The mild steel sheet used for the study were of composition (wt %) Mn (0.64), P (0.06), C (0.19), S (0.05), Ni (0.09), Cr (0.08), Mo (0.02), Cu (0.27), Si (0.26) and the rest Fe. The sheet was cut into coupons of dimension 2 × 0.08 × 5 cm and 4 × 0.08 × 5 for the hydrogen evolution and weight loss measurements. Before measurements the mild steel coupons were mechanically polished with series of emery paper of up to 400 grits, degreased with absolute ethanol, dried in acetone and preserved in desiccators. Distilled water was used to prepare all experimental solutions. All other chemicals used were of Analar grade and all weighing was done with Mettler P 1210 digital analytical balance.

The required plant parts Viscum album (Mistletoe) was collected from a local bush in Ikot Ekpene, Akwa Ibom State, Nigeria. These were cut into small chips and dried in an N53C―Genlab laboratory oven at 323 K and ground into powder form. 80 g of the powder was extracted continually with 250 cm³ of absolute ethanol in a soxhlet extractor for 24 hours. The extracted jell was heated in a water bath (at 333 K) until most of the ethanol evaporated. 10 g of the ethanol extract was dissolved in 1 liter of 5.0 M H₂SO₄ solution. The resultant solution was kept for 24 hours, filtered and stored. From the stock solution (10.0 g/L), inhibitor test solutions were prepared at concentrations of 0.5, 1.0, 2.5, 4.0, 5.0 and 7.5 g/L. Phytochemical screening was carried out on the ethanol extracts of Viscum album following the methods previously described by [16] and Trease & Evans [17] . The plant extracts were screened for alkaloids, saponins, tannins, flavonoids, phytates, phenols, oxalate and glycosides. The same procedure was repeated to obtain various concentrations of PEVA, SEVA and FEVA. Gasometric and weight loss experiments were then carried out using different concentrations of the extracts at different temperatures.

Gasometric assembly was used for the measurement of hydrogen gas evolution from corrosion reaction. The assembly was coupled and the connections were greased to avoid the escape of the gas. 100 cm³ of the corrodent (5.0 M H₂SO₄ solution) was introduced into the reaction chamber connected to a burette through a delivery tube. A sample of the polished mild steel coupon was dropped into the solution and the reaction chamber quickly closed to prevent any escape of hydrogen. The volume of the hydrogen gas evolved from the corrosion reaction was monitored by the depression (in cm³) in the paraffin oil level. This depression in the paraffin oil level was recorded every minute for 60 minutes (1 hour). The same experiment was repeated in the presence of 0.5, 1.0, 2.5, 4.0, 5.0, 7.5 and 10.0 g/L ethanol extracts of Viscum album (EEVA) at 303 K. Also the experiments were carried out with 2.5 g/L EEVA at 313, 323 and 333 ± 2 K. The same experiment was repeated in the presence of different concentrations (0.5, 1.0 and 2.5 g/L) of SEVA, FEVA and PEVA at 303 K. Then also, the experiments were repeated with 2.5 g/L of each extract at 313, 323 and 333 ± 2 K.

In the weight loss experiment, a weighed mild steel coupon was suspended in a beaker containing 100 cm³ of 5.0 M H₂SO₄ solution with the aid of glass rod and hook at room temperature. The mild steel coupon was completely immersed in the solution and after 1 hour the mild steel coupon was retrieved, washed, scrubbed with bristle brush by immersing it in 20% NaOH containing 20 g of zinc-dust. After washing, the mild steel coupon was rinsed with absolute ethanol, dipped in acetone, air dried, re-weighed and re-immersed in the corrodent. The weight loss of the mild steel coupon was evaluated in grams as the difference between the initial and final weight of the coupon. This process was repeated progressively for 6 hours. The same experimental procedure was carried out in the presence of ethanol extracts of Viscum album using concentrations of 0.5, 1.0, 2.5, 4.0, 5.0, 7.5 and 10.0 g/L. Experiments were carried out on mild steel coupons at room temperature 303 K only for the weight loss study.

Phytochemical screening was carried out on the ethanol extracts of Viscum album. The results obtained are shown in Table 1 and this indicates that the plant extracts contain glycosides, phytates, saponins, flavonoids and tannins. These compounds are heterogeneous organic compounds containing nitrogen, oxygen, sulphur and/or aromatic ring in the molecular structures. Organic heterogeneous compounds containing these elements have been reported to be efficient corrosion inhibitors [19] [24] . Thus, in the systems studied, these compounds are believed to contribute significantly to the inhibition of the corrosion reaction of the mild steel in acid media.

The variation of the weight loss for mild steel coupons with immersion time in 5.0 M H₂SO₄ solutions in the absence and presence of EEVA at 303 K is shown in Table 2. From the results, it is observed that weight loss increases with time but decrease with increase in the concentration of the extracts. This behavior reflects the inhibitory effect of the extract toward the acid corrosion of the steel.

Notes: +; moderately present in the extracts, ++; highly present, −; absent in the extract and ND, not determined.

The rates of corrosion (in mg cm⁻²・hr⁻¹) of the mild steel in the acid medium in the absence and presence of the plant extracts are presented in Table 3. It is clearly seen that the corrosion rate decreases with increase in the inhibitor concentration, indicating that the extent of inhibition is dependent on the amount of extract present. From the corrosion rate values, the inhibition efficiency (%I) was determined using Equation (1).

% I = ( C R blank − C R inh ) × 100 C R blank (1)

where CR_blank and CR_inh are the corrosion rates in the absence and presence of the plant extracts. The results obtained are given in Table 3. It is observed that the inhibition efficiency increases as the added inhibitor concentrations are increased. This suggests that the phytochemical components of the extracts are adsorbed onto the mild steel surface resulting in the blocking of the reaction sites, thus protecting the mild steel surface from the attack of the corrosion active ions in the acid medium.

The acid corrosion of mild steel is characterized by evolution of hydrogen and the rate of corrosion is proportional to the amount of hydrogen evolved [5] [7] , thus the volume of hydrogen evolved (V_H) during the corrosion of mild steel in 5.0 M H₂SO₄ solutions in the absence and presence of the ethanol extracts from Viscum album at 303 K as a function of time is depicted in Figure 1. The presence of the extract decreases the volume of hydrogen evolved during the corrosion reaction compared to the blank system. Also, similar trends were obtained at 313 K, 323 K and 333 K during the corrosion reaction for the extract of Viscum album with a concentration of 0.5, 1.0, 2.5, and 5.0 g/L. The rate of hydrogen evolved was deduced from the slopes of the plots in Figure 1 and that for the elevated temperatures [18] and presented in Table 4. It is seen that the rate of hydrogen evolution decreases with increase in the extract concentrations and temperature. From the rate of hydrogen evolution, the inhibition efficiencies were determined using Equation (2). The results obtained are shown in Table 4 at 303 K and Table 5 at (313, 323 and 333 K). The inhibition efficiency was

observed to increase with an increase in the extracts concentration and temperature (Table 5). This observation however, suggests a chemical interaction between the metal surface and the phyto chemical components of EEVA.

% I = ( R H B − R H I ) × 100 R H B (2)

where R_HB and R_HI are the rates of hydrogen evolution in the absence and presence

of plant extract. From the rate of hydrogen evolution, the inhibition efficiencies of different concentrations of flavonoids extract of Viscum album (FEVA), saponins extracts of Viscum album (SEVA), phytates extracts of Viscum album (PEVA) in 5.0 M H₂SO₄ solution at 303 K were determined and presented in Table 6. The results obtained indicate that inhibition efficiencies generally increased with increase in extracts concentration for all the extracts. The highest inhibition efficiency of 85.3% obtained at 5.0 g/L SEVA, indicates that the dominant inhibitive effect of Viscum album may be attributed to SEVA while the lowest %I of 0.9% was obtained at 1.0 g/L of FEVA. The Inhibition efficiency of EEVA was generally higher than that of SEVA, FEVA and PEVA at lower concentrations (0.5 - 2.5 g/L), this suggests a synergistic interaction between the phytochemicals in Viscum album at low concentration. However, at higher concentrations of 5.0 g/L the efficiency of PEVA (81.5%) and SEVA (85.3%) became more pronounced.

Comparison of the variation of inhibition efficiency with inhibitor concentration for EEVA, PEVA, SEVA and FEVA in 5.0 M H₂SO₄ at 303 K is presented in Figure 2. The order of the efficiency at low concentrations obey the following trend EEVA > PEVA > SEVA > FEVA and at higher concentration the order was SEVA > PEVA > EEVA > FEVA. This indicates synergism at lower concentration and the opposite at higher concentration.

The volume of hydrogen evolved during corrosion of mild steel in 5.0 M H₂SO₄ containing 2.5 g/L of PEVA at different temperatures is shown in Figure 3. From this figure, it is seen that the volume of hydrogen gas evolved increases with increase in temperature. Similar trends were obtained for FEVA and SEVA as shown in Figure 4 and Figure 5.

This is attributed to the increase in reaction rate which is known to increase with temperature. At elevated temperatures more of the reactant molecules gain enough energy required to overcome the energy barrier need to form products,

thus leading to the higher rates of hydrogen evolution. From the rate of hydrogen evolution, the inhibition efficiencies for mild steel in 5.0 M H₂SO₄ solution in the absence (blank) and presence of 2.5 g/L of EEVA, FEVA, PEVA and SEVA at different temperatures were determined and presented in Table 7. The results obtained indicate that the inhibition efficiencies increase with an increase in temperature. The maximum %I, 91.8% was obtained at 333 K for 2.5 g/L concentration of EEVA and the minimum 31.4% at 303 K for 2.5 g/L concentration of FEVA. This suggests adsorption of the extracts via chemical interactions between the heterogeneous inhibitor molecules and the metal surface. The inhibitory effects observed for all the extracts can be attributed to the presence of hetero atoms in the molecular structure of the phytochemicals which provide good

adsorption sites unto the metal surface [20] .

The observed corrosion inhibition of mild steel in 5.0 M H₂SO₄ solutions in the absence and presence of EEVA, PEVA, SEVA and FEVA can be explained by the adsorption of the components of the plant extract on the metal surface. The surface coverage values obtained from this study were applied to different adsorption isotherm equations. From the results, it was found that the experimental data fitted the Temkin adsorption isotherm shown in Figure 6 which may be formulated by Equation (3).

f ( θ , x ) exp ( − 2 a θ ) = K a d s C (3)

where, f(θ,x) is the configuration factor which depends upon the physical model and the assumption underlying the derivation of the isotherm, c is the inhibitor concentration, θ is the degree of surface coverage and K_ads is the adsorption-desorption equilibrium constant, and “a” is an interaction parameter. The calculated values of K_ads and “a” are as shown in Table 8. The positive values of “a” indicate attractive forces between adsorbed molecules while negative values indicate repulsive forces between the adsorbed molecules.

It can be seen in Table 8 that the value of “a” is negative indicating that repulsion exists in the adsorption layer. It can also be observed from Table 8 that the correlation factors (R²) for the isotherms in the presence of EEVA and SEVA are 0.9804 and 0.9990. This value is close to unity, signifying adherence to the principles underlying the derivation of the Temkin adsorption isotherm. It is generally known that K_ads denotes the strength between the adsorbate and adsorbent. Large values of K_ads imply more efficient adsorption and hence better inhibition efficiency [7] . In the present case, K_ads values for PEVA and SEVA is higher than those of EEVA and FEVA. This implies that PEVA and SEVA are better adsorbed [7] .

The rate of a chemical reaction increases with increase in temperature. Increase in temperature was found to increase the inhibition efficiencies of the plant

extracts indicating chemical adsorption. In order to further support that chemisorption is proposed, the activation parameters for the mild steel in 5.0 M H₂SO₄ solutions in the absence and presence of the extracts from Viscum album were calculated using the Arrhenius equation and the transition state equation given by Equations (4) and (5), respectively.

K = A exp ( − E a R T ) (4)

K = R T N h exp ( Δ S 0 R ) exp ( − Δ H 0 R T ) (5)

where, R is the universal gas constant, N is the Avogadro number, h is the Plank’s constant, E_a is the activation energy, T is the absolute temperature, and ΔH_o and ΔS_o are the standard enthalpy and entropy of activation, respectively. The values of K were taken to be equal to the rate of hydrogen evolution [18] [19] . The Arhenius plot of logK versus 1/T, for mild steel in 5.0 M H₂SO₄ solutions containing EEVA, PEVA, FEVA and SEVA shown in Figure 7 gave straight lines. The calculated activation energies obtained from the slope of the plots are listed in Table 9. The results showed that E_a value in the presence of EEVA, PEVA, FEVA and SEVA is lower compared to the blank system. This behavior is an indication of chemical adsorption of the inhibitor on the surface of the metal [20] [21] . The lower E_a value in the presence of the inhibitor indicates that the inhibitor will be effective at higher temperature and efficiencies will increase at low temperature. The enthalpy and the entropy of activation

values for the dissolution of mild steel in 5.0 M H₂SO₄ solutions obtained from linear transition state plots of log(K/T) data versus 1/T shown in Figure 8 are presented in Table 9. The lower value for ΔH_o in the presence of the plant extract is indicative of lower protection efficiency observed for the system.

Generally, the positive value of ΔH_o obtained indicates an endothermic nature of the dissolution of mild steel in the presence of the inhibitors. The endothermic adsorption process can be attributable to chemisorption [22] .

The ΔS_o value in the absence and presence of plant extract is negative. This implies that the activation complex in the rate determining step represents association rather than dissolution step, meaning that a decrease in disordering takes place on going from reactants to the activated complex. The negative value for entropy also indicates the non-spontaneous nature of the dissolution as a result of the addition of the inhibitors [23] [24] [25] .