Mild Steel Corrosion Inhibition by Eremomastax polysperma Leaf Extract in Acidic Medium

The inhibition of mild steel corrosion in 1 M H2SO4 solution by Eremomastax polysperma leaf extract has been studied using weight loss and hydrogen evolution techniques. The results obtained show that Eremomastax polysperma leaf extract is a good inhibitor of mild steel corrosion in H2SO4 solution. The inhibition efficiency increased with increase in extract concentration but decreased with increase in temperature. Physical adsorption has been proposed for the adsorption of the leaf extract on mild steel surface. Thermodynamic parameters reveal that the corrosion inhibition process in the presence of the extract was endothermic and occurred spontaneously. The adsorption of Eremomastax polysperma leaf extract on mild steel surface best fitted the Langmuir adsorption isotherm.


INTRODUCTION
The present day high operational cost of extracting metals from their ores has made it imperative to protect existing metallic structures from corrosion. Corrosion weakens the mechanical strength of metals leading to equipment breakdown or failure [1]. The use of corrosion inhibitors for the protection of metals in aggressive media is a standard practice globally. Compounds containing nitrogen, sulphur, oxygen and/or phosphorous have been reported as good inhibitors of mild steel corrosion in acidic media [2][3][4][5][6]. The use of traditional inhibitors (i.e. synthesised organic and inorganic compounds) is now limited due to environmental safety concerns, as some of them are toxic and environmentally -unfriendly. The desire for biodegradable, cheap, renewable and environmentally -friendly corrosion inhibitors has led to the extraction of inhibitors from natural products, especially of plant origin. Plant extracts contain phytochemicals such as saponins, alkaloids, terpenes, tannins, etc, which are rich sources of organic nitrogen, sulphur and oxygen. Many workers have reported high inhibition efficiencies of mild steel corrosion by leaves extracts in acidic medium [1,[7][8][9][10]. Unfortunately, some of the reported plants are known to grow only in certain regions of the world. Importation of extracts of such plants for use in other regions of the world is not only expensive but time consuming. Hence, the need for the local sourcing of efficient eco-friendly corrosion inhibitors cannot be over-emphasised.
Eremomastax polysperma is a medicinal plant in the family Acanthaceae. It is locally called Edem iduduot by the Efik and Ibibio speaking people of Nigeria. The traditional medicinal uses of the plant in eastern Nigeria have been documented [11][12][13]. The phytochemical screening of Eremomastax polysperma leaf extract showed the presence of tannins, phenol, flavonoid, alkaloid, saponin and sterol [14].
The aim of this work was to evaluate the inhibitory effect of Eremomastax polysperma leaf extract on mild steel corrosion in H 2 SO 4 solution. H 2 SO 4 solution is widely used industrially in the pickling of iron and steel because it is less fumy than other mineral acids. Additionally, the choice of 30°C -60°C for this work was to simulate the temperature for the pickling process.

Test Materials
Mild steel sheet used for this work was obtained in Calabar, Nigeria. It had the following chemical composition (weight %): C (0.12), Mn (0.85), S (0.06), P (0.05), Si (0.09) and Fe (98.83). The sheet was mechanically press -cut into 4 cm x 5 cm coupons, and polished to mirror finish using different grades of silicon carbide papers. The coupons were degreased in absolute ethanol, dipped in acetone before airdrying. They were then stored in a moisture -free desiccator before use in corrosion studies.

Preparation of Eremomastax polysperma Leaf Extract
Fresh leaves of Eremomastax polysperma were collected from a farm in Nung Oku Ibesikpo, Akwa Ibom State, Nigeria and authenticated by a plant taxonomist in the Department of Botany and Ecological Studies, University of Uyo, Nigeria. They were washed and air -dried at 30°C for seven days. They were then ground to powder. The dried ground sample of Eremomastax polysperma leaves was macerated with 90% ethanol for seven days at room temperature in a glass trough with cover. The mixture was then filtered. The filtrate was evaporated at 40°C in a water bath to constant weight, leaving a dark green extract (paste) in the beaker.

Weight Loss Method
Previously weighed mild steel coupons were suspended with the aid of glass hooks and rods and immersed in 100 ml of 1 M H 2 SO 4 solution (blank) and in 1 M H 2 SO 4 solution containing 1.0 g/L -4.0 g/L Eremomastax polysperma leaf extract (inhibitor) in open beakers. In each experiment, one mild steel coupon per beaker was used. The beakers were then placed in a thermostatic water bath maintained at 30°C, 40°C, 50°C, and 60°C, respectively. The mild steel coupons were retrieved from the test solutions after four (4) hours and scrubbed with bristle brush under running water. They were dipped in acetone and air -dried before reweighing. The weight loss was recorded and used to compute the inhibition efficiency, I(%) [15]: where W 0 and W 1 are the weight losses of the mild steel coupons in the absence and presence of extract, respectively, in 1 M H 2 SO 4 at the same temperature.
The corrosion rate (CR) was calculated using the formula [16]: where W is the weight loss (mg), A is the total surface area (cm 2 ) while t is the exposure time (hours).

Hydrogen Evolution Method
The reaction vessel and procedure for measuring the corrosion process by this method are as described in literature [17]. A 100 cm 3 of 1 M H 2 SO 4 solution was introduced into the reaction vessel connected to a burette through a delivery tube. The initial volume of air in the burette was recorded. Two mild steel coupons weighing 8.0 g were dropped into the 1 M H 2 SO 4 solution and the reaction vessel quickly closed to prevent any escape of hydrogen gas. The volume of H 2 gas evolved from the corrosion reaction was monitored by the depression (in cm 3 ) in the paraffin oil level. The depression in paraffin oil level was monitored every 60 seconds for 100 minutes. The same experiment was repeated in the presence of 1.0 g/L -4.0 g/L Eremomastax polysperma leaf extract in 1 M H 2 SO 4 solution.
The inhibition efficiency I(%) was calculated using the equation [18]: where V 0 and V 1 are the volumes of H 2 gas evolved in the absence and presence of inhibitor, respectively, at a specified time.

Effect of Eremomastax polysperma
Leaf Extract Concentration on Inhibition Efficiency Fig. 1 shows that at a particular temperature the inhibition efficiency increased with increase in the concentration of Eremomastax polysperma leaf extract. The maximum inhibition efficiency of the leaf extract was 90.42% at extract concentration of 4.0 g/L at 30°C. An increase in inhibition efficiency with increase in inhibitor concentration indicates a strong interaction between the mild steel surface and the inhibitor [19].  Fig. 2 illustrates the effect of Eremomastax polysperma leaf extract on the volume of H 2 gas evolved in the corrosion of mild steel in 1 M H 2 SO 4 . It is observed that, at a given time, as the leaf extract concentration increases, the volume of H 2 gas evolved decreases. This indicates that the extract inhibits the corrosion process by adsorbing on the metal surface, forming protective thin films which reduce/stop the electron transfer process on the metal surface [20]. Table 1 contains the calculated values of inhibition efficiency for the inhibition process containing Eremomastax polysperma leaf extract. Table 1 reveals that the inhibition efficiency increased with increase in the concentration of Eremomastax polysperma leaf extract. The inhibition efficiencies obtained by both the weight loss and hydrogen evolution methods followed a similar trend.

Effect of Temperature on Inhibition Efficiency
Table 2 reveals an increase in the corrosion rate of mild steel in H 2 SO 4 solution with increase in temperature. This shows that the reactants acquired more energy and overcame the activation energy barrier more easily with an increase in temperature than at lower temperatures. Additionally, Table 2 shows that the inhibition efficiency decreased with increase in temperature at the extract concentrations studied. This indicates weakening of adsorption bonds between mild steel surface and inhibitor as well as a physical adsorption process [21].   The values of the activation energy (E a ) for mild steel corrosion in 1 M H 2 SO 4 solution in the presence and absence of Eremomastax polysperma leaf extract, respectively, were obtained using the alternative formulation of Arrhenius equation [20]:

Fig. 2. Variation of volume of H 2 gas evolved (cm 3 ) with time (min) for mild steel corrosion in 1 M H 2 SO 4 in the absence and presence of Eremomastax polysperma leaf extract at 30°C
where CR is the corrosion rate, R is the universal gas constant, T is the absolute temperature while A is the pre-exponential factor.
The activation energies (E a ) of mild steel corrosion in 1M H 2 SO 4 solution, with and without inhibitors, were obtained from the gradients of ln CR vs. 1/T plots (Fig. 3) and the results are presented in Table 3. Table 3 shows that the E a values in the presence of the leaf extract were higher than the E a value of the blank (42.8578 kJ mol -1 ). The increase in the E a values in the presence of the extract indicates physical adsorption while the reverse signifies chemical adsorption [22,23].
The values of enthalpy of activation (ΔH°a ds ) and entropy of activation (ΔS°a ds ) were obtained from an alternative formulation of the transition state equation [24]: where CR is the corrosion rate, T is the absolute temperature, R is the universal gas constant, h is the Planck's constant, and N is the Avogadro's number. Fig. 4 shows linear plots of ln (CR/T) vs. 1/T with gradients of (-ΔH°a ds /R) and intercepts of [ln (R/Nh) + ΔS°a ds /R] from which the values of ΔH°a ds and ΔS°a ds were calculated and listed in Table 3. The positive values of ΔH°a ds indicate that mild steel corrosion inhibition was an endothermic process. Additionally, the negative values of ΔS°a ds show a decrease in the disorderliness of the system, indicating an ordered layer of extract on mild steel surface.

Adsorption Isotherm
The best fit for the adsorption of Eremomastax polysperma leaf extract on mild steel surface was obtained by the modified Langmuir isotherm defined as: where C is the inhibitor concentration, θ is the degree of surface coverage while K ads is the equilibrium adsorption constant. The degree of surface coverage of mild steel by Eremomastax polysperma leaf extract is presented in Table 4. It is noticed that the degree of surface coverage increased with increase in extract concentration but decreased with increase in temperature.
Linear plot of C/θ vs. C (Fig. 5) shows that the adsorption of Eremomastax polysperma leaf extract on mild steel surface in 1 M H 2 SO 4 solution obeyed the Langmuir adsorption isotherm. The values of K ads were evaluated from the intercept of the graph and presented in Table 5. K ads is related to the standard free energy of adsorption (ΔG°a ds ) by the formula [25]: where 55.5 is the molar concentration of water in the solution in mol dm -3 .
The negative values of ΔG°a ds reveal that the mild steel corrosion inhibition process by Eremomastax polysperma leaf extract occurred spontaneously. Generally, values of ΔG°a ds less negative than -20 kJ mol -1 indicate physical adsorption while those more negative than -40 kJ mol -1 indicate chemical adsorption [26][27][28]. Consequently, the values of ΔG°a ds obtained in this work being less negative than -20 kJ mol -1 coupled with a decrease in the inhibition efficiency with increase in temperature indicates a physical adsorption process.

CONCLUSION
On the basis of this study, the following conclusions could be drawn: Eremomastax polysperma leaf extract appreciably inhibited the corrosion of mild steel in H 2 SO 4 solution. The inhibition efficiency increased with increase in Eremomastax polysperma leaf extract concentration but decreased with increase in temperature. Based on a decrease in inhibition efficiency with temperature, higher E a values in the extract compared to the blank and the ΔG°a ds values being less negative than -20 kJ mol -1 , physically adsorption has been proposed for the adsorption of