Corrosion Inhibition of Aluminum in 0.5 M HCl by Garlic aqueous extract

Saedah R. Al-Mhyawi¹

King Abd El-Aziz university, Girls College of Education, Chemistry Department, Jeddah, KSA^.

DOI : http://dx.doi.org/10.13005/ojc/300218

The inhibition efficiency of extract of Garlic on aluminium in hydrochloric acid solutions has been evaluated by weight loss techniques. Values of inhibition efficiency obtained are dependent upon the concentration of inhibitor and temperature. Generally, inhibition was found to increase with inhibitor concentration, half-life, activation energy but decrease with temperature and first-order rate constant at the temperatures studied. Physical adsorption mechanism has been proposed for the inhibition and Langmuir , Temkin adsorption isotherm was obeyed. Garlic is an inhibitor of aluminium corrosion in 0.5  M hydrochloric acid solution.The values of standard free energy of adsorption suggest that the adsorption of inhibitor on aluminium surface occurred by physisorption mechanism. the negative sign of the Free Energy of adsorption indicates that the adsorption of the inhibitors on the aluminum surface was a spontaneous process.the negative values of enthalpy of adsorption (ΔH) suggest that the chemical reaction involved in the adsorption of the inhibitors on the metal surface is an exothermic process, hence increase in the reaction temperature of the medium will decrease the inhibition efficiency.

Corossion; inhibition; Langmur; Temkin adsorption isotherm; half-life; activation energy; free energy.

having dimension R =1.37 L=5.38 cm with exposed area of 26.090417 cm². Befor all measurements,the specimen was mechanically polished.This was achieved by using a series of emery papers with different grade (320-600-1200), starting with a coarse one and proceeding in steps to fine grades. After this process the specimen was washed with double-distilled  water, degreased with aceton  and finally , dried with a stream of air .The specimen was then weighed and immersed in the test solution.

preparation of extract (inhibitor) and Test Solution

The aggressive of 0.5 M HCl was prepared by dilution of AR grade(%)  HCl with distilled water.

The inhibitor used was acetone extract of Garlic (allium) skin. The skin of Garlic (allium) bulb was obtained locally from Panda market,jeddah. The Garlic (allium)  skin was boiled with 50 ml acetone and 50ml water mixture. The resulting Garlic (allium) solution was heated to dryness; the powder obtained was then scrapped out and stored in a sample bottle. Six different concentrations (0.0025g/dm³,0.0050 g/dm³,0.0075g/dm3, 0.0100g/dm³, 0.0125g/dm³ and 0.0250g/dm³) of the extract were prepared with 0.5 M hydrochloric acid solution and were used for all measurement .

Figure 1 A Click here to View Figure

Methods

The corrosion rate of Al in 0.5 M HCl followed by using chemical methods . This method is done in, weight loss method(WL).

Weight loss study using the  Garlic (allium) acetone extract

Experiments were conducted for the immersion times 30,60,90,120,150,180,210 mins. Preweighted aluminium specimens were suspended for different immersion periods in 0.5 HCl with and without the inhibitor in different concentrations ranging from concentrations (0.0025, 0.0050, 0.0075, 0.0100, 0.0125 and 0.0250 g/dm3) of the extract into seven separate beakers maintained at 303, 313and 323K.  .After the specified time the coupons were removed from test solution, thoroughly washed with double distilled water, dried well and then weighted ^21,22. The percentage of inhibitor efficiency (IE%) for various concentrations of inhibitor were calculated as ²³

%IE = [ (ΔM_u-ΔM_i) / ΔM_u] x 100    (1)

Where ΔM_u is mass loss without inhibitor and Δ M_i is mass loss with inhibitor.

Results And Discussion

Effect of Reaction Conditions on Corrosion Rate and Inhibition Efficiency of Garlic Extract

Effect of inhibitor Concentration (Gravimetric results)

The weight losses (gravimetric measurements) for the Aluminium  in 0.5 M HCL containing different concentrations of the Garlic  extracts as a function of time are presented in Figures. 1,2,3. The results show that weight losses increase with increase in time but decrease with increase in concentration of Garlic  extracts.

As shown in Figures 1,2,3, the inhibition efficiency of the extracts on the mild steel increases with increasing concentration of the plant extracts. This is expected because as the concentration of the plant extracts increases, the fraction of the surface covered by the adsorbed molecule also increases which results into an increase in the inhibition efficiency. The inhibition efficiency increases progressively as the concentration of the extracts increases However, the inhibition efficiency decreases with increase in the temperature of the reaction medium.

At 313 K, as the concentration of Garlic skin extract increases from 0.0025 to 0.0250 g/dm3, the weight losses of the aluminium coupons reduce as shown in Figure 2.This shows us that Garlic skin extract is still effective in inhibiting the corrosion of aluminium at 313 K. The weight loss of the aluminium coupons still reduced with increasing extract concentration as seen in Fig. 3. This depicts that, even at 323K, Garlic skin extract inhibits the corrosion of aluminium in hydrochloric acid solution.

The decrease is due to the inhibitive effects of Garlic  extracts  and these effects increase with increase in Garlic  extracts  concentration. This behavior indicates that the Garlic  extracts is  strongly inhibit the corrosion of the aluminium and the degree of inhibition is concentration dependent.

Fig1:Variation of Weight Loss with Time for aluminium Coupons in 0.5 M HCl Solution Containing Different Concentrations of Garlice extract at 303. Click here to View Figure

Fig2: Variation of Weight Loss with Time for aluminium Coupons in 0.5 M HCl Solution Containing Different Concentrations of Garlice extract at 313 K.  Click here to View Figure

Fig 3: Variation of Weight Loss with Time for aluminium Coupons in 0.5 M HCl Solution Containing Different Concentrations of Garlice extract at 323 K. Click here to View Figure

There is a progressive increase in weight loss as the temperature is increased from 303K to 323K (Figures 1,2,3). This signifies that the dissolution of the metals increased at higher temperatures. This observation is attributed to the general rule guiding the rate of chemical reaction, which says that chemical reaction increases with increasing temperatures. Also an increased temperature favors the formation of activated molecules, which may be doubled in number, with 10^oC rise in temperature,thereby increasing the reaction rate. This is because the reactant molecules gain more energy and are able to overcome the energy barrier more   rapidly²⁴. An increase in temperature may also increase the solubility of the protective films on the metals, thus increasing the susceptibility of the metal to corrosion ²⁵. The solubility of oxygen gas decreases with increase in temperature.

Thus oxygen concentration is expected to be more at higher temperature which in this case is higher at 323K than at 313 and 303K. The presence of high concentration of oxygen thereby causes the metal to corrode faster. Also for solids, solubility generally increases with increasing temperature. This explains why the protective film which is solid becomes more soluble as the temperature is increased.

Effect of Temperature on Inhibition Efficiency

The effect of temperature on the inhibition efficiency for aluminium in 0.5 M HCl solution in the absence and presence of different concentration of Garlice extract at temperature ranging from 303K to 323 K was obtained by weight loss measurements is displayed graphically in Figure 4.The inhibition efficiencies are found to decrease with increasing the solution temperature from 303 to 323 K.This behavior can be interpreted on the basic that relatively high temperature the increase in temperature results in the desorption of the inhibitor molecules from the surface of aluminium²⁶. Previous investigators showed that the corrosion rate increases with increase in temperature ²⁷, which results into a decrease in the inhibition efficiency, Therefore, decreasing the reaction temperature favours the inhibition efficiency of acid extracts of Garlice extract on aluminium in hydrochloric acid.

Figure 4 also portrays an increase in inhibition efficiency of Garlice extract as the concentration of the extract increases in the acid solution.

However,aluminium corrosion rate in absence and presence of  the Garlice extract obeys Arrhenius type reaction ^28.

1npwl = 1nA – E_a/ RT——-(2)

where r_WL is the corrosion rate obtained from WL measurements,E_a is the apparent activation energy,R is the molar gas constant,T is the absolute temperature and A is the frequency factor

In order to calculate the apparent activation energy of aluminium corrosion in the absence and presence of inhibitor,a plot of lnr_WL vs.T^-1was done as shown in Figure 5.The slope (E_a/RT) of the straight lines was used to calculate the apparent activation energy while the intercept(ln A) was used to calculate the frequency factor.Table 2 gives the calculated kinetic parameters.

As was shown in Table 2,the activation energy of aluminium corrosion in the inhibited solution using the Garlice extract was endothermic and has greater than that of the uninhibited solution,suggesting that higher energy barrier for the corrosion process in the inhibited solution associated with physical adsorption or weak chemical bonding between the inhibitor species and Garlice aluminium surface^29.

Table 2:  Activation Energy values with the various concentrations of the extract

Fig4: Variation of Inhibition Efficiency with Inhibitor Concentration for aluminium Coupons in 0.5 M HCl Solution Containing Garlice extract at Three Different  Temperatures. Click here to View Figure

Fig5: Arrhenius plots for aluminium corrosion in 0.5 M HCl in absence and presence of different concentration of Garlice extract at 120 mint Click here to View Figure

Adsorption Parameters

Adsorption isotherms are very important in determining the mechanism of organo-electrochemical reaction. The inhibition of the corrosion of aluminum in 0.5M HCl medium with addition of different concentrations of the extract can be explained by the adsorption of the components of the plant extract on the metal surface.

The principal step in the accomplishment of inhibitiors action in acid solution is usually established to be the adsorption on the metal surface.This leads to the assumption that the corrosion reactions are prohibited from occurring over the area(or active sites) of the metal surface enclosed by adsorbed inhibitior species,whereas these corrosion reactions take place on the inhibitor-free region³⁰ .

The surface coverage(θ) data are very useful on discussing the adsorption characteristics.When the fraction of surface covered is determined as a function of the concentraction at constant remperature,adsorption isotherm could be evaluated at equilibrium condition.Attemts were made to fit the θ values to various isotherm including Langmuir,Temkin,Frumkin,ElAwady,Freundlich and Flory-Huggins ect.The best fit was obtained with Langmuir isotherm as suggested by the plot between C/θ and C (as shown in Figure 6) and the linear correlation coefficient of the fitted data was close to 1,indicating that the adsorption of the inhibitor molecules obey the Langmuir’s adsorption isotherm as expressed   as

Cθ^-1=1/K_ads.+C_inh.                (3)

Where C_inh. is the inhibitor concentration and K_ads.is the equilibrium constant for adsorption/desorption process of the inhibitor molecules on the metal surface. K_ads. Values were calculated from the intercept of the plot for adsorption process and related with the free energy change of adsorption ,∆G⁰_ads,by the following equation

,∆G⁰_ads= -RT ln(k_ads.C_water)     (4)

The Langmuir adsorption parameters and the values of ,∆G⁰_ads in thepresenceofGarlice extract are given in table 3.Inspection of Table 3 reveals that the negative values of ,∆G⁰_ads point to the strength of the adsorbed layer on the Aluminum surface and naturalness of the adsorption process with the formation of an adsorbed film on the Aluminium surface ³¹ .

The plot of θ  vs log C was linear for Garlice extract (see Figute 7). The plots support the assertion that the mechanism of corrosion inhibition is due to the formation and maintenance of a protective film on the metalsurface and that the additive covers both the anodic and cathodic sites through uniform adsorption ³². The fit of the experimental data to these isotherms provide evidence for the role of adsorption in the observed inhibitive effect of the Garlice extract.

Fig6: Langmuir isotherm for Garlice extract: Click here to View Figure

Fig7: Temkin isotherm for Garlice extract Click here to View Figure

Table 3:Adsorption Equilibrum constant (Kads) at different Temperatures

Thermodynamic Study

Determination of Enthalpy and Entropy

Thermodynamic parameters such as enthalpy (ΔH) and entropy (ΔS) of activation of  corrosion process may be evaluated from the effect of temperature. The enthalpy and entropy of activation of corrosion process was calculated from the equation

Log CR/T = log (R/nh ) + ∆S/2.303R-∆H/2.303RT……………..(5)

Where „CR‟ is the corrosion rate, „T‟ is the absolute temperature , „R‟ is the molar gas constant, „n‟ is Avogadro‟s constant, and „h‟ is the Planck‟s constant. A plot of log CR/T vs 1/T is a straight line graph (see Figure 8) with a slope of (-∆H/2.303R ) and an intercept of (log (log (R/nh ) + ∆S/2.303R ). From the slope and  the intercept, ΔH and ΔS were calculated as reported by Abiola et al  ^13,33.

The results presented in Table 4 show that the enthalpy of activation values were all negative for Garlice extract which reflects the exothermic nature of the aluminum dissolution process. Also, the entropies of activation were negative indicating that the activation complex represents association steps and that the reaction was spontaneous and feasible. These results were in excellent agreement with the previous study ³⁴.

Free Energy of Adsorption

The free energy of adsorption values, ΔG°_ads,were obtained by using equation (8) and the values obtained are presemted in Table 5.

ΔG^°_ads= -2.303RT log (55.5K_ads) ………………………………… (6)

Results obtained indicate that the values of ΔG^°_ads are negative in all cases, showing that the reaction is spontaneous ³³ and that the Garlice extracts are strongly adsorbed on the aluminum  surface by physical adsorption. Previous investigators showed that the values of ΔG°_ads up to -20 kJmol^-1 are consistent with electrostatic interaction between charged molecules and a charged metal (which indicates physisorption), while those more negative than -20 KJ mol-1 involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate type of bond (which indicates chemisorption) . This observation also supports the earlier assertion that a physical adsorption is proposed as a result of electrostatic attraction between charged metal surface and charged species in the bulk of the solution. The ΔG values which are less than 10.5 kJ/mol for various inhibitor concentrations (Table 5) revealed the decreased rate of corrosion reaction which is also substantiated by other studies ^13,33.

Fig8: Plot of logCR/T  vs 1/T for  Garlice extract  Click here to View Figure

 Kinetic Study

 Determination of Rate Constant

The corrosion reaction is a heterogeneous one, composed of anodic and cathodic reactions with the same or different rate. It is on this basis that kinetic analysis of the data is considered necessary.

In this present study, the initial weight of aluminum coupon at time, t is designated Wi, the weight loss is ∆W and the weight change at time t, (Wi – ∆W). The plots of log (Wi – ∆W) against time (min) at 303 K and other temperatures studied, showed a linear variation which confirms a first order reaction kinetics with respect to the corrosion of aluminium corrosion in 0.5 M HCl solutions in the presence of the Garlice extract (Figure 9). The rate constant, half-life time and activation energy were calculated as shown in our earlier report ³⁵. There is a

Table 4:Enthalpy and Entropy values of the reaction with various concentrations of the extracts Concentration of Extract (g.dm3) ΔH (KJ mol-1) ΔS (J mol-1K-1)

Table 5: The Free energy of Adsorptions at various Temperatures

general decrease in the rate constants from 303K – 323K with increasing concentrations of the red onion skin extract  (Table 6).

Determination of Half Life

The values of half-life, t_1/2, were calculated using the equation:

t_1/2 =0.693/k

Fig9: Variation of Log (Wi – ∆W) with time (mins) for aluminium coupons in 0.5M HCl solution containing Garlice extract at 303K.  Click here to View Figure

The increase in half-life (_t1/2) shown when the the Garlice extract is present further supports the inhibition of aluminium in 0.5 M HCl by the additives. The increase in half life indicates more protection of the metals by the the Garlice extract. On the basis of the experimentally determined activation energy value (50.45 kJ mol^-1 at 303 – 313 K), the additive is physically adsorbed on the coupons.Therefore, it is probable that a multilayer protective coverage on the entire aluminium surface was obtained ¹³.

Phytochemical Constituents

Garlic contains at least 33 sulfur compounds like aliin, allicin, ajoene, allylpropl, diallyl, trisulfide, sallylcysteine, vinyldithiines, S-allylmercaptocystein, and others. Besides sulfure compounds garlic contains 17 amino acids and their glycosides, arginine and others. Minerals such as selenium and enzymes like allinase, peroxidases, myrosinase, and others. Garlic contains a higher concentration of sulfur compounds than any other Allium species. The structure of the main representative compounds in Garlic are shown in Figure 10.

Fig10: The structure of some compounds present in Garlic plant  Click here to View Figure

Table 6:Kinetic data for aluminium in 2M HCl containing red onion skin extract from weight loss measurement.

The above said phytochemical constituents present in Garlic  having many active center such as sulfer,oxygen and nitrogen,which are regarded as centers of adsorption.Where the results of temperatures study have shown that mechanical adsorption takes place through physical adsorption,this effect of the extract cannot be ascribed to a definite component.It was found that aluminum surface in acid media have a positive charge³⁶,which leads to electrostatic attraction of the negatively charge species(in this case Cl^– ions).In aqueous acidic solutions main constituents of Garlic exist either as neutral molecules or as protonated molecules(cations).So in view of the above the adsorption mechanism may occur as follows

•  First,the acid’s anions(Cl^–) adsorb physically on the positively charged metal surface,resulting surface charge modification significantly to negative charge.
• Second,the protonated molecules are physically attracted to the anions layer which is formed on the metal surface.

The degree of protection increases with increase in extract concentration due to higher degree of surface coverage resulting from enhanced inhibitor adsorption.

From Figure 10  reveal of the chemical structures of these phytochemical constituents reveal that these compounds are easily hydrolysable and the compounds can be adsorbed on the metal surface via the lone pair of electrons present on their oxygen which make a barrier for charge and mass transfer leading to decrease the interaction of the metal with the corrosive environment. As a result, the corrosion rate of the metal was decreased. The formation of film layer essentially blocks the discharge of H⁺ and dissolution of the metal ions. Due to electrostatic interaction, the protonated constituent‟s molecules are adsorbed (physisorption) and high inhibition is expected.

Conclusion

1. The acid extract of Garlic acts as good and efficient inhibitor for the corrosion of aluminum in hydrochloric acid medium.
2. Inhibition efficiency increases with the increase of extract concentration and decreases with rise in temperature .
3. The adsorption of different concentrations of the plant extract on the surface of the aluminum in 0.5 M HCl acid followed both Langmuir and Temkin adsorption isotherm.
4. The values of standard free energy of adsorption suggest that the adsorption of inhibitor on aluminium surface occurred by physisorption mechanism. the negative sign of the Free Energy of adsorption indicates that the adsorption of the inhibitors on the aluminum surface was a spontaneous process.
5. The negative values of enthalpy of adsorption (ΔH) suggest that the chemical reaction involved in the adsorption of the inhibitors on the metal surface is an exothermic process, hence increase in the reaction temperature of the medium will decrease the inhibition efficiency.
6. The negative values of entropy of adsorption indicate that the reaction was spontaneous and feasible.
7. Mechanism of physical adsorption is proposed and a first order type of reaction is obtained from the kinetic treatment of the data.

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