Corrosion Inhibition of Aluminium in Acid Media By Citrullus Colocynthis Extract

Inhibition of corrosion of aluminium in acid solution by methanol extract of Citrullus colocynthis plant has been studied using mass loss and thermometric measurements. It has been found that the plant extract act as a good corrosion inhibitor for aluminium in all concentrations of sulphuric and hydrochloric acid solution. The inhibition action depends on the concentration of acid and inhibitor. Results for mass loss and thermometric measurement indicate that inhibition efficiency increase with increasing inhibitor concentration. The inhibition action of the plant extract is discussed in view of Langmuir adsorption isotherm. It has been observed that the adsorption of the extract on aluminium surface is a spontaneous process. The plant extract provides a good protection to aluminium against corrosion.


Introduction
Corrosion is a natural phenomenon, which can be considered either chemical or electrochemical in nature, degrades the metallic properties of metal and alloys make them unfit for specific role.Aluminium is a soft, durable, lightweight, malleable metal with appearance ranging from silvery to dull gray, depending on the surface roughness.Aluminium is nonmagnetic and nonsparking.Aluminium and its alloy recommended for building purpose and for various internal outfits, at various industries and highly polluted places.Aluminium is remarkable for the metal's low density and for its ability to resist corrosion to some extent due to the phenomenon of passivation.But its corrosion takes place in aqueous acidic conditions.
The corrosion of aluminium and its alloy in acid solutions have extensively been studied [1][2][3] .Some chemicals as corrosion inhibitor are currently used in industry to prevent or to reduce the corrosion rates of metals in acid media.Due to toxic nature and high cost of these chemicals it is necessary to develop environmentally acceptable and less expensive inhibitors.Some natural products have been evaluated as potential corrosion inhibitors for different metals [4][5] .Natural product of plant origin contain different organic compounds (viz-Alkaloids, Steroids, Amino acids, Tannins, Flavanoids etc.) and most are known to have inhibitive action [6][7] .
The present communication describes the influence of various concentration of the extract of Citrullus colocynthis plant on corrosion behavior of aluminium in acid media by mass loss and thermometric measurements.

Chemical constituents of plant
The plant has very high medicinal value 8 .The plant contains three antitumor ingredients: cucurbitacin B, cucurbitacin E and the D-glcoside of beta-sitosterol.The pulp contains colocynthin, extractive, a fixed oil, a resinous substance insoluble in ether, gum, pectin, water and calcium and magnesium phosphates.Seeds contain the phyto sterolin, two hydrocarbons, an alkaloid, glycoside and tannin 9 .Roots contain hentriacontane and saponin.

Experimental
The Citrullus colocynthis extract was obtained by drying the plant part, then finely powered and extracted with boiling in methanol.The solvent was distilled off and the residue was treated with activated charcoal to remove gangue and pure extract of Citrullus colocynthis was used as probable corrosion inhibitor for aluminium metal.Inhibition efficiency was calculated by two methods.

Mass loss method 10
Rectangular size specimens of aluminium metal foils of dimension 2.5 x 1.5 x 0.03 cm containing a small hole of about 3 mm diameter near the upper edge were employed for the experiment.Specimens were cleaned by buffing to produce a mirror finish with the help of emery paper and then weighed.Each specimen was suspended by a V-shaped glass hook in a beaker containing 50 mL of test solution (acidic) at room temperature and left exposed to air.The acidic solution was prepared by using doubly distilled water and evaporation losses were made up with doubly distilled water.After the experiment, specimens were took out of the beaker, cleaned with benzene and weighed again.A set of duplicate experiments were performed in each case and mean values of the mass loss were calculated.All chemicals used were of analytical reagent quality.The degree of surface coverage (θ) can be calculated as: θ = ∆Mu-∆Mi / ∆Mu Where, ∆Mu and ∆Mi are the mass loss of the metal in uninhibited and inhibited acid respectively.The corrosion rate in mmpy (millimiles per year) can be obtained by the following equation: Mass loss x 87.6 Corrosion rate, mmpy = Area ×Time × Metal density Where mass loss is expressed in mg, area is expressed in cm² of metal surface exposed, time is expressed in hours of exposure, and metal density is expressed in gms/cm³and 87.6 is a conversion factor.The percentage inhibition efficiency (η%) was calculated as : Thermometric method 11 Inhibition efficiencies were also determined using a thermometric technique.This involved the immersion of single specimen measuring 2.5 x 1.5 x 0.03 cm in a reaction chamber containing 50 mL of test solution.Temperature changes were measured at interval of one minute using a thermometer with a precision of 0.01 K.The temperature first increased slowly, then rapidly and attained a maximum value and then start decreasing.The highest temperature was recorded.Percentage inhibition efficiency was calculated as: (η %) = 100 (RN free -RN i ) / RN free Where RN i and RN free are the reaction number in the presence and absence of inhibitors respectively and RN (K/min) is defined as: RN = (Tm-To) / t Where Tm and to are the maximum and initial temperature respectively and t is the time required to reach the maximum temperature.

Results and Discussion
Values of mass loss, surface coverage, percentage inhibition efficiency (η%) and corrosion rate at different concentrations of acid and inhibitor are given in Table 1 and 2. It is observed that the inhibition efficiency increases with increasing acid and inhibitor concentration because the inhibitor ionizes more easily under more acidic solution and is more easily adsorbed on the metal surface, thus increasing the inhibition efficiency.All the inhibitors reduce the corrosion rate to a significant extent.The maximum efficiencies are obtained at 1 N hydrochloric acid concentration.The inhibitor shows efficiencies in the range from 71.84% to a maximum 87.29% for methanol extract (Table 2).Inhibition efficiency values were also determined by using thermometric techniques.Temperature change for aluminium in 3 N, 4 N and 5 N hydrochloric acid and sulphuric acid solution were recorded at various inhibitor concentrations.However no significant temperature changes were recorded at an acid concentration of 1 N and 2 N hydrochloric acid solution.Therefore, use of the thermometric method was restricted to 3 N-5 N hydrochloric acid and sulphuric acid solution.The results summarized in Table 3 and 4 are in broad agreement with those obtained from mass loss measurement.The variation of reaction number with inhibitor concentrations, presented graphically shows essentially linear behavior with the negative slop indicating that the reaction number decreases with increasing inhibitor concentration.Adsorption plays an important role in the inhibition of metallic corrosion by inhibitors.Many investigators have used the Langmuir adsorption isotherm to study inhibitors characteristics 12 assuming that the inhibitors adsorbed on the metal surface decrease the surface area available for corrosion.Langmuir adsorption isotherm should give a straight line of unit gradient for the plot of log [θ /1-θ] versus log C. log [θ /1-θ] = log A + log C -(θ / 2.3RT) Where, θ is degree of surface coverage, A is a temperature independent constant and C is a concentration of the inhibitor.The corresponding plot (Figures 1-4) is linear but the gradients are not equal to unity as would be expected for the Langmuir adsorption isotherm equation.This deviation from the unity may be explained on the basis of the interaction among the adsorbed species on the metal surface.

Conclusion
The study of the extract of plant Citrullus colocynthis in different acid concentration solutions have proved them to be effective inhibitors of the corrosion of aluminium giving up to 87.29% inhibition efficiency.Its adsorption on aluminium obeyed Langmuir adsorption isotherm.The results shows that the plant is the rich sources of ingredients which have very high inhibition efficiency have become important as an environmentally acceptable, readily available, less toxic and renewable source for wide range of inhibitors.

Figure 1 .
Figure 1.Langmuir adsorption isotherm of leaves extract for aluminium in 0.5N H 2 SO 4 with inhibitor concentration

Figure 2 .
Figure 2. Langmuir adsorption isotherm of leaves extract for aluminium in 1.0N H 2 SO 4 with inhibitor concentration

Figure 3 .
Figure 3. Langmuir adsorption isotherm of leaves extract for aluminium in 0.5N HCl with inhibitor concentration

Figure 4 .
Figure 4. Langmuir adsorption isotherm of leaves extract for aluminium in 1N HCl with inhibitor concentration.

Table 1 .
Mass loss, surface coverage, inhibition efficiency and corrosion rate for aluminium in 0.5 N and 1.0 N H 2 SO 4 solution with methanolic extract of Citrullus colocynthis for 24 h at 298±0.1 K

Table 2 .
Mass loss, surface coverage, inhibition efficiency and corrosion rate for aluminium in 0.5 N and 1.0N HCl solution with methanolic extract of Citrullus colocynthis for 24 h at 298±0.1K

Table 3 .
Reaction number (RN) and percentage inhibition efficiency (η%) for aluminium in H 2 SO 4 solution with given inhibitor extract at 298 ±0.1K

Table 4 .
Reaction number (RN) and inhibition efficiency (η%) for aluminium in HCl solution with given inhibitor extract at 298 ±0.1K