CORROSION INHIBITION PERFORMANCE OF ACACIA NOLITICA POD EXTRACT ON ALUMINIUM ALLOY IN 1M ALKALINE SOLUTION

The effect of extract of Acacia Nolitica pod on the corrosion inhibition of LM 6 aluminium alloy in 1M NaOH was studied using weight loss, gasometric, and open circuit potential techniques. Corrosion rates were found to reduce in the presence of the inhibitor. The inhibition efficiency was found to increase with increase in the concentration of the inhibitor and decreased with increase in the temperature. Thermodynamic parameters showed that the adsorption of the inhibitor on the metal surface is a spontaneous process and that the adsorption was via a physisorption mechanism. The adsorption process fitted perfectly with the Langmuir adsorption isotherm indicating that the extract was strongly adsorbed on the aluminium alloy surface. Morphology of the surface was examined by scanning electron microscopy (SEM) in the absence and presence of 0.5%v/v of the used inhibitor which confirmed the existence of a protective film of inhibitor molecule on the metal surface.


INTRODUCTION
Corrosion studies of aluminium and aluminium alloys have received considerable attention by researchers because of their wide industrial applications and economic considerations (Oguzie, 2007). Aluminium and aluminium alloys have emerged as alternative materials in aerospace and in some chemical processing industries. Due to their wide applications, aluminium alloys frequently come in contact with aggressive medium such as: acids or bases during pickling, de-scaling, electrochemical etching and extensively used in many chemical process industries. Alkaline solution is more corrosive in nature for aluminium than other corrosive media (Padmalatha & Deepa, 2017;Qurashi et al., 2017). Therefore, it is desirable to study the corrosion and protection of aluminium in alkaline medium. Aluminium has been reported to have high resistance to corrosion as a result of the oxide film on its surface and this gives it stability (Qurashi et al., 2017)). It is reported that due to the presence of OHion protective oxide film dissolves in alkaline solution and negative potential develops on the aluminium surface (Abiola & Otaigbe, 2008). The shift towards green technology has helped to sustain the growing interest in the use of agricultural plants and their derivatives as sources for the development of inhibitors (Alaleme et al., 2012;Alaleme et al., 2016). Their advantages include low cost of processing, availability, and biodegradability (Loto et al., 2014). Also, some of the plants used as green inhibitors are weeds with very little economic or established medicinal values (Hui, et al., 2013). From literature, the corrosion inhibition characteristics of several plants have been explored as reported elsewhere (Belkhaouda et al., 2013;Loto et al., 2014;Alaneme, 2014;Alaneme & Olusegun, 2012); Deepa et al., 2013;Johnsirani et al., 2013;Alaneme et al., 2015;Shalabi et al., 2014;Abubakaret et al., 2013;Alaleme et al., 2016;Chaubey et al., 2016). In the present work, aqueous extract of the pods of Acacia Nolitica (AN) was selected for the corrosion inhibition study on LM 6 aluminium alloy in 1M NaOH solution using gasometric, open circuit potential and weight loss measurement respectively. The experimental results were supported by SEM investigations.

Chemicals and reagents
All chemicals and reagents used in this study were of analytical grade. LM 6 aluminium alloy was supplied by Orkila chemicals Ltd, Ikeja, Lagos State, Nigeria. The 1M NaOH solution was prepared by dissolving 40 g of sodium hydroxide pellets in 1000 ml of double distilled water as reported elsewhere (Chaubey et al., 2016).

Preparation of pod extract
Acacia Nolitica (AN) Pods were obtained within the environs of Ahmadu Bello University, Zaria, Nigeria. The pods of the plant were gathered, sun dried and pulverized. 40 g of the pulverized pods were taken into 400 ml of prepared 1M NaOH solution in a reagent bottle. The mixture was immersed in a water bath maintained at 70 o C for 3 hours, after which it was removed from the water bath and allowed to stand for some time before filtered to obtain the extract. Stock solutions of inhibitors of 0.1, 0.2, 0.3, 0.4, and 0.5%(v/v) AN pod extract in 1M NaOH were prepared from the filtrate.

Weight loss measurement
Aluminium alloy coupons with dimensions of (2cm×3cm×1.3cm) and chemical composition as per BS1490:1988 of (wt%) 85.95 Al, 0.1 Cu, 0.1 Mg, 12 Si, 0.6 Fe, 0.5 Mn, 0.1 Ni, 0.1 Zn, and 0.45 others, were polished using different grades of silicon carbide impregnated emery paper (120, 400, 800, and 1000 grits). Weight loss studies were carried out under total immersion conditions in beakers maintained at 30 o C. In each experiment, 5 cleaned coupons were weighed (m1) and immersed completely in the inhibited alkaline solution. At ambient temperature (30 o C), the coupons were retrieved at intervals of 2 days progressively for 10 days; this is to allow for a measurable amount of the specimen to react in the solution. Finally, the samples were washed with running water to remove any corroded product(s), dipped in acetone, dried and re-weighed (m2). The weight loss (W) was taken as the difference between the weight at a given time and the initial weight of the test coupon. The corrosion rate (C.R) in millimeters penetration per year (mm/y) was calculated, from equation 1 (Callister, 1996).
Where W is the weight loss (mg), A is the total surface area of the coupons in cm 2 , T is time of exposure in hours, ρ is the density of aluminium alloy in g/cm 3 .

Gasometric measurement
The gasometric setup is basically an apparatus that measures the volume of gas evolved from a reaction as described by Onuchukwu (Njoku, 2014). The reaction vessel was connected to a burette via a delivery tube, which was in turn connected to a reservoir of paraffin oil. 50 ml of the test solution was introduced into the reaction vessel for blank determinations and the initial volumes of air in the burette was taken against that of the paraffin oil and recorded. Thereafter, two aluminium alloy coupons were introduced into the reaction vessel and the flask quickly closed. The volume of hydrogen gas evolved by the corrosion reaction was observed by the drop in the volume of the paraffin oil level in the gasometric gauge. The progress of the corrosion reaction was monitored by careful volumetric measurement of the evolved hydrogen gas at fixed time intervals. The temperature of the experiment was controlled at 30 and 60 o C. The experiments were performed separately employing 0.1 and 0.5% (v/v) inhibitor concentrations in 1M NaOH solution.

Open circuit potential
The potential of aluminium alloy electrode was measured against saturated calomel electrode (SCE) in 1M NaOH solution in absence and presence of 0.5% v/v of the used inhibitors at 30°C. All measurements were carried out using Multi-tester until steady-state potentials are reached; the experiment was conducted as reported by Abd El-Hameed (2011).

Micrograph examination
The specimens used for surface morphological examination were immersed in the 1M NaOH solution containing maximum concentration of the inhibitor and blank for 10 days, it was then removed, rinsed quickly with running water dipped in acetone and dried. Scanning Electron microscope (SEM) EVO MA-10 with Energy-dispersive X-ray spectroscopy (EDX) manufactured by Carl, at the National Geological Survey Institute, Kaduna, Nigeria, was used for the analysis. Sample coupons were attached to multi-stub specimen holder with the use of double sided conductive carbon tape, after which, they were mounted onto the specimen chamber, while the column was put at vacuum. After reaching the vacuum target, the electron gun was switch on which passed accelerating voltage of 20kV and probe current of 227pA through the samples at a working distance of 7.0mm. Micrographs of the coupons were taken in three magnifications along the span of the specimens (Chaubey et al., 2016).

Weight loss measurement Effect of inhibitor concentration
Corrosion rate of the aluminium alloy coupons versus time of immersion was plotted in Figure 1. It is evident from the figure that in all cases, the corrosion rate increases with increase in the time of immersion. The curves obtained in the presence of AN additives falls significantly below that in the absence of inhibitor. In this respect, the increase of the additive concentration was accompanied by a decrease of corrosion rate and an increase in the percentage of inhibition as reported by similar observation (Qurashi et al., 2017).

Effect of temperature
To elucidate the mechanism of inhibition of the corrosion process, weight loss measurements were performed at different temperatures 303, 313, 323, 333K in the presence of 0.5% v/v AN pod extracts in 1 M NaOH. The effect of temperature on the corrosion rate and inhibitor efficiency of the aluminium alloy is graphically represented in Figure 2. It can be seen in the figure that corrosion rate increased with increase in temperature, which may be due to the desorption of some adsorbed molecules from the surface of the aluminium alloy at higher temperature due to which greater area of the metal is exposed to the alkaline environment. The inhibitor efficiency decreased with increase in temperature from 303-333K, leading to the conclusion that the protective film of these compounds formed on the aluminium alloy surface is less stable at higher temperature; which may be attributed to desorption of the inhibitor molecules from the surface at higher temperature and agrees with an earlier 156 research by (Oguzie, 2006). The percentage inhibition efficiency (IE) and degree of surface coverage Ө of the AN pod extract at 0.5% v/v was calculated using the following equations: where (C.R)o and (C.R) are the corrosion rates in (mm/y) without and with different concentrations of the inhibitor.
The apparent activation energy (Ea) was calculated using the Arrhenius equation.
where Ea is the activation energy of the reaction, A is the Arrhenius pre exponential factor, R is the gas constant, C.R is the corrosion rate, and T is the absolute temperature. Figure 3 depicts the plots of log C.R vs 1/T and the values of (Ea) obtained from the slope of the plots are given in Table 1. It was observed that the value of Ea for uninhibited system is 40.05 kJ/mol, which is lower than values observed for the inhibited system. Higher values of Ea for inhibited system are explained by the formation of a physical barrier to charge and mass transfer created by the adsorbed inhibitor molecules (Chaubey et al., 2015). The above Ea values in the presence of inhibitors are larger than in the absence, but lower than 80kJ/mol, which is the threshold value for chemical adsorption. This suggests physical adsorption of inhibitor molecules.

Adsorption isotherm
Langmuir adsorption isotherm as shown in Figure 4, was employed to determine whether the inhibition of aluminium alloy corrosion is due to either physisorption which is the adsorption of molecules of AN pod extract or formation of complex insoluble layer of aluminium alloy on the surface which can act as a barrier in between the corrosive medium and the aluminium alloy surface or through chemisorption which involves the displacement of water molecules from the metal surface and electrons sharing between aluminium alloy and oxygen atom. Langmuir relationship is represented by equation 5.

= 1 + (5)
where K is the equilibrium constant of adsorption, C is the inhibitor concentration %v/v, and θ is the degree of surface coverage (Okewale & Oliatan, 2017). It is seen that the corrosion rate data of AN pod extract fitted to Langmuir adsorption isotherm strongly, this is very obvious from the correlation co-efficient (R 2 )value of 0.985 this suggests that Acacia Nolitica pod extract molecules form a monolayer on the aluminium alloy surface.  inhibitor efficiencies even at low concentration of AN pod extract. In all the cases, the dissolution of aluminium alloy was characterized by a linear increase in the evolution of hydrogen with time. The reaction rate was characterized by differentiating the volume of hydrogen evolved with time and was obtained from the slope of the linear portions of Figure  5(a) and 5(b). Table 2 shows the values of corrosion rates obtained for the test solution. The result shows that corrosion rate reduced in the presence of the inhibitor and was found to decrease with increasing AN pod extract concentration as reported elsewhere (Alaneme & Olusegun, 2012).

Inhibitor efficiency
For the gasometric experiments, the degree of surface coverage ( ) and the inhibition efficiency (IE, %) of AN pod extract on aluminium alloy in alkaline media was evaluated from equations 6 and 7.
where Vfree and Vinh are the corrosion rates in the absence and presence respectively of a given concentration of AN pod extract. Table 3 shows values of the degree of surface coverage ( ) and the inhibition efficiency (IE, %) obtained for different concentrations of AN pod extract using the gasometric technique. The result shows that the extract retarded alkaline corrosion of the aluminium alloy, inhibition efficiencies increased with increasing inhibitor concentration and decreased with increase in temperature.

Open circuit potential measurement
The potential of aluminium alloy electrodes immersed in 1M NaOH solution was measured as a function of immersion time in the absence and presence of AN pod extract as shown in Figure 6. It is clear that the potential of aluminium alloy electrode immersed in 1M NaOH solution (blank curve) tends towards more negative potential firstly. This behaviour was reported by West (1970), which represents the breakdown of the pre-immersion air formed oxide film presents on the surface. It is shown from the figure that addition of the inhibitor molecules shifted the potential to more positive direction (less negative) and as the concentration of the inhibitor increases, the corrosion potential was shifted to more noble direction. In all curves the steady-state values are always more negative than the immersion potential suggesting that before the steady state condition is achieved the aluminium oxide film has to dissolve.

Scanning electron microscopy
Morphological images shown in Figure 7a and 7b shows surface corrosion of aluminium alloy decreased remarkably in the presence of the inhibitor. Inspections of the figures reveal that there is severe damage, clear pits and cavities on the surface of aluminium alloy in the absence of inhibitor than in its presence. This confirms that metal surface is fully covered with adsorbed inhibitor molecules.

CONCLUSION
The corrosion behaviour and adsorption characteristics of varied concentrations of Acacia Nolitica pod extract in 1M NaOH solution containing aluminium alloy substrate was investigated in this study. From the analyses of the results obtained, Corrosion rates reduced in the presence of the inhibitor and were found to decrease with increasing AN pod extract concentration. Protection efficiency by AN pod extract is found to increase with increase in inhibitor concentration but decreased with increase in temperature, suggesting that the protective film of this compound formed on the aluminium alloy surface is less stable at higher temperature. Activation energies were higher in the presence of the inhibitor suggesting physisorption mechanism in alkaline media. The optical examination of the aluminium alloy specimen show that the inhibitor displayed an effective inhibitive tendency against stress corrosion cracking of the materials in the aggressive solution Considered