¹ Department of Chemistry, Faculty of Science, Yarmouk University, Irbid, Jordan

² Department of Biology, Faculty of Science, Yarmouk University, Irbid, Jordan

Abstract

The effect of the extract of Lupinus varius l. on corrosion of Al in 1 M NaOH solution using the weight loss technique was investigated. Lupinus varius l. extract inhibited the corrosion of Al in 1 M NaOH solution and the inhibition efficiency increased with increasing the concentration of the extract and decreased with increasing temperature. The adsorption of the inhibitor molecules on Al surface was in accordance with Langmuir and Temkin adsorption isotherms. A first-order kinetic relationship with respect to Al was obtained with and without the extract from the kinetics treatment of the data.

Keywords: aluminum, Lupinus varius l., inhibition efficiency, basic corrosion, weight loss technique.

Introduction

Several previous studies showed that naturally occurring substances of plant origin are successfully used as inhibitors of corrosion [1-7]. Previously, the corrosion of Al in alkaline solution has been investigated in the presence of organic and inorganic compounds [6-11], and in the presence of some plants extract, such as Gum Arabic [12], Pachylobus edulis and Raphia hookeri [13]. This work aims to studying the effect of Lupinus varius l. extract on the corrosion of Al in alkaline solution. Lupinus varius l. belongs to the family of Leguminosae. L. varius L. ssp. orientalis Franco et Silva (=Lupinusdigitatus Forssk, Lupinus pilosus L., Lupinus hispanicus and Lupinusmicroanthus) is an annual herb, growing in the Mediterranean, North Africa, South Europe, West Syria, and Palestine region [14,15]. Previous investigations of this species revealed the presence of several quinolizidine as well as dipiperidine alkaloids [16,17]. It has been shown that the inhibitory action of some plants solution extract is due to the presence of tannin, organic acids, amino acids, alkaloids and pigments in their chemical constiutions [18]. Alkaloids such as papaverine, strychnine, quinine and nicotine, were studied as corrosion inhibitors in acidic medium [5]. It has been shown that the inhibitive effect of the extract of some plants is due to the adsorption of molecules of phytochemicals present in the plant on the surface of the metal [19-21]; theses extract protect the metal surface and thus do not permit the corrosion process to take place.

Weight loss measurement was used to calculate the inhibition efficiency of the L. varius l. extract. The effect of temperature on the corrosion reaction rate in free and inhibited basic solutions was also investigated.

Experimental

Preparation of specimen

The weight loss measurements were carried out in a test tube placed in a thermostat water bath. The solution volume was 10 mL. Test specimens of 99.96% aluminium foil (length = 2 cm, width = 1 cm, thickness = 0.03 cm), were degreased in acetone, chemically polished for 30 s in a solution containing 85 % conc. H₃PO₄ and 15 % conc. HNO₃ at 85 °C, rinsed well in deionized water, etched for 20 s in a solution containing 40 gL-1 sodium hydroxide at 40 °C, rinsed again and immersed for 20 s in 1:1 v/v HNO₃ at room temperature. After rinsing in deionized water and drying in a stream of air at room temperature, the specimens were kept in a desiccator.

Inhibitor material

A stock solution of the inhibitor material was prepared by refluxing 15.0 g of dry L.varius l. Powder with 250 mL of 1 M NaOH for 3 hours. The refluxed solution was allowed to stand overnight and filtered through ordinary filter paper. From this solution, different concentrations of inhibitor solutions ranging from 20 to 100% were diluted.

Weight loss method

Pre-weighed aluminium specimens (in triplicate) were suspended for 1 hour in 1 M NaOH with and without the inhibitor in different volume ranging from 2 to 10 mL of extract. After the specified time, the coupons were removed from test solution, thoroughly washed with acetone solution and deionised water, dried well and then reweighed.

Results and discussion

where W_B and W_A are the weights of metal before and after exposure to the corrosive solution, respectively.

The rate of reaction of Al 1 M NaOH decreases with time.

The effect of temperature on the corrosion of Al in 1 M NaOH over the temperature range of (25-50 °C) in the absence and presence of different concentrations of the L varius l. extract has been studied. The weight loss is found to increase with increasing temperature.

The percentage inhibition efficiency (%I) and the degree of surface coverage (Θ) of the investigated L. varius l. extract were computed from the following equations:

where ΔW_free and ΔW_inh are weight losses of metal in the absence and presence of inhibitor, respectively.

where R is the corrosion rate, A is the constant frequency factor and Ea is the apparent activation energy.

It is clear that, the activation energy increases in presence of L. varius l. extract and consequently the rate of corrosion reaction is decreased.

An alternative formulation of the Arrhenius equation is the transition state equation [25]:

where h is Planck's constant, N is Avogadro's number, ΔS* is the entropy of activation and ΔH* is the enthalpy of activation.

where R is the gas constant, Θ1and Θ2 are the degree of surface coverage at temperatures T1 and T2, respectively.

The calculated Qads values are ranged from -58.84 to -113.80 kJ/mol. This negative value indicates that the adsorption of L. varius l. extract on the surface of Al metal is exothermic [15].

This is because a straight line is obtained when Log(C/Θ) is plotted against logC and the linear correlation coefficient of the fitted data is very close to 1. This indicates that the adsorption of L. varius l. extract molecules obeys the Langmuir adsorption model [10,18] expressed as

The relationship between the equilibrium constant, K, of adsorption and the free energy of adsorption, ΔG_ads, is given by the following expression [3, 16]

where Wi is the initial weight of aluminum specimen, ΔW is the weight loss of aluminum specimen at time t, [Wi - ΔW] is the residual weight of aluminum coupon at time t and k is the first-order rate constant. The linear plots obtained with correlation coefficients close to 1 confirm a first-order kinetics for the corrosion of aluminum in 1 M NaOH solution in the presence and absence of Lupinus varius l. extract.

Conclusions

From the experimental results obtained in the present study, the following conclusions could be drawn:

1. L. varius l. extract acts as inhibitor for aluminum corrosion in NaOH solution.

2. Inhibition efficiency increased with increase in concentration of the L. varius l. extract but decreased with increase in temperature. Phytochemical constituents in the extract play a very vital role in the inhibiting action.

3. Activation energies were higher in the presence of the exudates gum suggesting physisorption mechanism.

4. The present study provides new information on the inhibiting characteristic of a L. varius l. extract under the specified conditions. The adsorption of L. varius l. extract fits into Langmuir isotherm and Temkin isotherm models and a first-order kinetics relationship was obtained from the kinetics treatment of the data of weight loss measurements.

References