Thermodynamic and Kinetic Parameters of Methanolic Extract of Leaves of Cochlospermum Tinctorium as Eco-friendly Inhibitor for Corrosion of Mild Steel in HCL Medium

© Nigerian Journal of Pure and Applied Sciences ISSN: 0795 25070 Vol. 6, 2014, pp. 83-90 NJPAS re a u nd P f A o p l p a li n e r d u o S J c i n e a n i c r e e s gi N electrons and electron around the atoms (Arora et al., 2007). Quite a number of extracts from plants have been investigated (Avwiri and Igho, 2003; El-Etre, 2003; Orubite and Oforka, 2003; Benabdellah et al, 2006; Oguzie, 2006; Eddy et al., 2008; Eddy and Ebenso, 2008; Adejo et al., 2010, Nnanna et al., 2012, Adejo et al., 2013). Cochlospermeae is a well-known plant family in herbal medicine. The species Cochlospermum tinctorium, the plant of interest in this work, is a widely distributed plant within the savanna areas of West and Central Africa (Etuket al., 2009). This plant is called balagarde in Hausa, abanzi in Igbo, cewatu in Yoruba, ukpatafulo in Idoma and kpavande in Tiv, all these being the tribes in Nigeria, signifying the importance of the plant in the country. In Mali and Ivory Coast the plant is widely used for the treatment of jaundice, malaria, abdominal pain, wound healing, haemorrhoids, intestinal worms, bilharizias and hepatitis (Togolaet al., 2005). Nergard et al., (2005) has shown that pectic polysaccharides obtained from the plant are responsible for its bioactivities. In this study, methanol extract of leaves of the plant was put into a different use; as inhibitor of corrosion of mild steel in HCl solution. The thermodynamic, kinetic and adsorptive parameters of the inhibition process were evaluated at two temperatures of 303 Kand311 K, a narrow temperature range employed to maintain the molecular structure(s) of the component(s) responsible for the inhibition process in the course of the experiment. Materials and Method Material preparations A sample of mild steel of composition (%W); Mn(0.56), P (0.04), C(0.27), Si(0.25), S(0.04) with rest being Fe, was obtained from Department of Mechanical Engineering, University of Agriculture, Makurdi-Nigeria. Coupons of the dimension of 2 cm x 2 cm x 0.13 cm (with a 2.5 mm hole drilled through an edge of each) were prepared as described in Eddy and Ebenso (2008). Leaves of the plant were shade-dried, ground and 20 g soaked in 200 ml of absolute methanol in a 250 ml volumetric flask, which was properly corked and kept, with occasional shaking, for 48 hours. The extract obtained, therefrom, was filtered and the methanol was carefully evaporated at 321 K, to available any possible structural changes to any component of the extract, using Clifton Model thermostated water bath. Stock solutions of the extract of the concentrations of 0.1, 0.2, 3 0.3, 0.4 and 0.5 g/dm were prepared in 2 M HCl. Analytical grade reagents were all used, with double distilled used for all solution preparations. Method The Weight loss measurement was carried out as reported in Orubite-Okorosaye and Oforka (2004) at 303 Kand 311 K in the thermostated water bath. The first part of the experiment involved weighing the coupon using ae Adam AFP weighing balance (4100 Model) and suspending it in a beaker containing the corrodant, which was a 100 ml of the acid in 250 ml beaker, using a thread passed through the hole to tie it to a retort stand. The coupon was retrieved from the solution at an interval of 2 hours for 12 hours. The procedure was repeated using solution of different concentrations of the extract introduced into the corrodant. At retrieval, each coupon was dipped immediately into a saturated ammonium acetate solution at room temperature to terminate the reaction, washed with distilled water and then dried in acetone, kept in a desiccator to reach ambient temperature and carefully re-weighed. The weight loss was calculated using the expression, (1) where W is weight loss of coupon, W is the i weight before immersion and W the final f weight of the coupon after retrieval. The inhibition efficiency, %IE, and degree of surface coverage, è, were calculated 84 Thermodynamic and Kinetic Parameters of Methanolic Extract of Leaves of Cochlospermum ...


Introduction
Humanity has now come to terms with the repercussions and the environmental accidents that our behaviour has impacted on the earth, putting it in a compromising future. Chemical weapons, pesticides, fertilizers, industrial effluents, paints and organic solvents have changed the health of plants and w o r l d i n h a b i t a n t s t o t h e p o i n t o f contaminating even the most remote places on the planet (Hodgeset al., 2010). As a result, man at the moment is environmentally conscious, trying to be prudent and transparent in the use of chemicals. This has resulted in the search for eco-friendly chemicals within the scientific and engineering circles. The focus on plant extracts, away from synthetic inhibitors, as eco-friendly inhibitors for metal corrosion prevention is due mainly to economic reasons, strict environmental r e g u l a t i o n s , r e n e w a b i l i t y a n d biodegradability (Arora et al., 2007;Selviet al., 2009, Alaneme andOlusegun, 2012). The extracts such natural substances contain mixtures of compounds having oxygen, nitrogen, sulphur and phosphorus which help in corrosion inhibition process (Jai and Ali, 2008). The inhibition efficiency of the compounds containing these elements in their structures as corrosion inhibitors can be attributed to the number of mobile electron pairs present, the orbital character of free Thermodynamic, kinetic and adsorptive parameters of methanolic extract of leaves of Cochlospermum tinctorium as eco-friendly inhibitor for the corrosion of mild steel in HCl medium were evaluated through the weight loss method at 303 K and 311 K. The inhibition efficiency, %IE, increased with increase in extract concentration, but found to be better at 303 K than 311 K. The decrease in %IE with increase in temperature with the average value of 81.88 kJ/mol for activation energy, E , and the values NJPAS re a u nd P  f  A  o  p  l  p  a  li  n  e  r  d  u  o  S  J  c i  n  e  a  n  i  c  r  e  e s g i N electrons and electron around the atoms (Arora et al., 2007). Quite a number of extracts from plants have been investigated (Avwiri and Igho, 2003;El-Etre, 2003;Orubite and Oforka, 2003;Benabdellah et al, 2006;Oguzie, 2006;Eddy et al., 2008;Eddy and Ebenso, 2008;Adejo et al., 2010, Nnanna et al., 2012, Adejo et al., 2013. Cochlospermeae is a well-known plant family in herbal medicine. The species Cochlospermum tinctorium, the plant of interest in this work, is a widely distributed plant within the savanna areas of West and Central Africa (Etuket al., 2009). This plant is called balagarde in Hausa, abanzi in Igbo, cewatu in Yoruba, ukpatafulo in Idoma and kpavande in Tiv, all these being the tribes in Nigeria, signifying the importance of the plant in the country. In Mali and Ivory Coast the plant is widely used for the treatment of jaundice, malaria, abdominal pain, wound healing, haemorrhoids, intestinal worms, bilharizias and hepatitis (Togolaet al., 2005). Nergard et al., (2005) has shown that pectic polysaccharides obtained from the plant are responsible for its bioactivities.
In this study, methanol extract of leaves of the plant was put into a different use; as inhibitor of corrosion of mild steel in HCl solution. The thermodynamic, kinetic and adsorptive parameters of the inhibition process were evaluated at two temperatures of 303 Kand311 K, a narrow temperature range employed to maintain the molecular structure(s) of the component(s) responsible for the inhibition process in the course of the experiment.

Material preparations
A sample of mild steel of composition (%W); Mn(0.56), P (0.04), C(0.27), Si(0.25), S(0.04) with rest being Fe, was obtained from Department of Mechanical Engineering, University of Agriculture, Makurdi-Nigeria. Coupons of the dimension of 2 cm x 2 cm x 0.13 cm (with a 2.5 mm hole drilled through an edge of each) were prepared as described in Eddy and Ebenso (2008).
Leaves of the plant were shade-dried, ground and 20 g soaked in 200 ml of absolute methanol in a 250 ml volumetric flask, which was properly corked and kept, with occasional shaking, for 48 hours. The extract obtained, therefrom, was filtered and the methanol was carefully evaporated at 321 K, to available any possible structural changes to any component of the extract, using Clifton Model thermostated water bath. Stock solutions of the extract of the concentrations of 0.1, 0.2, 3 0.3, 0.4 and 0.5 g/dm were prepared in 2 M HCl. Analytical grade reagents were all used, with double distilled used for all solution preparations.

Method
The Weight loss measurement was carried out as reported in Orubite-Okorosaye and Oforka (2004) at 303 Kand 311 K in the thermostated water bath. The first part of the experiment involved weighing the coupon using ae Adam AFP weighing balance (4100 Model) and suspending it in a beaker containing the corrodant, which was a 100 ml of the acid in 250 ml beaker, using a thread passed through the hole to tie it to a retort stand. The coupon was retrieved from the solution at an interval of 2 hours for 12 hours. The procedure was repeated using solution of different concentrations of the extract introduced into the corrodant. At retrieval, each coupon was dipped immediately into a saturated ammonium acetate solution at room temperature to terminate the reaction, washed with distilled water and then dried in acetone, kept in a desiccator to reach ambient temperature and carefully re-weighed. The weight loss was calculated using the expression, where W is weight loss of coupon, W is the i weight before immersion and W the final f weight of the coupon after retrieval. The inhibition efficiency, %IE, and degree of surface coverage, è, were calculated by the use of equations (2) and (3), respectively. (2) ( 3) whereW and W are weight losses of the 1 2 coupon in the presence and absence of inhibitor, respectively, at the same temperature.
The corrosion rate, CR, of the mild steel was obtained using the NACE (1984)  (4) whereW is weight loss (mg), D the density of 3 the mild steel (g/cm ), A the surface area of the 2 coupon in cm and T the exposure time in hours and mdd is mg/dm/day. The extract has inhibitive effect on the corrosion of mild steel in HCl medium as it can clearly be seen from Table 1 that the corrosion rate decreases and IE% increases with increase in the extract concentration at the two temperatures. However, the inhibitor performance is higher at the lower temperature, suggestive of physical adsorption mechanism for adsorption of the inhibitor onto the surface of the metal (Siakaet al., 2012).

Thermodynamic parameters
The activation energy, E , of the a corrosion process was evaluated by the used of Arrhenius equation (equation 5) (Atkins, 1982;Eddy et al., 2008).
whereCR is corrosion rate, A the Arrhenius parameter, E the activation energy of the a corrosion reaction, R the gas constant and T the temperature in Kelvin. A linearised form of equation (5) is given thus; For corrosion rates at two temperatures (T 1 and T ) the expression below is obtained from 2 equation (6).
Corrosion rates were evaluated at 303 K and 311 K. In spite of the closeness of the temperatures it was found there exist a significant difference in the corrosion rate between the two temperatures. Calculated corrosion rate and inhibition efficiency at the two temperatures are shown in Table 1, while  Table 2 shows the values of activation, heat of adsorption, free energy and half-life for the corrosion process. C is the concentration of the extract and 55.5 is concentration of water expressed in moles 3 (i.e. ~ 1000 g/dm ) (Bastidas et al., 2005). Calculated values of ÄG are all ads negative, an indication of spontaneity of adsorption process. However, ÄG values are ads more negative at 303 K than 311 K, implying that the spontaneity of the adsorption process and hence the stability of the adsorbed layer of the inhibitor is higher at lower temperature (Atkins, 1982).

Kinetic Consideration
Studies have shown that the corrosion reaction of metals is of first-order kinetics (Okafor et al., 2010;Nair et al., 2010) and therefore first-order equations were employed. Values of the half-life, t in hours 1/2, for the corrosion process in presence of the inhibitor were evaluated by the use of equation (10) (Atkins, 1982;Onen and Nwufo, 2010 where k is the rate constant for the corrosion reaction, which was obtained by the use of the integrated rate equation for first-order reaction (equation 11).
A is the initial weight of the steel sample and o A is the weight left after the corrosion t reaction. The t was found to increase with 1/2 increase in the inhibitor concentration (Table  2), implying a decrease in k with increase in the concentration. This means that the inhibition process increased with the concentration of the inhibitor. Values of t 1/2 were found to be higher at 303 K than at 311 K, indicative of the fact that the corrosion reaction increased at the higher temperature, Table 2. Evaluated Values of Activation Energy ( E a ), Heat of Adsorption ( Q ads ), Free Energy (ÄG ads ) and Half-life (t 1/2 ) for the Inhibition Process.
The value of Ea was found to be higher in the presence of the extract than pure corrodant. E values in the presence of the a inhibitor ranged from 71.74 to 93.37 kJ/mol. The values of E are close to 80 kJ/mol, the a threshold value for physical adsorption mechanism, which is clearly supportive of physical adsorption mechanism (Eddy et al., 2008).
Q values evaluated through equation ads (8) are all negative (average = -87.01kJ/mol), an evidence of exothermic process for the adsorption of this extract onto the mild steel surface. These values are large suggesting that the heat of adsorption for the process was quite large. The negative values also show that the adsorption and hence the efficiency of the inhibitor will decrease with increase in temperature (Eddy and Ebenso, 2008).  another evidence in support of physical adsorption mechanism as earlier stated.

Adsorption Consideration
C o m m o n a d s o r p t i o n i s o t h e r m s frequently used in trying to understand the mechanism of inhibition of corrosion reactions are the Langmuir, Frumkin, Freundlich, Temkin, Flory-Huggins, El-Awady and Bockris-Swinkels. A general equation to represent these isotherms is given by equation (12) (Eddy et al., 2008).
wheref(è, x) is configuration factor that is dependent upon the physical model and the assumption underlying the derivation of the isotherm, è the degree of surface coverage, C the concentration of inhibitor in the electrolyte, x is the size ratio, The adsorptive parameters obtained from the isotherm plots given in Table 3. From the 2 coefficient of determination, R values it is , evidently clear that the Langmuir isotherm is most applicable model for the adsorption of this extract onto the mild steel surface, while Freundlich and El-Awady are applicable at 311 K. The adherence of the adsorption to the Langmuir isotherm has been argued by Orubite-Okorosaye and Oforka (2004) to signify that the extract covered both the anodic and cathodic sites on the metal surface. The argument is quite plausible considering the fact that Langmuir isotherm involves monolayer over the entire surface. The positive value of K signifies favourable adsorption, and judging from the values at the two temperatures, the adsorption process was more favourable at the lower temperature.
The parameter in the Freundlich isotherm is related to intensity of adsorption and it varies with heterogeneity of the material (Chatterjeeet al, 2009). The values of Freundlich isotherm constants and K F obtained are 0.7853 and 2.4444 and 0.0819 and 0.8287 at 303 K and 311 K, respectively. n values are within the normal beneficial F adsorption of between 1 and 10 (Adejo et al., 2008), but higher at higher temperature with 2 even a better R . This is, however, in invariance with the observation in the case of Langmuir. According to Khadom et al (2010), typical value of n is 0.6, which implies that F this adsorptionprocess cannot be reasonably 3 The negative sign of áin Frumkin isotherm signifies existence of molecular 2 interaction within the adsorbed layer. The R are relatively low, and therefore, this adsorption cannot also be reasonably modeled 2 by the isotherm. The fact that R values are better in case of Langmuir isotherm than Freundlich and Temkin isotherms implies that the adsorption is monolayer and close to ideal.
K corresponding to equilibrium constant for the adsorption isotherm in the case of El-Awady and is given by, where y is the number of inhibitor molecules occupying one active site of the adsorbent. Value of y > 1 implies the formation of multilayers of inhibitor on the surface of metal; value of y < 1 indicates the formation of monolayer on the metal surface (Niamienet al., 2012;Umasankareswari and Jeyaraj, 2012). y values are less than unity at the two temperatures, an indication of monolayer adsorption. 1/y is the number of surface active sites occupied by one inhibitor molecules (Ayobe et al., 2012), and its values are 2.6709 and 1.3321 at 303 K and 311 K, respectively.

Conclusion
This study has demonstrated that Cochlospermum tinctorium methanolic leaves extract has good inhibitive action against the corrosion of mild in HCl medium. The inhibition action, which was found to be spontaneous and exothermic, was better at lower temperature and can be better modeled by Langmuir isotherm.