Introduction

Recently, the concept of green, sustainable, and environmental friendly materials became a trend that captured the attention of several researchers [1]. Nowadays, one of the most fundamentals of sustainability is the growing interests of "referring back to nature" and to the green natural materials acquired from the extracts of various plant leaves, stems, vegetable skins, and roots for using them in artificial sectors [2]. Basically, one of these successful implementations inspired by nature is the use of such green materials as a protective layer on metal surfaces against severe corrosive conditions [3]. Most of the synthetic corrosion inhibitors are not only can cause toxicity to living organisms, but also can cause damage to the environment [4]. Furthermore, the low cost of the green inhibitors extracted from sustainable sources, their high inhibition efficiency values to protect the metal surface against severe corrosive conditions, and their environmental friendliness make them unusual alternatives for many toxic and expensive inhibitors [5]. Many studies have confirmed the high inhibition efficiency of promising green inhibitors for metal surfaces like; sustainable onion skin extract [6], Eucalyptus leaves extract [7], and beetroot extract [8]. On the other hand, betalain pigments are available in the tuberous part of beetroots (Beta vulgaris L.). They can be subdivided into two components: betacyanins responsible for red-violet pigments and betaxanthins responsible for orange-yellow colouring [9]. Betanin (betanidin 5-O-β-D-glucoside) is the most abundant betacyanins and is the major constituent of the red-violet pigments and approved for use as natural colourant in food products, cosmetics and pharmaceuticals [10, 11]. In fact, more than 200000 tonnes of beetroots rich in betalain pigments are produced in Western Europe per annum [12], most of which are peeled and its skin are wasted. In our study, beetroot disposable waste and sustainable source were used to extract betalain pigments, which then were used as potential green and environmentally friendly film coating for aluminium surface. The green coating was examined with severe corrosive 1.0 M HCl solution, and different optimization conditions were investigated to affirm its high inhibition efficiency. Moreover, various adsorption isotherms investigated equilibrium constants and adsorption parameters. Dynamic and thermodynamic parameters were also evaluated. Eventually, surface topology of the coated and uncoated aluminium surfaces was examined and evaluated using scanning electron microscope technique.

Experimental

Materials

Fresh beetroots was purchased from local market in Irbid city (Jordan) and well washed with distilled water, air-dried at ambient temperature. Ultra-gradient ethanol was purchased from Carlo Erba (France). Hydrochloric acid HCl (37%) was purchased from Carlo Erba (France). Deionized water was used in all experiments.

Extraction of betalain pigment

The fresh beetroots were thoroughly washed with deionized water, air-dried at ambient temperature, peeled and cut into small slices. The beetroot slices were soaked in 200 mL of 1:1 ethanol–water mixture at pH = 6.5 and 20 °C for 24 h until the red-violet betalain pigments were extracted. The solvent was evaporated at 40 °C, and betalain pigments were collected [13]. Different betalain pigments concentrations were prepared from stock solution (i.e. 300, 600, 900, and 1200 ppm) and stored until used.

Surface of aluminium

Rectangular sheets of aluminium with dimension of length = 30 cm, width = 10 cm, and height = 0.15 cm were abraded with emery paper up to 1200 grade and used to make smaller aluminium coupons. The purity of aluminium sheet = 99.7%. The Al-sheet was cut to smaller coupons with dimensions of length = 3 cm, width = 1 cm and height = 0.15 cm. The coupons were washed with deionized water and ethanol, respectively, and allowed to dry at ambient temperature. Later on, betalain pigments were adsorbed on the coupon's surface and the inhibition efficiency of the coating process was tested and evaluated using severe corrosive 1.0 M HCl solution. Aluminium coupons were soaked in different betalain concentrations of solutions (i.e. 300, 600, 900, and 1200 ppm) for one hour at different temperatures (i.e. 10, 20, and 30 °C) and different turbulent flows (i.e. 50, 100, 200, 300, 400, 500 rpm). After time completion, coupons were removed from solution and allowed to dry overnight and stored. Later on, the coupons were exposed to severe corrosive 1.0 M HCl solution for one hour, and the inhibition efficiency value (%IE) was determined as follows:

$$\% IE = \left( {\frac{{W_{0} - W_{i} }}{{W_{0} }}} \right)\; \times \;100$$
(1)

where \(w_{0}\) and \(w_{i}\) were the weight loss (in kg) of naked and coated aluminium coupons, respectively. The optimum conditions used for best inhibition efficiency values are available in Fig. 1.

Figure 1
figure 1

Schematic presentation of betalain pigments film coating over aluminium surface before and after been exposed to severe corrosive 1.0 M HCl solution

Full size image

Characterization techniques

Attenuated total reflectance Fourier transform infrared: Bruker alpha FTIR spectrometer armed with attenuated total reflectance unit was used to record the well-deconvoluted vibrational spectra in the 4000–400 cm−1 range. UV–Vis spectrophotometer: Shimadzu UV–Vis 2401 spectrophotometer was used to record the change of absorbance in 200–800 nm range. Heating control device was supplied with instrument to monitor absorbance change at elevated temperatures. Scanning electron microscope: Images of the aluminium surface were monitored at different magnifications using FEI Quanta model scanning electron microscope. Calibrated Radwag Wagi Elektroniczne (Poland) Semi-micron balance with a precision of 10 μg was adjusted and used to determine the weight loss measurements. All measurements were carried out in triplicate values, averaged, and used as ± SD values.

Results and discussion

Structural characterization of betalain pigments

Natural beetroot extracts of betalain pigments consist of betacyanins and betaxanthins components; betacyanins components are red–purple colour and consist of betanin and betanidin, whereas the betaxanthins components are yellow-orange and consist of indicaxanthin and betalamic acid, respectively (Fig. 1a). The chemical structure of betalain pigments was represented using attenuated total reflectance Fourier transform infrared spectrum as described in Fig. 2b; the stretching frequency of 3414 cm−1 showed the presence of secondary amine group (N–H). The O–H stretching frequency appeared as broad band at a maximum of 3354 cm−1, and the aromatic C–H stretching appeared at 3103 cm−1. The bands at 2977, 2929, and 2874 cm−1 indicated aliphatic C–H symmetry in stretching mode existing in glycosidic groups attached to betanin subgroups. The carbonyl group (C=O) of aldehyde in betalamic acid appeared at 1723 cm−1. The band at C=N bond existing in betanin, betanidin, and indicaxanthin pigments appeared at 1624 cm−1. The asymmetric and symmetric COO− frequencies existing in the four pigments appeared at 1610 and 1406 cm−1, respectively. The conjugated C=C band existing in the four pigments appeared at 1595 cm−1. Finally, the COC stretching of the glycosidic group of betanin appeared at 1044 cm−1, which manifested the backbone structure of the betalain pigments [14, 15].

Figure 2
figure 2

a Chemical components of betalain pigments, b attenuated total reflectance Fourier transform Infrared spectrum of betalain pigments, c UV–Vis spectrum of betalain pigments at different concentrations

Full size image

Likewise, the different constituents of the betalain pigments were qualitatively determined using UV–Vis spectrophotometry as shown in Fig. 2c. Different absorption peaks in the range of 400–600 nm for different concentrations (300, 600, 900, or 1200) ppm were observed. The yellow-orange betaxanthins show one shoulder peak at 454 nm, which corresponds to betalamic acid pigment, and one sharp peak at 482 nm, which corresponds to indicaxanthin pigment. On the other hand, the red–purple betacyanins show two λmax peaks at 535 and 542 nm, which correspond to betanin and betanidin pigments, respectively. Such sharp peaks result from π − π* transitions of the four different betalain pigments, namely betanin, betanidin, betalamic acid and indicaxanthin coexisting with each other [16,17,18,19].

Optimum adsorption conditions of betalain pigments

The optimization of the different conditions meant to achieve optimum conditions for best inhibition of aluminium surface against severe corrosive HCl solution. For this purpose, different parameters such as betalain concentration, operating temperature, and turbulent flow were optimized for maximum inhibition efficiency values against severe corrosive HCl solution. Table 1 illustrates different green inhibitors for different metals with their maximum inhibition efficiency values.

Table 1 Maximum inhibition efficiency on metal surfaces using different green inhibitors
Full size table

The change of inhibition efficiency with betalain concentration is shown in Fig. 3a. The %IE value exponentially increased as betalain concentration increased. Clearly, the increased betalain concentration implied better coverage of aluminium surface, lower weight loss, better inhibition, and better resistance against corrosive HCl molecules. Ultimate inhibition efficiency was obtained at 1200 ppm with 98.4% value. Apparently, the intermolecular forces of betalain molecules with aluminium surface were much more favourable than between betalain molecules with themselves, which pave the way for layer-by-layer adsorption, better geometrical layer formation, and hence better resistance of the layer against corrosive HCl molecules. On the other hand, the effect of operating temperature had an adverse effect on the adsorption process (Fig. 3b), as temperature increased the %IE values deceased to 69.5% for the same inhibitor concentration. This can be explained due to large kinetic energy of aluminium surface that dismisses the betalain molecules away from aluminium surface leading to poorer coverage of the surface and hence poor inhibition against severe HCl molecules. Moreover, turbulent flow is an additional parameter that plays essential role in adsorption of the inhibitor molecules on the surface of aluminium. It measures the frequent structural settlement of the inhibitor on the surface of aluminium, and how successful the formed layer can protect the metal surface against corrosive HCl molecules. Obviously, the increase of turbulent flow led to increased inhibition efficiency (93.7%) up to 200 rpm, which then was sharply decreased to 46.7% at 500 rpm (Fig. 3c). In the range of 50–200 rpm, the inhibitor molecules gain enough kinetic energy to successfully settle on the metal surface, and the frequent speed of solution facilitated the adsorption of the inhibitor on the metal surface. However, in the range of 300–500 rpm a significant reduction in %IE due to high turbulent flow was obtained [27]. High turbulent flow implied high kinetic energy of inhibitor molecules during together with irregular flow of solution that allow desorption and encourage dislocation of inhibitor molecules away from metal surface and hence poor film coating and poor inhibition efficiency value.

Figure 3
figure 3

a Change of inhibition efficiency versus betalain pigments concentration at different temperatures, b change of inhibition efficiency versus temperature at different betalain concentrations, c change of inhibition efficiency versus turbulent flow at 1200 ppm and 10 °C

Full size image

Adsorption properties

Different adsorption isotherms were evaluated to fit to our data such as Langmuir, Temkin, and Freundlich, [28,29,30,31,32,33,34] respectively. Langmuir isotherm was described by the following equation;

$$\frac{\theta }{1 - \theta } = K_{L} C$$
(2)

where KL was the equilibrium constant of adsorption process and C was the betalain pigments concentration used (in ppm). The θ value is the fraction of surface covered by adsorbed molecules and can be determined as follows;

$$\theta = \frac{{W_{0} - W_{i} }}{{W_{0} }}$$
(3)

where \(w_{0}\) and \(w_{i}\) were the weight losses (in kg) of metal surface in the absence and in the presence of betalain pigments, respectively. The linearized form of Langmuir isotherm was as follows:

$$\frac{C}{\theta } = \frac{1}{{K_{L} }} + C$$
(4)

The linear plot of C/θ versus C can be used to determine equilibrium constant (KL) values from reciprocal of intercept value. On the other hand, Temkin adsorption isotherm equation was as follows:

$$\exp \left( { - 2\alpha \theta } \right) = K_{{\text{T}}} C$$
(5)

where α-value was the molecular interaction parameter, KT was the binding constant of the adsorbate with the metal surface. The linear form of Temkin model is:

$$\theta = - \frac{1}{2\alpha }\ln C - \frac{1}{2\alpha }\ln K_{{\text{T}}}$$
(6)

The linear plot of θ versus ln C can be used to determine α and KL from slope and intercept values, respectively. Freundlich adsorption isotherm equation was as follows:

$$\theta = K_{{\text{F}}} C^{1/n}$$
(7)

where the 1/n value describes how easy the adsorption is, easy adsorption when 0 < 1/n < 1, moderate when 1/n = 1 or difficult adsorption when 1/n > 1. KF was the binding constant that describes how strong the betalain pigments bind to the aluminium surface. The 1.n and KF constant values were determined from slope and intercept of the linearized form of Freundlich as follows:

$$\ln \theta = \frac{1}{n}\ln C + \ln K_{{\text{F}}}$$
(8)

Delicate examination of the various adsorption isotherms of our data revealed linear behaviour as described in Fig. 4a–c. Different equilibrium constant values and adsorption parameters that describe the adsorption of betalain pigments on the surface of aluminium are summarized in Table 2.

Figure 4
figure 4

Adsorption isotherms of betalain pigments on aluminium surface: a Langmuir isotherm at different temperatures, b Temkin isotherm at different temperatures, c Freundlich isotherm at different temperatures, d dynamic parameters; ln RC versus reciprocal temperature (1/T) at different concentrations, e thermodynamic parameters; ln KL versus reciprocal temperature (1/T)

Full size image
Table 2 Equilibrium constant values and adsorption parameters of betalain pigments on aluminium surface using different isotherms
Full size table

Obviously, the equilibrium constant values (KL) decreased with increasing temperature, which indicates that desorption process dominates at elevated temperatures. Similarly, the Temkin and Freundlich binding constants (KT and KF) were in favour of poor adsorption of betalain pigments on the surface of aluminium at elevated temperature. The α-values were very close to zero value, which indicated good molecular interaction parameter of adsorbate with adsorbent [35]. Moreover, the values of 1/n were in the range of 0.1–0.3, which strongly indicates excellent and easy adsorption process of betalain pigments on the surface of aluminium [36]. Conclusively, the equilibrium constant values and the different adsorption parameters were in favour of successful adsorption and high affinity of betalain pigments to aluminium surface.

Dynamic parameters

The rate of corrosion (RC) is a very crucial parameter that can describe the success of inhibition process of aluminium surface against corrosive HCl. The rate of corrosion was determined from the following relation:

$$R_{C} = \frac{W}{Vt}$$
(9)

where W was the weight loss of an aluminium coupon, W/V value was the normalized weight loss (in kg/m3) per coupon exposed to corrosive 1.0 M HCl solution for one hour. The rate of corrosion was determined for each W/V value, and the apparent activation energy (Ea) for corrosion process was determined from the Arrhenius equation [37, 38]:

$$R_{{\text{C}}} = A \exp \left( { - \frac{{E_{{\text{a}}} }}{{{\text{RT}}}}} \right)$$
(10)

The apparent activation energy was determined from the slope of ln RC versus reciprocal temperature (1/T) plot, and R was the universal gas constant (8.314 J/mol.K). Figure 4d and Table 3 illustrate the dynamic parameters of the adsorption of betalain inhibitor on the aluminium surface. The rate of corrosion increased as temperature increased at different inhibitor concentrations. However, the apparent activation energy increased fourfold at 1200 ppm with respect to 300 ppm. This clearly indicated that layer-by-layer arrangement of the inhibitor on the surface of aluminium occurred, which allowed better coating and consistent inhibition layers, and hence higher resistance against corrosive HCl molecules.

Table 3 Dynamic parameters of betalain adsorption on the aluminium surface
Full size table

Thermodynamic investigations

Thermodynamic parameters such as ΔG°, ΔH°, or ΔS° describe the type of sorption process (i.e. physisorption or chemisorption), energy enthalpy change, and entropy changes during the adsorption of betalain pigments on the surface of aluminium. Gibbs free energy (ΔG°) can be calculated from the relations [39, 40];

$${{\varvec{\Delta}}}G^\circ = - {\text{RTln}}(55.5K_{L} )$$
(11)

Where KL is the Langmuir equilibrium constant determined from Eq. 4. R is the universal gas constant (8.314 J/mol. K), and T is the operating temperature of adsorption (in K). As shown in Table 4, the Gibbs energy (ΔG°) values of the adsorption of betalain pigments on the aluminium surface demonstrated − 31 kJ/mol for the different temperatures. This clearly indicated that the adsorption of betalain pigments on the surface of aluminium was of physisorption nature [41].

Table 4 Thermodynamic parameters of betalain adsorption on the aluminium surface
Full size table

Substitution of ΔG = ΔH − ΔS and rearrangement of equation 9 could be used to yield a linearized form of the equation, which was used to determine the ΔH° and ΔS° thermodynamic parameters as follows [42]:

$$\ln K_{L} = - \frac{{\Delta H^{^\circ } }}{{{\text{RT}}}} + \left( {\frac{{\Delta S^{^\circ } }}{R} - \ln 55.5} \right)$$
(12)

The plot of ln KL versus 1/T can result with a straight line, for which the enthalpy change (ΔH°) and entropy change (ΔS°) can be determined from slope \(\left( { - \frac{{\Delta H^{^\circ } }}{R}} \right)\) and intercept \(\left( {\frac{{\Delta S^{^\circ } }}{R} - \ln 55.5} \right)\), respectively. The ΔH° and the ΔS° values were − 37.0 kJ/mol and − 20.5 J/mol.K, respectively, as described in Table 4 and Fig. 4e. Negative enthalpy change suggested exothermic nature of the metal dissolution process [43]. Furthermore, negative sign of entropy change suggested less entropy of betalain molecules adsorbed on the surface with respect to free betalain molecules in the solution [42]. Conclusively, the dynamic and thermodynamic parameters suggest successful adsorption of betalain molecules on the surface of aluminium based on electrostatic interaction of partially negative oxygen atoms of betalain with partially positive aluminium ions, leading to the formation of successful protective layers of betalain pigments.

Surface morphology

Morphologic changes of metal surfaces monitored by scanning electron microscope technique gained a great resonance and global attention due to microscale and nanoscale images that describe the corrosion punctures and scratches pre- and post-exposure to severe corrosive 1.0 M HCl solution. Furthermore, it can show how firm the coating material can resist acidic or saline environment [44,45,46]. The as-received aluminium coupon and film-coated aluminium surface pre- and post-exposure to 1.0 M HCl solution are explicitly shown in Fig. 5. Figure 5a demonstrates the great damage of the as-received aluminium surface (which cannot be seen by naked eye) due to bad humid storage, accidents through transportation, and atmospheric erosion. Around 2–10 μm microsized corrosion pitting, punctures, and cracks appeared on the surface. Such pitting could widen and become larger in time period due to atmospheric erosion. On the other hand, film coating of aluminium surface using 900 ppm betalain pigments can be seen in Fig. 4b, c, respectively. Obviously, the treatment with betalain pigment through film coating could yield a smooth surface of aluminium with no cracks, punctures, or corrosion pitting. Entire corrosion pitting and cracks that appeared in Fig. 4a have been well protected and cured by the betalain film coating. This gives a nice clue that betalain molecules were able to act as filler and fill the punctures and cracks appeared on aluminium surface, and to establish consistent and smooth film coating on the aluminium surface. However, the firmness and the durability of the film coating must be exposed to severe corrosive environment to examine and evaluate it. Therefore, the film-coated aluminium coupons were exposed to severe corrosive 1.0 M HCl solution for one hour (Fig. 4d, e). Apparently, the outcomes were stimulating, the uniformity and smoothness of the film coating did not alter, the surface was slightly scratched, and number of corrosion pitting and punctures was strongly minimized. These remarks recommend that betalain pigments coating was firm and durable during the exposure to severe and corrosive 1.0 M HCl solution and hence provides evidence on the technical feasibility for the use of betalain pigments as consistent and effective film coating of aluminium surface.

Figure 5
figure 5

Scanning electron microscope images of a the as-received aluminium surface, b and c 900 ppm betalain film coating over aluminium, d and e 900 ppm betalain film coating over aluminium after been exposed to severe 1.0 M HCl for 1 h

Full size image

Conclusions

Betalain pigments extracted from green beetroots were used as sustainable eco-friendly film coating for aluminium surface. Chemical components of different functional groups forming betalain pigments were characterized using attenuated total reflectance Fourier transform infrared techniques, and the four pigments: betanin, betanidin betalamic acid, and indicaxanthin, were identified using UV–Vis spectrophotometry. Surface morphology of film coating of betalain pigments over aluminium surface revealed smooth, well-cured and filler-like coating, whereas after been exposed to corrosive 1.0 M HCl firm, durable and consistent film coatings were obtained. Various operating conditions examined to meet highest inhibition efficiency values revealed optimum conditions as follows: betalain concentration = 1200 ppm, operating temperature = 10 °C, turbulent flow = 200 rpm, and extreme inhibition efficiency = 98.4%. Several adsorption isotherms revealed equilibrium constant values and adsorption parameters in favour of the adsorption process. Dynamic investigations disclosed decay of rate of corrosion (RC) and increase of activation energy values (Ea) at high betalain concentration, Moreover, the thermodynamic parameters showed physisorption process, exothermic enthalpy, and low entropy properties. The extraordinary advantages of abundance, eco-friendly, low cost, sustainability, and technical feasibility of betalain pigments adapt it to be an uprising sustainable film coating for aluminium surfaces.