Plant extract use as corrosion. inhibitor for copper in acidic and ammoniacal medium

Transcription

1 Chapter - IV B Plant extract use as corrosion inhibitor for copper in acidic and ammoniacal medium Azadirachta indica (leaves and bark) extract has been used as corrosion inhibitor for copper in acidic and ammoniacal medium Cryptostegia grandiflora (leaves and stem) extract used as corrosion inhibitor for copper in acidic (HCl and HNO 3 ) medium Calotropis procera (leaves) extract used as corrosion inhibitor for copper in acidic and ammoniacal medium References

2 PART IV-B PLANT EXTRACT USED AS CORROSION INHIBITOR FOR COPPER IN ACIDIC AND AMMONIACAL MEDIUM Plants are sources of naturally occurring compounds, some with complex molecular structures and having different chemical, biological, and physical properties. The naturally occurring compounds are mostly used because they are environmentally acceptable, cost effective, and have abundant availability. These advantages are the reason for use of extracts of plants and their products as corrosion inhibitors for metals and alloys under different environment. Different plant extracts can be used as corrosion inhibitors commonly known as green corrosion inhibitors. Green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds. Some research groups have reported the successful use of naturally occurring substances to inhibit the corrosion of metals in acidic and alkaline environment [13-17]. Following plant have been used as corrosion study namely ; 4-B.1. Azadirachta indica (leaves and bark) extract 4.B.2. Calotropis procera (leaves) 4.B.3. Cryptostegia grandiflora (leaves and stem) 4-B.1 Azadirachta indica (leaves and bark) extract has been used as corrosion inhibitor for copper in acidic and ammoniacal medium Plant description:- It is popularly known as the miracle tree. It is known as Nimba in India. The Sanskrit name of neem is Arishtha meaning the reliever of the sickness. 206

3 Classification :- Kingdom - Plantae Division - Magnoliophyta Class - Magnoliopsida Order - Sapindales Family - Meliaceae Genus - Azadirachta Species - A. indica Scientific Name - Azadirachta indica Common name - Neem Azadirachta indica holds medicinal value. Each part of Azadirachta indica is used in the medicines. It has been used in Ayurvedic medicines for more than 4000 years. Azadirachta indica oil extracted from its seeds is used in medicines, pest control and cosmetics etc. Its leaves are used to treat Chickenpox. According to the Hindus, it is believed that the Goddess of the chickenpox, Sithala lives in the Neem tree. Azadirachta indica tea is usually taken to reduce the headache and fever. Its flowers are used to cure intestinal problems. Azadirachta indica bark acts as an analgesic and can cure high fever as of malaria. Even the skin diseases can be cured from the Azadirachta indica leaves. People in India use its twigs to brush their teeth. Azadirachta indica is considered as the useful tree in rehabilitating the waste land areas [18-19]. Azadirachta indica seed pulp is useful for methane gas production. It is also useful as carbohydrate which is rich base for other industrial fermentations. Azadirachta indica bark contains tannins which are used in tanning and dyeing. Azadirachta indica cake is widely used in India as fertilizer for sugarcane, vegetable and other cash crops. Alcoholic extract of Neem leaves and bark have been used to corrosion study. 207

4 The extracts of Azadirachta indica have been reported as inhibitor for metals. In present section Azadirachta indica leaves and bark extracts use as corrosion inhibitor for copper metal in acidic and ammoniacal medium.the leaves of the Azadirachta indica plant are the most versatile and most easily available resource. Leaf pastes and extracts are used in skin care products, hair oils, in neem toothpastes and mouth washs, and they also have lots of medicinal uses. Azadirachta indica bark contains 14% tannin.the Azadirachta indica bark contains a higher concentration of active ingredients than the leaves, and is especially high in ingredients with antiseptic and anti-inflammatory action. 4.B.1.1 Experimental part 1. Inhibitor solution- Azadirachta indica extract solution were used as inhibitor. Extract of leaves and bark of Azadirachta indica was prepared and use against inhibition study for copper in acidic (HCl and HNO 3 ) and ammoniacal medium. Azadirachta indica leaves and bark were collected, washed with water and dried in shade. Dried leaves and barks were soaked separately in alcohol solution for 72 hours at room temperature. Alcoholic solution was filtered using whatmann filter paper. Filtrate was concentrated from ethyl alcohol solution by heating in condenser and remaining solvent evaporated by keeping the solution for 24 hours. Further it was dried in open. This crude was stored and used to prepare stock solution for inhibition study. Different concentration solutions 0.1g/l, 0.3g/l,0.5 g/l and 0.7 g/l were used as inhibitor solution. 2. Medium Hydrochloric acid, nitric acid and ammoniacal medium have been used for inhibition study. Nitric acid (0.5N), Hydrochloric acid (1N) and ammoniacal solution (2N ) were prepared by dilution of AR grade HNO 3, HCl and NH 3 and used as test solution. 3. Metal specimen: - Rectangular size copper specimen having dimension (2.5(length) X 2.5(width) X 0.01(thickness)) cm and a hole near one of edge were used for inhibition study. Specimen were cleaned by emery paper washed with acetone and distilled water, dried and then weighed. 208

5 4 Methodology: - Weight loss method has been used to study inhibition property. Experiment was performed using rectangular copper specimen having hole on upper edge. Specimen polish was cleaned with emery paper, washed with distilled water and acetone, dried in oven. Each specimen was weighed using an analytical balance of gm accuracy. These specimens were suspended with V shape capillary in beaker containing 100 ml test solution with and without different concentration of leaves and bark extracts. All these beakers were placed in thermostat so that the temperature of beaker solution becomes constant. After 24 hour, all beakers were taken out from thermostat; each suspended specimen was washed, dried and weighed again. Inhibition study was carried out at four different temperatures 303K, 313 K, 323K, and 333K. Experiment was repeated using different concentration of extracts at different temperature range. Duplicate set of every experiment were performed so that accurate values of weight loss could be calculate. Weight loss value has been used to calculate corrosion rate, inhibition efficiency, activation energy, enthalpy, entropy, and Gibbs free energy. (a) Weight loss measurement The simplest and most accurate method of estimating the corrosion rate is weight loss analysis. A weighed specimen of the metal or alloy under consideration is introduced into the corrosive environment and later removed after a reasonable time interval. The specimen is then cleaned of all corrosion products and is reweighed. Weight loss of copper specimen after dipping in test solution with and without extracts in different concentration was calculated. The weight loss is converted to an average corrosion rate (CR), as follows, = (1) Where ΔW is the average weight loss of copper specimen, A is the total area of one copper sheet specimen, and t is the immersion time, CR is corrosion rate in mg cm 2 h 1 209

6 When metal corrosion is slow and average data are acceptable, weight loss monitoring is the preferred technique. The percent inhibition efficiency (IE%) was calculated for the extracts under investigation according to the following equation [20] Where.. = (2) I.E. = Inhibition efficiency W 0 = Weight loss in absence of inhibitor W = Weight loss in presence of inhibitor Surface coverage was calculated by using following formula = 0 0 (3) Weight loss can be a measure for corrosion if specimen are of the same size and have been tested for the experiment as loss in weight per unit area per unit time. The initial specimen weight were recorded using an analytical balance before immersion in 150 ml beakers containing 100 ml of corrosive medium without and with different concentrations of leaves and bark extracts of Azadirachta indica. The specimens were taken out, washed, dried, and reweighed accurately. The average weight loss of the three parallel copper specimens could be obtained. The corrosion rates of the copper coupons have been determined for 24 hour immersion period at temperature range from 303K to 333 K using Eq(1) and the percentage inhibition efficiency was calculated according the relationship Eq(2). The value of percentage inhibition efficiency (IE) and corrosion rate (CR) obtained from weight loss method at different concentrations of Azadirachta indica leaves and bark extract in acidic and ammoniacal solution at temperature range from 303 to 333 K are summarized in table1. 210

7 Table 1 : Inhibition efficacy and corrosion rate of Azadirachta indica extracts for copper Plant part Leaves Medium HNO 3 HCl NH 3 Tem(K) Con.of Corr. Corr. Corr. Corr. Wt.loss I.E Wt.loss I.E Wt.loss I.E Wt.loss extract(ppm) rate rate rate rate I.E Blank Blank Blank

8 Plant part Medium Tem(K) Con.of Corr. Corr. Corr. Corr. Wt.loss I.E Wt.loss I.E Wt.loss I.E Wt.loss extract(ppm) rate rate rate rate I.E Blank HNO Bark Blank NH

9 4.B.1.2 Result and discussion 1. Effect of concentration of extract on inhibition efficiency Corrosion inhibition efficiency of Azadirachta indica Azadirachta indica leaves and bark extract solutions have been calculated using weight loss measurement after 24 hours of immersion at various temperatures ranging from 303 to 333K. In blank sample no extract solution was used and other solution contained concentration 0.1g/l, 0.3g/l,0.5 g/l and 0.7 g/l of extract. Table 1 shows the calculated values of inhibition efficiency and corrosion rate in hydrochloric acid, nitric acid and ammoniacal medium.with increasing concentration of extracts inhibition efficiency increases. It is clear from table 1 that in ammoniacal medium leaves and bark extract show maximum inhibiton efficiency. Leaves extract show maximum efficiency 95% at 0.7 g/l concentration and bark extracts show 92 % inhibition efficiency in ammoniacal medium. For leaves extract maximum efficiency in hydrochloric acid medium is 80% and in nitric acid medium is 67%. For bark extracts 75% inhibition efficiency show at 0.7g/l concentration. Corrosion rate decreases with increase in concentration of extracts which show that inhibition efficiency increases due to decrease in corrosion rate. 2. Effect of solution temperature on inhibition efficiency Corrosion inhibition studies have been carried out at temperature range from 303 K to 333K and calculated values are reported in table 1. Results reveal that with increasing temperature inhibition efficiency decreases which is due to deabsorption of inhibitor molecules on metal surface. Maximum inhibition efficiency was shown at 303K temperature for leaves and bark extracts. While going to higher temperature from 303 to 333 K inhibition efficiency decreases. For leaves extracts inhibition efficiencies decreases from 95% to 91% in ammoniacal medium with increase in temperature. For bark extracts inhibition efficiencies decreases from 92% to 87 % with increase in temperature. In hydrochloric and nitric acid medium inhibition efficiency decreases with increase in temperature for leaves and bark extracts. The temperatures dependence on IE have been shown by plotting graph between temperature and inhibition efficiency at various concentration as show in figure 4.B.1.1to 4.B

10 Figure 4.B.1.1-Leaf extract in HNO 3 Medium Figure 4.B.1.2-Leaf extract in HCl Medium Figure 4.B.1.3-Leaf extract in NH 3 Medium Figure 4.B.1.4-Bark extract in HNO 3 Medium Figure 4.B.1.5-Bark extract in NH 3 Medium Figure 4.B.1.1 to 4.B Effect of temperature and concentration on inhibition efficiency of leaf and bark extracts of Azadirachta indica 214

11 4.B.1.3 Determination of thermodynamic parameters 4.B Activation energy In understanding mechanism of inhibition, thermodynamic parameters play an important role. Arrhenius equation expresses the variation in corrosion rate with time [21] as expressed by log CR = A + E a RT (4) Where CR is the corrosion rate, A is the frequency factor, E a is the apparent activation energy, R is the molar gas constant (8.314 K 1 mol 1 ) and T is the absolute temperature. Figure 4.B.1.6 to 4.B.1.10 show the Arrhenius plots of log CR against 1/T. Where a Straight line is obtained with a slope (-E a /RT). Using value of slope activation energy for various leaves and bark extracts of Azadirachta indica have been calculated. Corresponding values were summarized in table-2. Activation energy is lower in absence of extracts while increase concentration of extract of leaves and bark activation energies increases. For leaves extracts activation energy increases from 28 to 38 KJ/mol in HNO 3 medium, from 29 to 42 KJ/mol in HCl medium and 36 to 54 KJ/mol in NH 3 medium. For bark extracts activation energies increases from 37 to 43 KJ/mol in HNO 3 medium and from 35 to 52 KJ/mol in NH 3 medium. Activation energy is lower in absence of inhibitor but with increasing concentration of leaves and bark extracts value of activation energy increases. The increasing value of activation energy with concentration of extract shows that there is an adsorption of leaves and bark extracts on copper surface that blocks the activation site of copper specimen against corrosion, so corrosion rate decrease with increase in concentration of extract. Further Corrosion rate decrease with increase in temperature since deadsorption of adsorbed molecule takes place. Inhibition efficiency and corrosion rate decreases with temperature which shows adsorption of extracts on copper surface is a physio-adsorption process. 215

12 Figure 4.B.1.6 to 4.B.1.10 Arrhenious plot of leaf and bark extracts of Azadirachta indica 216

13 It is obvious from data listed in table 2 that, E a of the inhibited solution in this study increases by increasing the extracts concentration indicating strong adsorption of the inhibitor molecules at the metal surface. The range of E a values (28 56 KJ/mol) are lower than the threshold value of 80 KJ/mol, required for chemical adsorption. This means that the adsorption of leaves and bark extracts on copper surface is an physical adsorption. It is evident from table 2 that the value of E a increased on addition of extract solution in comparison to the uninhibited solution. 4.B Determination of enthalpy and entropy Kinetic parameters including enthalpy and entropy were calculated using transition state equation [22] as express by CR = RT Nh S exp R H exp RT (5) Where, CR = Corrosion rate, h = Plank s constant, N =Avogadro s number R = Universal gas constant, T = Absolute temperature. Enthalpy (ΔH) = Slope X 2.302X R Entropy (ΔS) = X R (Intercept-log(R/Nh)) 217

14 Figure 4.B.1.11 to 4.B.1.15 Transition state plot of leaf and bark extracts of Azadirachta indica 218

15 Table 2 : Thermodynamic parameters at different concentration of Azadirachta indica extracts Part of plant Leaf Bark Medium HNO 3 HCl NH 3 HNO 3 NH 3 Concentration of extracts (ppm) E a (KJ/mol) H (KJ/mol) S (J/Mol) Blank Blank Blank Blank Blank Figure 4.B.1.11 to 4.B.1.15 show the transition state plots for copper corrosion in the absence and presence of different concentrations of leaves and bark extracts of Azadirachta indica. The curve plotted between log CR against 1/T shows a straight line. Enthalpies and entropies were calculated using slope and intercept of these lines. Calculated values of enthalpies and entropies are listed in table

16 From table 2 it is clearly observed that ΔH values are positive and ΔS values are large and negative with increasing concentration of extracts. ΔH value moves towards higher positive value that shows higher inhibition efficiency of inhibitors. Enthalpies increase with an increase in the concentration of leaves and bark extract of Azadirachta indica. The entropy of activation (ΔS) was negative both in the absence and presence of inhibitor. This indicates that in the rate determining step activated complex represents association rather than dissociation, indicating that there is decrease in disorder. 4.B.1.4 Adsorption isotherm The primary step in the action of inhibitors in acid solution is generally agreed to be the adsorption on the metal surface. This involves the assumption that the corrosion reactions are prevented from occurring over the area (or active sites) of the metal surface covered by adsorbed inhibitor species, whereas these corrosion reactions occur normally on the inhibitor-free area. The fraction of surface covered with inhibitor species can be calculate by following equation θ = IE 100 (6) 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 concentration at constant temperature, adsorption isotherm could be evaluated at equilibrium condition. The dependence of the fraction of the surface covered θ on the concentration C of the inhibitor was tested graphically by fitting it to Langmuir s isotherm C θ = 1 k + C (8) For testing the adsorption isotherm obeyed by this system, a graphic relation between the inhibitor concentration C and C/θ at different temperatures, is drawn and represented in figure 4.B.1.16 to 4.B Straight line with almost unit slope and correlation coefficient are nearby one, were obtained indicating that the system follows Langmuir adsorption isotherm. 220

17 Figure 4.B.1.16 to 4.B.1.20 Langmuir adsorption isotherm of leaf and bark extracts of Azadirachta indica 221

18 where, K is adsorption constant. The K values can be calculated from the intercept lines on the C/θ axis. This is related to the standard free energy of adsorption (ΔG) by following equation- K = exp G RT (8) Where K is the adsorption equilibrium constant, R is the gas constant (8.314J/K/mol), T is the absolute temperature in Kelvin and the value 55.5 is the concentration of water in solution expressed in mol/l The free energy of adsorption ΔG, which can characterize the interaction of adsorbed molecules and metal surface, was calculated using the equation. ΔG = - RT ln (55.5 x K ) (9) The values of ΔG for the inhibitor on the surface of copper are given in table 3. The negative values of ΔG indicate the stability of the adsorbed layer on copper surface and spontaneity of the adsorption process. ΔG decreased (become more negative) with increasing temperature, indicating the occurrence of endothermic process. The negative value of ΔG ensures the adsorption process and stability of the absorbed layer on the electrode surface. The stability of the adsorbed layer decreases with increase in temperature. This is clearly seen from the decrease in the absolute value ΔG with rise in temperature. Calculated values of Gibbs free energy are less than the threshold value ( 40 KJ/mol) required for chemical adsorption and the IE% decreased with increasing temperature. These support the mechanism of physical adsorption. 222

19 Table 3 : Value of ΔG at different temperature for Azadirachta indica extracts Part of plant Medium Temperature R 2 G(KJ/mol) HNO Leaf Bark HCl NH 3 HNO 3 NH

20 4-B.2. Calotropis procera (leaves) extract used as corrosion inhibitor for copper in acidic and ammoniacal medium Plant discription :- Calotropis procera commonly known as Aak is used in many Ayurvedic formulations like Arkelavana. The medicinal potential of Calotropis procera has been known to traditional system of medicine. The use of the plants, plant extracts and pure compounds isolated from natural sources has always provided a foundation for modern pharmaceutical compounds Classification:- Kingdom - Plantae Division - Magnoliophyta Class - Magnoliopsida Order - Gentianales Family - Asclepiadaceae Genus - Calotropis Species - Calotropis procera Scientific Name - Calotropis procera Common name - Aak Calotropis procera holds medicinal value. Leaves found to be effective for treating elephantiasis. Flowers are useful against cough and improving appetite. Mixture of its latex, turmeric and sesame oil is useful in treating scabies. Plant works as a powerful cardiac stimulant (probably due to the cardiac glycosides present in the latex). Dry leaf powder used for treating wounds and boils. Juice squeezed from fresh leaves is used to treat bite wounds in rural areas. Phytochemical screening of ethanol leaf extracts of Calotropis procera revealed the presence of Glycosides, Protein, Triterpenoids, Steroids, Flavonoids.Calotropis glycoside are calotropin, calotoxin, calactin, uscharidin and voruscharin [23-25]. The presence of these components in this species is an indication that it may have some medicinal potential. Two new cardenolides (1 and 2) along with 12 known compounds were isolated from the dichloromethane extract of the leaves of Calotropis procera. 224

21 The extracts of Calotropis procera have been reported as inhibitor for metal. In present section Calotropis procera leaves extract have been used as corrosion inhibitor for copper metal in acidic and ammoniacal medium. Calotropis procera plant are the most versatile and most easily available resource. 4.B.2.1- Experimental part 1. Inhibitor solution- Calotropis procera extract solution has been used as inhibitor. Extract of leaves of Calotropis procera were prepared and used against inhibition of corrosion for copper in acidic (HCl and HNO 3 ) and ammoniacal medium. Calotropis procera leaves were collected, washed with water and dried in shade. Dried leaves were soaked in alcohol solution for 72 hours at room temperature. Alcoholic solution was filtered using whatmann filter paper. Filtrate was concentrated from ethyl alcohol solution by heating in condenser and remaining solvent evaporated by keeping the solution for 24 hours. Further it was dried in open. This crude was stored and used to prepare stock solution for inhibition study. Different concentration solutions viz; 0.1g/l, 0.3g/l, 0.5 g/l and 0.7 g/l were used as inhibitor solution. 2. Medium Hydrochloric acid, nitric acid and ammoniacal medium have been used for inhibition study. Nitric acid (0.5N), Hydrochloric acid (1N) and Ammoniacal solution (2N) were prepared by dilution of AR grade HNO 3, HCl and NH 3 and used as test solution. 225

22 3. Metal specimen: - Rectangular size copper specimen having dimension (2.5(length) X 2.5(width) X 0.01(thickness)) cm and a hole near one of edge were used for inhibition study. Specimen were cleaned by emery paper washed with acetone and distilled water, dried and then weighed. 4. Methodology: - Weight loss method has been used to study inhibition property. Experiment was performed using rectangular copper specimen having hole on upper edge. Specimen polish was cleaned with emery paper, washed with distilled water and acetone, dried in oven. Each specimen was weighed using an analytical balance of gm accuracy. These specimens were suspended with V shape capillary in beaker containing 100 ml test solution with and without different concentration of leaves extracts. All these beakers were placed in thermostat so that the temperature of beaker solution becomes constant. After 24 hour, all beakers were taken out from thermostat, each suspended specimen was washed, dried and weighed again. Inhibition study was carried out at four different temperatures 303K, 313 K, 323K and 333K. Experiment was repeated using different concentration of extracts at different temperature range. Duplicate set of every experiment were performed so that accurate values of weight loss could be calculate. Weight loss value has been used to calculate corrosion rate, inhibition efficiency, activation energy, enthalpy, entropy, and Gibbs free energy. (a) Weight loss measurement The simplest and most accurate method of estimating the corrosion rate is weight loss analysis. A weighed specimen of the metal or alloy under consideration is introduced into the corrosive environment and later removed after a reasonable time interval. The specimen is then cleaned of all corrosion products and is reweighed. Weight loss of copper specimen after dipping in test solution with and without extracts in different concentration was calculated. The weight loss is converted to an average corrosion rate (CR), as follows, 226

23 = (1) Where ΔW is the average weight loss of copper specimen, A is the total area of one copper sheet specimen, and t is the immersion time, CR is corrosion rate in mg cm 2 h 1 When metal corrosion is slow and average data are acceptable, weight loss monitoring is the preferred technique. The percent inhibition efficiency (IE%) was calculated for the extracts under investigation according to the following equation Where I.E. = Inhibition efficiency.. = (2) W 0 = Weight loss in absence of inhibitor W = Weight loss in presence of inhibitor Surface coverage was calculated by using following formula = 0 0 (3) Weight loss can be a measure for corrosion if specimen are of the same size and have been tested for the experiment as loss in weight per unit area per unit time. The initial specimen weight were recorded using an analytical balance before immersion in 150 ml beakers containing 100 ml of corrosive medium without and with different concentrations of leaves extracts of Calotropis procera. The specimens were taken out, washed, dried, and reweighed accurately. The average weight loss of the three parallel copper specimens could be obtained. The corrosion rates of the copper coupons have been determined for 24 hour immersion period at temperature range from 303K to 333 K using Eq(1) and the percentage inhibition efficiency was calculated according the relationship Eq(2) The value of percentage inhibition efficiency (IE) and corrosion rate (CR) 227

24 Table 4 : Inhibition efficacy and corrosion rate of Leaves extracts of Calotropis procera extracts for copper Medium HNO 3 HCl NH 3 Tem(K) Con.of extract (ppm) Wt.loss Corr. rate I.E Wt.loss Corr. rate I.E Wt.loss Corr. rate I.E Wt.loss Blank Blank Blank Corr. rate I.E 228

25 obtained from weight loss method at different concentrations of Calotropis procera leaves extract in acidic and ammoniacal solution at temperature range from 303 to 333 K are summarized in table1. 4.B.2.2 Result and discussion 1. Effect of concentration of extract on inhibition efficiency Corrosion inhibition efficiency of Calotropis procera leaves extract solutions have been calculated using weight loss measurement after 24 hours of immersion at various temperatures ranging from 303 to 333K. In blank sample no extract solution was used and other solution contained concentration 0.1g/l, 0.3g/l,0.5 g/l and 0.7 g/l of extract. Table 1 shows the calculated values of inhibition efficiency and corrosion rate in hydrochloric acid, nitric acid and ammoniacal medium. With increasing concentration of extracts inhibition efficiency increases. It is clear from table 1 that in ammoniacal medium leaves extract show maximum inhibiton efficiency 85% at 0.7 g/l concentration. For leaves extract maximum efficiency in hydrochloric acid medium is 71% and in nitric acid medium is 68%. Corrosion rate decreases with increase in concentration of extracts which show that inhibition efficiency increases due to decrease in corrosion rate. 2. Effect of solution temperature on inhibition efficiency Corrosion inhibition studies have been carried out at temperature range from 303 K to 333K and calculated values are reported in table1. Results reveal that with increase in temperature inhibition efficiency decreases which is due to deabsorption of inhibitor molecules on metal surface. Maximum inhibition efficiency was shown at 303K temperature for leaves extracts. While going to higher temperature from 303 to 333 K inhibition efficiency decreases. For leaves extracts inhibition efficiencies decrease from 85% to 78% in ammoniacal medium with increase in temperature. In hydrochloric acid medium inhibition efficiencies decrease from 71% to 66% and nitric acid medium inhibition efficiency decrease from 68% to 59%with increase in temperature for leaves extracts. The temperatures dependence on IE have been shown by plotting graph between temperature and inhibition efficiency at various concentration as show in figure 4.B.2.1to 4.B

26 Figure 4.B.2.1 to 4.B Effect of temperature and concentration on inhibition efficiency of leaf extracts of Calotropis procera 4.B.2.3 Determination of thermodynamic parameters 4.B Activation energy In understanding mechanism of inhibition, thermodynamic parameters play an important role. Arrhenius equation expresses the variation in corrosion rate with time [26] as expressed by log CR = A + E a RT (4) Where CR is the corrosion rate, A is the frequency factor, E a is the apparent activation energy, R is the molar gas constant (8.314 K 1 mol 1 ) and T is the absolute temperature. 230

27 Figure 4.B.2.4 to 4.B.2.6 Arrhenious plot of leaf extracts of Calotropis procera Figure 4.B.2.4 to 4.B.2.6 show the Arrhenius plots of log CR against 1/T. Where a Straight line is obtained with a slope (-E a /RT). Using value of slope activation energy for various leaves extracts of Calotropis procera have been calculated. Corresponding values were summarized in table-2. Activation energy is lower in absence of extracts while increase concentration of extract of leaves activation energies increases. For leaves extracts activation energy increases from 28 to 35 KJ/mol in HNO 3 medium, from 35 to 40 KJ/mol in HCl medium and 31 to 43 KJ/mol in NH 3 medium. Activation energy is lower in absence of inhibitor but with increasing concentration of leaves extracts value of activation energy increases. The increasing value of activation energy with concentration of extract shows that there is an adsorption of leaves extracts on copper surface that blocks the activation site of copper specimen against corrosion, so corrosion rate decreases with increase in concentration of extract. Further Corrosion rate decreases with increase in temperature since deadsorption of adsorbed molecule takes place. Inhibition efficiency and corrosion rate decreases with temperature 231

28 which shows adsorbtion of extracts on copper surface is a physio-adsorption process. It is obvious from data listed in table 2 that, E a of the inhibited solution in this study increases by increasing the extracts concentration indicating strong adsorption of the inhibitor molecules at the metal surface. The range of E a values (28 43 KJ/mol) are lower than the threshold value of 80 KJ/mol, required for chemical adsorption. This means that the adsorption of leaves and bark extracts on copper surface is an physical adsorption. It is evident from table 2 that the value of E a increased on addition of extract solution in comparison to the uninhibited solution. 4.B Determination of enthalpy and entropy Kinetic parameters including enthalpy and entropy were calculated using transition state equation [27] as express by CR = RT Nh S exp R H exp RT (5) Where, CR = Corrosion rate, h = Plank s constant, N =Avogadro s number R = Universal gas constant, T = Absolute temperature. Enthalpy (ΔH) = Slope X 2.302X R Entropy (ΔS) = X R (Intercept-log(R/Nh)) Figure 4.B.2.7 to 4.B.2.9 show the transition state plots for copper corrosion in the absence and presence of different concentrations of leaves extracts of Calotropis procera. The curve plotted between log CR against 1/T shows a straight line. Enthalpies and entropies were calculated using slope and intercept of these lines. Calculated values of enthalpies and entropies are listed in table

29 Figure 4.B.2.7 to 4.B.2.9 Transition state plot of leaf extracts of Calotropis procera Table 5 : Thermodynamic parameters at different concentration of Calotropis Part of plant Leaves procera extracts Medium HNO 3 HCl NH 3 Concentration of extracts (ppm) E a (KJ/mol) H (KJ/mol S (J/Mol) Blank Blank Blank

30 From table 2 it is clearly observed that ΔH values are positive and ΔS values are large and negative with increasing concentration of extracts. ΔH value moves towards higher positive value that shows higher inhibition efficiency of inhibitors. Enthalpies increase with an increase in the concentration of extract of Calotropis procera. The entropy of activation (ΔS) was negative both in the absence and presence of inhibitor. This indicates that in the rate determining step activated complex represents association rather than dissociation, indicating that there is decrease in disorder. 4.B.2.4 Adsorption isotherm The primary step in the action of inhibitors in acid solution is generally agreed to be the adsorption on the metal surface. This involves the assumption that the corrosion reactions are prevented from occurring over the area (or active sites) of the metal surface covered by adsorbed inhibitor species, whereas these corrosion reactions occur normally on the inhibitor-free area. The fraction of surface covered with inhibitor species can be calculate by following equation θ = IE 100 (6) 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 concentration at constant temperature, adsorption isotherm could be evaluated at equilibrium condition. The dependence of the fraction of the surface covered θ on the concentration C of the inhibitor was tested graphically by fitting it to Langmuir s isotherm C θ = 1 k + C (8) For testing the adsorption isotherm obeyed by this system, a graphic relation between the inhibitor concentration C and C/θ at different temperatures, is drawn and represented in figure 4.B.2.10 to 4.B.2.12 Straight line with almost unit slope and correlation coefficient are nearby one, were obtained indicating that the system follows Langmuir adsorption isotherm 234

31 Figure 4.B.2.9 to 4.B.2.12 Langmuir adsorption isotherm of leaf extracts of Calotropis procera where, K is adsorption constant. The K values can be calculated from the intercept lines on the C/θ axis. This is related to the standard free energy of adsorption (ΔG) by following equation- K = exp G RT (8) Where K is the adsorption equilibrium constant, R is the gas constant (8.314J/K/mol), T is the absolute temperature in Kelvin and the value 55.5 is the concentration of water in solution expressed in mol/l The free energy of adsorption ΔG, which can characterize the interaction of adsorbed molecules and metal surface, was calculated using the equation. ΔG = - RT ln (55.5 x K ) (9) 235

32 The values of ΔG for the inhibitor on the surface of copper are given in table 3. The negative values of ΔG indicate the stability of the adsorbed layer on copper surface and spontaneity of the adsorption process. ΔG decreased (become more negative) with increasing temperature, indicating the occurrence of endothermic process. The negative value of ΔG ensures the adsorption process and stability of the absorbed layer on the electrode surface. The stability of the adsorbed layer decreases with increase in temperature. This is clearly seen from the decrease in the absolute value ΔG with rise in temperature. Calculated values of Gibbs free energy are less than the threshold value ( 40 KJ/mol) required for chemical adsorption and the IE% decreased with increasing temperature. These support the mechanism of physical adsorption. Table 6 : Value of ΔG at different temperature for Calotropis procera extracts Part of plant Medium Temperature R 2 G(KJ/mol) HNO Leaf HCl NH

33 4.B.3. Cryptostegia grandiflora (leaves and stem) extract used as corrosion inhibitor for copper in acidic (HCl and HNO 3 ) medium Plant discription :- Cryptostegia grandiflora, commonly known as rubber vine. A rubber vine can grow up to 2 metres tall as a shrub, but when it is supported on other vegetation as a vine, it can reach up to 30 metres in length. Classification:- Kingdom - Plantae Division - Magnoliophyta Class - Magnoliopsida Order Gentianales Family - Apocynaceae Genus - Cryptostegia Species - C. grandiflora Scientific Name - Cryptostegia grandiflora Common name - India Rubber Vine The plant has a wide use in manufacturing of rubber and as a source of hydrocarbon fuels from its latex. From the leaves of Cryptostegia grandiflora, four new cardiacglycosides have been isolated [28-29] 1) Cryptostigmin I - Oleandrigenin3-O-b-glucopyranosyl-(1-4)-b-cymaropyranosyl-(1-4)-digitoxopyranoside, 2) Cryptostigmin II- oleandrigenin3-o-b-glucopyranosyl-(1-4)-arhamnopyranoside, 3) Cryptostigmin III- 16-Propionylgitoxigenin3-O-b-glucopyranosyl-(1-4)-arhamnopyranoside, 4) Cryptostigmin IV- Olean-drigenin3-O-b-glucopyranosyl-(1!6)-bglucopyranosyl-(1-4)-b-cymaropyranosyl-(1-4)-b-digitoxopyranoside, 237

34 From the flowers of Cryptostegia grandiflora two cardenolides oleandrigenin and gitoxigenin as well as, two flavonoid glycosides hyperoside and astragalin and their aglycones querceti and kaempferol were isolated. While, amyrin, lupeol,sitosterol and sitosterol 3-O-Dglucoside, 2,4,6-trihydroxy benzophenone-2-d-glucopyranoside were isolated from the latex of fresh unripe fruits 4.B.3.1 Experimental part 1. Inhibitor solution- Cryptostegia grandiflora extract solution were used as inhibitor. Extract of leaves and stem of Cryptostegia grandiflora were prepared and use against inhibition of corrosion of copper in acidic (HCl and HNO 3 ) medium. Cryptostegia grandiflora leaves and stem were collected, washed with water and dried in shade. Dried leaves and stems were soaked separately in alcohol solution for 72 hours at room temperature. Alcoholic solution was filtered using whatmann filter paper. Filtrate was concentrated from ethyl alcohol solution by heating in condenser and remaining solvent evaporated by keeping the solution for 24 hours. Further it was dried in open. This crude was stored and used to prepare stock solution for inhibition study. Different concentration solutions viz ; 0.1g/l, 0.3g/l,0.5 g/l and 0.7 g/l were used as inhibitor solution. 2. Medium Hydrochloric acid, nitric acid medium have been used for inhibition study. Nitric acid (0.5N) and Hydrochloric acid (1N) solution were prepared by dilution of AR grade HNO 3 and HCl and used as test solution. 3. Metal specimen: - Rectangular size copper specimen having dimension (2.5(length) X 2.5(width) X 0.01(thickness)) cm and a hole near one of edge were used for inhibition study. Specimen were cleaned by emery paper washed with acetone and distilled water, dried and then weighed. 4. Methodology: - Weight loss method has been used to study inhibition property. Experiment was performed using rectangular copper specimen having hole on upper edge. Specimen polish was cleaned with emery 238

35 paper, washed with distilled water and acetone, dried in oven. Each specimen was weighed using an analytical balance of gm accuracy. These specimens were suspended with V shape capillary in beaker containing 100 ml test solution with and without different concentration of leaves and stem extracts. All these beakers were placed in thermostat so that the temperature of beaker solution becomes constant. After 24 hour, all beakers were taken out from thermostat, each suspended specimen was washed, dried and weighed again. Inhibition study was carried out at four different temperatures 303K, 313 K, 323K and 333K. Experiment was repeated using different concentration of extracts at different temperature range. Duplicate set of every experiment were performed so that accurate values of weight loss could be calculate. Weight loss value has been used to calculate corrosion rate, inhibition efficiency, activation energy, enthalpy, entropy, and Gibbs free energy. (a) Weight loss measurement The simplest and most accurate method of estimating the corrosion rate is weight loss analysis. A weighed specimen of the metal or alloy under consideration is introduced into the corrosive environment and later removed after a reasonable time interval. The specimen is then cleaned of all corrosion products and is reweighed. Weight loss of copper specimen after dipping in test solution with and without extracts in different concentration was calculated. The weight loss is converted to an average corrosion rate (CR), as follows, = (1) Where ΔW is the average weight loss of copper specimen, A is the total area of one copper sheet specimen, and t is the immersion time, CR is corrosion rate in mg cm 2 h 1 When metal corrosion is slow and average data are acceptable, weight loss monitoring is the preferred technique. The percent inhibition efficiency (IE%) was calculated for the extracts under investigation according to the following equation [30] 239

36 .. = (2) Where I.E. = Inhibition efficiency W 0 = Weight loss in absence of inhibitor W = Weight loss in presence of inhibitor Surface coverage was calculated by using following formula = 0 0 (3) Weight loss can be a measure for corrosion if specimen are of the same size and have been tested for the experiment as loss in weight per unit area per unit time. The initial specimen weight were recorded using an analytical balance before immersion in 150 ml beakers containing 100 ml of corrosive medium without and with different concentrations of leaves and stem extracts of Cryptostegia grandifloras. The specimens were taken out, washed, dried, and reweighed accurately. The average weight loss of the three parallel copper specimens could be obtained. The corrosion rates of the copper coupons have been determined for 24 hour immersion period at temperature range from 303K to 333 K using Eq(1) and the percentage inhibition efficiency was calculated according the relationship Eq(2). The value of percentage inhibition efficiency (IE) and corrosion rate (CR) obtained from weight loss method at different concentrations of Cryptostegia grandiflora leave and stem extract in acidic solution at temperature range from 303 to 333 K are summarized in table1. 4.B.3.2 Result and discussion 1. Effect of concentration of extract on inhibition efficiency Corrosion inhibition efficiency of Cryptostegia grandiflora leaves and stem extract solutions have been calculated using weight loss measurement after 24 hours of immersion at various temperatures ranging from 303 to 333K. In 240

37 blank sample no extract solution was used and other solution contained concentration 0.1g/l, 0.3g/l,0.5 g/l and 0.7 g/l of extract. Table 1 shows the calculated values of inhibition efficiency and corrosion rate in hydrochloric acid, nitric acid medium. With increasing concentration of extracts inhibition efficiency increases. It is clear from table 1 that inhydrochloric acid medium leaves and stem extract show maximum inhibiton efficiency. Leaves extract show maximum efficiency 73% at 0.7 g/l concentration and stem extracts show 69 % inhibition efficiency in hydrochloric acid medium. For leaves extract maximum efficiency in nitric acid medium is 62% and for stem extracts 57% inhibition efficiency show at 0.7g/l concentration. Corrosion rate decreases with increase in concentration of extracts which show that inhibition efficiency increases due to decrease in corrosion rate. 2. Effect of solution temperature on inhibition efficiency Corrosion inhibition studies have been carried out at temperature range from 303 K to 333K and calculated values are reported in table 1. Results reveal that with increasing temperature inhibition efficiency decreases which is due to deabsorption of inhibitor molecules on metal surface. Maximum inhibition efficiency was shown at 303K temperature for leaves and stem extracts. While going to higher temperature from 303 to 333 K inhibition efficiency decrease. For leaves extracts inhibition efficiencies decrease from 73% to 60 % in hydrochloric acid medium with increase in temperature. For stem extracts inhibition efficiencies decrease from 69% to 58 % with increase in temperature. In nitric acid medium inhibition efficiency decrease with increase in temperature for leaves and stem extracts. The temperatures dependence on IE have been shown by plotting graph between temperature and inhibition efficiency at various concentration as show in figure 4.B.3.1to 4.B

38 Table 7 : Inhibition efficacy and corrosion rate of Cryptostegia Grandiflora for copper Plant part Leaves Stem Medium HNO 3 HCl HNO 3 HCl Tem(K) Con.of Corr. Corr. Corr. Corr. Wt.loss I.E Wt.loss I.E Wt.loss I.E Wt.loss extract(ppm) rate rate rate rate I.E Blank Blank Blank Blank