Assessment of anti-corrosion potentials of extract of Ficus asperifolia -Miq (Moraceae) on mild steel in acidic medium

The inhibitory potentials of ethanolic extracts and four fractions of the stem bark of Ficus asperifolia – Miq. for the corrosion of mild steel were assessed using weight loss method, Fourier atomic absorption spectroscopy, scanning electron microscopy (SEM). The effects of varying immersion period, concentration of the inhibitors and temperatures for mild steel corrosion in 5 M HCl acid solution were carried out. A detailed kinetic reaction, thermodynamic (Ea, ∆H, ∆S) and adsorption isotherm studies were obtained. The crude extract of F. asperifolia (EFA) has the highest inhibition efficiency of 55%. The inhibitory efficiency of the four fractions of F. asperifolia (EFA) compared with the crude extract was in the order ethyl acetate > n-hexane > butanol > EFA > Aqueous. The inhibition efficiency was found to increase with increase in the extracts concentration but decreased with temperature. Corrosion rate increases with time but decreases with extracts concentration. The kinetic studies of the data followed a first order reaction. Thermodynamic studies revealed that corrosion inhibition may be due to spontaneous mixed adsorption of the plant constituents on the metal surface. Experimental data were fitted to Langmuir, Freundlich and Temkin adsorption isotherms. The plant investigated showed a good potential for green corrosion inhibitors. Investigation of phytochemical constituents showed that the extracts contain alkaloids, flavonoids, saponins, tannins anthraquinones and reducing sugars.


INTRODUCTION
Mild steel has found urge and umpteen importance in the developing countries, emerging economies as well as developed world as it forms the backbone of industrialization. One of the most challenging and difficult tasks for industries are the protection of metals from corrosion (Al-Otaibi et al., 2012). Acidic solutions are widely used in industries and the areas of application are acid pickling, industrial acid cleaning, acid descaling and oil-well cleaning (Schmitt, 1984;Ashassi-Sorkhabi et al., 2009). Corrosion inhibitors are needed to reduce the corrosion rates of metallic materials in these acidic media (Lagrenee et al, 2002). Phyto-inhibitors now attract more attention in the research world over the synthetic counterparts due to their low level or no toxicity, eco-*Corresponding author. E-mail: fcfatherray2010@yahoo.co.uk.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License friendliness, availability, affordability and efficient nature of this class of inhibitors (Al-Otaibi et al., 2012). The synthetic inhibitors are now generating greater environmental concerns due to toxic effect of the synthetic compounds on human and animal life during their production and use.
The successful use of phytoconstituents as corrosion inhibitors in acidic media have been reported by some researchers (Olasehinde et al., 2013, Fouda et al., 2014, Devarayan et al., 2012, Abdulrahmanet al., 2011, Ten et al., 2012, Chen et al., 2013, Behpour and Mohammadi, 2012Negm et al., 2012). Efforts were made in this study to establish the inhibitory efficiency of F. asperifolia (Miq-Moraceace) stem bark extracts and the respective fractions; towards activity-directed purification of the plant extracts for better or improved inhibitory activities.

Materials preparation
Mild steel of known elemental composition (%) was used for this study: Mn (0.56), S (0.03), P (0.02), C (0.12), Si (0.17), Cu (0.28) and the rest Fe. It was mechanically pressed cut into coupons of dimensions 18 × 16 × 4 mm before carrying out corrosion test on them. The coupons were used as supplied without any form of polishing but surface treatment of the coupons by degreasing in absolute ethanol and drying in acetone were carried out. The coupons were then preserved under air tight dessicator with activated dessicant to avoid contamination before their use in the corrosion studies James and Akaranta, 2009). All the solvents used for this study were redistilled before usage to ensure purity.

Collection, authentication, preparation and extraction of plant materials
The stem bark of the plant, F. asperifolia was collected in June, 2014 at Ile-Ife, Nigeria. The plant material was authenticated at the Botany Department, Obafemi Awolowo University, Ile-Ife. The stem bark was cut into pieces, washed thoroughly with water, and dried using vacuum drying oven at 40°C. The sample was pulverized using Buchi milling machine (1200 rpm) and stored for Fadare et al. 9 further analysis. Powdered stem bark of F. asperifolia was soaked in 80% ethanol for 72 h. It was filtered and extract was concentrated in-vacuo to dryness using rotary evaporator and the weight of the extract was determined. Five different concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 g/100 mL) of the extract were prepared with 5 M hydrochloric acid solution and were used for all the experiments.

Chemical analysis
Qualitative and quantitative analysis of the crude extracts of F. asperifolia was carried out according to the method reported by Onyeka and Nwabekwe (2007).

Solvent partitioning: modified Kupchan partitioning method
The ethanolic extract (40.0 g) of F. asperifolia was suspended with 200 mL distilled water each, and successively partitioned into n-Hexane (1500 ml), ethylacetate (2800 ml), and n-Butanol (400 ml) based on the polarity of the extracts, and were in turn concentrated in vacuo to dryness to give four partitioned fractions (Wagenen et al., 1993). Five different concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 g/100 ml) of each of the fractions were prepared with 5 M hydrochloric acid solution and were tested for corrosion inhibitory activities.

Gravimetric study
In the gravimetric study, a known weight metal (mild steel) coupon was completely immersed in 100 mL of the test solutions, the respective concentration of the blank (5 M HCl) and the inhibitor solutions for 4 h after which it was retrieved, washed with distilled water, alcohol, dried and weighed. The difference in weight for a period of 4 h was taken as weight loss.
From the weight loss results, corrosion rate (CR) and percentage inhibition efficiency (I.E%) were calculated using Equations 1 and 2. (1) where is weight loss, that is, , is the area of the mild steel coupon (in cm 2 ) and is immersion time (in hours) where and correspond to the corrosion rates in the presence and absence of inhibitor respectively.
The surface coverage was calculated by using Equation 3: (3)

Kinetic studies
In order to investigate the order of the reactions and evaluate the mass loss, the experiments were carried out at room temperature at various inhibitor concentrations of 0.1 to 0.5 g/100 mL extract in 5 M HCl. The pre-weighed coupons were immersed in 100 mL of the respective inhibitor/blank solutions. The coupons were retrieved from the solutions, washed in distilled water, alcohol, dried and weighed at 24 h interval progressively for 7 days. The order of reaction and half life were determined from the results obtained using Equations 4 and 5 respectively. (4) where is the initial weight of the metal, is the final weight, is the duration of immersion (time) or exposure of the metal to acidic medium, and is the rate constant.
From the rate constant , the half life (t1/2) can be calculated using: (5)

Temperature variation
The pre weighed coupons were dipped in 100 mL of the various concentrations of the inhibitors/blank solution ranging from 0.1 to 0.5 g/100 mL in 5 M HCl and maintained at 303, 313, 323 and 333k in a thermostatic water bath for 4 h. This was replicated. After which it was retrieved, rinsed in distilled water, degreased in ethanol, dried in acetone and weighed. The results obtained were fitted into different isotherms and the thermodynamic parameters were calculated.

Atomic absorption spectroscopy (AAS) analysis
Atomic absorption analysis was conducted by using atomic absorption spectrometer model PerkinElmer AAnalyst 400. This was carried out to determine the concentration of iron (II) ions in 5 M HCl after 4 h immersion time in the presence of the extracts of F. asperiofolia. The pre-weighed coupon was immersed in a solution of HCl containing the plant extract for 4 h to examine the amount of Iron (II) in the acidic solution.
The calibration curve of iron (II) ions was drawn before analyzing the electrolyte solution. All samples containing iron (II) were diluted with distilled water to ensure that the concentration of metal ions is within the range of the calibration curve.

Scanning electron microscopy (SEM) analysis
The surface morphological analysis of mild steel after 4 h immersion in 5M HCl containing 1% w /v EFA extract was studied to understand the changes that occur during the corrosion of mild steel in the presence and absence of stem bark extracts using scanning electron microscope (Model: Aspex 3020).

Phytochemical constituents of F. asperifolia
Corrosion inhibition performance of organic compounds can be evaluated using electrochemical and chemical techniques (Obi-Egbedi and Obot, 2010). Presence of secondary metabolites in plants extracts has been identified to be responsible for the corrosion inhibitory properties on metal surface (Vijayalakshmi et al., 2011;Ating et al., 2010, Okafor et al., 2012 Phytochemical screening of F. asperifolia using qualitative methods revealed the presence of Alkaloids, saponnin, tannins, anthraquinones, cardiac glycosides and flavonoids while the quantitative phytochemical analysis revealed that flavonoid, saponin, tannin, cardiac glycoside and terpenoid has 52., 72.4, 31.2, 11.5 and 29.8 mg/ml respectively ( Table 1). The chemical structures of most of these phyto-constituents contained electron rich bond or hetero atoms that facilitate their electron donating ability; hence the inhibition of the corrosion of mild steel by ethanolic extracts of F. asperifolia is attributed to the phyto-constituents of the extract. Similar inferences have been reported by some researchers for the inhibition of the corrosion of mild steel by ethanol extract of some plants (Bendahou et al., 2006;Ebenso et al., 2008). Possession of π-electrons or suitable functional groups may facilitate the transfer of charge from the inhibitor's molecule to the charged metal surface (physical adsorption) or transfer of electron from the inhibitor's molecule to the vacant-orbital of the metal (chemical adsorption) (Eddy et al., 2011). Ashassi-Sorkhabi et al. (2009) implicated tannins in their study as a corrosion inhibitor. The inhibitive properties of tannins have been attributed to the reaction of the polyphenolic fraction of the tannins moieties, which ensures effective protection of the metal surfaces (Loto, 1998;Anauda et al., 2005;Nnanna et al., 2010).
Flavonoids are widely distributed in plants and give the flowers and fruits of many plants their vibrant colours. Flavonoids have antioxidant activity, anti-allergic, anti-cancer, anti-inflammatory and anti-viral activities. Al-qudah (2011) established in his study, using three different types of flavonoids and about 92% inhibitory efficiency of these flavonoids was reported.
Terpenoids and other constituents also posses functional groups which are capable of chelating with metal ions and thus facilitate strong coordination on the metal surface (Oguzie, 2008).
Some of these phyto-constituents observed in the plant of study have been implicated in one corrosion inhibitory activity or the other. It was reported (Schmitt, 1984) that as a rule of thumb, N-containing compounds such as amines, amino acids and quaternary ammonium salts exert their best efficiencies in prevention corrosion of iron and steel in HCl solutions while they are relatively ineffective in H 2 SO 4 and this behaviour is in good agreement with the obtained results in this study. An acceleration behavior was reported for mild steel in H 2 SO 4 in the presence of some phenols and this behavior tends to decrease with the increase in the inhibitor concentration, while they behave very well as corrosion inhibitors in HCl solutions (Rengamani et al., 1994). These observations are similar to that obtained in this study.

Effect of time on weight loss
For the chemical methods, a weight loss measurement is   ideally suited for long term immersion test. Corroborative results between weight loss and other techniques have been reported (Shukla and Quraishi, 2009;Singh and Quraishi, 2010;Obi-Egbedi and Obot, 2010). De Sauza and Spinelli (2009) reported that weight loss provides more reliable results than electrochemical techniques for the determination of corrosion rates and inhibition efficiency. Weight loss method is widely used for inhibition efficiency test (Obi-Egbedi and Obot, 2010;Musa et al., 2010;Khadom et al., 2010;Bouklahet al., 2006;Mercer, 1985). The simplicity and reliability of the measurement offered by the weight loss method are such that the technique forms the baseline method of measurement in many corrosion monitoring programmes (Obi-Egbedi and Obot, 2010; Afidah and Kassim, 2008). Figure 1 shows the variation in weight-loss of mild steel in 5 M HCl medium in the absence and presence of different concentration of the F. asperifolia stem bark extract as a function of time. The concentration of the extract varied from 0.1 to 0.5% w / v , and the time of immersion was varied from 0-168 hours at room temperature. From the results obtained, it was observed that weight-loss increase (up to 2.015 g) with increase in immersion time but decrease with increase in concentration of the extract to as low as 0.896 g at 0.5% w / v EFA concentration. The decrease observed in the weight loss with increase in the concentration of the extracts suggest an increased surface coverage on the metal and the adsorption of the phyto-constituents on the metal surface creating a barrier between the metal and the acidic medium. This enhances the inhibition of the metal from corroding due to the acidic environment hence reduction in weight loss with respect to the metal in the blank solution.
The plot of weight loss against time for the corrosion inhibition of mild steel in the presence and absence of the four fractions of the extract of F. asperifolia (EFA) namely: n-hexane, ethyl acetate, butanol and aqueous are shown in Figures 2 to 5.
From the plots, the weight loss increased with increase in immersion time but decreased with inhibitor concentration. The four fractions of F. asperifoila (EFA) followed similar trend with the crude extract but some of the fractions showed a greater inhibitory efficiency against corrosion compared to the crude extract.
Figures 6 and 7 compared the weight loss as well as the inhibitory efficiency within the four fractions and the crude extract of F. asperifoila (EFA). The result revealed that ethyl acetate fraction has the highest inhibitory efficiency amidst the four fractions and also better than the crude extract. This was followed by n-hexane, butanol, EFA and aqueous respectively. This suggests an increase in surface coverage by the phyto-constituents of the extract and the phyto-protective activity of the extract (Obi-Egbedi  and Obot, 2010). It was also observed that further purification of the crude extract suggest better and greater inhibitory efficiency as well as reduction in the amount of inhibitor required per time for corrosion control.

Effect of extract concentration on inhibition efficiency
The effect of F. asperifolia extract concentrations on the    the concentrations of the extract. There was an increase in the percentage Inhibitory efficiency of F. asperifolia from 29.6 to 55.5%. This is in agreement with other studies (James and Akaranta, 2009;Olasehinde et al., 2013;Owate et al., 2014). These results show that the plant under study is good green inhibitor against corrosion of mild steel. There is reduction in the concentrations of Fe

Effect of temperature on corrosion rate
Corrosion rate is a function of weight loss. Temperature is an important parameter in studies on metal dissolution studies (de Sauza and Spinelli, 2009). The corrosion rate in acid solutions, for example, increases exponentially with temperature increase because the hydrogen evolution over potential decreases (Popova et al., 2003).
To assess the effect of temperature on corrosion and corrosion inhibitive process, weight loss experiments were performed at 10 K intervals in the temperature range 303 to 333 K in uninhibited acid (5 M HCl) and in inhibited solutions containing different concentrations of EFA as presented in Figure 9 to evaluate the stability of adsorbed layer/film of inhibitor on mild steel surface (Olasehinde et al., 2013). Corrosion rate of mild steel increased with increase in temperature but decreases with increase in the concentration of the extracts (Figure 9). There are similar observation reported in the literatures (Umoru et al., 2006;Ating et al., 2010;Vijayalakshmi et al., 2011;Okafor et al., 2012). The rate of corrosion of mild steel increases as a result of increase in  the average kinetic energy of the reacting molecules. However, the corrosion rate is much decreased for higher concentration of inhibited acid solution than the solution with low concentration of inhibitors. The decrease in the corrosion rate can be attributed to mitigating effect of the plant extract on the corrosion rate of the mild steel.

Effect of temperature on inhibition efficiency
To evaluate the stability of adsorbate on the mild steel surface, weight loss measurements were carried out in the temperature range of 303 to 333 K in the presence and absence of extract of F. asperifolia. Corrosion rate has a corresponding relationship with inhibition efficiency and from the plot (Figure 10), it was observed that inhibition efficiency increase with increase in the concentration of the extract but decrease with increasing temperature due to increased rate of dissolution process of mild steel and partial desorption of the phyto-inhibitor from the mild steel surface with temperature. Therefore, decrease in inhibition efficiency with increase in temperature suggests a physisorptions mechanism of the inhibitor components onto the mild steel surface (Ating et al., 2010). This reveals the extracts under investigation as potent inhibitors for corrosion of mild steel in 5 M HCl (Olasehinde et al., 2013;Blaedel and Meloche, 1963).

Kinetic studies
In order to assess the stability of inhibitive behavior of the extracts on a time scale, weight loss measurement were performed in 5 M HCl acid solution in absence and presence of the extract EFA at 0.1 to 0.5% w / v concentrations for 1 to 7 days at room temperature. The data obtained from the plots of variation of weight loss against time with respect to concentration were fitted into Equation (4) to test for the order of corrosion reaction in the presence of EFA. Representative plot for the first order reaction in different concentrations of the inhibitor is shown in Figure 11. The plots were linear with R 2 values of 0.9873, 0.9855, 0.9975, 0.9929 and 0.9906 at 0.1, 0.2, 0.3, 0.4 and 0.5% respectively. This indicates that a first order kinetic is applicable to the corrosion of mild steel in the presence of extracts of F. asperifolia as inhibitor. Also, the half lives (t 1/2 ) of the corrosion of mild steel in various media were calculated using Equation (5) (Nkuzinna et al., 2011). The values of rate constant (k) obtained from the slope of the plots of equation (4) and the values half life calculated from equation (5) is presented in Table 2.
From the results obtained the rate constant decreases with increase in the concentration of the extracts of EFA. Also, it is significant to note that half life increases with increase in concentration of the extract.
As the half-life increases, the corrosion rate decreases, which is an indication that more protection of the metals by the phyto-constituents of F. asperifolia extract. The findings were in accordance with James and Akaranta, 2009;Okaforet al., 2010;Olasehinde, 2013.

Scanning electron microscopy (SEM)
The surfaces of the mild steel strip used for the study were examined using scanning electron microscope model Aspex 3020 with 100X magnification. SEM micrographs of the mild steel before and after immersion in 5 M HCl solution in the absence and presence of inhibitors (EFA and EEC) are presented in Plates 1 to 3.
Surface analysis using scanning electron microscope provided more information on the level of attack as well as inhibition strength of the extract on the surface of the mild steel. The surface of the mild steel before immersion (Plate 1) appeared fine and smooth with a clear uniform pattern. This shows absence of any corrosion products formed on the metal surface (Johnsirani et al., 2012).
Plate 2 shows the micrograph of the mild steel immersed in 5M HCl solution in the absence of inhibitor for 4 h. The surface of the treated coupon shows etching composed of white and dark areas (Villamil et al., 1999;Fouda et al., 2014), the white areas represent the ferrite phase and the dark areas represent the pearlite (mixture of ferrite and cementite (FeC 3 ) in a lamellar form). It indicates clear disarrangement in the surface formation of the mild steel due to high metal dissolution rate and more active sites available for corrosion (Ji et al., 2012;Johnsirani et al., 2012).
The micrograph of the mild steels immersed in 5 M HCl solution containing 0.1% w / v EFA (Plate 3) shows that the surfaces were smoother when compared to the one without inhibitors. The plate shows less extensive attack in the presence of the extract than in the uninhibited acid. This implies that the extracts components adsorbed on the metal surface forming protective film over the surface of the mild steel thereby decreasing the mild steel dissolution in acidic medium. The protective film acts as a barrier between the steel and the acidic medium and thus retards the corrosion reactions (Loto et al., 2011). This is in line with various studies on plant extracts James and Akaranta, 2009;Dahmani et al., 2010;Sherine et al., 2010;Dauda et al., 2013;Nnanna et al., 2010;Fouda et al., 2014).

Thermodynamics studies
From Arrhenius equation (6), the logarithm of corrosion rate is a linear function of temperature. This was used to calculate the activation energy of the reactions in the presence and absence of the extract of EFA.

(6)
Where C R is the corrosion rate, E a is the apparent activation energy, R is the molar gas constant, T is the absolute temperature and A is the Arrhenius pre-exponential factor. The Arrhenius plot obtained gave a straight line graph as shown in Figure 12 with a slope of -E a /2.303R. The value of corrosion rate is jointly decided by the activation energy and the pre-exponential factor (Khadom et al., 2009). Analysis of the temperature dependence of inhibition efficiency as well as comparison of corrosion apparent activation energies in absence and presence of inhibitor gives some insight into the possible mechanism of inhibitor adsorption (Owate et al., 2014). A decrease in inhibition efficiency with the rise in temperature with analogous increase in corrosion activation energy in the presence of inhibitor compared to its absence is frequently interpreted as being suggestive of formation of an adsorption film of physical (electrostatic) nature. The reverse effect, corresponding to an increase in inhibition efficiency with rise in temperature and lower activation energy in the presence of inhibitor, suggests a chemisorptions mechanism (Popova et al., 2003;Ebenso, 2003;Oguzie et al., 2004). Popova et al. (2003) gave the relationship between the temperature, percentage inhibition efficiency (I.E %) of an inhibitor and the activation energy in the presence of an inhibitor thus: 1. For an inhibitor whose I.E (%) decreases with temperature increase, the value of activation energy (E a ) found is greater than that in the uninhibited solution (blank solution). 2. For an inhibitor whose I.E (%) does not change with temperature variation, the value of activation energy (E a ) does not change in the presence or absence of inhibitors. 3. For an inhibitor whose I.E (%) increases with temperature increase, the value of activation energy (E a ) found is less than that in the uninhibited solution (blank solution).
While the higher value of the activation energy (E a ) of the process in an inhibitor's presence when compared to that in its absence is attributed to its physical adsorption, its chemisorptions is pronounced in the opposite case.
The values of activation energy obtained in this study as presented in Table 3, shows clearly that E a values in the presence of different concentrations of the extract of EFA are higher than that of the uninhibited solution (blank solution). This increase in the presence of the extract of EFA denotes physical adsorption (Ebenso, 2003). This further buttress the increasing inhibition efficiency observed with increasing temperature for the adsorption of the extracts of EFA. An alternative formulation of Arrhenius equation (Fouda et al., 2012) which is also referred to as Transition State (Equation 7): where is the corrosion rate at absolute temperature , is universal gas constant, is the Avogadro's constant, is the Plank's constant, is the entropy of activation and is the enthalpy of activation. Figure 14 shows a plot of Log ( /T) against 1/T, straight lines were obtained with a slope of (∆H o /2.303R) and an intercept of log / + /2.303R, from which the values of and were calculated. This is presented in Table 4.
From the Table, the activation energy (E a ) for F. asperifolia extract ranges from 35.989 to 45.009 kJmol -1 . The average activation energy value obtained for blank is 22.12 kJ/mol. This value is lower than that obtained for a system containing different concentrations of EFA. This result suggests that EFA is adsorbed on the surface of mild steel by physical means (Popova et al., 2003;Awad, 2006).
The data showed that the positive values for ∆H enthalpy of activation both in the absence and presence of the extracts reflect the endothermic nature of the mild steel dissolution process . It is evident that the activation enthalpies vary in the same trend as the activation energies. The value of enthalpy of activation (∆H) for F. asperifolia extracts ranges from 53.4205 to 59.356 kJmol -1 . The values of entropies of activation (∆S) for F. asperifolia extract range from -105.408 to -142.546 kJmol -1 implies that the activated complex in the rate determining step represents association rather than dissociation, meaning that there is a reduction in disorderliness on going from reactant to activated complex. Similar observations were made by Oguzie (2008), Tao et al. (2009), Fouda et al. (2009), Obi-Egbedi and Obot (2012 and Ansari et al. (2014).
The standard free energy of adsorption, ∆G o ads , which can characterize the interaction of adsorption molecules and metal surface, was calculated using Equation 8 (8) This can also be expressed as: (9) K ads values for extracts of EFA was calculated from the intercepts ( Figure 13) and presented in Table 4. ∆G o ads values were calculated using Equation 9 and also presented in Table 4. The negative values of Gibbs free energy ensures the spontaneity of adsorption process and stability of the adsorbed layer on the surface of mild steel (Shukla et al., 2011). The negative values of ∆G o ads of F. asperifolia extract indicate spontaneous adsorption of the plant extract under study onto the mild steel surface (Ahmad et al., 2010) and the strong interaction that exist between the inhibitor molecules and metal surface (Obi-Egbedi and Obot, 2008).
The values of ∆G o ads for F. asperifolia extract are below -40 KJmol -1 but above -20 kJmol -1 . This is an indication that the adsorption mechanism of the plant extract on mild steel in 5 M HCl acid solution at the temperatures 303 to 333K may be physisorption. Generally, the value of ∆G o ads up to -20 kJ/mol are consistent with electrostatic interaction between charged molecules and a charged metal (which indicates physical adsorption) while those more negative than -40 kJ/mol involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond (which indicates chemisorption) (Bhajiwala and Vashi, 2001;Ebenso, 2003;Ahmad et al., 2010;Obi-Egbedi and Obot, 2008). Results of the study have shown that extract of EFA inhibits the acid induced corrosion of mild steel by virtue of adsorption of its constituents onto the mild steel surface. These mechanism of adsorption observed could be attributed to the fact that EFA contains many phyto-constituents in which some have the tendency to adsorb chemically and others physically (Ating et al., 2010). This observation may be attributed to the fact that adsorbed organic molecules can influence the behavior of electrochemical reactions involved in corrosion process in several ways. The action of organic inhibitors depends on the type of interactions between the substance and the metallic surface. The interactions can bring about a change either in electrochemical mechanism or in the surface available for the process (Ahmad et al., 2010;Obi-Egbedi and Obot, 2008).

Adsorption isotherm
Adsorption isotherms are very important to understand the mechanism of heterogenous organo-electrochemical reactions (Bockris and Khan, 1993). Adsorption of organic adsorbate at metal/solution interface can be represented by a substitutional adsorption process between organic molecules in the aqueous solution (Org (sol) ) and water molecules on the metallic surface Org (ads) (Langmuir, 1918;Fouda et al., 2014) Org (sol) + xH 2 O (ads) Org (ads) + H 2 O (sol) where Org (sol) and Org (ads) are the organic molecules in the aqueous solution and those adsorbed on the metallic surface respectively, H 2 O (ads) are water molecules on metallic surface, x is the size ratio representing the number of water molecules replaced by one molecule of organic adsorbate. When equilibrium is reached, it becomes possible to obtain different expressions for the adsorption isotherm plots (Fouda et al., 2014). These factors can be relayed to Equation (10).
where is the configurational factor which depends upon the physical model and the assumptions underlying the derivation of the isotherm, is the degree of surface coverage, is the inhibitor concentration in the electrolyte, is the size ratio, a is molecular interaction parameter and k is the equilibrium constant of the adsorption process .
The surface coverage ( ) values help in understanding adsorption characteristics. The degree of surface coverage values for different concentrations of the extract of EFA obtained at different temperatures from weight loss measurement were calculated using Equation (11).
Surface coverage (11) Inhibition efficiency (%) is directly proportional to the fraction of the surface coverage ( ) by the adsorbed molecule. Therefore, surface coverage with the extract concentration specifies the adsorption isotherm that describes the system given the relationship between the coverage of interface with the adsorbed species and the concentration of species in solution. The value of the surface coverage ( ) at different concentrations of the inhibitors in 5 M HCl solution for each extract was made to fit to various adsorption isotherms. There are many isotherm models to describe the inhibitor/metal interactions, three of these models were used namely: Langmuir, Freundlich and Temkin.

Langmuir isotherm
Langmuir adsorption isotherm describes quantitatively the formation of a monolayer adsorbate on the outer surface of the adsorbent, and after that no further adsorption takes place. The model assumes uniform energies of adsorption onto the surface and no transmigration of adsorbate in the plane of the surface. Langmuir equation is an ideal isotherm for physical and chemical adsorption where there is no interaction between the adsorbate and the adsorbent .
Assumptions of Langmuir relate the concentration of the adsorbate in the bulk of the electrolyte (C) to the degree of surface coverage ( ) according to Equation 12: where C inh represents the inhibitor concentration, is the surface coverage and K ads is the adsorption equilibrium constant. The plots of C/ against C gives a linear plot as shown in Figure 14 indicate the adsorption of the inhibitor (EFA) on the surface of mild steel is consistent with Langmuir isotherm. From the data calculated in Table 4, it's clear that Langmuir isotherm is favourable. The correlation coefficient (R 2 ) of the adsorption isotherm data shows that Langmuir isotherm is best fitted into the study with R 2 ranges from 0.9897 to 0.9914. The values of equilibrium constant K ads are all positive, implicitly, indicative of a favourable adsorption. Similar reports were published by Nnanna et al. (2010), Mejeha et al. (2010), Akalezi et al. (2013) and Adejo et al. (2014).

Freundlich isotherm
Freundlich adsorption isotherm is commonly used to describe the adsorption characteristics for the heterogeneous surface (Hutson and Yang, 2000). The empirical equation proposed by Freundlich encompasses the heterogeneity of sites and the exponential distribution of sites and their energies (Figure 15).
Freundlich Isotherm:  where is the Freundlich isotherm constant, is adsorption intensity; is the inhibitor concentration, is the surface coverage.
Linearizing equation 13, gives: The constant is an approximate indicator of adsorption capacity, while 1/ is a function of the strength of adsorption in the adsorption process (Weber and Chakravarti, 1974). If = 1 then the partition between the two phases are independent of the concentration. If value of 1/ is below one it indicates a normal adsorption.
On the other hand, 1/ being above one indicates cooperative adsorption (Mohan and Karthikeyan, 1997). As the temperature increases, the constants k and n change to reflect the empirical observation that the quantity adsorbed rises more slowly and higher pressures are required to saturate the surface. However, and n are parameters characteristic of the sorbent-sorbate system, which must be determined by data fitting and whereas linear regression is generally used to determine the parameters of kinetic and isotherm models (Guadalupe et al., 2008). Specifically, the linear least-squares method and the linearly transformed equations have been widely applied to correlate sorption data where 1/n is a heterogeneity parameter, the smaller 1/n, the greater the expected heterogeneity. This expression reduces to a linear adsorption isotherm when 1/n = 1. If n lies between one and ten, this indicates a favorable sorption process (Goldberg, 2005). From the data in Table 5, the values of 1/n ranges from 0.0059 to 0.0169 indicating the adsorption of EFA unto the metal surfaces is favourable and the R 2 values range from 0.9178 to 0.9273.

Temkin isotherm
The Temkin isotherm equation assumes that the heat of adsorption of all the molecules in layer decreases linearly with coverage due to adsorbent-adsorbate interactions and that the adsorption is characterized by a uniform distribution of the bonding energies, up to some maximum binding energy (Temkin and Pyzhev, 1940;Oladoja et al., 2008).
The Temkin isotherm is given as: (15) where A (l/g) is the equilibrium binding constant, corresponding to the maximum binding energy and constant b is related to the heat of adsorption. A plot of ϴ against InC (Figure 16) enables the determination of the isotherm constants B and A from the slope and the intercept of the linear plot. The Temkin equilibrium adsorption plot relates the mild steel and the liquid phase concentration of HCl at equilibrium. The values of b calculated are between 9.8673 and 9.9517 for EFA as shown in Table 5. The regression values of the extract tend towards unity (Oladoja et al., 2008;Adejoet al., 2014).

Conclusion
This study has shown that ethanolic extract of F. asperifolia as well as its fractions (n-hexane, ethyl acetate, butanol and aqueous) are found to be effective corrosion inhibitors for mild steel in acidic medium. The reaction was found to best obey Langmuir adsorption isotherm from the fit of the experimental data at all concentrations and temperature ranges studied.