Corrosion Inhibition of Aluminium by Capparis decidua in Acidic Media

The inhibition efficiency of ethanolic extract of different parts of Capparis decidua (Ker) in acidic medium has been evaluated by mass loss and thermometric methods. Values of inhibition efficiency obtained from the two methods are in good agreement and are dependent upon the concentration of inhibitor and acid.


Introduction
Aluminium being various corrosive agents subject an industrially important metal to corrosion in service, of which aqueous acids are most harmful.The corrosion of aluminium and its alloys in acidic solution has been studied.The effect of nitrogen containing heterocyclic compounds on the dissolution of aluminium has also been evaluated.Organic compounds containing nitrogen have been found to function as very effective corrosion inhibitors.The efficiency of these compounds as corrosion inhibitors can be attributed to the number of mobile electron pair present, the orbital character of free electrons and electron density around the nitrogen atoms.In the present investigation the inhibitive effect have been evaluated of ethanolic extract of fruit, stem bark and root bark of Capparis decidua.The extracts of Capparis decidua work as green inhibitor, which are nontoxic and biodegradable.Ker is commonly used in the Indian system of medicine.Its stem bark, root bark, fruit, seeds, young shoots and even buds are endowed with medicinal properties.The seeds are cooling, laxative, refrigerant, leaves and young shoots are purgative, stem bark is cooling astringent and fruit is laxative and digestive.Because of the toxic nature and high cost of some chemicals currently in use it is necessary to develop environmentally acceptable and less expensive inhibitors.Natural product can be considered as a good source for this purpose.To reduce the corrosion problem in environment inhibitive effects of various naturally occurring substances like Datura stramonium 2 , Calotropis gigantea, Capparis decidua 3 , Prosopis juliflora 4

Experimental
2 Kg of plant material in natural condition was air dried for 8 to 10 days in shade.Then grained and powdered.300g of finely powdered dried material was taken in a 1000mL round bottom flask and sufficient quantity of ethyl alcohol was added to cover the powder completely.The RBF was covered with stopper and left for 48h.Then the resulting paste was refluxed for 48h.Then it was filtered.The reaction procedure for extraction was repeated for maximum extraction.The filtrate was collected and then separated ethyl alcohol with the help of distillation unit.To clean the stuff obtained it was boiled with activated charcoal (2g) to remove gung and pure plant extract is obtained.2kg of plant material in natural condition was air dried for 8 to 10 days in shade.Then grained and powdered.300g of finely powdered dried material was taken in a 1000ml round bottom flask and sufficient quantity of ethyl alcohol was added to cover the powder completely.The RBF was covered with stopper and left for 48h.Then the resulting paste was refluxed for 48hours.Then it was filtered.The reaction procedure for extraction was repeated for maximum extraction.The filtrate was collected and then separated ethyl alcohol with the help of distillation unit.To clean the stuff obtained it was boiled with activated charcoal (2g) to remove gung and pure plant extract is obtained.

Specimen preparation
Rectangular specimens of aluminium of dimension 2.5X1.5X0.02cm containing a small hole of 2mm diameter near the upper edge were employed for the determination of corrosion rate.Specimens were cleaned by buffing to produce a mirror finish with the help of emery paper and were then degreased.Each specimen was suspended by a glass hook and immersed in a beaker containing 50mL of test solution at 23 0 C and left exposed to air.Evaporational losses were made up with doubly distilled water.After the test specimens were cleaned with benzene.Duplicate experiments were performed in each case and mean values of the Weight loss were calculated.

Test solution preparation
The acidic solution was prepared by using doubly distilled water.All chemicals were used of analytical reagent quality.
The percentage inhibition efficiency was calculated 5 as I=100( Mu-Mi/ Mu) Where Mu and Mi are the mass loss of the metal is uninhibited and inhibited solution respectively.
The degree of surface coverage ( ) can be calculated as = Mu-Mi/ Mu Where surface coverage and Mu and Mi are the mass loss of the metal in uninhibited and inhibited acid.
The corrosion rate is mmpy (mili miles per year) can be obtained by the following equation Corrosion rate (mmpy) = (Mass loss*87.6)/ (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, metal density is expressed in g/cm³and 87.6 is conversion factor.Inhibition efficiency was also determined using a thermometric technique.This involved the immersion of single specimens measuring 2.5X1.5X0.02cm 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 0.5ºC.The temperatures increased slowly at first then rapidly and attained a maximum temperature was recorded percentage inhibition efficiencies were calculated 6 as η=100(RNfree-RNi)/RNfree Where RNi and RNfree are the reaction number in the presence and absence of inhibitors respectively and RN ( º C/min) is defined as 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
The inhibition efficiency (%) calculated from the mass loss measurement for hydrochloric acid and sulphuric acid and inhibitor are given in tables.It is observed that the inhibition efficiency increases with increase in the concentration of inhibitor and decreases with increases in acid strength.The corrosion rate decreases with increases in concentration of inhibitor.The maximum effeciency was obtained in low acid concentration.The inhibitors have shown the efficiency in the range.Capparis decidua fruit extract shows minimum 51.99% inhibition efficiency and maximum 98.49% for 0.5N hydrochloric acid but 1N hydrochloric acid solution show minimum efficiency 32.27% and maximum 65.17% and 2N hydrochloric acid solution show minimum efficiency 33.95% and maximum 55.56%.Capparis decidua stem bark extract shows minimum 38.92% inhibition efficiency and maximum 93.52% for 0.5N for hydrochloric acid but 1N hydrochloric acid solution show minimum efficiency 27.07% and maximum 64.09% and 2N hydrochloric acid solution show minimum efficiency 25.42% and maximum 53.22%.Capparis decidua root bark extract shows minimum 31.05%inhibition efficiency and maximum 94.44% for 0.5N for hydrochloric acid but 1N hydrochloric acid solution show minimum efficiency 27.74% and maximum 63.10 % and 2N hydrochloric acid solution show minimum 36.38 % and maximum 57.68% (Table 3).Capparis decidua fruit extract shows inhibition effeciency minimum 23.10% and maximum 77.84% for 0.5N for sulphuric acid but 1N sulphuric acid solution show minimum 24.08% and maximum 62.20% and 2N sulphuric acid solution show minimum 46.07%and maximum 55.62%.Capparis decidua stem bark extract shows inhibition effeciency minimum 24.52% and maximum 76.23% for 0.5N for sulphuric acid but 1N sulphuric acid solution show minimum 15.86% and maximum 56.62% and 2N sulphuric acid solution show minimum 48.19% and maximum 56.12%.Generally the adsorption of organic molecules involves O, N and S atoms.This process may block active sites hence, decrease the corrosion rate.In the present study it is assuming that the plant extract are adsorbed on the metal surface and decreases the surface area available for cathodic and anodic reaction to take place.
have been evaluated as effective corrosion inhibitors.The average composition of ripe fruit is as following

Table 1 .
Mass loss and corrosion rate (mmpy) for aluminium in hydrochloric acid solution with given inhibitor addition at room temperature for Capparis decidua.

Table 2 .
Mass loss and corrosion rate (Mmpy) for aluminium in sulphuric acid solution with given inhibitor addition at room temperature for Capparis decidua.

Table 3 .
Surface coverage and inhibition efficiency for aluminium in hydrochloric acid solution with given inhibitor addition at room temperature for Capparis decidua.

Table 4 .
Surface coverage and inhibition efficiency for aluminium in sulphuric acid solution with given inhibitor addition at room temperature for Capparis decidua.

Table 5 .
Reaction number and inhibition efficiency for aluminium in hydrochloric acid solution with given inhibitor addition at room temperature for Capparis decidua

Table 6 .
Reaction number and inhibition efficiency for aluminium in sulphuric acid solution with given inhibitor addition at room temperature for Capparis decidua