Secondary Plant Metabolites of Natural Product Origin—Strongylodon macrobotrys as Pitting Corrosion Inhibitors of Steel around Heavy Salt Deposits in Gabu, Nigeria

Investigation into the Inhibition of pitting corrosion in mild steel around heavy salt deposits by some selected secondary plant metabolites—alkaloid extract (AESML), saponin extract (SESML and flavonoid extract (FESML) of natural product origin—Strongylodon macrobotrys was successfully completed with the aid of electrochemical impedance spectroscopy, potentiodynamic polarization, gravimetric and gasometric experimentation. The research proved that the selected secondary plant metabolites were excellent inhibitors of mild steel in the salt water environment as inhibition efficiency was recorded at 99.2%, 92.6% and 84.7% for AESML, SESML and FESML. The inhibitors showed higher inhibition at lower temperature due to frequent scale redeposition from agitation in temperature rise and loss in collision of the molecules. The potentiodynamic polarization result confirmed the reduction in the loss of electrons at the anode by the inhibitors that would have trigger oxidation reaction that causes the anode to corrode. Charge transfer resistance reflected the maximum inhibition efficiency obtained for mild steel at maximum concentration and the decrease in double layer capacitance is due to the decrease of the area where electrolyte is present due to the formation of inhibitor film. Thermodynamic investigation shows that the inhibitor has the potential of increasing the energy of the intermediate, reducing both the number of collisions, and number of particles that have enough energy to react and also number of corrosion reaction particles with the correct orientation. The adsorption isotherm consideration shows physical adsorption mechanism with binding constant increasing with increasing temperature.

pleted with the aid of electrochemical impedance spectroscopy, potentiodynamic polarization, gravimetric and gasometric experimentation. The research proved that the selected secondary plant metabolites were excellent inhibitors of mild steel in the salt water environment as inhibition efficiency was recorded at 99.2%, 92.6% and 84.7% for AESML, SESML and FESML. The inhibitors showed higher inhibition at lower temperature due to frequent scale redeposition from agitation in temperature rise and loss in collision of the molecules. The potentiodynamic polarization result confirmed the reduction in the loss of electrons at the anode by the inhibitors that would have trigger oxidation reaction that causes the anode to corrode. Charge transfer resistance reflected the maximum inhibition efficiency obtained for mild steel at maximum concentration and the decrease in double layer capacitance is due to the decrease of the area where electrolyte is present due to the formation of inhibitor film. Thermodynamic investigation shows that the inhibitor has the potential of increasing the energy of the intermediate, reducing both the number of collisions, and number of particles that have enough energy to react and also number of corrosion reaction particles with the correct orientation. The adsorption isotherm consideration shows physical adsorption me-

Introduction
Corrosion is the world's most destructive factor of materials most importantly metals, and one very important factor that slows the advancement of industrialization and economic advancement. Normal steel is an alloy of iron and contains up to 1.5% carbon, sometimes with traces of a few other metals [1]. Due to increased use of metals in modern world, corrosion can cause enormous loss if it occurs. Lives had been lost to corrosion effects; building and bridges have collapsed; leakages of both waste and valuables materials attributed to rust of containers have been verified; breakdown of very valuable and expensive industrial machines has been recorded; road accidents attributed to wears and tears of bolts and nuts at joints have also been established, etc. [2]. From the above mentioned dangers of corrosion, one can indisputably see corrosion as an electrochemical process that causes a material exposed to unfavorable environmental conditions to degrade spontaneously with time leading to threat. There are mainly two factors that affect corrosion-metallic and environmental factors.
Metallic factors range from the position of metal in galvanic series, purity of metal, relative areas of anode and cathode, physical state of metal, etc. while the environmental factors include temperature, humidity, atmospheric impurity, pH value, etc. [1] [3]. The struggle to combat corrosion effects on materials especially metals has come a long way ranging from the use of different protective measures especially environmental modifications, metal selection and surface conditions, cathodic protection, coating, plating to the use of synthesizes inorganic materials that became unhealthy to the environment after usage, and now eco-friendly secondary plant metabolites of natural product [4] [5] [6] [7]. Secondary metabolites are chemicals produced by plants for which no role has yet been found in growth, photosynthesis, reproduction, or other primary functions [8] [9]. They can be classified on the basis of chemical (for example, having rings, containing a sugar), composition (containing nitrogen or not), their solubility in various solvents, or the pathway by which they are synthesized. A simple classification includes three main groups: the terpenes (made from mevalonic acid, composed mostly entirely of carbon and hydrogen), phenolics (made from simple sugars, containing benzene rings, hydrogen and oxygen), and nitrogen-containing compounds (extremely diverse, may also contain sulphur) [ [15]. Strongylodon macrobotrys as shown in Figure 1, is a striking plant from the pea family

Preparation of Standard Solution and Metal Dressing
A raw water sample which was collected in 1000 ml container and evaporated left a salt deposit (NaCl) of about 12 g. Hence the raw water which was now been described with a concentration of 0.2 M standard solution was used for the experimentation and serial dilution into the various inhibitor concentrations of 0.5, 1.0, 2.0, 3.5, 5.0 g/L was carried out, while the raw water was used as the control (0.2 M NaCl). The metal used for this work was obtained from a dilapidated overhead water tank in Gabu secondary school and was resized in the dimension 3.5 cm × 0.1 cm × 3.5 cm for gravimetric analysis, 2.0 cm × 0.1 cm × 2.0 cm for gasometric analysis and 1 cm × 1 cm for electrochemical methods. All the resized metals were adequately polished with electronic UNIPOL-820 metallographic polishing machine to a mirror surface and stored in a moisture free desiccator.

Extraction of Crude Extracts of Strongylodon macrobotrys
After preliminary preparation of the leaves, a MEMMERT WNB-14 laboratory Oven was employed in the drying of the leaves at a temperature low enough (500˚C) to avoid loss of volatile organic constituents present. Small surface area of the dried leaves was obtained by grinding using mortar and pistil followed by a grinding engine then sieved. A sucxhlet extractor and 500 ml ethanol as extraction solvent was adopted for the extraction of the crude ethanol extracts ahead of the preparation of the stock solution of various experimental secondary metabolites. The crude extract obtained after extraction was evaporated in a water bath and stored for use.

Extraction of Alkaloids
Extraction of alkaloid extracts of Strongylodon macrobotrys was possible with the use of diethyl ether, 0.5 M HCl acid and ammonia. A 1000 ml separating funnel was used to host 50 g of crude ethanol extract of Strongylodon macrobotrys and 250 ml of 0.5 M HCl and diethyl ether each. The funnel was stoppered properly and the content shook to attain homogeneity. The mixture was allowed to stand for 3 hours after which the tailing content was separated and the top portion basified with ammonia and 250 ml of diethyl ether added for separation and kept for another 3 hours. The mixture was partitioned in the separating funnel and the tailing content was the collected and evaporated over a water bath. The remaining content after evaporation was the alkaloids.

Extraction of Saponins
The extraction of saponins was possible through the help of metahanol, chloroform and n-butanol. The crude ethanol extract crude (50 g) was digested with 25 ml of methanol and heated using a water bath for 3 hours. This was followed immediately with the addition of 30 ml of chloroform to enable organic portion separation. The methanol fraction obtained was then diluted with 15 ml n-butanol, mixed properly and heated gently to evaporate the n-butanol until dryness to obtain a crystalline soapy extract used as saponins.

Extraction of Flavonoids
Extraction of flavonoids followed the use of 80 g of the powdered leaves and extracted with 250 ml ethanol at ambient temperature for 2 hours. The digested solution was then digested in 100 ml of hexane solution to remove lipids and filtered then evaporated at 600˚C to dryness. This followed the weighing of the extract and the flavonoid extract amount measured. The solution was filtered and the filtrate was evaporated to dryness over water bath at 500˚C.

Gravimetric Experimentation
Hundred ml graduated beakers were used for this experimentation.

Gasometric Experimentation
Gasometric assembly majorly measures the evolution of hydrogen gas as response to level of corrosion by acid or alkaline solution in the presence of heat. Experiment was carried out in triplicate and result averaged for accuracy.

Electrochemical Experimentation
The EIS was investigated at ambient temperature in a triple electrode cell compartment using Gamry Reference 600 potentiostart/galvanostart inclusive of a Gamry framework EIS300 system. Echem analyst software was used to analyze the fitting of the data. A saturated calomel (SCE) electrode was introduced as the reference electrode and a 1 cm 2 platinum foil was introduced as a counter elec- where Ro ct and Ri ct represent the charge transfer resistance with and without the inhibitors.
B. U. Ugi et al.

Potentiodynamic Polarization Experimentation
The behavior of resized steel in the presence of each of AESML, SESML and FESML was investigated by drawing up the anodic site and cathodic site (Tafel) plots. Analysis was carried out on standard alkaline with varied partitions of inhibitors by altering the eV between −250 to +250 mV following a scan rate of 1 mV/sec.
Corrosion current densities values were obtained using Equation (3): where Io corr and Ii corr represent the corrosion current density with and without the extract concentrations.  Figure   2(a), Figure 2

Gasometric Analysis and Results
Data showing the effect of temperature on both the corrosion rate of steel and the surface coverage and inhibition efficiency of various extracted secondary metabolites of Strongylodon macrobotrys leaves in the presence of water of heavy salt deposit are presented in Table 2 [36]. Inhibition efficiency was however seen to take the progression, FESML < SESML < AESML.         [53]. This can also be confirmed from the anodic and cathodic slope values in Table 3.

Electrochemical Impedance Spectroscopy Analysis and Result
It is obvious while considering the plots in Figures 5(a) [54]. The data for the electrohemical parameters obtained using Nyquist curves are presented in Table 4. However, data of the double-layer capacitance for the semicircle were calculated using Equations (5) and (6).
where Z ′′ is immaginay component of impedance at any frequency inside the semicircle and ω is the angular frequency.   Hence, here f max describe the maximun frequency of the semicircle and the π is 3.142. However, the data for the IE R % were obtained from the fitting of the charge transfer resistance values into Equation (7) 0 0 % 100 where 0 ct R and i ct R correspond to the charge ransfer values with and without the test solutions. From Table 4, also showed gradual decrease in the C dl data with inhibitors increase. This could be as a result of the inability of the double layer to allow the line up of charges and storage of electrical energy within [33] [48] [53] [55], consequence upon large surface area coverage effect of the inhibitor on the metal-surface interface. The decrease of double layer capacitance may also be due to the decrease of the area where electrolyte is present due to the formation of inhibitor film [19] [27] [53]. As more and more of either of these inhibitor molecules adsorbed on the surface, it can be seen that the inhibitor film capacitance becomes much lower references. This can explain why the C dl of the AESML is decreasing more compared to the rest informing its higher inhibition effectiveness.

Enthalpy, Entropy, Free Energy and Activation Energy Evaluation
Activation energy Ea is strictly combined with kinetics of chemical reactions.
where k is the rate coefficient, A is the collision constatnt, R is the universal gas constatnt, T is the temperature (in Kelvin), Ea is the amount of energy required to ensure that a reaction happens.
By taking the log of Equation (8), Equation (9) was obtained.
The plots obtained from the fitting of temperature dependent data into Equation (9) are shown in Figures 6(a)-(c). According to Equation (9), it is expected that the graph of lnCR vs 1/T should have slope and intercept equal to Ea/R and logA, respectively. In chemical kinetics, Ea is the height of the potential barrier separating the products and the reactants [11] [15]- [20] [42] [51]. According to   exp exp where the gas constant is R, the temperature is T, the plank constant is h, ΔHads the enthalpy and ΔSads the entropy change.
Taking logarithm of Equation (10), yields Equation (11) Figures 7(a)-(c). From Table 5, values of enthalpy of adsorption were seen to be positive which implies that heat flows (energy) from the surrounding into the reacting system (endothermic reaction).      [59]. This has been confirmed already from the result of entropy of adsorption.

Adsorption Explanations
In order to ascertain the nature of adsorption at the inhibitor/metal interface, adsorption isotherm, specifically Langmuir isotherm was employed. Langmuir adsorption Isotherm plots and data generated from the plots are presented in  Table 6, respectively. The Langmuir adsorption isotherm equation used for the plot of C θ against Concentration is shown in Equation (12).
where C represent extract concentration while K is the binding constant of corrosion inhibition process. It was noticed from Table 6 that the binding constant values were increasing with increased temperature. This explains a greater binding affinity of the inhibitor molecules to the metal surface and also the fact that the inhibition will be in its best efficiency at lower temperatures and this is demonstrated in the gasometric experimentation [13] [19] [37] [56]. It also shows that the inhibitors are physically adsorbed on the metal surfaces. The regression values (R 2 ) were very close to unity, an indication revealing the good fitting of the data to the isotherm and the fact that all the adsorbed inhibitor molecules are in contact with the surface layer of the adsorbent (mild steel) [42] [43] [44] [56] [59]. The slope value for the Langmuir plots is greater than unity, indicating a strong relationship between the species adsorbed.  2) The inhibitors are proved to be thermodynamically feasible, very stabled and spontaneous in the forward direction as the Gibbs free energy of adsorption was less negative.
3) Both inhibitors were very active and adsorbed effectively as the charged transfer resistance increased and corrosion current density for the inhibitor/surface interface decreased appreciably.

4)
The regression values (R 2 ) were very close to unity, revealing the good fitting of the data to the Langmuir isotherm and the fact that all the adsorbed inhibitor molecules are in contact with the surface layer of the adsorbent. 5) Thermodynamic parameters confirmed that the inhibitors have the potential of influencing the incoming energy in the system resulting in higher poten-

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.