Corrosion Inhibition of Aluminium in Acidic Medium by Different Extracts of Ocimum gratissimum

Aims: This study is aimed at using plant extracts to prevent corrosion of aluminium in acidic medium. The inhibition efficiency of different extracts of 1M HCl, ethanol and distilled water were determined. Study Design: Gravimetric method was used for the analysis. Place and Duration of Study: Department of Chemistry, Federal University of Technology, Owerri, Nigeria, between March and November, 2011. Methodology: Aluminium sheets of AA1060 and purity 98.98% was used. Each sheet was mechanically press-cut into coupons of dimension 3 cm x 3 cm. The solvents used for extraction of ocimum gratissimum leaf were 1M HCl, ethanol and distilled water, respectively. A hole with diameter 0.5 cm was drilled in each aluminium coupon and suspended in beakers containing test solution using glass hook. The weight loss was determined by retrieving Results: The percentage of inhibition efficiency (% I.E) was calculated at 303 K and 333 K, respectively. The result indicates that % I.E increases as concentration of inhibitor increases. The trend of inhibition efficiency was in order: Distilled H 2 O > C 2 H 5 OH > 1M HCl. The result indicates that increase in temperature decreases inhibition efficiency and degree of surface coverage. The result shows that apparent activation energy E a increases as inhibitor concentration increases. The activation energy of distilled water extract range between 45.02 to 79.90 kJ/mol. While the activation energies of ethanol and 1 M HCl extracts of the inhibitor ranges between 35.12 to 72.93 kJ/mol and 26.60 to 65.25 kJ/mol, respectively. The experimental data obtained corroborated with Langmuir and Flory Huggins adsorption isotherms. Conclusion: This study indicates that different extracts of Ocimum gratissimum inhibits aluminium surface in presence of 1M HCl. The result of the analysis shows that inhibition efficiency and degree of surface coverage decreases as temperature increases. The activation energy of different extracts increases as concentration of inhibitor increases. The negative values of ∆G ads shows that adsorption of inhibitor on surface of aluminium is spontaneous.


INTRODUCTION
Aluminium is a hard, strong, white metal. It is highly electropositive and resistant to corrosion because a hard, tough film of oxide is formed on the surface (Cotton and Wilkinson, 1972). The surface film is amphoteric, hence the metal could dissolve readily in both strong acid and alkaline media. Hydrochloric acid solutions are normally used for pickling of aluminium and electrochemical etching processes that normally lead to substantial loss of metal to corrosion. Aluminium is extensively used in industry as well as domestic applications.
Attempts have been made to reduce aluminium surface film dissolution and hence protect the metal in aggressive acid and alkaline media. Some of the methods employed to reduce corrosion of aluminium is application of sulphur, oxygen or nitrogen containing organic compounds as corrosion inhibitors to hinder corrosion reaction and thus reduce corrosion rate (Moussa et al., 1998;Madkour et al., 1999;Ebenso et al., 2001;Aytac et al., 2005). These organic compounds function by forming a protective adsorption layer on aluminium surface which isolates the corroding metal from action of corrodent.
Corrosion behaviour of aluminium has been studied in acid media. It has been reported that addition of halide salt to sulphuric acid solution containing organic inhibitor, effectively inhibits iron corrosion (Oguzie et al., 2004;Elewady et al., 2008). Halides have been reported to inhibit corrosion of some metals in strong acids and this effect depends on ionic size and charge. The use of chemical inhibitors has been a concern about its toxicity which affects the living organism as well as poisoning of environment. The use of plant extracts to prevent corrosion has become important because they are environmentally acceptable, readily available and renewable source for a wide range of inhibitors. Plant extracts are rich source of naturally synthesized chemical compounds that can be extracted by simple methods with low cost (Abdel-Gaber et al., 2008). These extracts are biodegradable and do not contain heavy metals or other toxic compounds. The use of plant extracts as corrosion inhibitor of metals in acidic and alkaline media have been reported (Abiola et al., 2007;Kliskic et al., 2000;Umoren and Ebenso, 2008;Oguzie, 2005;Eddy and Ebenso, 2008).
Ocimum gratissimum (scent leaf) is widely distributed in tropical Africa especially Nigeria. It belongs to lamiacea family and most abundant of genus Ocimum. The Igbos of Eastern part of Nigeria calls it Nchonwu. The Yorubas of Western part of Nigeria calls it Efinrin ajase, whereas Hausas of Northern part of Nigeria call it Daidoya (USDA, 2008). It is a perennial plant and woody at base. It has an average height of 1-3 m high and leaves are broad, narrowly ovate, usually 5-13 cm long and 3-9 cm wide. It is scented shrub with lime-green fuzzy leaves. It is used in Nigeria for nutritional and therapeutic purposes. In coastal area of Nigeria it is used for treatment of epilepsy, high fever and diarrhea, while in savannah areas, decoctions of its leaves are used to treat mental illness. The Igbos of Eastern Nigeria use Ocimum gratissimum for management of baby's cord by keeping the wound sterile. It is used as antifungal, antibacterial and for seasoning of foods.

Material Preparation
Aluminium sheets of AAI060 and purity 98.98% were obtained from Material and Metallurgical Engineering Workshop, Federal University of Technology, Owerri, Nigeria. Each sheet was 0.1 cm in thickness and mechanically press-cut into coupons of dimension 3 cm x 3 cm. The coupons were descaled using wire brush and degreased in absolute ethanol, dried in acetone, weighed and stored in moisture-free desicator prior to use. The solvents used for extraction of Ocimum gratissimum leaf were 1 M HCl, ethanol and distilled water, respectively. All the solvents used in this study were of analytical reagent grade.

Extraction of Plant
Sample leaves of Ocimum gratissimum were obtained from Ihiagwa market in Owerri, Imo State, Nigeria. The leaf samples were washed and dried under the sunlight and ground to fine powder. 10 g of leaf extract were extracted with Soxhlet extractor for 3 hours using 1 M HCl, ethanol and distilled water, respectively. After extraction the samples were cooled and filtered. The filtrates obtained were used to prepare inhibitor concentrations between 20 mg/l and 100 mg/l in HCl corrodents.

Gravimetric Method
In weight loss experiment, a hole with diameter 0.5 cm was drilled in each aluminium coupon so that it could hang freely in solution. The aluminium coupons were suspended in beakers containing 200 ml of test solution maintained at 303--333 K with glass hooks and rods in a thermo-stated water bath. The weight loss was determined by retrieving coupons from test solutions at 3 hours intervals. The coupons after retrieving from test solution were scrubbed with bristle brush under running water, dried in acetone and weighed (Ebenso et al., 2001). The difference in weight was taken as weight loss of aluminium. From weight loss, the inhibition efficiency (%I.E) of extract and corrosion rate (CR) of aluminium were calculated using equations 1 and 2, respectively. % I.E = (1 -w 2 /w 1 ) x 100 (1) Where w 1 , and w 2 are weight loss of aluminium in absence and presence of inhibitor, respectively. A is area of coupon in cm 2 , T is period of immersion in hours and ∆w = w 1 -w 2 . The degree of surface coverage was calculated using: Where θ is surface coverage; C Rinh is corrosion rate for aluminium in presence of inhibitor, C RBC is corrosion rate for aluminium in the absence of inhibitor.

Inhibition Efficiency and Surface Coverage
The percentage of inhibition efficiency (% I.E) was calculated and represented in tables 1 and 2 at 303k and 333k, respectively. The result revealed that different extracts of inhibitor inhibits corrosion of Al sample in hydrochloric acid solution. The result indicates that % I.E increases as concentration of inhibitor increases. A parameter (θ), which represents part of metal surface covered by inhibitor molecules were calculated for different inhibitor concentrations and represented in tables 1 and 2. The result shows that surface coverage (θ) increases as inhibitor concentration increases. The increase in inhibition efficiency and surface coverage as concentration of extracts increases may be due to the presence of complex chemical composition of extracts. The increase in inhibition efficiency with increase in inhibitor concentration suggests that inhibitor molecule were adsorbed at aluminium sample /solution interface where adsorbed species mechanically screen the coated part of aluminium surface from action of corrosive medium.

Adsorption Consideration
The adsorption of inhibitor on surface of corroding aluminium sample may be regarded as a substitution process between inhibitor compound in aqueous phase and water molecules adsorbed on aluminium surface: Inh aq + xH 2 O ads Inh ads + xH 2 O aq

M H C l E t h a n o l D i s t i l l e d w a t e r
Where x is size ratio, the number of water molecules displaced by one molecule of organic inhibitor. When the equilibrium as described in Eq (4) is reached, it is possible to obtain different forms of adsorption isotherm.
In the present study Langmuir adsorption isotherm was found to be suitable for the experimental findings and had been used to describe the adsorption characteristic of inhibitor. Langmuir adsorption isotherm is expressed in Eq (5) (Schockry et al., 1998).
where C inh is inhibitor concentration, K is equilibrium constant of adsorption, θ is degree of surface coverage.
Figs. 5 and 6 show plot of C inh /θ versus C inh at 303 k and 333 k, respectively for 1M HCl, ethanol and distilled water extracts of inhibitor. Figs. 5 and 6 were linear indicating that adsorption of inhibitor is consistent with assumptions of Langmuir adsorption isotherm meaning there is no interaction between adsorbed species.
Adsorption of inhibitor extracts on Al surface was found to follow Flory -Huggins isotherm expressed in Eq (6) (Oguzie et al., 2004): Where θ is degree of surface coverage, C inh is inhibitor concentration, x is number of water molecules replaced by one inhibitor molecule and K c is equilibrium constant for adsorption process. Figs. 7, 8 and 9 show Flory -Huggins adsorption isotherm for different extracts of inhibitor using 1M HCl, ethanol and distilled water at 303 k and 333 k, respectively. The calculated values for x and k c for 1 M HCl extract of inhibitor were 1.00 and 0.007 for 303 k and 0.40 and 0.74 for 333 k, respectively. The values for x and K c obtained with ethanol extract of inhibitor were 1.0 and 0.006 for 303 k, and 0.30 and 0.63 for 333 k, respectively. Also calculated values for x and k c with distilled water extract of inhibitor were 1.51 and 0.005 for 303 k, and 0.10 and 0.58 for 333 k, respectively. The result indicates that values of x are unity for both 1 M HCl and ethanol extracts, while distilled water extract of inhibitor is more than unity, indicating that each molecule of inhibitor is attached to one active site on Al surface.
The values of free energy of adsorption, G ads of 1 M HCl, ethanol and distilled water extracts of inhibitor on Al surface were calculated using Eq (7) (Noor, 2007): Where R is gas constant, T is temperature and K is equilibrium constant of adsorption, given as K = /1-and 55.5 is concentration of water in solution. The values of ∆G ads were negative in 303k and 333k indicating spontaneous adsorption of inhibitor on surface of aluminium, and the propose mechanism is physical adsorption ( G ads < 40 kJ/mol) (Ebenso, 2004;Bilgic and Sahin, 2001).

Effect of Temperature
The influence of temperature on weight loss of Al sample in 1 M HCl in presence and absence of different concentrations of inhibitor has been depicted in figs. 1 and 2. The increase in temperature decreases inhibition efficiency and degree of surface coverage. The reasons for this observation have been explained earlier in this study. In order to calculate apparent activation energy, E a , for corrosion reaction of Al in absence and presence of various concentrations of inhibitor, the Arrhenius equation was used: log (ℓ 2 /ℓ 1 ) = E a / 2.303R [1/T 1 -1/T 2 ] Where ℓ 1 and ℓ 2 are corrosion rates at temperatures T 1 and T 2 , respectively, and R is gas constant.
The heat of adsorption (Q ads ) of inhibitor on surface of Al was evaluated using equation (9) (Bhajiwala and Vashi, 2001).
Q ads = 2.303R [log (θ 2 /1-θ 2 ) -log (θ 1 /1-θ 1 )] x T 1 T 2 /T 2 -T 1 Where 1 and 2 are degree of surface coverage at temperatures T 1 and T 2 . The calculated values of apparent activation energy E a , and heat of adsorption, Q ads of different inhibitor extracts are shown in Tables 3, 4 and 5, respectively.  The results shown in tables 3, 4 and 5 of different inhibitor extract indicates that apparent activation energy E a , increases as inhibitor concentration increases. Activation energy of distilled water extract ranged between 45.02 to 79.90 KJ/mol. While the activation energies of ethanol and 1 M HCl extracts of inhibitor ranged between 35.12 to 72.93 KJ/mol and 26.60 to 65.25 KJ/mol, respectively. Since corrosion primarily occurs at surface sites free of adsorbed inhibitor, the higher E a values in inhibited solutions imply that extracts mechanically screen the active sites of Al surface thereby decreasing the surface area available for corrosion. The order of apparent activation energy, E a of different extracts was in order: distilled H 2 O > C 2 H 5 OH> 1 M HCl.
The heat of adsorption, Q ads were all negative and ranged from -86.68 to -102.90 KJ/mol for distilled water extract, while ethanol and 1 M HCl extracts ranged from -71.22 to -102.04 KJ/mol and --84.04 to --98.95 KJ/mol, respectively. The negative Q ads values indicate that adsorption and hence inhibition efficiency decreases as temperature increases, supporting the earlier proposed physisorption mechanism (Bhajiwala and Vashi, 2001).

CONCLUSION
This study indicates that extract of Ocimum gratissimum inhibits Al surface in presence of 1 M HCl. The corrosion process is inhibited by adsorption of extracts on aluminium surface. Inhibition efficiency increase with increase in inhibitor concentrations. The result of the analysis shows that inhibition efficiency and degree of surface coverage decreases as temperature increases. Activation energies of different extracts increase as concentration of inhibitor increases. The negative values of G ads shows that adsorption of inhibitor on surface of aluminium is spontaneous. The adsorption behaviour was approximated by Langmuir and Flory -Huggins isotherms. The trend of inhibition efficiency of different extracts was in order: Distilled H 2 O > C 2 H 5 OH>1M HCl.