Introduction: Corrosion is a fundamental process that significantly affects both economic and structural safety, particularly in metals. Among the various methods for corrosion protection, the use of inhibitors is one of the most practical approaches, especially in acidic environments [1]. Most commonly used acid inhibitors are organic compounds containing π-bonds and heteroatoms such as sulfur, nitrogen, oxygen, or phosphorus, which enable adsorption onto the metal surface [2–6].
Inorganic inhibitors, on the other hand, must possess chemical properties that allow them to oxidize the metal and form a passive protective layer. Organic inhibitors require specific structural features—such as large molecular size, double bonds, or reactive functional groups—that facilitate the formation of a tightly bound protective film over the metal surface [1].
Beyond molecular structure, economic and environmental considerations are crucial. Given the substantial financial losses associated with corrosion, effective inhibitors should be inexpensive. Additionally, with increasing awareness of green chemistry, there is a growing demand for nontoxic and environmentally friendly alternatives. Plants offer an abundant source of such safe, cost-effective inhibitors. Green corrosion inhibitors are typically biodegradable and free from heavy metals or other toxic substances.
Several studies have demonstrated the successful application of naturally occurring substances for corrosion inhibition in both acidic and alkaline environments. Various natural products—including low-grade gram flour, honey, onion, potato, gelatin, and extracts from plant roots, leaves, seeds, and gums—have been reported as effective inhibitors. Most investigations have focused on steel and nickel, while studies on aluminum have been limited to mild acidic or basic solutions (millimolar concentrations). Recent research [2–9] continues to explore the use of naturally occurring compounds as green inhibitors, showing promising results. For instance, Khillah extract was reported to inhibit steel corrosion in HCl solutions with up to 99% efficiency [2], while Opuntia extract achieved approximately 96% inhibition for aluminum under similar conditions.
The aim of the present study is to investigate the potential of natural plant extracts as eco-friendly corrosion inhibitors for metals in acidic media. This work focuses on evaluating their inhibition efficiency, understanding the adsorption behavior on metal surfaces, and exploring the structural features responsible for their effectiveness. Such studies provide valuable insights for developing cost-effective, sustainable, and environmentally benign corrosion protection strategies for industrial applications.
2. Experimental
2.1 Materials and solutions
Chemical composition of carbon steel specimens (weight %) is: C 0.2, Mn 0.91, P 0.007, Si 0.002 and the rest is Fe. The carbon specimens before immersion in the test solutions each of them is abraded with emery papers from 300 to 1200 grade size to obtain a smooth surface washed with bidistilled water and then dried using filter papers. AR grade hydrochloric acid (25 %) was used for preparing the corrosive solutions. Appropriate concentration of aggressive solutions used (1 M HCl) was prepared by dilution using bidistilled water.
2.1Weight loss measurements
For weight loss measurements, rectangular C-steel specimens of size 20 x 20 x 2 mm were immersed in 100 ml inhibited and uninhibited solutions and allow to stand for several intervals at 25°C in water thermostat. Therefore, the weight losses given by:
Δm = (m1-m2) (1)
where m1 and m2 are the weights of metal before and after exposure to the corrosive solution, respectively. The percentage inhibition efficiency (% IE) and the degree of surface coverage (θ) of the investigated compounds were calculated from equation (2):
% IE = θ x 100 = [1 – ( Δminh/ Δmfree)] x 100 (2)
Where Δmfree and Δminh are the weight losses per unit area in the absence and presence of additives, respectively
Fig.1: Weight loss-time curves for the dissolution of C-steel in 1 M HCl in the absence and presence of different concentrations of ginger extract at 25 oC
Table 1. Corrosion rate (C.R.) in (mg cm-2 min-1) and inhibition efficiency data obtained from weight loss measurements for carbon steel in 1 M HCl solution in the absence and presence of different concentrations of green inhibitors at 25°C
| Compounds | Conc., g/l | Corrosion Rate (CR), mg cm-2 min-1 | θ | % IE |
| 1 M HCl | 0.0500 | —- | —- | |
| Turmeric | 2 | 0.01166 | 0.7668 | 76.68 |
| 4 | 0.00804 | 0.8393 | 83.93 | |
| 6 | 0.00591 | 0.8824 | 88.24 | |
| 8 | 0.005424 | 0.8915 | 89.15 | |
| 10 | 0.003427 | 0.9315 | 93.15 | |
| Ginger | 2 | 0.0122 | 0.756 | 75.6 |
| 4 | 0.0101 | 0.798 | 79.8 | |
| 6 | 0.00712 | 0.8576 | 85.76 | |
| 8 | 0.00633 | 0.8734 | 87.34 | |
| 10 | 0.00588 | 0.8824 | 88.24 | |
3. Results and Discussion
3.1 Weight – loss measurements
Figure 1-8 shows the effect of concentration of ginger and turmeric extract on the weight loss vs. time of C-steel at 25°C , 35°C , 45°C and 55°C .. It is obvious that the weight loss of C-steel in presence of green inhibitors varies linearly with time, and is much lower than that obtained in blank solution. The linearity obtained indicated the absence of insoluble surface film during corrosion and that the inhibitors were first adsorbed onto the metal surface and, therefore, impede the corrosion process [14]. The calculated values of the percentage inhibition efficiency (% IE) at different concentrations of green inhibitors extract in 1 M HCl at different temperatures (25- 45°C) are given in Tables 1 and 2. From these Tables, the inhibition efficiency increases by increasing green inhibitors concentrations and decreases by increasing in temperature. This behavior could be attributed to the increase of the number of adsorbed molecules at the metal surface and desorption of adsorbed molecules from the metal surface occurs by raising the temperature.
Fig.2: Weight loss-time curves for the dissolution of C-steel in 1 M HCl in the absence and presence of different concentrations of ginger extract at 35 oC
Table 2. Data of weight loss measurements for carbon steel in 1 M HCl solution in the absence and presence of different concentrations of green inhibitors at 35-55°C.
| Compound | Conc., g/l | 35°C | 45°C | 55°C | |||
| θ | % IE | θ | % IE | θ | % IE | ||
| Turmeric | 2 | 0.7490 | 74.9 | 0.7014 | 70.14 | 0.6992 | 69.92 |
| 4 | 0.7818 | 78.18 | 0.7145 | 71.45 | 0.7069 | 70.69 | |
| 6 | 0.8055 | 80.55 | 0.7523 | 75.23 | 0.7225 | 72.25 | |
| 8 | 0.8357 | 83.57 | 0.7718 | 77.18 | 0.7284 | 72.84 | |
| 10 | 0.858 | 85.8 | 0.794 | 79.4 | 0.776 | 77.6 | |
| Ginger | 2 | 0.5012 | 50.12 | 0.4550 | 45.50 | 0.4211 | 42.11 |
| 4 | 0.532 | 53.2 | 0.471 | 47.1 | 0.455 | 45.5 | |
| 6 | 0.705 | 70.5 | 0.602 | 60.2 | 0.549 | 54.9 | |
| 8 | 0.772 | 77.2 | 0.662 | 66.2 | 0.602 | 60.2 | |
| 10 | 0.842 | 84.2 | 0.713 | 71.3 | 0.638 | 63.8 | |
3.2 Adsorption isotherms
Basic information on the interaction between the inhibitors and the C-steel can be provided by the adsorption isotherm. Two main types of interaction can describe the adsorption of the organic compound: physical adsorption and chemical adsorption. These are influenced by the chemical structure of the inhibitor, the type of the electrolyte, the charge and nature of the metal. The surface coverage, θ, of the metal surface by the adsorbed inhibitor was evaluated from weight loss measurements using equation (2). The θ values of different inhibitor concentrations at 25°C were tested by fitting to various isotherms including, Frumkin, Langmuir, Temkin and Flory-Huggins. By far the best fit was obtained with the Langmuir isotherm is given as [15]:
C /θ = 1/Kads + C (5)
A plot of (C /θ) against C, for all concentrations of inhibitors (Figure 9-10) a straight line relationship was obtained in all cases with correlation coefficients (R2) in more than 0.994. Where C is the inhibitor concentration and Kads is the equilibrium constant of adsorption process and is related to the standard free energy of adsorption ΔG˚ads by equation (6):
Kads = 1/55.5 exp (-ΔG°ads/RT) (6)
The value of 55.5 is the concentration of water in solution expressed in mole per liter, R is the universal gas constant and T is the absolute temperature. The deviation of the slope from unity as observed from this study could be interpreted that there are interactions between adsorbed species on the metal surface as well as changes in adsorption heat with increasing surface coverage [16, 17], factors that were ignored in the derivation of Langmuir isotherm. The negative ΔG°ads values (Table 3) are consistent with the spontaneity of the adsorption process and the stability of the adsorbed layer on the C-steel surface [18]. It is generally accepted that the values of ΔG°ads up to -20 kJ mol-1 the types of adsorption were regarded as physisorption, the inhibition acts due to the electrostatic interaction between the charged molecules and the charged metal, while the values around -40 kJ mol-1 or smaller, were seen as
chemisorptions, which is due to the charge sharing or a transfer from the inhibitor molecules to the metal surface to form covalent bond [19-20]. The ΔG°ads values obtained in this study range from –36.2 to –36.7 kJ mol-1. It suggested that the adsorption mechanism of investigated inhibitors on C-steel in 1 M HCl solution was typical of physisorption.
Fig.3: Weight loss-time curves for the dissolution of C-steel in 1 M HCl absence and presence of different concentrations of ginger extract at 45 oC
Table 3. Thermodynamic parameters for the adsorption of three compounds on carbon steel surface in 1 M HCl at different temperatures
| -∆S◦ads, J mol-1k-1 | -∆H◦ads, kJ mol-1 | -∆G°ads, kJ mol-1 | Kads x104, M-1 | Temp., K | Compounds |
| 109.56 | 77.23 | 43.89 | 85.71 | 298 | Turmeric |
| 107.72 | 43.90 | 62.53 | 308 | ||
| 107.78 | 43.34 | 40.23 | 318 | ||
| 110.12 | 42.07 | 28.81 | 328 | ||
| 71.91 | 59.81 | 42.93 | 60.21 | 298 | Ginger |
| 74.24 | 41.86 | 28.64 | 308 | ||
| 75.78 | 41.02 | 17.27 | 318 | ||
| 74.43 | 41.06 | 13.31 | 328 |
3.3 Kinetic-thermodynamic corrosion parameters
As noticed previously, the adsorption process was well elucidating by using a thermodynamic model, in addition a kinetic-thermodynamic model was another tool to explain the mechanism of corrosion inhibition for an inhibitor. The apparent effective activation energies (E*a) for the corrosion reaction of C-steel in HCl in the absence and presence of different concentrations of investigated compounds were calculated from Arrhenius-type equation [21]:
k=A exp (-Ea* / RT) (7)
where A is the Arrhenius pre-exponential factor. A plot of log k (corrosion rate) vs. 1 / T gave straight lines as shown in (Figure 11). The entropy of activation (ΔS*) and the enthalpy of activation (ΔH*) for the intermediate complex in the transition state for the corrosion of C-steel in HCl in the absence and presence of different concentrations of investigated compounds were obtained by applying the transition-state equation [22-24]
k = RT / Nh exp (∆S*/R) exp (-∆H* / RT) (8)
where h is the Planck’s constant and N is the Avogadro’s number
A plot of log k (corrosion rate) / T vs. 1 / T should give a straight lines (Figure 11), with a slope of (-∆H* / 2.303R), and an intercept of [(log (RT / Nh) + (∆S*/2.303R)] [25-26], from which the values of ∆H* and ∆S* were calculated, respectively. (Table 4) exhibited values of apparent activation energy, apparent enthalpies ∆H* and entropies ∆S* for C-steel dissolution in 1 M HCl solution in the absence and presence of different investigated compounds. The presence of inhibitors decreased the activation energies of C-steel indicating strong adsorption of the inhibitor molecules on the metal surface and the presence of these additives induces the adsorption of theses additives on the surface of C-steel. Values of the entropy of activation ΔS* in the absence and in presence of the studied compounds are negative .This implies that the activated complex in the rate determining step represents an association rather
than a dissociation step [27]. This means that the activated molecules were in higher order state than that at the initial stage [28-29].
CONCLUSIONS
From the overall experimental results the following conclusions can be deduced:
Green inhibitors (ginger and turmeric ) are good inhibitors and act as mixed type but mainly act as mixed type inhibitors for carbon steel corrosion in 1 M HCl solution. The results obtained from weight loss measurements showed that the inhibiting action increases with the inhibitors concentration and decreases with the increasing in temperature. Double layer capacitances decrease with respect to blank solution when green inhibitors extract are added. This fact confirms the adsorption of the investigated compounds molecules on the carbon steel surface. The adsorption of green inhibitors on the carbon steel surface at different temperature was found to obey the Langmuir adsorption isotherm and this adsorption is phsicosorption.
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