The aloe-vera leaf has a bark covering its entire surface. Beneath the bark is a thin vascular layer in the form of a yellow gel. Inside is a white pulp [14] . The leaf structure is shown in Figure 1.

Aloe vera extract is used in this study as a corrosion inhibitor for concrete reinforcing steel. The Aloe vera gel extraction protocol is illustrated in Figure 2.

The Aloe vera leaves were sourced from a supermarket in France. They were then carefully cleaned and washed with demineralized water. Using a knife, the leaf bark was removed and the pulp was washed with demineralized water. The pulp is then ground using a blender, and the liquid filtered through a 0.80 mm sieve. Finally, the filtrate is poured into a plastic bottle lined with two layers of aluminum foil and stored in a refrigerator at 9˚C.

Aloe vera gel contains over 70% saccharides and polysaccharides [15] , particularly in the form of linear glucose and mannose chains known as polymannans due to the high presence of mannose. The chemical structures of some of the most important compounds in Aloe vera gel are shown in Figure 3.

Aloe vera is composed of several high molecular weight heterocyclic compounds [10] [17] containing numerous oxygen atoms in the functional groups and aromatic ring. In most inhibitors, π-electrons and heteroatoms (e.g., O, N) are responsible for adsorbing inhibitor molecules onto the metal surface and protecting against corrosion [18] .

For corrosion measurements, we used 12 mm diameter high adhesion (HA) rebar. The surface exposed to the aggressive solution is equal to 1.13cm² and the rest is covered with epoxy insulation. The surface of the bar is polished with abrasive paper (80 grit), rinsed with distilled water and air-dried before each electrochemical test. A corrosive 0.5M NaCl solution was chosen to simulate sea water.

The experimental set-up used for electrochemical testing is illustrated in Figure 4, and comprises a Potentiostat/Galvanostat connected to an electrochemical cell and controlled by a computer running EC-Lab software. It displays the evolution of the various curves and automatically provides the electrochemical parameters.

The cell consists of a working electrode (WE, HA steel bar), a counter electrode (CE, stainless steel rod) and a saturated calomel reference electrode (RE, SCE).

Measurements begin by monitoring the open-circuit free corrosion potential (OCP) until a quasi-stationary regime is established after 6 hours. Tafel polarization measurements are then started, with a potential sweep around this OCP value. The sweep is performed in a potential range between −0.25 V and +0.25 V/OCP at a speed of 0.1 mV/s. The inhibitory efficiency is calculated from the polarization resistances according to the following formula [19] :

IE = i cor − i cor o i cor × 100

with i cor o and i cor are the current density in the absence and presence of inhibitor, respectively.

Impedance measurements were carried out at free-corrosion potential with an amplitude of 15 mV. Potential sweeps are performed at variable frequencies in the range 200 kHz - 10 mHz with 5 points/decade. All electrochemical curves are recorded using EC-Lab software, which is used to find the equivalent electrical circuit and its various components. Inhibitory efficacy was evaluated using equation [19] :

IE = R ct − R ct o R ct × 100

R ct o and R ct represent the charge transfer resistance in the absence and presence of inhibitor, respectively.

Figure 5 shows corrosion potential monitoring in the absence and presence of different inhibitor concentrations in a corrosive NaCl solution for 6 hours at 20˚C. Examination of Figure 5 shows that the addition of inhibitor shifts the corrosion potential towards anodic values.

In the absence of inhibitor in the corrosive solution, the corrosion potential monitoring curve shows two phases. The first is marked by a decrease in corrosion potential in the first few seconds, from −0.64 V/SCE to −0.72 V/SCE, before stabilizing after 1 h 45 mins. This decrease is attributed to metal dissolution. The second phase is characterized by numerous disturbances on the curve which can be attributed to corrosion of the sample with the formation of corrosion products.

In the presence of Aloe vera in the NaCl solution, there is a considerable reduction in the first phase (from 0 to around 25 min.), i.e. a decrease in metal dissolution. The disturbances seem to stabilize in the presence of aloe vera, indicating a probable protection of the framework by partial adsorption of Aloe vera molecules on the bar surface.

Nevertheless, we note that a non-stationary regime was established after 6 h. Tafel polarization and electrochemical impedance spectroscopy measurements are carried out after 6h of zero-current armature holding in the different solutions.

Figure 6 illustrates the polarization curves of rebar exposed to 0.5 M NaCl solution and different contents (10% to 40%) of Aloe vera plant extract for 6 h. Tafel polarization parameters such as polarization resistance (R_p), free corrosion potential (E_cor), corrosion current density (i_corr), Tafel anodic slope (βa) and Tafel cathodic slope (βc) are presented in Table 1.

The polarization curves show that the presence of Aloe vera extract in the salt solution reduces the anodic and cathodic current intensities. The inhibitory effect of Aloe vera is most pronounced at the 20% level. This grade has the lowest cathodic and anodic branch intensities. In fact, in the presence of inhibiting molecules, a protective layer could form on the surface, blocking the active sites and modifying the surface state. This would lead to a shift of the anodic and/or cathodic branches towards lower current densities [20] . In addition, corrosion potentials shift towards less negative values (anodic side) compared to the control. These findings confirm the anodic character of aloe vera.

Cathodic polarization curves show a slight decrease in current densities in the presence of the inhibitor. However, negligible changes in Tafel slopes are observed in the presence of Aloe vera compared to the uninhibited solution. This shows that the reduction reaction at the steel surface is not altered by the addition of Aloe vera. The decrease in cathodic current density in the presence of Aloe vera extract could be linked to a blocking of cathodic sites without altering the oxygen reduction reaction.

In contrast to the evolution of cathodic branches, a decrease in anodic current densities is observed for solutions containing the inhibitor. This behavior is similar to that observed by Linter and Burstein [21] . It is clear from Table 1 that the addition of Aloe vera extract to the NaCl solution resulted in a decrease in current density and corrosion rate compared to the solution without the inhibitor. Polarization resistance increased from 2.6 kΩ∙cm² (control) to 28 kΩ∙cm² for a 20% content, corresponding to the highest efficiency of 90.4%.

The corresponding EIS spectra, illustrated in Figure 7, show a single capacitive loop, indicating that charge transfer is the dominant phenomenon controlling the corrosion process in the absence and presence of different levels of Aloe vera extract [12] .

The diameter of the semicircles is greater in the presence of Aloe vera extract, indicating better resistance to charge transfer (Rtc) in the presence of Aloe vera extract. However, a higher charge transfer resistance was observed for 20% aloe vera. The equivalent electrical circuit adopted for simulating electrochemical impedance diagrams is shown in Figure 8, and comprises a solution resistor (R_s ) connected in series with a constant phase element (CPE) connected in parallel with a charge transfer resistor (Rtc). Parameters extracted from EIS data in the presence and absence of Aloe vera are shown in Table 2.

The double-layer capacity is determined from the following equation [22] :

C dl = ( RQ ) 1 / n R

Q is used to characterize the EPC according to the values of the coefficient n and to take into account the non-ideal behavior of the capacitance if its value is close to unity.

Values of the empirical coefficient n are close to 1 and range from 0.643 to 0.928. In these cases, EPC can be equated with pure capacitance, with deviation from unity reflecting surface inhomogeneities [23] . Charge transfer resistances are greater in the presence of Aloe vera extract than in the absence of aloe vera. This reflects a decrease in the kinetics of electrochemical reactions in the presence of aloe vera. Charge transfer resistances initially increased from 3.5 for control to 18.5 Ω∙cm² for 10% aloe and then to 29.0 Ω∙cm² for 20% aloe vera. In a second phase, charge transfer resistances decreased for 30% (13.4 Ω∙cm²) and 40% (12.5 Ω∙cm²). This variation indicates a maximum value of 29 Ω∙cm² which corresponds to the optimum content of 20% for which the best inhibition efficiency of 80.7% is recorded. Finally, in parallel with the increase in charge transfer resistance in the presence of the inhibitor, we observe a decrease in double-layer capacity, reflecting adsorption of the inhibitor to the metal surface [13] .

Figure 9 shows photos of the samples after corrosion testing. An orange-colored layer formed on the surface of the reinforcement in the absence of Aloe vera (NaCl + 0% aloe vera). This could be attributed to corrosion products. On the other hand, in the presence of aloe vera, a layer of Aloe vera (AV) products was partially formed on the surface of the reinforcement. The image of the surface of the framework containing a 20% content reveals an almost uniform layer of Aloe vera products. These visual results are in agreement with the electrochemical results.

The absorption mechanism proposed by Mehdipour, shown in Figure 10, illustrates the corrosion and corrosion inhibition process of Aloe vera extracts. Considering that the surface of the framework in the NaCl solution is largely positively charged [24] , an attractive force would be created between the Cl⁻ ions in the solution and the metal surface. The inhibitor being active anticorrosive would also be attracted to the electrode surface by electrostatic force between the positively charged surface and the O atoms or π electrons of the heterocyclic ring. This mechanism well explains the behavior of anodic slopes and the decrease in doubling capacity through the formation of inhibitory complexes, blocking anodic sites (Fe → Fe²⁺ + 2e⁻). It has been shown that physical adsorption could be the crucial step that could then accelerate the formation of the

inhibitor complex on the metal surface [25] . The strong presence of oxygenated functional groups and heterocyclic groups with π electrons in Aloe vera molecules will contribute to the protective mechanism [13] . Studies have mentioned that aloe vera’s anticorrosive properties may be due to Aloe vera components such as aloin and aloe-emodin [9] .