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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.54 n.1 Concepción  2009 

J. Chil. Chem. Soc, 54, N° 1 (2009); págs: 83-88





Department of Pure andApplied Chemistry, Maharshi Dayanand Saraswati University, Ajmer, Rajasthan (India) 305009. *e-mail-


The alcoholic extracts of leaves, latex and fruit from the Calotropis procera and Calotropis gigantea are tested for corrosion inhibition in basic solution by mass loss method and thermometric method. In the present investigation the extract reduces the corrosion rate of mild steel in basic solution. The inhibition efficiency increases as the extract concentration is increased. The alcoholic extract of Calotropis is found effective corrosion inhibitor in basic media and give up to 80.89% efficiency.

Key words: Mild steel, Mass loss method, Thermometric, Corrosion , Adsorption.


Mild steel is widely used in chemical industries due to its low cost and easy availability for fabrication reaction of vessel, tanks, pipeline and boiler, because it suffer from severe corrosion in aggressive environments, it has to be protective. The corrosion of metal in aqueous solution occurs in two steps oxidation and reduction. Oxidation reaction takes place at anode, whereas reduction takes place at cathode. The cathodic reaction may be either by evolution of hydrogen and absorption of oxygen.

Mild steel is a reactive metal to reduce the corrosion problem in these environment inhibitive effects of various naturally occurring substance like Datura stramonium1, Tannin beet root, Tamarind, Tealeaves, Pomegranate juice, Saponin2, Embellica officinalis, Terminalia bellerica, a mixture of the later three Spindus trifolianus, Acacia concianna, Swerti aungustifolia and quinoline based cinchona alkaloids as well as very popular ayurvadic powder Mahasudarshana chuma, Prosopis juliflora3, Caparis decidua4, Adhotoda-vasica, Vinca-rosea, Heencf, Eugenia jambolans, pomegranate and Peeis6,7, Tannins, Caffeine, Prosopis cineraria 8 and Ficus relegeosa9 have been evaluated as effective corrosion inhibitor. Calotropis is used as a traditional medicinal plant with unique properties. Calotropis is used alone or with other medicinalis to treat common disease. Generally organic compounds having heteroatom O, N and S are found to have basicity and electrón density thus assist in corrosion inhibitor. O, N and S are the active center for the adsorption on the metal surface10-11.

Mass loss can be determined gravimetrically, volumetrically and radiometrically, all are the direct measures of corrosion of these, gravimetrically or mass loss methods are most used for inhibitor testing.


The Calotropis extract was obtained by dried, then finally powdered and extracted with boiling ethanol. The solvent is distilled off and the residue is treated with the inorganic acid where the base is extracted as their soluble salts. The free acids are liberated by the addition of base and extracted with various solvents, e.g. chloroform etc. The mixture of bases thus obtained is separated by various methods into the individual compounds. To cleanthe stuff obtained it was boiled with activated charcoal (2gm) to remove gung and puré plant extract was obtained.

(a) Specimen preparation:

Rectangular specimens of mild steel of dimension 2.5*1.5*0.02Cm3 containing a small hole of 0.2Cm 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 pumic powder and degreased. Each specimen was suspended by a glass hook and immersed in a beaker containing 50mL of test solution at 23·c and left exposed to air to 24 hours. Evaporational losses were made up with deionized water. After the test specimens were cleaned with benzene. Duplícate experiments were performed in each case and mean values of the mass loss were calculated.

(b) Test solution preparation:

The basic solution was prepared by using deionized water. All chemicals were used of analytical reagent quality.

The percentage inhibition efficiency was calaulated as12

I =100(ΔMu-ΔMi/ΔMu)----------------------------- (1)

Where ΔMu and ΔMi are the mass loss of the metal is uninhibited and inhibited solution respectively.

The degree of surface coverage (0) can be calculated as

θ = ΔMu-ΔMi/ΔMu-------------------------------------- (2)

Where 0 surface coverage and ΔMu and ΔMi are the mass loss of the metal in uninhibited and inhibited basic solution.

The corrosion rate is mmpy (mili meter penetration per year) can be obtained by the following equation

Corrosion rate (mmpy) = Weight loss*87.6/Area*Time*Metal density------(3)

where mass loss is expressed in gm, area is expressed in cm2 of metal surface exposed, time is expressed in hours of exposure, metal density is expressed in gm/cm3and 87.6 is conversion factor.

Inhibition efficiency was also determined using a thermometric technique. This involved the inmersion of single specimens measuring 2.5* 1.5*0.02 Cm3 in a reaction chamber containing 50 MI of test solution. Temperature changes were measured at interval of one minute using a thermometer with a precision of 0.5°c.The temperature increased slowly at first then rapidly and attained a maximum temperature was recorded percentage inhibition efficiencies were calculated as13

η =100(RNfree-RNi)/RNfree ----------------------------(4)

Where RNi and RNfree are the reaction number in the presence and absence of inhibitors respectively and RN (°c/min) is defined as

RN=(Tm-To)/t ---------------------------------------- (5)

Where Tm and To are the maximum and initial temperature respectively and t is the time required to reach the maximum temperature.


The inhibition efficiency (%) calculated from the mass loss measurement for sodium hydroxide solution and inhibitors are given in tables (1-4). It is observed that the inhibition efficiency increases with increase in the concentration of inhibition and decreases with increases in acid strength. The corrosion rate decreases with increases in concentration of inhibitor. The maximum efficiency was obtained in low acid concentration. The inhibitors have shown the efficiency in the range. Calotropis procera show minimum 31.77% and maximum 78.95% whereas latex shows minimum 33.41% and maximum 80.18% but fruit shows 32.19% minimum and 79.88% maximum for 0.5M sodium hydroxide solution instead of Calotropis gigantean shows minimum 31.35% and maximum 78.57% whereas latex shows minimum 33.88% and maximum 80.89% but fruit shows 33.44% minimum and 80.33% maximum for 0.5M sodium hydroxide solution but Calotropis procera in 1M sodium hydroxide solution show minimum 50.09% and maximum 79.16% with leaves while latex shows minimum 48.05% and maximum 79.92% whereas fruit shows minimum 49.95% and maximum 78.97% inhibition efficiency instead of Calotropis gigantea show minimum 31.35% and maximum 80.89% inhibition efficiency with leaves extract but latex reveals 48.53% minimum and 79.68 maximum efficiency and fruit shows 50.37% minimum and 79.37% maximum inhibition efficiency towards sodium hydroxide solution.


Inhibition efficiency values were also determined by the thermometric method (Tables 5-6). Temperature changes for mild steel in 3M, 4M and 5M sodium hydroxide solution were recorded at various inhibitor concentrations. However, no significance temperature changes were obtained for 1M and 2M sodium hydroxide solution. Therefore, the thermometric method was used for 3M, 4M and 5M sodium hydroxide solution. For lower concentration of sodium hydroxide solution have lower reaction number but inhibition efficiency is higher instead of higher concentration has its vice-versa. Calotropis procera leaves extract show minimum inhibition efficiency 28.26% and 71.73% but latex shows minimum 21.73% and maximum 73.91% while fruit shows minimum 26.08% and 76.08% maximum inhibition efficiency for 3M sodium hydroxide solution. Calotropis procera leaves extract show minimum inhibition efficiency 36.84% and 73.68% but latex shows minimum 34.73% and maximum 71.57% while fruit shows minimum 38.94% and 75.78% maximum inhibition efficiency for 4M sodium hydroxide solution. Calotropis procera leaves extract show minimum inhibition efficiency 33.33% and 70.73% but latex shows minimum 30.08% and maximum 66.66% while fruit shows minimum 34.95% and 73.14% maximum inhibition efficiency for 5M sodium hydroxide solution instead of Calotropis gigantea leaves extract show minimum inhibition efficiency 30.43% and 78.26% but latex shows minimum 23.91% and maximum 73.91% while fruit shows minimum 28.26% and 80.43% maximum inhibition efficiency for 3M sodium hydroxide solution. Calotropis gigantea leaves extract show minimum inhibition efficiency 38.94% and 75.78% but latex shows minimum 35.78% and maximum 72.63% while fruit shows minimum 42.10% and 76.84% maximum inhibition efficiency for 4M sodium hydroxide solution. Calotropis gigantea leaves extract show minimum inhibition efficiency 34.14% and 72.35% but latex shows minimum 31.70% and maximum 68.39% while fruit shows minimum 35.77% and 73.98% maximum inhibition efficiency for 5M sodium hydroxide solution.


Generally the adsorption of organic molecules involves O, N and S atoms. This process may block active sites henee may decrease the corrosion rate. In that the plant extract uscharin adsorbed on the metal surface and decreases the surface area available for cathodic and anodic reaction to take place. In the present study it is assumed that the plant extract are adsorbed on the metal surface and decreases the surface area available for cathodic and anodic reaction to take place. The nitrogen, sulphur and oxygen of uscharin may be responsible for the adsorption. Organic inhibitors, with active portions contain generally large C-H chains or rings with positively charged amine nitrogen group at the one end.

Organic corrosion inhibitors14 may function as:

1.- Chemisorption of the molecule on the metallic surface.
2.- Complexing of the molecule with the metal ion which remains in solid state.
3.- Neutralising the corrodant.
4.- Adsorbing the corrodent.

They offered large coverage due to the long hydrocarbon chain and by the presence of OH group, being hydrophilic in nature; the OH group counteracted the effeets of chain length and ensured higher solubility.

Cathodic inhibitors slow down the reaction taking place at cathode (i.e. H2 evolution)

eg. organic inhibitors like amine, substituted urea's, heavy metal soap etc. decrease H2 evolution process. Mild steel salts deposits on cathode helps in decreasing the 02 absorption process.

Absorption of oxygen: This type of corrosion oceurs generally in aqueous solution.

The Fe2+ and OH- ions diffuse towards each other (Smaller Fe2+ diffuse more rapidly) and forms Fe(OH)2

If oxygen is present

Yellow rust

If oxygen is less, then it forms black Fe304 (Anhydrous magnetite)

If oxygen absorption type of corrosion , the anodic area are small, while the cathodic are large.

Mild steel when exposed to alkaline solution, temperature and stress causes due to caustic embrittlement i.e. in experimental condition sodium hydroxide react with mild steel to form sodium ferroate. This decomposes and sodium hydroxide is regenerated which causes further dissolution of mild steel. All metal abo ve hydrogen in electrochemical series have a tendeney to get dissolve in solution releasing hydrogen.

Figures (2-5) shows inhibition efficieney increases with increase in the concentration of inhibition and decrease with increase in acidic strength.


It has been observed that the fruit extract of Calotropis procera and Calotropis gigantea has maximum inhibition efficieney compared to the leaves and latex extraets. This process increased the adsorptivity of the fruit extract on the corroding site of the metal. This explains that the higher inhibition efficieney displayed by the fruit extract for 0.6% concentration.

Figures (6-7) shows the surface analysis of metal by Scanning Electron Microscopy was carried out on Model- ZEISS EVO 50. The surface morphological characteristics of the blank and inhibited mild steel were analyzed at magnification of 2.0 KX operated at an accelerating voltage of 20 KV. Scanning electrón microscopy reveals that plant extract adsorbed on metal surface that decreases the metal surface for corrosion attack. SEM provides a two-dimensional projection or a two-dimensional image of a sample.



1.     Plant inhibitors inhibited mild steel corrosion in basic solutions.

2.     Corrosion inhibition of mild steel in basic solution is under anodic control.

3.     Inhibition efficiency of plant extracts increases with increase in concentration.

4.     The mass loss measurements are in good agreement with electrochemical method.

5.     The surface analysis has shown that inhibition of corrosion by plant extract is due to formation of layer on the mild steel surface.



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The authors are thankful to Head of Department of Puré and Applied Chemistry, Maharshi Dayanand Sarswati University for providing necessary facilities and express thanks to the director IIT Delhi, India for providing Scanning Electrón Microscopy analysis.

(Received 27 July 2008 - Accepted 28 August 2008)

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