The contribution of cinnamon extract on leaching of aluminum (Al) cook wares was investigated using two aluminum alloys (Indian and Egyptian) and pure Al. The cinnamon (Cin) was extracted by heating the Cin sticks at 90 °C in distilled water for an hour to make the 10% stock solution. This study was done in aqueous solutions in presence and absence of 1% NaCl using weight loss at 90 °C. Moreover surface study (SEM and EDX) and electrochemical methods (Open Circuit Potential and Tafel plot) were applied. The addition of Cin solutions to 1% NaCl decreased the corrosion rates in weight loss and electrochemical method compared to 1% NaCl solutions which showed an inhibitive property of Cin solution. The inhibition was found to obey the modified Langmuir isotherm with a negative Langmuir ΔGads indicating the spontaneous nature of adsorption even at 90 °C. The EDX surface analysis of the Al surface immersed in Cin + NaCl revealed the composition of the pits formed. From Tafel method, it was found that the corrosion current density of pure Al was more than that of the Egyptian alloy. The apparent activation energy values for Cin, Cin + NaCl and NaCl solution were evaluated and discussed.
Aluminum (Al) was regarded a neurotoxin agent because it was associated with some diseases like dialysis encephalopathy and bone disorder due to its accumulation in brain, bones, and liver [
Cinnamon is one of the most popular spices used by mankind. The genus Cinnamomum (family: Lauraceae) consists of 250 species of trees and shrubs distributed in Southeast Asia, China and Australia [
It is well known that Al dissolution is highly dependent on pH, temperature, and the presence of complexing agents. There is a continuous use of Al cook wares in different countries despite its association with serious health problems. Corrosion control of metals is an important activity of technical, economical, and environmental importance. The use of inhibitors is one of the best options of protecting metals and alloys against corrosion. Some inhibitors retard corrosion by adsorption to form a thin invisible film while others form visible precipitates which coat the metal and protect it from aggressive attack. Other inhibitors, when added to an environment, retard corrosion but do not interact directly with metal surface [
The toxicity of inorganic corrosion inhibitors to the environment has prompted the search for green corrosion inhibitors for metals as they are biodegradable and do not contain heavy metals or other toxic compounds. Naturally occurring organic substances and plant extract are widely used as corrosion inhibitors [
The purpose of this paper is to study the leaching behavior of two Al alloys used for Al cook wares (Egyptian and Indian alloys) in aqueous solutions containing cinnamon extract in presence and absence of 1% NaCl at concentrations 1 - 5 w/w at 90˚C to imitate real cooking conditions. Weight loss method, surface study and electrochemical methods (Open Circuit Potential and Tafel plots) were applied. For the electrochemical methods pure Aluminum (Al) was used for comparison in addition to the Egyptian and Indian alloys.
The cinnamon sticks used in this study were bought from the local market. Pure Al (99.99%) from Good fellow was used in addition to two kinds of Al cook wares chosen from the local market from Egypt (Eg alloy) and from India (In alloy). NaCl (AR) was purchased from BDH.
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, Ess ± 250 mV at a rate of 1 mV/s.
Corrosion rate law (CR) was used to calculate Al leaching using the following equation:
where ∆w is the weight loss of Al sample calculated from subtracting the original sample weight minus the sample weight after the immersion experiments (mg); A is the exposed surface area of Al sample (cm2) and t is the immersion time (hour). The alloying elements of the two Al alloys used for Al cook wares (Egyptian and Indian alloys) was determined as shown in
The percentage inhibition efficiency (IE%) and surface coverage (θ) is calculated from applying the following equations:
where CR° and CR are the corrosion rate in the presence and absence of pure Cin.
The weight loss measurements were performed at 90˚C to imitate real cooking conditions. The study was performed in tab water (Tab W) and distilled water (DW). The pH of the solutions in the present study was in the range of 5.4 - 5.5 without NaCl and about 6.3 - 6.5 in presence of NaCl. There was no significant change in pH after the leaching experiments. The study was done in cinnamon solutions w/without NaCl prepared from 10% w/v stock solution: 1%, 2%, 3%, 4%, 5% w/v as shown in
・ The extent of Al leaching in Cin solutions is much less than Al leaching due to distilled water only which reflects the inhibitive effect of Cin solutions.
・ For both alloys, the addition of Cin to NaCl in DW and tab W decreased the Al leaching in all concentrations compared to NaCl only. The percentage of surface coverage of the In alloy is smaller than the Eg alloy. The
% | Ti | Sn | Co | Cu | Cr | Ni | Mo | Fe | Mg | Si |
---|---|---|---|---|---|---|---|---|---|---|
In. | 0.19 | 0.99 | 0.36 | 0.21 | 0.19 | 0.14 | 0.09 | 0.65 | 0.67 | 0.56 |
Eg. | 0.92 | 0.59 | 0.10 | 0.12 | - | 0.20 | 0.06 | 0.50 | 0.46 | 0.60 |
Solution | %C (w/v) | Eg alloy | In alloy | ||||
---|---|---|---|---|---|---|---|
CR*E4 (g/cm2・days) | IE% | Ɵ | CR*E4 (g/cm2・days) | IE% | θ | ||
Dis. W only | ---- | 3.78 | --- | --- | 3.78 | --- | --- |
Tab W only | --- | 8.92 | --- | --- | 10.50 | --- | --- |
NaCl, Dis W | 1 | 7.04 | --- | --- | 8.35 | --- | --- |
NaCl, Tab W | 1 | 10.71 | --- | --- | 11.39 | --- | --- |
Cin (DW) | 1 | 1.34 | --- | --- | 1.44 | --- | --- |
2 | 3.51 | --- | --- | 2.34 | --- | --- | |
3 | 2.95 | --- | --- | 2.79 | --- | --- | |
4 | 2.41 | --- | --- | 2.80 | --- | --- | |
5 | 3.17 | --- | --- | 3.62 | --- | --- | |
Cin + NaCl (DW) | 1 | 3.07 | 56.4 | 0.564 | 6.94 | 16.89 | 0.17 |
2 | 2.76 | 60.5 | 0.605 | 7.07 | 15.33 | 0.15 | |
3 | 2.62 | 62.8 | 0.628 | 6.75 | 19.16 | 0.19 | |
4 | 2.76 | 60.8 | 0.608 | 6.36 | 23.83 | 0.24 | |
5 | 2.75 | 61.1 | 0.61 | 6.28 | 24.79 | 0.25 | |
Cin + NaCl (Tap W) | 1 | 5.00 | 53.3 | 0.533 | 8.55 | 24.93 | 0.25 |
2 | 6.99 | 34.7 | 0.347 | 9.68 | 15.01 | 0.15 | |
3 | 7.32 | 31.7 | 0.317 | 9.46 | 16.94 | 0.17 | |
4 | 7.80 | 27.2 | 0.272 | 9.44 | 17.12 | 0.17 | |
5 | 5.84 | 45.5 | 0.455 | 9.50 | 16.59 | 0.17 |
highest IE% was for 3% w/v where the surface coverage was 62.8. The surface coverage of 1% - 5% w/v using the Eg alloy range between 0.56 - 0.63. On the other hand, the surface coverage of 1% - 5% w/v using the In alloy range between 0.17 - 0.25.
・ From the gravimetric measurement, it is shown that using Tab water (which contained many ions Cu2+, F−, Mg2+) instead of distilled water, increased the CR (Al leaching). This result may be attributed to the aggressive effect of these ions and agrees with other studies [
Different adsorption isotherms were tested in order to find the best one which are:
1) Freundlich and Frumkin isotherms did not fit our data.
2) Langmuir adsorption isotherm gave a good fit to our data (R2 = 0.996).
Langmuir isotherm may be formulated as:
where C is the concentration of inhibitor (ppm), θ is the surface coverage and K is the equilibrium constant of the adsorption process.
From the present study, it was found in
Although there was a good fit to the Langmuir graph as shown in
The value of Kads = 10.97 L/g was calculated from the reciprocal of the intercept with the value of n = 1.58. The free-energy of adsorption (ΔG°ads) was calculated according to the equation:
where 55.5 is the water concentration, R is the universal gas constant, and T is the temperature in Kelvin (363K).
From the adsorption isotherm, the negative value of ΔG°ads indicates that the adsorption process of the cinnamon constituent is spontaneous while adherence to the modified Langmuir adsorption isotherm indicates physisorption with mono coverage. The value being of the order less than 20 kJ/mol indicates that cinnamon was physically adsorbed on the Al surface [
Physisorption involves electrostatic forces between the ionic charges at the metal/interface; this type of adsorption is stable only at relatively low temperatures. On the other hand, chemisorption involves charge sharing or charge transfer from the inhibitor molecules to the metal surface to form a coordinate type bond which is more stable at higher temperatures. Chemisorption has much stronger adsorption energy than physical adsorption [
From the surface study of the, using 5% Cin and theIn alloy, the presence of the white crystals in
that when using the In alloy, the CR of 5% Cin equals 3.62 × 10−4 g/cm2・days and the CR of 5% Cin + NaCl equals 6.28 × 10−4 g/cm2・days with 24.7% IE%. The difference in CR of the two alloys in
The In alloy was taken as an example to investigate the effect of Cin solution because the CR is higher than the Eg alloy. After WL experiments for 5% Cin solution, the In alloy sample was washed with distilled water only and dried. The surface showed a few white depositions after immersion in 5% Cin solution for 1 hour, as shown in
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
The values of Ess 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
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
From the electrochemical methods, the Eg alloy showed an increase in the values of Ess at 60˚C which indicated strengthening of the pre immersion Al oxide layer [
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 Ess 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 [
Tafel method was performed to measure the current density (Icorr) after OCP measurements reached Ess. To study the effect of concentration, the temperature was kept constant at 60˚C. The results are depicted in
1) For pure Al, the Icorr values of Cin solution w/without NaCl are about 1.99 µA/cm2. The constant value of
T (˚C) | 10 | 25 | 45 | 60 |
---|---|---|---|---|
Ess (mV) Cin | −674 | −687 | −779 | −728 |
Ess (mV) Cin + NaCl | −723 | −805 | −893 | −965 |
Alloy | T (˚C) | %C (w/v) | Ba (mV/dec) | −Bc (mV/dec) | Icorr (µA/cm2) |
---|---|---|---|---|---|
Pure Al | 60 | 2 | 106 | 73 | 1.99 |
5 | 97 | 79 | 1.96 | ||
7 | 97 | 79 | 1.93 | ||
10 | 91 | 61 | 1.99 | ||
In Al | 60 | 2 | 86 | 75 | 0.75 |
5 | 84 | 67 | 1.04 | ||
7 | 97 | 107 | 1.25 | ||
10 | 100 | 88 | 1.97 | ||
Eg Al | 60 | 2 | 93 | 85 | 0.73 |
5 | 107 | 65 | 0.85 | ||
7 | 83 | 88 | 1.08 | ||
10 | 128 | 77 | 1.31 |
Icorr 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 Icorr are less than that of the In alloy. The values of Ba increased slightly with increasing concentration while for Bc values there were no regular trend. For the In alloy the values of Icorr increased with increasing concentration. The trend was similar to the trend for Ess. The values of Ba increased but there were no obvious trend for values of Bc.
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
・ For NaCl solutions the Icorr 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.
Sol. | T (˚C) | Ba (mV/dec) | −Bc (mV/dec) | Icorr (µA/cm2) | %IE | CR (mm/day) |
---|---|---|---|---|---|---|
1% NaCl | 10 | 19 | 104 | 0.22 | -- | 0.933 |
25 | 22 | 84 | 0.64 | -- | 2.71 | |
45 | 17 | 256 | 0.91 | -- | 3.86 | |
60 | 21 | 236 | 2.94 | -- | 12.47 | |
5% Cin only | 10 | 79 | 72 | 0.13 | -- | 0.55 |
25 | 75 | 62 | 0.23 | -- | 0.98 | |
45 | 77 | 85 | 0.39 | -- | 1.66 | |
60 | 97 | 79 | 1.02 | -- | 4.34 | |
5% Cin + NaCl | 10 | 70 | 67 | 0.14 | 36 | 0.61 |
25 | 67 | 52 | 0.37 | 42 | 1.54 | |
45 | 67 | 67 | 0.40 | 56 | 1.68 | |
60 | 67 | 75 | 1.14 | 61 | 4.81 |
・ For 5% Cin solution, the values of Icorr increased with increasing temperature. There was a slight increase in Ba values but there were no obvious trend for Bc.
・ For 5% Cin + NaCl, the values of Icorr increased with increasing temperature. There were no obvious trend for the values of Ba and Bc. The values of Icorr of Cin + NaCl were comparable to the values of Icorr 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
From the slopes of the graphs, the value of Ea was calculated and the results are listed in
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
From Tafel method, the values of apparent activation energy (Ea), ΔS and ΔH show the following trend:
NaCl > Cin > Cin + NaCl
The values of Ea for Cin solution was 32.45 kJ /mol. The values for Ea were 36.45 and 28.44 kJ/mol for NaCl and Cin + NaCl solutions respectively. The low value of Ea of Cin solution w/without NaCl compared to NaCl indicates the ease of adsorption of Cin constituents on Al surface. The decrease in Ea after adding Cin to NaCl may indicate a competition between Cin constituents to be adsorbed and Chloride ions to dissolve Al as Ea of Cin + NaCl is smaller than Ea 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
NaCl > Cin > Cin + NaCl
Sol. | pH | Eac (kJ/mol) | ΔH (kJ/mol) | ΔS (J/K.mol) |
---|---|---|---|---|
Cin | 5.00 ± 0.25 | 32.52 | 28.03 | −151.1 |
NaCl | 6.40 ± 0.20 | 36.55 | 34.00 | −124.5 |
Cin + NaCl | 6.40 ± 0.20 | 28.44 | 25.84 | −158.0 |
The decrease in Ea after adding Cin to NaCl may indicated a competition between Cin constituents to be adsorbed on Al surface as Ea of Cin + NaCl is smaller than Ea 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 Ea of Obot et al. in 2011 [
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 Ea 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 [
From WL result, the Al leaching of Cin solutions in DW and Tab W of both the In and Eg alloys at 90˚C was less than the Al leaching without Cin solutions. The addition of Cin solutions in DW or Tab W to NaCl decreased the Al leaching in all concentrations for both Al alloys compared to NaCl solution alone. Comparing the Al dissolution in pure NaCl solution with Cin + NaCl showed that Cin solution has inhibitive effect using both alloys but the IE% was higher in the Eg alloy. The results of Surface Study are in agreement with WL result.
From Tafel method it was found that current density at 60˚C using Al alloys showed the following trend:
Pure Al > In Al > Eg Al
This indicates that some of the alloying elements in the Eg alloy played an inhibitive role in decreasing Al leaching. From Tafel method, the values of Ea, ΔS and ΔH show the following trend:
NaCl > Cin > Cin + NaCl
The solutions of Cin + NaCl were less than those of NaCl only which are in agreement with WL results that Cin is a good inhibitor of Al dissolution. In conclusion using cinnamon in cooking may reduce the extent of Al leaching.
This research project was supported by a grant from the “Research Center of the Female Scientific and Medical Colleges”, Deanship of Scientific Research, King Saud University.
Layla A.Al Juhaiman,Raesah A.Al-Shihry,Hassan M.Al Hazmei, (2015) The Inhibition Effect of Cinnamon Extract on Leaching of Aluminum Cook Wares in NaCl Solutions at Quasi-Cooking Condition. Journal of Surface Engineered Materials and Advanced Technology,05,177-189. doi: 10.4236/jsemat.2015.54020