The best method which was adopted was heating cinnamon sticks at 90˚C in distilled water for an hour to make 10% w/v solution. This method is suitable because it resembles real cooking condition. The cinnamon extract stock solution was used to prepare the following concentrations (1%, 2%, 3%, 4%, 5%). These concentrations are close to those used in real cooking.

To study Al leaching of Al alloys weight loss method (WL) was employed in cinnamon solutions w/without NaCl. The mean of two to three replicate experiments was reported. The two Al cook wares were cut into rectangular specimens of dimensions 2 × 2.2 cm and 2 mm thickness. The average exposed area was 9.0 ± 0.10 cm². The following procedure was done prior to each experiment: wet polishing with emery paper (400 - 1200) grade, washing with distilled water, then ultrasonic cleaning in acetone for 2 minutes, then air drying. The Al samples were weighed in a sensitive balance then immediately immersed in the hot cinnamon solution for 1 hour.

Ultrasonic cleaner from Cole Palmer was used to clean the Al specimens before corrosion experiments. A sensitive balance (±0.0001 g from Metler Toledo) was used to weigh the Al specimens before and after corrosion experiments. After the leaching experiments the Al samples were immersed in a cleaning solution of 2% CrO₃ + 5% H₃PO₄ at 80˚C for 10 minutes to remove the reaction products. Finally it was washed with distilled water (DW) and acetone then it was weighed again to determine the weight loss. All experiments were performed in aerated solutions and maintained at 90˚C ± 1˚C.

After the leaching experiments some of the Al samples were kept for surface study. The surface morphology of Al samples was studied using a Scanning Electron Microscope (SEM) from (Joel-JSM-6380). Moreover the Al surface after weight loss was analyzed by Energy Diffraction X-Ray (EDX).

For the electrochemical studies, two cells were used to estimate the small changes of Al leaching using a galvanostat/potentiostat (from ACM). The first one was a three electrode cell and the working electrode was Pure Al (99.99%) with an exposed surface area of 0.20 cm². The second cell was a sample holder from Radiometer and the working electrode was cut into circular disks from Al cook wares with 1.4 cm diameter and an exposed area of 1.13 cm². The reference electrode for both cells was saturated Calomel electrode (SCE) and the auxiliary electrode was Platinum. All electrochemical experiments were performed in aerated solutions and maintained at 60˚C ± 1˚C using a circulating water bath (from Haak). After running open circuit potential for 1 hour, Tafel measurements were done to obtain the corrosion current density by scanning the potential, E_ss ± 250 mV at a rate of 1 mV/s.

To detect any small changes in Al leaching, pure Al was used in addition to the two Al alloys. Two methods were applied: Open Circuit Potential Method followed by Tafel. All experiments were performed in aerated solutions at 60˚C ± 1˚C. As an example, the effect of change of temperature on OCP of pure Al for 5% Cin solutions w/without NaCl is shown in Figure 4 and Figure 5.

The values of E_ss decreased with increasing temperatures with and without NaCl. The effect of changing the temperature from 10˚C - 60˚C on OCP was studied only for pure Al at 5% Cin solutions w/without NaCl as

shown in Table 3. The values of E_ss decreased after addition of chloride ions and decreased with increasing temperature. The electrochemical behavior of Al in presence of NaCl and the effect of temperature using the following equation was studied [22] :

When applying Equation (5) for pure Al at 5% Cin + NaCl, it gave a linear relation for temperature (R² = 0.992) with a negative slope indicating thinning of the pre-immersion oxide film with increasing temperature as shown in Figure 5.

From the electrochemical methods, the Eg alloy showed an increase in the values of E_ss at 60˚C which indicated strengthening of the pre immersion Al oxide layer [25] . The change of the potential with time from OCP method reflected the effect of attack of the constituents of the solution. The resulting E_ss is a mixed potential of all the anodic and cathodic reactions. It is known that E_ss depends on O₂ concentration, composition of Al, pH and temperature [25] . The values of E_ss for 5% - 10% Cin solutions were almost constant at 710 - 715 mV. For the In alloy it showed the same trend as the pure Al. On the contrary, the Eg alloy showed an increase in the values of E_ss at 60˚C which indicated strengthening of the pre immersion Al oxide layer [25] . The values of E_ss decreased after addition of chloride ions and decreased with increasing temperature which indicated thinning of the pre-immersion oxide film. This phenomenon resembled the behaviour found for 8-hydroxy quinoline by Garrigues et al. [26] . The pH of the present study was 6.40 ± 0.20 in presence of NaCl solutions which is close to the neutral range. Mazhar et al., explained the competition between OH^? and Cl^? in neutral solutions which is explained in the following equation [27] :

The use of aerated solutions in the present study means that the cathodic reaction of oxygen reduction was applicable (Jones, 1995). Assuming that this was the cathodic reaction with the possible anodic reaction:

The resulting mixed potential is close to E_ss obtained in the present study. The attack of Al₂O₃ may occur through voids and defected sites. A previous model by McCafferty considered that the penetration of Cl⁻ ions can occur by film dissolution or by migration through the oxygen vacancies [21] . These defected sights are more prone to attack at high temperatures. Moreover using labeled chloride ions Kolice had shown that Cl⁻ ions are incorporated to a distance of 10 - 15 Å from the interphase of oxide/solution [20] chloride ions were adsorbed chemically on Al oxide film and reacted as partners in Al dissolution by forming oxy chloride ionic complexes.

Tafel method was performed to measure the current density (I_corr) after OCP measurements reached E_ss. To study the effect of concentration, the temperature was kept constant at 60˚C. The results are depicted in Figure 6. The following results were obtained as shown in Table 4:

1) For pure Al, the I_corr values of Cin solution w/without NaCl are about 1.99 µA/cm². The constant value of

I_corr regardless of the increase in concentration indicates that the effective parameter in pure Al dissolution is the temperature.

For the Eg Alloy, the values of I_corr are less than that of the In alloy. The values of B_a increased slightly with increasing concentration while for B_c values there were no regular trend. For the In alloy the values of I_corr increased with increasing concentration. The trend was similar to the trend for E_ss. The values of B_a increased but there were no obvious trend for values of B_c.

To study the effect of increasing the temperature on Tafel method, the electrochemical parameters were recorded for 1% NaCl pure Al at 5% Cin solutions w/without NaCl at different temperatures as shown in Table 5. The following observations may be drawn:

・ For NaCl solutions the I_corr values were larger than their values for Cin w/without NaCl at the same temperature. This result agrees with the aggressive role of Cl⁻ ions in Al dissolution in neutral solutions.

・ For 5% Cin solution, the values of I_corr increased with increasing temperature. There was a slight increase in B_a values but there were no obvious trend for B_c.

・ For 5% Cin + NaCl, the values of I_corr increased with increasing temperature. There were no obvious trend for the values of B_a and B_c. The values of I_corr of Cin + NaCl were comparable to the values of I_corr of C in solutions for each degree of temperatures. The EI% values range between 36% - 61% which is in accordance with WL results.

To gain more insight about adsorption, the parameters of Tafel method were used to calculate some thermodynamic parameters for 1% NaCl and Cin w/without NaCl using the following Arrhenius equation:

Using Table 5, and applying Equation (8) to calculate E_a of pure Al in Cin solutions w/without NaCl gave straight lines. The apparent activation energy values were calculated by plotting ln I_corr versus 1/T.

From the slopes of the graphs, the value of E_a was calculated and the results are listed in Table 6.

Evaluating the thermodynamic parameters was done using the transition state equation,

where h is Planck’s constant, N is Avogadro’s number, ΔS is the entropy of the activation, and ΔH is the enthalpy of activation.

Drawing ln CR/T vs. 1/T gave a straight line with a slope of −∆H_°/R and an intercept of (ln R/Nh) + ∆S_°/R) as shown in Figure 7 for Cin w/without NaCl solution. The calculated parameters at 5% concentrations of the inhibitor are collected in Table 6.

From Tafel method, the values of apparent activation energy (E_a), ΔS and ΔH show the following trend:

NaCl > Cin > Cin + NaCl

The values of E_a for Cin solution was 32.45 kJ /mol. The values for E_a were 36.45 and 28.44 kJ/mol for NaCl and Cin + NaCl solutions respectively. The low value of E_a of Cin solution w/without NaCl compared to NaCl indicates the ease of adsorption of Cin constituents on Al surface. The decrease in E_a after adding Cin to NaCl may indicate a competition between Cin constituents to be adsorbed and Chloride ions to dissolve Al as E_a of Cin + NaCl is smaller than E_a of Cin alone. This is in agreement with the trend of CR from WL results that Cin is a good inhibitor of Al dissolution in presence of NaCl.

From Table 6, the trend for the thermodynamic values (ΔH, ΔS) and E_a is:

NaCl > Cin > Cin + NaCl

The decrease in E_a after adding Cin to NaCl may indicated a competition between Cin constituents to be adsorbed on Al surface as E_a of Cin + NaCl is smaller than E_a of Cin alone. This is in agreement with the trend of CR from WL results that Cin is a good inhibitor of Al dissolution in presence of NaCl. This result agrees with the trend of E_a of Obot et al. in 2011 [17] and also with a previous study by Al Juhaiman et al. about Polyvinyl Pyrrolidone inhibition of carbon steel in neutral medium containing NaCl [28] . However, some researchers reported a large increase of E_a for the blank + inhibitor compared to the blank although the CR of the metal decreased in presence of the inhibitor [11] [12] [15] . From the present study, the value of E_a for Cin solution shown in Table 6 was 32.45 kJ/mol. The values for E_a were 36.45 and 28.44 kJ/mol for NaCl and Cin + NaCl solutions respectively. The low value of E_a of Cin solution w/without NaCl compared to NaCl indicated the ease of adsorption of Cin constituents on Al surface. It was surprising that the value of E_a of Cin + NaCl is less than the E_a of Cin alone. It is a well-known fact that metal dissolution is pH dependent as shown from Pourbaix diagrams. Thus we think that the pH difference between the two solutions and Kolice et al. findings in 2001 may explain this ambiguity. The oxide film thickness on Al surface is strongly dependent upon the solution pH. Using XRF, Kolice et al. found “a slight thickness variation in the passive region of aluminum (4 < Ph < 10) which represented a film thickness of 40 - 45 Å” [20] . The pH of the present study was 6.40 ± 0.20 in presence of NaCl and 5.00 ± 0.25 in presence of Cin solution. From Table 2, Cin solution has inhibitive properties depicted in the lower CR compared to Cin + NaCl and NaCl solutions. Thus we may postulate that due to the aggressive role of chloride ions, the thickness of the Al oxide film was less thicker for Cin + NaCl compared to Cin solutions which needed a lower E_a value as found in Table 6.

The difference in ΔH and ΔS values between Cin and Cin + NaCl solutions may be explained by the difference in pH using the same reasoning discussed for E_a values. It is known that the positive sign of the enthalpies reflect the endothermic nature of the metal dissolution process. It is noticed that ΔH of Cin + NaCl solution is less than ΔH of NaCl. That means that the adsorption process in presence of the inhibitor needs less energy compared to the blank which makes Al dissolution less favored and agrees with the results in WL and Tafel methods.

The elevated temperature had an adverse effect on adsorption process, as the intermolecular and intramolecular forces were weakened [10] [15] . For the change of entropy, Table 6 shows that ΔS of NaCl solution is larger than that of Cin w/without NaCl which means the increase in disorder in NaCl solutions than in presence of the inhibitor. The decrease in the value of ΔS for Cin w/without NaCl compared to NaCl only is logical as the inhibitor contains many constituents which may be bound to the Al surface by physical bonds as revealed from obeying the modified Langmuir adsorption isotherm which decrease the randomness in the system (increase the order). Although the present trend agree with some studies [17] [28] [29] ; but the findings disagree with other studies as well [11] [12] [15] [30] .