Chemistry Department, Yarmouk University, Irbid – Jordan
Received date: 13/06/2015; Accepted date: 19/11/2015; Published date: 23/11/2015
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The inhibition effect of Cleome droserifolia leaves extract on the corrosion of aluminum metal in 1M NaOH solution has been investigated using the weight loss technique as a function of extract concentrations and temperatures at 25, 35, 45 and 55°C. Also, the surface morphology of Al metal was analyzed using scanning electron microscope. The plant extract of 14 g/l at 35°C has the highest inhibition value for the corrosion of aluminum metal with 78.6% inhibition efficiency. The values of the inhibition efficiency increase with increasing of the extract concentration and found to be temperature dependent. Entropy and enthalpy of activation were calculated and discussed. The adsorption of Cleome droserifolia extract on Al-surface obeys Temkin adsorption isotherm with high regression coefficients.
Cleome droserifolia, Temkin isotherm, Aluminum, NaOH, Corrosion, Terpenoids.
The plant, Cleome droserifolia (CD), grows in stony regions in Jordan and it has been subjected to severe overexploitation to be used in folk medicine for diabetes [1-6]. According to literature, the extract of the (CD) plant composed from multi natural organic compounds such as substituted thiocyanates, carotols, flavonoids and terpenoids. These naturally occurring organic substances are rich with hetero atoms such as oxygen, nitrogen, and sulfur substituents. Some of these naturally occurring substances, such as terpenoids are large branched and diverse class of organic compounds with different molecular formulas such as C15H26O2 (25 mg/ml), C34H52O7 (5 mg/ml) and C35H58O6 (7 mg/ml) .
According to literature, the extract of naturally occurring substances played an important role in the corrosion chemistry of metals and alloys due to their inhibition effect, low cost and renewable sources of materials. A lot of investigations were done on the effect of leaves extract or seeds of different naturally occurring substances as corrosion inhibitors of different metals and alloys [8-11].
Corrosion of aluminum and its alloys has been the subject of numerous studies concerning the inhibition of aluminum corrosion using organic substances [12-14]. Herein, the effect of (CD) leaves extract on the corrosion of Al metal in 1M NaOH solution was studied as a function of temperature and leaves extract concentration using the weight loss method. The surface morphology of Al metal was analyzed by using electron scanning microscope.
Stock solution of (CD) plant leaves extracts was obtained by drying the plant for 4 hours in an oven at 80ºC and grinded to powder 7.0 g of the dried powder leaves was refluxed in 500 ml of 1M sodium hydroxide for 2 hours. The refluxed solution was filtrated to remove any contamination and then stored. The concentrations of the test solutions were 2.8, 5.6, 8.4, 11.2 and 14 g/l in 1M NaOH. Pure aluminum metal sheets (Aldrich), of dimensions 3.0 × 1.0 × 0.03 cm with a purity of 99.96% were cleaned using etching solution composed from 85% H3PO4 and 15% HNO3 at 80ºC, washed with distilled water, then placed in a solution of 40% (wt/v) sodium hydroxide at 50ºC for 20 seconds. The aluminum specimens rinsed again with distilled water, immersed in 1:1 (v/v) HNO3, washed with distilled water and then dried and stored in desiccator. For weight loss determination, the aluminum specimens were hanged and immersed in the vessel of a test solution containing 10 ml of 1M NaOH and then with adding different extract concentrations to the 1M NaOH (2.8-14 g/L) at 25-55ºC using water thermostat controlled to ± 0.5ºC. After 1 hour, the aluminum specimens of each test were taken out and washed in acetone then rinsed with distilled water, dried and reweighed by using analytical balance with precision of 0.0001 g. Duplicate measurements were performed for each test solution to insure reproducibility.
The weight loss of the aluminum metal in the corrosive solution (1M NaOH) is given by equation 1.
ΔW=Wa – Wb ………….. (1)
Where Wb and Wa are the weight of Al metal before and after immersion in the test corrosive solution, respectively.
The variations of the weight loss (mg) of Al in 1M NaOH solution at different immersion times containing various concentrations of (CD) extract at 25ºC are shown in Figure 1.
According to Figure 1, initial addition of the (CD) extract solution (up to 5.0 g/l) in 1M NaOH has a remarkable effect on decreasing the weight loss values of Al metal in the corrosive NaOH medium, i.e., the corrosion inhibition strengthened, further increase of the (CD) extract concentration (>5.0 g/l) was followed by very small decrease in the weight loss values. The inhibition effect of the (CD) extract may be resulted from the adsorption of the available various naturally occurring organic compounds in the (CD) extract solution at the surface of Al metal through adsorption of atoms with excessive π electrons such as sulfur, nitrogen, and/or oxygen atoms at the surface of Al metal, leading to a decrease in the corrosion rate of Al metal in 1M NaOH solution, thus the Al surface is efficiently separated from the corrosive NaOH medium  especially at low (CD) extract solutions. Also, the weight loss values of Al increase as the immersion time increases at the same (CD) concentration, due to the corrosion of Al with NaOH solution. As a result, these naturally occurring organic compounds create a barrier between the metal surface and the corrosive media leading to an inhibition in the rate of corrosion.
The effect of temperatures on the corrosion of Al in 1M NaOH, after 1 hour immersion time over the temperature range from 25 to 55ºC in the absence and presence of different concentrations of (CD) extract solutions has been studied and presented in Figure 2. The weight loss values of Al metal were found to increase with increasing temperature and decrease as the concentrations of (CD) extract increase at the same temperature. The same behavior was obtained at 35, 45 and 55ºC in comparison with that obtained at 25ºC, where a remarkable drop in the weight loss values was obtained initially and almost kept constant at concentrations higher than 5.0 g/l
The percentage inhibition efficiency (%I) and the degree of surface coverage (Θ) of Al metal in 1M NaOH in the presence and absence of (CD) extract solutions were computed from equations 2 and 3. The calculated values of inhibition efficiency (%I) and the surface coverage (Θ) are collected in Tables 1 and 2, respectively.
|Extract conc. (g/l)||%I at 25 ºC||%I at 35 ºC||%I at 45 ºC||%I at 55 ºC|
Table 1. %I at different (CD) extract solutions and temperatures after 1 h immersion time.
|Extract conc. (g/l)||Θ at 25μC||Θ at 35 μC||θ at 45 μC||Θ at 55 μC|
Table 2. Θ values at different extract (CD) concentrations and temperatures after 1 h immersion time.
%I=(ΔW)/Wb × 100 (2)
Table 1 shows the percentage inhibition efficiency of the (CD) extract inhibitors at various concentrations in 1M NaOH after 1 h immersion time at different temperatures (25-55ºC). The Results showed that the values of the inhibition efficiency increased as the concentration of the (CD) extract increased from 2.8 to 14 g/l (Figure 3). The maximum inhibition efficiency was 78.8% at 35ºC for 14 g/l of extract concentration. According to Table 1 and Figure 3, the followings were observed:
a) At the same temperature, the %I values increase as the concentrations of (CD) extract concentration increases.
b) At the same (CD) extract concentration, the %I values were increased as temperature increases from 25 to 35ºC reaching a maximum at 35ºC followed by a decrease at 45 and 55ºC, respectively.
c) Above 5.0 g/l extract concetration, the %I increase slightly with increasing extract concentration at all temperatures.
The above mentioned observations could be explained as
1- Adsorption of the naturally organic compounds in the (CD) extract solutions via the rich π electron of N, S and/or O atoms was occurred over the temperature range from 25 to 55ºC and at the different (CD) extract concentrations through an increase in the %I values, thus the Al- surface is efficiently separated from the corrosive media .
2- The presence of large and branched naturally occurring molecules such as terpenoids in the (CD) extract solution will be adsorbed partially at the Al-surface (steric effect), causing weak adsorption. Dilute extract solutions (below 5.0 g/l) containing low concentrations of terpenoids will be adsorbed freely at the surface of Al metal, producing much higher inhibition effect than at high concentration
3- The above mentioned steric effect also explains the trend of inhibitions observed at 45 and 55ºC (negative Qad values, ), where adsorption became more weaker than that obtained at 35ºC and 25ºC (positive Qad vlues), due to an increase in kinetic energy of molecules with increasing temperature, which overcome the interaction between adsorbent-adsorbate. The same explanations can be obtained from the variations of (Θ) with temperature and (CD) extract concentrations.
The corrosion rate (CR) of Al is calculated according to equation 4:
Where A and t are the surface area of Al metal and t the immersion time, respectively. Logarithmic plot of the corrosion rate versus the reciprocal of temperature (T) in Kelvin gives straight lines (Figure 4) according to the following Arrhenius equation :
Where A, R and Ea are the pre-exponential factor, universal gas constant (8.314 J/K mol) and the apparent activation energy, respectively. The values of Ea were calculated and listed in Table 3. The Ea for the reaction of Al metal in 1M NaOH solution was found to be 57.3 kJ/mol. Upon addition of the (CD) extract solution, the values of Ea were dropped and reaching 43.7 kJ/mol at 14.0 g/l extract concentration. The value of Ea is higher in the uninhibited solution (in NaOH) compared to that obtained with (CD) extract solutions, this implies that the (CD) extracts functioned through adsorption at the surface of Al metal via mixed inhibition mechanisms under the studied conditions .
|Extract conc. (g/l)||r2||Ea (kJ/mol)||r2||ΔH* (kJ/mol)||ΔS*(J/mol.K)||Qads (kJ/mol) ΔT1ΔT2|
Table 3. Activation parameters for the dissolution of Al in 1M NaOH in with and without of (CD) extract after 1 h immersion time.
An alternative formulation of the Arrhenius equation is Eyring formula  for the transition state (equation 6)
CR=(KbT/h) exp(ΔS*/R) exp (-ΔH*/RT) (6)
Figure 5 shows a plot of log (CR/T) against (1/T, K-1). Straight lines were obtained and from their slopes and intercepts, the values of ΔH* and ΔS* were calculated and listed in Table 3. The values of ΔH* were endothermic nature due to Al metal dissolution. It has been mentioned elsewhere [8-11] that physisorbtion prevailed with ΔH* values less than 48kJ/mol, whereas, ΔH* values approaching 100 kJ/mol indicating chemisorptions . In this study, the values of ΔH* were less than 48kJ/mol confirming physical adsorption. The values of ΔS* in the absence of the (CD) extract inhibitors are negative and became more negative in the presence of the (CD) extract solutions. This could be attributed to the libration of large fraction of more disordered water molecules that were adsorbed on the surface of the metal and replaced with less disordered organic compounds in the (CD) extract solution. This implies that the activation complex is the rate determining step, representing association rather than dissociation mechanism (adsorption at the surface of Al metal by the various naturally organic compounds), indicating that an increase in order takes place on going from reactant to the activated complex .
The heat of adsorbed materials (Qads) at the surface of Al metal in the presence of the (CD) extract inhibitor values of the adsorbed materials at the surface of Al metal is calculated by the following equation .
Qads=2.303R [log (Θ2/1- Θ2)-log (Θ1/1- Θ 1)] × (T2T1/T-2T1) (7)
To explain the previous behavior, where maximum adsorption took place at 35ºC, two different temperature ranges were used upon applying equation 7. The first temperature range (ΔT1), in which %I increases with increasing temperature from 25 to 35ºC, while the second temperature range (ΔT2), where %I decreases with increasing temperature from 45 to 55ºC. Qads values (Table 2) in the first temperature range were positive and decreases with increasing (CD) extract concentration. In the second temperature range, the values of Qads were small negative values and increases with increasing (CD) extract concentration.
The positive values of Qads suggests an increase in adsorption efficiency in the first temperature range (ΔT1) and a decrease in adsorption efficiency in the second temperature region range (ΔT2) . Generally, the adsorption efficiency is greater in the first temperature than in the second one.
The inhibition effect of (CD) extract solutions for the reaction of Al metal in 1.0M NaOH solution obeys Temkin adsorption isotherm relationship  as represented by equation 8.
Θ=A+B log C (8)
Where A and B are constants, respectively.
This isotherm takes the account of adsorbate–adsorbent interaction and it assumes that the activation adsorption heat (function of temperature) of all molecules in the layer would decreased linearly rather than logarithmic with increasing coverage .
The variation of Θ vs log C is shown in Figure 6, where straight lines with high R2 values (>0.96), which confirms the applicability of Temkin adsorption isotherm.
Scanning electron microscope was used to investigate the topologic changes of Al surface in presence and absence of (CD) plant extract in 1.0 M NaOH after 1 h immersion time. The surface of Al metal infected remarkably in the presence of corrosive NaOH solution only as shown in Figure 7A through the formation of an irregular voids along the surface of the metal sheet, indicating clear attack of Al metal by NaOH solution. On the other hand, Figure 7B showed the image for the surface of Al metal in 11.2 g/l (CD) extract solution, the surface of Al metal is smoother than that obtained in the corrosive NaOH medium, due to the formation of a protective layer at the surface of Al metal.
The following main Conclusions can be deduced from the present study:
• The (CD) extract was found to be a good inhibitor for Al metal in 1M NaOH solution.
• The %I values were increased with increasing the (CD) extract concentration at the same temperature to attain a maximum value of 78.8% (14.0 g/l) at 35ºC.
• The %I values of %I and Qad were found to be temperature dependent, where maximum adsorption obtained at 35ºC
• The Ea values for Al metal in NaOH solution in the presence of the (CD) inhibitor were less than Ea values obtained for Al metal in NaOH solution without the (CD) inhibitor.
• Temkin adsorption isotherm was found to fit the data obtained for the adsorption of (CD) extract at the Al-surface