin via FT-IR. R.G.T. conducted the surface characterization after corrosion test and J.G.G.R. gave the experimental arrangement to analyze the aluminum surface under corrosion conditions, and R.L.S wrote and organized the main manuscript.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

The crystallographic datasets generated and/or analysed during the current study are available in the Crystallography Open Database (COD) repository, [https://www.crystallography.net/cod/], deposition numbers 1000017, 1502689, 2211226, and 1011081 for Al₂O₃, Al, AlO(OH), and Al(OH)₃, respectively. Figures 2 and 8, and 9 were drawn by Martha Patricia Hernández Valencia using Biorender and GaussView 5.0 software.

Declarations

Competing interests

The authors declare no competing interests.